Nice. A related result from the neutron star collision is that the speed of light and the speed of gravity are the same, to within something like 1 part in 10^15. Both light and gravity waves from an event 130 million light years away came in less than 2 seconds apart.
How do we know that the 2 types of waves came 2 seconds apart? Didn't we have to hunt for the light source after the gravitational wave detection, which must have taken some time? Or were there cameras already trained on this part of the sky?
Edit: Ok, read the linked article, and the explanation is the detection of gamma radiation, which is part of the electromagnetic spectrum 1.7 seconds after the gravitational waves. Not sure which satellite sensor picked them up, but I assume gamma ray observatories are not as directional as light telescopes, since there isn't a huge background of gamma emissions.
The instrument in question is the gamma ray burst monitor (GBM) on the Fermi gammay ray telescope. The GBM does provide some directional data but because it's an all-sky instrument it is fairly crude but not terrifically so, it's able to determine the direction of a signal to within several degrees.
I had a physics professor that ran a Gamma Ray observatory in a high altitude balloon. (He accidentally spotted a few nuclear powered satellites in the process!) The way it worked is that instead of having a lens/mirror and an aperture pointed at a specific part of the sky it had a cube of sensors. Based on which sensors are triggered you can determine direction because high energy Gamma rays cause a cascade of different radiation when they hit a target. (Compton Scattering I think...) The downside of this method is the angular resolution is super low.
Some Gamma ray telescopes are directional like Swift, but yeah, these Gamma Ray Bursts can be detected by non-directional observatories.
Thank you, very interesting. Were those nuclear satellites known ones, as in this article: https://www.businessinsider.com/nuclear-powered-satellites-s...?
And were the Gamma ray sensors behind a collimator (basically lead plates with through-holes that only allow a very narrow angle of rays to be detected, e.g. used in Gamma cameras for nuclear medicine). Or were they in a 3D arrangement, and the timing of detection gave the direction? You said cube, so I assume it's the latter, but wanted to be sure.
He didn't mention which satellites they were, I think he was under the impression they were Russian military satellites. Unfortunately this was all during my lower division courses so I didn't get a chance to work with him. When I was doing my senior project with a different professor we used different types of detectors for gamma ray spectroscopy. The two we had were these little sodium iodide detectors and a larger liquid nitrogen cooled gallium arsenide based detector. Pretty much just chunks of metal or salt that caused Compton scattering and were detected by photon multipliers or single photon counting modules. They often had multiple channels and were not very sensitive.
It's been six years so my memory is kinda fuzzy. >_<
Is there nothing that will slow gravity down? Does that mean that gravity is essentially faster than light? I mean not technically, but for all practical cases it would be slightly faster?
Regarding your first question, the only way we could even determine if it a meaningful question to ask is if we demonstrated conclusively if gravitons exist or not, which would require a quantum theory of gravity that we currently just don't have. Great question though.
Regarding your second question, yes. And as we increase our number of gravity wave followed shortly by EM wave detections, I'm willing to bet that the difference between the two will become a very useful tool in the cosmic distance ladder toolbox.
Electromagnetic radiation is never actually slowed down, but the effective speed of light in a medium is more a function of a statistical averaging effect as some radiation is absorbed and re-emitted. So a wave packet that might be very narrow, representing a distant pulse, will tend to spread out as it travels through a medium. The net effect will be that the maximum of the impulse will happen further after the (undetectable) leading edge of the packet, leading to the apparent slow-down of electromagnetic radiation.
For gravitational waves the same effect will happen, as the waves pass through matter with its own stress-energy that will absorb some of the incoming radiation and re-emit it. But gravitation interaction with matter is so much weaker that it would take a significant amount of concentrated stress-energy or order to have the same effect. Sparse matter in interstellar space is too diffuse to experience tidal effects even at the small magnitudes of gravitational waves.
Gravity follows the space just as light does nothing slows it down other than space curvature which doesn’t technically slows it down just can increase its path.
