Do gravitational waves leave a mark on the Universe?

Daniel and Jorge talk about whether gravitational waves cause permanent distortions of space.

See omnystudio.com/listener for privacy information.

Episode Transcript

Speaker 1 (00:08):
Hey, Jorgey, where were you when you heard about the
first gravitational wave discovery? I don't know. Probably home in
my pajamas. That's like ninety nine percent of my life.
Are you telling me you don't remember? How could you
forget such a pivotal moment in science history? Well? I
remember pivotal moments in history, just not it's a physical history.

(00:32):
Was this a very significant moment for you? Oh? Of
course it was huge. People have been looking for gravitational
ways for decades and I was personally one of the skeptics,
so it was mind blowing when they actually saw one.
Because you've proven wrong is at the historical event here.
It certainly left an imprint on me, like a big
footstep in your brain. Hi am or Handway, cartoonists and

(01:10):
the creator of PhD comics. Hi. I'm Daniel. I'm a
particle physicist and a professor at UC Irvine, and I
love to be wrong about physics discoveries, but only physics discoveries.
What does your spouse think about that? I don't have
the expertise to be intelligently wrong about anything else. I'm
just wrong about other stuff. I see. There's a difference
between being intelligently wrong and regular wrong. But that doesn't

(01:33):
seem right when it comes to big ideas in physics.
I sometimes disagree with the mainstream and think, oh, that
will never happen, or that's not going to work, or
the universe can't be that way I saw. I'm a
bit more skeptical sometimes than others. But then when the
universe comes through and delivers an incredible result or discovery,
I'm happy that it did. We can sound like physics
is just a bunch of people throwing out a random

(01:54):
ideas and arguing about it until the universe reveals its Well,
you know that's not the complete process, but generating random
ideas is part of the process. The next step is
sort of a filter like does that idea make any sense?
And can it describe our universe at all? But there
is definitely a step where you're just like brainstorming random craziness.
Maybe this, maybe that, Maybe it's all just kittens all

(02:17):
the way down. Sounds like the cuts me out, but
welcome for our podcast, Daniel and Jorge Explain the Universe,
a production of iHeartRadio in which we do our best
not to be wrong about what we do and do
not understand about the nature of the universe. One thing
we're not skeptical about is humanity's ability to understand the
nature of the universe, to cast our brains out into

(02:38):
the cosmos, to wiggle it along with gravitational waves, to
send it flying into the hearts of neutron stars and
deep down into the frothing craziness of quantum mechanics. We
will keep pushing and pushing until we understand the entire
universe and explain most of it to you. Yeah, although, Daniel,
if you like being wrong so much, why don't you
do it more often? Oh, you'd be surprised. It's not

(02:58):
a rare event, almost a hobby. But it is an
amazing and wonderful universe. It's so big and so incredibly
vast that it kind of makes you wonder what out
there can have an impact on it. If anything, we
are definitely still understanding all the ways the universe works,
the way energy slashes through it, the way particles fly
through space, what space even is if you have to

(03:21):
even have space in our universe. There are so many
very basic questions that we don't have answers to about
the very nature of the cosmos. In which we reside
and what that means for human existence, what it means
to be alive in this crazy cosmos, and you know
how we should spend our lives. And so we keep
digging into the very firmament of the universe we find
ourselves in to try to understand its basic nature. What

(03:44):
is this place we find ourselves in. That's right, because
there are a lot of questions we can ask about
the universe, and the universe is always happy to provide
its interesting answers. The universe is strange and weird and
sometimes very unexpected, none more so than some of the
amazing discoveries we've made in the last few decades. Yeah,
and we can be surprised by the universe sort of

(04:04):
in two different ways. One way is to come up
with a new idea for how the universe works, a
theory of description of the mathematics underlying the universe, and
then go out and search for those things. For example,
Einstein's general relativity predicted black holes before we even saw them.
It suggested these should be a real thing. They should
be out there in the universe. They should be happening.

(04:25):
You should be able to find them, and people search
for them for decades and then finally eventually actually found them,
despite a large fraction in the community being skeptical that
they existed at all. So those folks were surprised because
what was predicted was actually out there. Well, we found something,
But I thought that you told me last time that
we're not one hundred percent sure they're black holes. Oh,

(04:46):
you're absolutely right. We're not one hundred percent sure or
basically anything we know about the universe, but specifically black holes.
We've never been close enough to one and never seen
the heart of one to be sure that there's really
a general relativistic singularity something. They're very compact and very massive,
very much distorting space and time. We don't know if

(05:06):
it's actually a black hole or a dark star, or
a fuzzball or something else quantum mechanical. That's like you're saying,
the only thing we can be sure about is that
we're not sure. Science is a process we hope gradually
bending towards the truth. As we refine our mental model
for what we think is happening out there in the universe,
we can come up with clever ways to test it,

(05:28):
and sometimes the universe says, yep, you are totally right.
Good job, and sometimes the universe says something else completely
different is going on. Yeah, and speaking of bending the
universe and our minds, another interesting discovery that was made
in the last few years that literally has made ripples
across the science landscape and the universe itself are gravitational waves.

