Updated by DK 2001.
Original by Don Koks 1998.
It's often said that special relativity is based on two postulates: that all inertial frames are of equal validity, and that light travels at the same speed in all inertial frames. But in real world scenarios, objects almost never travel at constant velocity, and so we might never find an inertial frame in which such an object is at rest. To allow us to make predictions about how accelerating objects behave, we need to introduce a third postulate.
This is often called the "clock postulate", but it applies to much more than just clocks, and in fact it underpins much of advanced relativity, both special and general, as well as the notion of covariance (that is, writing the equations of physics in a frame-independent way).
The clock postulate can be stated in the following way. First, we take the rate that our frame's clocks count out their time, and compare that to the rate that a moving clock counts out its time. Before the clock postulate was ever thought of, all that was known was that when the moving clock has a constant velocity v (measured relative to the speed of light c), this ratio of rates is the Lorentz factor
gamma = 1/sqrt(1-v2)The clock postulate generalises this to say that even when the moving clock accelerates, the ratio of the rate of our clocks compared to its rate is still the above quantity. That is, it only depends on v, and does not depend on any derivatives of v, such as acceleration. So this says that an accelerating clock will count out its time in such a way that at any one moment, its timing has slowed by a factor (gamma) that only depends on its current speed; its acceleration has no effect at all.
In other words, the accelerated clock's rate is identical to the clock rate in a "momentarily comoving inertial frame" (MCIF), which we can imagine is holding an inertial clock that for a brief moment slows to a stop alongside of the accelerated clock, so that their relative velocity is momentarily zero. At that moment they are ticking at the same rate. A moment later, the accelerated clock has a new MCIF, again one that is moving momentarily to match its speed, and there is a new inertial clock that briefly slows to a stop alongside of the accelerated clock. And again, the rates of accelerated clock and the new inertial one will momentarily be the same.
So the clock postulate says that the rate of an accelerated clock doesn't depend on its acceleration. But note: the clock postulate does not say that the rate of timing of a moving clock is unaffected by its acceleration. Of course it is affected! Its acceleration changes its speed, and its speed determines how fast it counts out its time (i.e. by the factor gamma). If that seems confusing, think of an everyday analogy. If you ride your bicycle on an icy morning, you get very cold due to the wind chill factor. The faster you go, the colder your hands get. This wind chill is a function of your speed, but not your acceleration. Nevertheless, it is affected by your acceleration, because your acceleration changes your speed. But regardless of whether you have a low or a high acceleration, the only thing that matters as far as your cold hands are concerned is what your current speed is. So the wind chill factor does not depend on your acceleration, but it certainly is affected by your acceleration.
The clock postulate also implies that the amount of shortening of a moving rod is independent of its acceleration. And also, that the so-called "relativistic mass" of a moving object (an outdated term but still useful) also doesn't depend on its acceleration.
Although the clock postulate is just that, a postulate, it has been verified experimentally up to extraordinarily high accelerations, as much as 1018 g in fact (see the faq What is the experimental basis of Special Relativity?). Of course, the postulate also speaks of more than acceleration, it speaks of all derivatives of v with respect to time. But still it can be shown to be a reasonable thing to assume, because it leads to something that has been tested in other ways.
This something is the idea of a spacetime metric. Typically, when learning special relativity, at some stage we note that the "interval" between two events, delta t2 - delta x2 - delta y2 - delta z2, is independent of the inertial frame in which we make our measurements. So although relativity has taught us to throw away our ideas of the absoluteness of space and time, even so this idea of something else which is observer-independent leads to a new absolute thing called spacetime. And since the interval is frame independent, we can also say immediately that the time delta T elapsed on an inertial clock connecting two events is just delta T2 = delta t2 - delta x2 - delta y2 - delta z2, since in its own inertial frame, the clock doesn't move.
But, what is the time as shown on a noninertial (i.e. accelerated) clock connecting two widely separated events? There's no reason why it should be delta t2 - delta x2 - delta y2 - delta z2 (and it certainly isn't), because that expression only looks at the end points and doesn't take into account how the clock moved. In fact, what we do to get the right answer is this: we assume the clock postulate holds. So, when an accelerating clock moves from one event to another that is infinitesimally close, we can say that the infinitesimal time dT elapsed is given by
dT2 = dt2 - dx2 - dy2 - dz2since this is the time that elapses on a clock in the MCIF. And we can now integrate this dT along the accelerated clock's worldline to get the actual elapsed time that it shows.
This sort of idea is exactly analogous to the idea of calculating the length of a curve by dividing it into a large number of short segments, each of which is almost straight, and then adding up the lengths of each of those using Pythagoras: dl2 = dx2 + dy2 + dz2. That's an exercise in calculus. Sure, each of those short segments is definitely not straight--it still has some curvature that doesn't go away as we break it into smaller and smaller parts. But the total error we introduce by taking the segments as being straight gets ever smaller as we make the number of segments larger and larger; again, that's standard calculus in action.
So, the idea of calculating the time elapsed on a clock that accelerates from one event to another is just the same. We're dividing its path into small segments that the clock postulate lets us work with. And just as the curvatures of those small segments in the previous paragraph never become zero (and yet we can ignore them), so too the accelerations of the clock along its path never become zero either--and yet the clock postulate allows us to ignore them too.
