In previous chapters we have seen how our views of thenature of time have changed over the years. Up to thebeginning of this century people believed in an absolute time.
That is, each event could be labeled by a number called “time”
in a unique way, and all good clocks would agree on the timeinterval between two events. However, the discovery that thespeed of light appeared the same to every observer, no matterhow he was moving, led to the theory of relativity - and inthat one had to abandon the idea that there was a uniqueabsolute time. Instead, each observer would have his ownmeasure of time as recorded by a clock that he carried: clockscarried by different observers would not necessarily agree. Thustime became a more personal concept, relative to the observerwho measured it.
When one tried to unify¹ gravity with quantum mechanics,one had to introduce the idea of “imaginary” time. Imaginarytime is indistinguishable from directions in space. If one can gonorth, one can turn around and head south; equally, if onecan go forward in imaginary time, one ought to be able toturn round and go backward. This means that there can beno important difference between the forward and backwarddirections of imaginary time. On the other hand, when onelooks at “real” time, there’s a very big difference between theforward and backward directions, as we all know. Where doesthis difference between the past and the future come from?
Why do we remember the past but not the future?
The laws of science do not distinguish between the past andthe future. More precisely², as explained earlier, the laws ofscience are unchanged under the combination of operations (orsymmetries) known as C, P, and T. (C means changingparticles for antiparticles. P means taking the mirror image, soleft and right are interchanged. And T means reversing thedirection of motion of all particles: in effect, running the motionbackward.) The laws of science that govern the behavior ofmatter under all normal situations are unchanged under thecombination of the two operations C and P on their own. Inother words, life would be just the same for the inhabitants ofanother planet who were both mirror images of us and whowere made of antimatter, rather than matter.
If the laws of science are unchanged by the combination ofoperations C and P, and also by the combination C, P, and T,they must also be unchanged under the operation T alone. Yetthere is a big difference between the forward and backwarddirections of real time in ordinary life. Imagine a cup of waterfalling off a table and breaking into pieces on the floor. If youtake a film of this, you can easily tell whether it is being runforward or backward. If you run it backward you will see thepieces suddenly gather themselves together off the floor andjump back to form a whole cup on the table. You can tell thatthe film is being run backward because this kind of behavior isnever observed in ordinary life. If it were, crockerymanufacturers would go out of business.

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