(05:50):
The ripples and the fabric of space itself that are
made by super heavy objects moving really really fast. That's right,
And actually gravitational waves are made by everybody. Anything with
mass accelerating is making a gravitational wave. Hold up your
hand right now and wiggle it back and forth and boom,
you just created a gravitational wave which is rippling through
the universe. More like a wave wave. Let maybe when

(06:13):
you do it with your arm or what do you
call it, like a small wave in a small pond,
A ripple, A ripple. Yeah, I'm not judging anybody's waves
by their size. You know, size doesn't matter when it
comes to gravitational waves. But the point is that everything
out there that has mass and the accelerates changes the
shape of space and ripples that information their existence their

(06:34):
gravity throughout the universe in a way very similar to
how an electron creates an electric field, and if you
wiggle it, it creates a wiggling electric field, which is
basically a photon sending its message out there through space.
So it's amazing that we understand gravity well enough to
talk about how it wiggles and how it ripples, and

(06:55):
how space can bend and flex and do all sorts
of crazy things. Yeah, because if as we've talked about before,
gravity is not like a force something that attracts you
things with an actual kind of bending of space. That's
how physicists see it. And so when you have things
moving through space, they cause ripples in this bending, sort
of like moving through molasses or water right exactly. Everything

(07:17):
that has mass has a gravitational field, and then if
you move that mass, the gravitational field moves, but it
doesn't move instantaneously. So the gravitational field that's like one
light second away from you doesn't change immediately if you
wiggle your mass, but it does change very very close
to the mass, and then that change ripples out at
the speed of light. So as you move a mass

(07:39):
back and forth, that affects the gravitational field, and the
information about you having moved it propagates through the gravitational
field at the speed of light. That's basically what a
gravitational wave is. It's just the information about gravity being
updated because something has been accelerated. Yeah, I feel like
it's almost like if you had special gravitational glasses or

(08:01):
something that lets you see gravitation waste, you would see
them rippling all around you. Right, it'd be almost like
a super noisy environment that you're in with gravitational waves
being generated by everything and bouncing and well I don't
know if they bounce, but being rippled out in all
directions by everything. Yeah, that's a really insightful comment. Because
we don't have gravitational glasses, Like, we can't see directly

(08:23):
the curvature of space. That's what's really happening. When you
create a gravitational field, Really you're bending space around an
object to change the path of things. But because we
can't see that, you can't look at a chunk of
space and say how bent it is. All we can
see is the effect of it on stuff. That's why
originally we thought gravity was a force, because it looks

(08:43):
like there's a force because we can't see the curvature
directly that's causing it. So really gravity is what we
call an apparent force. It's like a force you have
to add to our description in order to account for
the motion that we see, because we didn't understand that
it was just due to the curvature of space, because
we can't see that directly. As you say, we don't
have gravitational glasses. Some of them are really really big

(09:05):
enough to register in detectors, and some of them very
very minute. Yeah, well we don't have gravitational glasses, but
we do have gravitational ears or gravitational microphones. In recent years,
we've been able to set up incredibly large and accurate
experiments that can sense these gravitational waves coming to us
from space and the rest of the universe. And it

(09:26):
was a huge discovery. It really did rock the world
of physics to accomplish this. You know, Einstein predicted gravitational
waves would exist, but he also predicted that there would
be undetectable, that the effect would be too small for
humanity to ever notice. So he was right that they exist,
but he was wrong that we couldn't discover them. It
was an incredible technological feat really just like amazing engineering

(09:51):
of an experiment to build something sensitive enough to detect
this very very slight effect of gravitational waves. Yeah, I'm
always impressed by engine years, by anything they do, anything
that works at least. Yeah, So they've detected gravitational waves
and we're learning more about them. But how much do
we know about gravitational waves? Are they everything we think

(10:11):
they are or are they maybe weirder than we think
they are? So today on the podcast, we'll be asking
the question do gravitational waves last forever? I love the
gravitational waves as mind bending in brain rippling as they
are continue to provide questions and ideas and new mysteries

(10:34):
for us to explore. Yeah, they bend spaced and time
and our minds at our brains. Well, this is an
interesting question at Daniel. Do gravitational waves kind of last
forever or at least believe a lasting imprint on the
universe forever forever? It's kind of a big word or
a long word, what's the right gadgetor it sort of
stretches your mind to imagine something lasting forever. But here,

(10:57):
because we've described gravitational waves as sort of ripple through space,
it's like an update of the gravitational field as it
propagates through the universe. You imagine that when the thing
that makes the gravitational wave stops wiggling, that the gravitational
waves stop also, that they sort of pass through you
and then on to the rest of the universe without
leaving any sort of permanent effect. Yeah, sort of like

(11:19):
if you're out in the ocean bobbing on the waves,
and the waves it's going to go through you and
they keep going on after you. Right. The question is
does the same happen to gravitational waves or do they
leave some sort of lasting mark on spacetime as they
propagate through like footprints in the sand. Well, ocean waves
usually leave a bit of a seasickness in me that

(11:41):
last a good bit of time, and then you deposit
something over the edge of the boat that leaves the
mark in the ocean. Yes, it's all the cycle of
life or the cycle of the universe, the beautiful circle
of the universe. I think that's part of the water cycle. Right. Well,
this is an interesting question. Do gravitation ways leave a
lasting imprint on the universe? Do they last forever or

(12:03):
do they fade away at some point? And so, as usual,
we were wondering how many people out there had thought
about this question, had thought about gravitational waves and what
they do. So thanks very much to everybody who volunteers
for this segment of the podcast, and you out there
who have never volunteered, who have never written in, who
haven't heard your own voice on the podcast. We want
to hear from you right to us. Two questions at

(12:25):
Daniel and Jorgey dot com. It's easy and fun. Think
about it for a second. Do you think gravitational waves
leave a lasting imprint on the universe. Here's what people
had to say. My guess is yes, but I don't
have a clue how this imprint would look like. It's
an interesting question. Well, if the analogy would regular wave holds,

(12:47):
you know, you would expect that will never truly disappear.
They will just get smaller and small smaller. The question
is is there some sort of pixelation or discrete level
where they basically disappear or not? So I don't know.
I'd be curious to learn that. Well, the electromagnetic force,

(13:08):
I believe, leaves an imprint on the universe in the
form of the cosmic microwave background radiation. So if electromagnetism
can do that. I don't see any reason that different
fundamental force like gravity couldn't do the same thing in
this case, leaving an impression on the universe with gravitational waves.
So yeah, I definitely think it's possible. I would imagine
possibly indirectly, just with how gravity influences mass in the universe.