Gravity cannot travel faster than light in vacuum traveling faster than light in a medium is possible and happens all the time in nuclear reactors for example.
Since interstellar space isn’t empty light can lag slightly behind a gravitational wave but it does not affect the ultimate speed limit of the universe.
The neutron stars were orbiting each other very fast to make the gravity waves. After they collided the gravity waves stopped and light waves were produced.
I would suspect that the light was affected by a large well on the way. If gravitational waves behave like normal waves then a gravity well shouldn't delay them much but the light would get affected and might change course a bit... enough to delay it for 2 seconds.
The fact that the event was 130 million light years away makes me imagine this chaotic, turbulent network of gravitational waves just ripping through pockets of interstellar space...
I wonder how close you would need to be to "feel" the gravitational wave pass through you. Imagine being in the epicenter of a cluster of them.
I think 0.1G of acceleration difference would be felt as center of inertia shifts from center of mass. Presumably while you're moving. Even more so if weightless.
If the wave is long enough or small amplitude enough you wouldn't feel it.
For something like the neutron star collision, it would be relatively far but not far enough to not get fried by EM radiation.
I don't get this. Shouldn't the speed of gravity be instant? If it has the same speed as light, the earth would gravitate towards the location of the sun roughly eight minutes ago (like we see the sun basically where it was eight minutes ago) in stead of where it actually is, and thus not hold its orbit.
Looking at just the Earth-Sun interaction, Earth is moving in a gravitational field that is unchanging. It's not that Earth is being pulled towards where the sun was eight minutes ago, but rather that it continues to move in free-fall in a region of space-time that is curved in a time-invariant manner.
If the sun were to vanish completely, then an observer above the elliptic of the solar system would see Earth transition from an apparently curved orbit to a straight-line path approximately 8 minutes after the sun's vanishing.
Thank you for your answer. I've heard the "it's an unchanging gravitational field" argument before, but it doesn't really explain the question for me. If the suns gravity is a field, then why is the movement of that field (which is locked to the suns mass I presume) instantaneously "known" to the earth, so the earth "free falls" in the curved space time in the right direction holding its orbit. Why doesn't the earth, at 1 au, free fall into the space time curvature that was created by the sun 8 minutes ago? It certainly appears the earth "knows" where the sun is now, which would imply instant information travel.
I also think you contradict yourself in your last statement, in which you say that the gravitational field of the sun does take 8 minutes to reach earth.
“Instant” is not a concept that makes sense with relativity. One observer’s “instant” is another observer’s “back in time.”
I’m not totally sure why this doesn’t result in spiraling into the sun as you describe, but I think it’s because the sun’s gravitational field is basically constant, and it’s changes which propagate at the speed of light.
Why wouldn't it? It's not obvious to me one way or the other: it's a small perturbation, over billions of years, of the same period as the period of the orbit itself. It's plausible that it might be unstable.
Remember that Newtonian concepts of gravity such as the equation g = Gm1m2/r^2 are only strictly valid within the Newtonian framework which, among other things, does assume that the effect of gravity is instantaneous. To derive accurate equations of motion under finite propagation speed of gravity, you need the machinery of general relativity. Of course, in mundane situations like the Earth-Sun system the two models give nigh identical results.
Thank you for your answer. True, Newton basically uses "instant gravity", and his equations really do work. I don't know enough about general relativity I must say, I've just never been able to really get it.
His equations work in the limit of slow speeds and low masses; GR keeps giving valid predictions in cases Newtonian mechanics breaks down. Famously, Newton cannot explain the slight observed precession of Mercury’s orbit which is something GR predicts precisely. But the effect is tiny and in almost all cases you don’t need a full relativistic treatment to predict the motions of Solar System bodies sufficiently accurately.
One quite mundane situation where one does need GR is compensating for the clock drift of GPS satellites caused by their height and speed relative to a ground-based observer (time is relative, not absolute as Newton thought!) Because positioning critically relies on very precise clocks, a non-adjusted satellite positioning system would very quickly start giving completely inaccurate results.
