Why can't two laser beams collide?
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exchemist
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This article explains it, but not really in layman’s language: https://en.wikipedia.org/wiki/Two-photon_physics
Как это связано?
Pinball1970
Valued Senior Member
If there is no mass (or charge) then you do not get the kinds of phenomena / interactions you get in particle accelerators with massive particles.
Photons are bosons so do not obey Pauli's exclusion principle, that means they can occupy the same quantum state.
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DaveC426913
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As pinball points out, photons are bosons, they are not like atoms, which are fermions.
If you fill a box with atoms the box will eventually get full. This is because atoms can't occupy the same space (Pauli's Exclusion Principle).
However, you can fill a box with as much light as you want, because photons do not obey the PEP.
If you fill a box with atoms the box will eventually get full. This is because atoms can't occupy the same space (Pauli's Exclusion Principle).
However, you can fill a box with as much light as you want, because photons do not obey the PEP.
А вы можете посчитать, сколько фотонов находится в коробке? И как они вообще там смогут находиться, они же двигаются со скоростью света?
Как это мешает им сталкиваться?
DaveC426913
Valued Senior Member
In essence. You don't literally count photons.
Why not? They bounce around inside a mirrored box, even as they move at the speed of light.
DaveC426913
Valued Senior Member
Turn the question on its head. The better question is "why do some particles collide at all?"
The answer is: the only thing that stops particles from passing through each other is Pauli's Exclusion Principle, which only applies to certain types of particles. We call these types of particles hadrons. Protons, electrons and neutrons are all hadrons. You can't have two of them occupy of the same space at the same time (nor can they have the same quantum state). PEP is what makes atoms bounce off each other and what makes your body able to touch a table. it's also what makes electrons fall into their restrifcted orbitals - the basis of all chemistry.
Photons are a group of particles we call bosons. They do not obey PEP, and therefore do not simply bounce off each other. They can pass right through each other (excepting some special circumstances. It is possible to make two photons collide head-on).
exchemist
Valued Senior Member
Electrostatic charge has a lot to do with as well, though, doesn’t it? PEP just says they can’t occupy the same quantum state. One can envisage 2 different quantum states which have a degree of overlap in physical location, surely? This must be true of the different orbitals occupied in an atom, for instance.
Сонаправленные лучи в лазере притягиваются, или отталкиваются, по вашему мнению?
DaveC426913
Valued Senior Member
Neither.
Т.е., два встречно направленных фотона не проявляют магнитного притяжения друг к другу, а параллельно летящие в одном направлении фотоны не проявляют магнитного отталкивания друг от друга, по вашему мнению?
Pinball1970
Valued Senior Member
There is no charge so not in that way. The big difference between a laser and normal light is that the photos are coherent in a laser. That means the waves are the same wavelength and in phase. Physicists use lasers for a lot of stuff. If the exact wavelength and amplitude is known, you know a lot about the energies/momenta involved when they interact with the sample.
Researchers used lasers in the fairly recent fusion experiments, lots of energy involved because the photons are all in synch.
А при чём тут заряд?
Pinball1970
Valued Senior Member
Charge has everything to do with it. The reason why neutrinos do not interact with matter readily is because a they have a minute mass and have no charge.
Photons, up to recent studies, have put the lower bound on mass to a very very low number. With no mass therefore no attraction, no repulsion and the fact they can literally be in the same place, in the same way, at the same time, means they do not affect each other.
EDIT: Upper bound on the photon mass is 9.52 × 10-46 kilograms
Пин, а разве фотоны не безмассовые? И каким образом они могут находиться в одном и том же месте, в одно и то же время?
Pinball1970
Valued Senior Member
They could be massless but how do you confirm that? I weigh about 197 lbs and you want to weigh me to check.
I get on your scale and I weigh zero, that is because your scales begin at 1000kg.
So you develop different scales because you do not believe i weigh nothing.
Now your lowest measurement, your lower bound is 100kg and you are very happy because you have improved your accuracy by a factor of ten.
I get on, I still weigh nothing.
This is where physicists are right now, their instruments are extremely sensitive BUT you cannot weigh zero and you cannot weigh something that is beyond the limit of your instrumentation.
Pinball1970
Valued Senior Member
What about Einstein's E=mc sqr? The whole point is that if you have mass then you cannot travel at c right?
That's what I thought too.
Turns out that is not necessarily true, my details are fuzzy, I would have to consult with a complicated thread on another forum.
From memory it got tricky.
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