Special relativity and the Photon

In summary: Instead, photons have a relativistic mass due to their energy, but it is finite and calculable. Therefore, the concept of infinite mass for photons does not apply and they must be treated differently from sub-light speed particles.
  • #1
discord73
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If as an object's speed approaches the speed of light from an observer's point of view, its mass appears to increase, then shouldn't a photon appear to have an infinite mass?
 
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  • #2
I think the crux of where this argument fails is that you're basing it on "approaching the speed of light". Obviously, photons never approach the speed of light, so the logic doesn't really apply to them. Basically, you have to treat photons different from all sub-light speed particles in that sense.

This is a crumby answer, I think, and someone else can probably come by and do much better, but that's my input for now.
 
  • #3
My understanding is that the mass does not increase, because photons have no rest mass to begin with. Photons do not approach the speed of light c because they do not accelerate -- they always travel at c when measured in a vacuum, from the moment of emission. They do have what is considered a relativistic mass defined as m=E/c^2 according to Einstein's formula, but that is a different kind of mass. It does however allow photons to interact with other mass gravitationally. Since their energy can vary according to wavelength but is not infinite, their relativistic mass is finite and calculable.
 
  • #4
discord73 said:
If as an object's speed approaches the speed of light from an observer's point of view, its mass appears to increase, then shouldn't a photon appear to have an infinite mass?
The "relativistic" mass of an object with non-zero rest (invariant) mass increases with speed, but this doesn't apply to photons, which have zero rest mass.
 

1. What is special relativity and how does it relate to the photon?

Special relativity is a theory developed by Albert Einstein that describes the relationship between space and time. It explains how the laws of physics are the same for all observers in uniform motion and how the speed of light is constant regardless of the frame of reference. The photon, being a fundamental particle of light, is used as a key example to illustrate the principles of special relativity.

2. How does the concept of time dilation apply to the photon?

According to special relativity, time is relative and can appear to pass at different rates for different observers depending on their relative velocities. This means that time dilation, the slowing down of time, applies to the photon as well. For an observer traveling at the speed of light, time would appear to stand still, and for a stationary observer, time would appear to pass normally for the photon.

3. Can the photon have mass according to special relativity?

No, according to special relativity, the photon is a massless particle that travels at the speed of light. This is because as an object approaches the speed of light, its mass increases infinitely, making it impossible for the photon to have any mass.

4. How does special relativity explain the concept of length contraction for a photon?

Special relativity states that as an object approaches the speed of light, its length appears to contract in the direction of its motion. This means that for a photon traveling at the speed of light, its length would appear to be zero. This is because, at the speed of light, time and space are completely intertwined, and there would be no distance to travel for the photon.

5. Can the principles of special relativity be applied to objects other than the photon?

Yes, the principles of special relativity can be applied to any object that is traveling at a significant fraction of the speed of light. This includes subatomic particles, spacecraft, and even high-speed trains. Special relativity is a fundamental theory of physics and applies to all objects in the universe, not just the photon.

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