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Faster Than Light? Nope.

August 3, 2000

There has been a lot of discussion lately about scientists that have demonstrated faster than light travel (FTL, also called superluminal motion). Let's get right to the point: this is a gross exaggeration. Nothing has moved faster than light, despite the claims of many people.

A little background: as far as we know, the speed of light in a vacuum is the ultimate speed limit. It travels at about 300,000 kilometers per second. Einstein and others have shown mathematically that it would take literally infinite energy to move a piece of matter at the speed of light. It can also be shown that anything moving faster than light will in essence be traveling backward in time, which also has a host of problems. We therefore conclude that nothing with mass can travel at the speed of light, and nothing at all can travel faster.

There can be illusions of FTL. Some astrophysical objects appear to move faster than light, but this is a trick of geometry. I won't go into detail here, but explanations can be found at and at the sci.astro FAQ (scroll down to question H.08; some trigonometry is included in the explanation).

Still, nothing can actually travel faster than light. So what about this news story? Basically, these guys played a trick on light. A single pulse of light can be made up of many different pulses, each with a different wavelength. This is like a musical chord, which is made up of many different pitches of sound. By passing this pulse of light through a medium, the wavelengths can get mixed up, and reorder themselves when they re-emerge from the medium, restoring the original pulse. However, in very special cases, you get what's called anomalous dispersion, and the wavelengths reorder themselves differently. They do it in such a way that it appears that the pulse has left the medium before it completely enters it! But this is an illusion, and is no more real than the apparent bending of a spoon when it sits in a glass of water (and, as a matter of fact, they are both rooted in the same phenomenon).

The scientists themselves have a diagram of this on their website. Ned Wright, a cosmologist, also has a good diagram of this on his website. It's clear that nothing has actually moved FTL, despite the claims of media and others. It's only an illusion. No information has actually been sent FTL.

So why was this a huge news item? That's a decent question, and I am not sure about the answer. I personally don't see why it's so newsworthy; while it's cool, and anomalous dispersion is an interesting effect, the scientists involved must have known the media would latch onto the FTL aspects. All this has done is caused massive confusion and given people false hope that we have had a breakthrough in superluminal travel, when nothing of the sort has happened.

I'm sorry to be a party pooper, but the days of Star Trek are still a long way ahead of us, if they are even possible at all.

©2008 Phil Plait. All Rights Reserved.

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