35
votes
Why during annihilation of an electron and positron 2 gamma rays are produced instead of 1?
This has a simple answer: the process $e^++e^-\to\gamma$ cannot satisfy both momentum and energy conservation at the same time. To see, this let's choose a reference frame in which the total momentum ...
- 8,582
26
votes
Electron leaving the atom
For a photon to give rise to a real (not virtual) electron/positron pair it must possess an energy of slightly greater than one million electron volts. This is a very energetic photon indeed. In ...
- 97.8k
16
votes
Electron leaving the atom
The answer is, it does happen. Just at largely different energies. This picture (taken from this thesis, page 10) summarizes it quite nicely for scattering on Cu (Copper) atoms:
Photoelectric ...
- 1,628
15
votes
Accepted
How are proton-antiproton pairs produced?
Before we get to the actual answer, there's something that needs to be clarified about the reaction as you've written it down. The reaction $\gamma\to e^-+e^+$ is not a valid reaction, because it ...
- 35.8k
15
votes
On the (non)linearity of electromagnetism
Maxwell equations are linear... but they are also incomplete - they involve sources (i.e., currents and charges) that cannot be determined from these equations. To really solve Maxwell equations these ...
- 65k
13
votes
Accepted
Why does pair production only occur in an electric but not a magnetic field?
For some easy intuition, think about what happens in classical electrodynamics if you have a pair of equal-mass, opposite-charge particles initially moving parallel to each other in a background ...
- 55k
12
votes
Electron leaving the atom
This is actually a good question.
Because of conservation of momentum/energy, this cannot happen.
A single photon will not decay into an electron-positron pair such that they would exist as two ...
- 30.2k
12
votes
On the (non)linearity of electromagnetism
Very strong fields lead to vacuum-polarization, i.e. they "create" matter (at the very least electron-positron pairs). Even well before the creation of "real" particles there will ...
- 3,137
11
votes
Is it possible to create protons from photons?
Yes. This is possible from colliding real photons (i.e., on-shell photons). I try to show you how this is possible within the context of Standard Model of elementary particles.
Protons ($p$) are ...
- 4,630
9
votes
Why can't a single photon produce an electron-positron pair?
This has a very simple answer that also works the other way around (see my answer to Why during annihilation of an electron and positron 2 gamma rays are produced instead of 1?). The idea is that this ...
- 8,582
9
votes
If matter cannot be created or destroyed, how were scientists able to "create" matter out of light?
Matter can be created and destroyed. Matter is not a conserved quantity.
Conserved quantities include energy, momentum, charge, angular momentum, etc. Those cannot be created or destroyed, at least ...
- 109k
8
votes
Electron leaving the atom
In order to create an electron positron pair, a photon needs to have at least the energy of the mass of the two particles (511 keV * 2 = 1.02 MeV). The photons involved in the photoelectric effect are ...
- 7,777
8
votes
Is it possible to produce protons from vacuum polarization?
Except for the greater mass energy required, there is no major difference between how different charged particle-antiparticle pairs are produced by a strong electric field $\vec{E}$. The lowest-order ...
Buzz♦
- 17.1k
7
votes
Accepted
Is the vacuum spacetime around the Earth curved enough to make particles appear spontaneously?
someone … said spontaneous particle creation had been seen in a lab-vacuum.
This certainly refers to experiments with analog gravity such as sonic black holes. Those are non-gravitational systems ...
- 16.6k
7
votes
Accepted
Will the interference pattern still be observed with gamma rays in double slit experiment?
If you look at the table here of the wavelengths of electromagnetic waves you will see that gamma rays have very small wavelength,smaller than 10 pico meters. The smallest atom, the hydrogen atom has ...
- 235k
6
votes
Why during annihilation of an electron and positron 2 gamma rays are produced instead of 1?
The gamma photon should move with $c$, in any reference frame. Thus, it has to have a nonzero inertia.
But in the center of mass of the $e^-$ and the $e^+$, they have zero summed inertia.
Inertia is ...
- 8,338
6
votes
Accepted
Does the Uehling potential have any observable effect?
Yes. The Uehling potential due to vacuum polarization by virtual electron-positron pairs is the dominant contribution — 205.0073 meV — to the Lamb shift between the $2P_{1/2}$ and $2S_{1/2}$ states of ...
- 52.2k
6
votes
Is there anything that can be created from absolutely nothing?
On some level this is a philosophical question.
From the physical side, to the best of our knowledge, i.e. based on all experimental data to date, nature is constrained by certain conservation laws ...
- 1,741
6
votes
Could a single gamma ray photon break the Schwinger limit? If so, at what energy?
Consider an electorn-positron collision, and the subsequent production if photons. It is known that there must be at least two photons produced. This is because momentum must be conserved. In the ...
- 956
5
votes
Accepted
Is it possible to have a non mathematical explanation of the dependence of Pair Production cross section with energy?
A photon, no matter how much energy it has, will not turn into a pair of particles because of conservation of momentum and energy. The photon does not have a center of mass because it has no rest ...
- 235k
5
votes
Why during annihilation of an electron and positron 2 gamma rays are produced instead of 1?
The answers given above are incomplete, probably because the OP is misleading. Everyone discussed the case of pair annihilation (or creation, if you wish to consider time-reverse processes) in vacuo, ...
- 1,853
5
votes
Electron leaving the atom
how do we know that a photon in the photoelectric effect will actually interact with the electron not annihilate to form a positron and an electron, then the positron will interact with the electron ...
- 8,249
5
votes
Does the Schwinger effect prove matter and antimatter can be created from a vacuum?
Energy is not considered "nothing" anywhere in physics.
You need energy in order to create your strong field. It even has its own mass.
In fact, there are quite a few ways to create particle-...
- 8,249
5
votes
Negative energy particle effect on observable object
That is one of the issues I personally see with that paper. Logan J. Fisher already mentioned the issue with taking particles too literally, but also there is the matter that since they use path ...
- 23k
5
votes
Accepted
Negative energy particle effect on observable object
The paper is just plain wrong and not in a few subtleties but in its basic assumption of relevance of obtained local effective action for black hole radiance.
Since the paper puts a lot of emphasis on ...
- 16.6k
5
votes
Accepted
Average momentum of particle/anti-particle pairs
Your question hits on a lot of misconceptions, mostly derived from pop sci (YouTube) descriptions of virtual particles.
There are at least two classes of virtual particles. Your favorite content ...
- 39.6k
4
votes
Black holes and positive/negative-energy particles
I just watched PBS Space time on this, which also seems to answer your question.
https://www.youtube.com/watch?v=ztFovwCaOik
While I have no doubt the accepted answer is the most complete. I still ...
- 227
4
votes
Pair production- Why do the particles spiral away from each other?
This shows the scatter of a gamma on an electron in the atoms of the bubble chamber, ejecting it (called a spectator) and at the same time the gamma has enough energy to generate an e+e- pair.
why ...
- 235k
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