August 19, 2021 • 47 mins
Daniel and Jorge explain how the speed of light limits how events play out in the Universe.
Learn more about your ad-choices at https://www.iheartpodcastnetwork.com
See omnystudio.com/listener for privacy information.
Mark as Played
Transcript
Episode Transcript
Available transcripts are automatically generated. Complete accuracy is not guaranteed.
Speaker 1 (00:09):
Or Hey, do you ever wish that the speed of
light was faster? I guess I'm not dissatisfied with the
speed of light. I'm just glad it's not slower, because
you know, I don't want to wait ten minutes after
I flip on the light switch. But you know, if
it was faster, and then we could see like further
out into the universe. M m. That's true, and then
we might be able to travel further to explore the stars.
(00:30):
Also true. But there's another side to it, as always, well,
how could it be bad to have a faster speed
of light? Well, light speed was a million times faster
than technically any alien out there in the universe could
kill us instantly with a death ray. That's true. But
you know, witnessing an alien death ray would be pretty cool.
It would be pretty cool for about a nano second,
(00:51):
and then you'd be dead. Also, I'm pretty sure a
death ray would be hot and not cool. Might still
be worth it, just for that nana second, you would
know that AIDS existed. As my brain is getting FRIEDA.
(01:16):
I am or haemade cartoonists and the creator of PhD comics. Hi,
I'm Daniel, I'm a particle physicist, and I don't spend
a lot of time worrying about alien death rays. You don't,
do you know something I don't. I know that the
speed of light protects us from most of the universe
shooting us with alien death rays. Really like it forms
a force field around us, protecting this like a blanket
(01:37):
helping us sleep at night. It just makes it kind
of ineffective. If you're going to shoot an alien death
ray millions of light years, then you're gonna kill your
target in millions of years. And like, who's geopolitical strategy
really has that long and arc to it? But what
if there are patient aliens? Then we're toast? Then we
might literally be toast. Welcome to our podcast, Daniel and
Jorge Explain the Universe, a production of I Heart Radio
(01:59):
in which we beam our joke rays into your brain,
hoping to transmit some of the incredible wonder and knowledge
about the universe, all the things that science has learned
about this Bonker's wondrous, beautiful, glittering cosmos, and all the
things that science is still working on, all the vague
questions about how the universe got here, where it's going,
and who lives in it, and how to actually build
(02:21):
a joke Ray. I mean, that's probably the next Nobel Prize.
The question it doesn't make you groan or does it
make you laugh? Is that where you're gonna ask when
the aliens come, I'm gonna be like, tell us the
secrets of spacetime and you're like, have you developed the
joke Ray yet? We'll think about what it would mean
for the mental health of humanity. You know, we could
just have a laugh about everything. Laughter is the best medicine.
So anytime you go to the doctor, they're just like, yep,
(02:43):
we're prescribing you five shots with the joke Ray. And
then as the aliens as they you know, enslave us
and possibly eat us, we could be laughing all the way.
But it is a bizarre and jokey universe because it
seems like a lot of it is beyond our control
or even beyond our ability to whoever interact with it.
That's true, and it's also sometimes beyond our ability to
(03:05):
really understand. We have one way of thinking about the
world where things move and slide and the way time works.
But about a hundred years ago we discovered that that
wasn't really the way it worked, that there were different
rules the things change when you move fast or when
space is bent, and that we needed new rules and
new ideas to understand what actually goes on in this universe. Yeah,
(03:26):
because I guess we realized a few hundred years ago
that the universe, the world doesn't really behave the way
it behaves like on an everyday basis, like when you're
juggling baseball or or you know, balancing a basketball. It's
like that's one way that we think of the world
is working. But really, like if you get down to it,
in some situations, it's totally different. You know. In fact,
there are really fundamentally different rules that govern how things
(03:48):
move and how time flows. It just happens to be
that in certain situations, like when you're moving really slow
and when you're far from a black hole, things look
a lot simpler. So we're living in a sort of
a simplified corner of a very weird, very complicated universe. Right,
It's almost like we're in a special case, like our
perception of the universe. It's like a small example of
(04:11):
all the ways that the universe could be. It's like
you're playing the warm up levels on some video game
before all the bad guys come in, just you can
sort of like figure out how the controls work. Right,
That's what we've been doing for physics for like thousands
of years, and now we finally understand the full scope
of the game. And wouldn't be great a good respond
in this universe? We can just keep trying, well, we
(04:31):
sort of do. I mean, like physicists start from where
previous generations of physicists ended, right, We don't have to
start from scratch on physics every single generation. So physics
sort of responds in our minds, right, And we're going
to run out of lives at some point. Do we
need to put in more quarters into the arcade of
the universe? That depends on the alien death right or
the joke ray Neither one might help us or not
(04:53):
help us. But yeah, the universe is kind of tricky,
and it's tricky not just in like extreme situations like
in a that hole or moving on close to the
speed of light, because who knows, like we could be
moving right now really fast compared to other aliens and
other galaxies right we are. We are moving really fast
compared to the Sun, and the Sun is moving really
fast compared to the center of the galaxy. But all
(05:14):
those things are kind of abstract. What really matters is
your speed relative to the people who are watching you
and relative to your clock. And so that's something that
special relativity has taught us, is that the world looks
different from different perspectives. Based on who's doing the measuring.
You can get kind of a different account of what's
going on and what's happened in the world. Yeah, you
(05:35):
might not agree with your relatives about relativity, but yeah,
things are kind of tricky in this universe, and so
even physicists sort of struggle with it, right, I mean,
it's sort of hard to rab anyone's head around these
topics and how it actually works. And and so physicists
have some ways to kind of think about how these
things work that might help other people. Yeah, because in
(05:57):
science we have our intuition, like give a fling about
how things might work and what might happen and how
things are put together. And then there's the mathematics. And
the mathematics is what actually happens, what really is going on,
the rules that really do control the universe. But sometimes
you're looking at like a page of equations and you're like,
I don't see it. I don't really get it. And
(06:17):
for me personally, it's easier to think about these things
like geometrically to understand like the relationship in space and
time of these things. So sometimes you can get more
of an intuition for weird things if you think about
their structure and space and time sort of geometrically, like
the shapes that are created by all of these ideas, right,
and then you draw cartoons about it, which is where
(06:40):
right exactly? It helps to draw cartoons, no joke. Cartoons
are a great way to visualize this stuff. That's why
you and I work together on a lot of these books,
and folks out there should check out our book We
Have No Idea, which has all sorts of really clever
cartoons visualizing really tricky concepts. Yeah, and our upcoming book
which is coming out in a few months frequently questions
(07:00):
about the universe, which you can pre order right now. Man,
we totally snuck in those book plugs, Daniel, totally unplanned.
I keep getting emails from listeners saying, you guys should
write a book. I'm like, what do we not talk
about the book enough on the podcast? I guess I
guess we don't. We should talk about it every time.
I think we should, honestly. Anyway, back to the podcast. Yeah,
back to the podcast. Let's talk about one of these
(07:21):
tools that physicists used to kind of wrap your head
around some of these tricky concepts about relativity and the
speed of light. So today on the podcast, we'll be
asking the question, what is a light cone? Daniel, this
is a light coming, I think, I guess you mean light,
not like not heavy cone is exactly light as in
(07:44):
photons is and that thing that speeds along at the
maximum speed of the universe. Now, this sounds sort of
like maybe a dessert, but maybe we're not a dessert
you want to eat or that would taste very good.
Sounds like the kind of dessert your mom would order
for you when what you really want is scubavice. Where
you really want is a heavy cone, the heavy cream cone.
You don't want the like fat free light cone. But yeah,
(08:07):
So this is a concept that comes up in special
relativity and it has some pretty interesting consequences about causality
and like what's actually real or like what's actually happening
or happened in the universe. Yeah, and it helps you
think about what's going on inside a black hole and
even possibly maybe allows you to figure out a way
(08:27):
to do real life time travel. Sounds like a very
powerful dessert here. All right, Well, before we go on,
we just want to give a quick shout out to Biorn,
right he sort of helped inspire this episode. Daniel. Yeah,
thanks Born from Sweden who sent me a link to
his Solar System inspired music which I listened to while
preparing for this podcast. You can find him on Spotify
(08:50):
at ord cloud Services. Check them out if you like
the sort of space ambiance music. Thanks a lot, Biorn
for sharing music. Yeah, and again that's or Clouds. That's
o RT cloud service is on Spotify, which is a
pun because it's like the Orc Cloud and also cloud services.
And what is the orc cloud done for us recently? Well,
I guess it delivers spacey music and you can upload
(09:10):
your pictures to it. You might never get them back,
but yeah, you can upload them. It probably takes like
twenty years just to get there, and then the download
speed it's probably even worse. All right, Well, we were curious,
as usually how many people out there are familiar with
the concept we're talking about today. So Daniel went out
there into the internet to ask people what is a
(09:32):
light cone? And if you are out there on the
internet and always wanted to participate in a podcast, please
write to us two questions at Daniel and Jorge dot
com and you can be unprepared to answer future questions
for the podcast. Right, And if you're not on the Internet,
please tell us your secret. I mean I would to
sometimes not be on the Internet. I guess we're on
(09:52):
the radio too, right, Daniel. I think some stations broadcasts,
so they might be listening on the radio. Yeah, So
thanks everybody who's out there listening to us on the
radio as well. We appreciate all of your support and
all of your listening. Did you know that we recently
passed fifteen million episode downloads? Wow? Really, it's amazing. That's
like fifty million hours that people have been listening to us.
(10:13):
That's like fifteen million banana jokes at least at least
because you know, we're a multi banana joke death Ray
a joke Ray here. We try to in our in
our best episodes, like a shotgun rate joke. Really, we
just splatter bananappeals all over the place and now I
imagining fifteen million tiny bananas coming out of the nozzle
of a gun at very high to be coming out
(10:35):
of our mouth, that's a little disturbing. But anyways, think
about it for a second. If someone asked you what
do you think a light cone is? What would you say?
Here's what people had to say. I would say, that's
what comes out of the shiny end of a flash light. Um.