(13:32):
But I'm unsure. All Right, lots of interesting ideas here,
going all the way way back to the cosmic microwave
background radiation. Yeah, lots of really great references here also,
like the discussion of maybe a minimum level of gravitational waves,
like getting into quantum gravity. Very cool stuff. Yeah, so
let's break it down, Daniel, Well, we already talked a
lot a lot about what is a gravitation a wave?

(13:54):
What else can we say about what a gravitation wave is?
I think it's worth exploring how you actually see a
gravity wave. You know, how you like build a detector
that can measure a ripple in space and in time
because it's a little bit subtle. You know, what we
see when a gravitational wave passes is our lengths distorted.
If you have, for example, two big long rulers and

(14:15):
they're perpendicular to each other. As a gravitational wave passes,
you'll see one of those rulers get shorter and then longer,
and the other one then get shorter and then longer.
So you see this sort of like oscillation in the
lengths of those arms of your l as it passes through.
But there's a ringle there because you can only detect
it if you build your arms the right way. Yeah,

(14:36):
it's sort of like a distortion of space that passes
through you, sort of like in the movies when they
try to depict like sound waves or like energy waves.
Do you see sort of a ripple in the image.
That's kind of what's happening. Right, It's like space itself
kind of short and singment tracks in one direction. And
the sort of mental steps to get there. Remember, are
that we have some object out there, like a really
massive black hole that's accelerating near another black hole. That's

(15:00):
why it's generating the gravitational waves. And we said the
gravitational waves are basically an update in the gravitational field
or the gravitational force, right, because as the object that's
generating the gravity is moving, its gravitational field is also moving.
But remember also that we think about gravity, not as
a force or having a field to it, but as
just bending of space. And so now instead of changing

(15:22):
the gravitational field, you're changing the curvature of space itself. Right,
That's why we call it a ripple in the fabric
of space time, because it's curvature in space that actually
is what causes gravity. But how do you actually measure that? Right?
How do you measure changes in distance of the universe itself? Well,
if you just build like a really long stick and

(15:43):
you hold it out, you won't measure any gravitational waves
because that stick is held together by atoms, which prefer
various bond lengths, and so as the gravitational wave passes through,
they'll resist a change in its length. It's like sort
of too strong. Instead, the way to detect gravitational waves
is to use something like beams of light. Instead of
having like a long physical ruler, just have like two

(16:04):
mirrors at the ends and bounced light back and forth.
And by measuring how long it takes light to go
back and forth, then you can measure how far apart
those mirrors are. So gravitational way that's propagating through space.
We'll sort of bring those two mirrors closer together and
then further apart, and that's what you're actually measuring. Yeah,
it's like having a ruler a mine out of empty space, basically, right.

(16:26):
Instead of having a ruler that's a solid object, you
just look at space and how long it takes a
laser to go through a space and then back again, right, exactly.
It's the same reason that we can't measure the expansion
of space or notice the expansion of space very well locally. Right,
people talk about how space is expanding, Why am I
not expanding? Why is the Solar System not expanding? That's

(16:47):
because there are forces in play to hold you together.
The atoms in your body hold you together even though
space is expanding out from under you, and gravity holds
the Solar System together even though space is trying to
expand it out. And so in the same way, to
measure the ripples in space, you need something which isn't
being held at a fixed distance. So you need these

(17:07):
things just sort of separated at a certain known distance
and then bounce light back and forth and see if
the travel time of light changes. There's one more sort
of experimental wrinkle there, which is that the changes are
so small it's very difficult to measure, so they have
to actually send light beams both directions come back and
then measure the difference in those travel times by using
interference of those photons. So it's like really virtuoso experimental technique.

(17:32):
I remember visiting these labs in the late nineties when
I was thinking about going account tech for grad school,
for example, and thinking they're never going to get this
to work. Oh my god, this is impossible. But I
was very glad when ten years later I was proven wrong. Yeah,
they're amazing experiments. And so let's get into some questions
I have about this, like, for example, why doesn't light
also get stretched out over this space? And also doesn't

(17:54):
the Earth itself count as a fixed ruler? So let's
get the into this and also whether these gravitational waves
have a lasting impact on the universe. First, let's take
a quick break, right, we're making waves with gravitational waves.

(18:20):
Are there weirder than they we think they are? And
do they leave a lasting imprint on the universe? Do
they last forever? So we've talked a little bit about
what gravitational waves are and a little bit about how
we measure them. The super tricky thing because you want
to measure a house space itself, it's expanding, but here
on Earth things are kind of held together by other forces.
Like you said, you can't really measure the bending of

(18:41):
space with a fixed ruler, but it doesn't the Earth
also count as a fixed ruler. Like if I have
a mirror here on a mirror over there, aren't they
held together by the floor. Yeah, if you attach your
mirrors to the Earth, then it's basically just like building
a really long ruler. It'd be very difficult to measure
gravitational waves. So when they build their experiment lego, they're
trying to isolate those mirrors from anything around them. So

