Neutrino vs. Photon Momentum at Fixed Energy

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Discussion Overview

The discussion centers on the comparison of momentum between neutrinos and photons when both are considered at the same energy level. Participants explore the implications of this comparison in the context of thrust generated by a spaceship engine that can emit either a photon beam or a neutrino beam, while also addressing the nature of the question itself.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants note that photons are massless and their momentum equals their energy, while neutrinos, being massive, have momentum that can vary depending on their energy.
  • There is a suggestion that the phrasing of the momentum comparison may imply that photons have more momentum than neutrinos for the same energy, which some find counterintuitive.
  • One participant expresses confusion over the statement that photon momentum equals energy, suggesting it only holds in specific units (where c=1) and prefers using MKS units.
  • A later reply discusses how energy is "locked up" in the mass of neutrinos, potentially affecting their momentum relative to photons when considering total relativistic energy.
  • Another participant clarifies that neutrinos are very light, and their momentum is typically very close to their energy, making them experimentally indistinguishable in many cases.
  • The equation relating energy, momentum, and mass is mentioned, specifically ##E^2 = m^2 + p^2##, to provide a mathematical context to the discussion.
  • There is a recognition of the tone in the conversation, with participants reflecting on how their responses may be perceived and the importance of constructive communication.

Areas of Agreement / Disagreement

Participants express differing views on the implications of momentum comparison between neutrinos and photons, with no clear consensus reached. Some participants challenge each other's interpretations and phrasing, indicating a contested nature of the discussion.

Contextual Notes

There are unresolved assumptions regarding the definitions of momentum and energy in different contexts, as well as the implications of mass in the momentum of neutrinos compared to photons. The discussion also highlights the potential for miscommunication in written exchanges.

Who May Find This Useful

This discussion may be of interest to those studying particle physics, astrophysics, or anyone curious about the properties of fundamental particles and their interactions, particularly in the context of energy and momentum.

bob012345
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I'm interested in knowing the ratio of momentum for a neutrino vs. a photon when both have the same energy. Alternatively, my spaceship engine can release 1GW of either a photon beam or a neutrino beam. How much relative thrust will the neutrino beam give me for the same energy (and power) as compared to the photon beam. Thanks.
 
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This is a serious question. The second part was just put there for fun to illustrate the nature of the issue. I wish it hadn't been moved to here from particle physics. Makes me feel like I asked a stupid question.
 
This depends on the energy. Photons are massless so their momentum is equal to their energy. Neutrinos are massive so their momentum is anywhere between zero and their energy - depending on the energy.
 
Orodruin said:
This depends on the energy. Photons are massless so their momentum is equal to their energy. Neutrinos are massive so their momentum is anywhere between zero and their energy - depending on the energy.
Makes it sound like photons have more momentum than neutrinos for the same energy. Seems backwards. I also have an issue with the statement that the photon momentum equals it's energy. Only in the units where c=1. Please use MKS.
 
bob012345 said:
Makes it sound like photons have more momentum than neutrinos for the same energy. Seems backwards.
If you are not going to accept the correct answer and calling it backwards without saying why then why do you bother to ask?
 
Orodruin said:
If you are not going to accept the correct answer and calling it backwards without saying why then why do you bother to ask?
I didn't say it WAS backwards, I said it SEEMS backwards. If I was clear I wouldn't have asked the question. Anyway, I see the answer now. When you create the neutrinos, a lot of the energy gets locked up in the mass and the momentum of that moving mass probably doesn't match the momentum of the photon if we are talking about the total relativistic energy of both entities being the same. So the photon always has the highest ratio of momentum to total relativistic energy of anything. Thanks.
 
Last edited:
bob012345 said:
When you create the neutrinos, a lot of the energy gets locked up in the mass and the momentum of that moving mass probably doesn't match the momentum of the photon if we are talking about the total relativistic energy of both entities being the same.
Well, ”a lot” is overstating it a bit, neutrinos are very light - so light that in all cases where we have studied them, their momenum has been so close to their energy that they have been experimentallt indistinguishable (only through a quantum interference phenomenon do we have knowledge of the neutrino mass being non-zero).

The appropriate equation relating energy, momentum, and mass is ##E^2 = m^2 + p^2##.

bob012345 said:
I didn't say it WAS backwards, I said it SEEMS backwards.
If you reread your post and think about how it comes across after someone spent time to give you an answer, try to reconstruct how it will be perceived. You gave no indication of a further question or about what you thought was strange. This reads very dismissive and not at all as if you are ready to accept the answer if convinced on the issues you find weird. Essentially everything that was missing from the cookiecutter post of such a person was the ”LOL” in the end.
 
Orodruin said:
Well, ”a lot” is overstating it a bit, neutrinos are very light - so light that in all cases where we have studied them, their momenum has been so close to their energy that they have been experimentallt indistinguishable (only through a quantum interference phenomenon do we have knowledge of the neutrino mass being non-zero).

The appropriate equation relating energy, momentum, and mass is ##E^2 = m^2 + p^2##.If you reread your post and think about how it comes across after someone spent time to give you an answer, try to reconstruct how it will be perceived. You gave no indication of a further question or about what you thought was strange. This reads very dismissive and not at all as if you are ready to accept the answer if convinced on the issues you find weird. Essentially everything that was missing from the cookiecutter post of such a person was the ”LOL” in the end.
I apologize for my tone. I didn't intend it to come across so strong. I thank you for your answers.
 
bob012345 said:
I apologize for my tone.
Don’t worry, we all write things that come across the wrong way from time to time. Myself included. What sets people apart is if they can realize and accept when it has happened and learn from it or not. (I myself still have to work very hard not to start many posts ”This is wrong.” when correcting posts I disagree with. I typically do not mean it as something personal, just factual, but it can really come across awkwardly if the recipient takes it the wrong way. Tone is not conveyed well in writing.)
 

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