# Thread: When does a photon turn into matter?

1. ## When does a photon turn into matter?

Under what conditions does a photon turn into a matter/anti-matter particle?

Let's suppose the photon has enough energy to create an electron/anti-electron pair, and it's merrily zooming through space at the speed of light, far from any other influences (no other photons, matter, or gravity near by.)

What determines if this photon turns into matter? Is this governed by some probability equation in QM? Or does the photon need to interact with something else to "trigger" the transformation into matter?

Thanks!

Rob

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Originally Posted by robross
Let's suppose the photon has enough energy to create an electron/anti-electron pair, and it's merrily zooming through space at the speed of light, far from any other influences (no other photons, matter, or gravity near by.)

What determines if this photon turns into matter? Is this governed by some probability equation in QM? Or does the photon need to interact with something else to "trigger" the transformation into matter?
Needs something else, because of conservation of momentum. Indeed, if there is one photon and no other photons, you can just pick an inertial frame where the photon is redshifted to have insufficient energy to create electron/positron pair.

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The photon needs to have sufficient energy relative to another particle,
which it interacts with. I believe the other particle can come out of the
interaction unchanged, although it's certain to get pushed some.

-- Jeff, in Minneapolis

4. You've got the correct line of thinking. The photon needs to interact with something else to produce the particle/antiparticle pair. The energy level has to be ≥ 2x the rest mass of an electron.

It is based on the reference frame of the particle that is being interacted with by the photon.

from wikipedia

The process can happen with with another photon but I have to admit I'm not sure how this actually occurs given that different frames would measure the photons energy levels differently thus what would be a photon with 2x the rest mass of a photon, or 1.022 MeV, in one frame could have less energy in a lower frame. I'm guessing that it works because while there is an infinite possibilities of frames that there is a lower limit to frames. Thus the energy levels would have to be sufficient in this frame then in all other frames the particles are created and conservation of energy and momentum are still conserved.

If anyone can explain this better I'd appreciate it.

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Originally Posted by WayneFrancis
The process can happen with with another photon but I have to admit I'm not sure how this actually occurs given that different frames would measure the photons energy levels differently thus what would be a photon with 2x the rest mass of a photon, or 1.022 MeV, in one frame could have less energy in a lower frame. I'm guessing that it works because while there is an infinite possibilities of frames that there is a lower limit to frames. Thus the energy levels would have to be sufficient in this frame then in all other frames the particles are created and conservation of energy and momentum are still conserved.

If anyone can explain this better I'd appreciate it.
Look at it this way: a positronium (slow-moving electron-positron pair) will, because of conservation of energy and momentum, annihilate into two photons each with energy exactly 511 keV and in exactly opposite direction. (Provided they have opposite spins) This means that if 2 511 keV photons can do the opposite and form electron and positron provided they move in exactly opposite direction.

Now, let us move into a frame where the positronium moves in exact direction of one photon. Then the photons still have to move in exact opposite direction, but one is blueshifted to energy above 511 keV and the other is redshifted to energy below 511 keV. The combined energy of the two is more than 1022 keV now.

Then, pick a frame where the positronium moves in exact right angle to the direction of photons. Now the photons are aberred so that they no longer move in exact opposite direction, and they are both blueshifted to above 511 keV.

And then again you can pick any other direction and speed for your frame. For any two directions and energies of photons, you can choose a frame where the photons have exact opposite direction and equal energy. Then if this energy is 511 keV or more, electron-positron pair can form. (Except if the directions of photons are exact same, because then they cannot interact at all).

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Originally Posted by WayneFrancis
The process can happen with with another photon but I have to admit I'm not sure how this actually occurs given that different frames would measure the photons energy levels differently thus what would be a photon with 2x the rest mass of a photon, or 1.022 MeV, in one frame could have less energy in a lower frame. I'm guessing that it works because while there is an infinite possibilities of frames that there is a lower limit to frames. Thus the energy levels would have to be sufficient in this frame then in all other frames the particles are created and conservation of energy and momentum are still conserved.

If anyone can explain this better I'd appreciate it.
Look at it this way: a positronium (slow-moving electron-positron pair) will, because of conservation of energy and momentum, annihilate into two photons each with energy exactly 511 keV and in exactly opposite direction. (Provided they have opposite spins) This means that if 2 511 keV photons can do the opposite and form electron and positron provided they move in exactly opposite direction.

