# The Naked Scientists Forum

### Author Topic: Light, Relativity  (Read 1216 times)

#### queries

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##### Light, Relativity
« on: 17/03/2013 07:28:22 »
Hey

When an object approaches the speed of light, it gains infinite mass, and hence slows down. Then why doesn't light become a victim of infinite mass?

Cheers

#### Bored chemist

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##### Re: Light, Relativity
« Reply #1 on: 17/03/2013 09:30:15 »
Because it has no mass to start with. Doubling the mass still leaves it with no mass and so on.

#### flr

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##### Re: Light, Relativity
« Reply #2 on: 17/03/2013 11:17:35 »
When an object approaches the speed of light, it gains infinite mass, and hence slows down. Then why doesn't light become a victim of infinite mass?

Because the rest mass does not change with the speed.
The photon is thought to have zero rest mass.

#### simplified

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##### Re: Light, Relativity
« Reply #3 on: 17/03/2013 14:09:32 »
Speed does not increase gravitational mass.

#### JP

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##### Re: Light, Relativity
« Reply #4 on: 17/03/2013 15:08:43 »
When an object approaches the speed of light, it gains infinite mass, and hence slows down. Then why doesn't light become a victim of infinite mass?

As others have pointed out, the mass that increases with speed is called relativistic mass, which is proportional to kinetic energy (energy of motion).  So it makes sense that as things move faster, they have more energy and hence more relativistic mass.  And the energy required to accelerate something to the speed is infinite.

However, light in a vacuum can never move at any speed other than the speed of light, so the rules of what happens when things  speed up or slow down don't apply to it, i.e. it's incorrect to assume it gains infinite relativistic mass as it moves at the speed of light because you can't talk about it going slower.

#### Pmb

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##### Re: Light, Relativity
« Reply #5 on: 28/03/2013 01:36:05 »
Hey

When an object approaches the speed of light, it gains infinite mass, and hence slows down. Then why doesn't light become a victim of infinite mass?

Cheers
As JP noted you're speaking of the so-called "relativistic mass." This is defined by the expression p = mv where p = momentum, m = relativistic mass and v = velocity. IF the particle has a non-zero proper mass (aka rest mass) then the mass is a function of speed. If it has zero proper mass then then it still has mass as given in terms of its momentum or energy.

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##### Re: Light, Relativity
« Reply #5 on: 28/03/2013 01:36:05 »