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philippeb8
2015-Mar-28, 05:45 PM
(I assume this is the correct forum to post this on but please forgive me if this is the wrong one)

If gravity is a particle then it must lose energy every time it pulls atoms in the opposite direction it travels.

This means gravity on top of the Everest mountain (9,000 m) should be lesser than at the same altitude but over the middle of the ocean.

This test cannot be simpler.

Shaula
2015-Mar-28, 08:58 PM
(I assume this is the correct forum to post this on but please forgive me if this is the wrong one)

If gravity is a particle then it must lose energy every time it pulls atoms in the opposite direction it travels.

This means gravity on top of the Everest mountain (9,000 m) should be lesser than at the same altitude but over the middle of the ocean.

This test cannot be simpler.
Or wronger. Gravitons as a vector boson simply do not behave in this way. Neither do photons, gluons etc.

philippeb8
2015-Mar-28, 09:59 PM
Or wronger. Gravitons as a vector boson simply do not behave in this way. Neither do photons, gluons etc.

Ok thanks, I'll read more about it.

I was going to say we might detect the absorption / emission ratio during a solar eclipse by calculating the final gravitational pull caused by the aligned Moon and the Sun added together:

g_measured = g_moon + (g_sun - g_absorbed_by_the_moon)

Shaula
2015-Mar-29, 05:30 AM
Again, that is not how it would work in standard QFT type models. Experiments have been done of this nature and set pretty strict limits on any possible effect present.

Cougar
2015-Mar-29, 01:27 PM
A problem I see with gravitons is when 3 bodies are in alignment, say A, B, C. "A" affects C, but a graviton would have to go through B to get to C. It's unreasonable that A's gravitons are "tagged" -- this one's for B, and this other one is for C.

Shaula
2015-Mar-29, 01:40 PM
A problem I see with gravitons is when 3 bodies are in alignment, say A, B, C. "A" affects C, but a graviton would have to go through B to get to C. It's unreasonable that A's gravitons are "tagged" -- this one's for B, and this other one is for C.
That is only really the case if you visualise each graviton as being shot out of one body at another. Instead of thinking of them as representing quantised interactions between two fields.

Ken G
2015-Mar-29, 06:19 PM
Ok thanks, I'll read more about it.

I was going to say we might detect the absorption / emission ratio during a solar eclipse by calculating the final gravitational pull caused by the aligned Moon and the Sun added together:

g_measured = g_moon + (g_sun - g_absorbed_by_the_moon)The problem with this is that intervening mass does not reduce gravity, it adds to it, because the intervening mass is its own source of gravitons. Your experiment at the top of mountains, or the eclipse, would actually be a way of measuring the mass of the atmosphere, or the Moon, but the presence of these effects would be expected whether gravity was a particle or not.

philippeb8
2015-Mar-31, 01:43 AM
The problem with this is that intervening mass does not reduce gravity, it adds to it, because the intervening mass is its own source of gravitons. Your experiment at the top of mountains, or the eclipse, would actually be a way of measuring the mass of the atmosphere, or the Moon, but the presence of these effects would be expected whether gravity was a particle or not.

Please let me reiterate the idea. It has to do with the laws of conservation of the energy. Considering all possible positions of the Moon at a 90 degree angle and a gravimeter on the surface of the Earth, it comes down to 6 equations, 6 unknowns:

g_1 = -g_m + g_e + (g_s - g_sae)
g_2 = -g_m - g_e + g_s
g_3 = g_e + g_m + g_s
g_4 = -g_e + g_m + (g_s - g_sam)
g_5 = -g_e + g_m
g_6 = g_e + (g_m - g_mae)

Where:
g_x = measured gravitational acceleration
g_m = gravitational acceleration of the Moon
g_e = gravitational acceleration of the Earth
g_s = gravitational acceleration of the Sun
g_sae = gravitational acceleration of the Sun absorbed by the Earth
g_sam = gravitational acceleration of the Sun absorbed by the Moon
g_mae = gravitational acceleration of the Moon absorbed by the Earth

I don't know if gravitons are quantum objects or particles but if they are particles then the idea is to solve the gravitational acceleration that is absorbed (the last 3).

grapes
2015-Mar-31, 04:34 AM
Please let me reiterate the idea. It has to do with the laws of conservation of the energy. Considering all possible positions of the Moon at a 90 degree angle and a gravimeter on the surface of the Earth, it comes down to 6 equations, 6 unknowns:

g_1 = -g_m + g_e + (g_s - g_sae)
g_2 = -g_m - g_e + g_s
g_3 = g_e + g_m + g_s
g_4 = -g_e + g_m + (g_s - g_sam)
g_5 = -g_e + g_m
g_6 = g_e + (g_m - g_mae)

