Do Photons Have a Gravitational Effect?

[PAllen]

No, Passionflower is correct. Consider an electron and a positron, together they have a four-momentum of about (1,0,0,0) MeV/c and therefore a mass of about 1 MeV/c². By conservation of four-momentum, after anhilation the resulting collection of photons also has a four-momentum of about (1,0,0,0) MeV/c and therefore a mass of about 1 MeV/c².

Interesting. That must lead to an interesting formula for adding/combining 4-momenta (I guess not surprising; you can't 'add' 4-velocities either). Is there a simple formula for adding two 4 momenta explaining how each may have 0 in the first component (using your convention; I've sometimes seen this as the last component), but combined they have something there?

Thinking further, it makes perfect sense - the analog of momentum cancels, so conservation must make the first component signficant.

 

[atyy]

I guess what is not clear to me is - could Nordstrom gravity account for all the data given in support of light causing spacetime curvature. The data is primarily EP data (which is implemented in Newton as inertial mass = gravitational mass), and Nordstrom gravity does have some form of EP, but light does not cause spacetime curvature in Nordstrom gravity (because Nordstrom gravity couples to the trace of the stress-energy tensor, and the EM field is traceless). Is the precise EP of Nordstrom gravity (but Will says Nordstrom has strong EP!?) different from the EP of GR?

 

[Dale]

Interesting. That must lead to an interesting formula for adding/combining 4-momenta (I guess not surprising; you can't 'add' 4-velocities either). Is there a simple formula for adding two 4 momenta explaining how each may have 0 in the first component (using your convention; I've sometimes seen this as the last component), but combined they have something there?

Thinking further, it makes perfect sense - the analog of momentum cancels, so conservation must make the first component signficant.

Sorry for the confusion, I should have been more explicit with the convention. So this is the convention with time as the first component (ct,x,y,z), for the four-momentum that results in (E/c,px,py,pz). So an electron and a positron each have a four-momentum of (.5,0,0,0) MeV/c so the total system is:
(.5,0,0,0)+(.5,0,0,0)=(1,0,0,0) MeV/c

When they anhilate they will generally produce two photons, for convenience let's say that one goes in the +x direction. Then for momentum to be conserved the other must go in the -x direction and they must be equal in energy.
(.5,.5,0,0)+(.5,-.5,0,0)=(1,0,0,0) MeV/c

Note that the mass of each individual photon (m²c²=E²/c²-p²) is 0, even though the mass of the system of photons is 1 MeV/c².

 

[threadmark]

Please! In this thread there has been nothing but manipulation of Albert’s general relativity to produce a theoretical scenario where imaginary particle states can produce gravitation. Its not the photon with gravitation that is bugging me, it’s the use of Albert’s notions in producing general relativity to prove your hypothesis of bosons produce gravity is false because the theory in its entirety didn’t explain theoretical zero mass particles producing gravity and your existence to interpret the information in such a way to try and change theoretical understandings of a particle. What are you suggesting number 1? Where in Einstein’s formulas does it produce an explanation of zero mass particles producing gravity 2. And please note that a wave’s propagation can be seen as action at a distance but not gravity. How can you compress a field for density not field compression or amplification which will create distortion not density . Propagation is not gravity. Waves like light (sigh), do not produce gravity

 

[PAllen]

I guess what is not clear to me is - could Nordstrom gravity account for all the data given in support of light causing spacetime curvature. The data is primarily EP data (which is implemented in Newton as inertial mass = gravitational mass), and Nordstrom gravity does have some form of EP, but light does not cause spacetime curvature in Nordstrom gravity (because Nordstrom gravity couples to the trace of the stress-energy tensor, and the EM field is traceless). Is the precise EP of Nordstrom gravity (but Will says Nordstrom has strong EP!?) different from the EP of GR?

Nordstrom says light isn't deflected at all by gravity; it says light doesn't produce gravity. Together, these are consistent with any ep.

 

[atyy]

Nordstrom says light isn't deflected at all by gravity; it says light doesn't produce gravity. Together, these are consistent with any ep.

Yes, we can rule out Nordstrom based on non-EP data such as global light deflection. So I guess we use EP plus global light deflection? Wow, this is delicate.