(19:03):
if the Earth moves or wiggles or shakes or anything happens,
it doesn't affect the location of the mirrors. But this
is one limitation of our experiments and why people are
hoping to build a new version that's actually out in
space that's free from all these effects of Earth's gravity
and Earth's bonds and all that kind of stuff. Yeah,
I was gonna say, like, isn't the perfect gratational detector

(19:27):
or something where it's floating in space, right, so one
end is not connected to the other end, Yes, exactly,
And they have a plan for that project, which they
hope is going to launch in about twenty thirty seven.
Three spacecraft out in space, millions of kilometers away from
each other, shooting lasers at each other to measure their
relative distances. It sounds like science fiction, but maybe one

(19:49):
day it will be true. Yeah, I mean, what could
go wrong? Spaceships and lasers. I mean that's the dream,
isn't it, fully operational spaceships exactly? That I need a
fully operational size experiment. Make sure the lasers are green.
I was just going to say the lasers would be
pretty faint so they wouldn't be damaging, But you know,
in order to see them millions of kilometers away, they're

(20:11):
actually going to have to be pretty bright. So I
hope nobody's eyeballs and gets in the path of those lasers. Yeah,
what could go wrong? Well, speaking of lasers, another question
I had was, when you're trying to measure this bending
of space using light, doesn't light also get bent by
the gravitational wave or the bending of space? Doesn't light
sort of in a way sort of speed up or
slow down? Yeah, super fascinating question and very confusing and

(20:35):
something that I struggle to get my mind around sometimes.
Because light is definitely affected by the curvature of space, right,
we know that light gets bent by masses. As it
goes by a huge blob of dark matter, for example,
it can get lens and that is how we see
the curvature space in the change of the direction of light.
But remember that nearby light always travels at the speed
of light, and that sort of defines what distance is. Actually.

(20:58):
Our definition of the meter now is like how far
light has traveled in a certain time slice. Because light
doesn't change its speed as space gets curved. It only
just changes its direction. It doesn't. Gravity also is kind
of affect time as well, right, Like if you're near
a black hole, time slows down, and you'd actually kind
of see light slowing down. You're absolutely right, there is

(21:18):
a gravitational time dilation effect. If you're in an area
with strong curvature, then clocks will run slower. So for example,
you're far away from a black hole and you're looking
at somebody who's holding a clock and they're near a
black hole, their time will slow down. Right. And also
if you are watching photons near a black hole, you

(21:39):
can see them going on all sorts of crazy speeds
because there's an ambiguity in how to even define the
velocity of things that are far away from you and
through some sort of space curvature. We talked about this
a few times on the podcast, that general relativity doesn't
even really allow you to define velocities of things that
are either very far away if the universe is expanding,
or through curved space, because there's sort of multiple ways

(22:02):
to bring that thing's velocity vector over to you to
measure its velocity. But here we're talking about like very
small deviations in space and very local deviations in space,
and so from our point of view, we don't expect
it to affect the speed of photons. Well, I'm sure
it all works out in the math, but I think
the point is that when you're using lasers to measure
these distances, as a gravitational wave is passing by, the

(22:26):
speed of light sort of remains the same, so you
can sort of measure the length of space change exactly.
And that's really what we're talking about here, is relative distances.
You know, when we talk about spatial curvature, we don't
mean curvature with respect to some external ruler, like your
mental picture is probably that misleading bowling ball in a

(22:46):
rubber sheet analogy where a mass is bending space into
some other dimension. Right where we have a two dimensional
universe and the mass is bending it in some third dimension.
Our bending of space that we talk about is intrinsic.
It's just a changing of relative distances between things. So
with space is curved between two points, that just means

(23:07):
that those two points are now closer together. And that's
exactly what we're measuring by sending a beam of light
through and saying, oh, how long does it take light
to get through or measuring that relative distance. Well, it's
super tricky, but humans and namely engineers have done it.
They've measured gravitational ways that come from these crazy events
out there in the universe, like two black holes circling

(23:28):
each other in a death spiral, and so you can
capture the moments right before these things are spinning super
fast and crashing into each other. And so that's the
kind of event in the universe that makes big gravitational ways,
maybe the biggest events that we know about. But then
the question is, like what happens to gravitational ways? Do
they keep rippling out there forever, or do they get

(23:48):
absorbed by things as they move through the universe? Right,
And your naive picture is probably thinking about like an
antenna broadcasting a radio signal just sort of propagates out
through space. Asking a radio signal that's sort of shortened time,
like a shout, like a hello, you know in your Hello,
broadcasts out through space and it passes through space and
then it fades, and once it's gone, you can't really

(24:10):
detect that it was there anymore. That's sort of probably
your mental image for a gravitational wave. It creates a
ripple in space as the black holes in spiral and
then collide, and then that ripple passes through the universe.
Maybe it's detected by clever humans and aliens on its path,
and then it just passes by them and leave space unchanged.
But I guess I have a question though. Don't do

(24:31):
gravitational waves they hit something, they just passed through, they
never get absorbed or even as I asked earlier, bounce
Now they absolutely do get absorbed, and they can do
complicated things like reflect and bounce. I think we talked
about that once when we were talking about gravitational waves
passing around black holes, they can get lensed, for example,
by black holes, and so there's all sorts of interesting

(24:54):
wave effects. But as you say, they can also get absorbed.
Like what happens when the gravitational wave wave passes through
the Earth is that it's squeezing the Earth a little bit,
and then it's squeezing at the other direction, and that
does take some energy, and so it's depositing a little
bit of energy, is actually heating up the Earth a
little bit. It's like a little tidal squeeze. So that

