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Old 04-09-2019, 09:37 AM
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Update on the speed of gravity


My staff report from 2003 on the speed of gravity is now on the front page again, but there are a few important updates to it. In particular, I mention LIGO and LISA, two instruments which might eventually detect gravitational waves. LISA, unfortunately, has been canceled, and if something like it ever eventually launches, we have no idea when it would be. But LIGO has, in the past few years, made multiple unambiguous detections of gravitational waves, from merging black holes. These detections included at least one where an optical counterpart was also detected: In other words, we "heard" gravitational waves from a far-distant event, and saw light from that same event, at the same time, thus directly demonstrating that the light and gravitational waves were moving at the same speed.
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Old 04-09-2019, 09:41 AM
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I was going to suggest this update but you beat me to it.


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Old 04-09-2019, 09:56 AM
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In other words, we "heard" gravitational waves from a far-distant event, and saw light from that same event, at the same time, thus directly demonstrating that the light and gravitational waves were moving at the same speed.
Does this give us a more precise measurement of the gravitational constant G? Or is most of the imprecision coming from the mass measurements?
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Old 04-09-2019, 10:39 AM
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Dumb question time, but what are gravitational waves "made" of? Are they EM waves? And what is the frequency or wavelength of the waves?
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Old 04-09-2019, 10:52 AM
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Pleonast, any masses determined from any of these observations are based on knowing Newton's constant to begin with, and there's no way to determine the masses independently of Newton's constant. Which is irrelevant, anyway, since although Newton's constant is the least-precisely known of the fundamental constants, we do still know it to four decimal places. But all of the masses in any of these events are known only to within tens of percents.

Crafter_Man, gravitational waves are most assuredly not EM waves. To the extent that it's meaningful to ask what they're "made of", the best answer that can be given is that they're made of spacetime itself. Or you could say that they're made of streams of gravitons in the same way that light is made of streams of photons, but our knowledge of quantum gravity is so rudimentary (basically nonexistent) that that description is not at all useful. I'm speaking literally when I say that we'll probably never detect an individual graviton.

Since gravitational waves have the same speed as light waves, they also have the same relationship between wavelength and frequency. But the typical frequencies of gravitational waves are much, much lower (or, equivalently, the wavelengths are much, much longer) than for EM waves: In principle, just like with light, there's no limit on either end, but we don't know of any source in the Universe that would produce gravitational waves above a few kilohertz (corresponding to wavelengths of hundreds of kilometers).
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Old 04-09-2019, 10:59 AM
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I'm speaking literally when I say that we'll probably never detect an individual graviton.
What would a hypothetical graviton detector even look like?

Is it possible to draw an analogy to low frequency radio waves? I mean, at some frequency it becomes difficult enough to detect and/or emit continuous waves. Single photons would be an even taller order. Yes?
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Old 04-09-2019, 11:13 AM
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That's part of the problem: With the relationship between frequency and the energy of individual particles, extremely low-frequency waves have an extremely low energy per particle. The other part is that gravity is a much weaker force than electromagnetism, which means that any gravitational phenomenon is much harder to detect than its electromagnetic equivalent, regardless of energy.
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Old 04-09-2019, 08:04 PM
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What would a hypothetical graviton detector even look like?

Is it possible to draw an analogy to low frequency radio waves? I mean, at some frequency it becomes difficult enough to detect and/or emit continuous waves. Single photons would be an even taller order. Yes?
You can, in the classical experiments, block light so that you are getting, at most, one photon every day. Which you can detect.

I think that apart from the difficulty of detecting a single 'graviton', you'd have difficulty blocking gravity in the same way that you can block light.

(Thank you for the update Chronos. I came here to look.)

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Old 04-10-2019, 08:44 AM
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Ligo was turned back on after an extended downtime for upgrades, etc. And it's already detected another set of waves.

Gravity waves detected, ho hum, amiright?
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Old 04-10-2019, 09:02 AM
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You can, in the classical experiments, block light so that you are getting, at most, one photon every day. Which you can detect.

I think that apart from the difficulty of detecting a single 'graviton', you'd have difficulty blocking gravity in the same way that you can block light.