 

[atyy]

Please! In this thread there has been nothing but manipulation of Albert’s general relativity to produce a theoretical scenario where imaginary particle states can produce gravitation. Its not the photon with gravitation that is bugging me, it’s the use of Albert’s notions in producing general relativity to prove your hypothesis of bosons produce gravity is false because the theory in its entirety didn’t explain theoretical zero mass particles producing gravity and your existence to interpret the information in such a way to try and change theoretical understandings of a particle. What are you suggesting number 1? Where in Einstein’s formulas does it produce an explanation of zero mass particles producing gravity 2. And please note that a wave’s propagation can be seen as action at a distance but not gravity. How can you compress a field for density not field compression or amplification which will create distortion not density . Propagation is not gravity. Waves like light (sigh), do not produce gravity

There is no controversy that GR predicts that light bends spacetime.

 

[PAllen]

According to GR a light wave packet produces gravity. This is called a pp-wave spacetime.

However, although I understand how attractive it is, I would caution such physicists about making that intuitive jump. A photon is not merely a small classical wave packet. It may not even be localized nor have any definite energy. Since a photon may not have a definite energy or locationit quickly becomes difficult to figure out how much curvature it should produce and where it should produce it.

Point granted. However, we can agree that 'rest mass being zero (while energy and momentum nonzero) is a false reason to argue that something doesn't produce gravity (as threadmark has argued). Thus, if a quantum gravity theory eventually says individual photons don't produce gravity, it will not be simply because their rest mass is zero.

 

[PAllen]

Yes, we can rule out Nordstrom based on non-EP data such as global light deflection. So I guess we use EP plus global light deflection? Wow, this is delicate.

I would say we don't rule out Nordstrom based on EP at all. It is the only theory other than GR that is indistinguishable from it in this way. For other theories of gravity, we may use EP to rule them out. For Nordstrom we simply use major conflict with experiment.

 

[Dale]

the theory in its entirety didn’t explain theoretical zero mass particles producing gravity ... Where in Einstein’s formulas does it produce an explanation of zero mass particles producing gravity 2.

The Aichelburg–Sexl ultraboost solution does exactly that for a classical localized zero mass particle. You really should read up on pp-wave spacetimes.

Waves like light (sigh), do not produce gravity

According to GR they do.

Are you claiming that GR is wrong, or do you just not understand GR?

 

[Dale]

Point granted. However, we can agree that 'rest mass being zero (while energy and momentum nonzero) is a false reason to argue that something doesn't produce gravity (as threadmark has argued). Thus, if a quantum gravity theory eventually says individual photons don't produce gravity, it will not be simply because their rest mass is zero.

Correct, it will be because of other more subtle issues, of which there are plenty!

 

[Passionflower]

Thus, if a quantum gravity theory eventually says individual photons don't produce gravity, it will not be simply because their rest mass is zero.

Perhaps, but I am definitely not absolutely sure of that. We really can't tell as such a theory does not even exist.

 

[atyy]

I would say we don't rule out Nordstrom based on EP at all. It is the only theory other than GR that is indistinguishable from it in this way. For other theories of gravity, we may use EP to rule them out. For Nordstrom we simply use major conflict with experiment.

Yes, that's what I meant. My second sentence was about trying to figure out if the EP data shows light bends spacetime, ie. use EP to get down to GR and Nordstrom, then use global light deflection to rule out Nordstrom in which light does not bend spacetime. (And yes, this seems like a very delicate chain of reasoning.)

 

[atyy]

We do have a quantum theory of gravity and it says photons cause spacetime curvature.

http://arxiv.org/abs/gr-qc/9512024

 

[PAllen]

Correct, it will be because of other more subtle issues, of which there are plenty!

I will throw out another intuitive argument to believe that photons probably produce gravity. That is, if a quantum gravity theory ends up being similar in key ways to QFT, and if gravity is mediated by gravitons, and if gravitons interact with all particles (a lot of ifs, but all fairly commonly assumed in many guesses about general features of quantum gravity), then it would seem to follow that photons must both produce and respond to gravity.

