# Equivalence Principle Test Kits

I'd like to see for myself the Equivalence Principle. Would there be a commercially available test kit like perhaps a small iron and cotton enclosed in vacuum tubes and they are to fall at same time and hit the bottom at same time with indicator lights to tell if they hit at same time or at bit different time. I'd like to test this in different environments too like inside caves or mountains.

Also is EP a Lorentz symmetry or Poincare symmetry or what symmetry does EP fall under and why is that?

phinds
Gold Member
2021 Award
I think you misunderstand what the equivalence principle is. It has nothing to do with the fact that objects in a vacuum but in a gravitational field fall at the same rate regardless of mass, it is ... well look it up for yourself.

But it's related. See Wikipedia:

Equivalence principle
In the physics of general relativity, the equivalence principle is any of several related concepts dealing with the equivalence of gravitational and inertial mass, and to Albert Einstein's observation that the gravitational "force" as experienced locally while standing on a massive body (such as the Earth) is actually the same as the pseudo-force experienced by an observer in a non-inertial (accelerated) frame of reference.

Development of gravitation theory
Something like the equivalence principle emerged in the late 16th and early 17th centuries, when Galileo expressed experimentally that the acceleration of a test mass due to gravitation is independent of the amount of mass being accelerated. These findings led to gravitational theory, in which the inertial and gravitational masses are identical.

The equivalence principle was properly introduced by Albert Einstein in 1907, when he observed that the acceleration of bodies towards the center of the Earth at a rate of 1g (g = 9.81 m/s2 being a standard reference of gravitational acceleration at the Earth's surface) is equivalent to the acceleration of an inertially moving body that would be observed on a rocket in free space being accelerated at a rate of 1g. Einstein stated it thus

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So Phinds. The consequence of it is the iron and cotton falling at same time. If this is not exactly called EP. Then what term do you use to describe it?

Simon Bridge
Homework Helper
The same time-to-fall for iron or cotton, afaik, has no formal name - it just is itself. It is not usually thought of as a consequence of the Einstein equivalence principle.
The Einstein equivalence principle is how come you feel heavier when riding an accelerating lift upwards... just as if you were under higher gravity.

The equal independence of gravity on the mass of an object was famously verified experimentally by Galileo.
Galileo made his own equipment and so should you - it's not expensive or difficult.
So make a hollow ball that you can put weights in and time how long it take to fall with different weights... or look up how Galileo did it.

The short answer to your question is that there is no commercially available test rig for what you are asking because the equipment is universally available and cheap.

An easier test would be to construct two pendulums (i.e., tie two things to pieces of string). Make the string lengths equal but let the weights at the bottom have different masses, or the same mass but different materials, or whatever. Time the period of each pendulum, or let them swing next to each other so you can visually compare their periods. If inertial mass is exactly proportional to gravitational mass, both pendulums should have the same period of oscillation.

Probably you want both weights to be relatively heavy to minimize the effects of air resistance and the weight of the string.

Newton wrote about performing such an experiment: http://www.mathpages.com/home/kmath582/kmath582.htm

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Simon Bridge
Homework Helper
Pendulums are easy to mess up - heavier weights can get a longer period due to stretching the string or a shorter period due to being taller along the string (raising the center of mass) ... you also have rotational motion creeping in due to non-ideal conditions.
To do this properly, you need Borda and Cassini's experiment... much more complicated than just dropping stuff or running weights down ramps.

The same time-to-fall for iron or cotton, afaik, has no formal name - it just is itself. It is not usually thought of as a consequence of the Einstein equivalence principle.
The Einstein equivalence principle is how come you feel heavier when riding an accelerating lift upwards... just as if you were under higher gravity.

The equal independence of gravity on the mass of an object was famously verified experimentally by Galileo.
Galileo made his own equipment and so should you - it's not expensive or difficult.
So make a hollow ball that you can put weights in and time how long it take to fall with different weights... or look up how Galileo did it.

The short answer to your question is that there is no commercially available test rig for what you are asking because the equipment is universally available and cheap.

