Gravitational waves should permanently distort space-time | Hacker News Reader

Gravitational waves should permanently distort space-time

quantamagazine.org

160 pointstheafh123 comments

Layman's question: are gravitational waves really "waves"?

When I think of waves, I think of systems governed by the wave equation [0].

But IIUC that requires a restoring force. I'm not sure what that would mean in the case of gravity.

E.g., if the moon suddenly lurched towards the Earth, we'd perceive an increase in gravity between the two. But that would be a semi-permanent change in the strength of that attraction between the two objects; not what I'd think of as a wave-like fluctuation.

[0] https://en.wikipedia.org/wiki/Wave_equation

Yes, gravitational wave are governed by the wave equation. A standard way to treat them is as a small perturbation around a known spacetime geometry. Then the metric would be $g = g_0 + \epsilon h$, where $g_0$ is the background, $\epsilon$ is a small parameter and $h$ is the perturbation.

In general Einstein's field equations govern the dynamics of $g$, and if you take the first order behaviour in $\epsilon$ around $g_0$ as flat space, then you recover the wave equation for $h$ as in the article you linked, with propagation speed $c$ the speed of light. (There are some additional subtleties about choice of gauge, but this is not physical).

I'm not sure what you mean with your moon example.

Thanks! I'm not ready to understand your explanation, but I can clarify the moon example at least.

Suppose the gravitational attraction between the earth and moon is 2e20 Newtons.

Now imagine something forces that attraction to strengthen from 2e20N to 3e20N. E.g. the Earth and moon get closer to each other, or the moon gets more massive somehow.

When we talk about "detecting gravity waves", I understand that to mean that on Earth we've managed to detect that increase from 2e20N to 3e20N.

My point was: perhaps that looks like the rising edge of a wave phenomenon, but it's not actually cyclic like I expect from a traditional "wave". So I couldn't understand why it's called a gravity wave.

Nitpick: gravity waves != gravitational waves.

Unfortunately, I don't know enough to explain the difference.

"Gravity waves" are ordinary water waves, or other waves of a fluid, where gravity provides the restoring force:

https://en.wikipedia.org/wiki/Gravity_wave

They have absolutely nothing to do with gravitational waves.

Nonetheless, in the gravitational wave community (i.e. LIGO) we don't usually ever talk about true "gravity waves" so we use the terms interchangably.

The exception is that "gravity waves" in the Earth and the atmosphere can couple to LIGO through ordinary Newtonian gravity, which is something we have to think about and exclude.

Gravitational waves do not behave in the way you have described.

> I understand that to mean that on Earth we've managed to detect that increase from 2e20N to 3e20N.

This not what how gravitational waves behave. This assumption is not correct and will confuse you.

If you're already comfortable with the Wave Equation, then it's somewhat straightforward. You start with some spacetime geometry (i.e. some "known metric") and then study small perturbations that obey the wave equation. The key is finding distributions of matter that, by Einstein's Equation, cause such perturbations to arise.

In differential geometry, the device called a "metric" let's you define things like angles and distances on some space. This is generally denoted as g with the Greek letters mu and nu as subcripts. Just to give you a feel of what's going on, I'll leave out the subscripts, but what OP's saying is that

    g = k + h

which says that g, the space-time metric, decomposes into some known, usually easy to work with metric k, plus some perturbations on top, h. In the math you put restrictions on h to make precise the "small perturbations" part.

Anyway, what Gravity Waves boil down to are situations (read distributions of matter) that cause h to obey the wave equation. In other words, gravitational waves are spacetimes that have a component that waves "on top" of a given reference "background" spacetime. Of course, you could cherry pick your background such that anything looks like waves on top, but typically we choose the background to be Minkowskian (i.e. flat).

FWIW, the above perturbative approach is called Linearized Gravity and is kind of a (nice) hack to restrict study of the really hairy non-linear Einstein Equation to cases that are manageable.

Gravitational waves are waves of the metric tensor. Einstein's field equations provide the "restoring force."

Importantly, gravitational waves are not waves of Newtonian gravity. Gravitational waves do not "push and pull" along the direction of propagation. They stretch and compress space along axes perpendicular to the direction of propagation.

> E.g., if the moon suddenly lurched towards the Earth, we'd perceive an increase in gravity between the two. But that would be a semi-permanent change in the strength of that attraction between the two objects; not what I'd think of as a wave-like fluctuation.

Indeed, gravitational waves do not work this way.

Unfortunately it is hard to explain gravitational waves without significant math.

In fairness, even Einstein himself waffled over whether gravitational waves would be a real effect predicted by the theory. Then it took nearly a century to detect them experimentally, and there were plenty of doubters along the way.

