# Light reaching the speed of light.

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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.

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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.

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

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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.

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

Massless exchange particles aren't considered matter.

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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.

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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?!?

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

Classical equations aren't valid under relativistic conditions.

edit for spelling

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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 $\gamma=\frac{1}{(1-v^2/c^2)^{1/2}}$.

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.

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