(Thank you for the update Chronos. I came here to look.)
For something like 10 Hz "light"? How do you do that, and has it been done?
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Old 04-10-2019, 09:16 AM
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So far as I know, the lowest frequency for which single photons have been detected is somewhere in the microwave range, but that's not my specialty.
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Old 04-10-2019, 01:43 PM
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What would a hypothetical graviton detector even look like?

Is it possible to draw an analogy to low frequency radio waves? I mean, at some frequency it becomes difficult enough to detect and/or emit continuous waves. Single photons would be an even taller order. Yes?
We can detect single photon events. The rod cells of the vertebrate eye are actually sensitive enough to detect single photons in the visible range although humans will not consciously observe it.

Gravitons, on the other hand, are too weak to plausibly be detected by any means, and we have no means to filter gravitons or expect them to appear in small quantities in nature.

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Old 04-10-2019, 04:00 PM
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Gravitons, on the other hand, are too weak to plausibly be detected by any means, and we have no means to filter gravitons or expect them to appear in small quantities in nature.
So all I need is some material that blocks out all incoming gravitons plus a source that produces a single graviton at a time and a device that measures said graviton and I'll win a Nobel Prize.

Hold my beer caffeine-free diet cola.
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Old 04-10-2019, 04:10 PM
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So all I need is some material that blocks out all incoming gravitons plus a source that produces a single graviton at a time and a device that measures said graviton and I'll win a Nobel Prize.

Hold my beer caffeine-free diet cola.
These guys from Cornell can get you started. I only skimmed, but I think it involves building a detector the mass of Jupiter in close orbit around a neutron star...while solving some other unsolved problems along the way in order to possibly get one detection a year.
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Old 04-11-2019, 03:11 AM
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Chronos, there's something that always puzzled me about this staff report.

Gravity is often illustrated in terms of a rubber sheet. Place a bowling ball in the middle of the sheet, and it deforms creating a slope. Put a marble some distance from the bowling ball, it will roll down the slope.

So, asking about the speed of gravity, isn't that like asking about the speed of slope? The slope doesn't have a speed, it's just there. Or Crafter_Man's question, what gravity waves are made of, isn't that like asking what slope is made of?

Would it make more sense to phrase the question in terms of how quickly the slope changes if you add or remove mass?

It's very likely I'm completely misunderstanding something here.
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Old 04-11-2019, 05:23 AM
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Gravity does act like a deformation of space, but it is an acceleration, not a static "slope" as such.
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Old 04-11-2019, 11:24 AM
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We can detect single photon events. The rod cells of the vertebrate eye are actually sensitive enough to detect single photons in the visible range although humans will not consciously observe it.

Gravitons, on the other hand, are too weak to plausibly be detected by any means, and we have no means to filter gravitons or expect them to appear in small quantities in nature.

Stranger
I know that we can detect single photon events and I understand that gravitons would have very low energy. I was trying to ask if we could draw an analogy to low frequency radio spectrum EM and how hard it would be to produce or detect single photons of such low, but, and I'm guessing here, higher energy than gravitons.

Chronos has come the closest to answering that question with the tidbit that the lowest energy single photons detected might be in the microwave region.

Microwaves have frequencies (and thus photon energies) millions to billions of times higher than those of the longest wavelength radio waves we can detect as waves.

I thought it possible one could use the challenges of detecting a single photon of Ultra Low Frequency EM-radiation to aid in understanding why single gravitons will never be detected, but based on my inability to convey that to people who actually understand physics that seems implausible.
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Old 04-11-2019, 11:29 AM
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Chronos, there's something that always puzzled me about this staff report.

Gravity is often illustrated in terms of a rubber sheet. Place a bowling ball in the middle of the sheet, and it deforms creating a slope. Put a marble some distance from the bowling ball, it will roll down the slope.

So, asking about the speed of gravity, isn't that like asking about the speed of slope? The slope doesn't have a speed, it's just there. Or Crafter_Man's question, what gravity waves are made of, isn't that like asking what slope is made of?

