# Thus, matter can be created out of two photons.

1. Dec 16, 2012

### Erathsmus

"Thus, matter can be created out of two photons."

Hello, and thank you for opening my thread!

I have read over the FAQ on this forum, looked over a few similair topics, and still feel that I lack understanding with this concept.

To me, the wiki stated below sounds like it is saying light + light can = mass.
On this Wiki page, it states "Thus, matter can be created out of two photons."
http://en.wikipedia.org/wiki/Matter_creation

On this page as well, they describe a collision with electrons and photons to produce matter from light itself. But I do not know whether the mass of the new particle created was greater than the mass of the electron. In other words, does this electron to proton collision produce a mass greater than the electron initially used?
http://www.sciencedaily.com/releases/1997/09/970918045841.htm

So by this, even though photons do not have mass, they can be collided to make something with mass?

Another concept I am having trouble with is the following:
"she slowed light, which travels in free space at a speed of 186,000 miles a second, to just 38 miles per hour in a cloud of ultracold atoms. Einstein and others have theorized that the speed of light in free space can't be changed"
From: http://www.photonics.com/Article.aspx?AID=28520

I thought the speed of light is to be considered a constant? How can they slow this down, or even stop it? And when stopped, what does this light become?

Thank you all for your help, it is much appreciated.

2. Dec 16, 2012

### justsomeguy

Re: "Thus, matter can be created out of two photons."

In a vacuum. The speed of light in a vacuum is a constant. It goes slower through media like water, glass, etc.

3. Dec 17, 2012

### Staff: Mentor

Re: "Thus, matter can be created out of two photons."

The total reaction is electron + photon + photon -> electron + positron + electron
The left side has a summed mass of 511 keV, the right side has three times this value.

Right. This is the time-reversed process of electron/positron annihilation.

4. Dec 18, 2012

### lpetrich

Re: "Thus, matter can be created out of two photons."

They do not have *rest mass*, but they have total mass-energy, and two photons together will effectively have a rest mass.

Consider two photons with energies E1,E2 and directions n1,n2. Their 4-momenta are:
E1*(1,n1)
E2*(1,n2)
Imagine them combined into one particle. That particle will have this total 4-momentum:
(E1+E2, E1*n1 + E2*n2)

Applying m2 = E2 - p2 for rest mass m, energy E, and 3-momentum p, we get

m2 = 2*E1*E2(1 - n1.n2)

If the photons are not moving collinearly, they will have a combined effective rest mass, even though the individual photons have zero rest mass.

Also, pair production from photons has been observed, like for an electron colliding with a nucleus. Their electromagnetic fields act as virtual photons, and those virtual photons and do electron-positron annihilation in reverse, making an electron and a positron.

5. Dec 18, 2012

### Erathsmus

Re: "Thus, matter can be created out of two photons."

Thank you all very much for helping me understand this.

One thing I did not quite understand what Ipetrich said though, was "Their electromagnetic fields act as virtual photons, and those virtual photons and do electron-positron annihilation in reverse, making an electron and a positron."

If you can briefly explain that to me, it would be much appreciated.