Where:
g_x = measured gravitational acceleration
g_m = gravitational acceleration of the Moon
g_e = gravitational acceleration of the Earth
g_s = gravitational acceleration of the Sun
g_sae = gravitational acceleration of the Sun absorbed by the Earth
g_sam = gravitational acceleration of the Sun absorbed by the Moon
g_mae = gravitational acceleration of the Moon absorbed by the Earth

I don't know if gravitons are quantum objects or particles but if they are particles then the idea is to solve the gravitational acceleration that is absorbed (the last 3).
How does your idea work with electromagnetic charge? Say, three like charges lined up in a row?

philippeb8
2015-Mar-31, 05:02 AM
How does your idea work with electromagnetic charge? Say, three like charges lined up in a row?

It's the same thing with electric charges but I am not sure how we can measure the electric field strength at a subatomic scale, unless we can increase the dimensions of the charges to improve the accuracy of the measurements.

Shaula
2015-Mar-31, 05:03 AM
It is also worth pointing out that this kind of screening is not required by quantised theories, nor would it be unique to them (classical fields can be screened too). You are definitely not testing the graviton theory by looking for this. You are testing whether mass screens gravity.

John Mendenhall
2015-Mar-31, 07:28 AM
It is also worth pointing out that this kind of screening is not required by quantised theories, nor would it be unique to them (classical fields can be screened too). You are definitely not testing the graviton theory by looking for this. You are testing whether mass screens gravity.

Fascinating. No shielding. No way to shield.

Every particle in the Universe is gravitationally 'aware' of every other particle in the Universe?

Grey
2015-Mar-31, 11:49 AM
Fascinating. No shielding. No way to shield.

Every particle in the Universe is gravitationally 'aware' of every other particle in the Universe?Pretty much. Gravitational interactions propagate at the speed of light, though, so there could be stuff in the universe far enough away that it doesn't affect us gravitationally, but then we wouldn't be able to see it either.

crosscountry
2015-Mar-31, 05:21 PM
Pretty much. Gravitational interactions propagate at the speed of light, though, so there could be stuff in the universe far enough away that it doesn't affect us gravitationally, but then we wouldn't be able to see it either.

Presumably some of those distant objects affect less distant objects, ones we can see. If that's the case, we won't feel their gravitational acceleration, but we can observe its effect on another body, and hence be aware. But this argument is very esoteric.

Shaula
2015-Mar-31, 05:28 PM
If you look at most other screening mechanisms they come about due to multiple charge types which can cancel each other out. Thus EM screening relies on a mechanism where something is able to interact with the field in some way so as to generate a counter-field. So on a macro-scale you see currents induced in superconductors that can shield the interior of a material from EM effects. On a micro-scale you get effects like the effective charge varying with energy scale due to vacuum polarisation (and I believe you get a similar but far more complicated version of this with the strong force due to the multiple charges - ditto the weak force). So at a guess (I am not expert enough in the theory to actually know this, I will highlight) you would not expect to see gravitational screening unless you could generate pairs of positive and negative mass particles (or more accurately positive and negative gravitational charge equivalents) that could then act in an analogous way to electron positron pairs in the EM case.

John Mendenhall
2015-Mar-31, 08:13 PM
Pretty much. Gravitational interactions propagate at the speed of light, though, so there could be stuff in the universe far enough away that it doesn't affect us gravitationally, but then we wouldn't be able to see it either.

IIRC, Mach thought that the distant stars might be responsible for inertia. I can see why. Too bad the issue remains unresolved.

Grey
2015-Apr-01, 12:39 PM
Presumably some of those distant objects affect less distant objects, ones we can see. If that's the case, we won't feel their gravitational acceleration, but we can observe its effect on another body, and hence be aware. But this argument is very esoteric.

It is esoteric, but it also doesn't work. ;). If there is time for gravity to propagate from galaxy A to galaxy B, and then for light to propagate from galaxy B to galaxy C (us), there is also time for light (or gravity) to propagate directly from galaxy A to galaxy C.

crosscountry
2015-Apr-01, 04:15 PM
It is esoteric, but it also doesn't work. ;). If there is time for gravity to propagate from galaxy A to galaxy B, and then for light to propagate from galaxy B to galaxy C (us), there is also time for light (or gravity) to propagate directly from galaxy A to galaxy C.