I believe this is a space time diagram where it
shows a three dimensional object and as you go up,
(11:01):
that's time passing, so it makes a calm like if
you look at the diagram of the Big Bang and
are we sure a tiny point? Then it slowly goes
up into a cone as time progresses. Um. I don't
know what a light cone is. I'm thinking about there
was some propulsion the idea that was. I think it
(11:21):
was largely theoretical, but it was to do with kind
of using light to push spacecraft and it would capture
the light using light sales. I don't know if that
would be to do with it. I remember seeing a
model in the Science Museum in London, but there was
a cone, but that was probably just because it was
made of plastic. Light corne is representation of how from
(11:47):
the light travels from a point. For instance, if you
have if I have a little flash light in my hand,
then I turned it on the lightbulb trouble in space
and time. Like cone, you can represent it as a cone.
(12:09):
That is something I've heard of before. Um, it's the
realm of like what you can see. So it's like
ninety three billion light years across as in our observable universe,
because that's like how far we could observes or something
into space, or like the photons that have reached us
(12:29):
from that distance. Although I don't understand how that's bigger
than thirteen point eight billion light years since that's the
age of the universe. But I'm still quite unclear on that.
But I know that the light cone is basically all
that we can see within our realm of light, anything
that can reach us in any form. I think a
light cone is an ice cream cone that's been left
(12:52):
out in the sun too long and all the ice
cream melted. A light cone is an ice cream cone
that has no chocolate ice cream minute whatsoever, which is
fundamentally flawed, and I think it violates one of the
laws of thermodynamics, because why would you want vanilla ice cream?
One chocolate is an option, so light cones make no sense.
All right. A lot of people went with my interpretation,
(13:14):
which is that it sounds like a dessert possibly and
that it's light. Yeah, and we also heard some people
throwing shade on vanilla ice cream. Wow, that was rough.
I know, how can you be down in vanilla ice cream?
It's like the nicest flavor gets along with everybody. That's true.
I mean I would always pick chocolate over vanilla. But
you know, if all you got is vanilla, I'm happy
(13:35):
to get. But if you take vanilla and sprinkle chocolate
on it, see that's the versatility of vanilla. Or take
chocolate and sprinkled chocolate on it, it's still better. But
you can't take chocolate and sprinkle vanilla on it. That's
when you do. That's vanilla doesn't add to anything. It's
just sort of like the substrate he lean. It makes
most cookies taste good. It's just sort of vanilla. That's
(13:55):
the reason why they call it vanilla vanilla, you know,
because it's so dependent and solid and she just always
there for you. Right. If somebody reads you're writing and
they describe it as a vanilla. Would you take that
as a compliment? Depends am I writing the bag of
an ice cream carton? Then it would meet its cool
But yeah, so a light cone, Daniel tell me, this
(14:17):
is something I was surprised to see in our topic
because it is it's like a graphical interpretation of physics, right,
And so it's a little tricky to describe on a podcast.
It's a little tricky to describe on a podcast. We'll
see how we do. But I thought it was useful
because I find that every time we talk about special relativity,
I have light cones in my mind. I got these
things in my brain. They really helped me think about
(14:39):
sort of the structure of space and time, and they
lead to a lot of interesting consequences. And so I
hope if we can somehow download the concept of a
light cone into the minds of our listeners, it will
help them think about special relativity. We need like a
light cone ray to shoot listen to people. Maybe, Yeah,
I think it's a podcast starring me and you're talking
about throwing out vananative. All right, well, m step us
(15:01):
through it. What is technically a light cone? So a
light cone defines the part of the universe that you
can influence, the part of the future that you can
visit geometrically. And you can think about it like if
you turn on a flashlight for a moment, so you
shine photons in every direction, they leave you in sort
(15:21):
of a spear. After one second, they are one light
second away from you. After one year, they are one
light year away from you. So imagine a sphere and
then that sphere sort of growing as time goes on, right,
So that set of spears versus time as those spears grow,
this is your future light cone. This is the part
of the universe that you can influence where your choices,
(15:45):
the things that you do as a person could have
sort of an effect downstream. It's very easy to understand why.
The reason is that everything that's outside of that growing
set of spears you can't influence because information would have
to travel faster and the speed of light to get there.
So it's a direct outcome of the fact that we
have a maximum speed limit to the universe. It divides
(16:06):
the universe into parts that you can influence and parts
you cannot. So the light cone is the part of
these universe, including the future. Right, we're talking space and
time here, the part of the future universe that you
can have any sort of influence over. So I turn
on a light like a light bulb, and that light
is growing comes out of the light bulb of growth
(16:27):
in a sphere, and you're saying, that's the light cone.
So it really should be called like a light sphere.
Maybe it'd be a better name, like the light sphere
is the sphere around you which you can reach with light.
The reason it's often called the light cone is that
it's hard to draw four dimensions on a sheet of paper.
The most we can really draw on a sheet of
(16:47):
papers three dimensions, you know, by having like some perspective,
and so you need to have time on there. So
you can really only have two other dimensions of space
on your sheet of paper. And so when you're drawing
these diagrams, people typically treats base as if it was
a two D surface instead of like our three D world,
just to sort of simplify it. So in two dimensions
the sphere we were just talking about, it becomes a circle. Right. Imagine,
(17:10):
for example, taking a smooth sheet of water and hitting
it somewhere. What happens you have a wave that leaves
where you hit it and it travels outwards. So now
you get a set of circles. Now, if you take
those set of circles and you extend them like vertically
above the surface in time and make time that vertical direction,
then what you get is a cone. Right. It starts
from a point when you hit the surface of the water,
(17:32):
and then as time goes on, that circle gets larger
and larger, and that defines a cone. So it's a
cone only in a sort of universe that has two
spatial dimensions and one time dimension. Right, It's kind of
like if you take a slice of those light spheres
we were talking about, Like if you take a slice
of it and you see those that circle on that
slice growing, and then you imagine that circle growing but
(17:56):
growing and also moving up maybe at the same time,
it kind of makes like a cone, yeah, exactly, and
it moves up because now we're imagining time to be
one of our axes. Right, we have two dimensions of
space and we have another direction, which is time. And
obviously things just flow forward in time at one second
per second, and so this circle just sort of grows
(18:17):
in time. If you instead had something which wasn't moving. Right,
if you just like drew a circle around you, then
that circle would slide through time and make a cylinder, right,
because it wouldn't be growing. But if it is growing,
then it makes a cone. It starts from the point
where you've made it, and it makes a cone. And
I guess if you're having trail visual license, you can
always just pop on Google images or something and look
(18:38):
for light cone. I imagine you'll get a picture of this, right.
How do people do podcast before Google image search anymore?
They probably avoided topics that involved geometry and and multiple dimensions.
But it's kind of an interesting concept, and I think
you as a physics you like it because it defines
kind of like a volume, right. It defines kind of
(19:00):
like a set of space time that is either inside
the cone or outside the cone. That's kind of the
power of this kind of visualization. Yeah, and it's not
just arbitrary, like you can make whatever shape you want
in spacetime. But this one is special. This one is
really important because everything that's inside your future light cone
are things that you can influence. Right. You can't send
(19:21):
the message to anything outside your future light cone, like
anything outside your future. Light cone by definition, is too
far away for any message to reach because you would
have to travel faster than the speed of light. And
if you can send a message, it means you can't
send any information. That means you can't influence it at all.
And one of the really tricky, complicated things about special
(19:41):
relativity is that it plays sometimes with the order of
events and it seems to have consequences for causality. So
this helps you sort of keep track of that the
part of the universe that you can influence, and also
looking backwards, the part of the universe that can influence you. Right,
all right, Well, it's get into some of these weird
consequences for because reality and simultaneity and alien death rays.
(20:03):
But first let's take a quick break. Alright. We're talking
about the speed of light, causality, the universe, and light cones,
which are sort of like a visualization in physics that
(20:26):
help you kind of figure out what you can't affect
and what you cannot affect because I guess that's something
is far enough away you can't affect it right now,
right because you have to wait for the speed of
light to get from here there or there to hear
it doesn't even have to be far away. Something that's
one foot away from you, like literally one foot away
(20:47):
from you right now. You can have no influence on
what's happening right there, right now, because even if you
shot your death ray, it would only affect what's happening
to foot away from you in the time it takes
for light to travel from the tip of your death
ray one foot. There's nothing you can do about what's
happening one foot away from you right now. Oh wow,
that's kind of weird, right to know that there's stuff
(21:09):
around me that is unaccessible to me. Yeah, I mean,
even if you're seeing something that happens a foot away
from you, that means it actually happened in the past,
so it's already done anyway. Right, So all you can
do is affect the future, right That seems obvious. But
you also can't affect the future everywhere, Like if you
want to change the future of Andromeda, you know intuitively,
you can't change the future of Andromeda tomorrow or the
(21:31):
next day, or even the next year. It would take
millions of years for any decision you make to affect
anything in Andromeda, because nothing you do will get there.
No choices you make can propagate that far before a
million years because it's a million light years away, right, Yeah,
I guess that's the big consequence of having a speed
limit in the universe is this idea that you know
(21:53):
there are things in the present, but you can't really
touch them, more affect them, or even know what they
are right now in the same president as you. That's right,
there's a limit on the information and how information travels.
It's really fascinating. You can only ever see one slice
of the universe, and your slice the universe might tell
a different story from other slices of the universe. And
(22:14):
this is really hard to get into your head because
you imagine that what we're doing with physics is trying
to build up like an absolute picture of the universe,
like what's really out there, what is really going on,
what happened in the universe, and what's going to happen,
And it's hard to accept that one you can never
really know everything about what's going on in the universe
because you're limited to the slice of information you're going
(22:36):
to get, but also that what you do learn about
the universe might not be like the absolute truth. It
might just be like what things look like from your perspective, right.
I think we talked about this once in the podcast.
It's like, imagine like how the human society was before
the Internet or before telecommunications, you know, when we had
to rely on writing letters and you know, waiting for
(22:58):
newspaper to be delivered to your door. It's like, there
could be things happening right now in other parts of
the Earth, but you know it'll be a long time
before you find out about them, or you know that
they actually happened at the same time that they happen.
That's right, and speeding up our communications has sort of
expanded our cone of understanding. Right, It's shrunk the time
it gets for information yet here, but not to zero.
(23:21):
Even with cell phones and speed of light electronics, you
still can't know exactly what's happening across the world right now.