The results one gets from intuition are generally incorrect in important ways. Here's a derivation of a wave equation from Einstein's field equations:

https://en.wikipedia.org/wiki/Linearized_gravity

A more helpful introduction for a layperson might be the paper titled "Gravitational Waves on the back of an envelope":

https://aapt.scitation.org/doi/10.1119/1.13627

But, infuriatingly, that paper does not seem to be open-access. Here's someone's scanned copy:

https://www.ru.ac.za/media/rhodesuniversity/content/mathemat...

> But, infuriatingly, that paper does not seem to be open-access.

Try sci-hub.

> They stretch and compress space along axes perpendicular to the direction of propagation.

Is it possible to compress space?

(Not a native speaker).

Space is a mathematical abstraction. You can do anything with, because it's just math. If you want to bend, curve, distort, rip space, or add more dimensions — go for it.

Physical medium should be compressible, like any other medium. Higgs «field» (let's call it Higgium — Higgs+Vacuum) is presented everywhere, because Higgs boson gives mass to every particle in the Visible Universe and beyond, so it should conduct gravitational waves. As demonstrated by LIGO/VIRGO, gravitational waves causes distortions in light travel. These distortions can be explained as changes of conductivity in Higgium, which can be caused by changes in density, so yes, it can be waves of compression.

I appreciate what you’re trying to get at, but it’s not right. Gravitational waves exist in GR even in theories with no matter / Higgs.

The Higgs is a scalar field; gravitational waves are spin-2.

Thank you for pointing it out.

Spin-0 particles are round particles, like o (like ball), or something that cannot rotate at all. Spin-2 particles are symmetrical particles, like 8 (like H2 molecule).

For spin-0, it can be changes in density only. For spin-2, it can be changes in orientation, in rotation speed, in rotation orientation, Dzhanibekov effect, wobbling, and some other if it has more complex shape, e.g. two spirals connected.

I believe this answer is nonsense.

> Is it possible to compress space?

The geodesic distance between inertial test masses increases and decreases.

Colloquially we say that space has stretched and compressed.

Geodesic distance can be measured by the time it takes light to travel between the points.

Inertial test masses can be simulated by hanging them from fine wires (LIGO) or by setting them up in an orbital constellation (LISA).

> Importantly, gravitational waves are not waves of Newtonian gravity. Gravitational waves do not "push and pull" along the direction of propagation. They stretch and compress space along axes perpendicular to the direction of propagation.

This may be, likely is, a stupid question, but how do you measure this if space itself is stretched or compressed? Won't any yardstick lying in that space also be stretched or compressed? Or is it a matter of this distortion of space changing geodesics through it so light following a straight line will end up in a different place? It seems that LIGO uses interferometry to measure a change in distance between two points, though, and I don't understand how that can be measured if space is stretched, only if space is added. Perhaps that is the answer right there: when space is stretched dimensionless particles traveling through that space are not stretched, because they are dimensionless. Or perhaps better, objects in space stretch, but their momentum vectors don't stretch.

Here is an introductory paper on that exact topic:

"If light waves are stretched by gravitational waves, how can we use light as a ruler to detect gravitational waves?" https://universe.sonoma.edu/moodle/pluginfile.php/89/mod_res...

The way I explain it: The light already inside the arm cavity is stretched. But newly entering light is not. We are basically using the light as clock to measure the length of the arm. When the arm is in the "stretched" state, light will take longer to make a roundtrip in the arm. It will accumulate slightly more phase, which is measured interferometrically.

It is important that the storage time of the light in the arm is short compared to the period of the gravitational wave.

On a similar note - if a huge gravity wave passed through the earth, say one that stretched the space I’m in by a factor of 10, would I feel anything?

> when space is stretched dimensionless particles traveling through that space are not stretched, because they are dimensionless. Or perhaps better, objects in space stretch, but their momentum vectors don't stretch.

This is an interesting idea, but I think it's not right.

To be honest, even as an experimentalist who worked on LIGO, I'm not sure how to treat the effect of the g.w. on photons in the arm. Instead I think of the entire arm cavity vacuum as a giant phase modulator.

The clever thing about interferometers is that they're actually measuring space and time in two different dimensions concurrently and then looking for changes in the interference pattern between light moving along each of the dimensions. Simplistically, imagine a gravitational wave propagating along one dimension of the interferometer (realistically gravitational waves will never be perfectly aligned with any direction of the interferometer). Space will be distorted in that dimension but not the other and we can notice the change in the resulting interference pattern. In practice, gravitational waves will come from all sorts of weird angles, but they will distort each of the two dimensions differently and allow us to figure what direction they were propagating by the interference pattern that's observed.

> The clever thing about interferometers is that they're actually measuring space and time in two different dimensions concurrently

I would say that we measure space in two different directions concurrently.

