light?

Ginja Ninja

with regards einstein's e=mc^2, we studied it a while ago in class (i only do phys/chem as it's a small school) i ub=nderstand the principles of it but

WHY exactly is c^2 used as the constant value in this calculation?

fillmore jive

as c is the value of light in a vacumm. the whole idea of this equaion is that mass is equivalant to energy, and that the c^2 constant is required to convert mass to energy in terms of units.

CreepingDeath

Ginja Ninja wrote: »

WHY exactly is c^2 used as the constant value in this calculation?

Because the atoms are square shaped.

Professor_Fink

Ginja Ninja wrote: »

WHY exactly is c^2 used as the constant value in this calculation?

Because in special relativity the speed of light (c) is constant in all reference frames. The whole of relativity is built upon this assumption. If a different velocity (f) was constant between frames, then we would have f^2 as the constant.

Delphi91

Professor_Fink wrote: »

Because in special relativity the speed of light (c) is constant in all reference frames. The whole of relativity is built upon this assumption. If a different velocity (f) was constant between frames, then we would have f^2 as the constant.

I'm not sure, but I think ginga was wondering why it's specifically c^squared as against, say, c^3 or c^(1/2), etc

Professor_Fink

Delphi91 wrote: »

I'm not sure, but I think ginga was wondering why it's specifically c^squared as against, say, c^3 or c^(1/2), etc

The calculation can be found here: http://en.wikipedia.org/wiki/Kinetic_energy#Relativistic_kinetic_energy_of_rigid_bodies

Trivially you can only have a velocity squared as the constant, since otherwise the units don't match.

Capt'n Midnight

chem

I have often wondered, If a craft was close to the speed of light. And it fired its guns (bullets/ laser) would the projectile not then be moving faster then the speed of light?

Spear

chem wrote: »

I have often wondered, If a craft was close to the speed of light. And it fired its guns (bullets/ laser) would the projectile not then be moving faster then the speed of light?

No, velocities do not add linearly.

stevenmu

chem wrote: »

I have often wondered, If a craft was close to the speed of light. And it fired its guns (bullets/ laser) would the projectile not then be moving faster then the speed of light?

That's what makes relativity so cool. If the craft fires it's lasers forward, observers on the craft will see the lasers move away from them at the speed of light. Somebody standing still off to the side watching this would see the lasers moving off at the speed of light, and if the craft was travelling at say 3/4s the speed of light, the laser would appear to be going 1/4 the speed of light faster than the craft.

There's obviously a discrepancy here in the speed of the laser being observed in the two frames of reference (one frame of reference being on the craft, the other being off to the side watching). Observers on the craft see the laser move away from them at the speed of light, observers to the side see it move away from the craft at 1/4 the speed of light. Relativity solves this by linking in time, if we say that time is slower for those who are travelling on the craft than for those who are at rest watching it, that allows those on the craft to experience the laser moving away at the speed of light while those who are at rest can watch it move away from the craft at 1/4 the speed of light. And it turns out this is exactly the case, the faster something travels, the slower it experiences time, you've probably heard the example that if someone was to take of from earth and travel at near the speed of light for 100 years they might come back and only 1 year will have passed for them.

Now ... have fun wondering what would happen if two craft flying directly towards each other, both travelling nearly the speed of light, fired their lasers at each other :pac:

Capt'n Midnight

stevenmu wrote:

Now ... have fun wondering what would happen if two craft flying directly towards each other, both travelling nearly the speed of light, fired their lasers at each other :pac:

Lots of blue shift so you would get a dose of gamma rays, you'd either turn into the hulk or die bald.

eagleye7

the way we were told in lectures had to do with assuming a light particle had a momentum = mass times velocity which brings in the c value. i did it last year sometime i could dig out my notes and explain if you want but it went somethin like:

imagine you have a laser in a box. when it fires a photon there should be a very small recoil, this means the box moves and unless the light has a momentum then the system defies the law of conservation of momentum, this brings in the fact that the light must have had some mass associated with it, and as a result mass has energy associated with it