Photon-Photon Pair Production

[steevey]

It produces photons but they are so energetic that PAIRS of photons collide and produce electron-antielectron pairs.

 

Ok, when do photons ever collide to form electrons? That makes no sense. Why wouldn't they form protons or neutrons ever if that's true?

 

!

Moderator Note

This has been moved from the pair-instability supernova thread

Edited February 5, 2011 by swansont
add modnote

[insane_alien]

Ok, when do photons ever collide to form electrons? That makes no sense. Why wouldn't they form protons or neutrons ever if that's true?

 

they don't need to collide to produce electron-positron pairs, if the photon is over 1.04MeV then it'll do it on its own quite happily.

 

protons and antiprotons require a much much more energetic photon so these are really really unlikely to happen.

[D H]

Ok, when do photons ever collide to form electrons? That makes no sense. Why wouldn't they form protons or neutrons ever if that's true?

Just because it doesn't make sense to you doesn't mean it doesn't make sense, period. It makes sense because energy and mass are two sides of the same coin: E=mc². An electron and positron can annihilate to form a pair of protons. The reverse is also true: A pair of protons whose combined energy is at least twice that of the rest mass of an electron can (with the help of a nearby nucleus) combine to form an electron-positron pair. It is this photon-photon interaction that fuels a pair instability supernova.

 

As far as photon-photon interactions producing protons or neutrons: That can happen, too. However, the photons now need to be a whole lot more energetic because protons and neutrons are a lot more massive than are electrons.

 

they don't need to collide to produce electron-positron pairs, if the photon is over 1.04MeV then it'll do it on its own quite happily.

A high energy photon still needs the help of a nucleus to make a real (as opposed to virtual) electron positron pair. The process that powers pair instability supernova involves pairs of photons with about half that much energy. Stars massive enough to produce 1.04 MeV photons apparently undergo photodisintegration before they can produce photons of that kind of energy.

 

 

 

 

 

[steevey]

Just because it doesn't make sense to you doesn't mean it doesn't make sense, period. It makes sense because energy and mass are two sides of the same coin: E=mc². An electron and positron can annihilate to form a pair of protons. The reverse is also true: A pair of protons whose combined energy is at least twice that of the rest mass of an electron can (with the help of a nearby nucleus) combine to form an electron-positron pair. It is this photon-photon interaction that fuels a pair instability supernova.

 

As far as photon-photon interactions producing protons or neutrons: That can happen, too. However, the photons now need to be a whole lot more energetic because protons and neutrons are a lot more massive than are electrons.

 

 

A high energy photon still needs the help of a nucleus to make a real (as opposed to virtual) electron positron pair. The process that powers pair instability supernova involves pairs of photons with about half that much energy. Stars massive enough to produce 1.04 MeV photons apparently undergo photodisintegration before they can produce photons of that kind of energy.

 

 

 

 

 

That's why I said "it doesn't make sense" and not "it's wrong".

 

But...

 

 

****m=E/c^2****

 

How does that happen with two photons colliding?

 

 

Also, so if I point two high energy laser beams at each other, I should get a lot of matter being created right? Because those would be very powerful beams with many photons, and it only takes two photons to create solid matter, so if there's 10^20 photons colliding every second, I should be creating matter equal to half that every second.

So why hasn't anyone ever taken advantage of this property of photons?

Edited February 2, 2011 by steevey

[D H]

Also, so if I point two high energy laser beams at each other, I should get a lot of matter being created right?

Yes and no.

 

The no answer first: It is the energy of the photons, not the beam, that counts. The violet end of the visible spectrum is the most energetic of visible light. The energy in a 380 nanometer photon is 3.2 electronvolts. An electron has a rest energy of 0.511 million electron volts (MeV), so what is needed is a pair of 0.511 MeV photons. Those are gamma rays, not visible light. Adding power to a laser doesn't change the frequency of the photons. All it does is change the intensity of the beam, the number of photons emitted per second. Note that without help, a collision between a pair of 0.511 MeV photons still won't produce an electron-positron pair. The photon-photon collision needs to occurs near a massive atomic nucleus before pair production can occur.

 

Now for the yes answer: Lasers can be used to pump energy into a plasma. Pump enough energy into the plasma and it will glow. Pump in more energy and the glow will increase in intensity and frequency. Pump in enough energy and it will emit X-rays, and with even more energy, gammas. Should those gammas hit a high-Z nucleus they will produce electron-positron pairs. A powerful enough laser can also create electron-positron pairs via a separate mechanism, the trident pair production mechanism.

Edited February 2, 2011 by D H

[steevey]

Yes and no.

