August 20, 2025 • 30 mins
Can we go faster than light? Jorge goes into overdrive with two physicists to see if we can ever catch up to the fastest thing in the Universe.
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Speaker 1 (00:00):
Hey, welcome to Sign Stuff, a production of iHeartRadio. I'm
hoorhe cham and today we are answering the question can
we go faster than lights? Like the fastest thing in
the universe? And what would happen if you tried to
go faster. We're going to talk to a couple of
experts about this, including a famous theoretical physicist and a
(00:23):
historian of special relativity. So buckle up, put your helmets on,
because we are going to ludicrous speed to answer the
question can we go faster than light?
Speaker 2 (00:44):
Hey?
Speaker 1 (00:44):
Everyone? Okay, So today's question is a pretty deep one.
It goes to the very heart of what the laws
of physics in our universe are, and it has huge
consequences for what it means to have an orderly and
logical existence and reality. It's such a huge question that
(01:04):
it even stumped Einstein. So I knew I needed a
lot of help, which is why I called two very
smart friends of mine. And according to them, there are
three ways to answer this question. The first way is
to think about how fast light can go. Here's my
conversation with my first friend, famous theoretical physicist, New York
(01:27):
Times bestselling author and host of the Mind Skate Podcast,
Professor Sean Carroll. Cool, and I might crack some jokes.
That's the tone of the episode. That's okay, that's allowed.
We're allowed to entertain the podcast. All right, Well, thank
you doctor Carrol for joining us. Thanks very much for
having me. Please tell us who you are and what
(01:48):
you did. My name is Sean Carroll. I'm a physicist
and philosopher at Johns Hopkins University. I think about where
the universe came from, what it's made of, what the
laws of physics are awesome. Have you ever gone faster
than this of light? I never have. I've gone stoller
in the speed of light many many times.
Speaker 2 (02:04):
I guess.
Speaker 1 (02:05):
Just to start us off, what is the speed of light?
It's kind of something you can think about at different
levels of sophistication. To be honest, the speed of light
is the speed that light travels at in empty space.
So one very quick thing to get straight is that
light itself can travel at different speeds. Light travels at
a different speed through air or water than it does
(02:27):
through empty space. So when we're being a little bit
more careful, we say the speed of light in vacuum.
That's the special thing that's what people really care about.
That means that it's a speed of light when there's
nothing in the way, kind of nothing for the light
to bump into. Okay, so the first thing to know
about how fast light moves is that it has a speed.
Speaker 2 (02:49):
It may seem.
Speaker 1 (02:50):
Instantaneous when we turn on a light or a flashlight,
but it does take time for light to go places,
and this is something you can measure. For example, you
can shoot a laser at the moon and then see
how long it takes before it bounces back. If you
did that, you'd see it takes about two point six seconds.
Speaker 2 (03:09):
Or you could send the.
Speaker 1 (03:10):
Radio signal, which is also light, to Mars and you
see it takes about fifteen minutes on average for the
signal to go there and come back. The second thing
to know about how fast light moves is that it
depends on what it's moving through. If it's moving through
nothing or a vacuum, then it moves at its maximum speed,
(03:32):
which is about three hundred thousand kilometers per second. At
that speed, light can go all the way around the
Earth seven times in one second. But if it moves
through air or water or something transparent, then light slows
down and the denser that thing is the slower light
(03:54):
will move. So in a vacuum, light moves at about
three hundred thousand kilometers per second, but through water it
moves two hundred and twenty five thousand kilometers per second.
If it moves through glass, it's two hundred thousand kilometers
per second, And if it's a diamond, light moves at
one hundred and twenty four thousand kilometers per second, or.
Speaker 2 (04:16):
Almost two and a half times slower.
Speaker 1 (04:19):
And that's because even though these things are transparent, the
atoms still interact with the light. So when we think
about light traveling through water or glass or air, we
think it's transparent, it's just going right through, right, But
in fact, the light is being affected by the medium
it's moving in, by the very gentle electromagnetic interactions with
(04:43):
all the atoms that it might bump into. That's why
when you pass light through a prism, it will be
broken up into different wavelengths. Right, it'll become it'll be
the pink Floyd Dark side of the Moon album cover,
and white light becomes all the colors of the rainbow.
It gets psychedelic. You know, I'm not going to say
how you should best experience dark side of the moon.
(05:03):
That up to personal choices. Now, a lot of people
think the reason that light slows down when going through
something like air or water is that it's bumping into
all the atoms that are there. But that's not quite right.
The reason is a little technical, but basically, light is
an electromagnetic wave, and when it goes through water or
(05:26):
a diamond, the wave makes the electrons in the atoms
that are there wiggle, and the wiggling causes a kind
of counter electromagnetic wave that when you add it to
the original light wave, it slows it down. And it's
a good problem set for undergraduates learning this stuff for
the first time, because it's actually quite miraculous. It's the
(05:49):
net effect of all of the atoms has a sort
of simple calculable effect on what the light is doing.
It changes its speed. Okay, what does mean for the
first way of answering the question can we go faster
than light? Is that you can imagine setting up an
unfair race. On one side, you would shoot light through
(06:11):
a long piece or rod of a dense material like
glass or diamond, so that the light would be going
at half or less of its normal speed, and next
to it, you would hop into a rocket with nothing
in front of it but empty space. Now, you still
have to go pretty fast, thousands of times faster than
(06:33):
any human has ever gone before. But technically you could
beat the light going through that rod. If you had
enough fuel, you could get up to a speed that's
faster than that light beam, Which means that technically, one
way to answer the question can we go faster than light?
Is yes, in an unfair race. All right, you're probably thinking, sure, Jorge,
(06:58):
but that's cheating. And I imagine you clicked on this
episode to find out if you could go faster than
light without cheating. That is, can you go faster than
the speed of light in a vacuum? After all, that's
the question that's going to make it practical or not
if he could ever go to distant stars or even
(07:18):
to other galaxies. But here the answer is a lot trickier, because,
as it turns out, the speed of light isn't just
how fast light can go. It sets a universal speed limit.
Here's how Professor Carroll puts it. The slightly more sophisticated
(07:38):
way of saying it is the universe has a speed
limit built into the laws of physics, there is a
fastest speed at which anything can go, and unlike any
other speed, this speed is absolute. It is not relative.