I'll let someone else answer your question, but if you enjoy thinking about these things, you might enjoy this vsauce video about the "speed of dark": https://www.youtube.com/watch?v=JTvcpdfGUtQ
I don't think that time shifting the gravity vector has a measurable effect on the orbit. Indeed, the vector is 1au*c late in real life, but we didn't know that for sure until the recent neutron star collision observation.
As you're always working with a fixed temporal distance, wouldn't that basically negate any instability? There'd never be a gravitational "correction", as that 8 minute offset would never shift.
Gravitational waves aren't how gravity propagates, however. They are really only relevant in exotic situations like mergers of massive compact objects, where spacetime is warped enough that orbits aren't stable and the bodies spiral inwards, radiating their excess gravitational potential energy in the form of gravitational waves.
In general relativity gravity is curvature of spacetime caused by mass-energy. Changes in curvature propagate at c, the speed of causality. Gravitational waves are a special case of those changes predicted by GR, inspiraling massive objects making the spacetime around them ”ring” like a bell.
An eventual theory of quantum gravity may entail another viewpoint of gravity as a quantum field and mediated by a hypothetical graviton. But as of now we don’t have a good idea of what such a theory might look like; unifying QM and GR is famously one of the great unsolved problems in physics.
"In general relativity gravity is curvature of spacetime caused by mass-energy. Changes in curvature propagate at c, the speed of causality."
I really hope you meant speed of light there.
Regardless, this thing is still a mystery to me. Why, if gravity propagates at c (or, better perhaps: Changes in the space-time curvature, which is caused by mass-energy, propagate at c), why doesn't the earth spiral into the sun or away from it? If the earth is attracted to the point where the sun was eight minutes ago and not to where it is right now, it's orbit should not be stable. It's a very big difference compared to the "gravity is instant" way of Newton.
I used the term "speed of causality" on purpose; that's what c is really about. The fact that the speed of light in vacuum happens to be c is just a side effect of photons being massless.
Thank you again for your of answering my questions. So.. well.. What is this speed of causality then? Can we put a number on it?
Besides this all, I still can't get anyone to give me a clear cut and dried answer on my original question. If gravity is instant in the Newtonian model, and that gives us stable orbits and all that, how does that translate to the Einsteinian model, where there's an upper speed limit of c? So, why would that even work, if the gravity of the sun takes 1au/c to get here and be felt by the earth? Or, is gravity somehow "above" this speed limit?
> The gravitational waves from the collision reverberated in LIGO the morning of Aug. 17, 2017, followed by detections of gamma-rays, X-rays, radio waves, and optical and infrared light. If gravity were leaking into other dimensions along the way, then the signal they measured in the gravitational wave detectors would have been weaker than expected. But it wasn't.
Do they really mean to say "followed by detections"? Don't gravitational waves propagate at c? Also, I'm puzzled by "weaker than expected". One issue is calculating expected intensities for gravitational waves vs electromagnetic radiation. The other is calibration of LIGO signals. It's not obvious that resulting uncertainties are small enough for drawing reliable conclusions about gravity leakage.
My understanding is that the detection order follows the underlying phenomena: first the neutron stars fell towards each other, producing gravitational waves, and only then did the stars collide, producing electromagnetic waves.
As I understand it gravitational waves aren't slowed down by anything whereas electromagnetic waves interact with matter along the way, which takes some amount of time.
There isn't much matter to interact with, but not quite zero either. Something like 1e-23 of the distance had some amount of matter in it (mostly very diffuse gas clouds, nothing more).
Yes. Even if the creation events of each part of the spectrum are at the exact same time (which is unlikely) there is going to be matter in between the event and us. That matter will slow the rate of light's propagation.
Good question and I don't know the answer, but this unknown still helps to define the limits of what we can definitely say is and isn't taking place.