There's still a time delay. And that's why it's cool
to see in some of these TV shows that they
really incorporate that. You know that if you're trying to
coordinate your army on Mars from your base on Earth,
it takes time for your messages to get there. If
there's a battle going on around you know, the atmosphere
(23:42):
of Jupiter that by the time you've heard about the beginning,
it's probably over already and there's nothing you can do
to change the outcome. I guess, like before when you
had a battle on you have to like send a
messenger or send a scout or something you had, there
was this just huge delay, and so now thinks they're faster,
but they're still limited, like there's still no simultaneity, Like
there's still no ability to know what is going on
(24:04):
right now somewhere else. Yeah, exactly. And the same concept
is light cone, which tells you, like what you can influence.
It also works in the other direction. You have like
a backwards light cone. If you think in the other direction,
like how close does somebody have to be to me
to shoot a death ray and hit me right now?
You know, if somebody is in Andromeda and they shoot
a death ray at you, you're safe for a few
(24:26):
million years, right Nobody in Andromeda can kill you right now.
Even if they shot their death ray a hundred thousand
years ago and it's been hurtling towards you for a
hundred thousand years, you're still safe. It's not going to
get here for a while. Because they are not in
your backwards light cone yet. But if they shot at
a million years ago, it would be hitting us right
now potentially. And if they shoot it right now, they
(24:48):
were still going to get hit, But it's going to
be a million years from now, that's right. If they
shoot it right now, it's not in our past light cone,
but our past light cone moves up this time access
and eventually they will be inside our ask light gone
and yes it will get here, right but you're safe
right now, right now, there is this past like cone
that dictates all the things that could be influencing what's
(25:08):
happening at your location, and somebody really far away, even
a foot away, can't influence what's happening to you right now.
They can only influence your future. Right well, they can't
influence you right now. But I mean doesn't mean you're
safe from a death ray, because they could have shot
it a while ago and you would still die from
the death ray arriving now, you're right, And the weirdest thing,
the worst thing about death rays is that they travel
(25:30):
at the fastest speed possible, so there's literally no way
to get any warning. Right the moment you see it
is the moment that arrives at the moment it impacts you.
But hopefully, you know, a million years ago, you know,
we were just cavemen and women, and you know we
didn't do anything to offendiate, hopefully, Or maybe it's a
natural process. Maybe the gamma rays from a supernova that
(25:51):
blew up, you know, a while ago, and those sterilizing
death rays are just now arriving here at Earth. Man,
this line makes me feel not at all safer. Daniel,
See how you can drop comfort from this light cone.
It's like a pretty flimsy protection against giant gamma ray bers.
It just means that somebody's got to plan ahead. If
they really want to fry us now, they're got to
(26:12):
either be close by or they have that planned ahead
so that athetic exploding start. Are you saying it's not
very good at planning so we're saved. Is that it's
a procrastinator saves us from procrastinators exactly? But what about
type A supernovas? Oh, the kind of plants ahead, there's
nothing we can do. Yeah, we're toast, all right. So
(26:35):
it sort of protects us, and I think it also
kind of affects what we might agree on. Is kind
of a consistent history of the universe. Right, Yeah, it's
really interesting. The whole universe can be divided into three regions.
You have your future like cone, your path like cone,
and then there's everything else. Right, and this is this
called this the absolute elsewhere. It sounds like an awesome
(26:58):
name for a Netflix show or something. So the absolute
elsewhere is the spot that you can't affect, you can't influence, right,
And that's really interesting because things that happened in the
absolute elsewhere, you might disagree with people about the order
in which they happen. Like you might look at a
series of events and say A happens before B, and
somebody else might look at it and say, no, no, no,
I think B happens before A. And you could both
(27:20):
be right. If these two things are not inside each
other's light cones, and these events don't have overlapping light cones,
then they can be differently ordered and not affect anything
from a causality point of view. WHOA, So I guess
I can have a light cone and you can have
a light cone. And if those two don't interact, is
that what you mean? Like if they don't touch, then
(27:40):
we were looking at literally like different universes. Well, they
will eventually touch, right, because their light cones will eventually impact,
and so there is some overlap in their light cone
far in the future where both of them can influence.
But it doesn't matter which one happened first. It doesn't
matter if A happened in then B, or B happened
and then A, because these two events are the events
(28:01):
themselves are not inside each other's light cones. Right, B
is not in a's light cone, and A is not
in b s light cone. Even though the two light
cones do overlap, if they are not inside each other's
light cones, then you can swap their order just by
changing your speed. If you decide to zip by them
in a spaceship as close to the speed of light,
you will see one happened before the other. If you
(28:22):
go in the other direction and nearly the speed of light,
you will see the opposite order of events. And it
all works out downstream. Because these two things aren't causally linked,
they're not inside each other's light cones, so you're free
to change their order of events. But then you know
the TV who are involved would have a different interpretation, right,
like one of them will see one thing happening before
(28:42):
the other. Yeah, exactly, But there is no absolute definition
of what happened. First, all the observers have their own interpretation,
and they're all correct because the answer depends on your position.
That's not true for two events where one is inside
the other one's light cone. If I shoot a joke
ray and I hit you in the head, then you laugh, right,
(29:03):
put it on joke mode, please, there's that. It's don
and then there's a joke open on banana mode. So
I hit the ray and the banana hits you in
the head. Right now, you are in my light cone.
Because the two events me pressing the button on the
gun and you having banana arrived in your mouth, they're
not at the same time. What happens before the other,
(29:23):
and there's a causal structure for them. I have to
press the button before the banana ray arrives at your head.
And that's okay, because the banana ray arriving at your
head is an event in my future. It's inside my
light cone. But for example, you and I both have
these guns and we point them in random directions or whatever,
and we try to fire them at the same time.
Some people might say we fired them at the same time.
(29:45):
Other people flying behind the spaceship might disagree about who
pressed their trigger first because they would be moving and
because of the way that light along the light takes
to get to them and the speed limit of the universe,
they might reconstruct the events in a different way. Yeah,
but it's not just because how long light takes to
get to them. Even if you factor that in, right,
(30:07):
even if you account for how long it takes the
signals and the images to appear, there's still a difference
in the order of events. And that's because time flows
differently based on where you are and how fast you
are going. So your speed affects how you see time flowing,
and so you can see one event happening before the other.
You can see Jorge presses button first, or somebody else
going in the other direction can see Daniel press the
(30:29):
button first, even if they account for the transmission delay
from light. But they would be wrong. I guess what
I mean is they would have their own interpretation, but
other people would have their different interpretation. Yeah, but nobody's
right or everybody's right. You know, if these two events
are not inside each other's light, cones right, their light
cones can overlap in the future, but the two events
themselves are not inside each other's light cones, then their
(30:51):
order is not well defined. And this is all because
the universe has a speed limit, right, Like, if it
didn't have a speed limit, then you could talk about
like absolute history of the universe. That's right exactly, And
that speed limit defines the size of this cone. Right. Imagine,
for example, if the speed of light was much much slower,
then your light cone would be narrower. The part of
(31:13):
the universe, the part of the future that you could influence,
would be a much smaller part of the universe. If
you shot death rays in every direction and the speed
of light was much smaller, then you would kill a
smaller fraction of the universe. Or if you shot joke raise,
you would make a smaller fraction in the universe laugh.
Then if the speed of light was ten times faster
than it is, then you could influence more of the universe,
(31:33):
and aliens out there with their joke raise could make
us laugh more effectively. So this number the speed of
light really changes the whole structure of causality of space. Right,
But what if you make like a light joke like
this vanilla is the best. Ha ha ha. That's a joke,
like a light vanilla joke. Yeah, that wouldn't harm anyone,
and people would just let it go by. But it
(31:54):
really is sort of like a cone. I'm thinking, like
a doc cone. You know those cones they put on
dogs so that data and harm themselves or just to
protect them. It really is like a cone that you
put on our we put on our heads that kind
of limits what we can see and what we can
interact with, right it sort of does. Now imagining you
with one of these on your head trying to eat
a banana, even though you can't, Daniel shoot me with
(32:17):
the bananas exactly, then you need me to shoot you
with the banana ray or I guess she went up
in the air and then catch it and the cone
would help me catch this. There you go, there you
go pretty soon be surrounded with bananas. I'd have a
ConA around my head filmed with bananas and I'll go
along with a scoop of vanilla on top, and then
for some childhood. But the amazing thing to me is
(32:38):
that we still don't know why the speed of light
is this number. You know, it has such a dramatic
effect on what part of the universe we can see
and what part of the universe we can influence, and
how the universe is sort of like tied together, you know,
what affects what It really changes everything. And it's just
like a number. We measure it. We don't know why
it is what it is. It could have been better,
it could have been smaller, for all we know. Our
(33:00):
laws of physics just have like an empty spot there.
If you're like right in the number you measure, there's
no reason it has to be this or that that
we know of. And that's like a real mystery, you
know, it it makes you wonder why it's this and not
something else. Alright, cool, Well, let's get into now what
consequences is this light cone has in our in the
universe and possibly in our ability to travel through time.
(33:22):
But first let's take another quick break. Are we're talking
about light counts and the speed of light and whether
or not we can affect things in the universe that
(33:43):
are happening right now, which is basically no, right, Like,
we can't affect anything else in the universe happening at
this moment, that's right, nothing except for what's happening right here,
right now, or stuff that's happening in the future. But
at this very moment, you can't affect anything except for
exactly the spot where you are. Well, technically nothing right
(34:03):
because even things that are one millimeter away from my
eye in the present moment, I can't even see or effect.
That's true. We have to go down to zero millimeters.
And that's only even only if you believe in free
will and if your brain operates instantaneously, So technically you're right.
Any decision you make right now can only influence the future,
except that maybe there's a possibility to use this concept
(34:24):
to do time travel. Daniel so explained that to me. Yes,
so what we talked about so far, this idea of
light cones these circles of light expanding out into the
universe and into your future. This is the way we
think about space. If it's not curved, so we call
space flat, if it's just sort of like has no
shape to it, if there's no bending of it that
affects like the path of photons. And we know that
(34:46):
space can be bent. We know that, for example, everything
with mass and energy bend space, like the Sun bend space.