> and then looking for changes in the interference pattern between light moving along each of the dimensions

This is true, but I like to push back against the use of the phrase "interference pattern." We're not looking at a "pattern", which to me brings to mind a complicated interference pattern resolved spatially. We do not resolve the "interference pattern" spatially. We measure the amplitude (er, power) of the light coming out of the interferometer with a photodiode (a single pixel, if you will.)

> In practice, gravitational waves will come from all sorts of weird angles, but they will distort each of the two dimensions differently

This is true. The detector's sensitivity to waves coming from different directions is the "antenna pattern."

> t they will distort each of the two dimensions differently and allow us to figure what direction they were propagating by the interference pattern that's observed.

With a single detector, we cannot determine the direction in which a g.w. is propagating. With a single detector and a transient (short-lived) source, we cannot tell the difference between a loud source in a direction where the detector is not very sensitive versus a quieter source in a direction where we have good sensitivity.

With a network of detectors and/or signals that persist for a long time (compared to the rotation of the earth, etc) we can resolve the source direction.

You might enjoy this video which goes into that question:

https://youtu.be/iphcyNWFD10

Sci Hub has it apparently: https://sci-hubtw.hkvisa.net/10.1119/1.13627

I'm hardly an expert in GR, but it seems to me that gravitational waves is a trivial consequence of limited speed of gravity propagation. It takes 8 mins for Sun's gravity to reach Earth. If Sun suddenly moves left and right, in 8 mins we'll feel a pull to the left and then to the right, so in effect gravity will feel like a couple waves passing thru us. The exact geometry of those waves is complex, but that hardly matters for intuitive understanding. The exact geometry of water ripples is also very complex, after all.

Unfortunately that’s not how they behave at all. See some of the references posted by others in other comments. Intuition doesn’t really work at all here.

Agree with jleahy.

I would add that considering counterfactual situations like "if the sun were to move instantly to the right" is of limited utility. That cannot happen (violates conservation of momentum). (Granted, other thought experiments have been quite influential in this field...)

Apart from some tidal forces we probably wouldn't feel a thing but rather just continue following the free falling trajectory.

Gravitational waves are also different from changes to the amount of gravitational force. This can be seen from the fact that gravitational waves affect the distances between objects, whereas the sun which exerts far more gravitational force has no measurable effect (at least, to my knowledge rotating objects don't change length noticeably).

I think it's actually more complicated than that. The strength of the sun's gravity goes as 1/r^2. Move it to the side a bit, and the direction of the gravitational field changes slightly. This direction change goes as 1/r, by the small angle approximation. So from your description, we'd expect the size of the ripple to go as 1/r^3. But in the case of actual gravitational waves, the field strength of the ripple goes as 1/r. That's why we can detect gravitational waves from distant black holes, but we can't detect their static gravitational fields, which go as 1/r^2.

Electromagnetic waves are similar. The Lienard-Wiechert field formulas [1] have 2 terms. The first term describes the delayed field, and is proportional to 1/r^2, while the second describes waves and is proportional to 1/r.

[1] https://en.wikipedia.org/wiki/Li%C3%A9nard%E2%80%93Wiechert_...

The Wikipedia page on gravitational memory effect [1] gives links to some actual papers, which might be more illuminating than the rather garbled description in this article.

https://en.wikipedia.org/wiki/Gravitational_memory_effect

"persistent changes in the relative position of pairs of points in space due to the passing of a gravitational wave."

So the idea isn't that space itself is storing information, just that a gravitational wave doesn't put everything back where it found it. Presumably if you cleared out all the stuff, there would be no memory left behind.

That's my understanding, yes. So the "permanently distort spacetime" part in the article's title is misleading.

What you are both saying seems incompatible not just merely the title but also with the text of the article, where is states, "Earth’s gravitational pull also tends to restore LIGO’s mirrors to their original position, erasing its memory. So even though the kinks in space-time are permanent, the changes in the mirror position — which enables us to measure the kinks — are not." In other words, the article is claiming that there are two things being affected, spacetime itself, and the objects embedded in spacetime, but only the objects are temporarily displaced, spacetime itself is permanently displaced. At least, that's my reading.

But I wonder, notwithstanding the article's assertion, are objects really completely restored back to their former position in spacetime? Spatial coordinates, sure, but timewise? I don't claim to know the answer, just posing this as a question, but imagine a really powerful gravitational wave going through a large object. Wouldn't it create some kind of timewise interference pattern in the object where at the peaks of oscillation, the constituent subatomic particles would feel a stronger gravitational force and age more slowly than the particles at the troughs of oscillation? So after the wave had passed, you'd be left with an object that has areas of divergent age?

