Can a free particle ever emit a particle












4














I have found this question here Can a free particle absorb/emit photons?, along with other reseources that show a free particle connot emit a photon (in a vacuum).



Now, I am 90% sure it does, but does this result hold generally, ie not just photons/ photons but in a medium?



If you go in the rest frame no matter what it emits energy is not conserved. The reason I am questioning this is that an exam question said 'if it can emit it find the angle relative to the motion of the electron'. Seems odd that they would give away marks for just repeating that it can't.






special-relativity







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  • 1




    How about a free neutron decaying to a proton, electron & antineutrino?
    – PM 2Ring
    4 hours ago






  • 1




    But then it changes rest mass, it doesn't just emit things
    – Toby Peterken
    4 hours ago
















4














I have found this question here Can a free particle absorb/emit photons?, along with other reseources that show a free particle connot emit a photon (in a vacuum).



Now, I am 90% sure it does, but does this result hold generally, ie not just photons/ photons but in a medium?



If you go in the rest frame no matter what it emits energy is not conserved. The reason I am questioning this is that an exam question said 'if it can emit it find the angle relative to the motion of the electron'. Seems odd that they would give away marks for just repeating that it can't.






special-relativity







share|cite|improve this question


















  • 1




    How about a free neutron decaying to a proton, electron & antineutrino?
    – PM 2Ring
    4 hours ago






  • 1




    But then it changes rest mass, it doesn't just emit things
    – Toby Peterken
    4 hours ago














4












4








4


2





I have found this question here Can a free particle absorb/emit photons?, along with other reseources that show a free particle connot emit a photon (in a vacuum).



Now, I am 90% sure it does, but does this result hold generally, ie not just photons/ photons but in a medium?



If you go in the rest frame no matter what it emits energy is not conserved. The reason I am questioning this is that an exam question said 'if it can emit it find the angle relative to the motion of the electron'. Seems odd that they would give away marks for just repeating that it can't.






special-relativity







share|cite|improve this question













I have found this question here Can a free particle absorb/emit photons?, along with other reseources that show a free particle connot emit a photon (in a vacuum).



Now, I am 90% sure it does, but does this result hold generally, ie not just photons/ photons but in a medium?



If you go in the rest frame no matter what it emits energy is not conserved. The reason I am questioning this is that an exam question said 'if it can emit it find the angle relative to the motion of the electron'. Seems odd that they would give away marks for just repeating that it can't.






special-relativity




special-relativity






share|cite|improve this question













share|cite|improve this question











share|cite|improve this question




share|cite|improve this question










asked 5 hours ago









Toby Peterken

341113




341113








  • 1




    How about a free neutron decaying to a proton, electron & antineutrino?
    – PM 2Ring
    4 hours ago






  • 1




    But then it changes rest mass, it doesn't just emit things
    – Toby Peterken
    4 hours ago














  • 1




    How about a free neutron decaying to a proton, electron & antineutrino?
    – PM 2Ring
    4 hours ago






  • 1




    But then it changes rest mass, it doesn't just emit things
    – Toby Peterken
    4 hours ago








1




1




How about a free neutron decaying to a proton, electron & antineutrino?
– PM 2Ring
4 hours ago




How about a free neutron decaying to a proton, electron & antineutrino?
– PM 2Ring
4 hours ago




1




1




But then it changes rest mass, it doesn't just emit things
– Toby Peterken
4 hours ago




But then it changes rest mass, it doesn't just emit things
– Toby Peterken
4 hours ago










1 Answer
1






active

oldest

votes


















8














True and true - there is no contradiction here.



The complete answer is contained in the answer to linked question:




This is because energy and momentum are not both conserved if a free
charged particle (say, an electron) emits a photon




and the comment to that answer:




Note that if the particle has internal structure, this argument can
fail. For example, atomic nuclei emit gamma rays. This is because they
have more than one internal state with different energies.