It doesn't matter how you're moving through the universe, you
are going to measure the same speed limit relative to you.
(08:01):
It just so happens that light travels at this speed
when it's moving in vacuum. But other things do as well.
Gravitational waves, for example, also move at the speed of light.
So it's really a feature of space time itself. It's
a crucially important feature. It says that there are things
to our past, there are things to our future, and
there are things sort of sideways to us, things that
(08:24):
are neither in our past and our future, because we
can't get there without going faster than the speed of light,
which is not allowed. All right, when we come back,
we're gonna dig into everything Professor Carroll just said, and
we're gonna go into the mind of Albert Einstein as
he wondered the same question we are asking today, what
(08:44):
would happen if he could catch up to a lightbeam.
Stay with us, we'll be right back.
Speaker 2 (08:59):
We come back.
Speaker 1 (09:01):
Okay, we're answering the question, can we go faster than light.
And as I mentioned, there are three ways to answer
this question. The first is to cheat. You can slow
down light by making it go through something like water
or a diamond, and you could then technically beat it
in a race. The second way to answer this question
(09:22):
is to ask if we can go faster than the
fastest light can go, which is the speed of light
in a vacuum. And here the answer is trickier because
if you ask any physicists on Earth this question, they'll
probably tell you the answer is no. And so for
this segment, I thought i'd take you through the history
of how physicists arrived at this answer, because it definitely
(09:45):
wasn't by trying to go faster than light.
Speaker 3 (09:52):
It sort of came in waves. It wasn't always clear
to people for all time that light even had a
finite speed. Some people wonder if light simply was instant,
as if it traveled at an infinite speed, and a
bunch of people wondered, maybe that's not right.
Speaker 1 (10:06):
That's my other friend, Professor David Kaiser. He's a theoretical
physicist and a historian of science at the Massachusetts Institute
of Technology or MIT. According to doctor Kaiser, it took
a few hundred years since humans started doing serious science
in the fifteen hundreds, with people like Galileo to figure
out that light wasn't instantaneous and then it moved at
(10:29):
a certain speed. And that's at the stage for a
young teenager named Albert Einstein to ask a very simple question.
Speaker 3 (10:39):
That's what sets up a pretty amazing intellectual journey that
leads to people like a young Albert Einstein and many others.
Right around the turn of the twentieth century. In the
early nineteen hundreds, Einstein was wondering, even as a teenager
really before he was doing very formal schooling. In this
he wondered, what would happen if I could catch up
to a light wave? What would I see?
Speaker 1 (11:00):
What teenager Einstein wondered was, well, if light doesn't move
infinitely fast, if it moves at a set speed, what
would happen if you went faster and faster, could you
eventually catch up to a beam of light? And if
you did, what would it look like?
Speaker 3 (11:19):
And so he had these wonderful, very visual, kind of
imaginary thought experiments in his head as a sixteen year old,
and he realized, well, if there's a surfer on an
ocean wave, and they're going at the same speed. If
they look from side to side, at least momentarily, they'll
see this wave kind of frozen, right, because they'll be
moving with the crest of the wave. There's some time
they would see basically a wave frozen in time, not
(11:42):
zipping past them. And Einstein wondered, well, why can't I
do that with a LightWave, or what would happen if
I if I zoomed up and traveled at the same
speed as a light wave, but I see a frozen
wave like that frozen ocean wave as a surfer.
Speaker 1 (11:55):
In other words, Einstein wondered, as a sixteen year old,
if you can catch to light, would you essentially freeze
light or make it look like it's standing still? And
this question bothered Einstein. Okay, so then Einstein had this
idea that he nurtured since he was sixteen. And by
the way, when I was sixteen, I was definitely not
(12:17):
thinking about the nature of the universe. Not too loud,
that's right here we are.
Speaker 3 (12:23):
So then what happened and then he got bothered by
that he was reading for fun as a teenager and
early college did there's still pretty new theory worked out
by people like James Clark, Maxwell and Hendrick Herritt's and others.
That was just scoring like win after win in describing
all these other things about light. It was like better
than anyone even imagined. It would match all these experiments
(12:45):
to high accuracy. It was a theory not only of
optics but also electric field magnetic fields. But in none
of those solutions was there a sense that you could
slow light down to zero. Wow, there was no mathematical
solution of these otherwise beautiful and very powerful equations that
showed a LightWave frozen in space.
Speaker 1 (13:04):
So for young Einstein, this idea of going faster than
light and catching up to a lightbeam and seeing it
frozen wasn't allowed according to the other theories of the day.
It seemed to be true according to lots of experiments,
and so he came up with a pretty radical solution.
Speaker 3 (13:24):
And so Einstein got bugged by that, and he wouldn't
let it go. Whereas other teenagers might think about other things.
For one, he got there and he realized one way
to remove this uncomfortable paradox would be to say, what
if the speed of light always has the same value,
even if I'm moving fast. What if every person who's
even in motion would measure the speed of light to
(13:46):
have that same speed of speed of light in vacuum.
If that's the case, that if he was careful to
say if whoa, then no one could ever catch up
with a light wave.
Speaker 1 (13:53):
That's a huge leap. I mean it almost sounds like
a crazy idea.
Speaker 3 (13:58):
It is not what most people were I think, that's
for sure.
Speaker 1 (14:02):
Okay, here's Einstein's idea. What if you did try to
catch up to a lightbeam, you might start going faster
and faster behind it, trying to catch up to it,
and you would expect it to get closer to you.
But what if instead it never does. What if the
light beam always moves away from you at the speed
of light, no matter how fast you go.
Speaker 2 (14:24):
And this is very strange.
Speaker 1 (14:25):
And different than what we experience in our everyday life.
In our daily lives, we're used to speeds adding up.
For example, if you're driving down the highway at sixty
miles per hour and your kid in the back seat
throws a toy at you at ten miles per hour,
then to someone outside the car, they would see that
toy going at seventy or sixty plus ten miles per hour.
(14:49):
But Einstein said, what if that doesn't happen for light?
For example, what if you're going sixty miles an hour
in your car and you turn on the headlights. Expect
the light from your headlights to go the speed of
light plus to sixty miles an hour that the car
is moving. But Einstein said, nope, that light still only
(15:10):
goes at the speed of light. You and the car
would see the light go at the speed of light,
and someone standing on the highway would also see it
going at the speed of light. And this weird but
simple idea that light always seems to go at the
speed of light no matter who looks at it basically
revolutionized our understanding.