Would it make more sense to phrase the question in terms of how quickly the slope changes if you add or remove mass?

It's very likely I'm completely misunderstanding something here.
What's wrong with "the speed of slope"? The slope "isn't just there", it moves with whatever object is making it, and it doesn't move instantaneously when the object accelerates.

Wiggle that bowling ball back and forth, to stick with the analogy, and you'll create moving slopes in the sheet. It's tricky to do it in a way that really mimics gravity waves, since the wave energy will be dampened quickly by the sheet in a way space-time doesn't, but the speed with which does waves in the sheet spread out would be analogous to the speed of gravity.

Last edited by naita; 04-11-2019 at 11:29 AM.
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Old 04-11-2019, 12:52 PM
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What's wrong with "the speed of slope"? The slope "isn't just there", it moves with whatever object is making it, and it doesn't move instantaneously when the object accelerates.
I know, hence my question : "Would it make more sense to phrase the question in terms of how quickly the slope changes if you add or remove mass?"

Maybe I should have added: "or move the bowling ball"
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Old 04-11-2019, 01:20 PM
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I know, hence my question : "Would it make more sense to phrase the question in terms of how quickly the slope changes if you add or remove mass?"

Maybe I should have added: "or move the bowling ball"
The point is, that's the speed of gravity. I thought my analogy might help explain why your question doesn't make sense to me.

Here's a different approach, which might fail as well.

Just as there is always gravity around in the real universe, there is also always electromagnetic radiation, so what we observe is just changes in the level of that radiation (or disturbances in the electromagnetic field if you're into that). How fast those changes propagate is called the speed of light, not the speed of changes in light intensity.
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Old 04-11-2019, 01:30 PM
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Gravity is often illustrated in terms of a rubber sheet. Place a bowling ball in the middle of the sheet, and it deforms creating a slope. Put a marble some distance from the bowling ball, it will roll down the slope.

So, asking about the speed of gravity, isn't that like asking about the speed of slope? The slope doesn't have a speed, it's just there. Or Crafter_Man's question, what gravity waves are made of, isn't that like asking what slope is made of?

Would it make more sense to phrase the question in terms of how quickly the slope changes if you add or remove mass?

It's very likely I'm completely misunderstanding something here.
The “rubber sheet” analogy is useful for visualizing the geometric effects of mass on space-time (which is fundamental to the concept of Einstein gravity) but it shouldn’t be extended to any dynamic phenomena relating to the conveyance or gravity itself. For one, it is (as far as we can tell) impossible to arbitrarily add or remove mass from space-time; doing so would create a discontinuity that would break general relativity. For another, while space-time of Einstein gravity is static, a quantum theory of gravity requires a continuous exchange of gravitons to mediate the apparent force of gravity. That exchange occurs (hypothetically, and as apparently verified by the recent measurements) at c. That has nothing to do with the level of acceleration of gravity (which is the slope of the fabric in the rubber sheet illustration); it is just a constant regardless of the amount of gravitons exchanged. The energy and/or number of gravitons is what controls the intensity of gravity.

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Old 04-11-2019, 03:05 PM
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I should also mention that people have in fact looked for gravitational waves at frequencies above the kilohertz range. This is for two reasons: There might in fact be some sources in the Universe that we don't know about that produce waves in that range, and high-frequency gravitational wave detectors are much easier to build than the ones like LIGO. But to the surprise of nobody, including their builders, none of these high-frequency detectors have found anything.
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Old 04-11-2019, 10:14 PM
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So all I need is some material that blocks out all incoming gravitons plus a source that produces a single graviton at a time and a device that measures said graviton and I'll win a Nobel Prize.

Hold my beer caffeine-free diet cola.
How about a partnership:
I will dance around a laboratory, generating gravitational waves.
I'll leave it up to you to figure out some of the fiddly technical details like graviton shielding and detection apparatus.
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Old 04-11-2019, 11:09 PM
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How large is a graviton, though? In QM, we study particle-wave dualities on a picosopic scale, but why should we assume that gravitons exist on a similar scale? Could they not be enormous entities?
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Old 04-12-2019, 08:11 PM
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This is great! Thanks to all of you and especially you, Chronos.