My impressions of the connection between the test and EP came from the so called Eotvos experiment in such (wiki):

"The Eötvös experiment was a famous physics experiment that measured the correlation between inertial mass and gravitational mass, demonstrating that the two were one and the same, something that had long been suspected but never demonstrated with the same accuracy. The earliest experiments were done by Isaac Newton (1642–1727) and improved upon by Friedrich Wilhelm Bessel (1784–1846).[1] A much more accurate experiment using a torsion balance was carried out by Loránd Eötvös starting around 1885, with further improvements in a lengthy run between 1906 and 1909. Eötvös's team followed this with a series of similar but more accurate experiments, as well as experiments with different types of materials and in different locations around the Earth, all of which demonstrated the same equivalence in mass. In turn, these experiments led to the modern understanding of the equivalence principle encoded in general relativity, which states that the gravitational and inertial masses are the same."

But Simon. If the Equivalence Principle didn't occur in nature. The iron and cotton won't fall at same time, would it? so they are kinda related...

Simon Bridge
Homework Helper
The words "the equivalence principle" has a special meaning in physics - it refers (usually) to the principle in general relativity where gravitation is equivalent to an accelerating reference frame. It is not usually understood to mean the equivalence between inertial and gravitational mass. Sometimes you do find lectures etc talking about the equivalence of inertial and gravitational mass as the weak equivalence principle. Notice that the passage you quoted is careful to distinguish them?

It is easy to see how someone could get confused.

That passage also points out that the modern understanding of gravitational effects, like how come gravitational and inertial mass are the same, is now understood in terms of the equivalence principle. It does not say that these experiments demonstrate the Einstein equivalence principle - which is what you [seemed to be saying] you wanted the apparatus to do.

(Note: the Eotvos experiment does not demonstrate that different masses fall in the same time.)

Of course the fall-time and the Einstein equivalence principle are related ... they are both about gravity.
But demonstrating the first does not demonstrate the second. Otherwise GR would have been invented much earlier.

You can purchase commercial Borda Pendulum apparatus if you are keen - or construct two of them cheaply.
You can reproduce the Galileo experiments (ramps and balls, not leaning towers) cheaply.
The usual high-school demo is just to drop stuff off high places and time the fall.
There are other cheaper experiments you can do.

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PeterDonis
Mentor
Would there be a commercially available test kit

NASA sent a "test kit" to the Moon and verified that objects of very different masses (a feather and a hammer) fall at the same rate there; you can watch the video here:

http://science.nasa.gov/science-news/science-at-nasa/2007/18may_equivalenceprinciple/

Note that NASA attributes this to the "equivalence principle", illustrating (as has already been remarked on in this thread) that that term has several different possible meanings. Whether you call it that or not, though, this NASA experiment tests what you said you wanted to test in the OP.

Simon Bridge
Homework Helper
Although the NASA test kit (hammer and feather) are commercially available (purchase separately), the lab-space may be difficult to access without some more substantial financial outlay. But I suppose if someone were really really keen...

MIT has a setup too:
http://video.mit.edu/watch/feather-and-coin-in-a-vacuum-6407/
... again, all the apparatus is commercially available but I don't think they are sold as kits - but by the look of the one in the vid, maybe.

PeterDonis
Mentor
Although the NASA test kit (hammer and feather) are commercially available (purchase separately), the lab-space may be difficult to access without some more substantial financial outlay.

Good point. :D

MIT has a setup too

Simon Bridge
Homework Helper
... yah. The expensive part will be the pump - I wonder how good-a vacuum you need for a good effect like that?

The words "the equivalence principle" has a special meaning in physics - it refers (usually) to the principle in general relativity where gravitation is equivalent to an accelerating reference frame. It is not usually understood to mean the equivalence between inertial and gravitational mass. Sometimes you do find lectures etc talking about the equivalence of inertial and gravitational mass as the weak equivalence principle. Notice that the passage you quoted is careful to distinguish them?

It is easy to see how someone could get confused.

