Gravitational waves provide dose of reality about extra dimensions
phys.org
phys.org
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.
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 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.
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.
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.
Aww, this is not the answer I was hoping for. :(
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.
Some Gamma ray telescopes are directional like Swift, but yeah, these Gamma Ray Bursts can be detected by non-directional observatories.
It's been six years so my memory is kinda fuzzy. >_<
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.
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.
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.
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?
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.
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.
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.
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.
Edit: And I didn't see Animats' comment that EM radiation was delayed ~2 seconds. But also that the speed difference was very small.
It's entirely possible that displacement in these dimensions corresponds to an increase in energy in the system that offsets the "lost" energy.
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 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!!
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).
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.
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.
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.
Physicists tend to drop constant factors when they do calculations, I’ve noticed.
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.
Could it also not mean that gravity is "just" a 3D (or 3+1D) phenomenon?
For example, what does gravity mean in 2D?
Can we rule out that our 3D† universe is not a rarity in a larger, 4D universe, much like how true-2D planes are a rarity in our 3D universe?
† D as in spatial dimension, excluding time.
Or is this just confirming what everyone expected all along?
https://en.wikipedia.org/wiki/Large_extra_dimension
It doesn't say anything about compact extra dimensions (the tiny curled up ones you usually hear about in string theory).