That's why the Earth is moving around it instead of
just flying off into deep space. We know the Eube
and in the space because you have mass, and so
everything out there changes the shape of space, which means
it changes how light goes through it. Remember that gravity
(35:09):
is not just like a force that tugs on things,
because light, for example, has no mass, but gravity can
affect its path because gravity changes the very shape of
space and light travels through that space. And so we
know that things are more complicated when space gets curved.
So what we talked about so far is only in
the event that the universe is empty and there's no curvature,
(35:31):
there's no mass or anything to change the shape. So
in flat space, when there's a lot of mass around,
things get curved and things get pretty different and weird.
These light cones, which you can imagine sort of like
going up and going down now, start to get tilted. Right.
Is this something we can actually see? Right? You can
see light getting bent out there in space by heavy objects. Yeah,
(35:52):
it's called gravitational lensing. If there's like a really big
heavy object between us and some source of light, like
a background galaxy, then as those photons come towards Earth,
they could bent around that massive object. And this is
one way we can detect, for example, the existence of
dark matter and try to figure out, like roughly where
it is based on how it distorts the light that's
(36:12):
passing near it or through it. So it's definitely a
real thing that's happening. It's something we've observed. We're sure
it is true. So if a space can bend light,
or land space can sort of affect the trajectory of light,
that would make sense that it would bend these light
cones that we have to kind of figure out what
we can see and cannot see in the universe exactly.
(36:32):
Because the shape of the light cone is defined by
where light goes right where light can reach, and so
space is bent in a way that light can't get somewhere,
then the light cones are also bent. So this idea
of them being like simple cones is only in flat
space near something really massive, they bend towards the mass, right,
because light moves towards mass. Right. If you have, for example,
(36:55):
the Sun, and you shoot a photon near the Sun,
it will get curved sort of in the dire action
of the Sun. And if you have a black hole,
then it's going to get curved room towards the black hole.
So as you get closer and closer something massive, these
light cones tilt more and more. Well, they start to
look like maybe horns and not so much cones or yeah,
or like bow ties or something yeah, or like a bugle,
(37:17):
remember that snack, the little like cones, curved cones. Yeah, exactly, Yeah,
light bugles. Will call it light bugles exactly. And so
inside a black hole, of course, you can think about
what happens to a photon. While a photon inside a
black hole always goes towards the center, not because gravity
is so strong, but because the shape of space requires it,
(37:37):
because there's only one direction inside a black hole. So
inside a black hole, these things have tilted totally sideways.
You shoot a photon and it goes just towards the singularity.
All the light cones are now tilted towards the center.
You hear people say sometimes this thing that inside a
black hole, space and time switch roles. Right, This is
like a geometric way to understand that, because now instead
(38:01):
of time flowing up, it's sort of flowing sideways. Right,
Because the light cone has tilted over by forty five degrees. Well,
I guess I don't understand it at all. So the
light cone tilts, and somehow time and space which positions?
Is that what you just said? Yeah, because your future,
(38:21):
all possible futures are the singularity, and so the only
way to move forward in time is to move in
this one direction of space. And so sort of like
moving through space towards the singularity is the same thing
as moving forwards in time. And you can graph that
geometrically by thinking about the light zone being tilted over
(38:42):
from being vertical to being horizontal. It's like there's only
space and time become one way and everything goes in
the same direction. Yeah, exactly, all right, And so then
how does that relate to time travel? You said it
might be possible to do time travel. It might be
possible to do time travel if you use these cones
you and construct this really weird structure. Right, So we
(39:03):
talked about how near something really heavy, these cones can
tilt like up to forty five degrees right towards the
center of a black hole. Now, imagine if you could
have a cone that tilted more than forty five degrees,
And what that means is that you're tilting so that
part of the light cone is now in the past, right,
So it's possible if you could do this, it would
(39:25):
mean that these particles are moving not just into the future,
but like the future is now dipping into the past.
What you mean you you bend the light cone so
much it wraps around and touches kind of the past
light cone, m exactly, and you think, is this possible?
Doesn't that contradict itself? Will remember that, like the definition
(39:45):
of time and the definition of space depends a little
bit on how fast you're going and the environment you're in,
and that gets even more complicated when space is bent.
And so what you can have is like a particle
moving forward into it's future like it's local time still
says I'm moving forward in the future one second, then
the next second, and the next second. But it could
(40:07):
be moving into the past from the point of view
of another observer. So like from a distant observer who's
watching this from far away, whose space isn't curved, they
could see that particle moving into like their past, even
though if it's still moving into its own future. Whoa wait,
are you saying that space and time actually curve around itself,
or it only looks curved around to a different observer,
(40:29):
the structure of it is actually occurred, and people think
in some weird configurations, if there's a very special arrangement
of mass, then you might be able to construct a
structure of space that allows particles to have their local future,
their personal timeline extend into like the past from the
point of view of other observers. From the point of
(40:51):
view of other observers, but the particle itsels wouldn't be
traveling into the past, or like me, I wouldn't be
traveling into the past. Well, it's like into their own future.
But that's sort of like what happens on a time
travel show. Right, If you are on a time travel
show and you want to go into the past, you're
still living your life forward on your timeline. Right, you
were forty six years old and seven days and two minutes,
(41:13):
And then when you go into the past, you're not
like younger again, right, You're just like keep going forward.
Now you're forty seven years, two days and five minutes
and seven minutes and ten minutes. So what you want
is your own timeline to continue, but you want the
structure of space around you to change that your timeline,
your future now goes into like the rest of the
universe's past. So this is called in physics, it's called
(41:35):
a closed time like curve because if you can tilt
a bunch of these cones just the right way, then
you can make this path that loops on around itself
and comes back to the same point where it originated.
So it's like shooting a light beam and having it
hit you in the back of the head, almost like
a wormhole kind of in time, almost like a wormhole. Yeah,
it's not actually a wormhole, because a wormhole is like
(41:57):
a shortcut between two distant locations. This is like a
structure in space. It's like a loop in space time. Right.
It is a complicated reconnection and rearrangement of how space works.
But it's technically a little bit different from a wormhole. Right,
wouldn't you be stuck in this loop like you'd be
be like groundhogs day? For this particle, it's just reliving,
going around and around and around. It thinks it's going
(42:18):
in the future, but really it's like revisiting the same
time over and over and over. Exactly. It's a structure
that reinforces itself, and it's only stable if it does
revisit itself. And so you're right. You wouldn't like get
to go back and choose a different flavor of ice
cream or set your death ray to banana instead of
death right. You have to make the same choices over
and over again. It's like this self supporting structure in
(42:39):
space and time so not recommended. Well, unless you want
to keep watching the same shows maybe, or if you
really like vanilla and you want to eat only vanilla
flavor for the rest of your lives for eternity. Yeah, exactly.
And this is something which is theoretically conceivable, like people
have worked this out on paper and it seems like
it might work. But again, general relativity doesn't tell us
(43:02):
like how to bend space to make this happen. People like,
if you could bend space in this particular way, then
we think this could happen. That doesn't mean necessarily we
know how to make space bend in that particular way.
You know, people have ideas, they have this crazy structure
that's like an infinite spinning cylinder of dust, and they
(43:23):
do the calculations and they think maybe that would allow
for these closed timelike curves to happen, but other people
think it's impossible, there's no way it could work, and
that we have to sort of throw up our hands
and say, let's figure it out when we get the
unified theory of everything. Well, you're saying that it's according
to the equations, it's possible, or like you know, anything
that can bend, if you bend it enough, this will happen,
(43:45):
But we don't know if it's physically possible to do it.
We don't know if it's physically possible. If you believe
general relativity is absolutely true and you can construct an
infinite spinning cylinder of dust, then you should believe this
is physically possible and can actually happen. Personally, I don't
think general relativity is a true theory of nature because
it ignores quantum mechanics, and so it can't be correct,
(44:07):
and we need some updated version. And it might be
that in that updated version and incorporates quantum mechanics, this
doesn't work anymore. So this could be like sort of
like a singularity of a black hole, like a sign
that the theory is wrong because it predicts this sort
of nonsense result. Right, Because I guess if it's a
closed loop. How would you even get into that loop? Right? Yeah,
you have to have somebody else build that infinite spinning
(44:27):
cylinder for you in the infinite past, because you'll always
be in that loop. So it might exist and there
might be a particle stuck in it or from the
beginning of time, but you wouldn't be able to get
in or out of it. That's right. If it exists,
it would be stuck forever. But you know, now we're
talking about like construction of a time loop, so we're
talking about the variation of the loop as a function
(44:49):
of time. So it's like an extra level of meta
time there, which gets tricky. You can just use the
same vanilla equations, you get to go extra spicy or
sprinkle some bugles on for some extra crunching iss. All right,
So that's the light on one, I guess, and I
guess you're saying it's one way that that it helps
you think about all these weird situations of space and
(45:10):
black holes and bending time and spacetime looping around itself.
You use that in in your everyday you know, theoretical
musings I do, and also when people write to me
and they ask me some like crazy question about special
relativity involving you know, spaceships and snowballs whatever. I usually
end up drawing light cones so I can understand, like, well,
(45:31):
this happened there, What will this person see or is
it possible for them to know this. It's just a
nice way to sort of like separate things because in
physics problems because much simpler when you can separate and
say these things are independent from those things, so I
can just deal with it on its own. They're not
like entangled. So a light cone shows you how to
like disentangle all the causal threads of the universe into
(45:51):
a bunch of different packets. Right, Yeah, everything is better
with the snow cone. That's our next episode. What is
a physics snow cone? It's a commort physic You say
no all the time, it's no, can I travel back
in times? No? Cyes c Yes? All right, Well, hopefully
(46:12):
it helps all of you out there and think about
cantality and what we can and cannot affect about the
present and maybe makes you appreciate a little bit more
where you are and what we're able to see of
this big, crazy universe. Well, thanks for joining us, see
you next time. Thanks for listening, and remember that Daniel
(46:39):
and Jorge explained. The Universe is a production of I
heart Radio. Or more podcast from my heart Radio, visit
the i heart Radio app, Apple Podcasts, or wherever you
listen to your favorite shows.
Or Hey, do you ever wish that the speed of
light was faster? I guess I'm not dissatisfied with the
speed of light. I'm just glad it's not slower, because
you know, I don't want to wait ten minutes after
I flip on the light switch. But you know, if
it was faster, and then we could see like further
out into the universe. M m. That's true, and then
we might be able to travel further to explore the stars.