I mean even with standard GR we experience aging on a gradient, in that our feet are somewhat younger than our heads thanks to the gravitational pull of the Earth. ( https://www.theatlantic.com/technology/archive/2010/09/study... )

> What you are both saying seems incompatible not just merely the title but also with the text of the article

I would not trust the text of the article. Pop science articles are often untrustworthy even on much simpler topics. Actual papers are much better sources.

> are objects really completely restored back to their former position in spacetime?

Of course not; that's impossible. Any talk about restoring objects to their original position can only mean position in space. (And since "position in space" depends on a particular choice of reference frame, one then needs to ask what reference frame.)

> the constituent subatomic particles would feel a stronger gravitational force and age more slowly than the particles at the troughs of oscillation?

The "rate of aging" does not depend on "gravitational force". In situations where such concepts as "gravitational force" and "gravitational potential" are even applicable (which will not include very strong gravitational waves in any case), the "rate of aging" depends on gravitational potential. (For example, in the vicinity of the Earth, one's altitude above the Earth affects "rate of aging".) However, one can still assess differential aging in more general situations, just not with the rule of thumb you gave.

> after the wave had passed, you'd be left with an object that has areas of divergent age?

This is possible, but, as noted above, it cannot be assessed with the rule of thumb you gave. You would have to know the details of the spacetime geometry of the gravitational wave (and you would have to be prepared for some pretty heavy duty numerical computations).

Wow, the citations in the article are bigger than the text.

The title reminded me of that Star Trek episode, in which they realize, that warp drive usage has a permanent effect on space time and can tear terrible rifts open, which could destroy basically everything and then decided, that throughout the whole federation there would be speed limits or so.

>which could destroy basically everything

https://en.wikipedia.org/wiki/False_vacuum_decay

I guess it's not worth worrying about, because if it happened, it would happen at the speed of light and be impossible to see coming.

Earth-Sun system should radiate about 200 watts of gravitational waves continuously. One 20th that should be 10 watts of permanent deformation.

Bad news is 10 watts is not much, spread thru a large volume. Good news is its "easy" to measure the infra red radiation from Pioneer and Voyager space probes running less than a KW point source of energy.

I wonder if the gravity probe A and B missions would show those permanent deformations in their data.

Permanent deformation should be detected in large scale long term orbits, eventually?

What does this deformation mean physically? I thought matter makes space curve. Does the deformation mean the curvature get changed?

So I wonder if spacetime has a kind of "entropy" where it can be stretched-out but never put back exactly how it was. Possibly explaining inflation.

No inflation is very different, it's pervasive in every direction, it's not localized like your idea would mean.

Plus, it's not like galaxies are temporarily pulling on a memory foam, before releasing the pressure, so I m not sure why you think inflation (such a bad name, people think balloon) is so close to that model.

It looks like Big Shrink (BS) theory is easier to understand: we are shrinking, our rulers are shrinking, so cosmic distances are looking bigger in every direction. It explains why our Universe is flat and why we cannot find a Universe-big source of energy for expanding of the Universe.

But what energy is making us shrink?

From the article:

> “The memory is nothing but the change in the gravitational potential,” said Thorne, “but it’s a relativistic gravitational potential.” The energy of a passing gravitational wave creates a change in the gravitational potential; that change in potential distorts space-time, even after the wave has passed.

Gravitational waves are that energy.

Conservation of energy gets a bit screwy with these kinds of cosmic models. Partially because there isn't a single direction of 'time', but also because all kinds of energy gets stored in the fabric of space-time which can cancel out all kinds of things.

Not to mention that there's no reason to assume conservation of energy still holds if the laws of physics simply change over time (which would be the case in the simplest possible theory were all interaction distances simply shrink over time).

This doesn't make sense either - if I were shrinking and you were shrinking, then the distance between us would appear to be growing larger, but it's not. Gravitationally bound systems don't expand, only the galaxies themselves seem to be moving away from one another. Combined with no mechanism to explain the shrinking, nor any reason why the various other laws of physics don't seem to be affected and it doesn't seem any easier to understand at all.

Just curious, since gravity works over infinite distances (as I understand things):

When do two masses stop being gravitationally bound? Is that when each mass's relative speed exceeds the escape velocity of the other mass?

No, that can't be right: they could still end up in orbit - obviously gravitationally bound.

One complication is that since the universe is homogenous and infinite, there’s a boundary that is expanding away from us faster than the speed of light and this gravity as well.

The observable universe is of finite mass and size, while the total universe is of infinite or finite mass and size. On a macro scale, the total universe probably looks similar. It's definitely not micro homogeneous or entropy would be infinite and heat death would have already occurred. Heat death won't be the biggest problem because of the likelihood of the Big Rip.