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Old 04-13-2019, 10:28 AM
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Fundamental particles don't have size. They have two properties that can behave sort of analogously to size, namely wavelength and cross-section. The wavelength of typical gravitational waves (and hence also of the particles that make them up, to the extent that the particles can be said to have wavelength) is, as already mentioned, huge. And the cross-sections measure the degree to which they interact with various other particles: That'll vary from one interaction to another, but we know from experiment that any cross-section involving gravitons will be very tiny indeed, dozens of orders of magnitude smaller than any of the standard fundamental particles.
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Old 04-13-2019, 10:53 AM
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Since we were looking at a black hole, if gravity waves is caused by the mass of the BH singularity (if it is a singularity) and gravity waves are part of the fabric of spacetime itself, why are we detecting light and gravity waves at the same time from the same event , as light can not escape from the BH, (or at least not directly) and gravity waves apparently can escape?

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Old 04-13-2019, 11:07 AM
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A black hole just sitting there won't emit gravitational waves, any more than it emits light. Two black holes merging will produce gravitational waves, but that comes from the entire system, not from either one of the holes individually. And most of that system is outside of event horizons.
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Old 04-13-2019, 11:56 AM
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Nothing “just sits there”. Everything in space is in motion, so a black hole (or any other body) will invariably generate wave. Gravity is such a weak force, though, that the amplitude and frequency of most gravitational perturbance is effectively immeasurable.
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Old 04-13-2019, 12:06 PM
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Nothing “just sits there”. Everything in space is in motion, so a black hole (or any other body) will invariably generate wave. Gravity is such a weak force, though, that the amplitude and frequency of most gravitational perturbance is effectively immeasurable.
How would a hypothetical object just floating/moving through space radiate gravitational waves or gravitational "perturbance"? Of course something like a body orbiting another body is a different story.
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Old 04-13-2019, 01:09 PM
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When two black holes collide, it is unlikely that nothing outside of the event horizon of either, or both of them will not also collide. Probably at fairly high speeds, and high temperatures. That event takes place at the same time as the gravity wave generation, and is "visible". By recording the spectrum of the region as a whole, one can obtain fairly detailed information about the movements of the black holes without actually seeing them, because the effect the spectrum of the infalling, and colliding ordinary matter. Given the recent increase in resolving power of "virtual" collectors created from data streams from many sources, and the recently developed algorithms which can render them into images (as well as data with other information about the process) the evaluation of "when did the gravity wave generating event take place" is now increasingly likely to be exact to a more narrow margin of error than the distances of the objects themselves. The results imply closely that gravity waves travel at c.

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Old 04-13-2019, 01:14 PM
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Oh, by the way, if we build another one of these gravity wave detectors, we need
a new acronym, so its name will be SLUGGO.

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Old 04-13-2019, 02:30 PM
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Nothing “just sits there”. Everything in space is in motion, so a black hole (or any other body) will invariably generate wave. Gravity is such a weak force, though, that the amplitude and frequency of most gravitational perturbance is effectively immeasurable.
Unaccelerated motion does not generate gravitational waves. In fact, spherically or rationally symmetric acceleration won't generate gravitational waves either, so a star pulsing in size or s Ringworld won't generate gravitational waves either. The Earth isn't exactly symmetrical so some tiny waves are generated by its rotation but a black hole is symmetric and won't generate waves except while interacting with another body.
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Old 04-13-2019, 02:36 PM
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Nm

Last edited by Andy L; 04-13-2019 at 02:37 PM. Reason: Accidental duplication
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Old 04-13-2019, 02:49 PM
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How would a hypothetical object just floating/moving through space radiate gravitational waves or gravitational "perturbance"? Of course something like a body orbiting another body is a different story.
I picture spacetime as a sort of array of gradients. Go out to some distant point in space and place a baseball at that point with zero momentum (which is not really a meaningful value) and it will start moving “down” the gradient.

The gradient is the vector sum of all the gravitational influences on a point in space. But the anchors of all of those vectors are in motion, so all the point gradients in spacetime are constantly changing.