Applying these two principles we arrive at:




  1. A single free electron cannot emit a photon

  2. A single free hydrogen atom with electron in an excited state can emit a photon by spontaneous emission

  3. A single free neutron can emit a particle by a nuclear decay process






share|cite|improve this answer

















  • 2




    What about lepton decay, say the Tau particle decaying into an electron and a few neutrinos? I think it's not that much about elementarity but rather about the existence of a final state which in particular has a lower total mass than the initial state. edit: However, the argument makes sense if you modify it a little: An elementary particle can only decay into different particles which would not fit into the "emission" picture. In order to emit something, it needs to be a compound particle such that some interaction energy is involved which can be used to ensure energy conservation.
    – Photon
    3 hours ago








  • 2




    @Photon OK yes there is more to it: the original linked question referred to a single particle emitting a single photon without changing. The two quotes I gave are still correct.
    – Bruce Greetham
    3 hours ago












  • Yes, energy-momentum (4-momentum) should be conserved, that's why I wrote "in particular". My only point was that it is necessary to differentiate more clearly between a decay and a particle emission. But maybe your answer does so sufficiently, I might have read it not carefully enough and identified decay and particle emission which is clearly wrong here. My bad, sorry for the possible confusion!
    – Photon
    3 hours ago






  • 1




    @Photon I think your comment was very pertinent - I learnt from it thanks.
    – Bruce Greetham
    3 hours ago










  • In your final example (v1), do you have in mind neutron beta decay? That is a different argument since there are multiple, massive decay products emitted, and beta emission can occur in structureless particles like the muon and tau. If you meant that a free neutron may emit a photon, that's not correct for the same reason that a free electron can't emit a photon. If you meant that a free nucleus may relax by emitting a photon due to its internal structure, you should edit to clarify.
    – rob♦
    15 mins ago











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1 Answer
1






active

oldest

votes








1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes









8














True and true - there is no contradiction here.



The complete answer is contained in the answer to linked question:




This is because energy and momentum are not both conserved if a free
charged particle (say, an electron) emits a photon




and the comment to that answer:




Note that if the particle has internal structure, this argument can
fail. For example, atomic nuclei emit gamma rays. This is because they
have more than one internal state with different energies.




Applying these two principles we arrive at:




  1. A single free electron cannot emit a photon

  2. A single free hydrogen atom with electron in an excited state can emit a photon by spontaneous emission

  3. A single free neutron can emit a particle by a nuclear decay process






share|cite|improve this answer

















  • 2




    What about lepton decay, say the Tau particle decaying into an electron and a few neutrinos? I think it's not that much about elementarity but rather about the existence of a final state which in particular has a lower total mass than the initial state. edit: However, the argument makes sense if you modify it a little: An elementary particle can only decay into different particles which would not fit into the "emission" picture. In order to emit something, it needs to be a compound particle such that some interaction energy is involved which can be used to ensure energy conservation.
    – Photon
    3 hours ago








  • 2




    @Photon OK yes there is more to it: the original linked question referred to a single particle emitting a single photon without changing. The two quotes I gave are still correct.
    – Bruce Greetham
    3 hours ago












  • Yes, energy-momentum (4-momentum) should be conserved, that's why I wrote "in particular". My only point was that it is necessary to differentiate more clearly between a decay and a particle emission. But maybe your answer does so sufficiently, I might have read it not carefully enough and identified decay and particle emission which is clearly wrong here. My bad, sorry for the possible confusion!
    – Photon
    3 hours ago






  • 1




    @Photon I think your comment was very pertinent - I learnt from it thanks.
    – Bruce Greetham
    3 hours ago










  • In your final example (v1), do you have in mind neutron beta decay? That is a different argument since there are multiple, massive decay products emitted, and beta emission can occur in structureless particles like the muon and tau. If you meant that a free neutron may emit a photon, that's not correct for the same reason that a free electron can't emit a photon. If you meant that a free nucleus may relax by emitting a photon due to its internal structure, you should edit to clarify.
    – rob♦
    15 mins ago
















8














True and true - there is no contradiction here.



The complete answer is contained in the answer to linked question:




This is because energy and momentum are not both conserved if a free
charged particle (say, an electron) emits a photon




and the comment to that answer:




Note that if the particle has internal structure, this argument can
fail. For example, atomic nuclei emit gamma rays. This is because they
have more than one internal state with different energies.




