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tionis
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What would happen if I were to fly toward a gravitational wave pulse at relativistic speed? Would I be destroyed by the Doppler-shifted pulse? Would the wave become visible?
tionis said:What would happen if I were to fly toward a gravitational wave pulse at relativistic speed? Would I be destroyed by the Doppler-shifted pulse? Would the wave become visible?
tionis said:or perhaps even boost them depending of where the GW pulse came from?
The GW approaches you at c, just like the light from the distant Galaxies. So before the GW reaches you, you also won't see any light that met it.tionis said:Second, would there be any visible effects that would betray the presence of an approaching GW pulse such as the lensing of background stars or galaxies while approaching the GW pulse at close to c?
A.T. said:The GW approaches you at c, just like the light from the distant Galaxies. So before the GW reaches you, you also won't see any light that met it.
You can user other detectors than your eyes. And stars emit frequencies below the visible range, which would be shifted into that range.tionis said:I've forgotten that I couldn't see any stars or light because it would have been Doppler-shifted to the invisible part of the spectrum as I approached c.
A.T. said:You can user other detectors than your eyes. And stars emit frequencies below the visible range, which would be shifted into that range.
No. There's only one spacetime in the universe; in some regions it is highly curved and in other regions it is less highly curved. Solutions like the Kerr and Minkowski spacetimes may be more or less good approximations to the local curvature in a particular region, but neither comes even remotely close to describing the curvature in the local neighborhood of two colliding black holes.tionis said:IOW, would it be correct to say that the recently detected GWs occurred in Kerr spacetime (curved) and were detected in Minkoswki (flat) space?
pervect said:Can we calculate this classically, or do we need a theory of quantum gravity?
tionis said:GWs are spacetime in motion
tionis said:Why doesn't this particular propagating spacetime metric have an assigned name to it?
PeterDonis said:It does; it's called a gravitational wave.
tionis said:I don't quite understand when you say spacetime doesn't move, but has ripples in it. What is the difference?
tionis said:these ripples move at c
tionis said:it appears that we treat time, when measuring GWs pulses, as an independent parameter, don't we?
tionis said:Is it correct to say that what we detected was a p-p wave spacetime?
tionis said:Are physicists going to define GWs by the spacetime they originate in?
tionis said:would you please address my post #15
PeterDonis said:But saying that the 4-dimensional geometry itself "moves" doesn't even make sense. Where would it move to?
PeterDonis said:If we consider the "ripples" as things moving within spacetime, yes, they move at c. But there is another way of viewing them: they are just part of the 4-dimensional geometry of spacetime, and the crests and troughs of the ripples lie along particular kinds of curves (null curves) in that 4-dimensional geometry.
PeterDonis said:No. "Time" is one of the 4 dimensions; when we say something "moves", we are being sloppy. What we should say, to be perfectly precise, is that the something we are interested in is described by a curve, or a family of curves, in the 4-dimensional geometry. "Time" and "space" are just ways of labeling the points on the curves.
PeterDonis said:No. We didn't detect a "spacetime". We detected a particular piece of the geometry of the spacetime we are already in.
PeterDonis said:GWs don't "originate in a spacetime". "Spacetime" means the entire 4-dimensional geometry of the universe--not just the universe at a single instant, but the entire history of the universe. There aren't different spacetimes for different parts of the universe; there is just one spacetime, the entire universe.
PeterDonis said:(Not all 4-dimensional geometries have ripples in them; but a spacetime that contains gravitational waves does.)
PeterDonis said:I think pervect is correct that the GWs would still have the same amplitude, regardless of how you were moving when they passed you. That means that, in general, their effect would still be extremely tiny; you would need extremely sensitive instruments, like LIGO, to detect them. The only difference would be their frequency and wavelength.
I think it is possible, as pervect speculated, that if you were moving relative to the GWs in such a way that their frequency happened to be at a resonance, for example some kind of resonant frequency for atoms, then their effect could be larger. However, I don't know how we could determine what those resonant frequencies would be. They wouldn't be the same as the frequencies of light that are emitted and absorbed by atoms, because those frequencies are determined by the electromagnetic interaction between the electrons and the nucleus, not by anything involving gravity.
tionis said:What about dark energy? Where is the Universe going in such a hurry?
tionis said:It looks as if you are describing a completely different spacetime.
tionis said:so these ''null curves'' are not moving?
tionis said:this family of curves don't move?
tionis said:That particular piece of geometry got from there to here
tionis said:I think I'm completely over my head on this spacetime thing
PeterDonis said:Only after you have a good understanding of how all this works in SR would I recommend trying to understand it in the context of GR.