 

The no answer first: It is the energy of the photons, not the beam, that counts. The violet end of the visible spectrum is the most energetic of visible light. The energy in a 380 nanometer photon is 3.2 electronvolts. An electron has a rest energy of 0.511 million electron volts (MeV), so what is needed is a pair of 0.511 MeV photons. Those are gamma rays, not visible light. Adding power to a laser doesn't change the frequency of the photons. All it does is change the intensity of the beam, the number of photons emitted per second. Note that without help, a collision between a pair of 0.511 MeV photons still won't produce an electron-positron pair. The photon-photon collision needs to occurs near a massive atomic nucleus before pair production can occur.

 

Now for the yes answer: Lasers can be used to pump energy into a plasma. Pump enough energy into the plasma and it will glow. Pump in more energy and the glow will increase in intensity and frequency. Pump in enough energy and it will emit X-rays, and with even more energy, gammas. Should those gammas hit a high-Z nucleus they will produce electron-positron pairs. A powerful enough laser can also create electron-positron pairs via a separate mechanism, the trident pair production mechanism.

 

How come two flashlights pointed at each other won't create matter? And why haven't places like the Fermi lab done this to more accurately study what particles matter is made of? Because if you can create electrons from a couple pieces of pure energy (which I still don't get how), I think that would really tell us something about the anatomy of an electron. But I can't find any reports anywhere, whether it be in books or on the internet about documented experiments using high energy photons to create matter.

Edited February 2, 2011 by steevey

[D H]

Steevey, two questions.

 

1. What is the energy of a photon in a flashlight beam?

2. You have an incredible research wonderful tool at your fingertips. Have you used it to do any research into this topic?

[steevey]

Steevey, two questions.

 

1. What is the energy of a photon in a flashlight beam?

2. You have an incredible research wonderful tool at your fingertips. Have you used it to do any research into this topic?

 

1. Low, but so what? Why does that amount matter? Why wouldn't it create smaller pieces of matter smaller than electrons if it was two flashlights?

2. Yeah I've googled it a bunch of times and didn't find anything on photons colliding and creating matter. The only things I've found is that physicists don't really know what happens when photons collide or that the photons don't really interact or scatter at all when they collide. They might just create a different photon or equalibraite or something like that.

 

But also, how does m=E/c^2 happen with two photons colliding? Because E/c^2 is what's required to make matter from energy, at least according to Einstein.

Edited February 3, 2011 by steevey

[D H]

1. Low, but so what? Why does that amount matter? Why wouldn't it create smaller pieces of matter smaller than electrons if it was two flashlights?

What "smaller pieces of matter smaller than electrons"? Matter is quantized.

 

 

2. Yeah I've googled it a bunch of times and didn't find anything on photons colliding and creating matter.

Then you haven't looked very hard or you haven't used the right terms.

 

There is a reason sciences consistently use specific terms. Those specific terms have meaning. You will have much better results if you use the right words. Try these searches:

1. Pair instability supernova
   
2. Pair production
   
3. Threshold energy

[steevey]

What "smaller pieces of matter smaller than electrons"? Matter is quantized.

 

 

 

Then you haven't looked very hard or you haven't used the right terms.

 

There is a reason sciences consistently use specific terms. Those specific terms have meaning. You will have much better results if you use the right words. Try these searches:

1. Pair instability supernova
2. Pair production
3. Threshold energy

 

So your saying there can't possibly be anything smaller than an electron? Cause there's a lot of theories containing particle smaller than an electron. What about the singularity of a black hole, which really isn't even a theory?

 

 

Still, never says only two photons make matter, its always a photon and a nucleus or a photon and some other piece of matter.

 

Threshold energy doesn't really help this at all either.

 

Why don't you find a page that says something like "when two single high energy photons collide with just each other (and not with anything else at all in any way), an electron pair is created" because I can't find one anywhere.

Edited February 3, 2011 by steevey

[D H]

So your saying there can't possibly be anything smaller than an electron? Cause there's a lot of theories containing particle smaller than an electron. What about the singularity of a black hole, which really isn't even a theory?

You must be kidding. The singularity at the heart of a stellar black hole has the mass of a star, which is just a bit larger than the mass of an electron.

 

In addition to photons (which are massless), there are three elementary particles less massive than electrons: The neutrinos. Photons can also produce neutrino-antineutrino pairs. However, the cross-section is rather low, and even if/when this kind of pair production occurs, it won't slow the transport of energy from the center of a star. Electron-positron pair production leads to the pair instability supernova because the end effect is to slow the energy transport. The onset of electron-positron pair production triggers a chain reaction. This production reduces energy transport, which results in increased energy at the heart of the star. This increased energy results in even greater amount of electron-positron pair production, which results in an even greater reduces energy transport, and so on.

