[tzs]

When do gravitational waves actually arrive from distant objects relative to light from those objects?

Generally the space between us and distant objects isn't actually a perfect vacuum. It should have an index of refraction greater than 1, and it should vary by frequency. Light from a distant object should arrive here spread out in time by frequency, and the earliest should arrive a little later than something moving at the speed of light would arrive.

Is there something like the index of refraction for gravity waves? If not then we should see gravity waves from an event before we see any light from the event. If there is, then it should be possible for gravity waves to arrive before, at the same time, or after light from the same event depending on the frequency of the gravity wave and the light.

[AnotherGoodName]

Every experiment so far has detected gravitational waves a tiny bit before they detected light based evidence. Consistent with the light being slowed by the refraction in that very small amount of matter that exists in the interstellar medium and gravity passing through that dust more or less unaffected.

Of course going faster than light which is being slowed by absorption and re-emission isn't the same as breaking the speed of light since light itself is going slower than the speed of light in this case.

So yes you're right that it isn't exactly the same arrival time but we're not talking about curvature differences here, we're talking about physical interactions that the light undergoes that gravity doesn't.

[piyh]

We need to replace faster than light with faster than gravity

[spsoto]

I don’t have a physics background but I’ve always seen “c” as the speed of causality. The light happens to go at that speed in the absence of gravitational disturbances. Gravity and others fields should also move at this maximum speed.

That said, I’m still trying to come to terms with the fact that breaking this speed limit just means that causality would be potentially broken. Isn’t that just something we axiomatically believed based on experience and we just haven’t observed otherwise?

[Twisol]

My (mostly layperson's) understanding is that our laws of physics demand that causality would be broken; it's not taken as an axiom.

Because of how the three dimensions of space and one dimension of time are put together, you can think of there being a balance or trade between motion in space and motion in time. If you aren't moving in space, you're moving through time at the maximum possible "rate". The more rapidly you move through space, the slower you move through time. This trade bottoms out at "c", at which point you're not moving through time at all. (Since motion is impossible without time passing, "c" itself is unachievable; you can only approach it asymptotically. Something about massless particles makes "motion" not a thing in the first place, I think, meaning they can actually propagate at exactly "c" as seen by an observer.)

You can visualize this as a dial on an X-Y graph which starts out pointing in the Y direction, and as you speed up, it turns more toward the X direction. When you're pointing completely in the X direction, you're moving "at the speed of light", purely in space and not at all through time. If you turn the dial even further, you're trading some of that speed back for motion in time... but in the opposite direction.

Of course, this is all super-handwavey; most importantly, velocity has to be measured relative to an observer, so all of this about rates has to be anchored relative to an observer. (But this is also precisely why massless particles propagate at the same rate regardless of observer -- insert timey-wimey Doctor Who reference.)

Greg Egan has a lovely trilogy, Orthogonal, set in a universe where space and time don't have this trade (formally, the sign on the time variable in some critical equation is flipped to match the spatial dimensions). He has some great material on the exact physics of such a world. [0]

[0] https://www.gregegan.net/ORTHOGONAL/00/PM.html

[evanb]

Light can take any speed---when it is traveling through a medium. Capital-T 'The' speed of light is the speed of light through a vacuum, and equal to 'The' speed of gravity.

[tsegratis]

May I ask since gravity obeys curvature like light, why do we see gravity from black holes, when it is the curvature that stops us seeing light?

Maybe the gravity emanates from outside the event horizon, but then why would it pull us inside?

Thanks

[AnotherGoodName]

>We study static, spherically symmetric black hole solutions of the Einstein equations with a positive cosmological constant and a conformally coupled self interacting scalar field. Exact solutions for this model found by Mart´ınez, Troncoso, and Zanelli, (MTZ)

>The final conclusion of our analysis is that there appear to be no physically acceptable stable solutions of the MTZ system

https://arxiv.org/pdf/0710.1735.pdf

Basically it's a huge hole in black hole theory right now. It should be made clear though that both gravity is self interacting and black holes do exist. It's just when you get down to specifics it's a case of "we don't know how to make this work".

[tsegratis]

Thanks for the reply

I assume punching a hole in spacetime, punches an equivalent hole in maths aswell

Let me ask a few simpler questions first, my main question is at the end

This punched hole might be like measuring angles with a differential. When the difference between the measured points hits zero, the other end of the equation hits infinity and the angle becomes meaningless

So would a true vertical curvature in spacetime equivalently require an infinite amount of mass?

They say that at the event horizon the deformation is so strong that from a black hole all paths lead inwards. But isn't gravity commutative? A.k.a. coming from inside, vertical curvature is reached. But if the curvature is vertical, then presumably there is also no way into a black hole?

---

So main question; could we just say that vertical curvature is impossible, and black holes are simply extreeeeme curvature to the extent that a 1.7second difference between light waves and gravitational waves over 130million years is enough to stop light escaping, but not gravity?

Is that solution too simple, what am i missing?

Thanks

[AnotherGoodName]

The 1.7second difference isn't from any actual speed difference. It's from light hitting things on its way here that gravity interacts weakly with and thus doesn't hit. So that wouldn't explain no light at all escaping while 100% of gravity does from a black hole.

