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salem44dream
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21 Jan 2013, 9:12 pm

This board seems to be an incredible resource for answers to scientific questions, so I thought I'd start a topic on how little we know about gravity. All I have are questions.

How close are we to discovering the "graviton," the particle or wave that supposedly accounts for gravity?

Let's say if a giant star of enormous mass were to just disappear (I know that can't happen, but let's just say it did). Is it true that the sudden disappearance of that star's gravitational effect would be felt instantly in all places in the galaxy that was its home? That is, would the force the star had been exerting on different bodies throughout that galaxy disappear at exactly the same moment without any lapse of time?



ruveyn
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21 Jan 2013, 10:01 pm

salem44dream wrote:
This board seems to be an incredible resource for answers to scientific questions, so I thought I'd start a topic on how little we know about gravity. All I have are questions.

How close are we to discovering the "graviton," the particle or wave that supposedly accounts for gravity?



At this juncture a graviton has not been detected. In addition all of the attempts a quantum gravity yield up infinities that cannot be resolved. It was thought String Theory my be the solution, but there are 10^500 (or so) possible string theories. So there is no way to pick a good one out based on the inadequate empirical evidence we possess. String Theory is probably a looser.

ruveyn



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21 Jan 2013, 10:08 pm

Salem44dream wrote:
How close are we to discovering the "graviton," the particle or wave that supposedly accounts for gravity?

We're 30 years away, just like we are with nuclear fusion.

For those who don't know, a "Graviton" is a hypothetical particle (testable, but not tested yet) having no mass and no charge, and that mediates (carries) the gravitational force. The graviton is a boson. The existence of a graviton has not yet been confirmed experimentally, although string theory predicts the existence of gravitons as closed strings with the minimum possible energy. It is also theorized that gravitons interact with leptons and quarks.

A "Boson" is an integer-spin particle (spin = 2) which mediates forces between fermions. Odd spin bosons mediate repulsive forces; even spin bosons mediate attractive forces. Bosons of the same type are indistinguishable and have symmetric wavefunctions. Bosons obey Bose-Einstein statistics.

"Fermions" are odd half-integer spin particles that act on each other by exchanging bosons. Examples include leptons (such as the electron), neutrons, protons and quarks. They are indistinguishable, have antisymmetric wave functions, and obey Fermi-Dirac statistics.

Unambiguous detection of individual gravitons, though not prohibited by any fundamental law, is impossible with any physically reasonable detector. The reason is the extremely low cross section for the interaction of gravitons with matter. For example, a detector with the mass of Jupiter and 100% efficiency, placed in close orbit around a neutron star, would only be expected to observe one graviton every 10 years, even under the most favorable conditions. It would be impossible to discriminate these events from the background of neutrinos, since the dimensions of the required neutrino shield would ensure collapse into a black hole.

However, experiments to detect gravitational waves, which may be viewed as coherent states of many gravitons, are underway (e.g., LIGO and VIRGO). Although these experiments cannot detect individual gravitons, they might provide information about certain properties of the graviton. For example, if gravitational waves were observed to propagate slower than c (the speed of light in a vacuum), that would imply that the graviton has mass.

References:

Rothman, T.; Boughn, S. (2006). "Can Gravitons be Detected?". Foundations of Physics 36 (12)

Will, C. M. (1998). "Bounding the mass of the graviton using gravitational-wave observations of inspiralling compact binaries". Physical Review D 57 (4): 2061–2068.


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Guilliman
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21 Jan 2013, 11:05 pm

Gravity follows the speed of light limit according to general relativity or is instant according to the Newtonian model.

http://math.ucr.edu/home/baez/physics/R ... speed.html

It hard to find a proper source on this. Gravity is not so much understood.



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22 Jan 2013, 12:23 am

One explanation (an idea, not a theory or an hypothesis) is that the Higgs Boson itself ret*ds the flow of time, that the Higgs Boson is present in all matter, and that the more matter in one location, the greater the retardation of time in that location. However, this retardation becomes less as distance from matter increases.

It is this gradient of time that causes the effect we perceive as gravity - slower time gradients distort space in such a way was to cause the faster flow of time in free space to push objects toward each other.

Thus, it is not so much that gravity is an attractive force, but that the flow of time in free space is a repulsive force that pushes bits of matter together.