 

 

/Still, never says only two photons make matter, its always a photon and a nucleus or a photon and some other piece of matter. ... Why don't you find a page that says something like "when two single high energy photons collide with just each other (and not with anything else at all in any way), an electron pair is created" because I can't find one anywhere.

That is a red herring argument; use of logical fallacies is strongly discouraged at this site. Have you read what I have written? Look at post #7, where I said

Note that without help, a collision between a pair of 0.511 MeV photons still won't produce an electron-positron pair. The photon-photon collision needs to occurs near a massive atomic nucleus before pair production can occur.

 

 

 

 

[steevey]

You must be kidding. The singularity at the heart of a stellar black hole has the mass of a star, which is just a bit larger than the mass of an electron.

 

In addition to photons (which are massless), there are three elementary particles less massive than electrons: The neutrinos. Photons can also produce neutrino-antineutrino pairs. However, the cross-section is rather low, and even if/when this kind of pair production occurs, it won't slow the transport of energy from the center of a star. Electron-positron pair production leads to the pair instability supernova because the end effect is to slow the energy transport. The onset of electron-positron pair production triggers a chain reaction. This production reduces energy transport, which results in increased energy at the heart of the star. This increased energy results in even greater amount of electron-positron pair production, which results in an even greater reduces energy transport, and so on.

 

 

 

That is a red herring argument; use of logical fallacies is strongly discouraged at this site. Have you read what I have written? Look at post #7, where I said

 

 

 

 

 

 

 

The problem with the fact that you wrote it is the fact that I can't find and I guess you can't find any proof to back up the mechanism that powers this version of it. It can't be scientific consensus if there's no proof for the way it occurs.

 

 

http://en.wikipedia....ility_supernova

http://www.wisegeek....y-supernova.htm

 

Nothing about photon-photon collisions.

 

The evidence for a pair instability nova in general doesn't seem to be so good either. There is no way this is scientific consensus and that's why I point out logical fallacies. If it's something that's been proven but there isn't an explanation, like things in quantum mechanics, you can't really argue with that and shouldn't with those things since they are still proven to happen. But this, this is something you can argue against existing.

If there isn't enough evidence to prove this exists, you shouldn't bother telling people it's a real thing.

 

You must be kidding. The singularity at the heart of a stellar black hole has the mass of a star, which is just a bit larger than the mass of an electron.

 

 

The mass of a black hole is only a little bit more massive than an electron... Last I heard an electron has a mass of something like 10^-32 kilograms, and a star like the sun has like 10^12(+ or - a power or two) kilograms. That statement doesn't make sense to me because of that.

Edited February 4, 2011 by steevey

[D H]

The problem with the fact that you wrote it is the fact that I can't find and I guess you can't find any proof to back up the mechanism that powers this version of it. It can't be scientific consensus if there's no proof for the way it occurs.

 

 

http://en.wikipedia....ility_supernova

http://www.wisegeek....y-supernova.htm

 

Nothing about photon-photon collisions.

Are you serious? Can you read?

 

In the wikipedia article, the word "gamma" occurs 32 times, the term "pair production" occurs six times, and the words electron and positron occur five times each. The article links to "pair production by gammas".

 

The evidence for a pair instability nova in general doesn't seem to be so good either. There is no way this is scientific consensus and that's why I point out logical fallacies. If it's something that's been proven but there isn't an explanation, like things in quantum mechanics, you can't really argue with that and shouldn't with those things since they are still proven to happen. But this, this is something you can argue against existing.

As I said before, just because you don't understand something doesn't mean physicists and astronomers don't.

 

The mass of a black hole is only a little bit more massive than an electron... Last I heard an electron has a mass of something like 10^-32 kilograms, and a star like the sun has like 10^12(+ or - a power or two) kilograms. That statement doesn't make sense to me because of that.

I. Give. Up.

[steevey]

Are you serious? Can you read?

 

In the wikipedia article, the word "gamma" occurs 32 times, the term "pair production" occurs six times, and the words electron and positron occur five times each. The article links to "pair production by gammas".

 

 

As I said before, just because you don't understand something doesn't mean physicists and astronomers don't.

 

 

I. Give. Up.

 

So what if the word gamma occurs? Those sites state a collision between a "gamma" ray photon and a NUCLEUS.

 

Not only that, I never said I don't understand the process, I said there isn't enough evidence to back up how it works on this topic. Maybe there is proof somewhere, but it doesn't matter unless you find it.