 

[threadmark]

Point granted. However, we can agree that 'rest mass being zero (while energy and momentum nonzero) is a false reason to argue that something doesn't produce gravity (as threadmark has argued). Thus, if a quantum gravity theory eventually says individual photons don't produce gravity, it will not be simply because their rest mass is zero.

No, it would be propagation and momentum that world say it wouldn’t affect space time because
(A ) a light wave …sigh is not a constant state and its bodies to create this light wave…sigh constantly shift positions, so they would not occupy the same point in space. Although a solar system is constantly moving, it to would never pass the same point in space twice. But because its state is coherent its affect on space time is constant. Where you are all saying that the theoretical occurrence of photons somehow have a moment of vector symmetry would create gravity. I would humor this by saying fluctuation in space time but that’s it

(B) the photon has no (mass)

 

[Passionflower]

We do have a quantum theory of gravity and it says photons cause spacetime curvature.

http://arxiv.org/abs/gr-qc/9512024

That looks like a darn interesting article!

 

[Dale]

No, it would be propagation and momentum that world say it wouldn’t affect space time because
(A ) a light wave …sigh is not a constant state and its bodies to create this light wave…sigh constantly shift positions, so they would not occupy the same point in space. Although a solar system is constantly moving, it to would never pass the same point in space twice. But because its state is coherent its affect on space time is constant. Where you are all saying that the theoretical occurrence of photons somehow have a moment of vector symmetry would create gravity. I would humor this by saying fluctuation in space time but that’s it

(B) the photon has no (mass)

threadmark, I wish you would learn to write coherent sentences. I can't parse this post either. I get the feeling that there is a severe language barrier.

Can you answer my previous question, are you claiming that GR is wrong?

 

[threadmark]

No, I am saying you are all wrong. Every person adding to this discussion in affirmative is wrong. I agree that a state of momentary coherence/vector symmetry in photons would create a fluctuation at best but I don’t think you understand the difference between a curvature and a fluctuation. A gravitational field in my opinion and I have done math to prove this is... Gravitational fields exerts equal amount of pressure on the propagating body due to momentum shifts. This can not be the case due to Alberts notation that if the sun was to instantly vanish you would feel the affects instantly, you should note that he was suggesting that gravity is a state produced from mass density and is dependant on it. Though I think he is wrong in this notation, Its no ground to throw the theory out all together. My opinion\formulas prove that gravity is a state of space time not the density and complete dependence of a body of mass and the creation of this field is time dependant on the coherence of matter. So if a body of mass instantly vanished, the gravity would remain continuing the velocity decreasing its magnitude or field strength by the same amount as it was created so the field would take the same time disperse as it did to create. That is my theory and would not say Albert’s theory is wrong he just hasn’t had evidence to create formulas that would predict this occurrence. Yes I have dyslexia it means numbers and words get jumbled. But I assure you my white boards provent me from making to manymistakes in formulation and math so if its on the board i can correct the numbers just not in my head. I am sorry you can't understand me.

 

[Passionflower]

My opinion\formulas prove...

Ok, let's cut this short shows us your formulas.

 

[pervect]

No, I am saying you are all wrong. Every person adding to this discussion in affirmative is wrong. I agree that a state of momentary coherence/vector symmetry in photons would create a fluctuation at best but I don’t think you understand the difference between a curvature and a fluctuation.

Do you have a reference to support your viewpoint? Or is this some personal theory? Have you looked at the references I supplied?

 

[threadmark]

the page is not formated like i need it to. I am tired I am going to bed

 

[PAllen]

This can not be the case due to Alberts notation that if the sun was to instantly vanish you would feel the affects instantly, .

This is false, according to Einstein and every reputable expert on general relativity. If you believe that this is what Einstein's theory says, you have essentially no understanding of it.

1) This scenario cannot actually be set up in GR; there are no solutions where matter simply disappears;

2) If you set up a solution where matter distribution rapidly changes, the change in curvature/gravity propagates with speed c.

3) There is a significant, limited sense in which gravity appears to propagate instantaneously. The direction of attraction is to the quadratically extrapolated position of a gravitating source. This means that a gravitating body must have changing acceleration before you could (in principle) detect the finite propagation speed of gravity. Here is a famous paper explaining this:

http://arxiv.org/abs/gr-qc/9909087

------

 

[Vanadium 50]

I think it is a bad idea to claim to know how a quantum theory of gravity would function.

