Ask a Physicist

If you were traveling at the speed of light and turned on the lights what
would appear to happen? Also --- Does darkness have a traveling speed?
Submitted by Sherry Ocasio from Reading, Pennsylvania

My experiments indicate that darkness leaves the refrigerator at least as fast as I can open the door. But darkness leaving is just light moving in, so it's not a surprise that I can't catch it. Light moves very fast.

And light always moves at the same speed (if it's not bouncing around in matter), a speed you can never catch up with. If you try, maybe by strapping yourself to a rocket, your friend standing back at the starting line sees you accelerating away and gaining speed. But your friend also sees you getting heavier, in fact infinitely heavy as you approach the speed of light relative to the starting line, so you would need an infinite amount of rocket fuel to get up to light speed with respect to your friend.

But you don't feel yourself getting heavier. In fact, as you see your friend moving backwards very fast relative to you, you see your friend getting heavier. For that matter, you and your friend see each other's watches ticking more slowly; and you see each other shrinking along the direction of travel. This is may seem inconsistent (why don't you see your friend getting
lighter?) but it's actually got a simple explanation. It's basically a geometric effect, like perspective. If you walk away from your friend at a more normal speed, you see your friend getting smaller. Your friend sees you getting smaller too rather than bigger.

Since you can't quite get to light speed, let's ask: What would you see if you were traveling through a room very close to light speed and flipped the switch on your way by? Actually, you would outrun the electric signal on its way from the switch to the bulb. That signal is a propagating electric field like light, and would run at light speed through a perfect conductor. But even the best wires would slow it down significantly on its way to the bulb.

So you wouldn't see the lights go on in the room until you were long gone. Looks like darkness wins this one.