 

This theory shouldn't be considered scientific consensus on site nor has it been in the real world.

Edited February 4, 2011 by steevey

[D H]

So what if the word gamma occurs? Those sites state a collision between a "gamma" ray photon and a NUCLEUS.

 

SCREEEECH! That is the sound of the moving goalpost that is emanating from your post.

 

Let's go back to something I wrote earlier, this time in a nice big font so you can read it:

 

Note that without help, a collision between a pair of 0.511 MeV photons still won't produce an electron-positron pair. The photon-photon collision needs to occurs near a massive atomic nucleus before pair production can occur.

 

A single photon, no matter how energetic, cannot produce a real (as opposed to virtual) electron-positron pair on its own. It needs the help of some other particle. A pair of photons on their own can produce an electron-positron pair, but the cross section is rather low. The thermal photons in a star do not have the energy needed to result in pair production in vacuo. The presence of a nearby nucleus increases the cross section by orders of magnitude. There just happens to be a whole lot of nuclei in the center of a star.

 

 

 

Not only that, I never said I don't understand the process, ...

It is rather obvious that you do not understand the process, and it appears that you do not want to understand the process.

 

 

I said there isn't enough evidence to back up how it works on this topic.

Evidence? Are you truly claiming that electron-positron pair production is only "theoretical"? Are you serious?

 

The evidence that electron-positron pair production does occur is overwhelming. It is an uninteresting and rather annoying process to particle physicists. Due to the EM cascade, the vast majority of events seen in a collider are the production of electron-positron pairs and the annihilation events that inevitably follow. All of those EM cascade events have to be removed from the picture so that particle physicists can see the interactions of interest. Lately, physicists have used pair production as a means of producing billions (hundreds of billions) of positrons.

Edited February 4, 2011 by D H

[steevey]

A pair of photons on their own can produce an electron-positron pair

 

 

 

Dude, that's what I was arguing against, there's no proof for that anywhere that I can find. You honestly didn't know that when I kept emphasizing "two photons" and "only two photons" that I meant two photons on their own without any other matter? Yeah of course photons can create matter when they hit matter like a nucleus. THAT can produce an electron position pair. However, the statement that only two photons can create matter still isn't proven.

 

Just so you can make sure if you even bothered to read this much of my post: I already knew that when high energy photons hit a *NUCLEUS*, that they can produce matter and anti-matter. HOWEVER, THERE IS NO PROOF KNOWN TO ME that in a collision of JUST two photons without ANY other matter, that matter is created from the collision of JUST those two photons regardless of how powerful the photons are.

 

SCREEEECH! That is the sound of the moving goalpost

 

 

WAIT A MINUTE!!! Your arguing just to seem smart and not to actually iron out true information?

Edited February 4, 2011 by steevey

[D H]

Steevey, you are moving the goalposts. You are making it as if the issue is whether two photons by themselves can result in pair production. That is not what is happening inside stars. The issue here is electron-positron pair production in stars.

 

Without the help of a nearby nucleus, a single photon cannot produce a real (as opposed to virtual) electron positron pair. One way to look at it is that the nucleus catalyzes the reaction. Two photons can directly produce an electron positron pair per the following Feynman diagram:

 

 

However, the cross section of this reaction at the energy levels available inside stars is extremely low. Just as a nearby nucleus makes it possible for a single photon to produce an electron-positron pair, the low cross section for an isolated photon-photon interaction becomes very high when the interaction takes place in the vicinity of a nucleus. The nucleus once again catalyzes the reaction.

 

That single photon pair production cannot happen in vacuo / photon-photon pair production is unlikely in vacuo is a red herring. Photons, with the help of a nearby nucleus, do produce electron-positron pairs. That there are lots of nuclei present in the center of a star is what makes your argument a red herring.

 

What is happening inside this pair instability supernova are the reactions γ+Z → Z+e^-+e⁺ and 2γ+Z → Z+e^-+e⁺. The photons are gone, replaced by electron-positron pairs. That is pair production. It is this pair production that slow the transport of energy out of the center of the star.

[steevey]

You are making it as if the issue is whether two photons by themselves can result in pair production.

 

Uhh, that's actually exactly what my issue was. I thought you were trying to argue that that was true...

 

 

 

[D H]

You are making it as if the issue is whether two photons by themselves can result in pair production.

 

Uhh, that's actually exactly what my issue was. I thought you were trying to argue that that was true...

It is true. Physicists observe these reactions all the time in colliders. To a particle physicist these are uninteresting and somewhat annoying reactions. Uninteresting because the physics is very well known, somewhat annoying because the electromagnetic cascade acts to hide the interesting physics.