But we don't need a quantum theory of gravity to answer this question. All we need is QED in curved spacetime, and that is no problem at all.

1. Observationally, we know single photons fall.
2. GR says that momentum is conserved (in this case).
3. Therefore, single photons must gravitate.

That conclusion can only be escaped by asserting GR is incorrect. Alternatively, one can put in a single photon in T and again GR shows the effect on G. Again, the conclusion can only be escaped by asserting GR is incorrect.

 

[Passionflower]

But we don't need a quantum theory of gravity to answer this question. All we need is QED in curved spacetime, and that is no problem at all.

1. Observationally, we know single photons fall.
2. GR says that momentum is conserved (in this case).
3. Therefore, single photons must gravitate.

That conclusion can only be escaped by asserting GR is incorrect. Alternatively, one can put in a single photon in T and again GR shows the effect on G. Again, the conclusion can only be escaped by asserting GR is incorrect.

I do not believe the topic is whether photons gravitate, the topic is whether a single photon can create a gravitational field, these are in my mind two different things.

 

[cragar]

When we have pair production , a photon turns into an electron and positron , I would like to think that the gravitational field of the electron and positron came from the photon,
And vice versa if we collide an electron and positron and we get a photon out, It seems like the G field of the electron and positron would get transferred to the photon.

 

[D H]

I do not believe the topic is whether photons gravitate, the topic is whether a single photon can create a gravitational field, these are in my mind two different things.

Caveat: I am not an expert in GR.

My understanding is that a single photon does not create a gravitational field. A collection of photons can, however. The intrinsic mass of a collection of particles is

m = \frac{\sqrt{E^2 - p^2c^2}}{c^2}

where E is the total energy of the collection of particles and p² is the square of the net momentum of the collection. While energy and momentum are both frame-dependent quantities, the difference E² - p²c² is frame independent.

For a single photon, E=pc, so the intrinsic mass of a single photon is zero. Now consider photons created by an electron-positron annihilation. In the rest frame of the (former) electron-positron pair, the created photons have zero net momentum. The total energy of these photons is equal to the 2m_ec² plus any kinetic energy of the electron-positron pair. The intrinsic mass of the photons created by this annihilation event is identically equal to the intrinsic mass of the electron + positron system prior to the annihilation event.

 

[Dale]

No, I am saying you are all wrong.

If you believe that GR is correct and that we are wrong then you do not understand GR. I recommend further study. In particular, you should learn about pp-wave spacetimes.

 

[Dale]

But we don't need a quantum theory of gravity to answer this question. All we need is QED in curved spacetime, and that is no problem at all.

1. Observationally, we know single photons fall.
2. GR says that momentum is conserved (in this case).
3. Therefore, single photons must gravitate.

That conclusion can only be escaped by asserting GR is incorrect.

I like this. That is a good point.

Alternatively, one can put in a single photon in T and again GR shows the effect on G.

This one is the dangerous approach. How can you define T when the photon does not have a definite energy, momentum, and location at any given time? Actually, I am not even sure that spacetime can be represented by a manifold at sufficiently small scales due to quantum effects.

 

[Vanadium 50]

A single photon does create a gravitational field, because T is non-zero. If T is non-zero, so is G.

 

[Dale]

If T is non-zero then what is T?

 

[Driftwood1]

"everything" distorts/curves/interrupts etc space-time

thats what gravity is

 

[pervect]

While it is true that photons do not have mass, it is incorrect to conclude that because they don't have a mass, they can't cause gravity. Photons don't need mass to cause gravity, all they need is energy and momentum, which they do have, because it's the stress energy tensor that causes gravity and not "mass".

While GR is a classical theory, and doesn't have anything to do directly with photons per se, you can model a photon in GR as a packet of light. Which is essentially what Tolman et al did in the 1931 paper I mentioned earlier, with the results I mentioned - parallel light packets don't attract each other, there is no "self focusing" effect of light due to gravity, but beams in opposite directions do interact gravitationally.

