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Rogan Josh
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OK kiddies, time for a physics lesson.
Light has no mass. period. The masslessness of light is one of the best measured quantities we have, so if you want to believe anything a scientist tells you, believe this.
Secondly, your (Japher's, Azazel's) equation E=mc^2 is wrong. This is only true for a particle at rest, which a photon can never be (despite what CNN tells you). The real equation (applicable to everything) is:
E^2 = m^2 c^4 +p^2 c^2
where p is momentum. Since an object at rest has p=0, this gives E=mc^2 at rest. But for a photon, m=0, so the equation becomes a relation between energy and momentum:
E=p c
So Kramerman is completely correct. If they had actually 'stopped' light it would have amazing consequences. It would break the U(1) symmetry of electromagnetism and for a start.
But they didn't really stop light - this is just a journalist's take. They absorbed the light and emitted it again with no energy loss. This is perfectly ok, and violates no physics laws. In fact, it is well known that light travels more slowly through water for example - experiments to detect neutrinos use the 'warp boom' of the netrino passing through water to detect it - because the neutrino passes through the water faster then light, it gives off a lot of light in analogy to the sonic boom of an aircraft travelling faster than sound.
The fact that they can do it with no energy loss is quite cool, but certainly has no consequences for fundamental physics.
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Az
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MACEDONIA - It's the name of the sovereign country to the north of Greece
Apr 2000
time: 07:32
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Actually, when I am thinking about it, what's wrong about describing a moving particle's energy as E=gamma*mc^2
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Last edited by Az on 12-12-2003 at 03:08
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Rogan Josh
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Nothing, if it is a general well behaved particle like an electron. But for a photon gamma=infinity (as you pointed out) and m=0, so the equation
E=gamma mc^2
isn't very well defined (note that m=0 doesn't imply E=0, since gamma is infinite)
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Thorgal
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Tarifa, Cádiz.
Jan 2002
time: 05:32
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If light have no mass, why is it affected by gravity?
In fact photon has no mass at rest but it has innercial mass.
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Rogan Josh
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quote:
Originally posted by Thorgal
If light have no mass, why is it affected by gravity?
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Despite what Newton says, mass doesn't give gravity - it is energy that does. This is what General Relativity is all about. The energy warps space-time, not the mass. (Besides, in GR, you can always find a frame of reference where the light moves in a straight line)
quote:
In fact photon has no mass at rest but it has innercial mass. |
What do you mean by this?
Usually 'inertial mass' is the one in E^2= m^2c^4+p^2c^2 (ie the one that influences dynamics), while 'gravitational' is the gravity 'charge' (sort of). The success of GR is that it shows they are the same. Light has no inertial mass.
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Rogan Josh
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Start with E^2 = m^2 c^4 +p^2 c^2 = m^2 c^4 (1+p^2 /(m^2 c^2))
or in other words gamma = sqrt(1+p^2/(m^2 c^2))
But this entire manipulation is wrong if m=0 (since I divided by it). gamma is infinite because m=0, so the argument doesn't work.
On the other hand, it is perfectly valid to say that for a fixed p, if you want to get close to c, you better have small m. That argument is fine (which is almost what you are saying, but a bit weaker).
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12-12-2003 00:07
That's real honor.
stay with chemistry and get to drug discovery/manufacturing and you will hardly ever want for money, ppl will throw it at you!
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