(00:30):
Also true. But there's another side to it, as always, well,
how could it be bad to have a faster speed
of light? Well, light speed was a million times faster
than technically any alien out there in the universe could
kill us instantly with a death ray. That's true. But
you know, witnessing an alien death ray would be pretty cool.
It would be pretty cool for about a nano second,
(00:51):
and then you'd be dead. Also, I'm pretty sure a
death ray would be hot and not cool. Might still
be worth it, just for that nana second, you would
know that AIDS existed. As my brain is getting FRIEDA.
(01:16):
I am or haemade cartoonists and the creator of PhD comics. Hi,
I'm Daniel, I'm a particle physicist, and I don't spend
a lot of time worrying about alien death rays. You don't,
do you know something I don't. I know that the
speed of light protects us from most of the universe
shooting us with alien death rays. Really like it forms
a force field around us, protecting this like a blanket
(01:37):
helping us sleep at night. It just makes it kind
of ineffective. If you're going to shoot an alien death
ray millions of light years, then you're gonna kill your
target in millions of years. And like, who's geopolitical strategy
really has that long and arc to it? But what
if there are patient aliens? Then we're toast? Then we
might literally be toast. Welcome to our podcast, Daniel and
Jorge Explain the Universe, a production of I Heart Radio
(01:59):
in which we beam our joke rays into your brain,
hoping to transmit some of the incredible wonder and knowledge
about the universe, all the things that science has learned
about this Bonker's wondrous, beautiful, glittering cosmos, and all the
things that science is still working on, all the vague
questions about how the universe got here, where it's going,
and who lives in it, and how to actually build
(02:21):
a joke Ray. I mean, that's probably the next Nobel Prize.
The question it doesn't make you groan or does it
make you laugh? Is that where you're gonna ask when
the aliens come, I'm gonna be like, tell us the
secrets of spacetime and you're like, have you developed the
joke Ray yet? We'll think about what it would mean
for the mental health of humanity. You know, we could
just have a laugh about everything. Laughter is the best medicine.
So anytime you go to the doctor, they're just like, yep,
(02:43):
we're prescribing you five shots with the joke Ray. And
then as the aliens as they you know, enslave us
and possibly eat us, we could be laughing all the way.
But it is a bizarre and jokey universe because it
seems like a lot of it is beyond our control
or even beyond our ability to whoever interact with it.
That's true, and it's also sometimes beyond our ability to
(03:05):
really understand. We have one way of thinking about the
world where things move and slide and the way time works.
But about a hundred years ago we discovered that that
wasn't really the way it worked, that there were different
rules the things change when you move fast or when
space is bent, and that we needed new rules and
new ideas to understand what actually goes on in this universe. Yeah,
(03:26):
because I guess we realized a few hundred years ago
that the universe, the world doesn't really behave the way
it behaves like on an everyday basis, like when you're
juggling baseball or or you know, balancing a basketball. It's
like that's one way that we think of the world
is working. But really, like if you get down to it,
in some situations, it's totally different. You know. In fact,
there are really fundamentally different rules that govern how things
(03:48):
move and how time flows. It just happens to be
that in certain situations, like when you're moving really slow
and when you're far from a black hole, things look
a lot simpler. So we're living in a sort of
a simplified corner of a very weird, very complicated universe. Right,
It's almost like we're in a special case, like our
perception of the universe. It's like a small example of
(04:11):
all the ways that the universe could be. It's like
you're playing the warm up levels on some video game
before all the bad guys come in, just you can
sort of like figure out how the controls work. Right,
That's what we've been doing for physics for like thousands
of years, and now we finally understand the full scope
of the game. And wouldn't be great a good respond
in this universe? We can just keep trying, well, we
(04:31):
sort of do. I mean, like physicists start from where
previous generations of physicists ended, right, We don't have to
start from scratch on physics every single generation. So physics
sort of responds in our minds, right, And we're going
to run out of lives at some point. Do we
need to put in more quarters into the arcade of
the universe? That depends on the alien death right or
the joke ray Neither one might help us or not
(04:53):
help us. But yeah, the universe is kind of tricky,
and it's tricky not just in like extreme situations like
in a that hole or moving on close to the
speed of light, because who knows, like we could be
moving right now really fast compared to other aliens and
other galaxies right we are. We are moving really fast
compared to the Sun, and the Sun is moving really
fast compared to the center of the galaxy. But all
(05:14):
those things are kind of abstract. What really matters is
your speed relative to the people who are watching you
and relative to your clock. And so that's something that
special relativity has taught us, is that the world looks
different from different perspectives. Based on who's doing the measuring.
You can get kind of a different account of what's
going on and what's happened in the world. Yeah, you
(05:35):
might not agree with your relatives about relativity, but yeah,
things are kind of tricky in this universe, and so
even physicists sort of struggle with it, right, I mean,
it's sort of hard to rab anyone's head around these
topics and how it actually works. And and so physicists
have some ways to kind of think about how these
things work that might help other people. Yeah, because in
(05:57):
science we have our intuition, like give a fling about
how things might work and what might happen and how
things are put together. And then there's the mathematics. And
the mathematics is what actually happens, what really is going on,
the rules that really do control the universe. But sometimes
you're looking at like a page of equations and you're like,
I don't see it. I don't really get it. And
(06:17):
for me personally, it's easier to think about these things
like geometrically to understand like the relationship in space and
time of these things. So sometimes you can get more
of an intuition for weird things if you think about
their structure and space and time sort of geometrically, like
the shapes that are created by all of these ideas, right,
and then you draw cartoons about it, which is where
(06:40):
right exactly? It helps to draw cartoons, no joke. Cartoons
are a great way to visualize this stuff. That's why
you and I work together on a lot of these books,
and folks out there should check out our book We
Have No Idea, which has all sorts of really clever
cartoons visualizing really tricky concepts. Yeah, and our upcoming book
which is coming out in a few months frequently questions
(07:00):
about the universe, which you can pre order right now. Man,
we totally snuck in those book plugs, Daniel, totally unplanned.
I keep getting emails from listeners saying, you guys should
write a book. I'm like, what do we not talk
about the book enough on the podcast? I guess I
guess we don't. We should talk about it every time.
I think we should, honestly. Anyway, back to the podcast. Yeah,
back to the podcast. Let's talk about one of these
(07:21):
tools that physicists used to kind of wrap your head
around some of these tricky concepts about relativity and the
speed of light. So today on the podcast, we'll be
asking the question, what is a light cone? Daniel, this
is a light coming, I think, I guess you mean light,
not like not heavy cone is exactly light as in
(07:44):
photons is and that thing that speeds along at the
maximum speed of the universe. Now, this sounds sort of
like maybe a dessert, but maybe we're not a dessert
you want to eat or that would taste very good.
Sounds like the kind of dessert your mom would order
for you when what you really want is scubavice. Where
you really want is a heavy cone, the heavy cream cone.
You don't want the like fat free light cone. But yeah,
(08:07):
So this is a concept that comes up in special
relativity and it has some pretty interesting consequences about causality
and like what's actually real or like what's actually happening
or happened in the universe. Yeah, and it helps you
think about what's going on inside a black hole and
even possibly maybe allows you to figure out a way
(08:27):
to do real life time travel. Sounds like a very
powerful dessert here. All right, Well, before we go on,
we just want to give a quick shout out to Biorn,
right he sort of helped inspire this episode. Daniel. Yeah,
thanks Born from Sweden who sent me a link to
his Solar System inspired music which I listened to while
preparing for this podcast. You can find him on Spotify
(08:50):
at ord cloud Services. Check them out if you like
the sort of space ambiance music. Thanks a lot, Biorn
for sharing music. Yeah, and again that's or Clouds. That's
o RT cloud service is on Spotify, which is a
pun because it's like the Orc Cloud and also cloud services.
And what is the orc cloud done for us recently? Well,
I guess it delivers spacey music and you can upload
(09:10):
your pictures to it. You might never get them back,
but yeah, you can upload them. It probably takes like
twenty years just to get there, and then the download
speed it's probably even worse. All right, Well, we were curious,
as usually how many people out there are familiar with
the concept we're talking about today. So Daniel went out
there into the internet to ask people what is a
(09:32):
light cone? And if you are out there on the
internet and always wanted to participate in a podcast, please
write to us two questions at Daniel and Jorge dot
com and you can be unprepared to answer future questions
for the podcast. Right, And if you're not on the Internet,
please tell us your secret. I mean I would to
sometimes not be on the Internet. I guess we're on
(09:52):
the radio too, right, Daniel. I think some stations broadcasts,
so they might be listening on the radio. Yeah, So
thanks everybody who's out there listening to us on the
radio as well. We appreciate all of your support and
all of your listening. Did you know that we recently
passed fifteen million episode downloads? Wow? Really, it's amazing. That's
like fifty million hours that people have been listening to us.
(10:13):
That's like fifteen million banana jokes at least at least
because you know, we're a multi banana joke death Ray
a joke Ray here. We try to in our in
our best episodes, like a shotgun rate joke. Really, we
just splatter bananappeals all over the place and now I
imagining fifteen million tiny bananas coming out of the nozzle
of a gun at very high to be coming out
(10:35):
of our mouth, that's a little disturbing. But anyways, think
about it for a second. If someone asked you what
do you think a light cone is? What would you say?
Here's what people had to say. I would say, that's
what comes out of the shiny end of a flash light. Um.
I believe this is a space time diagram where it
shows a three dimensional object and as you go up,
(11:01):
that's time passing, so it makes a calm like if
you look at the diagram of the Big Bang and
are we sure a tiny point? Then it slowly goes
up into a cone as time progresses. Um. I don't
know what a light cone is. I'm thinking about there
was some propulsion the idea that was. I think it
(11:21):
was largely theoretical, but it was to do with kind
of using light to push spacecraft and it would capture
the light using light sales. I don't know if that
would be to do with it. I remember seeing a
model in the Science Museum in London, but there was
a cone, but that was probably just because it was
made of plastic. Light corne is representation of how from
(11:47):
the light travels from a point. For instance, if you
have if I have a little flash light in my hand,
then I turned it on the lightbulb trouble in space
and time. Like cone, you can represent it as a cone.