 

[Troponin]

Please! In this thread there has been nothing but manipulation of Albert’s general relativity to produce a theoretical scenario where imaginary particle states can produce gravitation. Its not the photon with gravitation that is bugging me, it’s the use of Albert’s notions in producing general relativity to prove your hypothesis of bosons produce gravity is false because the theory in its entirety didn’t explain theoretical zero mass particles producing gravity and your existence to interpret the information in such a way to try and change theoretical understandings of a particle. What are you suggesting number 1? Where in Einstein’s formulas does it produce an explanation of zero mass particles producing gravity 2. And please note that a wave’s propagation can be seen as action at a distance but not gravity. How can you compress a field for density not field compression or amplification which will create distortion not density . Propagation is not gravity. Waves like light (sigh), do not produce gravity

This thread is in the relativity section, and according to GR, "light" does produce gravity.

If you looked at a completely "empty" Universe that was void of any form of energy, matter, radiation, etc., in a 4 dimensional spacetime where

<br /> \mathbf{R(X,Y)Z}=R^{a}_{\ bcd}X^{c}Y^{d}Z^{b}=0=R_{bd} <br />

and then let a wave of radiation pass through then you get the interesting scenario where calculating the Ricci tensor will still give you zero
R_{bd}=0
but the Riemann tensor isn't required to vanish and you can find calculations where R^{a}_{\ bcd}X^{c}Y^{d}Z^{b}\neq 0, ie., the radiation causes curvature of the spacetime (in 4 or greater dimensions).The fact that GR predicts that light alters the spacetime geometry is much more general, but that is a situation where nothing exists to cause curvature, so "gravitational" curvature is entirely from the radiation.

 

[Vanadium 50]

If T is non-zero then what is T?

It's been a while, but I believe the upper left 2x2 is 1 and the other components are 0.

 

[bcrowell]

Please! In this thread there has been nothing but manipulation of Albert’s general relativity to produce a theoretical scenario where imaginary particle states can produce gravitation.

I posted information about experimental tests in #4, but your #8 showed no signs of having looked at that information. I posted more about experimental tests in #23, #26, and #31, but your #34 showed no signs of having looked at that, either. In #35, I asked whether you had read any of the four references to experimental peer-reviewed scientific papers I gave in #31. You replied in #37 without giving any signs that you had looked at any of those references. Therefore it is simply not true that this thread has consisted of nothing but theoretical arguments. You have been provided with evidence about experiments, but you've simply chosen not to read it.

 

[Dale]

It's been a while, but I believe the upper left 2x2 is 1 and the other components are 0.

I believe that is correct for a classical pulse of light at a definite location and of known (unit) energy density propagating in the +x direction. But that is not the same as an EM quantum with uncertain location and non-definite energy. What you are describing is simply GR for classical pulses of light, not for photons. Do you see the difference and why I am reluctant to make conclusions for a photon based on classical pulses of light?

 

[atyy]

There's a little bit about coupling quantum gravity to photons in section 3.2.1 of http://www.emis.de/journals/LRG/Articles/lrr-2004-5/ .

 

[Tantalos]

I like this. That is a good point.

This one is the dangerous approach. How can you define T when the photon does not have a definite energy, momentum, and location at any given time? Actually, I am not even sure that spacetime can be represented by a manifold at sufficiently small scales due to quantum effects.

The QM theory gives only probability distributions of photon location, its momentum and energy, but photon must have a definite energy, momentum and location in time that we cannot know by QM.

 

[pervect]

Energy, momentum, etc would only be expectations for a photon. But we expect that any quantum theory of light is going to have to include gravity in the classical limit, because we know that collections of photons do gravitate under certain circumstances, and we know which ones.

Furthermore, I already posted a link to a book by Zee which discusses how you get quantum gravity for the photon perturbatively - though it's outside the scope of GR.

So, I'd suggest continuing the QM side of the debate in the "beyond the standard model" forum.

I'd also encourage people interested to look at the reference by Zee, perhaps it got lost in the shufle.