Applying these two principles we arrive at:




  1. A single free electron cannot emit a photon

  2. A single free hydrogen atom with electron in an excited state can emit a photon by spontaneous emission

  3. A single free neutron can emit a particle by a nuclear decay process






share|cite|improve this answer

















  • 2




    What about lepton decay, say the Tau particle decaying into an electron and a few neutrinos? I think it's not that much about elementarity but rather about the existence of a final state which in particular has a lower total mass than the initial state. edit: However, the argument makes sense if you modify it a little: An elementary particle can only decay into different particles which would not fit into the "emission" picture. In order to emit something, it needs to be a compound particle such that some interaction energy is involved which can be used to ensure energy conservation.
    – Photon
    3 hours ago








  • 2




    @Photon OK yes there is more to it: the original linked question referred to a single particle emitting a single photon without changing. The two quotes I gave are still correct.
    – Bruce Greetham
    3 hours ago












  • Yes, energy-momentum (4-momentum) should be conserved, that's why I wrote "in particular". My only point was that it is necessary to differentiate more clearly between a decay and a particle emission. But maybe your answer does so sufficiently, I might have read it not carefully enough and identified decay and particle emission which is clearly wrong here. My bad, sorry for the possible confusion!
    – Photon
    3 hours ago






  • 1




    @Photon I think your comment was very pertinent - I learnt from it thanks.
    – Bruce Greetham
    3 hours ago










  • In your final example (v1), do you have in mind neutron beta decay? That is a different argument since there are multiple, massive decay products emitted, and beta emission can occur in structureless particles like the muon and tau. If you meant that a free neutron may emit a photon, that's not correct for the same reason that a free electron can't emit a photon. If you meant that a free nucleus may relax by emitting a photon due to its internal structure, you should edit to clarify.
    – rob♦
    15 mins ago














8












8








8






True and true - there is no contradiction here.



The complete answer is contained in the answer to linked question:




This is because energy and momentum are not both conserved if a free
charged particle (say, an electron) emits a photon




and the comment to that answer:




Note that if the particle has internal structure, this argument can
fail. For example, atomic nuclei emit gamma rays. This is because they
have more than one internal state with different energies.




Applying these two principles we arrive at:




  1. A single free electron cannot emit a photon

  2. A single free hydrogen atom with electron in an excited state can emit a photon by spontaneous emission

  3. A single free neutron can emit a particle by a nuclear decay process






share|cite|improve this answer












True and true - there is no contradiction here.



The complete answer is contained in the answer to linked question:




This is because energy and momentum are not both conserved if a free
charged particle (say, an electron) emits a photon




and the comment to that answer:




Note that if the particle has internal structure, this argument can
fail. For example, atomic nuclei emit gamma rays. This is because they
have more than one internal state with different energies.




Applying these two principles we arrive at:




  1. A single free electron cannot emit a photon

  2. A single free hydrogen atom with electron in an excited state can emit a photon by spontaneous emission

  3. A single free neutron can emit a particle by a nuclear decay process







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered 3 hours ago









Bruce Greetham

1,3651416




1,3651416








  • 2




    What about lepton decay, say the Tau particle decaying into an electron and a few neutrinos? I think it's not that much about elementarity but rather about the existence of a final state which in particular has a lower total mass than the initial state. edit: However, the argument makes sense if you modify it a little: An elementary particle can only decay into different particles which would not fit into the "emission" picture. In order to emit something, it needs to be a compound particle such that some interaction energy is involved which can be used to ensure energy conservation.
    – Photon
    3 hours ago








  • 2




    @Photon OK yes there is more to it: the original linked question referred to a single particle emitting a single photon without changing. The two quotes I gave are still correct.
    – Bruce Greetham
    3 hours ago












  • Yes, energy-momentum (4-momentum) should be conserved, that's why I wrote "in particular". My only point was that it is necessary to differentiate more clearly between a decay and a particle emission. But maybe your answer does so sufficiently, I might have read it not carefully enough and identified decay and particle emission which is clearly wrong here. My bad, sorry for the possible confusion!
    – Photon
    3 hours ago






  • 1




    @Photon I think your comment was very pertinent - I learnt from it thanks.
    – Bruce Greetham
    3 hours ago










  • In your final example (v1), do you have in mind neutron beta decay? That is a different argument since there are multiple, massive decay products emitted, and beta emission can occur in structureless particles like the muon and tau. If you meant that a free neutron may emit a photon, that's not correct for the same reason that a free electron can't emit a photon. If you meant that a free nucleus may relax by emitting a photon due to its internal structure, you should edit to clarify.
    – rob♦
    15 mins ago














