Scientists stop light

[Japher]

Thank you...

Will you still respect me in the morning?

[Az]

Actually, when I am thinking about it, what's wrong about describing a moving particle's energy as E=gamma*mc^2
?

[Rogan Josh]

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)

[Thorgal]

If light have no mass, why is it affected by gravity?

In fact photon has no mass at rest but it has innercial mass.

[Rogan Josh]

Originally posted by Thorgal
If light have no mass, why is it affected by gravity?

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)

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.

[Az]

Originally posted by Rogan Josh
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)

I know that. but I think it's perfectly legitimate to use it to illustrate why can't a particle with the speed of c have mass.

( of course, it is possible that the transformation itself is based on this fact, so I could be commiting a grave logical error, but this is irrelevant. )

[Rogan Josh]

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).

[Az]

fair enough. now I need some help with the applications of Quantum mechanics to chemistry.

[Ben Kenobi]

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

Well said Rogan.

[Rogan Josh]

Why bother with chemistry when you can do physics?

I just sent a funding proposal to the EU asking for 1 million euros.

[Japher]

Bah! 1 million stay with chemistry and get to drug discovery/manufacturing and you will hardly ever want for money, ppl will throw it at you!

[Az]

Why bother with chemistry when you can do physics?

You physicists are such arrogant bastards. . Well, honestly, chemists would never be able to do what they did (change the world) without the physicists.

I just sent a funding proposal to the EU asking for 1 million euros.

So, "research" in an exotic location, I suppose?

[Ramo]

Rogan, do you happen to know if anyone's doing particularly interesting research at Wurzburg (in general)?

[Japher]

So, "research" in an exotic location, I suppose?

I wonder if a chemist can get funding to a study on the affects of alcohol consumption in tropical locations? I don't think a physicist would even have a shot!

[Az]

dude, all the cool places are Enviro. Engineering! I took a class there once ( Air Pollution ) and the professor kept joking about his friends that go to Patagonia "to measure pollutants".