Speaker 2 (15:30):
Of the universe.
Speaker 1 (15:32):
Although it wasn't immediately a big hit, the.
Speaker 3 (15:37):
First reaction Einstein's work was no reaction at all. He
was out of the way. He wasn't in an academic position.
He was, you know, frankly, kind of a nobody put
it this way. He didn't even get a promotion at
the patent office.
Speaker 1 (15:51):
But eventually other people did notice Einstein's work.
Speaker 3 (15:56):
Nonetheless, more time goes by, he's able to get a
fewer people to read the work and take it seriously,
and a few of them actually were beginning to do
things like write their own review articles and pay more attention.
So over a period that stretches nearly to a decade,
more and more of the professional scientists begin to pay attention.
Speaker 1 (16:14):
Okay, you might be wondering, what does this have to
do with going faster than lights? Well, it turns out
that Einstein's sort of simple idea has some really weird
consequences for how the universe works. You might have heard
that time slows down when you're going really really fast.
That comes from Einstein's idea. Or that things look shorter
(16:35):
when they're going super fast. That's also from Einstein's crazy idea.
Speaker 3 (16:41):
And he's very clear, I'm going to assume this right
and then say what will follow from that? And even
more stream sounding things followed from that, even in that
first nineteen to oh five paper. So from this postulate
he deduces other consequences. If you're zipping by on the
train at some high speed, you and I will disagree
on whether two events happened at the same time. This
becomes known as the relativity of simultaneity. Another thing he
(17:03):
begins to realize is that we're going to disagree on
the measurements of lengths if we're moving at a relative speed.
So it's called length contraction. And the third one is
time dilation. So if we have identical clocks that we're holding,
but you're racing past me at some outrageously high speed,
I will watch your clock tick more slowly.
Speaker 1 (17:21):
The last main consequence of Einstein's idea is that nothing
can ever go faster than the speed of light. How
do we go from the speed of light, whether it's
the same in all directions, to concluding that it's a
speed limit for everything.
Speaker 3 (17:38):
Partly because from Einstein's own equations, as you try to
get closer and closer to the speed of light, the
difference in measured clock rate, all these mismatches in lengths
of meter six times on clocks, even also things like
the masses, the way we measure mass, These things all
rise and in fact become infinite. And that's like saying
it would take basically an infinite amount of energy to
(17:59):
go as fast as that light beam does, and I can't.
I have no source of infinite energy.
Speaker 1 (18:05):
Yeah, infinity can be a bummer, all right. When we
come back. We're gonna talk more about this explanation, whether
we've ever actually tried to go faster than the speed
of light, and then I'm gonna ask our experts what
they think would happen if we ever did break this
speed limit. So stay with us.
Speaker 2 (18:25):
You're listening to sign stuff.
Speaker 1 (18:35):
And we're back.
Speaker 2 (18:37):
Okay.
Speaker 1 (18:37):
We're asking the question can we go faster than light?
And so far we have two different answers. The first
is yes, you can go faster than light if you
slow down light by making it go through water or diamond.
The second answer seems to be no, if you mean
whether we can go faster than light in a vacum.
(19:00):
As we learn, it seems that the universe has a
speed limit, and the speed limit says nothing can ever
go faster than the speed of light in a vacuum. Now,
I know what you're thinking right now, which is have
we actually tried to go faster than the speed of light?
And the answer is yes. Since about the nineteen thirties,
(19:21):
scientists have been building particle accelerators. These are huge machines
that can take small particles like protons or electrons and
speed them up to go faster and faster. These accelerators
have been getting bigger and more.
Speaker 2 (19:35):
Powerful over the years.
Speaker 1 (19:36):
They've been pushing the limits of how fast we can
make something go. The largest accelerator right now, the Large
Hadron Collider in Geneva, Switzerland, is a seventeen mile long
circular tunnel that can speed protons up to ninety nine
point nine nine one percent of the speed of light.
(19:57):
What happens if you try to go faster, uh, not much.
As you get closer to the speed of light, it
gets harder and harder to make things go faster. To
actually reach the speed of light, it will take an
infinite amount of energy. Here's how doctor Carrol explains it.
(20:17):
All right, so we have this speed limit, and what
happens if you try to go faster than that? You
can try. The thing is that one of the equations
that Einstein bequeathed to us is a formula for how
much energy it takes to go at a certain speed.
We all have heard of equals mc squared. Right, energy
is vast times the speed of light squared. That's a
(20:39):
formula for what the energy is for an object with
zero velocity. If an object is moving with respect to us,
it also has what we call kinetic energy, and its
energy becomes bigger, and so there's a different formula as
that velocity goes closer and closer to the speed of light.
Einstein's formula for the energy says that the energy goes
(21:00):
to infinity. So the problem is that it would require
more than an infinite amount of energy to accelerate yourself
faster than the speed of light. It's one of the
ways of thinking about the fact that really you cannot
go faster than the speed of light. If you tried,
you would get closer and closer from our reference frame,
but you would never make it all the way there
(21:22):
is because you only have a finite amount of energy
to push you. Would you say then that the reason
we have a speed limit is because it takes an
infinite amount of energy to go faster than light. I
would say that fact, but I would say it the
other way around. I would say that it takes in
front of energy because we have a speed limit. It's
the speed limit that is really just fundamental. The existence
of a speed limit is really the defining feature of
(21:44):
space time, according to Einstein. Okay, and this brings us
to the big question, why does the universe seem to
have this speed limit. After all, it seems weird that
we live in a universe where you can't just go
faster and faster, that there is some sort of arbitrary
rule that curbs your speed and gives you strange phenomena
(22:08):
like time flowing differently for different people. I asked doctor
Carol this question. Okay, so now the question is why
do we have this speed limit in the universe. That's
an excellent question, And as we see, it is not
hard to bump up into questions where the answer is
there's no necessary reason why, Like maybe there's no such
(22:32):
thing as why it is a feature of the universe
that there is this speed limit. It doesn't seem necessary
if you think about all the possible laws of physics.
But since relativity came along, we know that the universe
has this feature, that there is a speed limit. But
do we have any kind of physical explanation for why
we have this speed limit? I don't know if that's
(22:53):
even a plausible question. You know, when you say what
is the physical explanation for something? When you really ask
is why is it? That it could not have been different?