(25:15):
helps the gravitational wave fade. Right. First of all, it's
fading because distances are increasing. It's spreading out through a
larger and larger distance, and so it goes like one
over distance squared. But also when it passes through matter,
it does deposit a little bit of its energy into
that matter. Yeah, and the universe is full of matter, right,
I mean not a huge amount, but like in the viewer,

(25:36):
to emit a gravitation wave in the middle of the
Milky wave, for example, it would have to go through
all of those stars in the milky wave before it
can go out to the rest of the universe. Yeah,
that's absolutely true. But now think about a simpler scenario.
Imagine you just have like two particles floating freely in space,
and they're exactly one light second apart. A gravitational wave
passes through and it sort of brings them closer together

(25:57):
and then further apart again when the gravitational wave is done,
because it leave them as the same distance as they
were originally, or is there some sort of permanent distortion there?
That's the question, M, Because I guess that gravitational wave
isn't just like something that stretches space. It kind of
contracts and stretches space. Right, that's kind of what a

(26:17):
wave is. It's like it's an upend adet M. Absolutely
it is, and gravity is really complicated stuff. It's much
more complicated than electromagnetism, for example, because it interacts with itself. Right,
you send a photon out through space, that photon flies
through space. It's a ripple in the electromagnetic field. But
photons don't talk to other photons and don't create other photons.

(26:39):
Like two photons, as we talked about on the podcast once,
don't directly interact with each other. They can do it
indirectly via other virtual particles. But photons themselves don't bounce
off other photons. That's not true for gravity. Everything that
has mass or energy creates gravity and influences everything else
with mass and energy. And gravitational waves have energy, which

(27:00):
means they create gravitational waves, and then those create gravitational waves,
and those create gravitational waves. So you have this sort
of like nonlinear effect where gravitational waves are constantly spewing
off other gravitational waves. Yeah, so it sounds like gravitational
waves do sort of leave a pretty obvious lasting impact
on things, right, Like if a gravitational wave was big

(27:22):
and then went through Earth, and like you said, it
squeeze Earth in one direction and then stretch it in
the other direction, and then the vice versa and in
went through, which does heat up the Earth a little bit,
which lasts for a while at least, right, Like, it
depots some energy and we keep that energy and you
can measure that energy. Yeah, that's certainly true. And there's
this other effect even for isolated stuff, you know, even

(27:44):
for two particles floating out in space, there's something called
the gravitational wave memory effect. It's like it changes space
as it passes through and leaves it changed. Right, those
two particles floating out in space, when they get wiggled together,
there's no energy deposit like when the Earth gets squeezed,
because there's no like bond between these two particles floating
out in space, and yet still there's a permanent effect

(28:07):
on those particles. This again, is called the gravitational wave
memory effect. It's like footprints in the sand. Once the
gravitational wave passes through, it changes things even after it's gone. Wait, wait, wait,
what's it called again? The gravitational wave memory effect? Okay, sorry,
I'd for gone for a second. I walked right into that.

(28:28):
It's called the podcaster or dementia effect. Well, I think,
I think what you're saying is that we know that
the gravitational ways leave an impact in stuff around the universe, right,
Like if it goes through stuff, it leaves a little
bit of energy because it had to squeeze it and
stretch it. But now I think maybe the question we're
really asking in this podcast is to gravitational waves leave

(28:50):
an imprint on like space itself, Like the space itself
gets scarred or marred or you know, imprinted by the
gravitation wave. You make it sound so negative, like it's
been vandalized by gravitational waves or something like it's been ruined, Like, man,
I'd built this space and then those crazy teenagers marred it. Well,

(29:10):
I mean space is so pristine and beautiful. Yeah, for
a bunch of ripples in it. You are kind of
changing the esthetics. Yeah, but you know space is also
filled with gravitation a wave. So the space we started
with has already been imprinted on by all the gravitational
waves that came before us. Okay, so then the question
we're really asking is do gravitational waves leave a lasting

(29:31):
imprint on space itself? And you're saying the scenario we
should be imagining is like two particles out there in space,
floating at a certain distance from each other, and they
don't interact any other way at all, Like there's no
electromagnetic forces between them, there's I mean there's gravity. Can
they have gravity between them? Well, they're far enough apart
that there is essentially no gravity. Yeah, okay, essentially no gravity,

(29:54):
but no weak force and a strong force. They're just
like two lonely particles out there in space. And then
gravitational waves comes through, does it change the space between
them permanently in a way that you could be like, hey,
something came through here. Yeah, really fun question. And in
the seventies, people who were playing with the equations of

(30:14):
general relativity and doing calculations discovered to their surprise that
it should. This is fun because it's like a theoretical surprise.
You know, we have equations for Einstein's general relativity, but
we don't always know the consequences of them. Sometimes when
you sit down and say what would happen in this scenario,
you run into something unexpected because to understand the effects

(30:36):
on the universe of these equations you have to do
some calculations. You have to set it up and say,
I'll let me see if I can predict this scenario.
And so this was discovered in the seventies, and then
a lot of progress has been made in the last
few decades. But all sorts of various different kinds of
memory effects. What's it call again, I forgot I have

(30:56):
a memorable name. Fool me once. This effect as a
name is called the effect, and so it's a thing.
It's like a theoretical thing that says that space does
get kind of altered permanently when a gravitation wave moves
through And so how does that work, How does it
effect work? How does it come up in the equations.
So there's actually a few different effects. There's a nonlinear
worm and the linear one. I think the most interesting

(31:18):
and least confusing to understand is the nonlinear effect. And
this comes up because gravity, as I was saying before,
couples to itself, like gravity creates more gravity, whereas photons
don't create more photons. Gravitational waves generate gravitational waves as
they pass through space, and so this creates a sort
of like nonlinear effect because the energy doesn't just fly

