# B Does an electron turn into a positron when hit by a photon?

1. Nov 27, 2016

### Jovian_Dsouza

I read somewhere that an electron travels forward in time and a positron travels backwards.And when a photon hits the electron the direction of time for it reverses and it becomes a positron.Does an electron really turn into a positron when hit by a photon? why?

2. Nov 27, 2016

### hilbert2

The sum of electrons and electron neutrinos minus the sum of positrons and electron antineutrinos must be the same before and after a particle collision process. Therefore you can't turn an electron to a positron like that. Where have you seen someone say that a photon can change the direction of time for some particle? Do you mean $e^+ + e^- \rightarrow e^- + e^+$ scattering that happens via photon exchange?

3. Nov 27, 2016

### vanhees71

Forget this nonsense about antiparticles being particles travelling backwards in time. The contrary is right: In relativistic QFT you have plane-wave modes with positive and negative energy, and then you write the field with help of annihilation and creation operators. Putting a creation operator for the negative-frequency modes leads to another particle with the opposite charge that moves forward in time with positive energy.

Then you cannot have the process $e^- + \gamma \rightarrow e^+$ since it violates energy-momentum and electric-charge conservation. What you can have is a bremsstrahlung pair-creation process like $\gamma + X \rightarrow e^+ +e^- +X$, where $X$ is some charged particle (or a heavy atomic nucleus).

4. Nov 27, 2016

### Jovian_Dsouza

Thanks but I still didn't get the concept .I am a student and need more explanation .could please explain in simple words

5. Nov 27, 2016

### secur

@hilbert2's question is surely simple enough for a student - or anyone - to answer. Even "I forget where I read it" would be better than nothing.

6. Nov 27, 2016

### ZapperZ

Staff Emeritus
Your understanding of the idea is wrong.

What I am guessing here is that you have a severe misunderstanding of the Feymann diagram. Unfortunately, your lack of exact reference does not allow us to diagnose what went wrong here.

In this forum, "I read somewhere" isn't a sufficient source reference. Please keep that in mind in your future post. In fact, paying attention to your source, and paying attention to the nature of your source, is a valuable lesson that you can learn from this forum.

Zz.

7. Nov 27, 2016

### vanhees71

It's obviously from a popular-science book, where the nonsense about "particles moving backwards in time" is unfortunately a common idea to popularize QFT, which cannot be explained with "simple words" but only with mathematics, which is available after a first course on quantum mechanics only. The arrows on the lines of particles indicate flow of charge, and that's why an anti-particle with an arrow pointing into the diagram is in fact an anti-particle moving outward, but that has nothing to do with "moving backwards in time". The in the very few first lectures on relativistic QT you learn that to the contrary in order to have particles only moving forward in time (causality) and at the same time with a Hamiltonian that is bounded from below (to guarantee the stability of matter in the theory) relativistic QT forces you to introduce antiparticles in the very way I described in my previous posting. Dirac came to the same conclusion in a much more complicated way, which shouldn't be taught anymore, because it leads to misunderstandings. My advice to any student of physics is to read a serious textbook about relativistic QFT. A good introductory one is

M. D. Schwartz, Quantum field theory and the Standard Model, Cambridge University Press, Cambridge, New York, 2014.

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