As that large body way over there moves on its path, its effect on a given point gradient will change, in a wave-like manner (or wake-like), and the change will propagate at the speed of light: you would have to adjust the influence vector behind where that body actually is, except, you can only observe that body based on what you can see, so where it was will be consistent with its gravitational effect.

Gravity is a very weak force, so the waves in spacetime are immeasurable subtle for almost everything. But it does appear that they are there, based on what the gravitational wave detectors tell us about the really massive things.

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Old 04-13-2019, 02:53 PM
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Unaccelerated motion does not generate gravitational waves. …
Give me a real example of unaccelerated motion.
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Old 04-13-2019, 06:50 PM
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Give me a real example of unaccelerated motion.
There was some guy who claimed that an object in motion, tends to remain in motion. That would be unaccelerated motion. Not sure how reliable that guy was, but I've heard him quoted a lot.

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Old 04-15-2019, 10:35 AM
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Give me a real example of unaccelerated motion.
Any object following a ballistic path, including anything in free orbit.
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Old 04-15-2019, 12:33 PM
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Any object following a ballistic path, including anything in free orbit.
An orbit is inherently accelerated. Gravitational attraction between any two massive objects with vectors that do not collide will accelerate those objects toward each other. The path resulting from those forces will cause the objects to orbit one of the foci of an ellipse.

If the original vectors are large enough, the path will be parabolic, or hyperbolic. The acceleration will still apply.

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Old 04-15-2019, 03:28 PM
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An orbit is inherently accelerated. Gravitational attraction between any two massive objects with vectors that do not collide will accelerate those objects toward each other. The path resulting from those forces will cause the objects to orbit one of the foci of an ellipse.

If the original vectors are large enough, the path will be parabolic, or hyperbolic. The acceleration will still apply.
Yes, that is correct when applying Newtonian gravitation, but doesn't work when discussing the speed of gravity or gravitational waves.

Gravity is a fictitious force in the same sense that centrifugal forces are. It's an artifact of the frame of reference. An object in free fall (that is a ballistic path or free orbit) is not experiencing any acceleration in their own frame of reference. This is what creates the phenomenon of "zero-gravity".

I'm not very good at explaining this; it's been too long since I've studied it. Some quick links that might explain it better, Proper Acceleration on Wikipedia and Curved Space on Physics of the Universe site.

On a slightly related tangent, if a system of orbiting objects were emitting gravitational waves, then the system would be losing energy.
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Old 04-15-2019, 03:51 PM
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On a slightly related tangent, if a system of orbiting objects were emitting gravitational waves, then the system would be losing energy.
That is how it works, yes. The Earth-Sun system emits gravitational waves and is losing energy.

https://en.wikipedia.org/wiki/Gravit...l_wave#Sources
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Old 04-15-2019, 04:14 PM
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Or more notably, the binary pulsar, which won its discoverers the Nobel Prize.
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Old 04-15-2019, 04:49 PM
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... it is (as far as we can tell) impossible to arbitrarily add or remove mass from space-time; doing so would create a discontinuity that would break general relativity.
But one can still posit a rapidly occurring event far away that changes the mass of a large object, and ask a meaningful question about how long it takes the gravitational effect to propagate. For example, the sun exploding, or a fast-moving massive object colliding and merging with it.

My own simplistic way of looking at it is that the speed of gravity has no material constraint, just like light in a vacuum has no material constraint, unlike the propagation of wave energy in a substance which is governed by the physical properties of the substance. But, it still has to obey the relativistic laws that define the nature of simultaneity and causality in spacetime, just as light does, which includes any kind of effect or information. Thus, if the sun suddenly exploded, the earth would feel the lessening of gravity and start to fly off into space at precisely the same time that we saw the event, about 8.3 minutes after it happened. Until then it would continue serenely in its usual orbit, because from our reference frame, the sun was still there!
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Old 04-15-2019, 06:50 PM
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If the Sun exploded, it'd take longer than that to feel the effects, because the Sun's mass isn't changed. If the explosion were spherically symmetrical, then the change in gravity would happen right as the spreading debris from the explosion passed us (with some of it probably hitting us in the process), and since that's matter moving, it'd have to be slower than c.
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Old 04-16-2019, 08:40 AM
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If the Sun exploded, it'd take longer than that to feel the effects, because the Sun's mass isn't changed. If the explosion were spherically symmetrical, then the change in gravity would happen right as the spreading debris from the explosion passed us (with some of it probably hitting us in the process), and since that's matter moving, it'd have to be slower than c.
Wouldn't the change in gravitational influence from the matter moving towards us affect us earlier than the change for matter moving away?