(31:41):
through the universe. It's sort of like deposits itself in
space itself as it goes along. It creates these little
mini gravitational waves that change space. Wait what as it
goes through stuff or even in empty space, even in
empty space, right. Gravitational waves themselves contain energy, and so
part of their energy goes into making other gravitational ways

(32:01):
as it goes along. Yeah, exactly, And so that's one
of the ways that they fade. And so if you
look at like the prediction for what should happen to
two particles as a gravitational wave passes through them. A
sort of canonical prediction you're familiar with from like Lego, etc.
Is that they get further and closer and further and closer,
and then they get back to their original location. If
you conclude all these effects of like gravitational waves generating

(32:23):
more gravitational waves, you see that they don't come back
to where they started. That there's a lasting effect on
the distance between these two particles, which basically means space
itself has been stretched permanently. Oh. Interesting, But I guess
the picture you're painting for us here is that the
gravitation away generates more ways. But then don't those ways
also fade away eventually? Where does the permanence come from?

(32:46):
The permanence comes from this nonlinearity. Right, they're fading, but
they're also generating new gravitational waves and generating new gravitational waves.
But each time it's smaller, isn't it? Each time it's smaller?
But you know, it's an infinite series. An infinite series.
Sometimes they'd diver, sometimes they go to zero row. In
this case, they add up to a constant. They add
up to a non zero value. Oh, I see it's
it's sort of like a permanent echo. Like if you're

(33:08):
in a close room and you say hello, hello, Hello, helloooo,
that hello never kind of goes away and it stays
where the gravitation wave went through. So it's sort of
like it it leaves a permanent echo wherever it goes. Yeah, exactly,
It's just like a permanent echo. And so that's really
kind of interesting because it's suggests that gravitational radiation is

(33:29):
fundamentally different from other kinds of radiation, right, It's not
as capable of transmitting energy because, as you say, it
deposits some of that in space as it goes along. Interesting,
So then that's what it does do space? And where
do there are two literal lonely particles come in? How
do those two particles notice that space space is now
full of gravitational echo because they don't go back to

(33:54):
their original distance. If they started out one light second
apart before the gravitation the wave has passed through, then
they wiggled out and in and out and in a
little bit. But when the gravitational wave has passed, they're
now like one point zero zero one light seconds apart,
whereas they used to be one light second apart. So
you can measure this, you can say, my gravitational waves

(34:14):
passed by, yet my particles are still further apart than
when they started. M We mean like the premanent echo
the lingers after the wave goes through actually results in
kind of stretching space between them exactly. And that's what
gravity is, right, is the stretching or its compression of space.
And so it's like made more space. It's like deposits

(34:36):
some of that energy into creating new space between these
two test particles, a very very tiny amount. Remember, gravitational
wave effects are very very subtle, which is why they
are so hard to see. The original experiment, Lego had
these arms where the mirrors were like kilometers apart, and
they saw the distance between those mirrors changed by one

(34:56):
one thousands of the width of a proton. Right, these
are really really tiny effects, and the gravitational wave memory
effect is even smaller than the gravitational wave effect itself.
Wait wait, wait, what's the gravitational memory effect? I forgot again.
I'm just kidding. So, like if you measured how far
these two lonely particles were before out in space, and

(35:20):
then what gravitational wave went through, and then you measured
it again, you would measure them to be a little
bit further apart than before. Exactly if there was nothing
else influencing them, no gravity, no bonds, nothing else but
just the shape of space, then yes, they would permanently
be further apart than when they started, even long after
the gravitational wave has passed through them. And it's like

(35:41):
a positive stretching effect. It's not a compression effect, right,
It's a positive stretching effect. It deposits energy, creates new space. Interesting,
and this is happening all over the universe. So are
you saying gravitational waves like black holes crashing into each other,
they're part of the reason the universe is banding? Or
does it contract in some places and expands in other places? Right?

(36:04):
Like when the gravitational waves get generated, does it compress space? Oh,
that's a really interesting question. Whether it overall, like integrated
overall of space, contributes to the expansion of space or
whether it cancels out somewhere. I'm not one hundred percent
sure of the answer, but I think that this is
only a positive contribution to the shape of space. And again,

(36:24):
this is really a tiny effect almost impossible to measure
much much smaller than the expansion of space due to
dark energy, But I think it would technically contribute to
the expansion of space. Yes, that if you had like
a universe where nothing was moving, so no gravitational waves
were created, versus a universe where things were swashing around
and gravitational waves would being made, that second one would

(36:46):
be expanding faster than the first one. Well, I wonder
if maybe it like does that energy has to sort
of come from somewhere, right, like when the black holes
get formed when they swirl around each other. I wonder
if that has an effect to shrink the local space.
But then it's strudges everything else out. You know what.
The energy, remember, comes from the masses of the objects
that generated. When two black holes merge, you have like

(37:07):
one of them is fifty solar masses and other is
fifty solar masses. The black hole that they result in
is not a hundred solar masses, it's like eighty because
they've generated an incredible amount of gravitational radiation, like twenty
solar masses worth. So that's where the energy comes from.
All right, Well, let's get into how you might measure
this interesting effect and also what it all means about

(37:29):
our understanding of gravity and the universe. But first, let's
take another quick break. All right, we're talking about what
was it we're talking about, Daniel, We're talking about moving

(37:51):
you to retirement homes. We're talking about the memory effect
of gravitational waves. This idea that gravitational waves, as they
propagate through the universe, they have a lasting effect on
space itself. It's stretching space, depositing little, tiny and everlasting
echoes of gravitational energy which stretch space and make it bigger.