Instead since light is redshifted as it exits a gravity well a better thought would be "is the almost but not quite black hole red-shifting light to the point of being impossible to detect?". After all light with almost 0hz frequency is basically non-interactive. It has a similar outcome. You could then have an 'almost black hole' that looks just like a real black hole but allows gravity to escape. https://arxiv.org/abs/2102.07769

[tsegratis]

Thanks, that was the question:

> You could then have an 'almost black hole' that looks just like a real black hole but allows gravity to escape

I wondered if that answer to the conundrum could apply to all black holes. I suppose not

For real black holes, I suppose we should say they are not true singularities where the event horizon curvature goes vertical, but simply that curvature goes beyond the speed of light, so the maths still makes sense

Thanks that is a lot more logical

So then the effect of gravity from a real black hole would be like the effect of a messy person after they've left the room, and the reason why the effect of a black hole is felt for much longer is because of time dilation, and gravity doesn't experience redshift?

[raattgift]

How did you come across these two arxiv preprints? Both are far from astrophysics. One is highly speculative theoretical physics. The other shoots down previous work that was highly speculative theoretical physics.

> astro-ph/2102.07769

This is about a particular model of dark matter that unlike in the standard cosmology is hot and has a particular radial profile within galaxies and outside galaxies undergoes a phase change to a uniformly distributed cold dark matter.

Tracing the gravitational collapse consequences of a theory whose characteristic matter distribution does not concord with observation (it breaks when the radial symmetry breaks, as in galaxy-galaxy mergers, lumpy galaxy clusters, and so on) is interesting but doesn't say much about astrophysics.

The preprint itself was the basis of a workshop talk on speculative physics, and the workship was literally titled in the form of a question ("What Comes Beyond Standard Models?")

FWIW, I had never before this heard of Bled Workshops in Physics, and I still don't know (after poking around in citeseer and the like) whether it is an event in Slovenia, or just named after Bled, Slovenia.

> hep-th/0710.1735

I don't understand why this is in hep-th rather than gr-qc as it is manifestly about a semiclassical model, with a peculiar form of quantum matter used to study gravitational collapse.

The paper is essentially an obituary for an idea for a toy quantum field on a classical curved spacetime that might work better than the simplest toy quantum field that has been in use since at least Hawking's 1974 work. The original work [hep-th/0205.319] introduces this toy model containing analogue to electromagnetism, and found that they could only form black holes under certain conditions. These additional complications, your linked paper's authors argue, aren't helpful even under those certain conditions, leading to things like naked singularities away from the horizon (p.20).

The paper's central purpose is to narrow the viability of this family of toy matter; in the authors' words (p.2.), "In this work we address the following question: Are there other static, spherically symmetric black hole solutions for the MTZ model, satisfying the dominant and strong energy condition between the event and cosmological horizon, besides MTZ1 and MTZ2? Using a combination of analytical and numerical methods we conclude that the answer to this question is negative." (In the very next paragraph they point out that MTZ2 has already been shown to be unstable with the addition of spherically symmetric masses, and that they will show that MTZ1 has the same problem).

The final paragraph of p.20 is pretty damning, and declares the low-energy-string-theory MTZ idea dead. ("M" is also one of the authors of the obituary). And so this raises my second question:

Why did you link this paper?

In my view does not support your statement that there is "a huge hole in black hole theory right now", but am certainly interested on what motivated your choice of that paper in the context of the questions tsegratis asked. Neither paper seems to go anywhere near answering those questions.

[tsegratis]

https://physics.stackexchange.com/questions/937/how-does-gra...

Lots of answers at link, most amusing one is:

> The total mass of the black hole must reside, completely, and only, in the self-energy of the curvature of spacetime around the hole!

> The answer to your question, then, is this: information about the mass of a black hole doesn't have to escape from within the black hole because there is no mass inside the black hole. All the mass is distributed in the field outside the hole. Therefore, no information needs to escape from inside

It seems the general answer is that fields and particles are not the same thing, and black holes can generate fields...

Since time stops within a black hole singularity, is entering one a good tip for escaping the end of the universe?

Below is some interesting background, on how a field is static, already defined at the creation of the black hole, and particles, if they happen, are just communicating changes in the field:

> A particle is an excitation of a field, not the field itself. In QED, if you set up a static central charge, and leave it there a very long time, it sets up a field E=kqr2. No photons. When another charge enters that region, it feels that force. Now, that second charge will scatter and accelerate, and there, you will have a e−−>e−+γ reaction due to that acceleration, (classically, the waves created by having a disturbance in the EM field) but you will not have a photon exchange with the central charge, at least not until it feels the field set up by our first charge, which will happen at some later time

> Now, consider the black hole. It is a static solution of Einstein's equations, sitting there happily. When it is intruded upon by a test mass, it already has set up its field. So, when something scatters off of it, it moves along the field set up by the black hole. Now, it will accelerate, and perhaps, "radiate a graviton", but the black hole will only feel that after the test particle's radiation field enters the black hole horizon, which it may do freely. But nowhere in this process, does a particle leave the black hole horizon

[wlesieutre]

https://arxiv.org/abs/1710.05833

> On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ∼1.7 s with respect to the merger time.

So there is a delay between arrival of gravitational waves and accompanying gamma ray burst, but I couldn't tell you if that's purely because light travels slower than it would in a perfect vacuum, because the gravitational waves are generated before the gamma ray burst, or a bit of both. The GRB being less than two seconds long, I would guess they both happened at close to the same time, and it does have a speed difference.

Coming from an object 130 million light years away, 1.7 seconds is a very small difference in speed.

[gizmo686]

Depending on frequency, the vaccumum of space is close enough to a vaccum. If the light needs to travel through something opaque, you generally just don't see it (although it may illuminate the dust)

We have measured the relative speed of gravity and light. The difference is constrained to be no more that about 10^-15 times the speed of light. This us based on a signal that travelled 130 million light years.

https://en.m.wikipedia.org/wiki/GW170817