We can't say "we're definitely not experiencing displacement in X dimensions" if we don't definitively know the rules of displacement are homogeneous in every dimension. Since we don't definitively know the number of dimensions, we also can't exhaustively test these rules.
We don't ever definitely know anything. That's why science and bayesian reasoning in general is so useful to begin with ;)
We can be more or less confident about how stuff works though. For instance, thermodynamics is something we are very confident of having gotten right, because there are plenty of ways to test if doesn't work, and those test fail all the time (so far, at least!).
So this article basically says: would it be easier to explain what we see if there were other dimensions? and the answer is it wouldn't, because we'd have to assume that things that we are very confident about (thermodynamics) are not correct, while simultaneously not providing any way to gain confidence in this new hypothesis (that there are new dimensions with different thermodynamic laws).
This might change some day: before spectrography was a thing, all the theories about star composition were non-falsiable either!!
I'm confused over the size of these extra dimensions... They say that they do not rule out small compact extra dimensions, but only large ones, the number they give is ~100 km. But this must surely be ruled out by ordinary gravitational experiments that establish the 1/r^2 law for gravity, since I would assume that that law would be seriously broken for extra dimensions of those sizes for even everyday physics.
The number that is quoted in some textbooks on String theory (but I do not have the refs.) is that gravity is 1/r^2 down to sizes of ~1 cm for extra dimensions.
Gravitational waves are the best litmus test we have for gravity on a large scale which is why they put to rest most of the modified gravity theories.
Newtonian gravity on large scales wasn’t “proven” explicitly in fact we have had to introduce dark matter to make it work to match observations, there is also the issue of empty space having weight which is attributed to various favtors depending on the theory in question.
The discussion about how many dimensions "really" exist is pointless because there is no other way to define "real" than to define it as the reality that I experience. And I experience 3 dimensions.
Physics is a collection of models expressed as formulas, plus recipes of how to translate these formulas to the reality that we experience.
If I have a formula with 5 dimensions in it that tells me how to build a teleporter then that's great but it doesn't change the fact that the world that I experience is 3 dimensional in space.
Holography attempts to explain 3D in terms of 2D, but that doesn't make 3D go away. People are still around after it was found out that we were made of cells.
It is a strike against the idea that over long distances, the spherical shell of "expanding and thinning" gravity can go down faster than 1/r^2 (which would come about from A=pi*r^2, the area of a sphere in three dimensions), achieving that goal by slipping in to dimensions higher than three. However now we have found that gravitational waves are about as strong as we expect it looks like gravity isn't leaving the world of 3+1D.
Sphere's have surface area and volume. Neither of those quantities is given by the formula for the area of a circle. I’m not a physicist and can’t speak to the merits of the rest of your post but this error of yours makes me doubt your post's veracity. Did you make a mistake in your parenthetical remark? Maybe it edit if you have.
A sphere has surface area and volume. I didn’t claim or imply that I thought area meant volume. It does not have area and the formula for the surface area of a sphere is not pi r^2. I mentioned this in my original post. Then there is the remark that whatshisface made: A = pi r^2, the area of a sphere in three dimensions. That is wrong and most likely a mistake.
When talking about dropping a constant factor are you referring to the 1/r^2 part or dropping a 4 in the statement A=pi r^2. Because if the latter then why keep pi? That’s a constant.
The constants perfectly divide out in this case, adding the pi was an afterthought so that people would recognize it as an area. Yes... It should have had a 4 as well. Too late to edit now.
the title of the article implies that there is new evidence for the existence of multiple dimensions, while the content of the article is about the opposite being the case
Hard to see how. "Dose of reality" usually means evidence to the contrary. Even if there was some ambiguity, the first line of the article as well as the caption of the opening image make it clear.
It's probably more cryptic than ambiguous, due to the decorative language. A lengthy terms and conditions is not ambiguous, yet it's very vexatious. This is neither ambiguous nor vexatious. Perhaps though, it's a little bit cryptic. A simple man must try a little harder. Spock would probably say the statement is intrinsically ambiguous due to the metaphor, however.