(38:12):
And so you're saying, Daniel, this effect is super duper
duper small. How can we even measure this? Can we
prove that this theory is right? So it is a
theory currently. It's a consequence of general relativity, and one
we've seen in the math, but we've never actually seen
it out there in the universe. Gravitational waves we have seen.
We know those are real. But this gravitational memory effect

(38:32):
that we keep forgetting what it's called this part has
not yet been seen. It's a theoretical prediction. And remember
that we have great confidence in general relativity because it's
been such a virtuoso description of how the universe works
and the nature of space itself. But we also think
that it's probably flawed because it can't describe the quantum
mechanical nature or the universe. So not everything that general

(38:54):
relativity predicts is guaranteed to be true, which is one
reason why we want to go out and test this.
But it's also much much smaller effect than gravitational waves.
It's predicted to be like twenty to fifty times smaller
than current gravitational waves effects we have measured. We don't
think that our current experiments like LEGO are going to
be capable of seeing this very easily, right, because, like

(39:16):
we talked about before, Ligo is a ruler which is
sort of attached to itself. It is a solid ruler
in a way. Right. Lego does a really good job
of trying to be independent from the Earth as much
as possible, But this is something that the Earth has
that Lego just cannot escape, and that's the Earth's gravity. Right.
We have these two mirrors where light is bouncing back
and forth between them. Imagine they're getting like squeezed a

(39:39):
little bit closer together or a little bit further apart.
And even if the gravitational wave wants to pass through
and leave them a little bit further apart. The Earth's
gravitational field will sort of pull them back, right, It
will pull them down and prevent them from staying a
little bit further apart. These mirrors are like suspended on cables, right,
So imagine like a pendulum that's been left little bit

(40:00):
away from equilibrium. If there's gravity there, it'll just swing
right back to the equilibrium position. So the Earth's gravity
sort of erases this gravitational memory effect in Lego saying
the Earth remembers. But I guess that doesn't make a
lot of sense to me, Like why would the Earth
care how far apart these mirrors are. Well, I don't

(40:21):
think the Earth has an opinion, Like it doesn't matter
to the Earth at all. But the Earth does have gravity,
and the effect of gravity will be to pull these
things back to where they were. In what way does
Earth's gravity pull the mirrors back? Like, why would the
Earth you want to put them back in the same position?
What was special about the original position? Well, the Earth

(40:41):
itself has these powerful bonds, and so we don't think
the Earth itself is changed, right, So the Earth still
has the original gravity that it had before. And these
mirrors are not floating in zero gravity, right, Their original
position was determined by the gravity of the Earth. So
they're going to end up back in that same position
if you don't apply somers to them. It's sort of
like if you went and pushed on one of those mirrors,

(41:03):
what would happen, Well, it would swing in one direction,
but then gravity would bring it back, right, but only
if you push it against Earth's gravity, Like if you
push it perpendicutor or to the side, the gravity Earth's
pulls is the same, isn't it? Like the Earth is
just pulling them towards the center of the Earth. Yeah,
so the mirrors are all being pulled down, right, all
being pulled down towards the center of the Earth. And

(41:24):
so if you give them a nudge away from that
line towards the center of the Earth, they're going to
naturally trend back towards the center of the Earth. But
if I push it along a circle around the Earth,
the gravity is the same, isn't it. Well, So remember
these mirrors are suspended on a cable with a fixed length, right,
and so if you push it then it's basically moving
it up. Just like if you have a ball on
a string and you push on the ball, the ball

(41:46):
goes up because the string is a fixed length, and
so now the Earth is going to pull it back
down to the equilibrium position. Oh, I see what you're saying.
The way Ligo is design these things are on pendulums,
and so the Earth's gravity wouldn't let you have a
permanent change in the space between them. Yeah, that's exactly right,
And in principle you might be able to observe it

(42:08):
before Earth brings some sort of back to the equilibrium position.
There's like a little bit of information there. The memory
effect might last for a little while before the Earth
erases it. But legos really just not set up to
make this kind of measurement. Instead, what you need is
the same kind of detector, but where everything is in
free fall right where there is no nearby massive object

(42:29):
pulling on everything. Basically you need this thing out in space.
I see. So there's just more of an excuse to
have space lasers, the operational space lasers. That's really it's
just a long con. When these guys in the seventies
were coming up with these calculations, they were just like
dot dot dot space lasers? How do we get there?

(42:53):
That's every physicist dream, isn't it? Shoot lasers in space?
Space lasers. It's pretty cool, though, space lasers. I mean,
that's pretty awesome. You got to say, right, that's yeah great.
Reagan thought that too. We're just going to shoot them
back and forth between these quiet little detectors in space.
I promise you, we're not going to shoot our eye out. Mom.
I feel like that kid in a Christmas story. Okay,

(43:15):
So then the idea is that you would have these
detectors out there in space, you know, kind of an
empty space, and then you would measure the distance between
them with super duper high accuracy. And then you would
not just measure the wave as it washes by, right,
but you might be able to measure like, oh, after
the wave went through. Now we're a little bit further apart,
which proves this idea of the what gravitational wave of memory?