Eventually you just say, look, that's the way it is. Okay,
that's more or less where we are with the speed
of light. It's just the way it is. It's annoying
that it's that way, because space opera becomes unrealistic when
(23:16):
it takes hundreds of years travel from one star to
another in your spaceship. But could it have been different?
As far as we know it could have been different.
We're just stuck with what we have. Okay. I get
this question all the time. So I wrote this book
for kids called Oliver's Great Big Universe, which you read,
Thank you for that. And I go around schools talking
about the book. And one of the biggest questions I
(23:37):
always get is what if I go faster than light?
And I always say you can't, you can't go. No, No,
I know, I know you can't. But what if you did?
And I say no, you can't, and they keep going no,
but what if he did? So John, why should I
tell these kids? So we think that there are laws
of physics, right, that it's not anything goes in the universe,
(23:58):
that apples fall down from trees. They don't just fly
up into the air when they fall off the tree. Right,
that the sun rises in the east, not in the West,
and that nothing can travel faster than the speed of light.
So it's not a choice that you have, or it's
not a matter of willpower or technological capability or anything
like that. You know, despite what you learn in Star
(24:19):
Trek and Star Wars, they're changing the laws of physics
because they want to have sort of a Western or
naval battle kind of feeling about what is going on
in outer space. But outer space is actually a very
different thing. So if you wet faster than the speed
of light, that would mean that everything we think we
(24:40):
know about the laws of physics is wrong. So if
someone asks what would happen, I got to say, Man,
I have no idea what would happen? Because you're telling
me I don't know anything other laws of physics, so
I have no way of saying what would happen. It
would go against everything we've observed as humans in the
history of humanity. Okay, what doctor Carl is saying is
(25:03):
that scientists don't actually know why we have the speed
limit in the universe. It's just a feature or a
rule that's built into it. There might be other universes
with a different speed limit, or even a universe out
there without the speed limit, where the speed of light
is infinite. But that's just not the universe we live in,
(25:27):
all right. I said at the beginning, there are three
ways to answer this question of whether we can go
faster than light, and so far we have a yes
if you cheat and slow down light. No, if you
follow the rules that we've observed about the universe. And
now the third way to answer this question is if
we put on our imagination heads and wonder about what
(25:50):
we don't know about the universe. To be a little
bit more fair, you know, physicists love to let their
imaginations run free. So we have imagined particles that could
go faster than the speed of light. We've even given
them a name. They're called tachions, and unlike ordinary particles
that can only go slower than the speed of light,
(26:13):
tachions can only go faster than the speed of light.
They can never slow down. Okay, so tachions are a
totally theoretical idea with no basis in experiments, but they
are technically sort of possible in the laws of physics,
and if they do exist, it would let.
Speaker 2 (26:32):
Us do some pretty amazing things.
Speaker 1 (26:36):
If you could go faster than the speed of light,
you could go to the past just as easily as
to the future, rather than me going fashion the speed
of light. Let's say I had a flashlight that could
shoot out tachions and they could go faster than the
speed of light. Right, I could send a signal to
myself yesterday, because going fashion in the speed of light
is just as bad as going to the past. Uh. Like,
(26:59):
if I shoot it at Alpha Centauri, will get there yesterday,
kind of, it very well could. And then if they
shoot it back, yeah, it'll get back two yesterdays ago.
There you go. So yeah, future me, if you have
a taki in flashlight, please beat me the numbers for
tomorrow's winning lottery ticket. All right, Like I said, tak
ins are purely theoretical, which puts this answer in the
(27:23):
maybe category. Until then, I guess we all have to
be law abiding citizens of the universe and live knowing
there is a speed limit to the universe, which our
expert argues is maybe not such a bad thing. You know,
the speed of light has a bad reputation because it
prevents us from being star wars. Right, it prevents us
(27:45):
from colonizing the galaxy and starting a galactic empire. But
I guess the lesson I would say is it also
brings a measure of control and predictability to the universe
that you wouldn't otherwise have. What it means is that
nothing you can do right here, right now instantaneously affects
things infinitely far away. So it's kind of a better
(28:08):
behaved universe. When there is a speed limit. You can't
instantly reach out and touch someone a gagillion light years away.
It seems less chaotic, I think, is what you're trying
to say, Like, not everyone's feeling everyone at the same time,
exactly right. It's a little more orderly. It takes time
for signals to travel. It calms down the universe to
have this speed limit on there. It's like if everyone
(28:30):
was forced to use mood and the like internet speed,
maybe our lives would be a little bit more orderly
and less chaotic. Yeah, the galaxy is not that big.
You just have to be patient. So maybe it takes
tens of thousands of years to cross it if you
go close to the speed of light, but that's nothing.
The universe is fourteen billion years old, you know, like,
be patient, we'll get there. The speed of light is
(28:52):
a good thing, I see, just adjust the perspective of time.
I'll tell that to the fourth graders that I talked to.
Just be patient. I'm sure that hell far good luck
with that. They'll be very understanding. I'm sure they always are,
all right. I think that answers our question for today.
Can we go faster than light? The answer seems to
be yes, sort of, No, not really maybe if there
(29:17):
are new things about the universe that we haven't discovered. Now,
why do we have the speed limit? We don't actually know.
That's just the way the universe we live in is.
If you live in the universe where this is not true,
send us a note. We'd love to take your experience
for a spin. Thanks for joining us. See you next time.
Speaker 2 (29:38):
You've been listening to science Stuff.
Speaker 1 (29:40):
The production of iHeartRadio written and produced by me or Hitchm,
edited by Rose Seguda, executive producer Jerry Rowland, and audio
engineer and mixer Kasey peckrom And you can follow me
on social media. Just search for PhD Comics and the
name of your favorite platform. Be sure to subscribe to
Sign Stuff on the iHeartRadio app, podcasts, or wherever you
(30:01):
get your podcasts, and please tell your friends we'll be
back next Wednesday with another episode. All right, that was fantastic.
I feel like I'd taken more than of your time
than I promise.
Speaker 3 (30:16):
Well, we've just discussed time is relative. Class, running slowly fine,
We always talked for ten minutes. It's fine.