(12:09):
That is something I've heard of before. Um, it's the
realm of like what you can see. So it's like
ninety three billion light years across as in our observable universe,
because that's like how far we could observes or something
into space, or like the photons that have reached us
(12:29):
from that distance. Although I don't understand how that's bigger
than thirteen point eight billion light years since that's the
age of the universe. But I'm still quite unclear on that.
But I know that the light cone is basically all
that we can see within our realm of light, anything
that can reach us in any form. I think a
light cone is an ice cream cone that's been left
(12:52):
out in the sun too long and all the ice
cream melted. A light cone is an ice cream cone
that has no chocolate ice cream minute whatsoever, which is
fundamentally flawed, and I think it violates one of the
laws of thermodynamics, because why would you want vanilla ice cream?
One chocolate is an option, so light cones make no sense.
All right. A lot of people went with my interpretation,
(13:14):
which is that it sounds like a dessert possibly and
that it's light. Yeah, and we also heard some people
throwing shade on vanilla ice cream. Wow, that was rough.
I know, how can you be down in vanilla ice cream?
It's like the nicest flavor gets along with everybody. That's true.
I mean I would always pick chocolate over vanilla. But
you know, if all you got is vanilla, I'm happy
(13:35):
to get. But if you take vanilla and sprinkle chocolate
on it, see that's the versatility of vanilla. Or take
chocolate and sprinkled chocolate on it, it's still better. But
you can't take chocolate and sprinkle vanilla on it. That's
when you do. That's vanilla doesn't add to anything. It's
just sort of like the substrate he lean. It makes
most cookies taste good. It's just sort of vanilla. That's
(13:55):
the reason why they call it vanilla vanilla, you know,
because it's so dependent and solid and she just always
there for you. Right. If somebody reads you're writing and
they describe it as a vanilla. Would you take that
as a compliment? Depends am I writing the bag of
an ice cream carton? Then it would meet its cool
But yeah, so a light cone, Daniel tell me, this
(14:17):
is something I was surprised to see in our topic
because it is it's like a graphical interpretation of physics, right,
And so it's a little tricky to describe on a podcast.
It's a little tricky to describe on a podcast. We'll
see how we do. But I thought it was useful
because I find that every time we talk about special relativity,
I have light cones in my mind. I got these
things in my brain. They really helped me think about
(14:39):
sort of the structure of space and time, and they
lead to a lot of interesting consequences. And so I
hope if we can somehow download the concept of a
light cone into the minds of our listeners, it will
help them think about special relativity. We need like a
light cone ray to shoot listen to people. Maybe, Yeah,
I think it's a podcast starring me and you're talking
about throwing out vananative. All right, well, m step us
(15:01):
through it. What is technically a light cone? So a
light cone defines the part of the universe that you
can influence, the part of the future that you can
visit geometrically. And you can think about it like if
you turn on a flashlight for a moment, so you
shine photons in every direction, they leave you in sort
(15:21):
of a spear. After one second, they are one light
second away from you. After one year, they are one
light year away from you. So imagine a sphere and
then that sphere sort of growing as time goes on, right,
So that set of spears versus time as those spears grow,
this is your future light cone. This is the part
of the universe that you can influence where your choices,
(15:45):
the things that you do as a person could have
sort of an effect downstream. It's very easy to understand why.
The reason is that everything that's outside of that growing
set of spears you can't influence because information would have
to travel faster and the speed of light to get there.
So it's a direct outcome of the fact that we
have a maximum speed limit to the universe. It divides
(16:06):
the universe into parts that you can influence and parts
you cannot. So the light cone is the part of
these universe, including the future. Right, we're talking space and
time here, the part of the future universe that you
can have any sort of influence over. So I turn
on a light like a light bulb, and that light
is growing comes out of the light bulb of growth
(16:27):
in a sphere, and you're saying, that's the light cone.
So it really should be called like a light sphere.
Maybe it'd be a better name, like the light sphere
is the sphere around you which you can reach with light.
The reason it's often called the light cone is that
it's hard to draw four dimensions on a sheet of paper.
The most we can really draw on a sheet of
(16:47):
papers three dimensions, you know, by having like some perspective,
and so you need to have time on there. So
you can really only have two other dimensions of space
on your sheet of paper. And so when you're drawing
these diagrams, people typically treats base as if it was
a two D surface instead of like our three D world,
just to sort of simplify it. So in two dimensions
the sphere we were just talking about, it becomes a circle. Right. Imagine,
(17:10):
for example, taking a smooth sheet of water and hitting
it somewhere. What happens you have a wave that leaves
where you hit it and it travels outwards. So now
you get a set of circles. Now, if you take
those set of circles and you extend them like vertically
above the surface in time and make time that vertical direction,
then what you get is a cone. Right. It starts
from a point when you hit the surface of the water,
(17:32):
and then as time goes on, that circle gets larger
and larger, and that defines a cone. So it's a
cone only in a sort of universe that has two
spatial dimensions and one time dimension. Right, It's kind of
like if you take a slice of those light spheres
we were talking about, Like if you take a slice
of it and you see those that circle on that
slice growing, and then you imagine that circle growing but
(17:56):
growing and also moving up maybe at the same time,
it kind of makes like a cone, yeah, exactly, and
it moves up because now we're imagining time to be
one of our axes. Right, we have two dimensions of
space and we have another direction, which is time. And
obviously things just flow forward in time at one second
per second, and so this circle just sort of grows
(18:17):
in time. If you instead had something which wasn't moving. Right,
if you just like drew a circle around you, then
that circle would slide through time and make a cylinder, right,
because it wouldn't be growing. But if it is growing,
then it makes a cone. It starts from the point
where you've made it, and it makes a cone. And
I guess if you're having trail visual license, you can
always just pop on Google images or something and look
(18:38):
for light cone. I imagine you'll get a picture of this, right.
How do people do podcast before Google image search anymore?
They probably avoided topics that involved geometry and and multiple dimensions.
But it's kind of an interesting concept, and I think
you as a physics you like it because it defines
kind of like a volume, right. It defines kind of
(19:00):
like a set of space time that is either inside
the cone or outside the cone. That's kind of the
power of this kind of visualization. Yeah, and it's not
just arbitrary, like you can make whatever shape you want
in spacetime. But this one is special. This one is
really important because everything that's inside your future light cone
are things that you can influence. Right. You can't send
(19:21):
the message to anything outside your future light cone, like
anything outside your future. Light cone by definition, is too
far away for any message to reach because you would
have to travel faster than the speed of light. And
if you can send a message, it means you can't
send any information. That means you can't influence it at all.
And one of the really tricky, complicated things about special
(19:41):
relativity is that it plays sometimes with the order of
events and it seems to have consequences for causality. So
this helps you sort of keep track of that the
part of the universe that you can influence, and also
looking backwards, the part of the universe that can influence you. Right,
all right, Well, it's get into some of these weird
consequences for because reality and simultaneity and alien death rays.
(20:03):
But first let's take a quick break. Alright. We're talking
about the speed of light, causality, the universe, and light cones,
which are sort of like a visualization in physics that
(20:26):
help you kind of figure out what you can't affect
and what you cannot affect because I guess that's something
is far enough away you can't affect it right now,
right because you have to wait for the speed of
light to get from here there or there to hear
it doesn't even have to be far away. Something that's
one foot away from you, like literally one foot away
(20:47):
from you right now. You can have no influence on
what's happening right there, right now, because even if you
shot your death ray, it would only affect what's happening
to foot away from you in the time it takes
for light to travel from the tip of your death
ray one foot. There's nothing you can do about what's
happening one foot away from you right now. Oh wow,
that's kind of weird, right to know that there's stuff
(21:09):
around me that is unaccessible to me. Yeah, I mean,
even if you're seeing something that happens a foot away
from you, that means it actually happened in the past,
so it's already done anyway. Right, So all you can
do is affect the future, right That seems obvious. But
you also can't affect the future everywhere, Like if you
want to change the future of Andromeda, you know intuitively,
you can't change the future of Andromeda tomorrow or the
(21:31):
next day, or even the next year. It would take
millions of years for any decision you make to affect
anything in Andromeda, because nothing you do will get there.
No choices you make can propagate that far before a
million years because it's a million light years away, right, Yeah,
I guess that's the big consequence of having a speed
limit in the universe is this idea that you know
(21:53):
there are things in the present, but you can't really
touch them, more affect them, or even know what they
are right now in the same president as you. That's right,
there's a limit on the information and how information travels.
It's really fascinating. You can only ever see one slice
of the universe, and your slice the universe might tell
a different story from other slices of the universe. And
(22:14):
this is really hard to get into your head because
you imagine that what we're doing with physics is trying
to build up like an absolute picture of the universe,
like what's really out there, what is really going on,
what happened in the universe, and what's going to happen,
And it's hard to accept that one you can never
really know everything about what's going on in the universe
because you're limited to the slice of information you're going
(22:36):
to get, but also that what you do learn about
the universe might not be like the absolute truth. It
might just be like what things look like from your perspective, right.
I think we talked about this once in the podcast.
It's like, imagine like how the human society was before
the Internet or before telecommunications, you know, when we had
to rely on writing letters and you know, waiting for
(22:58):
newspaper to be delivered to your door. It's like, there
could be things happening right now in other parts of
the Earth, but you know it'll be a long time
before you find out about them, or you know that
they actually happened at the same time that they happen.
That's right, and speeding up our communications has sort of
expanded our cone of understanding. Right, It's shrunk the time
it gets for information yet here, but not to zero.
(23:21):
Even with cell phones and speed of light electronics, you
still can't know exactly what's happening across the world right now.
There's still a time delay. And that's why it's cool
to see in some of these TV shows that they
really incorporate that. You know that if you're trying to
coordinate your army on Mars from your base on Earth,
it takes time for your messages to get there. If
there's a battle going on around you know, the atmosphere
(23:42):
of Jupiter that by the time you've heard about the beginning,
it's probably over already and there's nothing you can do
to change the outcome. I guess, like before when you
had a battle on you have to like send a
messenger or send a scout or something you had, there
was this just huge delay, and so now thinks they're faster,
but they're still limited, like there's still no simultaneity, Like
there's still no ability to know what is going on
(24:04):
right now somewhere else. Yeah, exactly. And the same concept
is light cone, which tells you, like what you can influence.