 

[johne1618]

No. According to GR the source of gravity is the stress-energy tensor. There are 10 independent components in the stress-energy tensor. Energy is only one of those 10 components.

http://en.wikipedia.org/wiki/Stress-energy_tensor

We do not have a working theory of quantum gravity at the time so I cannot answer your question wrt photons, however I can answer it wrt classical pulses of light. Pulses of light have energy, they also have momentum, so several of the components of stress energy tensor will be non-zero. So light can be a source of gravity.

I wonder what you think of the following thought experiment by Dimitry67 that seems to show that parallel beams of light will not converge. Thus it is as though light can't be a source of gravity.

Consider two massive objects, separated by some distance, flying in the same direction at velocity v according to an observer. In their inertial system they collide, say, in 1s. For the observer this process takes longer because of the time dilation. The faster the two objects are flying the longer it takes. In the limit where v --> c they never converge according to the observer.

 

[Dale]

I wonder what you think of the following thought experiment by Dimitry67 that seems to show that parallel beams of light will not converge. Thus it is as though light can't be a source of gravity.

Dimitry67 is correct, but you should not take the quote out of context. The bolded conclusion above does not follow, as you can clearly see by considering the full quote:

This is correct, light beams create gravity.

However, when it was discussed here about 1 or 2 y ago, I remember that someone (with much deeper knowledge of GR - I am just a layman) told me that:

2 parallel light beams going in the same direction do not attract (even they attract the surrounding objects)
2 parallel light beams going in opposite directions do attract.

The first fact might be clear if we look at 2 massive objects, separated by some distance, flying in the same direction. In their inertial system they collide, say, in 1s. For an external observer, this process would take longer because of the time dilation. The faster 2 objects are flying the longer it takes. You can think about the case N1 as a limit where v --> c (it takes forever)

 

[GrayGhost]

So,

What's the verdict? Threadmark has no idea, or Threadmark is ahead of his time?

What percentage of the universe's net gravitational field would be due to electromagnestism?

GrayGhost

 

[johne1618]

Dimitry67 is correct, but you should not take the quote out of context. The bolded conclusion above does not follow, as you can clearly see by considering the full quote:

I stand corrected - I did take Dimitry67's argument out of context.

I did it for two personal reasons I guess.

1/ I like the argument.

2/ I have a "pet theory" that the inertia of a particle with rest mass is caused by retarded gravitational waves impinging on the particle from all the other massive particles in the Universe. I believe that initially massless particles respond to this by following circular orbits whose rotation energy gives half the mass/energy of the particle (the rest being in the mutual gravitational energy between the particle and the rest of the Universe). I am trying to formalise Mach's Principle. I want to argue that light is different so that it does not pick up an inertia. I probably need to think about my theory more before I can decide what it says about light. I'm not using GR itself but an approximation to it called gravitomagnetism that is like Maxwell's theory. In fact I should probably just stick to trying to understand electromagnetically induced inertia for the moment as i feel on safer ground with EM. (I only have at best an undergraduate understanding of physics).

 

[Dale]

I have a "pet theory" ... I only have at best an undergraduate understanding of physics

Hmm.

 

[Dale]

What's the verdict? Threadmark has no idea, or Threadmark is ahead of his time?

Threadmark is wrong. Was there any ambiguity?

 

[bcrowell]

What percentage of the universe's net gravitational field would be due to electromagnestism?

If you mean the gravitational field as defined in Newtonian mechanics, that isn't an unambiguously well defined thing in GR. By the equivalence principle, the gravitational field at a given point can be anything you like, depending on your frame of reference. In the frame of an observer at rest with respect to the cosmic microwave background, the gravitational field is zero by symmetry (in a homogeneous and isotropic cosmological model).

Interpreting your question more loosely, the answer is that the universe was radiation-dominated at one time, then matter-dominated, and is now vacuum-dominated.

 

[GrayGhost]

No,

All I meant was that if photons gravitate, then they produce gravitation. The cosmos has some net collective gravitational field. Given photons are everywhere, I was just wondering what percentage of the cosmic gravitational field would owe to EM?

GrayGhost

 

[bcrowell]

The cosmos has some net collective gravitational field.

That's incorrect, for the reasons given in #97.

 

[GrayGhost]

That's incorrect, for the reasons given in #97.

hmm.