tumbledry

Special Relativity

In the New Yorker article Time Bandits lies the best plain English description of special relativity that I have ever run across (you owe it to yourself to read this carefully — it’s worth it):

Since light is an electromagnetic wave (this had been known since the nineteenth century), its speed is fixed by the laws of electromagnetism; those laws ought to be the same for all observers; and therefore everyone should see light moving at the same speed, regardless of the frame of reference. Still, it was bold of Einstein to embrace the light principle, for its consequences seemed downright absurd.

Suppose—to make things vivid—that the speed of light is a hundred miles an hour. Now suppose I am standing by the side of the road and I see a light beam pass by at this speed. Then I see you chasing after it in a car at sixty miles an hour. To me, it appears that the light beam is outpacing you by forty miles an hour. But you, from inside your car, must see the beam escaping you at a hundred miles an hour, just as you would if you were standing still: that is what the light principle demands. What if you gun your engine and speed up to ninety-nine miles an hour? Now I see the beam of light outpacing you by just one mile an hour. Yet to you, inside the car, the beam is still racing ahead at a hundred miles an hour, despite your increased speed. How can this be? Speed, of course, equals distance divided by time. Evidently, the faster you go in your car, the shorter your ruler must become and the slower your clock must tick relative to mine; that is the only way we can continue to agree on the speed of light. (If I were to pull out a pair of binoculars and look at your speeding car, I would actually see its length contracted and you moving in slow motion inside.) So Einstein set about recasting the laws of physics accordingly. To make these laws absolute, he made distance and time relative.

That’s one great explanation. The “shorter ruler, slower clock” phrase is particularly helpful. Another critical thing to remember is that this all occurs relative to the observer.

So, the slow speed of motion within the vehicle is only seen by the stationary person with binoculars… it is not experienced by the person within the vehicle traveling close to the speed of light.

Special relativity actually was illustrated quite wonderfully in the 1970s. Scientists used cesium-atomic clocks; these extremely accurate clocks base their resonant frequency on the decay of a cesium isotope, instead of the wildly inaccurate (by comparison) resonance of quartz. Current iterations of these clocks lose about 1 second every 17 to 30 million years. These clocks were put on jets which were flown around the world twice, in opposite directions. The idea was that the cesium clocks aboard the planes would record less time passing than the stationary clocks on the ground, which were used as a reference. To reference back to the New Yorker article: it would be as though you, sitting there, were watching these clocks tick slower because they were in the car chasing the light beam. This is, indeed, EXACTLY what happened. When the clocks were compared after the flights, the clocks aboard the planes recorded less time passing than did the clocks on the ground.

This type of almost incomprehensibly small difference in recorded time doesn’t seem that important, until you link together a global network of satellites designed to closely track your movements on the ground. You know… GPS. So, get this: if you just toss satellites up into orbit, they start seeing time passing differently than your computers on the ground, and the system gets all screwed up because all the computers can’t agree on time. So, scientists set the clock speed of the processors on the satellites to be 10.22999999543 MHz, and the speed of the earth-bound computers to be 10.23 MHz. If they hadn’t done this, along with a few other seemingly esoteric tweaks… your GPS in your car, the one that is supposed to make life so simple, wouldn’t work worth beans.

Science, even the crazy out-there relativistic science, matters in our daily lives.

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Nils +1

I geek out over this stuff. Awesome find Alex, and great discussion after the block quote!

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