It also works in the other direction. You have like
a backwards light cone. If you think in the other direction,
like how close does somebody have to be to me
to shoot a death ray and hit me right now?
You know, if somebody is in Andromeda and they shoot
a death ray at you, you're safe for a few
(24:26):
million years, right Nobody in Andromeda can kill you right now.
Even if they shot their death ray a hundred thousand
years ago and it's been hurtling towards you for a
hundred thousand years, you're still safe. It's not going to
get here for a while. Because they are not in
your backwards light cone yet. But if they shot at
a million years ago, it would be hitting us right
now potentially. And if they shoot it right now, they
(24:48):
were still going to get hit, But it's going to
be a million years from now, that's right. If they
shoot it right now, it's not in our past light cone,
but our past light cone moves up this time access
and eventually they will be inside our ask light gone
and yes it will get here, right but you're safe
right now, right now, there is this past like cone
that dictates all the things that could be influencing what's
(25:08):
happening at your location, and somebody really far away, even
a foot away, can't influence what's happening to you right now.
They can only influence your future. Right well, they can't
influence you right now. But I mean doesn't mean you're
safe from a death ray, because they could have shot
it a while ago and you would still die from
the death ray arriving now, you're right, And the weirdest thing,
the worst thing about death rays is that they travel
(25:30):
at the fastest speed possible, so there's literally no way
to get any warning. Right the moment you see it
is the moment that arrives at the moment it impacts you.
But hopefully, you know, a million years ago, you know,
we were just cavemen and women, and you know we
didn't do anything to offendiate, hopefully, Or maybe it's a
natural process. Maybe the gamma rays from a supernova that
(25:51):
blew up, you know, a while ago, and those sterilizing
death rays are just now arriving here at Earth. Man,
this line makes me feel not at all safer. Daniel,
See how you can drop comfort from this light cone.
It's like a pretty flimsy protection against giant gamma ray bers.
It just means that somebody's got to plan ahead. If
they really want to fry us now, they're got to
(26:12):
either be close by or they have that planned ahead
so that athetic exploding start. Are you saying it's not
very good at planning so we're saved. Is that it's
a procrastinator saves us from procrastinators exactly? But what about
type A supernovas? Oh, the kind of plants ahead, there's
nothing we can do. Yeah, we're toast, all right. So
(26:35):
it sort of protects us, and I think it also
kind of affects what we might agree on. Is kind
of a consistent history of the universe. Right, Yeah, it's
really interesting. The whole universe can be divided into three regions.
You have your future like cone, your path like cone,
and then there's everything else. Right, and this is this
called this the absolute elsewhere. It sounds like an awesome
(26:58):
name for a Netflix show or something. So the absolute
elsewhere is the spot that you can't affect, you can't influence, right,
And that's really interesting because things that happened in the
absolute elsewhere, you might disagree with people about the order
in which they happen. Like you might look at a
series of events and say A happens before B, and
somebody else might look at it and say, no, no, no,
I think B happens before A. And you could both
(27:20):
be right. If these two things are not inside each
other's light cones, and these events don't have overlapping light cones,
then they can be differently ordered and not affect anything
from a causality point of view. WHOA, So I guess
I can have a light cone and you can have
a light cone. And if those two don't interact, is
that what you mean? Like if they don't touch, then
(27:40):
we were looking at literally like different universes. Well, they
will eventually touch, right, because their light cones will eventually impact,
and so there is some overlap in their light cone
far in the future where both of them can influence.
But it doesn't matter which one happened first. It doesn't
matter if A happened in then B, or B happened
and then A, because these two events are the events
(28:01):
themselves are not inside each other's light cones. Right, B
is not in a's light cone, and A is not
in b s light cone. Even though the two light
cones do overlap, if they are not inside each other's
light cones, then you can swap their order just by
changing your speed. If you decide to zip by them
in a spaceship as close to the speed of light,
you will see one happened before the other. If you
(28:22):
go in the other direction and nearly the speed of light,
you will see the opposite order of events. And it
all works out downstream. Because these two things aren't causally linked,
they're not inside each other's light cones, so you're free
to change their order of events. But then you know
the TV who are involved would have a different interpretation, right,
like one of them will see one thing happening before
(28:42):
the other. Yeah, exactly, But there is no absolute definition
of what happened. First, all the observers have their own interpretation,
and they're all correct because the answer depends on your position.
That's not true for two events where one is inside
the other one's light cone. If I shoot a joke
ray and I hit you in the head, then you laugh, right,
(29:03):
put it on joke mode, please, there's that. It's don
and then there's a joke open on banana mode. So
I hit the ray and the banana hits you in
the head. Right now, you are in my light cone.
Because the two events me pressing the button on the
gun and you having banana arrived in your mouth, they're
not at the same time. What happens before the other,
(29:23):
and there's a causal structure for them. I have to
press the button before the banana ray arrives at your head.
And that's okay, because the banana ray arriving at your
head is an event in my future. It's inside my
light cone. But for example, you and I both have
these guns and we point them in random directions or whatever,
and we try to fire them at the same time.
Some people might say we fired them at the same time.
(29:45):
Other people flying behind the spaceship might disagree about who
pressed their trigger first because they would be moving and
because of the way that light along the light takes
to get to them and the speed limit of the universe,
they might reconstruct the events in a different way. Yeah,
but it's not just because how long light takes to
get to them. Even if you factor that in, right,
(30:07):
even if you account for how long it takes the
signals and the images to appear, there's still a difference
in the order of events. And that's because time flows
differently based on where you are and how fast you
are going. So your speed affects how you see time flowing,
and so you can see one event happening before the other.
You can see Jorge presses button first, or somebody else
going in the other direction can see Daniel press the
(30:29):
button first, even if they account for the transmission delay
from light. But they would be wrong. I guess what
I mean is they would have their own interpretation, but
other people would have their different interpretation. Yeah, but nobody's
right or everybody's right. You know, if these two events
are not inside each other's light, cones right, their light
cones can overlap in the future, but the two events
themselves are not inside each other's light cones, then their
(30:51):
order is not well defined. And this is all because
the universe has a speed limit, right, Like, if it
didn't have a speed limit, then you could talk about
like absolute history of the universe. That's right exactly, And
that speed limit defines the size of this cone. Right. Imagine,
for example, if the speed of light was much much slower,
then your light cone would be narrower. The part of
(31:13):
the universe, the part of the future that you could influence,
would be a much smaller part of the universe. If
you shot death rays in every direction and the speed
of light was much smaller, then you would kill a
smaller fraction of the universe. Or if you shot joke raise,
you would make a smaller fraction in the universe laugh.
Then if the speed of light was ten times faster
than it is, then you could influence more of the universe,
(31:33):
and aliens out there with their joke raise could make
us laugh more effectively. So this number the speed of
light really changes the whole structure of causality of space. Right,
But what if you make like a light joke like
this vanilla is the best. Ha ha ha. That's a joke,
like a light vanilla joke. Yeah, that wouldn't harm anyone,
and people would just let it go by. But it
(31:54):
really is sort of like a cone. I'm thinking, like
a doc cone. You know those cones they put on
dogs so that data and harm themselves or just to
protect them. It really is like a cone that you
put on our we put on our heads that kind
of limits what we can see and what we can
interact with, right it sort of does. Now imagining you
with one of these on your head trying to eat
a banana, even though you can't, Daniel shoot me with
(32:17):
the bananas exactly, then you need me to shoot you
with the banana ray or I guess she went up
in the air and then catch it and the cone
would help me catch this. There you go, there you
go pretty soon be surrounded with bananas. I'd have a
ConA around my head filmed with bananas and I'll go
along with a scoop of vanilla on top, and then
for some childhood. But the amazing thing to me is
(32:38):
that we still don't know why the speed of light
is this number. You know, it has such a dramatic
effect on what part of the universe we can see
and what part of the universe we can influence, and
how the universe is sort of like tied together, you know,
what affects what It really changes everything. And it's just
like a number. We measure it. We don't know why
it is what it is. It could have been better,
it could have been smaller, for all we know. Our
(33:00):
laws of physics just have like an empty spot there.
If you're like right in the number you measure, there's
no reason it has to be this or that that
we know of. And that's like a real mystery, you
know, it it makes you wonder why it's this and not
something else. Alright, cool, Well, let's get into now what
consequences is this light cone has in our in the
universe and possibly in our ability to travel through time.
(33:22):
But first let's take another quick break. Are we're talking
about light counts and the speed of light and whether
or not we can affect things in the universe that
(33:43):
are happening right now, which is basically no, right, Like,
we can't affect anything else in the universe happening at
this moment, that's right, nothing except for what's happening right here,
right now, or stuff that's happening in the future. But
at this very moment, you can't affect anything except for
exactly the spot where you are. Well, technically nothing right
(34:03):
because even things that are one millimeter away from my
eye in the present moment, I can't even see or effect.
That's true. We have to go down to zero millimeters.
And that's only even only if you believe in free
will and if your brain operates instantaneously, So technically you're right.
Any decision you make right now can only influence the future,
except that maybe there's a possibility to use this concept
(34:24):
to do time travel. Daniel so explained that to me. Yes,
so what we talked about so far, this idea of
light cones these circles of light expanding out into the
universe and into your future. This is the way we
think about space. If it's not curved, so we call
space flat, if it's just sort of like has no
shape to it, if there's no bending of it that
affects like the path of photons. And we know that
(34:46):
space can be bent. We know that, for example, everything
with mass and energy bend space, like the Sun bend space.
That's why the Earth is moving around it instead of
just flying off into deep space. We know the Eube
and in the space because you have mass, and so
everything out there changes the shape of space, which means
it changes how light goes through it. Remember that gravity
(35:09):
is not just like a force that tugs on things,
because light, for example, has no mass, but gravity can
affect its path because gravity changes the very shape of
space and light travels through that space. And so we
know that things are more complicated when space gets curved.
So what we talked about so far is only in
the event that the universe is empty and there's no curvature,
(35:31):
there's no mass or anything to change the shape. So
in flat space, when there's a lot of mass around,
things get curved and things get pretty different and weird.
These light cones, which you can imagine sort of like
going up and going down now, start to get tilted. Right.