(43:42):
I'm only saying that because maybe the listeners forgot, and
I want them to be confused the memory effect of
gravitational waves. Right, that's the idea, right, that is the idea,
And so there is actually a design for this. It's
not just physicists dreaming up space lasers. It's called LISA,
the Laser Interferometer Space Antenna, and it's basically three space

(44:04):
graft out there in a triangle. And you know, LIGO
is a few kilometers long. They're bouncing light back and forth.
This thing is going to be millions of kilometers long.
What so these are like space space? Oh yeah, these things?
I mean, why not make them really far apart, right
if you can? Because the further apart they are, the

(44:25):
more sensitive you are to really small gravitational waves. Not
only could this potentially detect the memory effect, it could
also measure the gravitational background effect, like, as you say,
every time you move your arm, you're creating gravitational waves,
and everything that's out there in space orbiting is creating
gravitational waves. We could sort of measure this sort of
background gravitational wave noise of the universe and get some

(44:50):
information about that using LISA. Oh wow, because I was
thinking this was more like maybe the gravitation will probe
be satellites that it did some relativity experiments around the Earth.
This is like out there in space base, like out
there between the planets, right, if you're talking about millions
of kilometers or is it still around Earth's orbit. It's
going to be orbiting the Sun sort of with the Earth,

(45:12):
and you know, millions of kilometers sounds really long, and
it is pretty big. It's bigger than the Earth, which
is kind of awesome. But these things will sort of
stay near the Earth so that we can talk to them,
communicate them, control them. They'll be orbiting the Sun sort
of with the Earth like a fully operational space station.
Not yet fully operational. This thing is just in the

(45:33):
planning stages. I think the earliest targeted launch date is
twenty thirty seven, which is like comfortably far away enough
in the future that nobody has to like actually build
anything today. So nobody's actually started constructing anything. I see.
We're still in the prequels. You're like in the Clone
Wars or Revel or a Rogue one. We're still drawing

(45:53):
pictures of Death Star on chalkboards at this point in
the story. We're not actually building any but you know,
if we build it, we will learn so much about
gravitational waves, gravitational memory, gravitational background. Maybe also even see
gravitational waves from the Big Bang, you know, ripples in space,
that were created in the very very early moments of

(46:14):
the universe. Wait, what, so it'd be so accurate and
so sensitive to gravitational waves that you would measure these
ripples from the beginning of the universe like those are
still around. Those we think are still around, sort of
the same way that like the cosmic microwave background radiation
is still around. These are photons from the very early universe,
but they don't go all the way back to the

(46:35):
very beginning of the universe. They only go back as
far as the universe has been transparent. Before some moment
in the early universe it was opaque, so you generated photons,
they just kind of absorbed. Those photons are not around anymore.
The oldest photons we can see are back when the
universe was transparent to light. But gravitational waves can go
through almost anything, right, So the universe is basically transparent

(46:57):
to gravitational waves, which means that if you could detect
very very faint gravitational waves, we could see all the
way back to well before that moment when the universe
was opaque and see signals and get information about the
very very early universe. So Lisa would be really powerful
sort of astrophysical and cosmologically m Yeah, you could hear

(47:18):
like the Big Bang itself, like the Bang, right, that's
the idea. Yeah, we might see ripples from inflation. You know,
that would be really awesome. All right, So we're building
these awesome space telescopes and we might measure and confirm
that gravitational waves do have this memory effect that leaves
kind of a standing echo across space. What would that
mean about our understanding of the universe. It would mean

(47:40):
that once again, general relativity is awesome and accurate. I
might also help us solve some puzzles we have about
like black holes. Remember that one mystery about black holes
is like where does the information go when something falls
into a black hole? Because we think that like black
holes don't release any information about what's inside them. And
the other hand, we also think black holes evaporative actually

(48:00):
due to hawking radiation, and so that information has to
go somewhere, but we don't really understand it. It's possible
that if gravitational waves are leaving permanent imprints on space,
then maybe things falling into a black hole are like
changing the space around a black hole in some important way,
leaving their information there. As they fall into the black hole,

(48:22):
so that it isn't actually destroyed. M interesting, like as
it falls in it leaves a little bit of a
graffiti in space itself before it gets destroyed by the
black hole. Yeah. Or maybe it's like a space angel,
you know, think about it in a positive way. It's
like making its mark on space. Space angela. You know,

(48:43):
you can make snow angels or sand angels. Can you
lie down in space and like wag your arms and
make a space angel? Oh? I see, I thought you
were talking about like actual space angels. I was like, yeah,
that's an interesting idea for space epic. No, that would
be a spiritual angel. I'm talking about a physical thing,
you know. I guess in principle, if you stand around

(49:03):
and wave your arms, you are making gravitational waves in
the shape of a space angel. I suppose. Yeah, And
technically it is permanent, like you are distorting space forever. Yeah,
you are, So be careful, everybody. What if I walk
around in a pattern that says Kilroy was here? Am
I marring the universe? Some future alien society will build

(49:25):
a very very sensitive gravitational wave detector and they will
read your message and they will wonder why did these
choose to send that? What does that mean? Didn't he
have anything better to do? All right, well, I think
that answers the question. Gravitational waves do leave an imprint
on the universe, on the stuff that it passes through,
and maybe theoretically it does also leave an imprint on

(49:49):
space itself, something that lasts forever, that echoes throughout eternity.
Then maybe some days some alien species or maybe us
in the future can read and learn about what what happened. Yeah,
if we are still doing this podcast in fifteen or
twenty years, then maybe we'll have an episode talking about
the discovery of gravitational wave memory. Yeah, assuming we remember

(50:10):
this episode, which I'm guessing probably not. All right, Well,
we hope you enjoyed that. Thanks for joining us, See
you next time. Thanks for listening, and remember that Daniel
and Jorge Explain the Universe is a production of iHeartRadio.

(50:32):
For more podcast from my Heart Radio, visit the iHeartRadio app,
Apple Podcasts, or wherever you listen to your favorite shows.