 

It is also off-topic. The topic of this thread is the cause of pair instability supernova, which is pair production with the aid of a nucleus.

[steevey]

It is true. Physicists observe these reactions all the time in colliders. To a particle physicist these are uninteresting and somewhat annoying reactions. Uninteresting because the physics is very well known, somewhat annoying because the electromagnetic cascade acts to hide the interesting physics.

 

It is also off-topic. The topic of this thread is the cause of pair instability supernova, which is pair production with the aid of a nucleus.

 

How come lower energy photons don't form high mass pieces of matter when they collide? And why can't I find a single shred of evidence anywhere other than people from this site that this phenomena occurs?

 

 

 

[Cap'n Refsmmat]

How come lower energy photons don't form high mass pieces of matter when they collide? And why can't I find a single shred of evidence anywhere other than people from this site that this phenomena occurs?

More energy is required to create more mass. That's mass-energy equivalence, as in [imath]E=mc^2[/imath].

 

http://en.wikipedia....Pair_production

 

Pair production refers to the creation of an elementary particle and its antiparticle, usually from a photon (or another neutral boson). For example an electron and its antiparticle, the positron, may be created.

 

http://dx.doi.org/10...hem.2005.10.008

 

Photons of energies above 2m_ec² (1.022 MeV) can interact with the Coulomb field of an atomic nucleus to be transformed into an electron–positron pair, the probability increasing with increasing photon energy, up to a plateau at high energies, and increasing with increasing atomic number approximately as the square of the nuclear charge (proton number).

 

(that's J.H. Hubbell, "Electron-positron pair production by photons: A historical overview", Radiation Physics and Chemistry, Volume 75, Issue 6, Pair Production, June 2006, Pages 614-623, if you can't get electronic access)

 

As it says, photons near a nucleus can collide and produce electrons and positrons.

[D H]

How come lower energy photons don't form high mass pieces of matter when they collide?

Because matter is quantized.

 

And why can't I find a single shred of evidence anywhere other than people from this site that this phenomena occurs?

The only possibility I can see is that you don't know how to use the internet.

 

Here is just a tiny sample of what you can find if you know how to look.

http://www.aps.org/m...d/LLNL_Chen.pdf

http://books.google....epage&q&f=false

http://www.hep.ucl.a...g-tutorial.html

 

 

How to search.

Certainly you have done this search: http://www.google.co...pair+production

 

For the most part production of electron-positron pairs is old hat. Sometimes it helps to tell google that you are interested in this old hat stuff:

http://www.google.co...pair+production

 

 

Restricting your search to .edu and .gov cites often helps eliminate a lot of the chaff:

http://www.google.com/search?q=electron+positron+pair+production+site:edu

http://www.google.co...uction+site:gov

 

There are also tools such as books.google.com and scholar.google.com. Use these. Other search engines can also be quite useful.

Edited February 5, 2011 by D H

[steevey]

Because matter is quantized.

 

 

The only possibility I can see is that you don't know how to use the internet.

 

Here is just a tiny sample of what you can find if you know how to look.

http://www.aps.org/m...d/LLNL_Chen.pdf

http://books.google....epage&q&f=false

http://www.hep.ucl.a...g-tutorial.html

 

 

How to search.

Certainly you have done this search: http://www.google.co...pair+production

 

For the most part production of electron-positron pairs is old hat. Sometimes it helps to tell google that you are interested in this old hat stuff:

http://www.google.co...pair+production

 

 

Restricting your search to .edu and .gov cites often helps eliminate a lot of the chaff:

http://www.google.co...uction+site:edu

http://www.google.co...uction+site:gov

 

There are also tools such as books.google.com and scholar.google.com. Use these. Other search engines can also be quite useful.

 

 

I thought you knew what I was saying, and it turns out you didn't and you were being condescending. Ok. here is as simple as I can make it:

 

JUST TWO AND ONLY TWO PHOTONS COLLIDING WITH NO INTERACTION WITH ANY PIECE OF MATTER IN THE ENTIRE UNIVERSE does not produce matter is what I'm saying.

Edited February 5, 2011 by steevey

[D H]

Ok. here is as simple as I can make it:

 

JUST TWO AND ONLY TWO PHOTONS COLLIDING WITH NO INTERACTION WITH ANY PIECE OF MATTER IN THE ENTIRE UNIVERSE does not produce matter is what I'm saying.

Ok. Here is as simple as I can make it: You are wrong (did you read the second link?), and moreover, you are completely off topic.

 

 

 

[swansont]

First calculations of rate of e+/e- pair production in photon-photon collision was done by Lev Landau in 1934

 

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