# Thread: Photon Collisions?

1. I have a question about photons. Not even sure if photons can collide but if they can then do they collide as waves, particles or both?  2.

3. Originally Posted by zinjanthropos I have a question about photons. Not even sure if photons can collide but if they can then do they collide as waves, particles or both?
Yes, a pair of photons can collide, and if their total energy is high enough, they can form a particle-antiparticle pair of some type (typically an electron-positron pair). As for whether they collide as waves, particles or both, quantum mechanically, there is no meaningful distinction between these options.  4. Personally, I would prefer the term "interact" rather than "collide". The latter just smacks too much of billiard balls and classical mechanics, which isn't the mental picture we would want to have of photons.  5. Another question about photons....do photons always move in a straight line or will they also move spirally or zigzag?  6. Originally Posted by PhDemon Markus may correct me here but my understanding is that photons move along geodesics (the shortest route between two points) -- these paths are "straight lines" in plane geometry but may be curved in curved spaces. Where space-time is curved by a large mass the photons may travel in "curved" paths for example in gravitaional lensing or in the fact that a black hole has an event horizon -- i.e. space-time is so curved all the geodesics inside the event horizon lead into the black hole. Not sure how you would make a photon travel on a zig-zag path though. Odd, I follow curves as well.  7. Originally Posted by PhDemon Not sure how you would make a photon travel on a zig-zag path though. Sorry, I was thinking, like a sine wave  8. Originally Posted by zinjanthropos Another question about photons....do photons always move in a straight line or will they also move spirally or zigzag?
They can't move zig-zag ( because such a trajectory is not smooth ), but they can move in spiral patterns around very massive objects. That is in fact what happens in the immediate vicinity of a black hole's event horizon.

Sorry, I was thinking, like a sine wave
The only scenario I can think of where this might happen is when a photon traverses a region where very intense gravitational waves are present; their trajectories may be distorted into wave-like patterns here.

Markus may correct me here but my understanding is that photons move along geodesics (the shortest route between two points) -- these paths are "straight lines" in plane geometry but may be curved in curved spaces.
Essentially correct. They are following a particular kind of geodesics, so-called null geodesics.  9. Monopoles could lead to some interesting photon interactions...   10. Originally Posted by PhDemon Markus may correct me here but my understanding is that photons move along geodesics (the shortest route between two points) -- these paths are "straight lines" in plane geometry but may be curved in curved spaces. Where space-time is curved by a large mass the photons may travel in "curved" paths for example in gravitaional lensing or in the fact that a black hole has an event horizon -- i.e. space-time is so curved all the geodesics inside the event horizon lead into the black hole. Not sure how you would make a photon travel on a zig-zag path though. That's a classical view which, because we are talking about individual photons, is incorrect. In quantum electrodynamics, a photon travels every possible path, the probability amplitudes for each path summing to produce an interference pattern. However, those paths that are furthest from the straight-line path tend to destructively interfere, thus the straight-line path is the path with the highest probability. This is explained in Richard Feynman's book, "QED: The Strange Theory of Light and Matter".

In the classical wave picture, one has Huygens' principle, which is applied mathematically to solve the homogeneous wave equation in four-dimensional spacetime.  11. Originally Posted by KJW That's a classical view
Sorry, maybe I got the questioner's intentions wrong, but I took the original question to be one about light rays in ( macroscopic ) GR space-time. Hence my classical answer without reference to quantum effects.  12. Originally Posted by Neverfly Monopoles could lead to some interesting photon interactions... I have a question someone with practical knowledge may be able to answer. -If you had one of those old fashioned fuse boxes (not a curcuit breaker installation) and then removed a fuse from it, would the remaining outlet represent a monopole? -If so, could there be experimental value in studying this?  13. Originally Posted by Michael Anteski would the remaining outlet represent a monopole?
No, a magnetic monopole is a hypothetical entity which carries isolated magnetic "charge" ( i.e. an isolated north or south pole ). Note that this goes contrary to the standard Maxwell equations.  14. Originally Posted by Michael Anteski  Originally Posted by Neverfly Monopoles could lead to some interesting photon interactions... I have a question someone with practical knowledge may be able to answer. -If you had one of those old fashioned fuse boxes (not a curcuit breaker installation) and then removed a fuse from it, would the remaining outlet represent a monopole? -If so, could there be experimental value in studying this?
As Markus has stated, magnetic monopoles do not exist, as far as we know. Dirac did point out that the existence of a monopole anywhere would necessitate a quantised electronic charge, which is what we observe, but you can't run the argument backwards -- the existence of quantised charge does not imply the existence of a magnetic monopole.

Now, as far as your removed-fuse-as-monopole idea, it's deeply flawed, independent of the considerations of the foregoing paragraph. Removing a fuse simply creates an open circuit in an AC power distribution system. That prevents current from flowing, and thus prevents the generation of a magnetic field. No magnetic field means you don't even get a dipole, let alone a monopole. Perhaps you are unaware of what a magnetic monopole is. It is a magnet that consists only of one pole (that's the mono part) -- a north pole or a south pole. Such a thing has never been found. All known magnets always have both a north and a south pole together, as first reported by Peregrinus in the 13th century.  15. I going to drift sideways for a minute.... Originally Posted by Markus Hanke They (photons) are following a particular kind of geodesics, so-called null geodesics.
A photon is traveling between two stars. It will follow a straight but curved line because of the two masses. Let's say I measure x amount of kilometres as the distance the photon travels from one star to another. The two masses then start moving towards each other in a straight line, will the distance the two masses travel be less than the distance the photon travelled? Or is it the same?  16. Originally Posted by zinjanthropos A photon is traveling between two stars. It will follow a straight but curved line because of the two masses. Let's say I measure x amount of kilometres as the distance the photon travels from one star to another. The two masses then start moving towards each other in a straight line, will the distance the two masses travel be less than the distance the photon travelled? Or is it the same?
As measured by whom, and how ?
For an outside far-away observer, the photon propagates along a null geodesic, whereas the two stars in free-fall towards each other will not. In any case, who measures what time and distance is observer-dependent, so there is no easy and globally valid answer to this question, particularly also since the stars themselves have an effect on the geometry of space-time around and between them.  17. Originally Posted by zinjanthropos I going to drift sideways for a minute.... Originally Posted by Markus Hanke They (photons) are following a particular kind of geodesics, so-called null geodesics.
A photon is traveling between two stars. It will follow a straight but curved line because of the two masses. Let's say I measure x amount of kilometres as the distance the photon travels from one star to another. The two masses then start moving towards each other in a straight line, will the distance the two masses travel be less than the distance the photon travelled? Or is it the same?
Am not sure if the masses can travel in a geodesic as did the photon(shortest path),but if they can it will be thesame distance as that which the photon covered#  18. Originally Posted by merumario Am not sure if the masses can travel in a geodesic as did the photon(shortest path),but if they can it will be thesame distance as that which the photon covered#
It's not that simple. Photons move on null geodesics, whereas the stars will free fall along time-like geodesics. Also, as I mentioned, the stars themselves distort space-time around them, so this whole process is rather non-trivial, I'm afraid.  19. Originally Posted by PhDemon "Non-trivial" one of my favourite phrases in science, a brilliant euphemism for "f***ing complicated!"
It has a scientific meaning other than showing complications#  20. Originally Posted by PhDemon I know, I was joking Φkªγ̲̣  Bookmarks
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