Is this something we can actually see? Right? You can
see light getting bent out there in space by heavy objects. Yeah,
(35:52):
it's called gravitational lensing. If there's like a really big
heavy object between us and some source of light, like
a background galaxy, then as those photons come towards Earth,
they could bent around that massive object. And this is
one way we can detect, for example, the existence of
dark matter and try to figure out, like roughly where
it is based on how it distorts the light that's
(36:12):
passing near it or through it. So it's definitely a
real thing that's happening. It's something we've observed. We're sure
it is true. So if a space can bend light,
or land space can sort of affect the trajectory of light,
that would make sense that it would bend these light
cones that we have to kind of figure out what
we can see and cannot see in the universe exactly.
(36:32):
Because the shape of the light cone is defined by
where light goes right where light can reach, and so
space is bent in a way that light can't get somewhere,
then the light cones are also bent. So this idea
of them being like simple cones is only in flat
space near something really massive, they bend towards the mass, right,
because light moves towards mass. Right. If you have, for example,
(36:55):
the Sun, and you shoot a photon near the Sun,
it will get curved sort of in the dire action
of the Sun. And if you have a black hole,
then it's going to get curved room towards the black hole.
So as you get closer and closer something massive, these
light cones tilt more and more. Well, they start to
look like maybe horns and not so much cones or yeah,
or like bow ties or something yeah, or like a bugle,
(37:17):
remember that snack, the little like cones, curved cones. Yeah, exactly, Yeah,
light bugles. Will call it light bugles exactly. And so
inside a black hole, of course, you can think about
what happens to a photon. While a photon inside a
black hole always goes towards the center, not because gravity
is so strong, but because the shape of space requires it,
(37:37):
because there's only one direction inside a black hole. So
inside a black hole, these things have tilted totally sideways.
You shoot a photon and it goes just towards the singularity.
All the light cones are now tilted towards the center.
You hear people say sometimes this thing that inside a
black hole, space and time switch roles. Right, This is
like a geometric way to understand that, because now instead
(38:01):
of time flowing up, it's sort of flowing sideways. Right,
Because the light cone has tilted over by forty five degrees. Well,
I guess I don't understand it at all. So the
light cone tilts, and somehow time and space which positions?
Is that what you just said? Yeah, because your future,
(38:21):
all possible futures are the singularity, and so the only
way to move forward in time is to move in
this one direction of space. And so sort of like
moving through space towards the singularity is the same thing
as moving forwards in time. And you can graph that
geometrically by thinking about the light zone being tilted over
(38:42):
from being vertical to being horizontal. It's like there's only
space and time become one way and everything goes in
the same direction. Yeah, exactly, all right, And so then
how does that relate to time travel? You said it
might be possible to do time travel. It might be
possible to do time travel if you use these cones
you and construct this really weird structure. Right, So we
(39:03):
talked about how near something really heavy, these cones can
tilt like up to forty five degrees right towards the
center of a black hole. Now, imagine if you could
have a cone that tilted more than forty five degrees,
And what that means is that you're tilting so that
part of the light cone is now in the past, right,
So it's possible if you could do this, it would
(39:25):
mean that these particles are moving not just into the future,
but like the future is now dipping into the past.
What you mean you you bend the light cone so
much it wraps around and touches kind of the past
light cone, m exactly, and you think, is this possible?
Doesn't that contradict itself? Will remember that, like the definition
(39:45):
of time and the definition of space depends a little
bit on how fast you're going and the environment you're in,
and that gets even more complicated when space is bent.
And so what you can have is like a particle
moving forward into it's future like it's local time still
says I'm moving forward in the future one second, then
the next second, and the next second. But it could
(40:07):
be moving into the past from the point of view
of another observer. So like from a distant observer who's
watching this from far away, whose space isn't curved, they
could see that particle moving into like their past, even
though if it's still moving into its own future. Whoa wait,
are you saying that space and time actually curve around itself,
or it only looks curved around to a different observer,
(40:29):
the structure of it is actually occurred, and people think
in some weird configurations, if there's a very special arrangement
of mass, then you might be able to construct a
structure of space that allows particles to have their local future,
their personal timeline extend into like the past from the
point of view of other observers. From the point of
(40:51):
view of other observers, but the particle itsels wouldn't be
traveling into the past, or like me, I wouldn't be
traveling into the past. Well, it's like into their own future.
But that's sort of like what happens on a time
travel show. Right, If you are on a time travel
show and you want to go into the past, you're
still living your life forward on your timeline. Right, you
were forty six years old and seven days and two minutes,
(41:13):
And then when you go into the past, you're not
like younger again, right, You're just like keep going forward.
Now you're forty seven years, two days and five minutes
and seven minutes and ten minutes. So what you want
is your own timeline to continue, but you want the
structure of space around you to change that your timeline,
your future now goes into like the rest of the
universe's past. So this is called in physics, it's called
(41:35):
a closed time like curve because if you can tilt
a bunch of these cones just the right way, then
you can make this path that loops on around itself
and comes back to the same point where it originated.
So it's like shooting a light beam and having it
hit you in the back of the head, almost like
a wormhole kind of in time, almost like a wormhole. Yeah,
it's not actually a wormhole, because a wormhole is like
(41:57):
a shortcut between two distant locations. This is like a
structure in space. It's like a loop in space time. Right.
It is a complicated reconnection and rearrangement of how space works.
But it's technically a little bit different from a wormhole. Right,
wouldn't you be stuck in this loop like you'd be
be like groundhogs day? For this particle, it's just reliving,
going around and around and around. It thinks it's going
(42:18):
in the future, but really it's like revisiting the same
time over and over and over. Exactly. It's a structure
that reinforces itself, and it's only stable if it does
revisit itself. And so you're right. You wouldn't like get
to go back and choose a different flavor of ice
cream or set your death ray to banana instead of
death right. You have to make the same choices over
and over again. It's like this self supporting structure in
(42:39):
space and time so not recommended. Well, unless you want
to keep watching the same shows maybe, or if you
really like vanilla and you want to eat only vanilla
flavor for the rest of your lives for eternity. Yeah, exactly.
And this is something which is theoretically conceivable, like people
have worked this out on paper and it seems like
it might work. But again, general relativity doesn't tell us
(43:02):
like how to bend space to make this happen. People like,
if you could bend space in this particular way, then
we think this could happen. That doesn't mean necessarily we
know how to make space bend in that particular way.
You know, people have ideas, they have this crazy structure
that's like an infinite spinning cylinder of dust, and they
(43:23):
do the calculations and they think maybe that would allow
for these closed timelike curves to happen, but other people
think it's impossible, there's no way it could work, and
that we have to sort of throw up our hands
and say, let's figure it out when we get the
unified theory of everything. Well, you're saying that it's according
to the equations, it's possible, or like you know, anything
that can bend, if you bend it enough, this will happen,
(43:45):
But we don't know if it's physically possible to do it.
We don't know if it's physically possible. If you believe
general relativity is absolutely true and you can construct an
infinite spinning cylinder of dust, then you should believe this
is physically possible and can actually happen. Personally, I don't
think general relativity is a true theory of nature because
it ignores quantum mechanics, and so it can't be correct,
(44:07):
and we need some updated version. And it might be
that in that updated version and incorporates quantum mechanics, this
doesn't work anymore. So this could be like sort of
like a singularity of a black hole, like a sign
that the theory is wrong because it predicts this sort
of nonsense result. Right, Because I guess if it's a
closed loop. How would you even get into that loop? Right? Yeah,
you have to have somebody else build that infinite spinning
(44:27):
cylinder for you in the infinite past, because you'll always
be in that loop. So it might exist and there
might be a particle stuck in it or from the
beginning of time, but you wouldn't be able to get
in or out of it. That's right. If it exists,
it would be stuck forever. But you know, now we're
talking about like construction of a time loop, so we're
talking about the variation of the loop as a function
(44:49):
of time. So it's like an extra level of meta
time there, which gets tricky. You can just use the
same vanilla equations, you get to go extra spicy or
sprinkle some bugles on for some extra crunching iss. All right,
So that's the light on one, I guess, and I
guess you're saying it's one way that that it helps
you think about all these weird situations of space and
(45:10):
black holes and bending time and spacetime looping around itself.
You use that in in your everyday you know, theoretical
musings I do, and also when people write to me
and they ask me some like crazy question about special
relativity involving you know, spaceships and snowballs whatever. I usually
end up drawing light cones so I can understand, like, well,
(45:31):
this happened there, What will this person see or is
it possible for them to know this. It's just a
nice way to sort of like separate things because in
physics problems because much simpler when you can separate and
say these things are independent from those things, so I
can just deal with it on its own. They're not
like entangled. So a light cone shows you how to
like disentangle all the causal threads of the universe into
(45:51):
a bunch of different packets. Right, Yeah, everything is better
with the snow cone. That's our next episode. What is
a physics snow cone? It's a commort physic You say
no all the time, it's no, can I travel back
in times? No? Cyes c Yes? All right, Well, hopefully
(46:12):
it helps all of you out there and think about
cantality and what we can and cannot affect about the
present and maybe makes you appreciate a little bit more
where you are and what we're able to see of
this big, crazy universe. Well, thanks for joining us, see
you next time. Thanks for listening, and remember that Daniel
(46:39):
and Jorge explained. The Universe is a production of I
heart Radio. Or more podcast from my heart Radio, visit
the i heart Radio app, Apple Podcasts, or wherever you
listen to your favorite shows.
Hosts And Creators
Daniel Whiteson
Kelly Weinersmith
Popular Podcasts
United States of Kennedy
United States of Kennedy is a podcast about our cultural fascination with the Kennedy dynasty. Every week, hosts Lyra Smith and George Civeris go into one aspect of the Kennedy story.
Stuff You Should Know
If you've ever wanted to know about champagne, satanism, the Stonewall Uprising, chaos theory, LSD, El Nino, true crime and Rosa Parks, then look no further. Josh and Chuck have you covered.
Dateline NBC
Current and classic episodes, featuring compelling true-crime mysteries, powerful documentaries and in-depth investigations. Follow now to get the latest episodes of Dateline NBC completely free, or subscribe to Dateline Premium for ad-free listening and exclusive bonus content: DatelinePremium.com