That passage also points out that the modern understanding of gravitational effects, like how come gravitational and inertial mass are the same, is now understood in terms of the equivalence principle. It does not say that these experiments demonstrate the Einstein equivalence principle - which is what you [seemed to be saying] you wanted the apparatus to do.

(Note: the Eotvos experiment does not demonstrate that different masses fall in the same time.)

Of course the fall-time and the Einstein equivalence principle are related ... they are both about gravity.
But demonstrating the first does not demonstrate the second. Otherwise GR would have been invented much earlier.

You can purchase commercial Borda Pendulum apparatus if you are keen - or construct two of them cheaply.
You can reproduce the Galileo experiments (ramps and balls, not leaning towers) cheaply.
The usual high-school demo is just to drop stuff off high places and time the fall.
There are other cheaper experiments you can do.

Thanks. I learnt the distinction between the weak EP and the Einstein EP.. but there is a third one called the Strong EP.. Wiki defined or differentiates them as:

Einstein EP
The outcome of any local non-gravitational experiment in a freely falling laboratory is independent of the velocity of the laboratory and its location in spacetime.
Strong EP
The outcome of any local experiment (gravitational or not) in a freely falling laboratory is independent of the velocity of the laboratory and its location in spacetime.

The difference between the above is the word "non-gravitational" in the former and "gravitational or not" in the latter meaning the latter is valid to all wheras the former is only valid to non-gravitational.

What does it really mean?

Staff Emeritus
2021 Award
The same time-to-fall for iron or cotton, afaik, has no formal name - it just is itself.

Universality of free fall.

Simon Bridge
Homework Helper
Wikipedia is not a recommended source.

You've probably seen the discussion:

http://en.wikipedia.org/wiki/Equivalence_principle
- notice there is a "cleanup needed" notice on this article. The "weak" section, in particular, seems to mix up bits of each of the other sections.
I think the wikipedia article is trying to distinguish the case where gravity is treated an extra force (so it needs a special rule to cope) vs being an effect of geometry (which dosn't) but it is difficult to be sure. Look elsewhere for clarification.

Also have a look at:
http://www.mathpages.com/home/kmath629/kmath629.htm
... takes a historical perspective, points out that the definitions change with time and context. It's more consistent than Wikipedia.

Cultural history perspective, closer examination:
http://einstein.stanford.edu/STEP/information/data/gravityhist2.html

An example of an introductory lesson on the same:
https://briankoberlein.com/2013/09/07/equivalent-principles/

TLDR: http://www.reference.com/browse/equivalence+principle [Broken]
http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/grel.html

@Vanadium50: thanks, I just found that out in the histories above.

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Wikipedia is not a recommended source.

You've probably seen the discussion:

http://en.wikipedia.org/wiki/Equivalence_principle
- notice there is a "cleanup needed" notice on this article. The "weak" section, in particular, seems to mix up bits of each of the other sections.
I think the wikipedia article is trying to distinguish the case where gravity is treated an extra force (so it needs a special rule to cope) vs being an effect of geometry (which dosn't) but it is difficult to be sure. Look elsewhere for clarification.

Also have a look at:
http://www.mathpages.com/home/kmath629/kmath629.htm
... takes a historical perspective, points out that the definitions change with time and context. It's more consistent than Wikipedia.

Cultural history perspective, closer examination:
http://einstein.stanford.edu/STEP/information/data/gravityhist2.html

An example of an introductory lesson on the same:
https://briankoberlein.com/2013/09/07/equivalent-principles/

TLDR: http://www.reference.com/browse/equivalence+principle [Broken]
http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/grel.html

@Vanadium50: thanks, I just found that out in the histories above.

Thanks for the references above. They have alternatives ideas of gravitons instead of geometry as cause of gravity. But the iron ball should have more gravitons interchange with earth versus the cotton and yet they are attracted to earth the same degree. Why do they fall together at same time in the theory of the gravitons? Do you know what are these people reasonings?

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Staff Emeritus
2021 Award
B. This "graviton theory" of yours is not the graviton theory of everybody else. I don't know where you got these ideas, but they are totally wrong.