In this case, bound means that the force of gravity is stronger than the expansion of space time. All of space is expanding, but things that are close (ie within a few hundred thousand light years) pull on eachother enough to resist being swept away by the "current" and only distant galaxies recede. But it's not like there is something special about our position, if you could magically teleport to another galaxy a few billion lightyears away the picture would be about the same: your own galaxy and those nearby don't go flying apart, but all the distant galaxies do.

I posit inflation is simply our perception of time dilation at scale.

How do you know and how can you be so sure?

It's called thinking out loud, this is still allowed, right, or should I assume criticism and hipster-like snark must be the defining marks of subject matter expertise?

The claim read like dragging a blackhole or massive object through space permanently alters it. I never said space was a rubber-band or a balloon, you did. Most people don't understand the Big Bang wasn't a point source per se either. Pick a name and run with it.

I read that whole article and all I came away with was, "Gravitational waves bend space time a little in a phenomenon called gravitational memory." I still have no idea by what mechanism though, other than guesses at some deeper super symmetry.

Can anyone dumb this down a little more for me? What holds on to the deformation? If spacetime can be deformed by a gravitational wave, then how can its original be entirely decided by the amount of and the arrangement of matter nearby. Meaning: if a wave passes by, unless it impacts the arrangement of matter in the locality, then what "holds" the deformation?

Spacetime is just 4D array: [x,y,z;t]. "Bending of spacetime" means that you need to apply a shader to this array.

If you want to talk about physics of the process, then you need to pick up a physical medium first, not an array of measurements.

I don't get in many depictions they show this planet/ball over a grid that's deforming downwards. I can understand how another mass would want to "fall" into that but is it actually that shape (has up/down) or it some kind of sphere. I also believe with regard to mass clumping, gravity is strongest near the surface of the Earth vs. inside where you could say it's equal/0 except the oblique part.

The rubber sheet analogy is criticised for this exact reason. Here’s a different visualisation that starts with what’s wrong with the rubber sheet: https://youtu.be/wrwgIjBUYVc

Oh yeah I can see the 3D sinking inwards

that's a neat video haven't seen that slicing idea before

That’s the best visualization of gravitation I’ve ever seen. Well done. I recommend it to anyone who’s been misled by the ubiquitous rubber sheet picture.

That's a great video. 7:50 is the important twist for me, and 10:25 really drives it home. I'll never forget that video now and it explains so much.

THere's still one "flaw" with this video: explaining that the grid "moves" is a little confusing. It doesn't move per-se, it .. evolves? ... over time. That's weird. I keep wanting to think the curves are static, but from t0->tn the grid pinches up. Yes, that's why they call it spacetime, but I have to stop and reset myself because how can the grid keep pinching up indefinitely but it doesn't it is just a concept. That is a stumbling block. 35 years after my last physics class...lol.

That's an amazing little video. thanks for posting. (Maybe that should go on to the main page, at some point)

Great video. Another one I like is "Why Gravity is NOT a Force" from Veritasium https://www.youtube.com/watch?v=XRr1kaXKBsU

It uses gravity force to demonstrate that gravity force is not a force. ;-)

That's clever. I'd suggest an improvement. When a sat has initial speed, show its small local reference frame, so we'd see that it always moves forward in its own reference frame, but the frame happens to be pulled to Earth.

Here's another (better, I think) attempt to explain it in plain language, from a few years ago:

https://nautil.us/issue/69/patterns/how-the-universe-remembe...

Could this prove/disprove whether there have been "prior" universes? In other words, the theory that after the heat death of the universe another universe will form... If gravitational waves permanently distort space time would that allow us to observe evidence of previous universes?

How can you change space-time? It's already got a time dimension, so you'd need another time dimension with respect to which to take the derivative. Or is the real claim here that gravitational waves distort space as part of their temporal dynamics?

We already know that spacetime has local curvature, so it’s possible that spacetimes curvature isn’t static. How this affects or interacts with time isn’t clear, since we don’t really understand what spacetime is or how it comes to be.

I think if both of you aren't aware, this talk on the concept of time orientability might be interesting exploration on this: https://yewtu.be/watch?v=Mk7vkhT0UCM

The detectors we have are just sensitive enough to detect the spin down when two objects 50+ times the size of the sun merge into a single new object. The gravity in the direction of the objects decreases permanently by the delta of the mass expended creating gravity waves, which is shift too small to measure.

I expect it may be possible in the future to confirm this, very small, persistent shift in spacetime, but not in my lifetime.

Gravitational wave detectors can only detect oscillatory signals, not static ones.

Would not be surprised if this cues some

    Expansion of the universe due to gravity lost in waves.

headlines.

But the distance scales and force magnitudes are so far out of my experience that even with the help of math I would be hard pressed to be convinced one way or another.

