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Author Topic: Can Photons Collide With Each Other?  (Read 10868 times)

Offline _Stefan_

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Can Photons Collide With Each Other?
« on: 01/06/2007 08:18:32 »
Can they? And if they can, what happens?

Failing that, can photons influence other photons in any way?

Also, I've heard of lasers in space used to keep space craft/satellites in the correct position by exerting enough force to move the craft. How is this possible? What's the physics behind it?

Thanks in advance :)


 

another_someone

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Can Photons Collide With Each Other?
« Reply #1 on: 01/06/2007 11:23:00 »
Photons are boson, and not fermions, which means two of them can occupy the same space without either excluding the other, so if they were to 'collide', they just pass through each other (unlike fermions, which would bounce off each other).

The way lasers apply force to a spacecraft (and it needs to be a very very powerful laser to apply appreciable force, or you use a very tiny force applied over a very very long time to have much appreciable change in energy of the vehicle) is through the interaction of photons with ordinary matter, not through photon/photon interaction.

Despite what I said about photons passing through each other unaffected, I think they must have some slight influence on each other to some degree.  Light (or other electromagnetic radiation) can be influenced by powerful magnetic fields, and electromagnetic radiation does carry its own magnetic field.  Thus it is conceivable that an intensely powerful low frequency radio signal might possibly effect the polarisation of a light beam, but maybe someone else can say to what degree, and in what manner, this might be true.
 

Offline lightarrow

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Can Photons Collide With Each Other?
« Reply #2 on: 01/06/2007 13:35:23 »
Can they? And if they can, what happens?
As another_someone said, photons cannot collide. According to Quantum Field Theory and the Standard Model, however, there could be a very slight probability of interaction, at Very high energies, but this hasn't been experimentally confirmed yet, as far as I know.
Quote
Failing that, can photons influence other photons in any way?
Yes, they can make interference, for example.
Quote
Also, I've heard of lasers in space used to keep space craft/satellites in the correct position by exerting enough force to move the craft. How is this possible? What's the physics behind it?
Light has momentum: p = E/c; E = energy.
Since dp/dt = F (force) it means that F = W/c; W = power. So, from the light beam's power in Watt, dividing by c (299,792,458 m/s), you immediately have the beam's push in Newton!
It results in a very tiny push, however: with 1 Giga Watt laser you achieve only 3.3 N (about 340 grams).
« Last Edit: 01/06/2007 13:41:50 by lightarrow »
 

Offline _Stefan_

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Can Photons Collide With Each Other?
« Reply #3 on: 01/06/2007 13:40:06 »
Thanks! :D
 

Offline G-1 Theory

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Can Photons Collide With Each Other?
« Reply #4 on: 25/07/2007 18:42:49 »
Dear Lightarrow;


Quote Lightarrow;
“Light has momentum: p = E/c; E = energy.
Since dp/dt = F (force) it means that F = W/c; W = power. So, from the light beam's power in Watt, dividing by c (299,792,458 m/s), you immediately have the beam's push in Newton!
It results in a very tiny push, however: with 1 Giga Watt laser you achieve only 3.3 N (about 340 grams).”

Some time back I saw a demonstration where a laser was used to power a spinning highly polished disk up to;  I believe it was 700 ft.
And only stopped going do to it ‘s spin slowed down and the disk became unstable and didn’t stay in line with the laser.






Ed
 

Offline Bored chemist

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Can Photons Collide With Each Other?
« Reply #5 on: 25/07/2007 19:04:53 »
I don't think that just relied on photon pressure. I think it was based on a concave mirror, the light was focussed to a point underneath the "disk" this ionised and heated the air pushing the disk upwards as the hot air expanded. The laser was pulsed and each pulse added to the last one so the effect was reasonably large.
Photons in free space don't interact much, but photons in the presence of matter can certainly influence one another.
 

Offline syhprum

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Can Photons Collide With Each Other?
« Reply #6 on: 25/07/2007 19:09:11 »
I would be suspicious of such a demonstration, unless conducted in a vacuum I would think any pressure exerted was more likely caused by heating and airflow
 

Offline G-1 Theory

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Can Photons Collide With Each Other?
« Reply #7 on: 25/07/2007 19:13:35 »
Go to NASA and type in laser power space eleveavator test.
Ed
 

Offline G-1 Theory

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Can Photons Collide With Each Other?
« Reply #8 on: 25/07/2007 19:36:52 »
It is the same as in the double slit experiment the em-fields on the edges of the slits and around the pin-hole camera redirect the in comeing light but where the light / photons collides they are directed again  leaving dark areas between the light bars.

Also see my posting called;  The opposite of the double slit experiment. Where the light from car #2 all but cancels out the lights of car #1.

Ed
 

Offline Bored chemist

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Can Photons Collide With Each Other?
« Reply #9 on: 25/07/2007 20:29:55 »
"It is the same as in the double slit experiment "
Er, no it's not really. It's more like burning ants with a magnifying glass on a sunny day.
 

