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mr d

energy to matter

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at what point could you transform energy into matter. if energy is traveling at the speed of light, what would occur if you slowed the speed of energy/light say by the use of a black hole till it was traveling below light speed. or use gravational forces of such to bend light in upon itself till energy began orbiting itself, perhaps creating sub-atomic particles in the process.

if all that exisited in the universe at the moment of the big bang was energy. what possible forces could have been used to transform it into matter. any other thoughts.

 

strange thoughts

 

mr d

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Gamma ray photons of sufficient energy can spontaneously turn into a positron and an electron. Look up "pair production".

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hmmm not sure but if e=mc^2 then couldn't m=e/c^2?

 

yes, but that just shows the relationship between mass and energy. It doesn't tell you how to actually do the deed.

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Matter is created all the time in particle accelerators, however as [Tycho?] has pointed out matter is produced in matter (electrons in this case) and anti-matter (positrons) pairs and so quickly annihilate each other and turn back to energy. So to create atoms and molecules et.c is impossible...so far.

 

This is one of the big problems about how matter managed to propogate succesfully to form our universe, and as yet there is no solid answer. You should maybe read up on Paul Dirac for more info on the above or visit the CERN website.

 

Something else you maybe interested in is the Higgs Boson / field which is (although undetected) one of the theories that could explain how particles acquire mass. Hope this helps.

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I readed that pair creation is stimulated when the gamma photon pass by an heavy atom nucleus. Is there other conditions that will stimulate the gamma photon to create pair ?

To create a pair the gamma photon need to have more energy than the equivalent mass of 2 electrons. Is it possible to combine 2 gamma photon each having the equivalent energy of 1.5 electron, to create a pair ?

Thanks

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Yes it is! In fact this is a proposal for a high energy particle physics collider. Collide two photons together and you can create particle-antiparticle pairs to study (and lots more!).

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So it is theorically possible, but was never observe. Why should we wait for giant collider. Can't we produce gamma ray with enought energy by other way ?

It's hard to imagine a photon collision: most of the time they pass throught each other...

Anyway photon being it's own anti particle it would be the same mechanism as the electron anti-electron anihilation.

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No, it has been observed. It is fairly easy to make photons energetic enough to make, say, an electron-positron pair. All you need is a centre-of-mass energy of 1.022MeV.

 

But to do interesting physics, producing new exotic matter (like the Higgs boson, or supersymmetric particles which are much heavier than the electron) you need very high energy photons (hundreds of GeV in energy). That is what the proposed photon collider would look at.

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All you need is a centre-of-mass energy of 1.022MeV.

Do you mean that I can collide two photons of 0.511MeV to produce an electron positron pair ?

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Yes. E=mc2 so the minimum energy you need to produces and electron positron pair is 2mec2. The mass of an electron (or positron) is 0.511MeV/c2.

 

In practice, you won't be able to observe the electron positron pair unless they are moving. 2mec2 would produce them at rest, so you really need a bit more than this do do anything useful.

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Let say we use .7MeV photons in a magnetic field so they separate quickly.

I was woundering if the angle between the 2 photons influence the pair creation rate.

Let 2 photons travelling in the same direction have 0 degre angle.

At 0 degree my guess is that the production would be very low.

At 90 degree I don't know if it's possible

At 180 degree (photons going in opposite direction) my guess is that it would be maximum.

So is there experiments been carried to mesure the rate of creation in function of the angle ?

Thanks for your answers!

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Let say we use .7MeV photons in a magnetic field so they separate quickly.

I was woundering if the angle between the 2 photons influence the pair creation rate.

Let 2 photons travelling in the same direction have 0 degre angle.

At 0 degree my guess is that the production would be very low.

At 90 degree I don't know if it's possible

At 180 degree (photons going in opposite direction) my guess is that it would be maximum.

So is there experiments been carried to mesure the rate of creation in function of the angle ?

Thanks for your answers!

 

The important thing is the centre-of-mass energy. This is the energy that the electron-positron pair have in the frame where their centre-of-mass is stationary. In this frame' date=' the photons are always colliding head on, and the centre-of-mass energy must be more than 2m[sub']e[/sub]/c2.

 

So when thinking about your angles, think about making a Lorentz transformation to the frame where the photons hit head on, and see what this energy is. For (near) 0 degrees, this will be a strong boost almost in the original direction of the photons, so in the centre-of-mass frame the photons would have very little energy and could not convert to an electron-positron pair. At 180 degrees we don't have to boost at all, so the energy will be much higher and we will have a much higher production rate. So your supposition was correct!

 

This sort of thing has been tested indirectly by colliding photons which are emitted from accelerating charged particles. The opposite reaction [math]e^+e^- \to \gamma \gamma[/math] has been exhaustively examined at many colliders, at many energies, the highest energy being the Large Electron-Positron (LEP) Collider at a little over 200GeV.

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For the LHC you have to remember that 7TeV is the energy of the entire proton though. Not all of the protons collide, only the constituents. At these energies the proton is mainly made of gluons, so the LHC is mainly a gluon collider. Typically the gluons which collide have about 600GeV energy, but there are some which have much higher energies.

 

For example, the LHC should be show the existance of squarks and gluinos (supersymmetric particles) as long as they are lighter than about 2.5TeV.

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Thanks again for your answers they are very clear!

A last question:

Is it true that one photon of more than 1.022MeV can spontaneously convert into an electron-positron pair ?

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Thanks again for your answers they are very clear!

A last question:

Is it true that one photon of more than 1.022MeV can spontaneously convert into an electron-positron pair ?

 

Not (quite) spontaneously - you need something else around to recoil and conserve momentum.

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Doesn't the Strong force, Weak force, and Electromagnetic Force all have to be combined into one force (GUT), then act upon Photons for them to spontaneosly turn into an electron and a positron?

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No. The conversion of photons to electron-positron pairs is purely QED. Admittedly the photon is a superposition of the one of the SU(2) gauge bosons and the U(1) hypercharge boson, but this is not a unification.

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I readed that pair creation is stimulated when the gamma photon pass by an heavy atom nucleus. Is there other conditions that will stimulate the gamma photon to create pair ?

To create a pair the gamma photon need to have more energy than the equivalent mass of 2 electrons. Is it possible to combine 2 gamma photon each having the equivalent energy of 1.5 electron' date=' to create a pair ?

Thanks[/quote']

Actually, i think it's really any opposing force strong enough. Magnetic fields can 'split' photons as well, into a positron and electron, and is often the process responsible for the misconception that magnetic fields are capable of bending light. They're not bending light, just altering the stream of electrons and positrons due to the fact that they're now charged particles.

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Magnetic fields are elctromagnetism too. The photon is invisible to all forces except electromagnetism and gravity.

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thank you for all the responces.

interesting information to look into.

have a feeling the process for the creation of matter must be simplier than is thought. as we do seem to have quit a lot of the stuff hanging around.

 

strange thoughts

 

mr d

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The problem is not creating matter (which is very easy). The problem is creating matter without creating anti-matter.

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