B. This "graviton theory" of yours is not the graviton theory of everybody else. I don't know where you got these ideas, but they are totally wrong.

I wrote this thread because I'm interested in violations of Lorentz invariance CPT, EP and there are many tests that search for them. I originally
saw this:

http://www.physics.indiana.edu/~kostelec/faq.html

I thought Equivalence Principle violation is part of Lorentz violations. What kind of spacetime symmetry does EP fall under.. is it Lorentz symmetry? Whatever. I read Lorentz violation is related to search for gravitons. And Im trying to understand how it is related to gravitons. If we have gravitons.. does it mean the geometry is not needed anymore? and gravitons artificaly created the geometry in the computations? In wiki:

"In physics, the graviton is a hypothetical elementary particle that mediates the force of gravitation in the framework of quantum field theory. If it exists, the graviton is expected to be massless (because the gravitational force appears to have unlimited range) and must be a spin-2 boson. The spin follows from the fact that the source of gravitation is the stress–energy tensor, a second-rank tensor (compared to electromagnetism's spin-1 photon, the source of which is the four-current, a first-rank tensor). Additionally, it can be shown that any massless spin-2 field would give rise to a force indistinguishable from gravitation, because a massless spin-2 field must couple to (interact with) the stress–energy tensor in the same way that the gravitational field does. Seeing as the graviton is hypothetical, its discovery would unite quantum theory with gravity.[4] This result suggests that, if a massless spin-2 particle is discovered, it must be the graviton, so that the only experimental verification needed for the graviton may simply be the discovery of a massless spin-2 particle.[5]"

A.T.
The words "the equivalence principle" has a special meaning in physics - it refers (usually) to the principle in general relativity where gravitation is equivalent to an accelerating reference frame.
And in an accelerating reference frame, all free falling object experience the same coordinate acceleration, which is exactly what the drop experiment demonstrates.

Staff Emeritus
2021 Award
I know that. What I don't know is where you got your statements in #16 from.

An airplane and cotton falls at same rate to earth. If gravity is caused by attraction between particles.. there are more particles in the airplane than the cotton so the airplane should attract the earth more. I guess this attraction is not what graviton is. I'm reading all archives now about gravitons in this forum. It seems some theorize them as just quanta of the gravitational waves. Meaning without gravitational wave, there are no gravitons. Since the airplane falling to earth doesn't produce gravitatational waves, there is no graviton exchange between the earth and the airplane.

Whatever, since gravity is not a force, why call it a fundamental force... we should have 3 fundamental forces only..

phinds
Gold Member
2021 Award
An airplane and cotton falls at same rate to earth. If gravity is caused by attraction between particles.. there are more particles in the airplane than the cotton so the airplane should attract the earth more. I guess this attraction is not what graviton is. I'm reading all archives now about gravitons in this forum. It seems some theorize them as just quanta of the gravitational waves. Meaning without gravitational wave, there are no gravitons. Since the airplane falling to earth doesn't produce gravitatational waves, there is no graviton exchange between the earth and the airplane.

Whatever, since gravity is not a force, why call it a fundamental force... we should have 3 fundamental forces only..
Yes, I would definitely suggest that you continue your reading. These things will be cleared up better that way than by some brief comments on an internet forum.

Yes, I would definitely suggest that you continue your reading. These things will be cleared up better that way than by some brief comments on an internet forum.

Ok. Going back to the Equivalence Principle.. so the airplane and cotton falling at same rate in vacuum is called the Weak Equivalance Principle (so calling it an EP is not far off at all). But then it is not so strange at all. Larger object has more inertia, larger object has greater amount of quantum fields.. so there should naturally be more resistance in the vacuum producing inertia.. this means the airplane and cotton could fall at same rate due to the cotton having much lighter inertia. Don't you think so?

Anyway. If we need to be forced to think Gravity is geometry and the airplane and cotton fall at same rate because they are in geodesic path in curved spacetime.. in other words, time is why the airplane falls down.. but in the Wheeler-Dewitt Equation, time is zero. So how could the geodesic thing occur when time is zero?