If you believe that fine bit of nonsense, then let me tell you sometime about the Magic Smoke Model of Electronics ...


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22 Jan 2013, 12:37 am

Fnord wrote:
One explanation (an idea, not a theory or an hypothesis) is that the Higgs Boson itself ret*ds the flow of time, that the Higgs Boson is present in all matter, and that the more matter in one location, the greater the retardation of time in that location. However, this retardation becomes less as distance from matter increases.

It is this gradient of time that causes the effect we perceive as gravity - slower time gradients distort space in such a way was to cause the faster flow of time in free space to push objects toward each other.

Thus, it is not so much that gravity is an attractive force, but that the flow of time in free space is a repulsive force that pushes bits of matter together.

If you believe that fine bit of nonsense, then let me tell you sometime about the Magic Smoke Model of Electronics ...

Yeah, it begins to smack a bit of phlogiston theory, doesn't it?

Of course, it could be that Einstein was right, and that gravity is a field effect caused by the distortion of space/time by mass/energy...


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22 Jan 2013, 12:45 am

Unless a practical test for gravitons can be made, we will have to be satisfied with a dumpload of crackpot ideas and admissions of ignorance.


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physicsnut42
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22 Jan 2013, 5:44 pm

salem44dream wrote:
This board seems to be an incredible resource for answers to scientific questions, so I thought I'd start a topic on how little we know about gravity. All I have are questions.

How close are we to discovering the "graviton," the particle or wave that supposedly accounts for gravity?

Let's say if a giant star of enormous mass were to just disappear (I know that can't happen, but let's just say it did). Is it true that the sudden disappearance of that star's gravitational effect would be felt instantly in all places in the galaxy that was its home? That is, would the force the star had been exerting on different bodies throughout that galaxy disappear at exactly the same moment without any lapse of time?


Gravity can, like everything else, only go up to the speed of light. "Gravity waves" (which they're actually building satellites to detect) travel at the speed of light. If such a star--actually, let's pretend it's the big black hole at the center of the Milky Way instead for convenience--if the black hole were to disappear, the effects would first be felt in the center of the galaxy, and the effect would gradually spread out. We would live on happily with no problems whatsoever for about 26,000 years (the distance in lightyears from us to the galactic center) until the gravity waves reached us. That's the present theory.

Ironically, the force we've known about the longest has puzzled us the most. As far as we know, there's no way to marry general relativity with the present Standard Model, which, as I like to put it, is the "Theory of Almost Everything". The "almost" referring to the exlusion of gravity.


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ruveyn
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22 Jan 2013, 8:09 pm

physicsnut42 wrote:

Ironically, the force we've known about the longest has puzzled us the most. As far as we know, there's no way to marry general relativity with the present Standard Model, which, as I like to put it, is the "Theory of Almost Everything". The "almost" referring to the exlusion of gravity.


One speculation is the most of gravity is "hiding" in extra dimensions which is why it is so weak in our usual four dimension. String Theory which is an 11 dimensional theory could accommodate gravitation and all the other forces. There is only one trouble. String Theory makes predictions that we cannot test and there is more than one String Theory. So which one is it and how do we test it empirically.

ruveyn



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22 Jan 2013, 9:50 pm

physicsnut42 wrote:
Gravity can, like everything else, only go up to the speed of light. "Gravity waves" (which they're actually building satellites to detect) travel at the speed of light. If such a star--actually, let's pretend it's the big black hole at the center of the Milky Way instead for convenience--if the black hole were to disappear, the effects would first be felt in the center of the galaxy, and the effect would gradually spread out. We would live on happily with no problems whatsoever for about 26,000 years (the distance in lightyears from us to the galactic center) until the gravity waves reached us. That's the present theory.


I guessed that's the present state of gravity/graviton theory, but isn't it true that this very thing hasn't been proven (that the effects of changes in gravity can't exceed the speed of light)? That there's really no way with our present technology to do so?



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23 Jan 2013, 5:25 pm

salem44dream wrote:
physicsnut42 wrote:
Gravity can, like everything else, only go up to the speed of light. "Gravity waves" (which they're actually building satellites to detect) travel at the speed of light. If such a star--actually, let's pretend it's the big black hole at the center of the Milky Way instead for convenience--if the black hole were to disappear, the effects would first be felt in the center of the galaxy, and the effect would gradually spread out. We would live on happily with no problems whatsoever for about 26,000 years (the distance in lightyears from us to the galactic center) until the gravity waves reached us. That's the present theory.