For simplicity's sake, say we have an object 1ly away with mass M affecting us with a gravitational pull of F. It is very large, we're very small and we are in effect in a circular orbit.

The object explodes into two parts with direction directly towards us and directly away, and a speed of 0.1 c. At the time two seconds (as observed by us) after the explosion we will observe, and experience the gravitational pull of, the two parts as being 1.182ly away and 0.778ly away, for a total gravitational pull of:

F/(2*1.182^2) + F/(2*0.778^2) = 1.184F

Last edited by naita; 04-16-2019 at 08:42 AM.
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Old 04-16-2019, 10:32 AM
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Are we neglecting the dynamics of explosion, though? The Sun is stable because of thermal/gravitational balance. For it to explode, it would have to become unbalanced, which means it would have to compress: all explosions are preceded by compression. This would change the profile of the gravity well, to a very small degree, over, probably, a relatively long period of time compared to the interval of the explosion. The Earth would experience a very subtle reduction in solar gravity, causing it to move very slightly outward prior to the explosion event.
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Old 04-17-2019, 04:47 PM
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If the explosion were spherically symmetric, then it woudn't matter. Outside of a spherically-symmetric mass distribution, the gravitational field (curvature, metric, whatever else you care to matter) depends only on the total mass. The gravitational field of the Sun is the same as the gravitational field of a one-solar-mass black hole (in fact, the Schwarzschild metric was originally developed to describe the gravitational field of stars; the theoretical prediction of black holes came when someone noticed that the metric got weird for very small objects).

Oh, and Triskadecamus, I'm not sure what SLUGGO is supposed to be an acronym for, but the next working gravitational wave detectors will be KAGRA and LIGO India. Or possibly GEO, but I have my doubts that they'll ever get anything useful out of that one. LIGO India will, as the name implies, be the same design as LIGO Hanford and LIGO Livingston, just placed on the other side of the world (there are significant advantages to spreading them out). There were at one time plans for another one in Australia and possibly somewhere in South America, but the funding didn't materialize. KAGRA (in Japan) is a different design, and in many ways better than LIGO, but it's new enough that it's taking them a while to get all the details worked out (just like it did for LIGO).
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Old 04-17-2019, 05:19 PM
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If the explosion were spherically symmetric, then it woudn't matter. Outside of a spherically-symmetric mass distribution, the gravitational field (curvature, metric, whatever else you care to matter) depends only on the total mass. The gravitational field of the Sun is the same as the gravitational field of a one-solar-mass black hole (in fact, the Schwarzschild metric was originally developed to describe the gravitational field of stars; the theoretical prediction of black holes came when someone noticed that the metric got weird for very small objects).
It does appear to matter if the sphere is changing shape and gravity has a finite speed though. You couldn't integrate over the actual shape of the mass, since the gravitational influence of the various bits take longer the further away they are, and since the bits are now moving, the apparent shape they form in aggregate is no longer spherical.
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Old 04-17-2019, 06:28 PM
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I'm not sure what SLUGGO is supposed to be an acronym for,
Neither do I !

But it would be nifty.

Southern Latitude Undersea (help me out here) Graviton Observatory?

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Last edited by Triskadecamus; 04-17-2019 at 06:28 PM.
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Old 04-17-2019, 06:33 PM
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Neither do I !

But it would be nifty.

Southern Latitude Undersea (help me out here) Graviton Observatory?
Southern Latitude Undersea Geolocating Graviton Observatory?
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