PeterDonis
Mentor
If gravity is caused by attraction between particles.. there are more particles in the airplane than the cotton so the airplane should attract the earth more.

It does; the force between the airplane and the Earth is larger than the force between the cotton and the Earth. But there are also more particles in the airplane, so it has more inertia and therefore accelerates less in response to a given force. The two effects exactly cancel out; that's what experiments like the Eotvos experiment are verifying. When people talk about inertial mass being equal to gravitational mass, that's what they're talking about.

I guess this attraction is not what graviton is.

The graviton is the (hypothesized) particle associated with the quantum aspects of gravity. But gravity is so weak as an interaction that we have no prospect of measuring quantum aspects of it any time soon.

It seems some theorize them as just quanta of the gravitational waves.

That's one aspect of them, yes; gravitational waves are fluctuations in spacetime curvature, and the quantum aspects of those fluctuations are one kind of graviton. But not the only kind; see below.

Since the airplane falling to earth doesn't produce gravitatational waves, there is no graviton exchange between the earth and the airplane.

This is not correct. The interaction between the Earth and the airplane involves virtual gravitons, just as any quantum interaction that appears as a static force involves virtual particles. (At least, that's what the best model we have of gravitons as the quantum aspect of gravity predicts. As I said above, we aren't going to be able to experimentally test any of this any time soon.)

since gravity is not a force, why call it a fundamental force... we should have 3 fundamental forces only..

From a quantum field theory point of view, gravity is an interaction like the others. (Note that I used the word "interaction", not "force"; it's a better word because it's more general. Not all aspects of the fundamental interactions appear as what we usually think of as a "force".)

Larger object has more inertia, larger object has greater amount of quantum fields.. so there should naturally be more resistance in the vacuum producing inertia.. this means the airplane and cotton could fall at same rate due to the cotton having much lighter inertia. Don't you think so?

Oh, definitely. See above. This is not a new idea. (Except for the part about "greater amount of quantum fields", which I don't understand.)

If we need to be forced to think Gravity is geometry and the airplane and cotton fall at same rate because they are in geodesic path in curved spacetime..

This is not inconsistent with viewing gravity as an interaction. It's just a different model. Both models are applicable within their domains of validity. From a quantum field theory point of view, the reason gravity can be viewed at the classical level as geometry is that the quantum interaction mediated by the graviton has certain properties (in particular, it couples to all mass-energy and has spin 2).

in other words, time is why the airplane falls down

I don't see how you get this from what you said in your previous sentence.

in the Wheeler-Dewitt Equation, time is zero

I think you are misunderstanding the Wheeler-DeWitt equation. I also think you need to get a better grounding in the basics before you tackle something as advanced as the Wheeler-DeWitt equation.

It does; the force between the airplane and the Earth is larger than the force between the cotton and the Earth. But there are also more particles in the airplane, so it has more inertia and therefore accelerates less in response to a given force. The two effects exactly cancel out; that's what experiments like the Eotvos experiment are verifying. When people talk about inertial mass being equal to gravitational mass, that's what they're talking about.

The graviton is the (hypothesized) particle associated with the quantum aspects of gravity. But gravity is so weak as an interaction that we have no prospect of measuring quantum aspects of it any time soon.

That's one aspect of them, yes; gravitational waves are fluctuations in spacetime curvature, and the quantum aspects of those fluctuations are one kind of graviton. But not the only kind; see below.

This is not correct. The interaction between the Earth and the airplane involves virtual gravitons, just as any quantum interaction that appears as a static force involves virtual particles. (At least, that's what the best model we have of gravitons as the quantum aspect of gravity predicts. As I said above, we aren't going to be able to experimentally test any of this any time soon.)

From a quantum field theory point of view, gravity is an interaction like the others. (Note that I used the word "interaction", not "force"; it's a better word because it's more general. Not all aspects of the fundamental interactions appear as what we usually think of as a "force".)

Oh, definitely. See above. This is not a new idea. (Except for the part about "greater amount of quantum fields", which I don't understand.)