Speaker 1 (30:22):
Yeah, that's right. When when a professor is putting at
sabbatical speed, time dilates and becomes meaningless
Speaker 3 (30:31):
You are exactly know, exactly that's right,
Hey, welcome to Sign Stuff, a production of iHeartRadio. I'm
hoorhe cham and today we are answering the question can
we go faster than lights? Like the fastest thing in
the universe? And what would happen if you tried to
go faster. We're going to talk to a couple of
experts about this, including a famous theoretical physicist and a
(00:23):
historian of special relativity. So buckle up, put your helmets on,
because we are going to ludicrous speed to answer the
question can we go faster than light?
Speaker 2 (00:44):
Hey?
Speaker 1 (00:44):
Everyone? Okay, So today's question is a pretty deep one.
It goes to the very heart of what the laws
of physics in our universe are, and it has huge
consequences for what it means to have an orderly and
logical existence and reality. It's such a huge question that
(01:04):
it even stumped Einstein. So I knew I needed a
lot of help, which is why I called two very
smart friends of mine. And according to them, there are
three ways to answer this question. The first way is
to think about how fast light can go. Here's my
conversation with my first friend, famous theoretical physicist, New York
(01:27):
Times bestselling author and host of the Mind Skate Podcast,
Professor Sean Carroll. Cool, and I might crack some jokes.
That's the tone of the episode. That's okay, that's allowed.
We're allowed to entertain the podcast. All right, Well, thank
you doctor Carrol for joining us. Thanks very much for
having me. Please tell us who you are and what
(01:48):
you did. My name is Sean Carroll. I'm a physicist
and philosopher at Johns Hopkins University. I think about where
the universe came from, what it's made of, what the
laws of physics are awesome. Have you ever gone faster
than this of light? I never have. I've gone stoller
in the speed of light many many times.
Speaker 2 (02:04):
I guess.
Speaker 1 (02:05):
Just to start us off, what is the speed of light?
It's kind of something you can think about at different
levels of sophistication. To be honest, the speed of light
is the speed that light travels at in empty space.
So one very quick thing to get straight is that
light itself can travel at different speeds. Light travels at
a different speed through air or water than it does
(02:27):
through empty space. So when we're being a little bit
more careful, we say the speed of light in vacuum.
That's the special thing that's what people really care about.
That means that it's a speed of light when there's
nothing in the way, kind of nothing for the light
to bump into. Okay, so the first thing to know
about how fast light moves is that it has a speed.
Speaker 2 (02:49):
It may seem.
Speaker 1 (02:50):
Instantaneous when we turn on a light or a flashlight,
but it does take time for light to go places,
and this is something you can measure. For example, you
can shoot a laser at the moon and then see
how long it takes before it bounces back. If you
did that, you'd see it takes about two point six seconds.
Speaker 2 (03:09):
Or you could send the.
Speaker 1 (03:10):
Radio signal, which is also light, to Mars and you
see it takes about fifteen minutes on average for the
signal to go there and come back. The second thing
to know about how fast light moves is that it
depends on what it's moving through. If it's moving through
nothing or a vacuum, then it moves at its maximum speed,
(03:32):
which is about three hundred thousand kilometers per second. At
that speed, light can go all the way around the
Earth seven times in one second. But if it moves
through air or water or something transparent, then light slows
down and the denser that thing is the slower light
(03:54):
will move. So in a vacuum, light moves at about
three hundred thousand kilometers per second, but through water it
moves two hundred and twenty five thousand kilometers per second.
If it moves through glass, it's two hundred thousand kilometers
per second, And if it's a diamond, light moves at
one hundred and twenty four thousand kilometers per second, or.
Speaker 2 (04:16):
Almost two and a half times slower.
Speaker 1 (04:19):
And that's because even though these things are transparent, the
atoms still interact with the light. So when we think
about light traveling through water or glass or air, we
think it's transparent, it's just going right through, right, But
in fact, the light is being affected by the medium
it's moving in, by the very gentle electromagnetic interactions with
(04:43):
all the atoms that it might bump into. That's why
when you pass light through a prism, it will be
broken up into different wavelengths. Right, it'll become it'll be
the pink Floyd Dark side of the Moon album cover,
and white light becomes all the colors of the rainbow.
It gets psychedelic. You know, I'm not going to say
how you should best experience dark side of the moon.
(05:03):
That up to personal choices. Now, a lot of people
think the reason that light slows down when going through
something like air or water is that it's bumping into
all the atoms that are there. But that's not quite right.
The reason is a little technical, but basically, light is
an electromagnetic wave, and when it goes through water or
(05:26):
a diamond, the wave makes the electrons in the atoms
that are there wiggle, and the wiggling causes a kind
of counter electromagnetic wave that when you add it to
the original light wave, it slows it down. And it's
a good problem set for undergraduates learning this stuff for
the first time, because it's actually quite miraculous. It's the
(05:49):
net effect of all of the atoms has a sort
of simple calculable effect on what the light is doing.
It changes its speed. Okay, what does mean for the
first way of answering the question can we go faster
than light? Is that you can imagine setting up an
unfair race. On one side, you would shoot light through
(06:11):
a long piece or rod of a dense material like
glass or diamond, so that the light would be going
at half or less of its normal speed, and next
to it, you would hop into a rocket with nothing
in front of it but empty space. Now, you still
have to go pretty fast, thousands of times faster than
(06:33):
any human has ever gone before. But technically you could
beat the light going through that rod. If you had
enough fuel, you could get up to a speed that's
faster than that light beam, Which means that technically, one
way to answer the question can we go faster than light?
Is yes, in an unfair race. All right, you're probably thinking, sure, Jorge,
(06:58):
but that's cheating. And I imagine you clicked on this
episode to find out if you could go faster than
light without cheating. That is, can you go faster than
the speed of light in a vacuum? After all, that's
the question that's going to make it practical or not
if he could ever go to distant stars or even
(07:18):
to other galaxies. But here the answer is a lot trickier, because,
as it turns out, the speed of light isn't just
how fast light can go. It sets a universal speed limit.
Here's how Professor Carroll puts it. The slightly more sophisticated
(07:38):
way of saying it is the universe has a speed
limit built into the laws of physics, there is a
fastest speed at which anything can go, and unlike any
other speed, this speed is absolute. It is not relative.
It doesn't matter how you're moving through the universe, you
are going to measure the same speed limit relative to you.