Now, let us move into a frame where the positronium moves in exact direction of one photon. Then the photons still have to move in exact opposite direction, but one is blueshifted to energy above 511 keV and the other is redshifted to energy below 511 keV. The combined energy of the two is more than 1022 keV now.

Then, pick a frame where the positronium moves in exact right angle to the direction of photons. Now the photons are aberred so that they no longer move in exact opposite direction, and they are both blueshifted to above 511 keV.

And then again you can pick any other direction and speed for your frame. For any two directions and energies of photons, you can choose a frame where the photons have exact opposite direction and equal energy. Then if this energy is 511 keV or more, electron-positron pair can form. (Except if the directions of photons are exact same, because then they cannot interact at all).

7. When does a photon become mater.. ? It never does. For when a interaction of electron and photon all sorts of things happen BUT. Photons do not change into any thing at all. Not mater, not antimatter, not dark mater, Not mater. When you mix things together they undergo changes. Sub atomic particles are almost famous for it... I do not like the term change... Its not what happens is it ?
Last edited by astromark; 2009-Jul-29 at 09:41 AM. Reason: edit spelling

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One simple way to look at why you need other matter around is to use the rest frame of the c.o.m. of the resultant particle pair. In this frame, momentum after the creation event is zero, but the photon clearly had momentum before, so this reaction breaks the conservation rule.

9. Originally Posted by astromark
When does a photon become mater.. ? It never does. For when a interaction of electron and photon all sorts of things happen BUT. Photons do not change into any thing at all. Not mater, not antimatter, not dark mater, Not mater. When you mix things together they undergo changes. Sub atomic particles are almost famous for it... I do not like the term change... Its not what happens is it ?
Well, "in the beginning" at some point there was only EM radiation filling the entire universe : no matter particles existed at all. If you're saying photons can't turn into matter unless they interact with a matter particle, we would not be here right now, would we?

Rob

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Rob,

Which point in time near the beginning are you referring to?

-- Jeff, in Minneapolis

11. You could take a look at this page; it gives some of the answers, although I can't vouch for it myself.
http://en.wikipedia.org/wiki/Matter_creation

I'm interested in this question because, at first sight, it would appear that you could in theory create matter/antimatter pairs simply by using opposing beams of carefully tuned photons.

However our laser physicist geezer at OA tells me that this process might work as a (fairly inefficient) method of creating positrons and electrons, but when you get to the energies required to produce (much more useful) protons and anti-protons, 'other processes dominate'.

I'd be interested to hear from anyone else on this matter, to see if this idea can be rescued in any way.

12. Oh... dear me... No. EM radiation filled the universe and then what ? I do not like to sound pompous but, The state of matter at the outset of the universe is not well understood is it., that's not a question. Its a fact. From that super hot plasma cloud did come everything and all we see today., but to say it was just photons is NOT right is it. The fact you seem to miss is that we do not know all the mechanics of mater formation at those super hot dense states. The road ahead is very unclear so to be imaginative is not absolute. You must except that some things are just beyond us.
By fistidious examination of data from such as the Large Hadron Collider might lead us to answer these questions with a little more knowledge comes the answers... We are as yet a long way from a absolute answer to how this universe became what it appears to be. To quote you 'Rob' The fact that we are here would seem to point to the fact that it can. But is it right to call it a sea of photons. I do not think so.

13. Originally Posted by Jeff Root
Rob,

Which point in time near the beginning are you referring to?

-- Jeff, in Minneapolis
In the inflationary cosmology model, just after the period of inflation comes the period of re-heating, in which the potential energy of inflation becomes EM radiation. So it's the period before which the universe is something unknown to us, and afterward it's filled with high-energy EM radiation, which is something we can plant a flag in the ground and say "we know how the universe can evolve from this state to our present state."

So "in the beginning" above refers to the period immediately after the re-heating.

Rob

14. It would seem then that you have reached the same point we all are attempting to say. The same conclusion as Jeff and I...and most of us are at.
the mainstream view is well reflected by the link...

http://en.wikipedia.org/wiki/Matter_creation

I fear you are asking us to make clear what is cloudy. I have no idea how or why the universe did what we think it did. Oddly this is the one question I do not seem to need to know... The fact as you so rightly observed.,' we are here.' Seems to have satisfied my quest for that answer.The need to know is evaporating.am I the lucky one...