This is not inconsistent with viewing gravity as an interaction. It's just a different model. Both models are applicable within their domains of validity. From a quantum field theory point of view, the reason gravity can be viewed at the classical level as geometry is that the quantum interaction mediated by the graviton has certain properties (in particular, it couples to all mass-energy and has spin 2).

I don't see how you get this from what you said in your previous sentence.

I think you are misunderstanding the Wheeler-DeWitt equation. I also think you need to get a better grounding in the basics before you tackle something as advanced as the Wheeler-DeWitt equation.

Hi, I've been reading the archives here on all gravitons entry for long time and there are many interesting ones. But there is one where I don't know the meaning of the acronym. It's in the thread "Why are we looking for graviton particles". What does "d.o.f" mean? depth of field? tom.stoer wrote:

"The reason why gravitons may be a useful concept is rather simple: in the weak gravity regime with gravitational waves propagating on a background metric the theory looks similar to a field theory which would result in a quantum theory of a spin-2 particle.

However the are mathematical reasons why this picture does not apply in general: the graviton as fundamental d.o.f. derived for the weak gravity regime fails to provide a consistent quantization of gravity in the non-perturbative regime where quantum effects will become relevant. Therefore all theories of quantum gravity are based on diffent quantization schemes (e.g. LQG) where gravitons are no longer fundamental d.o.f. but derived concepts applicable in a certain approximation, or they are based on a different classical setup (supergravity, strings) where there is hope that the above mentioned difficulties do not apply and where gravitons could play a fundamental role (in string theory the graviton is just one special oscillation of the string)

But in practice the regimes where quantum gravity becomes important are not accessable experimentally! All current searches for gravitational waves are entirely classical and do not deal with gravitons."

Peterdonis. So gravity as purely geometry may just be a temporary thought to make. It's like Newtonian physics as effective field theory. In Newtonian, their spacetime is a stage where things move with respect to it and diffeomorphism invariance doesn't exist. Likewise gravity as purely geometry only and nothing more is also an effective field theory or temporary. And we must not think it is the final thing. It's like we shouldn't treat women as simply geometry (because of their curves), the stone age men may think such but we have complex biology now. I concluded this after reading the following gem by jtbell in the thread (do you also think geometry is just temporary)?

"Theories of quantum gravity (which include gravitons) assume that the "warping" will turn out not to be the only explanation needed for gravity. People working on these theories hope that they will eventually be able to make predictions that differ from what classical general relativity would predict, and that can be tested by experiment. If this succeeds, then the new theories would supersede classical general relativity as the fundamental view of gravity. GR would probably continue to be used for practical calculations in areas where it already works, just as Newtonian gravity is still used for many practical calculations."

PeterDonis
Mentor
What does "d.o.f" mean?

Degrees of freedom.

People working on these theories hope that they will eventually be able to make predictions that differ from what classical general relativity would predict, and that can be tested by experiment. If this succeeds, then the new theories would supersede classical general relativity as the fundamental view of gravity. GR would probably continue to be used for practical calculations in areas where it already works, just as Newtonian gravity is still used for many practical calculations."

Yes, if this succeeds, then the new theories (whatever theories of quantum gravity turn out to be validated by experiment) would become the fundamental theory of gravity, and GR would be an effective theory built on them. But we're not anywhere close to that happening.

Degrees of freedom.

Yes, if this succeeds, then the new theories (whatever theories of quantum gravity turn out to be validated by experiment) would become the fundamental theory of gravity, and GR would be an effective theory built on them. But we're not anywhere close to that happening.

You are sounding like there is possibility GR is primary and it is quantum mechanics that has to be altered. Of all information in the archive, the following is what awakens me to the need for quantizing GR (this would be the last I'd post quote from others.. it's just that these are the clearest I've come across in all archive search). Bcrowell wrote:

"Historically, there was a long period during which Bohr and his followers wanted the atom to be quantized, but wanted the electromagnetic field to be classical. You get severe logical problems with this kind of theory. For example, say a classical wave of ultraviolet light with energy E impinges on a metal surface, and E is equal to 1.01 times the minimum energy needed in order to liberate one (quantized) electron from the surface (i.e., produce the photoelectric effect). By conservation of energy, the UV wave should only be able to eject one electron. But since the UV wave is purely classical, it can't carry the quantum-mechanical correlations that would tell electron A that it can't be ejected because atom B, far away, was the one that already got ejected. Therefore you get a violation of conservation of energy.

