Photons and gravity

This came up in the thread on consciousness and quantum mechanics, but it deserves its own thread.
I know that light is 'bent' by gravity, so it is affected by gravity. Einstein explains this by saying that spacetime is curved and the light is in fact moving in a straight line.

In the famous two split experiment the observer (a subject of discussion in the other thread) has no way of knowing which slit a photon has gone through. If this is the case, can we correctly conclude that the photon does not exert any gravitational pull on the universe, as, if it did, its path would be detectable by that means?

In general relativity, energy and mass are equivalent when it comes to bending spacetime, it's just the c² factor which comes into play because of our use of SI units. So a photon does curve spacetime, but also moves in a curved path due to the curvature of other mass/energy density distributions elsewhere.

Originally posted by KazetNagorra
In general relativity, energy and mass are equivalent when it comes to bending spacetime, it's just the c² factor which comes into play because of our use of SI units. So a photon does curve spacetime, but also moves in a curved path due to the curvature of other mass/energy density distributions elsewhere.

So why can't we detect photons using the fact that they curve space time? We can detect everything else that way.

Originally posted by twhitehead
So why can't we detect photons using the fact that they curve space time? We can detect everything else that way.

Because the curvature they cause is negligible for most purposes and hard to distinguish from other mass/energy density distributions.

Originally posted by KazetNagorra
Because the curvature they cause is negligible for most purposes and hard to distinguish from other mass/energy density distributions.

That is not good enough. If the curvature is possible to distinguish by any means whatsoever, the two slit experiment would not work and light would cease to behave like waves.

Originally posted by twhitehead
That is not good enough. If the curvature is possible to distinguish by any means whatsoever, the two slit experiment would not work and light would cease to behave like waves.

I don't get that - photons are always localized to some extent so it's possible to attribute an energy density to them (and the energy/mass density formalism can only be an approximation anyway since it fails at small length scales).

Originally posted by KazetNagorra
I don't get that - photons are always localized to some extent so it's possible to attribute an energy density to them (and the energy/mass density formalism can only be an approximation anyway since it fails at small length scales).

There may be some experiment possible using the difference between say, Thz waves (wavelengths longer than IR, shorter than radio waves) and gamma rays whose wavelength is extremely short, less than one nanometer. I am thinking a gamma ray would bend space more than Thz waves, maybe some experiment could differentiate between the two, (just supposerizing here🙂 Say two Thz beams crossing but not touching each other say at 30 degrees apart coming within one wavelength then diverging, and the same thing using two gamma ray beams. If you looked at the path closely a few km after they converged, maybe the gamma ray beam would have changed its direction of travel a bit, more than the Thz waves anyway. I was just thinking the two beams would differ in energy and thus the effect on space-time curvature.

Originally posted by KazetNagorra
I don't get that - photons are always localized to some extent so it's possible to attribute an energy density to them (and the energy/mass density formalism can only be an approximation anyway since it fails at small length scales).

The two slit experiment essentially claims that because we have no way of knowing which slit a photon went through, it therefore went through both - and interfered with itself - thus resulting in wavelike behavior and an interference pattern. (of course the claims by others in the other thread that a conscious entity is somehow involved seems wholly incompatible with this interpretation, so if anyone has another version lets hear it.).

One important point to note is that the 'slits' may be billions of light years apart (galaxy cluster lensing), so when you say that a photon is localized, do you realize that this could mean that it could be one one side of the galaxy or another?

It is my claim that if the photon, during its travel, affects space-time in any measurable way - regardless of whether or not we have equipment to detect it - then the universe would 'know' which route it took and the wavelike behavior would disappear.

If I am wrong, I would love and explanation.

I realize that the two slit experiment works for electrons too - and I would like an explanation as to why that is the case. ie why we cannot detect their gravitational pull.

Originally posted by twhitehead
I realize that the two slit experiment works for electrons too - and I would like an explanation as to why that is the case. ie why we cannot detect their gravitational pull.

