# If you were travelling at the speed of light and looked into a mirror would you see your reflection?

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I assume you mean that you and the mirror are both traveling together.

Regardless, the way modern physicists think of it, the question doesn’t quite make sense. The problem is the “traveling at the speed of light” part, because one of the conclusions of Einstein’s Special Theory of Relativity is that nothing can travel that fast.

Interestingly, though, it was consideration of a similar question that started Einstein down the path toward relativity… Light is best described by Maxwell’s Equations, which predict that any experiement will measure same speed for light in a vacuum. When Einstein thought about traveling at the speed of light, he realized that there must be a problem. In fact, Maxwell’s Equations don’t allow for a “standing wave” solution, which is what a light wave would look like if you were traveling at the speed of light.

So Einsten took things back to basics, which is what he was good at. Let’s start with what he actually said… His Special Theory of Relativity started with two postulates (or assumptions): 1) that the laws of physics are observed to be the same in any non-accelerated “reference frame” and 2) that the speed of light (c) is observed to be the same for all observers.

The first assumption basically says that as long as you’re traveling at a constant speed, all your experiments will give the same results. That seems reasonable. But the thing about the speed of light always being the same sounds a little odd: the Michelson-Morley experiment (and others) had detected no change in the speed of light when measured from a moving frame of reference, however, and as I already mentioned, Maxwell’s Equations yield a specific value for the velocity of light.

Starting from there, Einstein worked out how to add velocities together, and it comes out to be not simple addition, but a fraction that never gets larger than c. Basically, what happens is that the measurement of time and distance changes, so a physical object (e.g., you, a spaceship, or a mirror) can never achieve lightspeed.

So… That’s a long-winded way of saying that you and your mirror can’t travel at the speed of light. If we want to think about traveling very, very close to the speed of light, however, we realize that the light bouncing off the mirror still travels at lightspeed (as measured by the fast-moving mirror itself), so you would still see your reflection.

Ryan Wyatt

Rose Center for Earth & Space

New York, New York

• If you were able to travel at the speed of light (with a mirror held in front of you), the picture on the mirror would remain exactly as it was the moment you hit light speed. Since time stops when you're travelling at light speed, the image in the mirror wouldn't be able to change, as it would have no time to change.

Anyway, this is only a solution to an impossible problem, as nothing with mass can travel at the speed of light.

• Yes because the light relative to you is still traveling at the speed of light. That's the kind of thing that relativity talks about.

• Your question assumes a physical impossibility and is physically meaningless. Part of why Einstein concluded that you can't travel at the speed of light is because the laws of physics stop working for matter traveling at that speed.