If you substitute a gravitational wave for the electromagnetic wave, I think you get exactly the same kind of contradictions if you assume that gravity is a purely classical field interacting with quantized matter."

but in this paper Peterdonis. http://arxiv.org/abs/0802.1978

It appears there is possibility of coupling unquantized GR to quantized QM.

I mentioned all this because I'd like to hear your opinion. What do you think is the best road to quantum gravity?

If you will think of quantum mechanics Einstein style. Even though we get random collapse... but if you will do the experiment 100 times.. you will get a statistical distribution akin to thermodynamics. So what if the whole ensemble is classical and this is what can couple a completely classical GR? Have you or anyone heard anything like this?

PeterDonis
Mentor
You are sounding like there is possibility GR is primary and it is quantum mechanics that has to be altered.

Yes, I think this is a possibility, though I'm not sure how likely it is. There aren't many physicists who think this will happen, but there are some. Freeman Dyson, who was one of the key people who helped develop quantum electrodynamics, is one of them. See, for example, his article in this newsletter from the Institute for Advanced Study (where he's a member):

http://www.ias.edu/files/pdfs/publications/letter-2013-spring.pdf

What do you think is the best road to quantum gravity?

I don't really have an opinion at this point; we know too little and we aren't going to get any relevant experimental data any time soon.

Even though we get random collapse... but if you will do the experiment 100 times.. you will get a statistical distribution akin to thermodynamics

This sounds something like a version of GRW theory:

http://en.wikipedia.org/wiki/Ghirardi–Rimini–Weber_theory

At one point Roger Penrose, among others, was working on a theory that would link the frequency of spontaneous collapse of a superposition to the difference in spacetime curvature produced by the different states being superposed. I don't know if anything ever came of that. Again, our ability to do experiments in this regime is basically nonexistent, so it's really hard to pin down what theories we should be looking at; there are too many possibilities.

Yes, I think this is a possibility, though I'm not sure how likely it is. There aren't many physicists who think this will happen, but there are some. Freeman Dyson, who was one of the key people who helped develop quantum electrodynamics, is one of them. See, for example, his article in this newsletter from the Institute for Advanced Study (where he's a member):

http://www.ias.edu/files/pdfs/publications/letter-2013-spring.pdf

I don't really have an opinion at this point; we know too little and we aren't going to get any relevant experimental data any time soon.

This sounds something like a version of GRW theory:

http://en.wikipedia.org/wiki/Ghirardi–Rimini–Weber_theory

At one point Roger Penrose, among others, was working on a theory that would link the frequency of spontaneous collapse of a superposition to the difference in spacetime curvature produced by the different states being superposed. I don't know if anything ever came of that. Again, our ability to do experiments in this regime is basically nonexistent, so it's really hard to pin down what theories we should be looking at; there are too many possibilities.

I can't find any information in the archive after searching for "gravitational wave quantum gravity".

My question is. If quantum gravity were discovered and we have to make GR like Newtonian that is replaced.. what would happen to the gravitational wave? If gravity are excitations of spin 2 particles and not directly geometry. How would spin 2 particles radiate gravitational wave??

PeterDonis
Mentor
If quantum gravity were discovered and we have to make GR like Newtonian that is replaced.. what would happen to the gravitational wave?

Nothing at all. GR would still be the classical limit of whatever quantum gravity theory was underneath. So at the level at which we talk about gravitational waves, the classical level of GR, nothing would be changed.

If gravity are excitations of spin 2 particles and not directly geometry. How would spin 2 particles radiate gravitational wave??

The spin-2 particles wouldn't radiate the wave; they would be the wave, just as spin-1 particles (photons) are electromagnetic waves.