(08:01):
It just so happens that light travels at this speed
when it's moving in vacuum. But other things do as well.
Gravitational waves, for example, also move at the speed of light.
So it's really a feature of space time itself. It's
a crucially important feature. It says that there are things
to our past, there are things to our future, and
there are things sort of sideways to us, things that
(08:24):
are neither in our past and our future, because we
can't get there without going faster than the speed of light,
which is not allowed. All right, when we come back,
we're gonna dig into everything Professor Carroll just said, and
we're gonna go into the mind of Albert Einstein as
he wondered the same question we are asking today, what
(08:44):
would happen if he could catch up to a lightbeam.
Stay with us, we'll be right back.
Speaker 2 (08:59):
We come back.
Speaker 1 (09:01):
Okay, we're answering the question, can we go faster than light.
And as I mentioned, there are three ways to answer
this question. The first is to cheat. You can slow
down light by making it go through something like water
or a diamond, and you could then technically beat it
in a race. The second way to answer this question
(09:22):
is to ask if we can go faster than the
fastest light can go, which is the speed of light
in a vacuum. And here the answer is trickier because
if you ask any physicists on Earth this question, they'll
probably tell you the answer is no. And so for
this segment, I thought i'd take you through the history
of how physicists arrived at this answer, because it definitely
(09:45):
wasn't by trying to go faster than light.
Speaker 3 (09:52):
It sort of came in waves. It wasn't always clear
to people for all time that light even had a
finite speed. Some people wonder if light simply was instant,
as if it traveled at an infinite speed, and a
bunch of people wondered, maybe that's not right.
Speaker 1 (10:06):
That's my other friend, Professor David Kaiser. He's a theoretical
physicist and a historian of science at the Massachusetts Institute
of Technology or MIT. According to doctor Kaiser, it took
a few hundred years since humans started doing serious science
in the fifteen hundreds, with people like Galileo to figure
out that light wasn't instantaneous and then it moved at
(10:29):
a certain speed. And that's at the stage for a
young teenager named Albert Einstein to ask a very simple question.
Speaker 3 (10:39):
That's what sets up a pretty amazing intellectual journey that
leads to people like a young Albert Einstein and many others.
Right around the turn of the twentieth century. In the
early nineteen hundreds, Einstein was wondering, even as a teenager
really before he was doing very formal schooling. In this
he wondered, what would happen if I could catch up
to a light wave? What would I see?
Speaker 1 (11:00):
What teenager Einstein wondered was, well, if light doesn't move
infinitely fast, if it moves at a set speed, what
would happen if you went faster and faster, could you
eventually catch up to a beam of light? And if
you did, what would it look like?
Speaker 3 (11:19):
And so he had these wonderful, very visual, kind of
imaginary thought experiments in his head as a sixteen year old,
and he realized, well, if there's a surfer on an
ocean wave, and they're going at the same speed. If
they look from side to side, at least momentarily, they'll
see this wave kind of frozen, right, because they'll be
moving with the crest of the wave. There's some time
they would see basically a wave frozen in time, not
(11:42):
zipping past them. And Einstein wondered, well, why can't I
do that with a LightWave, or what would happen if
I if I zoomed up and traveled at the same
speed as a light wave, but I see a frozen
wave like that frozen ocean wave as a surfer.
Speaker 1 (11:55):
In other words, Einstein wondered, as a sixteen year old,
if you can catch to light, would you essentially freeze
light or make it look like it's standing still? And
this question bothered Einstein. Okay, so then Einstein had this
idea that he nurtured since he was sixteen. And by
the way, when I was sixteen, I was definitely not
(12:17):
thinking about the nature of the universe. Not too loud,
that's right here we are.
Speaker 3 (12:23):
So then what happened and then he got bothered by
that he was reading for fun as a teenager and
early college did there's still pretty new theory worked out
by people like James Clark, Maxwell and Hendrick Herritt's and others.
That was just scoring like win after win in describing
all these other things about light. It was like better
than anyone even imagined. It would match all these experiments
(12:45):
to high accuracy. It was a theory not only of
optics but also electric field magnetic fields. But in none
of those solutions was there a sense that you could
slow light down to zero. Wow, there was no mathematical
solution of these otherwise beautiful and very powerful equations that
showed a LightWave frozen in space.
Speaker 1 (13:04):
So for young Einstein, this idea of going faster than
light and catching up to a lightbeam and seeing it
frozen wasn't allowed according to the other theories of the day.
It seemed to be true according to lots of experiments,
and so he came up with a pretty radical solution.
Speaker 3 (13:24):
And so Einstein got bugged by that, and he wouldn't
let it go. Whereas other teenagers might think about other things.
For one, he got there and he realized one way
to remove this uncomfortable paradox would be to say, what
if the speed of light always has the same value,
even if I'm moving fast. What if every person who's
even in motion would measure the speed of light to
(13:46):
have that same speed of speed of light in vacuum.
If that's the case, that if he was careful to
say if whoa, then no one could ever catch up
with a light wave.
Speaker 1 (13:53):
That's a huge leap. I mean it almost sounds like
a crazy idea.
Speaker 3 (13:58):
It is not what most people were I think, that's
for sure.
Speaker 1 (14:02):
Okay, here's Einstein's idea. What if you did try to
catch up to a lightbeam, you might start going faster
and faster behind it, trying to catch up to it,
and you would expect it to get closer to you.
But what if instead it never does. What if the
light beam always moves away from you at the speed
of light, no matter how fast you go.
Speaker 2 (14:24):
And this is very strange.
Speaker 1 (14:25):
And different than what we experience in our everyday life.
In our daily lives, we're used to speeds adding up.
For example, if you're driving down the highway at sixty
miles per hour and your kid in the back seat
throws a toy at you at ten miles per hour,
then to someone outside the car, they would see that
toy going at seventy or sixty plus ten miles per hour.
(14:49):
But Einstein said, what if that doesn't happen for light?
For example, what if you're going sixty miles an hour
in your car and you turn on the headlights. Expect
the light from your headlights to go the speed of
light plus to sixty miles an hour that the car
is moving. But Einstein said, nope, that light still only
(15:10):
goes at the speed of light. You and the car
would see the light go at the speed of light,
and someone standing on the highway would also see it
going at the speed of light. And this weird but
simple idea that light always seems to go at the
speed of light no matter who looks at it basically
revolutionized our understanding.