This is why we can give some structure to spacetime, because it's possible to talk about the "length" of a curved worldline as being given by the time shown on a clock moving along that worldline, even though the clock itself is accelerating (since the worldline is curved--we are still only talking about flat space here). If the "length" between two infinitesimally separated spacetime points was something that depended on how a clock connecting them moved, then it wouldn't be an intrinsically geometric thing. The clock postulate geometrises relativity by saying that we don't need to consider how the clock moves; the time (squared) shown by any clock connecting two closely separated events is just dt2 - dx2 - dy2 - dz2, and it doesn't matter how the clock moves.
Ultimately, that's why the interval is written using infinitesimals. It has nothing to do with curved spacetime; we use infinitesimals even if the spacetime is flat. We use them because in this form the interval embodies the clock postulate. But now the grand thing is that this idea of spacetime structure allows us to make the transition to general relativity, and the great success of that theory lends plausibility to the parts that make it up--and one of these is the clock postulate.
The clock postulate is one of the ideas we use to build the theory of gravity known as general relativity. People sometimes think (and books sometimes say) that a proper description of accelerating clocks demands general relativity. That's not true at all. General relativity is built on a foundation that includes the clock postulate. Once we've built general relativity, we cannot then turn around and start using it to "explain" the physics of accelerating clocks! That would be circular reasoning. General relativity is built upon the ideas of special relativity, but the physics of accelerating clocks is completely handled by special relativity alone.
Sometimes people say "But if a clock's rate isn't affected by its acceleration, doesn't that mean the Equivalence Principle comes out wrong? If the Equivalence Principle says that a gravitational field is akin to acceleration, shouldn't that imply that a clock isn't affected by a gravitational field, even though the textbooks say it is?"
No, the Equivalence Principle is fine. Again, the confusion here is the same sort of thing as above where we spoke about the wind chill factor. Let's try to see what is happening. Imagine we have a rocket with no fuel. It sits on the launch pad with two occupants, a couple of astronauts who can't see outside and who believe that they are accelerating at 1 g in deep space, far from any gravity.
One of the astronauts sits at the base of the rocket, with the other at its top, and they both send a light beam to each other. Now, general relativity tells us that light loses energy as it climbs up a gravitational field, so we know that the top astronaut will see a redshifted signal. Likewise, the bottom astronaut will see a blueshifted signal, because the light coming down has fallen down the gravitational field and gained some energy en route.
How do the astronauts describe what is going on? They believe they're accelerating in deep space. The top astronaut reasons "By the time the light from the bottom astronaut reaches me, I'll have picked up some speed, so that I'll be receding from the light at a higher rate than previously as I receive it. So it should be redshifted--and yes, so it is!" The bottom astronaut reasons very similarly: "By the time the light from the top astronaut reaches me, I'll have picked up some speed, so that I'll be approaching the light at a higher rate than previously as I receive it. So it should be blueshifted--and yes, so it is!"
As you can see, they both got the right answer, care of the Equivalence Principle. But their analysis only used their speed, not their acceleration as such. So just like our wind chill factor above, applying the Equivalence Principle to the case of the rocket doesn't depend on acceleration per se, but it does depend on the result of acceleration: changing speeds!
On a more advanced note, the clock postulate can be generalised to say something about measurements we make in a noninertial frame. First, it tells us that noninertial objects only age and contract by the same gamma factor as that of their MCIF. So, any measurements we make in a noninertial frame that use clocks and rods, will be identical to measurements made in our MCIF. But we now choose to extend the clock postulate to include all measurements (though perhaps it can be argued that all measurements only ever use clocks and rods anyway). This idea leads onto "covariance", which is a way of using tensors to write the language of physics in a way that applies to all frames, noninertial as well as inertial. How is this done?
Take, for example, the electromagnetic field. Suppose some measurements were made in an inertial lab frame, and it was determined to have components Aa (up to a choice of gauge). Now, we know from other considerations that Aa is a four-vector. That means, it transforms between inertial frames just as do (dt, dx, dy, dz). In the language of tensors (where repeated up and down indices are summed over), we can say that from one inertial frame (t x y z) to another (t' x' y' z'), the Aa transforms like
a'
a' del x b
A = ---------- A ...(1)
b
del x
This much can be shown without the clock postulate, because it deals only with inertial frames. The question is, in this language, how does Aa look in a noninertial frame?
Suppose we are in a noninertial frame, and wish to know what values the field will take on for us. The clock postulate tells us that the field in our frame, Aa'', will be identical to the field measured by our MCIF, Aa'. But that frame is inertial, so we know how to calculate Aa': it's just given by equation (1) above. Next step: how can we relate our coordinates to the way that Aa transforms? After all, the MCIF always exists, but it's just a tool; we don't want to consider it explicitly.
Again, we make use of the clock postulate. It says that our measurements of distance and time are unaffected by any pseudo forces we feel, so that they are identical to those of our MCIF:
a'' a'
dx = dx
This is just another way of saying that
a'' a'
del x del x
--------- = ----------
b b
del x del x
so that we can put it all together to write
a'' a'
A = A (by our extended clock postulate)
a'
del x b
= --------- A (Lorentz transform between MCIF
b and lab frame)
del x
a''
del x b
= --------- A (the clock postulate again)
b
del x
or finally
a''
a'' del x b
A = --------- A
b
del x
which is just like (1) above, except now it holds in a noninertial frame! So, we've managed to write our original transformation law (1), which only held between two inertial frames, in a way that looks just the same as (1), except that it's using different coordinates, some from a noninertial frame. And this is the idea behind covariance and tensor language, that it's possible to write the laws of physics for arbitrary frames, even noninertial ones, in one way: the way of equation (1).