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

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• Jovian_Dsouza
In summary: But again, this is not suitable for students who are still in their first year of physics, so I recommend to read firstJ. J. Sakurai and J. Napolitano, Modern Quantum Mechanics, 2nd ed., Pearson, Addison-Wesley, San Francisco, 2011.​In summary, the conversation is discussing the concept of particles moving forward and backwards in time, and how this relates to the idea of antiparticles. The popular notion of a photon changing the direction of time for a particle is incorrect and goes against the principles of energy-momentum and electric-charge conservation. This idea often arises in popular science books, but it is important to study a serious textbook on relativistic quantum field theory
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?

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?

Forget this nonsense about antiparticles being particles traveling 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).

Jovian_Dsouza, bhobba and hilbert2
Thanks but I still didn't get the concept .I am a student and need more explanation .could please explain in simple words

Jovian_Dsouza said:
I read somewhere that ... when a photon hits the electron the direction of time for it reverses ...

hilbert2 said:
Where have you seen someone say that a photon can change the direction of time for some particle?

Jovian_Dsouza said:
I am a student ... please explain in simple words

@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.

Jovian_Dsouza said:
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?

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.

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.

Jovian_Dsouza

## 1. How does a photon interact with an electron?

When a photon (a particle of light) comes into contact with an electron, the two particles can interact in three different ways: absorption, scattering, or production of another particle.

## 2. What is the difference between an electron and a positron?

An electron is a negatively charged subatomic particle that is found in atoms. A positron, on the other hand, is the antiparticle of an electron, meaning it has the same mass as an electron but has a positive charge instead of a negative charge.

## 3. Can an electron really turn into a positron?

Yes, it is possible for an electron to turn into a positron. This process is known as electron-positron annihilation and occurs when a high-energy electron collides with a high-energy positron. The two particles annihilate each other, producing two or more photons.

## 4. How does a photon cause an electron to turn into a positron?

When a photon collides with an electron, it transfers its energy to the electron. This energy can then be used to create a positron and an electron-positron pair. The exact mechanism of this process is still being studied and is not fully understood.

## 5. Why is it important to study the interaction between photons and electrons?

The interaction between photons and electrons is a fundamental process in understanding the behavior of matter and energy. It plays a crucial role in fields such as quantum mechanics, particle physics, and astrophysics. Additionally, this interaction is essential in the development of technologies such as solar cells, lasers, and medical imaging devices.

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