Speaker 2 (15:30):
Of the universe.
Speaker 1 (15:32):
Although it wasn't immediately a big hit, the.
Speaker 3 (15:37):
First reaction Einstein's work was no reaction at all. He
was out of the way. He wasn't in an academic position.
He was, you know, frankly, kind of a nobody put
it this way. He didn't even get a promotion at
the patent office.
Speaker 1 (15:51):
But eventually other people did notice Einstein's work.
Speaker 3 (15:56):
Nonetheless, more time goes by, he's able to get a
fewer people to read the work and take it seriously,
and a few of them actually were beginning to do
things like write their own review articles and pay more attention.
So over a period that stretches nearly to a decade,
more and more of the professional scientists begin to pay attention.
Speaker 1 (16:14):
Okay, you might be wondering, what does this have to
do with going faster than lights? Well, it turns out
that Einstein's sort of simple idea has some really weird
consequences for how the universe works. You might have heard
that time slows down when you're going really really fast.
That comes from Einstein's idea. Or that things look shorter
(16:35):
when they're going super fast. That's also from Einstein's crazy idea.
Speaker 3 (16:41):
And he's very clear, I'm going to assume this right
and then say what will follow from that? And even
more stream sounding things followed from that, even in that
first nineteen to oh five paper. So from this postulate
he deduces other consequences. If you're zipping by on the
train at some high speed, you and I will disagree
on whether two events happened at the same time. This
becomes known as the relativity of simultaneity. Another thing he
(17:03):
begins to realize is that we're going to disagree on
the measurements of lengths if we're moving at a relative speed.
So it's called length contraction. And the third one is
time dilation. So if we have identical clocks that we're holding,
but you're racing past me at some outrageously high speed,
I will watch your clock tick more slowly.
Speaker 1 (17:21):
The last main consequence of Einstein's idea is that nothing
can ever go faster than the speed of light. How
do we go from the speed of light, whether it's
the same in all directions, to concluding that it's a
speed limit for everything.
Speaker 3 (17:38):
Partly because from Einstein's own equations, as you try to
get closer and closer to the speed of light, the
difference in measured clock rate, all these mismatches in lengths
of meter six times on clocks, even also things like
the masses, the way we measure mass, These things all
rise and in fact become infinite. And that's like saying
it would take basically an infinite amount of energy to
(17:59):
go as fast as that light beam does, and I can't.
I have no source of infinite energy.
Speaker 1 (18:05):
Yeah, infinity can be a bummer, all right. When we
come back. We're gonna talk more about this explanation, whether
we've ever actually tried to go faster than the speed
of light, and then I'm gonna ask our experts what
they think would happen if we ever did break this
speed limit. So stay with us.
Speaker 2 (18:25):
You're listening to sign stuff.
Speaker 1 (18:35):
And we're back.
Speaker 2 (18:37):
Okay.
Speaker 1 (18:37):
We're asking the question can we go faster than light?
And so far we have two different answers. The first
is yes, you can go faster than light if you
slow down light by making it go through water or diamond.
The second answer seems to be no, if you mean
whether we can go faster than light in a vacum.
(19:00):
As we learn, it seems that the universe has a
speed limit, and the speed limit says nothing can ever
go faster than the speed of light in a vacuum. Now,
I know what you're thinking right now, which is have
we actually tried to go faster than the speed of light?
And the answer is yes. Since about the nineteen thirties,
(19:21):
scientists have been building particle accelerators. These are huge machines
that can take small particles like protons or electrons and
speed them up to go faster and faster. These accelerators
have been getting bigger and more.
Speaker 2 (19:35):
Powerful over the years.
Speaker 1 (19:36):
They've been pushing the limits of how fast we can
make something go. The largest accelerator right now, the Large
Hadron Collider in Geneva, Switzerland, is a seventeen mile long
circular tunnel that can speed protons up to ninety nine
point nine nine one percent of the speed of light.
(19:57):
What happens if you try to go faster, uh, not much.
As you get closer to the speed of light, it
gets harder and harder to make things go faster. To
actually reach the speed of light, it will take an
infinite amount of energy. Here's how doctor Carrol explains it.
(20:17):
All right, so we have this speed limit, and what
happens if you try to go faster than that? You
can try. The thing is that one of the equations
that Einstein bequeathed to us is a formula for how
much energy it takes to go at a certain speed.
We all have heard of equals mc squared. Right, energy
is vast times the speed of light squared. That's a
(20:39):
formula for what the energy is for an object with
zero velocity. If an object is moving with respect to us,
it also has what we call kinetic energy, and its
energy becomes bigger, and so there's a different formula as
that velocity goes closer and closer to the speed of light.
Einstein's formula for the energy says that the energy goes
(21:00):
to infinity. So the problem is that it would require
more than an infinite amount of energy to accelerate yourself
faster than the speed of light. It's one of the
ways of thinking about the fact that really you cannot
go faster than the speed of light. If you tried,
you would get closer and closer from our reference frame,
but you would never make it all the way there
(21:22):
is because you only have a finite amount of energy
to push you. Would you say then that the reason
we have a speed limit is because it takes an
infinite amount of energy to go faster than light. I
would say that fact, but I would say it the
other way around. I would say that it takes in
front of energy because we have a speed limit. It's
the speed limit that is really just fundamental. The existence
of a speed limit is really the defining feature of
(21:44):
space time, according to Einstein. Okay, and this brings us
to the big question, why does the universe seem to
have this speed limit. After all, it seems weird that
we live in a universe where you can't just go
faster and faster, that there is some sort of arbitrary
rule that curbs your speed and gives you strange phenomena
(22:08):
like time flowing differently for different people. I asked doctor
Carol this question. Okay, so now the question is why
do we have this speed limit in the universe. That's
an excellent question, And as we see, it is not
hard to bump up into questions where the answer is
there's no necessary reason why, Like maybe there's no such
(22:32):
thing as why it is a feature of the universe
that there is this speed limit. It doesn't seem necessary
if you think about all the possible laws of physics.
But since relativity came along, we know that the universe
has this feature, that there is a speed limit. But
do we have any kind of physical explanation for why
we have this speed limit? I don't know if that's
(22:53):
even a plausible question. You know, when you say what
is the physical explanation for something? When you really ask
is why is it? That it could not have been different?