It's interesting how two posts recently about Newton's Method went viral here. it shows how despite how Quanta Magazine focuses on the most cutting edge of theoretical math, that applied math is still very important.

Lots of questionable assertions in this very speculative article.

"According to general relativity, every gravitational wave should leave an indelible imprint on the structure of space-time...."

All kinds of possibilities can be found in equations. The derivation of possibilities is no guarantee that they correspond with physical realities.

An earthquake leaves an undelible imprint on some matter in space-time. Different thing. The presence of stars is said to 'warp' space-time. Never saw a suggestion that the warp is permanent. Do stars orbiting a galactic center 'permanently warp' the space-time they pass through?

"How, exactly, will a passing wave distort space-time? The possibilities are literally infinite, and, puzzlingly, these possibilities are also equivalent to one another...."

Literally infinite? Colorful language, no math, no hint of why the possibilities are non-zero.

"While detecting the memory effect caused by a single gravitational wave is infeasible with current technology...."

Maybe so much speculative fiction based on no empirical evidence is no fit way to conduct science.

> Maybe so much speculative fiction based on no empirical evidence is no fit way to conduct science.

Well, if they can make predictions that could in principle be measured, then LIGO or some future version may become good enough to measure them (or observe their absence). That's actually one of the ways that science is conducted.

So as I was trying to imagine this I came up with an example that I think fits, can someone confirm my thoughts?

My idea is a square piece of fabric (space). You cut a single strand of fabric somewhere, which is the origin of the wave event. This affects the whole fabric as the destruction ripples throughout, and also results is a permanent scar/distortion on the fabric as a whole.

My understanding was more that spacetime isn't perfectly elastic, so when it's distorted by a gravitational wave it stores the equivalent of a gravitational charge - a bit like a piezoelectric crystal that stays very slightly permanently polarised after you stop squeezing it.

This would be amazing if true because spacetime would be rough on very tiny scales because of all the waves that passed through it.

Speculating wildly, this might even have observable quantum-level effects.

Offtopic. This would be a precursor for the typical generator in the future: a stone on a rope inside a stone cavity that resonates with gravity waves, converts the piezoelectric tension of spacetime into vibration, heats up as a result and boils a rusty cup of water.

Could this mean the possibility of stable wormholes that don't require any "exotic matter" to keep them from collapsing?

The article talks about the limitations of LIGO that make it impossible (currently) to detect such a small effect, but would the proposed Laser Interferometer Space Antenna[1] (LISA) probe be sensitive enough to measure this memory effect?

The proposal is to arrange three spacecrafts in an equilateral triangle with sides measuring 2.5 million km (for comparison, each "arm" of LIGO is 4 km long).

[1] https://en.wikipedia.org/wiki/Laser_Interferometer_Space_Ant...

I never understood how gravity waves can go through anything and not be weakened?

If the wave imparts energy on an object and makes it move, how is that energy still available for the next object the wave goes through?

I never understood how gravity waves propagate Mass causes gravity then what are these waves then? Is it dark matter thats not visible, not measurable and yet it has mass which causes ripple in space fabric

It's possible to move an object, and return it to its original location, without expending energy. Ocean waves do this all the time.

Based on conservation of energy, I would assume that friction can weaken a gravitational wave by turning some of its energy into heat.

Though if we're talking about permanently changing the location of an object, that theoretically requires a negligible amount of energy... reminds me of the magic drive in https://qntm.org/frontier

> Based on conservation of energy

It turns out that energy is not conserved globally in General Relativity.

https://math.ucr.edu/home/baez/physics/Relativity/GR/energy_...

> If the wave imparts energy on an object and makes it move

The objects don't move. The space between them stretches and compresses.

Now, if the gravitational wave moves through a large massive object like a star or planet, the electromagnetic forces of the "stuff" in the object will cause it to resist this stretching or compressing. I think this does cause a very very very tiny "backreaction" to the gravitational wave, weakening it slightly.

I've had that theory. Gravity wells remain for a time after the responsible mass has moved on. Maybe that explains dark matter/energy.

I like your theory, but it doesn't explain galaxies that don't seem to have any dark matter.

This is cool. Maybe it’s an explanation for mass. When you’re at rest (or at a constant velocity) you’re constantly making a dent in spacetime, that closes up eventually behind you. Go faster, the dent grows, so you need a force to push through that initial resistance of making the dent bigger.

If we step back and think about it, everything that happens changes the content of space-time; the universe does not cycle back to prior state (excluding, of course, certain concepts and time scales like going from a big bang to a big crunch).

Just like sea waves which are permanently changing(shaping) the structure of rocks they are hitting[0].

