The Time Illusion (Kindle Single)

by John Gribbin

Copyright © 2016 by John Gribbin

Once Upon a Time

Time’s arrow certainly does point in one direction, from what we call the Past towards what we call the Future. But does it move?

But what was the stage on which all this activity took place? Newton’s idea was that it all took place against a background of “absolute space”, which was the same everywhere and always at rest, almost literally like a stage. In the Newtonian Universe, the Earth, Moon, Sun and other objects move through this absolute space, and their speeds relative to absolute space are measured in terms of absolute time, which can be imagined as the regular ticking of some ultimate cosmic clock.

So, according to Newton, time and space are independent aspects of reality, separate from each other and unaffected by the events going on within time and space.

The Pointing Arrow

Much of the early investigation of thermodynamics was empirical, without a secure theoretical foundation. The theoretical understanding of what was going on was developed in the last quarter of the nineteenth century using statistical mechanics – an approach to thermodynamics that is based on applying the laws of statistics to the behaviour of large numbers of particles, such as the huge number of atoms or molecules present in a box of gas, each of them acting in accordance with Newton’s laws.

The second law can be expressed in different ways, but the most simple is the statement that heat cannot move from a cooler object to a hotter object without some outside influence.

Putting it another way, things wear out, and the second law is telling us that the Universe itself will one day wear out.

The natural effect of processes going on in the Universe is to move from a state of order to a state of disorder, unless there is an input of energy from outside

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.

Places where entropy seems to decrease locally always involve a greater increase in entropy elsewhere.

For the Universe as a whole, nineteenth-century physicists realised, entropy is always increasing.

If entropy is always increasing, how did the Universe get to be in the relatively low entropy state we see today? Indeed how did it get started in an even lower entropy state in the first place?

The second law itself seems to be only a statistical law, not an absolute truth, although this is not so much time running backwards as time repeating itself.

In round terms, the Poincaré cycle time is 10N seconds, where N is the number of particles (atoms or whatever) involved.

And as physicists have been known to quip, anything which is not forbidden is compulsory.

The statistics apply to large numbers of particles, such as atoms. But to an individual atom bouncing around in a box of gas and obeying the three laws of mechanics, there is no arrow of time.

A Reversible Arrow?

This has led to a puzzle known as the Boltzmann’s Brain paradox, although Boltzmann did not invent it and it is not a paradox. It starts from the reasonable point that deviations from thermodynamic equilibrium are more likely for smaller deviations, and less likely for larger deviations.

A positron, the positively charged counterpart to an electron, is absolutely identical to an electron which is travelling backwards in time, an idea which intrigued no less a physicist than Richard Feynman.

Thanks to this kind of property, the standard model of particle physics tells us that any process and its time-reversed equivalent are equally likely. So, again why does time seem to flow only in one direction?

The Schrödinger equation can be used to describe, or predict, the trajectory of an electron from A to B. The same equation also describes the trajectory of an electron from B to A.

if you are willing to take the maths on trust (if not, see https://arxiv.org/abs/1307.6167 and download the PDF). The key feature of quantum theory is that things are quantised – and this applies to processes, as well as to things like the amount of energy possessed by an electron in an atom. Instead of a smooth flow of time from the past to the future, there is a discrete series of events, some of which lie in the future and some of which lie in the past.

Marina Cortês, of the University of Edinburgh, and Lee Smolin, of Canada’s Perimeter Institute, may have done. They have developed what they call a causal set model, in which the Universe is composed of a series of events, each of which is different from all the other events in the Universe. Processes continually manufacture new events from the set of present events, but Cortês says that events “cannot unhappen”; reversing an event does not take you back to where you started, in effect erasing the process, but gives rise to a new event. Time continues to point, and flow, in the same direction. Each set of events can only influence events in the next set, and however you juggle the equations the “next” set is always in the future.

Zeno would ask, how do we move from the past through the present and into the future? If you had asked Albert Einstein that question, he would have told you that we don’t.

A Block of Relativity

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.

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.

Either Maxwell was wrong in some subtle way, and the speed of light was not an absolute constant; or Newton was wrong in some subtle way, and adding up two velocities v and u did not give the simple answer (v + u). In particular, it raised the ludicrous possibility that v + c = c.

Part of the beauty of the special theory is that for speeds that are much less than the speed of light, everything in Einstein’s world is the same as in Newton’s world. His equations “reduce” to Newton’s equations, a physicist would say, in the low-speed limit.

Everybody, and everything (including the Universe), has a world line through four-dimensional spacetime, a world line which traces their entire history.

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 fundamental feature of the block Universe is that nothing changes. The world lines are fixed.

Quantum Realities

What he abhorred is the idea that an electron (or other quantum entity) disappears from energy level A and appears at energy level B without passing through any intermediate stages, and without taking any time at all to make the transition. But that is what the equations tell us. And those equations work – they are the basis of the design of such practical things as lasers and computer chips, and in explaining the workings of DNA and heredity.

The equations say that you can have a mathematical description of an excited atom, and you can have a mathematical description of a system with exactly the same energy, shared between an unexcited atom and a photon. The equations also tell us what probability there is of finding the system in either state. But there is no mathematical description of a process which changes the excited atom into an unexcited atom plus a photon.

And in spite of some very careful experiments having been carried out to try to catch such quantum processes in action, nobody has ever seen an atom (or any other quantum system) in the act of changing from one state to another.

The “pot” which they watched contained a few thousand ions of beryllium, trapped by electric and magnetic fields.

The ions were held in this way in what is called a Penning Trap, at a temperature below 250 milliKelvin, almost at the absolute zero of temperature. At the start of the experiment, the ions were all in the same energy state, which the team referred to as Level 1. The ions could all be moved up in energy to a another state, called Level 2, by applying radio waves with a particular wavelength for exactly 256 milliseconds. This was the equivalent of making the pot boil. But what happens if you look to see what is going on during that 256 milliseconds? That time corresponds to there being an almost 100 per cent probability of the transition. After 128 milliseconds, the equations tell us, there is a 50:50 chance of the transition having taken place, so half the ions should be in Level 1 and half in Level 2, and so on for shorter time intervals. To test this, the team fired a brief flicker of carefully tuned laser light at the quantum pot. This had the effect of jumping ions in Level 1 up to another state, Level 3, from which they instantly fall back, emitting a characteristic photon. But ions in Level 2 are unaffected by the laser, so the number of photons detected revealed how many ions were still in Level 1. As expected, after 128 milliseconds half the ions were in Level 1, so the other half must have been in Level 2. After 64 milliseconds, just a quarter of the ions were in Level 2. But if the laser peeked 64 times (once every 4 milliseconds), almost all of the ions stayed i...