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Rob,

My information may not be as up-to-date as yours. I understand the
period after Inflation to be labeled the "Hadronic Era" because it was a
period in which pairs of hadrons (quark matter) were created by colliding
gamma rays as quickly as they annihilated. It lasted until the Universe
that hadrons could no longer be created. The Hadronic Era was followed
by the Leptonic Era, in which leptons (electrons and positrons, neutrinos
and antineutrinos) continued to be created as fast as they annihilated.
That lasted until the Universe was about one second old. After that,
most of the energy was in the photons. As the Universe expanded, the
energy density of the photons diminished until today the CMBR is barely
detectible.

There was never a time when "there was only EM radiation filling the
entire universe : no matter particles existed at all." The Universe was
very dense near the beginning, and mass is the densest form of energy.
So massive particles would have been favored at the earliest times.

-- Jeff, in Minneapolis

16. Originally Posted by astromark
It would seem then that you have reached the same point we all are attempting to say. The same conclusion as Jeff and I...and most of us are at.
the mainstream view is well reflected by the link...

http://en.wikipedia.org/wiki/Matter_creation

I fear you are asking us to make clear what is cloudy. I have no idea how or why the universe did what we think it did. Oddly this is the one question I do not seem to need to know... The fact as you so rightly observed.,' we are here.' Seems to have satisfied my quest for that answer.The need to know is evaporating.am I the lucky one...
Actually I was just answering Jeff's question. I'm not asking about the early universe in my OP, I'm just trying to understand the rules by which a photon( or photons) turns into matter. Obviously we don't see light from our Sun turning into random matter particles, so it's not a common occurrence. But it must still be happening regularly in places in the universe, since we can make it happen in particle accelerators, and yet there are more energetic particle collisions taking place near black holes/neutron stars, etc.

Rob

17. Originally Posted by Jeff Root
That lasted until the Universe was about one second old. After that,
most of the energy was in the photons. As the Universe expanded, the
energy density of the photons diminished until today the CMBR is barely
detectible.
The photons of the CMBR are not from when the universe was 1 second old, more like 300000 years old. They were emitted from electrons when the universe cooled enough for the plasma recombine into hydrogen (and a little helium) and become transparent.

18. Originally Posted by phunk
The photons of the CMBR are not from when the universe was 1 second old, more like 300000 years old. They were emitted from electrons when the universe cooled enough for the plasma recombine into hydrogen (and a little helium) and become transparent.
The cool kids refer to this as the "surface of last scattering."

Rob

19. Originally Posted by Jeff Root
Rob,

My information may not be as up-to-date as yours. I understand the
period after Inflation to be labeled the "Hadronic Era" because it was a
period in which pairs of hadrons (quark matter) were created by colliding
gamma rays as quickly as they annihilated. It lasted until the Universe
that hadrons could no longer be created. The Hadronic Era was followed
by the Leptonic Era, in which leptons (electrons and positrons, neutrinos
and antineutrinos) continued to be created as fast as they annihilated.
That lasted until the Universe was about one second old. After that,
most of the energy was in the photons. As the Universe expanded, the
energy density of the photons diminished until today the CMBR is barely
detectible.
I don't think we're in that much disagreement. However, before the age of quarks, the universe was too hot and energetic to allow quarks to form, so it had to expand and cool before matter could be created. So at this stage, it was filled primarily with photons.

There was never a time when "there was only EM radiation filling the
entire universe : no matter particles existed at all." The Universe was
very dense near the beginning, and mass is the densest form of energy.
So massive particles would have been favored at the earliest times.

-- Jeff, in Minneapolis
But at the extreme density and temperature of the earliest universe (shortly after 10^-41 seconds), any matter would have been immediately converted into energy by violently colliding with other particles of matter, since it would have been too hot for any matter particles to allow to condense, until around 10^-12 seconds when it became cool enough for free quarks to form and stick around.

Rob

20. Originally Posted by astromark
When does a photon become mater.. ? It never does. For when a interaction of electron and photon all sorts of things happen BUT. Photons do not change into any thing at all. Not mater, not antimatter, not dark mater, Not mater. When you mix things together they undergo changes. Sub atomic particles are almost famous for it... I do not like the term change... Its not what happens is it ?
Really? Guess these guys are wasting their time then.

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Originally Posted by eburacum45
...it would appear that you could in theory create matter/antimatter pairs simply by using opposing beams of carefully tuned photons.
Or photonic wave interactions which just happen to coincide - an act which I'm sure occurs in triplicate billions of times a second in our own solar system.

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