Light reaching the speed of light.

[Callipygous]

I have always heard light discribed as being both energy and matter. if its matter, it has to have mass. but according to E=MC^2, nothing with mass can reach the speed of light. so how can light reach light speed?

 

my understanding of this stuff is limited, so sorry if im missing something important.

[J.C.MacSwell]

I have always heard light discribed as being both energy and matter. if its matter' date=' it has to have mass. but according to E=MC^2, nothing with mass can reach the speed of light. so how can light reach light speed?

 

my understanding of this stuff is limited, so sorry if im missing something important.[/quote']

 

It can be considered a massless particle, but I don't think it technically can be considered matter.

[Callipygous]

its a particle, a physical thing, but it isnt matter? O.o

[J.C.MacSwell]

its a particle, a physical thing, but it isnt matter? O.o

 

I think that's right.

 

In another thread I claimed a photon absorbed by an atom added/created matter to the atom but was convinced that strictly speaking (particle physics definition) that was incorrect. That portion of the energy did not represent matter even though you have a mass increase.

[swansont]

its a particle, a physical thing, but it isnt matter? O.o

 

Massless exchange particles aren't considered matter.

[Tom Mattson]

I have always heard light discribed as being both energy and matter. if its matter' date=' it has to have mass. but according to E=MC^2, nothing with mass can reach the speed of light. so how can light reach light speed?

[/quote']

 

The problem is that you're only looking at half the equation. E=mc² only refers to massive particles at rest. It doesn't apply to light. The complete equation is:

 

E²=(pc)²+(mc²)²

 

If you consider a particle at rest (p=0), then you have E=mc². If you consider massless particles (m=0), then you have:

 

E=pc

 

which does describe light quanta.

[Tommio]

It is correct that light is both a wave and massless particle, which is why light travels in quanta.

 

Another way to look at this problem is that E=mc² means that does mean that only a photon can acheive c, the speed of light. -

 

E (at rest) = mc² so

 

m = E (at rest) / c² substituting in momentum

 

E (kenetic) = 1/2 pv where p = momentum (mv) so

 

E (total) =approx= pc which is exact for photons but not for mass

 

Which is an explanation of the previous post.

 

However when an 'object' is travelling faster than light it is a different matter as the theory states that the less energy the 'object' has - the faster it will travel.

 

WORK THAT ONE OUT?!?

[swansont]

E (kenetic) = 1/2 pv where p = momentum (mv)

 

Classical equations aren't valid under relativistic conditions.

 

_{edit for spelling}

[Tom Mattson]

E (kenetic) = 1/2 pv where p = momentum (mv) so

 

E (total) =approx= pc which is exact for photons but not for mass.

 

Swansont is right about this. The equation you wrote here is not valid in SR. In fact' date=' it does not make the point that you are trying to make. That is, it imposes no natural speed limit on massive bodies. The correct expression for KE is:

 

[math']K=mc^2(\gamma-1)[/math], where [math]\gamma=\frac{1}{(1-v^2/c^2)^{1/2}}[/math].

 

which does indeed approach infinity as v approaches c.

 

However when an 'object' is travelling faster than light it is a different matter as the theory states that the less energy the 'object' has - the faster it will travel.

 

WORK THAT ONE OUT?!?

 

You'd work it out by simultaneously allowing v>c and an imaginary mass in the above expression.