Pair production-annihilation causality

[modest]

-source

 

I'm very unschooled in particle physics...

 

The above is a spacetime diagram of Feynman's interpretation of an electron/positron production/annihilation. I'm wondering if there is no way for an experimenter to affect event A in such a way that an experimenter could recognize that effect at event B.

 

For example, if the person at B is providing high-energy photons to create pairs—he could catch the electrons to the far right of the diagram recognizing when a pair is created or how many are created. The person at A could control the availability of the electrons at the far left of the diagram presumably affecting when exactly the electron travels back and is available at B.

 

I'm sure this thinking of mine is based on some faulty understanding, it just seems that if A really does cause B as Feynman's diagram suggests then that opens up the possibility of communicating with the past. Yes/No?

 

~modest

[Erasmus00]

The way to read the above diagram is that a photon and an electron move forward in time. The photon pair produces an electrons/positron at B. The positron anihilates the electron and produces a photon at A. Does this help?

[freeztar]

Why are positrons not represented on the graph?

 

Shouldn't it be more like this?

 

 

Image Credit: http://www.upscale.utoronto.ca/GeneralInterest/Harrison/AntiMatter/AntiMatter.html

[modest]

As I understand it, the difference is between the Feynman and Dirac formalism. The site you reference also has this image:

 

 

which is the same as I give above except this one is moving from right to left. The explanation is what I find fascinating:

 

In 1949 Richard Feynman devised another theory of antimatter.

 

The spacetime diagram for pair production and annihilation appears to the right. An electron is travelling along from the lower right, interacts with some light energy and starts travelling backwards in time. An electron travelling backwards in time is what we call a positron. In the diagram, the electron travelling backwards in time interacts with some other light energy and starts travelling forwards in time again. Note that throughout, there is only one electron....

 

Feynman's theory is mathematically equivalent to Dirac's, although the interpretations are quite different. Which formalism a physicist uses when dealing with antimatter is usually a matter of which form has the simplest structure for the particular problem being solved.

 

Note that in Feynman's theory, there is no pair production or annihilation. Instead the electron is just interacting with electromagnetic radiation, i.e. light. Thus the whole process is just another aspect of the fact that accelerating electric charges radiate electric and magnetic fields; here the radiation process is sufficiently violent to reverse the direction of the electron's travel in time.

 

AntiMatter

 

If the probability of creating a pair at B is in any way caused at A then my unschooled mind starts thinking of communicating with the past.

 

Will, is this not a good interpretation of Feynman's theory?

 

~modest

 

EDIT: Sorry, I should also have given a source for the image in the OP. I've added it—it originated here.

[Erasmus00]

The key to Feynman's idea is that an electron going backward in time is mathematically equivalent to a positron moving forward in time. This lets us do something very helpful- we can always make sure that everything has positive total energy. (We reinterpret a particle with negative energy as its anti-particle moving forward in time).

 

It is this insistence on positive energy that I think removes any "communication with the past." Also consider that even interpreting the pair production as the light interacting with an electron from the future- this doesn't tell you WHERE in the future. The A and B events can be made unrelated (in fact, we have to sum over all possible places A and B could be to get the full amplitude for that process)

[modest]

The key to Feynman's idea is that an electron going backward in time is mathematically equivalent to a positron moving forward in time. This lets us do something very helpful- we can always make sure that everything has positive total energy. (We reinterpret a particle with negative energy as its anti-particle moving forward in time).

Yes, that makes sense.

 

I think I was just naively confusing symmetry with respect to time with the symmetry of causality. There are a lot of time-symmetric things that I wouldn't think could be used to communicate with the past. Like, a photon moving forward in time would be mathematically equivalent to the same photon moving backward, but I wouldn't think somebody deciding whether or not to absorb a photon would communicate anything to the person emitting it.

 

Kind of embarrasing now that I think about it ;)

 

~modest