Eventually you just say, look, that's the way it is. Okay,
that's more or less where we are with the speed
of light. It's just the way it is. It's annoying
that it's that way, because space opera becomes unrealistic when
(23:16):
it takes hundreds of years travel from one star to
another in your spaceship. But could it have been different?
As far as we know it could have been different.
We're just stuck with what we have. Okay. I get
this question all the time. So I wrote this book
for kids called Oliver's Great Big Universe, which you read,
Thank you for that. And I go around schools talking
about the book. And one of the biggest questions I
(23:37):
always get is what if I go faster than light?
And I always say you can't, you can't go. No, No,
I know, I know you can't. But what if you did?
And I say no, you can't, and they keep going no,
but what if he did? So John, why should I
tell these kids? So we think that there are laws
of physics, right, that it's not anything goes in the universe,
(23:58):
that apples fall down from trees. They don't just fly
up into the air when they fall off the tree. Right,
that the sun rises in the east, not in the West,
and that nothing can travel faster than the speed of light.
So it's not a choice that you have, or it's
not a matter of willpower or technological capability or anything
like that. You know, despite what you learn in Star
(24:19):
Trek and Star Wars, they're changing the laws of physics
because they want to have sort of a Western or
naval battle kind of feeling about what is going on
in outer space. But outer space is actually a very
different thing. So if you wet faster than the speed
of light, that would mean that everything we think we
(24:40):
know about the laws of physics is wrong. So if
someone asks what would happen, I got to say, Man,
I have no idea what would happen? Because you're telling
me I don't know anything other laws of physics, so
I have no way of saying what would happen. It
would go against everything we've observed as humans in the
history of humanity. Okay, what doctor Carl is saying is
(25:03):
that scientists don't actually know why we have the speed
limit in the universe. It's just a feature or a
rule that's built into it. There might be other universes
with a different speed limit, or even a universe out
there without the speed limit, where the speed of light
is infinite. But that's just not the universe we live in,
(25:27):
all right. I said at the beginning, there are three
ways to answer this question of whether we can go
faster than light, and so far we have a yes
if you cheat and slow down light. No, if you
follow the rules that we've observed about the universe. And
now the third way to answer this question is if
we put on our imagination heads and wonder about what
(25:50):
we don't know about the universe. To be a little
bit more fair, you know, physicists love to let their
imaginations run free. So we have imagined particles that could
go faster than the speed of light. We've even given
them a name. They're called tachions, and unlike ordinary particles
that can only go slower than the speed of light,
(26:13):
tachions can only go faster than the speed of light.
They can never slow down. Okay, so tachions are a
totally theoretical idea with no basis in experiments, but they
are technically sort of possible in the laws of physics,
and if they do exist, it would let.
Speaker 2 (26:32):
Us do some pretty amazing things.
Speaker 1 (26:36):
If you could go faster than the speed of light,
you could go to the past just as easily as
to the future, rather than me going fashion the speed
of light. Let's say I had a flashlight that could
shoot out tachions and they could go faster than the
speed of light. Right, I could send a signal to
myself yesterday, because going fashion in the speed of light
is just as bad as going to the past. Uh. Like,
(26:59):
if I shoot it at Alpha Centauri, will get there yesterday,
kind of, it very well could. And then if they
shoot it back, yeah, it'll get back two yesterdays ago.
There you go. So yeah, future me, if you have
a taki in flashlight, please beat me the numbers for
tomorrow's winning lottery ticket. All right, Like I said, tak
ins are purely theoretical, which puts this answer in the
(27:23):
maybe category. Until then, I guess we all have to
be law abiding citizens of the universe and live knowing
there is a speed limit to the universe, which our
expert argues is maybe not such a bad thing. You know,
the speed of light has a bad reputation because it
prevents us from being star wars. Right, it prevents us
(27:45):
from colonizing the galaxy and starting a galactic empire. But
I guess the lesson I would say is it also
brings a measure of control and predictability to the universe
that you wouldn't otherwise have. What it means is that
nothing you can do right here, right now instantaneously affects
things infinitely far away. So it's kind of a better
(28:08):
behaved universe. When there is a speed limit. You can't
instantly reach out and touch someone a gagillion light years away.
It seems less chaotic, I think, is what you're trying
to say, Like, not everyone's feeling everyone at the same time,
exactly right. It's a little more orderly. It takes time
for signals to travel. It calms down the universe to
have this speed limit on there. It's like if everyone
(28:30):
was forced to use mood and the like internet speed,
maybe our lives would be a little bit more orderly
and less chaotic. Yeah, the galaxy is not that big.
You just have to be patient. So maybe it takes
tens of thousands of years to cross it if you
go close to the speed of light, but that's nothing.
The universe is fourteen billion years old, you know, like,
be patient, we'll get there. The speed of light is
(28:52):
a good thing, I see, just adjust the perspective of time.
I'll tell that to the fourth graders that I talked to.
Just be patient. I'm sure that hell far good luck
with that. They'll be very understanding. I'm sure they always are,
all right. I think that answers our question for today.
Can we go faster than light? The answer seems to
be yes, sort of, No, not really maybe if there
(29:17):
are new things about the universe that we haven't discovered. Now,
why do we have the speed limit? We don't actually know.
That's just the way the universe we live in is.
If you live in the universe where this is not true,
send us a note. We'd love to take your experience
for a spin. Thanks for joining us. See you next time.
Speaker 2 (29:38):
You've been listening to science Stuff.
Speaker 1 (29:40):
The production of iHeartRadio written and produced by me or Hitchm,
edited by Rose Seguda, executive producer Jerry Rowland, and audio
engineer and mixer Kasey peckrom And you can follow me
on social media. Just search for PhD Comics and the
name of your favorite platform. Be sure to subscribe to
Sign Stuff on the iHeartRadio app, podcasts, or wherever you
(30:01):
get your podcasts, and please tell your friends we'll be
back next Wednesday with another episode. All right, that was fantastic.
I feel like I'd taken more than of your time
than I promise.
Speaker 3 (30:16):
Well, we've just discussed time is relative. Class, running slowly fine,
We always talked for ten minutes. It's fine.
Speaker 1 (30:22):
Yeah, that's right. When when a professor is putting at
sabbatical speed, time dilates and becomes meaningless
Speaker 3 (30:31):
You are exactly know, exactly that's right,
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