The Time Illusion (Kindle Single)

by John Gribbin

The second law of thermodynamics defines an (or the) arrow of time.

Disorder is measured in terms of a quantity called entropy. The more disordered a system is, the higher its entropy. So another way of expressing the second law is to say that entropy always increases, or at best stays the same. In other words, the future towards which the arrow of time points is the direction in which entropy is greater. There is more order in a more structured system, such as [water plus ice], than there is in a more uniform system, such as [water], because it takes more information to describe an ordered system.

The number which matched the speed of light was indeed ‘a’ constant, which became represented by the letter c. The equations seemed to be saying that the speed of light is always the same, wherever you measure it from. And this conflicts with classical mechanics. If you were moving towards a light source (a lamp, or a star, or any source of light) at a speed of 50 km per hour you would expect to measure the speed of the light coming towards you as c + 50 km/hr. If the light was coming from behind you, overtaking you as you moved away from the lamp, you would expect to measure its speed as c - 50 km/hr. But Maxwell’s equations make no allowance for this. They just say that if you measure the speed of light the answer you get will be c, regardless of where the light is coming from, or how you are moving relative to the light source.

Minkowski realised that Einstein’s equations were telling us that we can do very much the same in four dimensions, specifying some kind of four-dimensional length a by the equation a2 = x2 + y2 + z2 - (ct)2, where t denotes a time interval and c is the speed of light, so the two of them combined together make a distance. Time, or rather ct, appears as a fourth dimension (not like a fourth dimension; it is a fourth dimension), with two special features. The first is that minus sign, which makes time a kind of negative dimension. When space expands, the time shrinks, and vice-versa. The second is that c. It means that one second of time is equivalent to 300,000 kilometres of space, and this is why relativistic effects only become important if things are moving at a large fraction of the speed of light. The equation tells us that the four-dimensional length of an object, usually known as extension, stays the same however the object moves. If the length contracts, the time stretches, by an equivalent amount, and vice-versa.

There is nothing special about the present moment, the Now, or indeed about any other moment. Past, present and future are all on an equal footing, because there is no slice through spacetime which can be uniquely identified as “the present”. The Universe does not change, but it exists, as a fixed block of spacetime that contains all the things that have ever happened, and all the things that ever will happen. The flow of time is an illusion. Philosophers call this idea (or something so similar to this idea that as a physicist I cannot spot the differences) “eternalism”.

In his book The Fabric of Reality, Deutsch says: We think of causes as preceding their effects; we imagine the moving present arriving at causes before it arrives at their effects, and we imagine the effects flowing forwards with the present moment. Philosophically, the most important cause-and-effect processes are our conscious decisions and the consequent actions. The common-sense view is that we have free will; that we are sometimes in a position to affect future events . . . But: according to spacetime physics, the openness of the future is an illusion, and therefore causation and free will can be no more than illusions as well . . . In reality, we make no choices. Even as we think we are considering a choice, its outcome is already there, in the appropriate slice of spacetime, unchangeable like everything else in spacetime, and impervious to our deliberations.

It is with that, and the block Universe, in mind that we can understand why, when his old friend Michele Besso died in 1955, Einstein wrote in a letter of condolences to Besso’s family: Now he has departed from this strange world a little ahead of me. That means nothing. People like us, who believe in physics, know that the distinction between past, present and future is only a stubbornly persistent illusion. Einstein died on 18 April 1955, a few weeks after he wrote those words:“But our language, which has built in to it our subjective feeling of time passing, struggles to deal with these concepts.”

In other words, not only does it not seem to make sense to speak of an objective rate of flow of time; it also doesn’t make sense to speak of an objective direction of flow of time.

The image that emerges from quantum physics is similar in some ways to the way that the illusion that air, or water is a continuous fluid emerges. Myriad tiny particles separated by tiny gaps feels to you like a smooth fluid. Myriad quantum states separated by tiny gaps feels to you like a smooth flow of time. Zeno was right. The arrow of time points, but it does not move.