I understand your argument, but is it too simple? We can see things ~13 billion light years in all directions. Presumably, things near the edge of our visible universe can also see things ~13 billion light years in all directions, including opposite us. This is based on the idea that we are not the center of the universe.

So, object B, just at the edge for us, can detect what object A (just at the opposite edge for B) looked like ~13 billion years ago and is therefore affected by its gravity. In sum, it would take ~26 billion years for Object A's light to reach us, which is impossible because at that distance the object is moving faster than the speed of light (your premise).

crosscountry
2015-Apr-01, 04:20 PM
Another way to look at it is to find a galaxy that is just beyond our visible universe. It will and has always had a gravitational effect on the galaxy just inside our visible universe. Because the inner one is departing at nearly the speed of light, but not at it we can see it. However, the one just outside is moving away at beyond the speed of light, and we'll never see it, only its influence on the inner one.

Grey
2015-Apr-01, 11:59 PM
I understand your argument, but is it too simple? We can see things ~13 billion light years in all directions. Presumably, things near the edge of our visible universe can also see things ~13 billion light years in all directions, including opposite us. This is based on the idea that we are not the center of the universe.

So, object B, just at the edge for us, can detect what object A (just at the opposite edge for B) looked like ~13 billion years ago and is therefore affected by its gravity. In sum, it would take ~26 billion years for Object A's light to reach us, which is impossible because at that distance the object is moving faster than the speed of light (your premise).

But remember that we see that galaxy not as it is "now", but as it was 13 billion years ago. At that point, a galaxy 13 billion light years further away wouldn't have affected it yet. We really have no way of observing things that happened that far away, even indirectly.

crosscountry
2015-Apr-03, 12:13 AM
But remember that we see that galaxy not as it is "now", but as it was 13 billion years ago. At that point, a galaxy 13 billion light years further away wouldn't have affected it yet. We really have no way of observing things that happened that far away, even indirectly.

I remember that, but if two objects are side by side, they affect each other. One will influence the other gravitationally, even 13 billion years ago. Just because we cannot see the one doesn't mean it hasn't always been affecting the other.

Has the universe always been expanding faster than the speed of light? I don't believe that to be true. If not, then the light of that distant universe once passed by the Earth but is now slower than the expansion and invisible.

Something else to consider is that the Milky Way and the distant object were once in the same place. They influenced each other in the beginning.

Grey
2015-Apr-07, 03:29 PM
I remember that, but if two objects are side by side, they affect each other. One will influence the other gravitationally, even 13 billion years ago. Just because we cannot see the one doesn't mean it hasn't always been affecting the other. Well, if they're literally side by side, yes, but if one is a billion light years from the other, then gravitational influences take a billion years to propagate from one to the other.


Has the universe always been expanding faster than the speed of light? I don't believe that to be true. If not, then the light of that distant universe once passed by the Earth but is now slower than the expansion and invisible.Actually, I think you're under a misconception here. It's not that distant galaxies cannot be seen because they are receding faster than light. We can actually see galaxies that have an effective recession velocity faster than light (everything further away than about z = 1.4), and even galaxies that have had an effective recession velocity faster than light since they formed (everything further away than about z = 6). The reason we wouldn't be able to see a very distant galaxy is that there hasn't been time for the light to have gotten to us in the age of the universe since that galaxy formed. If that's the case, then there also hasn't been time for the gravity (travelling at the same speed as light) to get here either, and likewise not time for the gravitational influence to have propagated partway, and the light to have made the rest of the journey. An excellent paper discussing some of these misconceptions can be found here (http://arxiv.org/abs/astro-ph/0310808). I'd especially recommend section 3.3, discussing the observability of galaxies that have superluminal recession velocities.


Something else to consider is that the Milky Way and the distant object were once in the same place. They influenced each other in the beginning.Perhaps, although not certain. Current theory only says that everything in the universe was literally in the exact same place if you extrapolate back to that, something that we know current physical theory doesn't allow us to do. All we really know is that the universe was much hotter and denser, and the things we observe were much closer to us than they are now. But then this leads us to something more or less like the horizon problem (http://en.wikipedia.org/wiki/Horizon_problem). Cosmologists are agreed that, although we observe distant objects on both sides of the sky to be in equilibrium, they're too far apart to have been in causal contact at the beginning, so we postulate effects like inflation to account for that. But even though that means that distant parts of the universe were in causal contact very early, after inflation, that causal link no longer exists. So the matter that eventually formed a galaxy that's an extra few billion light years away may have once come into thermodynamic equilibrium with the matter of a closer galaxy that we can observe, but by the time it actually formed a galaxy, that link had long since been severed. Any gravitational effects it had as a galaxy would not be able to reach the nearer galaxy at the edge of our observable universe in time for those effects to be observable by us (or the more distant galaxy would be directly observable as well).