Offline JP

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« Reply #10 on: 25/07/2007 21:03:54 »
Can they? And if they can, what happens?
As another_someone said, photons cannot collide. According to Quantum Field Theory and the Standard Model, however, there could be a very slight probability of interaction, at Very high energies, but this hasn't been experimentally confirmed yet, as far as I know.

I'm not familiar with how to describe a "collision" in particle physics.  When you say photons can't "collide" here, you mean there's no potential energy interaction between photons (in the standard model, at least)? 
 

Offline G-1 Theory

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Can Photons Collide With Each Other?
« Reply #11 on: 26/07/2007 14:00:16 »
"It is the same as in the double slit experiment "
Er, no it's not really. It's more like burning ants with a magnifying glass on a sunny day.

Here is the dubble slite experiment showing that photons do have collisions and cause dark areas on the back of the box.



ED
« Last Edit: 05/08/2007 18:27:39 by ukmicky »
 

Offline lightarrow

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Can Photons Collide With Each Other?
« Reply #12 on: 26/07/2007 14:47:30 »
"It is the same as in the double slit experiment "
Er, no it's not really. It's more like burning ants with a magnifying glass on a sunny day.

Here is the dubble slite experiment showing that photons do have collisions and cause dark areas on the back of the box.



ED
This has nothing to do with photon collisions. Infact, you can have the same interference pattern even sending one photon at a time.
 

Offline lightarrow

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Can Photons Collide With Each Other?
« Reply #13 on: 26/07/2007 14:59:04 »
Can they? And if they can, what happens?
As another_someone said, photons cannot collide. According to Quantum Field Theory and the Standard Model, however, there could be a very slight probability of interaction, at Very high energies, but this hasn't been experimentally confirmed yet, as far as I know.

I'm not familiar with how to describe a "collision" in particle physics.  When you say photons can't "collide" here, you mean there's no potential energy interaction between photons (in the standard model, at least)? 
Unfortunately I'm not familiar too with it. QED and QFT ar still beyond my knowledge.
Look at here:
http://www.hep.ucl.ac.uk/opal/gammagamma/gg-tutorial.html
 

Offline G-1 Theory

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Can Photons Collide With Each Other?
« Reply #14 on: 26/07/2007 16:11:02 »

Quote Lightarrow

“This has nothing to do with photon collisions. Infact, you can have the same interference pattern even sending one photon at a time.”

Dear Lightarrow

To start with we are explaining Photon Collisions at a small distance inside the box, and yes this is explaining just that.

But, now that have brought up Interference Patterns and at a single photon at a time, the same experiment can be used to explain that also.



Ed
 

Offline G-1 Theory

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Can Photons Collide With Each Other?
« Reply #15 on: 26/07/2007 18:02:57 »
Now this brings to the probability of where the particles end up seems to behave a bit like a wave. This is called a wavefunction. Now where there are standing wave peaks in this wavefunction you are very likely to find your particle and where there are nodes (points of no amplitude) you are not going to find the particle. If you have two of these wavefunctions they will interfere like waves causing cool interference patterns. Hence although particles appear to arrive in places as clumps, their wavefuntions make them behave a lot like waves –

Hence wave-particle duality stuff...or the Probablility
Of where the particle will end up.

I believe that if you used difference elements in the plates that the holes are in and run a given timed test on each element that you would come with a very predictable pattern / known location for each particle,FOR EACH ELEMENT, thus doing away with this PROBABILITY STUFF !!!!!![/[/b]




eD
« Last Edit: 26/07/2007 18:06:45 by G-1 Theory »
 

Offline Bored chemist

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Can Photons Collide With Each Other?
« Reply #16 on: 26/07/2007 20:59:29 »
"Go to NASA and type in laser power space eleveavator test."
I did, it didn't help (with any spelling). That flying thing still has nothing to do with difraction (ie the 2 slit expt) and neither has anything to do with photon colisions as (as has been pointed out) it still works if you only use 1 photon at a time.
 

Offline lightarrow

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« Reply #17 on: 27/07/2007 08:45:34 »

Quote Lightarrow

“This has nothing to do with photon collisions. Infact, you can have the same interference pattern even sending one photon at a time.”

Dear Lightarrow

To start with we are explaining Photon Collisions at a small distance inside the box, and yes this is explaining just that.

But, now that have brought up Interference Patterns and at a single photon at a time, the same experiment can be used to explain that also.


As Soul Surfer has recently explained in another thread, the diffraction or the interference pattern doesn't depend on the nature of the slit's material, it only depends on:
1. width of each slit
2. distance between each slit
3. distance from screen 1 (with the slit) and screen 2 (where the pattern is observed
4. light wavelenght (provided it has spatial coherence).

If it depended on "the atomic system's vibration" of the slit's atoms, as you say, then the trajectory of each photon should be casual, and this means you wouldn't have an interference pattern, but only a diffuse bright spot on the last screen.
« Last Edit: 27/07/2007 08:48:56 by lightarrow »
 

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Can Photons Collide With Each Other?
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