The gravitational pull exists for both photons and electrons, even by gravitons themselves.
But we have to understand that the gravitational force is by far the weakest compared to the other three fundamental forces.
Our technology has not yet found out a way to detect the grav force from very light particles.

Originally posted by twhitehead
The two slit experiment essentially claims that because we have no way of knowing which slit a photon went through, it therefore went through both - and interfered with itself - thus resulting in wavelike behavior and an interference pattern. (of course the claims by others in the other thread that a conscious entity is somehow involved seems wholly inco ...[text shortened]... an explanation as to why that is the case. ie why we cannot detect their gravitational pull.

I'm no expert in astronomy, but in galaxy cluster lensing there is no interference between single photons light years apart, is there? It's just the effect of the curvature of spacetime, no?

Originally posted by FabianFnas
The gravitational pull exists for both photons and electrons, even by gravitons themselves.
But we have to understand that the gravitational force is by far the weakest compared to the other three fundamental forces.
Our technology has not yet found out a way to detect the grav force from very light particles.

A I stated earlier, our technology is irrelevant. What matters is whether or not it is theoretically possible, ie does the movement of the photon (or electron) have a an effect on the universe such that the universe 'knows' which path the photon or electron took.

Originally posted by KazetNagorra
I'm no expert in astronomy, but in galaxy cluster lensing there is no interference between single photons light years apart, is there? It's just the effect of the curvature of spacetime, no?

It seems that the experiment has not been successfully done, but it is a fairly well known thought experiment:
http://en.wikipedia.org/wiki/Wheeler's_delayed_choice_experiment
http://www.space.com/searchforlife/quantum_astronomy_050113.html

The second link indicates that the experiment might be carried out in the near future.

Originally posted by twhitehead
The two slit experiment essentially claims that because we have no way of knowing which slit a photon went through, it therefore went through both - and interfered with itself - thus resulting in wavelike behavior and an interference pattern. (of course the claims by others in the other thread that a conscious entity is somehow involved seems wholly inco ...[text shortened]... an explanation as to why that is the case. ie why we cannot detect their gravitational pull.

Oh methinks the agency of consiousness is always the case, but firs things first. The sole way I understand the DSE is to accept that in some way the particle changes into a wave or that it manifests its wave aspect when it is unobserved. Therefore I conclude that the wave squeeze itself through both slits and therefore interfares with itself on the other side -and I always keep in mind that we don't work with a physical wave but with a set of numbers.

Now, since the non-locality holds and the EPR is proved false, I conclude we cannot detect the photon’s gravitational pull mainly because in fact Einstein’s “elements of reality” do not exist -therefore the assertion of independent "elements of reality" that each of them has its own separate on-board bit of information is false.

So now I can in no time imagine a photon orbiting from the one "side" of the universe to the other on its wave trace, meeting its "starting" point at the same amplitude as its "starting" point. And I conclude that the wave/motion-like orbit of the photon has to be measured itself as a whole in order to determine how the gravity is affected
😵

Originally posted by KazetNagorra
I'm no expert in astronomy, but in galaxy cluster lensing there is no interference between single photons light years apart, is there? It's just the effect of the curvature of spacetime, no?

Except that when photons have sufficient energy, they can combine and make matter out of that energy so new particles with mass can be created out of massless particles.

Originally posted by black beetle
Now, since the non-locality holds and the EPR is proved false,

What does "EPR" stand for?
And is non-locality proven? By what, may I ask?

I conclude we cannot detect the photon’s gravitational pull mainly because in fact Einstein’s “elements of reality” do not exist -therefore the assertion of independent "elements of reality" that each of them has its own separate on-board bit of information is false.
Why do you always write in such a way that it is impossible to understand what you are saying? Can you state clearly whether or not a photon has gravitational pull and why it cant be detected.

So now I can in no time imagine a photon orbiting from the one "side" of the universe to the other on its wave trace, meeting its "starting" point at the same amplitude as its "starting" point. And I conclude that the wave/motion-like orbit of the photon has to be measured itself as a whole in order to determine how the gravity is affected
😵
Now you've lost me entirely.