# Time it will take when traveling at speed of light

#### welshvegan

There is such thing as a beam of light. If I shine a light from a flashlight, that is a beam of light, right? If not, then what is it? It also takes time to reach somewhere.

I will study the the spacetime diagrams. Thanks :)

#### Frame Dragger

There are threads about it every month, but have you ever heard a reasonable argument for why it should make sense to define a photon's point of view by doing a Lorentz transformation with speed v and taking the limit v→c? (I don't consider "Brian Greene did it" an actual argument. )
Can't argue with that... and "Brian Greene did it" has never passed these lips! lol.

#### Fredrik

Staff Emeritus
Gold Member
There is such thing as a beam of light. If I shine a light from a flashlight, that is a beam of light, right?
Right. There's nothing wrong with the phrase "a beam of light". The phrase that doesn't make sense is "for a beam of light". Massless particles don't have experiences. Massive particles don't either of course, but when we're dealing with a massive particle, there's at least a natural way to associate a coordinate system with its motion. Not so for massless particles, as I explained. So when we're dealing with a massive object, we can talk about its "experiences" even if it's incapable of having experiences. We define its "point of view" to be the description of events in terms of the coordinate system we associate with its motion.

If we can't associate a coordinate system with a particle's motion, we can't give meaning to a question like "How much time passes for such a particle?", or to any question or statement about anything for that particle.

It also takes time to reach somewhere.
Yes, in the coordinate system associated with your motion, and in the coordinate systems associated with the motion of other massive objects, but not in a coordinate system associated with its motion, because there's no such thing.

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