Can explosions move faster than the speed of light?

“Nothing travels faster than light, with the possible exception of bad news, which follows its own rules.” –Douglas Adams

Every once in a while, a star in our own galaxy can do something to surprise us. Over in the constellation of the Unicorn lived a quiet, run-of-the-mill star named V838 Monocerotis. But in early 2002, it brightened incredibly rapidly, and the before-and-after pictures were rather astonishing.

What was initially thought to be a nova turned out to be much, much more fascinating by time the Hubble Space telescope got around to looking at it in May of 2002.

Warning: what you’re about to see may shock you!

By time about four months had passed, this once-compact object now appeared to have an angular diameter of nearly a hundredth of a degree on the sky!

Sure, you say, but that’s incredibly tiny. Well, we kept watching it expand, all throughout the year. By September, we were looking at this.

By the end of October, the outer parts of this outburst had expanded even farther.

And finally, by the end of the year (December 2002), the entire original field-of-view was taken up by this massive, expanding cloud of gas!

For something thousands of light-years away to produce something of this size, it had to be expanding very, very quickly. Possibly, according to some estimates, maybe even approaching the speed of light!

But the puzzle got a lot more complicated. Rather than early estimates, which indicated that this star was about 2,000 light-years distant, more detailed models of the outburst combined with new data taken in 2005:

and again in 2006:

Showed that this object — V838 Monocerotis — is more like 20,000 to 36,000 light years away!

Does this mean, instead of being near the speed of light, this thing was actually expanding between 10 and 20 times the speed of light? After all, we pretty much watched the expansion unfold, and if we’ve figured out the distance, the speed is pretty easy to calculate.

And sure enough, those are the numbers we get. But we also know that nothing can move faster than the speed of light! So what’s going on here?

You are seeing a phenomenon known as a light echo. What happens is that, sure, you have an event that happens and emits a huge amount of light. The light coming directly from that event reaches you first, as shown in the very first image up top.

But what happens next? The light from other directions travels spherically outward, and does a lot of bouncing around — and gets absorbed and re-emitted — by all the gas and dust blown off of the star.

Light that follows the green path takes much, much longer to reach us than light that follows the straight, dotted white line. And light that follows the yellow path takes even longer! While it may not get slowed down as much as in the Sun, where light takes tens of thousands of years to get out, it can still take days or even weeks for the light to get bounced around and sent towards us.

What does all of this mean? That when that bounced-around light finally reaches us, it does so in stages, and the light we receive gives us the optical illusion of faster-than-light expansion!

But everything is actually expanding much slower than light speed; it’s just that the initial expansion of this star’s outer layers delay the light so significantly that it appears to expand so quickly.

The Universe is a subtle place, and will trick you, unless you pay close attention! So the answer is no, explosions can’t move faster than the speed of light. But if you can delay the early bursts of light in the beginning by much more than you delay the later ones, it looks like they do! And that’s the explanation behind what we see!