Light under light gravitational force

In summary: Two photons traveling side-by-side will not interact with each other, as they would need to share information to move faster than the speed of light. However, when two photons are launched into space, their trajectories are determined by the laws of physics. If they are launched at the same time, their trajectories will be parallel. If they are launched at different times, their trajectories will be bent towards each other due to the gravitational field.
  • #1
perfectobsession2004
4
0
Bonjour,

We can see light emitted by a star which is effectively behind a gravitational mass. Ok! The light path will curve. Or the light path will be as straight as the space surrounding the gravitational mass. Ok!

My question is:
When two parallel beams are emitted in the same parallel direction, will these beams will interact? Will they go straight? Will the reach each other? Will they swirl and stay distant? Will they swirl and reach each other?
For me, on earth! Or for me, on one of the beams!

In fact, what will happen to these beams?
P.O.
 
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  • #2
What do you think of this? My guess (with my limited knowledge) is this is what will happen.
 

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  • #3
No, I think the original poster is asking about whether two light beams will attract each other, without any massive object entering into the picture. There was a discussion about this here not too long ago:

https://www.physicsforums.com/showthread.php?t=174805
 
  • #4
perfectobsession2004 said:
Bonjour,

We can see light emitted by a star which is effectively behind a gravitational mass. Ok! The light path will curve. Or the light path will be as straight as the space surrounding the gravitational mass. Ok!

My question is:
When two parallel beams are emitted in the same parallel direction, will these beams will interact? Will they go straight? Will the reach each other? Will they swirl and stay distant? Will they swirl and reach each other?
For me, on earth! Or for me, on one of the beams!

In fact, what will happen to these beams?
P.O.

I believe parallel beams traveling in the same direction will not interact, while parallel beams traveling in opposite directions will tend to bend towards one another
 
  • #5
Merci for the other thread! Now, I have to be more specific about my interrogations!

Suppose, in free space with no other gravitational field, we have a photon and a photon's stream which is a succession of photons as near to be considered as a continuous line of photons.

First, at the image of bullets launched from guns, we could consider two photons, launched parallel at the same time, traveling at light speed.

Q: What will be photon's trajectories? Are they supposed to interact with each other? If so, will the gravitational field traveling at light speed transform into another field which creates swirl paths?

Second, at the image of bullet's streams launched from machine guns, we could consider two photon's stream, parallel at a certain time. Individual photons are traveling at light speed but each photon's stream is "static".

Q: What will be stream's trajectories? Are they supposed to interact, (differently from standalone photon) with each other? If so, will the radial gravitational field traveling at light speed transform into another rotational field which creates swirl paths?

Humm!?
 
  • #6
I think I can say with considerabl certainty that two photons traveling side-buy-side could not interact with each other, as such an interaction would require information to move faster than c to get from one to the other.
The other situation seems to be answered in the thread jtbell linked, since the stream of photons literally is a "pencil" of light.
 
  • #7
The photons do not attract - nor do parallel beams of light attract.

An oversimplified and non-rigorous description of why not might consider replacing the photon by a massive particle, and taking the limit as the velocity of the massive particle approaches c while keeping the energy constant and equal to the energy of the photon.

The mass of the massive particle approaches zero. Unlike the case of the photon, the massive particle has a rest frame, so we can see that the force in that rest frame approaches zero. No attraction in one frame implies no attraction in all frames.

The "gravitational field" associated with a light beam or pulse would not be a "swirl", but would be a pp wave.

Some references for PP waves:

http://en.wikipedia.org/wiki/Pp-wave_spacetime
http://arxiv.org/PS_cache/gr-qc/pdf/9811/9811052v1.pdf

see http://adsabs.harvard.edu/abs/1998gr.qc...11052F for the publication history of the arxiv article
 
  • #8
grol

triggernum5 said:
I believe parallel beams traveling in the same direction will not interact, while parallel beams traveling in opposite directions will tend to bend towards one another

why light bend towards each other when it trevels antiparallel?
 

1. What is "Light under light gravitational force"?

"Light under light gravitational force" is a phenomenon that occurs when light is affected by a gravitational field, but the force is so weak that the light's trajectory is not significantly altered. This is in contrast to objects with mass, which are more significantly affected by gravitational forces.

2. How does light behave under light gravitational force?

Under light gravitational force, light travels in a straight line with a constant speed. This is due to the fact that light is composed of massless particles called photons, which do not experience a change in velocity when subjected to a weak gravitational field.

3. Can light be bent under light gravitational force?

Technically, yes. But the amount of bending is so small that it is not observable. The effect of gravity on light is only significant when it is near an extremely massive object, such as a black hole.

4. How does the speed of light change under light gravitational force?

The speed of light remains constant under light gravitational force. This is one of the fundamental principles of physics known as the speed of light postulate. No matter the intensity of the gravitational field, the speed of light is always approximately 300,000,000 meters per second in a vacuum.

5. Does light have mass and therefore experience gravitational force?

Yes, light has energy and momentum, which can be thought of as a form of mass. However, the mass of light is so small that its effects on gravitational forces are negligible. This is why light is not significantly affected by weak gravitational fields.

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