PeterDonis said:The geometry of spacetime does not "change"; it is a 4-dimensional geometry that just "is". But that 4-dimensional geometry can certainly consist of wavelike "ripples" whose amplitude decreases as you move in spacetime from the vacuum region between the source and a detector, through the region of spacetime occupied by the detector, to the vacuum region beyond the detector. Correspondingly, the matter and energy distribution in the region of spacetime occupied by the detector is different "upstream" of the region where the GW passes through, vs. "downstream" of that region. Geometrically, this is just a particular geometry that is a perfectly good solution of the Einstein Field Equations--the EFE is what makes the connection between the spacetime geometry and the distribution of matter and energy. To us, located near the detector, it would look like a GW passing, being detected by the detector, and giving up some energy to it in the process. Now consider an alternative geometry, where the GW amplitude did not change at all from the vacuum region before the detector, through the detector, to the vacuum region after the detector. The point I'm making is that, in this case, there would be nothing in the detector region that corresponded to "detecting a GW"; the distribution of matter and energy in the region of spacetime occupied by the detector itself would be exactly what it would have been if no GW had passed through. This is why I said that the only way for a detector to detect a GW is for it to take some energy from it--if there is no exchange of energy, there is nothing in the matter and energy distribution of the detector that is affected by the spacetime geometry of the GW, so no GW is detected.
Reference https://www.physicsforums.com/threads/time-reversed-gw-emission.859974/
tionis said:Would it be correct to say that a GW is a change of the configuration of the energy-matter in a particular portion of spacetime without nothing ever traveling in between?
tionis said:what would be the correct, rigorous, non-mathematical way of describing what a GW is?
PeterDonis said:No, for two reasons. First, GWs are spacetime curvature; they're not "made of" energy-matter. Second, the "nothing ever traveling in between" makes it seem like you're describing a violation of energy conservation. GWs can't violate energy conservation any more than anything else can.
PeterDonis said:I don't think there is a rigorous, non-mathematical way of describing anything in physics. If you want the rigor, you need the math. That's why physicists use math to actually do physics (as opposed to describing it to non-physicists). The best short non-mathematical description of GWs is pretty much what I've already said: they are waves of spacetime curvature. But that isn't really rigorous.
tionis said:if the ripples of GWs carry the conserved energy of the source: couldn't that energy be converted into matter as per E=mc^2?
tionis said:I think I already asked if I would observe Hawking/Unruh radiation a few posts back
tionis said:I'm still nonplussed by how ''The geometry of spacetime does not "change"; it is a 4-dimensional geometry that just "is".
PeterDonis said:GWs that pass through matter can certainly deposit some of their energy in that matter, which would count as converting the GW energy to matter. I don't think, however, that it's possible for GW energy to be converted into matter in an empty region of space where there is no matter already. But I don't know for sure; I haven't seen any proof one way or the other.
I don't think so. Hawking radiation comes from a black hole horizon, which is a very different piece of spacetime geometry from a GW. Unruh radiation is observed by someone who has a very large proper acceleration; that has nothing to do with any particular spacetime geometry, it happens even in flat spacetime.
PeterDonis said:[Because, without realizing it, you are still thinking of "time" as something outside spacetime, instead of just as one of the dimensions within spacetime.
lbix said:OP: Here's a light cone: https://en.m.wikipedia.org/wiki/File:World_line.svg
Ignore the lower cone - it's not relevant here. The point of the upper cone is the two black holes colliding. As time goes on the gravitational waves spread out from that point, making ever larger circles - which form a cone. Think of a video of ripples on a pond. Then print each frame of the video and stack them on top of each other.
You are thinking of the video of the circles getting bigger. Peter is (approximately!) thinking of the frames stacked on top of each other making a cone, which doesn't change.
How literally you take either view is up to you. Either is a reasonable interpretation of the maths, but Peter's is one of the easiest ways to visualise it as a whole.
tionis said:OK, so the GWs give the matter they interact with kinetic energy and relativistic mass? Which one is it, or is it both?
tionis said:so the GWs give the matter they interact with kinetic energy and relativistic mass? Which one is it, or is it both?
tionis said:isn't my relativistic speed enough to make GWs make pairs of matter/antimmater
tionis said:I also think that you are divorcing spacetime from gravitational ripples somehow. I understand this is do to my ignorance, but I can't shake that feeling.
tionis said:When the black holes merge, they send ripples through the Jello in all directions, but the whole of spacetime doesn't change, meaning the block remains the same but with vibrations running through it which eventually reach the Earth at a far corner and go on to infinity. What is wrong with this analogy?
PeterDonis said:GWs don't move through spacetime; they are just part of the geometry of spacetime