HPS 0410  Einstein for Everyone 
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John D. Norton
Department of History and Philosophy of Science
University of Pittsburgh
Background reading: J. Schwartz and M. McGuinness, Einstein for Beginners. New York: Pantheon.. pp. 109  116.
Relativity theory tells us that a moving clock is slowed down and a moving rod is shrunk in the direction of its motion. If I am an inertial observer, I will find the effect to come about for the clocks and rods of a spaceship moving past at rapid speed. But if that spaceship is moving inertially, then, by the principle of relativity, the spaceship's observer must find the same thing for my clocks and rods. Relative to that observer, my clocks and rods move past at great speed. So that observer would find my clocks to be slowed and my rods to be shrunk in the direction of my motion.
Each finds the other's clocks slowed and rods shrunk. How can both be possible? Is there an inconsistency in the theory? If I am bigger than you, then you must be smaller than me. You cannot also be bigger than me. That's the problem.
That each finds the other's clocks slowed and rods shrunk is troubling. But is it a real paradox in the sense of there being a logical contradiction? If I walk away from you, simple perspective effects make it look to each of us that the other is getting smaller. I judge you to grow smaller; and you judge me to grow smaller. No one should think that this is a paradox.
http://en.wikipedia.org/wiki/File:08913Perspective_Run.jpg
That perspectival effect should not worry anyone. The car in the garage problem is an attempt to show that the relativistic effects are more serious than this simple perspectival effect. There is, it tries to show, a real contradiction; and we should not tolerate contradictions in a physical theory.
Here is how we might try to get a contradiction out
of the relativistic effect of each observer judging the other to have
shrunk. Imagine a car that fits perfectly
into a garage. The garage is a small free standing shed that is just as
long as the car. There is a door at the right and a door at the left of
the garage. The car fits exactlyas long as it is at rest. 
Now image that we drive the car at 86.6% speed of light through the garage from right to left. The doors have been opened at the right and the left of the garage to allow passage of the car. There is a garage attendant, who stands at rest with respect to the garage. Can the garage attendant close both doors so that, at least for a few brief moments, the car is fully enclosed within the garage?
According to the garage attendant, there
is no problem achieving this. At 86.6% the speed of light, the car has
shrunk to half of its length at rest. It fits in the garage handily.
The garage attendant can close both doors and trap the car inside. 
According to the car driver, however, matters are quite different.The car is at rest and the garage moves. The garage approaches the car at 86.6% the speed of light. So the car driver finds that it is the garage and not the car that has shrunk to half its length. The garage is now half as long as the car. The car driver says that there is no way the garage attendant can shut both doors and trap the car fully inside. 
Now this is a serious problem. Either the car can or cannot
be trapped fully within the garage, but not both. (Or so it would seem.)
There is a solution. It depends upon our remembering that that there is more in special relativity than the slowing of clocks and the shrinking of rods. We have already seen the relativity of simultaneity which will take on greater and greater importance in our assessment of the theory. It tells us that observers in relative motion can disagree on the timing of spatially separated events.  Note that an "event" in the context of relativity theory has a narrow meaning. It is something that happens at one place and at one time. Events are not spread out in space and time as might be the sort of events that we talk about in everyday talk. In relativity theory, an event happens at just one moment and one spot. 
The possibility of that disagreement is the key to the
problem of the car and the garage. A judgment of the simultaneity of events is
essential to any judgment of whether the car was
trapped in the garage by the closing of doors. The car driver and the
garage attendant disagree on whether the car is ever fully enclosed in the
garage simply because they disagree on the time order of two events.
The garage attendant says:
There are two events: "Left door shut": I closed the left door before the car struck it. "Right door shut": I closed the right door after the car passed. And these events happened at the same time. Therefore the car was fully enclosed. 
The car driver says: "There are two events. "Left door shut": You closed the left door before the car struck it. "Right door shut": You closed the right door after the car passed. But these events did not happen at the same time. You closed the left door first. Thenlateryou closed the right door after the front of the car had already burst through the closed left door. Therefore the car was never fully enclosed. 
Both agree that the two events "left door shut" and "right
door shut" happened. They disagree on the
time order in which they happened. But that time order is what is needed
to decide whether the car was fully enclosed in the garage. In a nutshell:
• The car can only be said to have been fully enclosed in the garage if
both doors were shut at the same time.
• There is no observer independent fact of the matter as to timing of
these events.
• Therefore there is no observer independent fact as to whether the car
was ever fully enclosed in the garage.
The problem of the car and the garage shows how judgments of lengths are entangled with judgments of simultaneity. This entanglement runs throughout special relativity. Indeed, one can understand all the odd kinematical effects as derived from it; for this reason, it was the first effect Einstein discussed in his 1905 paper.
For example, the relativity of simultaneity lies behind relativistic length contraction. To see this, consider how we might measure the length of a moving object. Take a car moving along a freeway at fancifully high speeds, so that relativistic effects come into play. I am standing by the roadside and want to know the car's lengthor at least its length relative to me.
I cannot just hold up a measuring rod and proceed in the normal way: that is, check which marks on the rod align with each end of the car. For the car is zooming past. By the time I have noted the alignment of the front of the car with, say, the 0 mark on the measuring rod, the car has long since zoomed off into distance. I will have had no chance to check where the rear of car aligned. I need a more refined procedure.
Here's one: as the car zooms by, I stand with a friend at the
roadside, each of us holding a raised flag, ready to plant into the roadside.
As the front of the car passes, I plant my flag
into the roadside; as the rear of the car passes my friend, my friend plants
his flag into the roadside. The car zooms away. But that doesn't matter
anymore. I have the information I need in the locations of the flags. I can use
my measuring rod to determine the distance between the flags. That is the
length of the moving car.