[0] https://en.wikipedia.org/wiki/Wave_pounding

I’m not sure that’s a good analogy. Spacetime isn’t necessarily pounding anything - it could be Spacetime itself that is distorted and curved by such a phenomonon. If spacetime is the medium of the waves, why are these waves hitting anything, and what?

>If spacetime is the medium of the waves, why are these waves hitting anything, and what?

"Gravitational waves are disturbances in the curvature of spacetime, generated by accelerated masses, that propagate as waves outward from their source at the speed of light." [0]

They are hitting everything what is on their way just like sea waves are.

[0] https://en.wikipedia.org/wiki/Gravitational_wave

It depends what “everything” is though. We exist in spacetime, so we are as much the medium of the wave as everything else is. What are the ‘rocks’ in the analogy? What’s ‘wearing’ like the rocks? We don’t know enough about the spacetime or what’s out of spacetime to say.

I'm not a physicist but it doesn't seem hard to understand: "Water waves, sound waves, and electromagnetic waves are able to carry energy, momentum, and angular momentum and by doing so they carry those away from the source. Gravitational waves perform the same function[0]."

So as water wave is hitting rock or any other object(mass/energy) on its way gravitational waves should hit also everything and anything on their way until they lose energy and collapse.

"Gravitational waves are constantly passing Earth; however, even the strongest have a minuscule effect and their sources are generally at a great distance. For example, the waves given off by the cataclysmic final merger of GW150914 reached Earth after travelling over a billion light-years, as a ripple in spacetime that changed the length of a 4 km LIGO* arm by a thousandth of the width of a proton, proportionally equivalent to changing the distance to the nearest star outside the Solar System by one hair's width[0]."

Gravitational waves are all around us but they have minimal impact whatsoever unless I suppose some large event(source) propagates strong gravitational waves near us.

*Laser Interferometer Gravitational-Wave Observatory

[0] https://en.wikipedia.org/wiki/Gravitational_wave

"I'm not a physicist but"... You should be. That was a nice postulation.

When we have a sound-wave it means air-pressure changes along the peaks and troughs of the wave.

When we have a gravitational wave what measurable quantity changes?

Ask a physicist but as far as I understood gravitational waves are dents in spacetime[0] and as they travel their energy, momentum, and angular momentum change over time because of interaction and collision with other "stuff" in space. Idk how you can measure any of that.

[0]https://www.esa.int/Science_Exploration/Space_Science/Gravit...

Is there a simple way to explain what a "dent in spacetime" means? How can it be measured.

I don't doubt that science and you all are right, I'm only asking questions in the hope that there is a simple explanation that would make a light-bulb go off in my head.

Gravitational waves change the definition of length and time. If you have some baseline pressure in a room, a pressure wave causes a small region to be briefly higher or lower pressure. If you have a meterstick and a stopwatch, a gravitational wave causes the meter to grow bigger or smaller and the stopwatch to run fast or slow. It's as if you are briefly closer to some immense object like a black hole which causes time dilation - indeed that's exactly what it is. No matter how ridiculously far away they are, as two celestial objects orbit each other they are continually moving closer and then further away from you, and while gravity drops off with distance it never goes to zero. You are feeling the gravitational pull of that celestial object on the other side of the universe getting infinitesimally stronger and weaker as the distance between you and it shrinks and grows, or more accurately as the distance between you and where it was all those millions of years ago when the waves were first emitted shrinks and grows.

So you're saying that measuring gravitational waves means we measure how much a given distant mass pulls us towards it and the value of that pull-force goes up and down like a wave?

Can we measure the "pull" of a specific distant mass?

Or do we measure something else like the change in distance between two points?

Well the pull of gravity is really just the warping of spacetime. A gravitational wave communicates the change in the gravitational field caused by an accelerating mass. We measure this by shooting lasers along two paths and comparing how long it takes light to traverse the paths, the warping of spacetime causes this value to change. You could use this value to directly calculate how much the pull of the distant object upon us varied. But we can only see the variation, not the absolute pull, of these distant objects; and even then only for extreme cases like black hole and neutron star pairs moments before they collide.

Good explanation thanks.

// we can only see the variation

Can we also see the direction from which the wave is coming from ?

>So as water wave is hitting rock or any other object(mass/energy) on its way gravitational waves should hit also everything and anything on their way until they lose energy and collapse.

Gravitational waves are vibrations in spacetime. They interact very weakly with matter. They travel at more or less the speed of light without 'hitting' any rocks. The question is what are the 'rocks' - since gravitational waves don't really interact with matter significantly, and we don't really understand the makeup or source of spacetime.

That was my original question to you, and the answer isn't obvious, because we don't actually know.

