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Should it be called matter if it's mass-less? Or is the article wrong and did this have a mass?

by djcapelis 16y ago

gowiththeflow 16y ago
Not a physics major, but my guess is E=mc^2 still applies.
- harshpotatoes 16y ago
  Yes and no. Yes, E=mc^2 does still apply, but this refers only to a particles rest energy. So in the rest frame of a photon, the photon has no energy, and thus has no mass (or at least that's the theory, in principle more difficult to test and for all intents and purposes it is very nearly zero and most probably is exactly zero). In our rest frame, the photon moves at the speed of light, so... E = mc^2 + kintetic energy + potential energy -> which basically reduces to E = hf (plank's constant * frequency of the light).
- jules 16y ago
  A physics major, this is correct, in the sense that if the photons in the "matter of photons" have total energy E, then it acts like an object of mass m (under the condition that the total momentum is zero, which I presume is approximately true because the photons are moving in random directions?). However, photons do not have rest mass. The formula for photons is E = pc, where p is the momentum of the photon. The full form is E^2 = (mc^2)^2 + (pc)^2, where m is the rest mass and p is the momentum. Taking p = 0 for an object at rest gives E = mc^2, taking m = 0 for a photon gives E = pc.
juiceandjuice 16y ago
Given that the definition of "matter" is slightly ambiguous, it's not really wrong, but it's not really correct either. Traditionally, matter usually has to have mass, but if you just think of matter as a volume of particles, then a collection of photons could be defined loosely as "matter".
lmkg 16y ago
The photons that make up the condensate have no rest mass, which is why they travel at the speed of light whenever they have a non-zero amount of kinetic energy. Anything that has any form of energy has mass, since those are just two different words for the same property. Because of this, the photons can be called "massless" (i.e. no rest mass) but still have mass, and the condensate could in theory have mass without any of its constituents having mass. The photons have mass from their individual kinetic energies, and the condensate could have mass beyond that from any potential energy required to get it into that state. Although, if the condensate is stable in any sense, it probably has less mass than the sum of the energies of its photons, with the difference being the loss of potential energy it gains from being stable. As a comparison, atomic nuclei are composed of particles with non-zero rest mass, and they tend to have less mass than the sum of their components because of their stability. The difference between the mass of the nuclei and its components is how much energy you have to dump into the nuclei to split it (plus enough kinetic energy so that they fly away before they reform). As to the question of what constitutes "matter"... I'm not sure that's actually defined in a way that's 100% free of contention. My personal opinion would by any configuration of particles that's stable. This new stuff could qualify.
- alecst 16y ago
  Photons have momentum. But is it not misleading to say that they have mass? You also say that E and m are just different words for the same property. This, I believe, is not fair. E (J) differs from m (kg) by a conversion factor of c (m/s) ^2. To give an analogy, I think it would be misleading to say that an electric field E(r) is a different word for some charge Q, because without one you cannot have the other, and they only differ by some conversion factor. Could you explain a little bit more thoroughly what you mean?
Estragon 16y ago
I don't really know what it means, but my understanding is that photons actually have a mass. E=mc^2 => m=E/c^2, and photons have energy...
- rorrr 16y ago
  Photons don't have a mass. If they did, they wouldn't be able to travel at the speed of light (it takes infinite energy to make any mass travel at the the speed of light). http://en.wikipedia.org/wiki/Photon#Experimental_checks_on_photon_mass http://en.wikipedia.org/wiki/Photon#Experimental_checks_on_p... E = mc^2 applies to matter, not photons.

Should it be called matter if it's mass-less? Or is the article wrong and did this have a mass?

gowiththeflow 16y ago

Not a physics major, but my guess is E=mc^2 still applies.

harshpotatoes 16y ago
Yes and no. Yes, E=mc^2 does still apply, but this refers only to a particles rest energy. So in the rest frame of a photon, the photon has no energy, and thus has no mass (or at least that's the theory, in principle more difficult to test and for all intents and purposes it is very nearly zero and most probably is exactly zero). In our rest frame, the photon moves at the speed of light, so... E = mc^2 + kintetic energy + potential energy -> which basically reduces to E = hf (plank's constant * frequency of the light).
jules 16y ago
A physics major, this is correct, in the sense that if the photons in the "matter of photons" have total energy E, then it acts like an object of mass m (under the condition that the total momentum is zero, which I presume is approximately true because the photons are moving in random directions?). However, photons do not have rest mass. The formula for photons is E = pc, where p is the momentum of the photon. The full form is E^2 = (mc^2)^2 + (pc)^2, where m is the rest mass and p is the momentum. Taking p = 0 for an object at rest gives E = mc^2, taking m = 0 for a photon gives E = pc.

juiceandjuice 16y ago

Given that the definition of "matter" is slightly ambiguous, it's not really wrong, but it's not really correct either. Traditionally, matter usually has to have mass, but if you just think of matter as a volume of particles, then a collection of photons could be defined loosely as "matter".

lmkg 16y ago

The photons that make up the condensate have no rest mass, which is why they travel at the speed of light whenever they have a non-zero amount of kinetic energy. Anything that has any form of energy has mass, since those are just two different words for the same property. Because of this, the photons can be called "massless" (i.e. no rest mass) but still have mass, and the condensate could in theory have mass without any of its constituents having mass. The photons have mass from their individual kinetic energies, and the condensate could have mass beyond that from any potential energy required to get it into that state. Although, if the condensate is stable in any sense, it probably has less mass than the sum of the energies of its photons, with the difference being the loss of potential energy it gains from being stable. As a comparison, atomic nuclei are composed of particles with non-zero rest mass, and they tend to have less mass than the sum of their components because of their stability. The difference between the mass of the nuclei and its components is how much energy you have to dump into the nuclei to split it (plus enough kinetic energy so that they fly away before they reform). As to the question of what constitutes "matter"... I'm not sure that's actually defined in a way that's 100% free of contention. My personal opinion would by any configuration of particles that's stable. This new stuff could qualify.

alecst 16y ago
Photons have momentum. But is it not misleading to say that they have mass? You also say that E and m are just different words for the same property. This, I believe, is not fair. E (J) differs from m (kg) by a conversion factor of c (m/s) ^2. To give an analogy, I think it would be misleading to say that an electric field E(r) is a different word for some charge Q, because without one you cannot have the other, and they only differ by some conversion factor. Could you explain a little bit more thoroughly what you mean?

Estragon 16y ago

I don't really know what it means, but my understanding is that photons actually have a mass. E=mc^2 => m=E/c^2, and photons have energy...

rorrr 16y ago
Photons don't have a mass. If they did, they wouldn't be able to travel at the speed of light (it takes infinite energy to make any mass travel at the the speed of light). http://en.wikipedia.org/wiki/Photon#Experimental_checks_on_photon_mass http://en.wikipedia.org/wiki/Photon#Experimental_checks_on_p... E = mc^2 applies to matter, not photons.