Exotic Matter

[tomBitonti]

To followup:

A consequence seems to be that looking at an EM field as the superposition of many fields from individual photons is not a complete description of the EM field.

For example, if the EM field was from a laser (coherent; all of the photons are in the same state), then there should be no scattering interactions where the laser crosses itself, where-as, if the EM field is no-coherent (say, a narrow beam from the sun), there will be a scattering interaction. Simply knowing the field strength and direction of travel doesn't seem to be sufficient.

Thx!
TomB

 

[freyar]

Yes. There is a careful consideration, though, of whether to add the amplitudes then square (interference) or if to square the amplitudes then add (no-interference). For indistinguishable contributions, we add the amplitudes first. For distinguishable contributions, we square first. I'm fitting this to the virtual particle photon-photon interaction, and am using the double slit case to test my understanding.

I think maybe you're getting a couple of different ideas mixed up. But the point is that two photons make a very different system than a single photon, as in my last response. The coherent/incoherent addition of amplitudes here is something of a red herring if you want to know if a single photon passing through a double slit experiment (with or without a mirror) can bounce off of itself.

 

[freyar]

To followup:

A consequence seems to be that looking at an EM field as the superposition of many fields from individual photons is not a complete description of the EM field.

For example, if the EM field was from a laser (coherent; all of the photons are in the same state), then there should be no scattering interactions where the laser crosses itself, where-as, if the EM field is no-coherent (say, a narrow beam from the sun), there will be a scattering interaction. Simply knowing the field strength and direction of travel doesn't seem to be sufficient.

If you take a laser beam, leave it on continuously, and send it though mirrors so that it crosses its own path, one laser photon that's gone through the mirrors absolutely can scatter from a photon that has not gone through the mirror. It would be an extremely rare event, but it can happen.

 

[Scott DeWar]

If you take a laser beam, leave it on continuously, and send it though mirrors so that it crosses its own path, one laser photon that's gone through the mirrors absolutely can scatter from a photon that has not gone through the mirror. It would be an extremely rare event, but it can happen.

Would there be a significant event when the one "Old" photon (post mirror reflect) collides and scatters from th3e "new" (pre-mirror) Photon? Would this be the event of Photon meeting with the electron - positron shindig and the gamma ray hangover? (Pardon the humorous bent, please)

also, can someone tell me why pre-mirror was not in my spell check library, but pee-mirror was?? on a side note, I do not want to know what a pee mirror is, so don't tell me especially it is used to detect exotic matter.

 

[Umbran]

For example, if the EM field was from a laser (coherent; all of the photons are in the same state), then there should be no scattering interactions where the laser crosses itself

As freyar has noted, there *can* be scattering interactions in this scenario. They are rare, but can happen.

 

[freyar]

Would there be a significant event when the one "Old" photon (post mirror reflect) collides and scatters from th3e "new" (pre-mirror) Photon? Would this be the event of Photon meeting with the electron - positron shindig and the gamma ray hangover? (Pardon the humorous bent, please)

Well, I'm not sure it would be significant enough to notice unless we looked quite carefully, but I think you have the right idea. Two photons could "bounce" off each other or even create an electron-positron pair (if they have enough energy).

also, can someone tell me why pre-mirror was not in my spell check library, but pee-mirror was?? on a side note, I do not want to know what a pee mirror is, so don't tell me especially it is used to detect exotic matter.

You have a pervy spell-checker?

 

[tomBitonti]

If you take a laser beam, leave it on continuously, and send it though mirrors so that it crosses its own path, one laser photon that's gone through the mirrors absolutely can scatter from a photon that has not gone through the mirror. It would be an extremely rare event, but it can happen.

Didn't get back to that point in time. A continuously firing laser will have a lot of different photon bundles (the wave front for a single photon and its coherent partners). Then, when different bundles interact they can scatter. But that is the same as scattering of non-coherent photons. The case of interest is a bundle induced by a single photon. Can that scatter with itself? (Will different photons in the bundle take different paths on the outgoing side of the double slit? I'm thinking yes, but it's something to think about more.)

Since a mirror is a problem, would an electron through an electric field also create a problem? The goal is to get the streams from the two slits to cross with a large scattering cross section. The streams overlap already, so this isn't necessary, just convenient to increase the cross section.

Thx!
TomB

 

[freyar]

Didn't get back to that point in time. A continuously firing laser will have a lot of different photon bundles (the wave front for a single photon and its coherent partners). Then, when different bundles interact they can scatter. But that is the same as scattering of non-coherent photons. The case of interest is a bundle induced by a single photon. Can that scatter with itself? (Will different photons in the bundle take different paths on the outgoing side of the double slit? I'm thinking yes, but it's something to think about more.)

I feel like I'm kind of talking across you or somehow I'm not getting my point over. The point is this: a single photon cannot interact ("bounce") off itself. In the case of the double-slit experiment, you seem to be worried that the photon can take multiple different paths and that the "photons" in the "different paths" hit each other, even though there is only one photon. That's not the case. In fact, if you have an experiment set up with a double-slit or mirrors or whatever, the way to define a photon includes all the interference patters from those "different paths." It's not a photon scattering off itself. I'll put it another way: scattering requires that two different particles trade energy and/or momentum between each other. That doesn't happen if there's only a single particle.

 

[Umbran]

Or, to put what freyar is saying a different way: The self-interference of the probability amplitude after it goes through the slits is *not equivalent* to an interaction between particles. The self-interference is *not* akin to having the particle go through both slits and scatter "off itself".

These are entirely different effects, and should not be conflated.

 

[tomBitonti]

I feel like I'm kind of talking across you or somehow I'm not getting my point over. The point is this: a single photon cannot interact ("bounce") off itself. In the case of the double-slit experiment, you seem to be worried that the photon can take multiple different paths and that the "photons" in the "different paths" hit each other, even though there is only one photon. That's not the case. In fact, if you have an experiment set up with a double-slit or mirrors or whatever, the way to define a photon includes all the interference patters from those "different paths." It's not a photon scattering off itself. I'll put it another way: scattering requires that two different particles trade energy and/or momentum between each other. That doesn't happen if there's only a single particle.

This: I fully believe that a photon doesn't interact with itself. But I'm not particularly interested in that specific result, other that it arising from a false but interesting line of reasoning. I'm much more interested in the reasoning used to reach the result. (My background is pure math, with some depth in formal systems. I rather like proofs. To me, the form of an argument is as much an thing to be understood as the topic of the argument itself.)

From the interaction of two photons, what seems to happen is two EM fields interacting. Looking only locally, the two EM fields don't look different than two parts of the field of a single particle. Or don't seem to look different. I'm missing something in my view that the fields look the same.

That is an "aha" moment for me. Looking just at EM fields doesn't seem to capture enough information.

For example, the field of two photons which are in the same state seemingly might look the same as a field from one, looking at just a portion of the field. This seems possible by confining the single photon to radiate in half of the angle of the two photons. This *seems* possible, but I might be missing something.

Then the difference between the two fields doesn't seem to arise until the entire fields are viewed, with the two photon field summing to twice the sum of the single photon field.

Thx!
TomB