>Gravitational waves are vibrations in spacetime. They interact very weakly with matter. They travel at more or less the speed of light without 'hitting' any rocks. The question is what are the 'rocks' - since gravitational waves don't really interact with matter significantly, and we don't really understand the makeup or source of spacetime.

They can have big impact: "The waves can also carry off linear momentum, a possibility that has some interesting implications for astrophysics. After two supermassive black holes coalesce, emission of linear momentum can produce a "kick" with amplitude as large as 4000 km/s. This is fast enough to eject the coalesced black hole completely from its host galaxy. Even if the kick is too small to eject the black hole completely, it can remove it temporarily from the nucleus of the galaxy, after which it will oscillate about the center, eventually coming to rest. A kicked black hole can also carry a star cluster with it, forming a hyper-compact stellar system. Or it may carry gas, allowing the recoiling black hole to appear temporarily as a "naked quasar"[1].

And idk what do you mean by "source of spacetime" you mean quantum origin[2]? https://knowablemagazine.org/article/physical-world/2019/qua...

[1] https://en.wikipedia.org/wiki/Gravitational_wave

[2] https://knowablemagazine.org/article/physical-world/2019/qua...

Correct, it's all just spacetime. This is what Einstein was after in the last years of his life - a unified geometric theory, where mass was not separate from the sheet, but rather, part of it - a deformation of it.

[1] https://www.arxiv.org/pdf/0706.0190v2

Water waves can crash into eachother. There's no reason that spacetime waves cannot either. You can get some hint of this if you look at the oscillations that comprise the Higgs field.

The negative curvature of gravitation should be cancelled out by the positive curvature of matter and energy.

https://en.wikipedia.org/wiki/Zero-energy_universe

We, and everything else, are stuff floating in the sea of spacetime. Just as a ship would bob up and down as an ocean wave passed along but is not itself part of the wave, so too are we affected by gravitational waves as the pass through spacetime. The difference is instead of up and down motion, the distance between two points in space and time expands and contracts.

>We, and everything else, are stuff floating in the sea of spacetime. Just as a ship would bob up and down as an ocean wave passed along but is not itself part of the wave, so too are we affected by gravitational waves as the pass through spacetime.

Exactly it's called Frame-dragging[0]: "Frame-dragging is an effect on spacetime, predicted by Albert Einstein's general theory of relativity, that is due to non-static stationary distributions of mass–energy. A stationary field is one that is in a steady state, but the masses causing that field may be non-static -rotating, for instance."

[0] https://en.wikipedia.org/wiki/Frame-dragging

I'll make a guess that matter is like bubbles in spacetime. For some reason the bubbles remain stable and due to cleverness of equations governing spacetime, it allows to form stable complex structures around those bubbles. This is even a verifyable hypothesis: check what happens with GR equations in topologies with tiny spherical holes.

Bubbles + thermal noise. Noise causes vibration, vibration makes bubbles stable. Vibration also causes bubbles to stick together due to Casimir effect.

I guess the question is what is spacetime made of, which is the big problem in physics right now. Whether it's loop quantum gravity, string theory, or one of the many other theories that exist, the question of what is spacetime is what people are trying to solve to figure out quantum gravity.

What is spacetime made of? Nobody knows yet.

More like the sea itself doesn't go completely flat after the wave passes. If the water is space, a rock is more like a break in space.

> But a gravitational wave has a longer reach than the force of gravity.

That can't be right - surely the gravitational field permeates the whole of space; and surely the wave and the force are the same thing?

> the wave and the force are the same thing?

Nope. This is a common misconception.

The difference between gravitational waves and gravity is a little bit like the difference between electromagnetic waves (emitted by an accelerating charge) and the electrostatic force (caused by a net charge at rest).

It's not possible to detect the effect of a charge far away, but easy to detect the propagating electromagnetic wave (light, ratio, etc).

Thanks - I can see the comparison with electromagnetism.

I'm no expert in the field, but if that's true, the space-time fabric must be quite irregular, and will become more so in the future.

So were talking about magnetic monopoles except with gravity this time?

Can we make memristors with this info?

What are the implications of this?

mmm haven't seen Apophysis fractal art in a long time!

Cue "Three Body Problem" trilogy references

Gravitational waves in the series are only mentioned briefly, as a telecommunication medium. IMO communicating with gravity waves is excessive, and they're not faster than electromagnetic radiation.

Obviously not the graviational waves, but I was also thinking abouth the warp bubbles used in FTL travelling (and its abuse) distorting space time forever

But all space travel in "The Three Body Problem" is subluminical? I can't even recall a single Liu Cixin story with FTL space travel.

The closest situation I can think of is ST TNG "Force of Nature", where warp travel was damaging subspace.

Damn! True, sorry about that!

This is clear evidence, these are angels, be not afraid.

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