By TOM SIEGFRIED / The Dallas Morning News
People who'd like to travel faster than the speed of light should try to set more realistic goals.
It would be easier to play more consecutive games than Cal Ripken Jr. Light's speed is not a record made to be broken. Nothing can go faster. Why? Because Einstein said so.
Well, not exactly. But his 1905 special theory of relativity has long been interpreted that way. More precisely, relativity supposedly says that no signal can transmit information faster than "c", the velocity of light in a vacuum. A faster signal would be able to carry a message backward in time, giving stock market speculators an unfair advantage.
As with many human laws, though, the speed-of-light limit has a loophole. You can send signals faster than light. But you have to do it with mirrors.
It's a trick named for Klaus Scharnhorst, the German physicist who discovered it in 1990. Maybe you could get light to go faster than c, he reasoned, if you messed with the vacuum in some way – greasing the skids, so to speak.
After all, light moves slower than c in air, slower still in water. That's because particles of light (photons) bump into atoms during flight. Atoms temporarily absorb photons and then spit them out, a process that slows light down substantially. Light is therefore at its fastest in vacuums, where there are no atoms around to absorb it.
But vacuums aren't exactly freeways without subatomic traffic, either. Even in empty space, photons and other particles pop into and out of existence, thanks to the rules of quantum physics.
Quantum physics allows short-term borrowing of energy to create these "virtual" particles, but the demand for payback is almost immediate. Light passing through a vacuum interacts with those virtual particles very briefly and so goes just a little more slowly than it would if the vacuum were completely particle-free.
Perhaps, though, light could go even faster if you could clear out some of the quantum clutter. And you can. Put two mirrors close together in a vacuum, and you can create what physicists call the Casimir effect. No longer will all possible light particles bounce back and forth in the space between the mirrors. Only light with wavelengths that fit in that space is allowed. Therefore, the vacuum between the mirrors will contain less junk. So light's velocity in the Casimir vacuum will be a little faster than c, a phenomenon known as the Scharnhorst effect.
Before you gasp, Einstein wouldn't roll over in his grave about this. The Scharnhorst effect does not really violate special relativity. In a new paper analyzing the effect, physicist Matt Visser of Washington University in St. Louis and two collaborators note that Einstein's theory really just demands that the speed of light be precisely the same no matter how fast you're moving when you're measuring it.
As long as you move in a straight line at a constant speed, you occupy a "reference frame" that is just as valid for making measurements (and deducing laws of physics) as anybody else's reference frame. Observers in all such reference frames should get identical answers when measuring the speed of light. If not, that would mean somebody's frame of reference is special.
But that's not the same thing as saying light's velocity is nature's ultimate speed limit. An invariant speed is not the same as a maximum speed.
"Contrary to widespread belief, special relativity only requires ... that there be an invariant speed," point out Dr. Visser and colleagues Stefano Liberati of the University of Maryland and Sebastiano Sonego of Universita di Udine in Italy. (Their paper is available at xxx.lanl.gov/abs/gr-qc/0107091.)
It's not easy to exceed the speed of light, though. If you start with an object with any mass at all, it would take an infinite amount of energy to get it to go as fast as c. Objects that begin life flying faster than light (called tachyons) are thinkable, but they seem to exist only in science fiction.
And experiments that supposedly show laser beams leaving a box before they enter have simply been misexplained. The waves in the beam get shifted, but no message-carrying signal in the light really moves faster than c.
In the Scharnhorst effect, though, c actually can be exceeded. But it's legal, because special relativity applies to equivalent reference frames. When you bring in the Casimir mirrors, you establish one frame of reference as special.
Dr. Visser and colleagues explain that a paradox arises only if a signal can go backward in all reference frames. In the Scharnhorst effect, light travels faster than c only in the one special reference frame. There is no prospect of sending a signal back in time.
Of course, you could try more complicated tricks – using multiple mirrors, for example, and moving them around. In that case the analysis is more difficult, but Dr. Visser and colleagues still think the world is safe from any pathological time-travel paradoxes.
In any event, the Scharnhorst speed advantage is not very substantial. If a Scharnhorst photon and an ordinary one had been racing since the birth of the universe, the faster one would now be ahead by only about the width of an atom.
So there's about as much hope of ever measuring that speed difference as there is of breaking Ripken's record.