  • 2




    What about lepton decay, say the Tau particle decaying into an electron and a few neutrinos? I think it's not that much about elementarity but rather about the existence of a final state which in particular has a lower total mass than the initial state. edit: However, the argument makes sense if you modify it a little: An elementary particle can only decay into different particles which would not fit into the "emission" picture. In order to emit something, it needs to be a compound particle such that some interaction energy is involved which can be used to ensure energy conservation.
    – Photon
    3 hours ago








  • 2




    @Photon OK yes there is more to it: the original linked question referred to a single particle emitting a single photon without changing. The two quotes I gave are still correct.
    – Bruce Greetham
    3 hours ago












  • Yes, energy-momentum (4-momentum) should be conserved, that's why I wrote "in particular". My only point was that it is necessary to differentiate more clearly between a decay and a particle emission. But maybe your answer does so sufficiently, I might have read it not carefully enough and identified decay and particle emission which is clearly wrong here. My bad, sorry for the possible confusion!
    – Photon
    3 hours ago






  • 1




    @Photon I think your comment was very pertinent - I learnt from it thanks.
    – Bruce Greetham
    3 hours ago










  • In your final example (v1), do you have in mind neutron beta decay? That is a different argument since there are multiple, massive decay products emitted, and beta emission can occur in structureless particles like the muon and tau. If you meant that a free neutron may emit a photon, that's not correct for the same reason that a free electron can't emit a photon. If you meant that a free nucleus may relax by emitting a photon due to its internal structure, you should edit to clarify.
    – rob♦
    15 mins ago








2




2




What about lepton decay, say the Tau particle decaying into an electron and a few neutrinos? I think it's not that much about elementarity but rather about the existence of a final state which in particular has a lower total mass than the initial state. edit: However, the argument makes sense if you modify it a little: An elementary particle can only decay into different particles which would not fit into the "emission" picture. In order to emit something, it needs to be a compound particle such that some interaction energy is involved which can be used to ensure energy conservation.
– Photon
3 hours ago






What about lepton decay, say the Tau particle decaying into an electron and a few neutrinos? I think it's not that much about elementarity but rather about the existence of a final state which in particular has a lower total mass than the initial state. edit: However, the argument makes sense if you modify it a little: An elementary particle can only decay into different particles which would not fit into the "emission" picture. In order to emit something, it needs to be a compound particle such that some interaction energy is involved which can be used to ensure energy conservation.
– Photon
3 hours ago






2




2




@Photon OK yes there is more to it: the original linked question referred to a single particle emitting a single photon without changing. The two quotes I gave are still correct.
– Bruce Greetham
3 hours ago






@Photon OK yes there is more to it: the original linked question referred to a single particle emitting a single photon without changing. The two quotes I gave are still correct.
– Bruce Greetham
3 hours ago














Yes, energy-momentum (4-momentum) should be conserved, that's why I wrote "in particular". My only point was that it is necessary to differentiate more clearly between a decay and a particle emission. But maybe your answer does so sufficiently, I might have read it not carefully enough and identified decay and particle emission which is clearly wrong here. My bad, sorry for the possible confusion!
– Photon
3 hours ago




Yes, energy-momentum (4-momentum) should be conserved, that's why I wrote "in particular". My only point was that it is necessary to differentiate more clearly between a decay and a particle emission. But maybe your answer does so sufficiently, I might have read it not carefully enough and identified decay and particle emission which is clearly wrong here. My bad, sorry for the possible confusion!
– Photon
3 hours ago




1




1




@Photon I think your comment was very pertinent - I learnt from it thanks.
– Bruce Greetham
3 hours ago




@Photon I think your comment was very pertinent - I learnt from it thanks.
– Bruce Greetham
3 hours ago












In your final example (v1), do you have in mind neutron beta decay? That is a different argument since there are multiple, massive decay products emitted, and beta emission can occur in structureless particles like the muon and tau. If you meant that a free neutron may emit a photon, that's not correct for the same reason that a free electron can't emit a photon. If you meant that a free nucleus may relax by emitting a photon due to its internal structure, you should edit to clarify.
– rob♦
15 mins ago




In your final example (v1), do you have in mind neutron beta decay? That is a different argument since there are multiple, massive decay products emitted, and beta emission can occur in structureless particles like the muon and tau. If you meant that a free neutron may emit a photon, that's not correct for the same reason that a free electron can't emit a photon. If you meant that a free nucleus may relax by emitting a photon due to its internal structure, you should edit to clarify.
– rob♦
15 mins ago


















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