It's worthwhile to note that the "as a galaxy" is an important note. The farthest thing we can see is the cosmic microwave background, which is at about z = 1100. The farthest galaxy we've currently found is at z = 7.5. The reason that we can't see any galaxies at, say, z = 10, is not such a hypothetical galaxy would be receding too fast to observe. It's that we'd be observing such a galaxy at such an early time that it wouldn't have formed yet. So we might be able to (in principle, at least) observe the gravitational effects on a galaxy at z = 7.5 of the matter that later formed a galaxy at z = 10, at that point, there wouldn't have been much difference between that matter and any of the surrounding matter. The intergalactic medium would have been much closer to homogeneous (indeed, if you go back to the microwave background, it was very homogeneous). You'll only get an observable gravitational influence once the matter has clumped a fair amount (i.e., formed into galaxies in the first place).

crosscountry
2015-Apr-09, 07:50 PM
Well, if they're literally side by side, yes, but if one is a billion light years from the other, then gravitational influences take a billion years to propagate from one to the other.

Actually, I think you're under a misconception here. It's not that distant galaxies cannot be seen because they are receding faster than light. We can actually see galaxies that have an effective recession velocity faster than light (everything further away than about z = 1.4), and even galaxies that have had an effective recession velocity faster than light since they formed (everything further away than about z = 6). The reason we wouldn't be able to see a very distant galaxy is that there hasn't been time for the light to have gotten to us in the age of the universe since that galaxy formed. If that's the case, then there also hasn't been time for the gravity (travelling at the same speed as light) to get here either, and likewise not time for the gravitational influence to have propagated partway, and the light to have made the rest of the journey. An excellent paper discussing some of these misconceptions can be found here (http://arxiv.org/abs/astro-ph/0310808). I'd especially recommend section 3.3, discussing the observability of galaxies that have superluminal recession velocities.



Thanks. I do understand that gravity moves at the speed of light and we can think of them as one in the same. Thus, if the galaxy's light hasn't reached us, neither has the gravity.

I was trying to imagine a scenario where it wouldn't be the case, but that's impossible.




Perhaps, although not certain. Current theory only says that everything in the universe was literally in the exact same place if you extrapolate back to that, something that we know current physical theory doesn't allow us to do. All we really know is that the universe was much hotter and denser, and the things we observe were much closer to us than they are now. But then this leads us to something more or less like the horizon problem (http://en.wikipedia.org/wiki/Horizon_problem). Cosmologists are agreed that, although we observe distant objects on both sides of the sky to be in equilibrium, they're too far apart to have been in causal contact at the beginning, so we postulate effects like inflation to account for that. But even though that means that distant parts of the universe were in causal contact very early, after inflation, that causal link no longer exists. So the matter that eventually formed a galaxy that's an extra few billion light years away may have once come into thermodynamic equilibrium with the matter of a closer galaxy that we can observe, but by the time it actually formed a galaxy, that link had long since been severed. Any gravitational effects it had as a galaxy would not be able to reach the nearer galaxy at the edge of our observable universe in time for those effects to be observable by us (or the more distant galaxy would be directly observable as well).

It's worthwhile to note that the "as a galaxy" is an important note. The farthest thing we can see is the cosmic microwave background, which is at about z = 1100. The farthest galaxy we've currently found is at z = 7.5. The reason that we can't see any galaxies at, say, z = 10, is not such a hypothetical galaxy would be receding too fast to observe. It's that we'd be observing such a galaxy at such an early time that it wouldn't have formed yet. So we might be able to (in principle, at least) observe the gravitational effects on a galaxy at z = 7.5 of the matter that later formed a galaxy at z = 10, at that point, there wouldn't have been much difference between that matter and any of the surrounding matter. The intergalactic medium would have been much closer to homogeneous (indeed, if you go back to the microwave background, it was very homogeneous). You'll only get an observable gravitational influence once the matter has clumped a fair amount (i.e., formed into galaxies in the first place).

This argument is nuanced, but it doesn't contradict what I was saying. You say, "there wouldn't have been much difference between that matter and any of the surrounding matter," but that is irrelevant. The effect is present, small or not. And that's why I made the point. Sure, it doesn't behave like a point source of mass, but that doesn't diminish the importance of it being there.