Einstein Wept

Stephen Prothero, in his rather interesting God Is Not One, illustrates the Jewish tradition of disputation with the following bizarre example:

It was a Jew, Albert Einstein, who showed via the theory of relativity that even scientific observations depend on your perspective.

It is certainly true that observations depend on context, but it didn’t take Einstein to figure it out; we might call it the Theory of Well Duh.  For example, physicists who did their experiments in a car trapped on a roundabout would arrive at laws like “All objects in the universe naturally move toward the passenger side.”  Roller-coasters are also considered suboptimal locations for your kinetics lab.

The funny thing, even funnier than Prothero’s idea that the Theory of Relativity is specifically Jewish, is that certain things don’t change depending on your frame of reference.  Indeed, Einstein himself suggested that it should be called the Theory of Invariance.  To see how this works, we have to start with the original idea of relativity, which is called Newtonian relativity but actually goes back even further, to Galileo.*

Back in the day, there was a series of commercials for some Yank Tank type vehicle,maybe a Cadillac or a Ford LTD.  The idea was that the ride was so smooth, you could conduct delicate operations in the backseat, such as cutting a big diamond (In the SNL parody, it was a rabbi doing a circumcision).Imagine now that you are in the ultimate version of such a car, going along a perfectly straight road (say, any road in North Dakota).  As you wake up from a nap (perhaps you’ve been having a bris under general anaesthesia), unless the car is accelerating, it will feel motionless, and you will have no way of telling whether you’re in fact standing still, going 20 mph, or doing 95 (the most likely, if you really are in ND).   If another LTD goes by in the other direction, apparently at 95 mph, there’s no way to tell which, if either, of you is standing still.

The point is that you can do whatever experiments you want in your LTD, and you won’t get any freakass stuff about objects being drawn to the passenger side.  You will get the same laws of physics as the person in the other LTD, as long as neither car is accelerating (which is the same as saying, as long as the passengers don’t feel pulled or pushed by any force).  Newtonian relativity states that all LTD frames of reference (if I may call them that) lead to the same laws, so there is no reason to choose one over the others.

I suppose I should specify that it is dark outside or the landscape is completely devoid of features (again, think North Dakota), since otherwise the land would seem like a preferred frame of reference.  You might object that you could still stick your hand out the window and judge your speed based on the flow of air, and something analogous was indeed tried in the later 1800s.    People thought that light waves were carried on a medium which they called ether (after the fifth element, or quintessence, that the ancients thought filled outer space).  Physicists decided to stick their hands out the window and feel how fast the ether was going by.

Just as an airplane flying with the wind will have higher groundspeed than an airplane flying against the wind (you’ve probably noticed that flights from Chicago to New York, say, are almost always shorter than flights from NY to Chicago), a beam of light traveling with the flow of ether should be faster than a beam sent against the direction of the ether.  So if you measured the speed of light in farious directions, you could find out how your LTD was moving compared to the ether; this would mess pretty seriously with Newtonian relativity, because the ether would serve as an absolute frame of reference that was more equal than the others,   Only an LTD moving along with the ether would find the speed of light to be the same in all directions.

By 1905, many experiments had been performed, sending light in various directions from various platforms moving at various speeds, and they had all come  up with exactly the same value for the speed of light.  This was super frustrating; it was as though Nature was fucking with the physicists’ minds twisting things so they could never measure the ether.  Then Uncle Albert had the brilliant idea of turning the whole thing upside-down: rather than sacrificing relativity to accommodate the supposed ether,  let’s punt the ether, which has never done anything for us anyway, and take the idea of relativity seriously.

This is the core of special relativity, from which everything else follows: the laws of physics are the same on any LTD, and one of these invariants is the speed of light.  All LTDs really are created equal.  As I said, the premise that the speed of light will always be found to have the same value, no matter who does the measuring, is the main idea of special relativity, and the famous weird phenomena like time dilation and

the contraction of objects in the direction of motion all follow from it.    I’ll just give one brief example, which should give you a feel for the way that the invariance of the speed of light can lead to unexpected results.

For this, we’re going to need a bigger vehicle, say a giant stretch  Hummer.  On a family vacation trip, two of the kids, Jane and Roy, have been fighting over whether to watch the Merv Griffin Show or Gilligan’s Island on the on-board TV, and have decided to settle it with a duel, water pistols at 10 paces or whatever.  Roy goes to the back of the Hummer, Jane to the front, and Grandpa, goes to the exact middle and pulls a tiny lantern out of his overalls pocket.  Each combatant is free to fire as soon as the light from the lantern reaches them.

To the participants, this is a fair arrangement. Remember, so far as they can tell, they are standing still, and all the distances and speeds are equal, so J and R will see the light at exactly the same time.  But now imagine a third person standing by the side of the road, say Peggy.  If Grandpa were throwing something ordinary, such as baseballs, to J and R, then Peggy would measure the speed of the bal thrown forward (to Jane) as much higher then the speed of the ball thrown back to Roy (say, the 50 mph of the throw plus the 95 mph of the vehicle, or 145 mph, versus 95-5- = 45 for the other ball).**  And this is necessary for the balls to arrive at the same time: Jane is receding, so her ball has to go faster to catch up to her, while Roy is coming forward to meet his ball.

But light can’t work that way.  Margaret has to perceive both beams of light as traveling at the same speed, so the light that is trying to catch up with Jane can’t be measured as going any faster than the light that Roy is coming to meet.  From Margaret’s standpoint, then, the light will seem to reach Roy first, confirming Peggy in her suspicion that Roy was always Grandpa’s favorite because he was born on Grandpa’s birthday.

Thus, events that are simultaneous in one frame of reference are not simultaneous in another equally valid frame of reference.  I guess you can see how this is, in some twisted way, the origin of Prothero’s characterization, in the same way that someone might describe king Lear as the story of an old guy who gets mad when his daughter takes away the keys to the car.

 

* I’m only going to talk about the Special Theory of Relativity, which is the easy one.  The General theory is also extremely cool, and bears, if possible, less resemblance to Prothero’s garbling.

** A friend of mine once provided a good illustration of this when he tried to shoot a layup in basketball, which he had not played before.  He didn’t realize that, since he was running toward the basket, the ball that was moving with him already had enough speed; it seemed motionless to him, so he added velocity by pushing the ball forward, and the two velocities, added up (from the point of view of the basket) were enoughto make the ball rocket off the backboard and sail almost to half-court.

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One Response to Einstein Wept

  1. beth mchugh says:

    And I loved this one too, Roy! Of course, what else would Grandpa have had but a tiny lantern in his overalls pocket, and a baseball at hand? What a great picture! And he might have been saying, “Now children, I wish you wouldn’t fuss at each other like that!”
    Thanks for the lesson and the enjoyment!

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