# Can a SINGLE photon produce an electron-positron pair?

• rwooduk
In summary, a photon cannot produce an electron-positron pair, but needs additional matter or light quanta.
rwooduk
A homework question is asking for a diagram illustrating a photon becoming a electron-positron pair (please don't post an image as it's homework).

Looking through the internet there seems to be some disagreement as to whether a single photon can create an electron-positron pair:

"Show that a single photon cannot produce an electron-positron pair, but needs additional matter or light quanta."

"For example you've got a 5MeV photon, so you think that there is plenty of energy to make e − e + pair. Now you make a boost along the direction of the photon momentum with v=0.99c and you get a 0.35MeV photon. That is not enough even for one electron. "

so a photon would have to have a massive amount of energy, but is it even possible?

if not why is this diagram on wiki?

i could just draw that diagram but if it's not a realistic possibility then would like to put in a comment as such.

Thanks for any comments / ideas on this.

rwooduk said:
A homework question is asking for a diagram illustrating a photon becoming a electron-positron pair (please don't post an image as it's homework).

Looking through the internet there seems to be some disagreement as to whether a single photon can create an electron-positron pair:

"Show that a single photon cannot produce an electron-positron pair, but needs additional matter or light quanta."

"For example you've got a 5MeV photon, so you think that there is plenty of energy to make e − e + pair. Now you make a boost along the direction of the photon momentum with v=0.99c and you get a 0.35MeV photon. That is not enough even for one electron. "

so a photon would have to have a massive amount of energy, but is it even possible?
You seemed to have missed the point of the example. Say the photon did have even more energy in your frame. Can't you always boost to a frame where the energy of the photon is too low for pair production?

vela said:
You seemed to have missed the point of the example. Say the photon did have even more energy in your frame. Can't you always boost to a frame where the energy of the photon is too low for pair production?

are you saying you could be in a frame where the energy required would be LESS to produce a positron electron pair? therefore it is possible?

How did you get that interpretation from the phrase "too low for pair production"?

Try writing down the equations using conservation of momentum and conservation of energy for a photon decaying into an electron-position pair. You should be able to show there's no solution to those equations. Think about what happens in the center-of-mass frame of the electron and positron.

Last edited:
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vela said:
How did you get that interpretation from the phrase "too low for pair production"?

hmm not sure, went a little abstract.

vela said:
Try writing down the equations using conservation of momentum and conservation of energy for a photon decaying into an electron-position pair. You should be able to show there's no solution to those equations. Think about what happens in the center-of-mass frame of the electron and positron.

Ahh, thanks I see now!

Also asked my tutor who says there would have to be nuclear recoil, i.e. it would have to interact with something to produce a electron positron pair, but that it is often ignored in a simple diagram.

Problem solved, thanks again!

## 1. What is a photon?

A photon is a fundamental particle of light and is the smallest possible unit of light. It has no mass and travels at the speed of light.

## 2. Can a single photon produce an electron-positron pair?

Yes, a single photon can produce an electron-positron pair through a process called pair production. This occurs when a photon with enough energy interacts with a strong electric field, such as the electric field of an atom's nucleus. The energy of the photon is converted into the mass of the electron-positron pair.

## 3. How much energy does a photon need to produce an electron-positron pair?

The minimum energy needed for a photon to produce an electron-positron pair is about 1.022 MeV (mega electron volts). This is equivalent to the rest mass energy of an electron and a positron combined.

## 4. Can this process occur in a vacuum?

Yes, pair production can occur in a vacuum as long as there is a strong enough electric field present. However, the probability of this happening is very low and usually only occurs in extreme conditions, such as near a black hole.

## 5. What are the implications of a single photon producing an electron-positron pair?

This process has significant implications in the field of quantum mechanics and demonstrates the dual nature of light as both a particle and a wave. It also plays a crucial role in understanding the creation and annihilation of matter in the universe.

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