And we don't mean the everyday, run-of-the-mill controlled explosion the sun is always doing. We mean BOOM, gone, vanished, no more sun! How do we know it's still there?

The answer? We don't. Not yet, anyway.

But wait! you cry. All I have to do is step outside and look up: there it is. Actually, all I really have to do is look out the window and make sure it's still daylight. The light, and the sun that produces it, are right there -- before our very eyes.

This is a sensible answer, but technically incorrect. The light from the sun is indeed right before our eyes; but the sun itself is ninety-three million miles away. Ninety-three million miles is a tremendous distance, so vast that light takes a little while to get from there to here. In fact, the sun that we see in the sky is an image that has been travelling through space for about eight minutes. So the sun itself could have vanished as many as seven minutes and fifty seconds ago and we'd be clueless.

But wait! you say. Wouldn't the earth spin out of orbit if that were true? Once again, no: since nothing can travel faster than the speed of light, not even the influence of gravity, there would be absolutely no way to tell that the sun was gone. For eight minutes we would continue orbiting, and basking in, an illusion.

But don't worry too much about this: after eight minutes we would be bound to figure it out.  

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A Moment of Science®