4 ms·

Interesting, thanks. So what's preventing "dark matter" from simply being, say, lots of photons traveling through intergalactic space then? That seems like the

Interesting, thanks. So what's preventing "dark matter" from simply being, say, lots of photons traveling through intergalactic space then? That seems like the next obvious candidate after ordinary matter.

gnaritas 9y ago
Photons lack mass right?
- posterboy 9y ago
  not exactly. As photons are constantly moving at the speed of light (citing the theory of relativity I don't really understand) they have no resting mass. But they have an impuls and create a force of impact (used in sun sails) from which a theoretical notion of ... mass (kinetic, perhaps?) can be derived. I'd like to know how it's derived originally, too though. edit: if E=mc² with E(photon)=f•h and f=c/λ, then m=h/(λc) in vacuum. ... I hope that's correct. What's interesting, [m]=J/[a]/m
  - gnaritas 9y ago
    Normally when discussing mass, people are referring to resting mass, not relativistic mass.
    - posterboy 9y ago
      normally, people don't discuss mass. rather they are simply heavy. most don't even really care why. and those who do, conflate the idea with weight often enough.
      - gnaritas 9y ago
        haha, very true.
- throwaway613834 9y ago
  Well I read [1] [2] that they do have gravity... which is probably a more direct answer to what you're asking. I'm not sure what counts as "mass" for a photon per se, except that I assume there's an effective mass via the energy. [1] https://www.quora.com/How-does-a-photon-exert-gravitational-pull/answer/Giordon-Stark https://www.quora.com/How-does-a-photon-exert-gravitational-... [2] https://physics.stackexchange.com/a/22878 https://physics.stackexchange.com/a/22878
gus_massa 9y ago
If dark matter where a lot of photons, then we could detect it. For example the experiments to detect the cosmic microwave background radiation https://en.wikipedia.org/wiki/Cosmic_microwave_background https://en.wikipedia.org/wiki/Cosmic_microwave_background can detect a very small amount of light that is the leftover of the radiation produced soon after the big bang. We have very good estimations of how many photons are out there.
- throwaway613834 9y ago
  I don't understand, how would you detect photons that are going in a direction orthogonal to you in intergalactic space? They have to be coming toward you for you to detect them, right?
  - eipipuz 9y ago
    An implicit assumption is that the universe is isotropic and homogenous at that scale. Meaning, it looks the same from any point and any direction. So you might not detect photons in one particular direction, but we should assume that they should be going in all directions. Otherwise that theory is just moving the goal from "where's this energy" to "why is it pointing towards that". CMB is consistent with, every direction is the same, so you would have to say "photons are going everywhere in this range, but towards that in this segment of the spectrum".
    - throwaway613834 9y ago
      But isn't it a hell of a lot less of a stretch to imagine there are more photons in other parts of the universe than here, than to imagine there is some sort of mysterious dark matter permeating all of the universe? If we're going to be unable to interact with whatever dark matter is to test may hypothesis, we might as well propose the hypothesis that requires the least radical changes to our fundamental theories of physics, no?
      - eipipuz 9y ago

gnaritas 9y ago

Photons lack mass right?

posterboy 9y ago
not exactly. As photons are constantly moving at the speed of light (citing the theory of relativity I don't really understand) they have no resting mass. But they have an impuls and create a force of impact (used in sun sails) from which a theoretical notion of ... mass (kinetic, perhaps?) can be derived. I'd like to know how it's derived originally, too though. edit: if E=mc² with E(photon)=f•h and f=c/λ, then m=h/(λc) in vacuum. ... I hope that's correct. What's interesting, [m]=J/[a]/m
- gnaritas 9y ago
  Normally when discussing mass, people are referring to resting mass, not relativistic mass.
  - posterboy 9y ago
    normally, people don't discuss mass. rather they are simply heavy. most don't even really care why. and those who do, conflate the idea with weight often enough.
    - gnaritas 9y ago
      haha, very true.
throwaway613834 9y ago
Well I read [1] [2] that they do have gravity... which is probably a more direct answer to what you're asking. I'm not sure what counts as "mass" for a photon per se, except that I assume there's an effective mass via the energy. [1] https://www.quora.com/How-does-a-photon-exert-gravitational-pull/answer/Giordon-Stark https://www.quora.com/How-does-a-photon-exert-gravitational-... [2] https://physics.stackexchange.com/a/22878 https://physics.stackexchange.com/a/22878

gus_massa 9y ago

If dark matter where a lot of photons, then we could detect it. For example the experiments to detect the cosmic microwave background radiation https://en.wikipedia.org/wiki/Cosmic_microwave_background https://en.wikipedia.org/wiki/Cosmic_microwave_background can detect a very small amount of light that is the leftover of the radiation produced soon after the big bang. We have very good estimations of how many photons are out there.

throwaway613834 9y ago
I don't understand, how would you detect photons that are going in a direction orthogonal to you in intergalactic space? They have to be coming toward you for you to detect them, right?
- eipipuz 9y ago
  An implicit assumption is that the universe is isotropic and homogenous at that scale. Meaning, it looks the same from any point and any direction. So you might not detect photons in one particular direction, but we should assume that they should be going in all directions. Otherwise that theory is just moving the goal from "where's this energy" to "why is it pointing towards that". CMB is consistent with, every direction is the same, so you would have to say "photons are going everywhere in this range, but towards that in this segment of the spectrum".
  - throwaway613834 9y ago
    But isn't it a hell of a lot less of a stretch to imagine there are more photons in other parts of the universe than here, than to imagine there is some sort of mysterious dark matter permeating all of the universe? If we're going to be unable to interact with whatever dark matter is to test may hypothesis, we might as well propose the hypothesis that requires the least radical changes to our fundamental theories of physics, no?
    - eipipuz 9y ago