What is essential to this procedure is that I and my friend
plant our flags at the same time. Otherwise
the distance between the two marks will not properly reflect the length of the
car.
But there's the catch. The car driver will disagree with my judgments of which events are simultaneous. The car driver will agree, of course, that there are two events, the planting of the two flags. But the car driver will not agree that I and my friend placed the marks simultaneously. Rather the car driver will find my friend and I to be rushing toward the car and the two flag plantings to have happened at different times. As the figure shows, the car driver will judge the planting of my flag at the front to have happened first; and the planting of my friend's flag at the rear to have happened later. 
Here's an animated version of this process.
Since my friend delayed the planting of the flag at the rear (in the car driver's judgment), the rear of the car advanced for some short time after I'd planted my flag at the front. Therefore (in the car driver's judgment) the distance we staked out with the flags is shorter than the length of the car and our determination of the length of the car is wrong. Hence we end up disagreeing about the length of the car.
The important point is that neither of us (driver and roadside observer) has made an error. There is no absolute fact as to which of us is really moving. Therefore there is no absolute fact as to which of our judgements of the timing of the two events is correct. Just as in the case of the car and the garage, we each judge the other as shrunken because we judge the simultaneity of events differently.
Similar considerations arise in judgments of the slowing of moving clocks. To see how the relativity of simultaneity underlies the relativistic slowing of clocks, we attend to a procedure we might use to measure the effect.
To judge the rate of a clock that passes me I need to be able to compare its reading with my wristwatch now and then compare its reading again later with my wristwatch after some time has passed. If the clock is running slow, I'll notice that its rate lags behind my wristwatch.
The catch in this simple procedure is that the clock is moving. I might find that both it and my wristwatch read the same time now, at the moment the clock passes. But the clock is moving rapidly. So after some time has elapsed, it has moved off into the distance.
How can I find out what the moving clock reads an hour from now when it is no longer anywhere near me? Here's one procedure: I set up many clocks at rest with respect to me throughout space. Then, one hour later, as the moving clock passes one of those clocks, a friend notes what the moving clock reads and what the local resting clock reads. From my friend's report, I can figure out whether the moving clock has slowed or not.
The figure shows the bare essentials of the moving clock and
all the other clocks spread out through space. The moving clock agrees with the reading of the
leftmost clockmy wristwatchas it passes by. However when it passes the
rightmost, it now reads much less. So I judge it to have slowed.
This procedure seems quite sound. So does that mean an observer who travels with the moving clock would agree
and judge the moving clock to have slowed? No! We have seen that relativity
theory requires that observer to judge my array of clocks to be running more
slowly! How can that be?
By now you know the answer. An essential part of the procedure is that all the clocks I laid out through space must be synchronized. That means that the events of each clock reading say "12 noon" must be simultaneous events. The relativity of simultaneity tells us that observers in relative motion may disagree on whether those events are simultaneous. Therefore observers in relative motion may disagree on whether clocks separated in space are properly synchronized. And that is what happens in this case.
The moving observer will judge my clocks not to be properly synchronized. As a result, the moving observer will regard my judgments of the rate of the moving clock to be defective. As before, there is no absolute fact as to whether the clocks are properly synchronized. Therefore there is no absolute fact as to whether the moving clock slows with respect to my clocks; or whether my clocks slow with respect to the moving clock.
Once you recognize how fully the relativity of simultaneity is bound up in the relativistic length contraction and clock slowing effects, it is easy to fall into a new misunderstanding. One might think that the effects are not really part of the world at all, but that they somehow come about solely because of the way we set our clocks.
An analogy: it is possible to board a transpacific flight in Sydney, Australia, on one day and, after 16 hours of travel, disembark in Los Angeles the day before! Is this time travel? Of course not. During the flight, you crossed the international date line. That the calendar reads a day earlier in Los Angeles is purely an artefact of how we set our clocks and calendars across the world.
http://en.wikipedia.org/wiki/File:Qantas_a380_vhoqa_takeoff_heathrow_arp.jpg 
http://aa.usno.navy.mil/faq/docs/international_date.php Historical positions of the International Date Line from "Notes on the History of the Date or Calendar Line," in The New Zealand Journal of Science and Technology, Vol. XI, pp. 385  388 
In the early 1910s, this issue entered the physics
literature in discussion of the geometry of a rotating disk. in 1911,
Vladimir Varicak offered the following diagnosis of the origin of
relativistic length contraction: It "is only an illusory, subjective appearance, caused by the manner of our regulation of clocks and measurement of length" "a psychological and not physical effect" 
The rotating disk has some odd properties. Its circumference is relativistically contracted but its radius is not, resulting in "Ehrenfest's paradox." But this is a topic for another time. 
Einstein's reply of the same year
read: "The question of whether the Lorentz contraction really exists or not is misleading. ...[it is] not real in so far as it does not exist for a comoving observer. ...[it is] real in so far as it can be demonstrated in principle by physical means by an observer that is not comoving" 
What I think Einstein is getting at is this. He is accusing Varicak of conflating two distinctions:
Real versus unreal 
Observer independent
versus observer dependent 

That we age is real. That we travel backwards in time when flying from Sydney to Los Angeles is unreal.  That an object spins on it axis is observer independent; it is verified by the presence of inertial forces. That an asteroid moves uniformly in space must be judged relative to another object. 
Varicak's point seems to be that being observer dependent makes an effect unreal. Einstein's response is that observer dependent effects can be both, according to the observer. You cannot infer from observer dependence to unreality. That the asteroid moves relative to us is both real and relative to us.