If a positron and an electron meet they annihilate each other and create a photon (a virtual photon). The photon will then release its energy and become a positron and an electron. Correct?
Can this photon be observed? If it is observed can it still return to a positron and electron? Can the photon be observed from more than one point?
Two photons, actually, each with 511 keV energy. These photons are very real and can be observed just like any other gamma ray photon. In fact, gamma ray telescopes have observed 511 keV radiation from many astronomical objects, indicating a presense of positrons in those objects.
These photons do not turn into an electron-positron pairs because 511 keV is only half the energy needed to create a pair. If it does interact with matter, it will either lose some of its energy through Compton scattering and/or get completely absorbed via the photoelectric effect. But if you have a 1022 keV photon, it can create an electron-positron pair when it interacts with matter. Even then, it’s not a spontaneous transition. The photon must interact with some other atom, otherwise momentum will not be conserved in all rest frames. (You can find a rest frame on which the resulting pair have zero net momentum, but even in this rest frame the original photon has a non-zero momentum, so momentum is not conserved in this rest frame.)
Actually, when an electron and positron collide, they annihilate each other and create two photons. The photons created are not virtual; they’re plain old everyday photons.
To create an electron/positron pair, a single photon with the same energy of the two aforementioned photons combined is needed.
Sorry it creates two photons. Can three individuals in different locations directly observe these two individual photons?
The two photons can be detected separately. I’m not sure what you mean by three individuals. It is possible for a photon to trigger one detector, exit that detector and go into another detector, but once it’s passed through the first detector it no longer has the same momentum and energy.
By the way, PET scanners use exactly this phenomenon. A radioactive isotope emits a positron which then annihilates with an electron, emitting a pair of 511 keV photons. PET stands for Positron Emission Tomography.
I meant, can two observers observe a photon at the same time?
Note that annihilation to two real photons is only one of many possible outcomes. Some of the possible interactions:
(Note: “-bar” below is my substitute for an overline which would typically indicate an antiparticle.)
e[sup]+[/sup] e[sup]-[/sup] --> e[sup]+[/sup] e[sup]-[/sup] (so-called Bhabha scattering)
e[sup]+[/sup] e[sup]-[/sup] --> l[sup]+[/sup] l[sup]-[/sup] (where l is either the [symbol]m[/symbol] or the [symbol]t[/symbol])
e[sup]+[/sup] e[sup]-[/sup] --> q q-bar (where q is any quark)
e[sup]+[/sup] e[sup]-[/sup] --> [symbol]n[/symbol] [symbol]n[/symbol]-bar (where [symbol]n[/symbol] is any flavor neutrino)
e[sup]+[/sup] e[sup]-[/sup] --> [symbol]g[/symbol] [symbol]g[/symbol]
e[sup]+[/sup] e[sup]-[/sup] --> Z[sup]0[/sup] Z[sup]0[/sup]
e[sup]+[/sup] e[sup]-[/sup] --> [symbol]g[/symbol] Z[sup]0[/sup]
e[sup]+[/sup] e[sup]-[/sup] --> W[sup]+[/sup] W[sup]-[/sup]
In the context of perturbation theory, these processes involve the exchanges of virtual particles, including virtual photons. These virtual particles are not real and not observable. However, as pointed out by scr4 and Joe Random, the above-listed real particles can indeed be detected.
Note, too, that some of these processes cannot happen without extra center-of-mass energy – energy perhaps provided by the RF cavities of the Stanford Linear Accelerator Center (for example).
And, one minor nitpick. The two photons that come from e[sup]+[/sup] e[sup]-[/sup] --> [symbol]g[/symbol] [symbol]g[/symbol] need not have 511 keV of energy each. That will be the case only if the initial positron and electron are at rest…
-P
Preface: a photon can only be observed through its interaction with a charged particle. Now…
If you are asking, “Can a photon interact with two particles at the same time?” the answer is no.
If you are asking, “Can a photon interact in such a way that a detectable photon continues on after the interaction?” the answer is yes. Taking the example of Compton scattering ([symbol]g[/symbol] e --> [symbol]g[/symbol] e)The kicked electron reveals the incoming photon, and the outgoing photon can travel onward to kick another electron later. However, this outgoing photon has a different momentum than the initial incoming photon, and depending on your mood, you may not want to call it the “same” photon.