I guessed that's the present state of gravity/graviton theory, but isn't it true that this very thing hasn't been proven (that the effects of changes in gravity can't exceed the speed of light)? That there's really no way with our present technology to do so?


Your absolutely right, but most experts would agree that it's very unlikely gravity doesn't obey the speed of light limit. Just like for a very long time, no one had seen DNA, but most people still believed it to exist (a terrible analogy but I think it gets the point across). If we follow Einstein's relativity (which itself has been proven time and time again since its publication in 1915), then the only thing that can break the speed of light limit is the very fabric of space itself, so you can cheat with stuff like the Alcubierre drive and go faster than light by moving space very fast, without actually moving yourself at all.

Thanks for correcting me, by the way. I tend to make mistakes with these things :oops: .


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salem44dream
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23 Jan 2013, 9:42 pm

I actually wasn't correcting you, but it might have seemed that way because I'm a little out of practice with scientific lingo and might not have realized how I had worded something. Just one more question, though ... to measure the "speed" of gravity, would you need a mechanism the size of a galaxy? (which obviously is out of reach for at least a few more epochs of time have passed by).



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23 Jan 2013, 10:14 pm

I think an easy way to test it is to move a satellite into the path of a moving large mass (moon/planet) and see how long the effects of the body coming closer to the satellite have on the satellite. We use the minimum range of attraction of the body. We're talking millions of kilometres and thousands of a second. So any satellite designed to test such a thing will need to be carefully calibrated. They'll probably launch least 2 to avoid data corruption.



salem44dream
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24 Jan 2013, 9:52 pm

Guilliman wrote:
I think an easy way to test it is to move a satellite into the path of a moving large mass (moon/planet) and see how long the effects of the body coming closer to the satellite have on the satellite. We use the minimum range of attraction of the body. We're talking millions of kilometres and thousands of a second. So any satellite designed to test such a thing will need to be carefully calibrated. They'll probably launch least 2 to avoid data corruption.


Or perhaps bringing the satellites to an abrupt halt as they approach a planet and see if the strength of gravity continues to increase even though the object has stopped. If there is a speed of gravity, the object will eventually equilibrate to the appropriate gravitational force at that distance from the planet. "Eventually" is a misnomer, though, because it should occur VERY quickly.

Two satellites doing this at the same time is a good idea, too.



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26 Jan 2013, 10:50 am

Just curious if anyone has heard of another completely different theory of gravity: that it a result of a form of pressure from the fields of all matter in the universe, or something. So, if you are floating next to a planet, then the planet is shielding you on one side from this pressure, causing the pressure from the other side to have a nett effect of pushing you to the planet. I've tried to think of a reason that this must be BS, and haven't yet found one.
Admittedly, my physics is not quite as 'up there' as some of yours seem to be. :wink:
I should probably go and find where I read this and try to think about exactly what might be causing this pressure.
I have instead been focusing on how such a pressure might behave, and whether or not it would act the way 'gravity' is observed to act. I think it would, mathematically at least.
I like to have an open mind. To me, his means that if if nothing has been PROVEN, (such as the existence of the graviton) then anything else is still possible, even if "everyone" does not agree.



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26 Jan 2013, 10:53 am

BlackSabre7 wrote:
Just curious if anyone has heard of another completely different theory of gravity: that it a result of a form of pressure from the fields of all matter in the universe, or something. So, if you are floating next to a planet, then the planet is shielding you on one side from this pressure, causing the pressure from the other side to have a nett effect of pushing you to the planet. I've tried to think of a reason that this must be BS, and haven't yet found one.
Admittedly, my physics is not quite as 'up there' as some of yours seem to be. :wink:
I should probably go and find where I read this and try to think about exactly what might be causing this pressure.
I have instead been focusing on how such a pressure might behave, and whether or not it would act the way 'gravity' is observed to act. I think it would, mathematically at least.
I like to have an open mind. To me, his means that if if nothing has been PROVEN, (such as the existence of the graviton) then anything else is still possible, even if "everyone" does not agree.


There is a version of one of the 10^500 possible string theories that unifies all the forces of nature. That includes the gravitation. Is this good news? Well, not a single one of the 10^500 string theories is capable of empirical falsification thus far.

ruveyn