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UND physicists explain Einstein's mind bender

Trying to explain the mind-bending nature of reality that the Theory of Relativity reveals, the two UND physicists asked the audience to imagine a car roaring down the road at half the speed of light.

Trying to explain the mind-bending nature of reality that the Theory of Relativity reveals, the two UND physicists asked the audience to imagine a car roaring down the road at half the speed of light.

When the driver turns on the headlights, common sense suggests that he would see a beam of photons -- the particles that light is made of -- shoot out ahead of him at, well, light speed.

Common sense further suggests that a pedestrian standing still by the road should see the photons fly by her at 1.5 times light speed followed shortly by the car.

William Schwalm, one of the physicists, told the audience last Tuesday that the driver sees the photons moving away from him at light speed. But the pedestrian, who is not moving, sees the photons moving away from her at light speed as well.

It's as if the two were experiencing different versions of reality.


Schwalm and Tim Young, the other physicists, have more mind-bending thought experiments ahead. For the next three Tuesdays, the duo will be delivering public lectures about the Theory of Relativity. The occasion is the 100th anniversary of a groundbreaking paper published by Albert Einstein.

A theory

The paper came out six years after Einstein unveiled the Theory of Relativity. According to Schwalm, it was significant because it was the first time the theory could be put to the test in the real world. The gravity field of a massive object, such as the sun, should bend light in the way the theory predicts, or Einstein would be wrong.

In 1919, a team of scientists using a solar eclipse to observe starlight discovered that they did bend as the theory said. The New York Times declared at the time: "Lights all askew in the heavens: Men of science more or less agog over results of eclipse observations."

Einstein's theory has two parts. General relativity deals with gravity and its effects, bending light, for example. Special relativity deals with the behavior of objects in motion, especially when they're moving at a significant fraction of the speed of light, which nothing can exceed.

"What was great about Einstein was he would have the audacity to try to make the equation work when he was coming up with, one after another, things that seemed utterly ridiculous," Schwalm told the audience. "Yet, he persevered, and it turns out the construction is not contradictory and many amazing things come out of it, all of which turned out to be true."

One thing that seems utterly ridiculous is idea that the physical laws of the universe would be consistent in every frame of reference. The driver, moving at a certain speed and direction, is in one frame. The pedestrian, moving at a certain speed and direction, is in another frame.

Light speed stays the same for both, which is why both saw light moving at light speed instead of faster or slower.



Schwalm and Young take this rule to an even-greater extreme with a common thought experiment called the barn-pole paradox.

One of the consequences of the Theory of Relativity is that the closer an object gets to light speed, relative to an observer, the more its length contracts, relative to the observer. In other words, the pedestrian will see a squished truck flying by. Conversely, the driver will see the pedestrian and everything else outside the truck as being squished. Relative to the driver, everything else is moving at half light speed.

The barn-pole paradox is this: A man with a 200-foot long pole is running towards a 100-foot long barn at close to light speed. He has a friend who, knowing the pole will be squished enough to fit inside the barn, wants to play a trick. The friend will wait until the man is in the middle of the barn to close both barn doors at once for a fraction of a second and then open them again.

To the friend, the trick will work. It will look as if the runner and the pole were completely in the barn. But the runner will say that it didn't look like that to him at all. He first saw the barn door in front of him close and open, then, when he had passed through the barn, he glanced behind him and saw the barn door in the back close and open.

Who was right?

Schwalm said both are. And, he hastened to add, it's not an optical illusion for either of them.

Both literally experienced reality differently.


A test

Einstein's theory is so important to physics, Schwalm said, because it's so logical in its description of reality. It would be a huge headache for physicists if it were proven to be flawed, he said, even if such an event would also be very exciting.

Coincidentally, a few weeks before he and Young delivered their first lecture, European scientists said they'd detected subatomic particles called neutrinos apparently moving faster than light speed, violating one of the iron rules of Einstein's theory. They were so startled they asked other scientists to investigate to see if they'd erred in some way.

Schwalm said he'd bet money that there was some error, and he thinks other physicists feel the same.

Young, who studies supernovas, said neutrinos are born in the heart of exploding stars and they've never been observed to exceed light speed. If they traveled as fast as the European scientists measured, the neutrinos from a supernova he observed in 1987 should've arrived in 1983, he said.


Asked if it wasn't very hard for physicists to explain the Theory of Relativity to laymen, Schwalm said it isn't. The theory can be demonstrated by logic that most nonphysicists can understand easily, he said.

Still, it seems at times that understanding the mathematical basis behind the theory is necessary. The things that the theory asks people to believe can seem almost magical, as if reality under the extreme conditions the theory describes is too far beyond ordinary human experience.


Consider the example of the car above. Now imagine that there are two cars playing chicken, each going three-quarters the speed of light.

Everyone knows from driver's ed that if you're driving 80 mph and you have a head-on collision with another car also going 80 mph, it's the equivalent of hitting a tree at 160 mph. Doesn't that mean the two cars going at 75 percent of light speed are actually traveling toward each other at 1.5 times the speed of light, violating Einstein's theory?

Schwalm and Young said they get a lot of these "what if" scenarios when they lecture.

In this case, the actual formula for determining closing speed is not quite what you learned in driver's ed. It's not the speed of car A plus the speed of car B, but (A + B) / (1 + ((A x B) / c2)). C is the speed of light -- 671 million mph -- and it's so huge in comparison to speeds most people can comprehend that, should you plug in the numbers, you'll find that you're just dividing A + B by 1.

The answer is the two cars are closing in at 96 percent light speed. No matter how close each gets to light speed, they will never be closing at more than light speed.

They'll probably look like pancakes, though.

Reach Tran at (701) 780-1248; (800) 477-6572, ext. 248; or send email to ttran@gfherald.com .

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