a brief history of time stephen hawking

a brief history of time book review – stephen hawking

Stephen Hawking’s A Brief History of Time is a well-written and fairly accessible explanation of some of the biggest questions we have – what is time, when did it start, why does it move in one direction and will it end? In the process of answering these questions, Hawking gives us a survey of the history of science as it relates to these questions, and also proposes his own solutions to some of the problems. I’m more comfortable in my understanding of the general history than I am in some of Hawking’s own contributions – I think I’ll need to loop back and read the book again once I’ve consolidated my understanding of the basics. For a non-scientist, this is complicated stuff!

What is time?

Before Einstein, time was always considered an absolute concept. Aristotle and Newton, for example, would have shared this idea of time. With absolute time it would be possible for two observers using identical clocks to be in total agreement when measuring the interval of time between two events.

The concept the time was absolute broke down when Einstein considered two observers at different points, and possibly travelling at different speeds, observing a single light source. Since the speed of light is constant, and since speed is measured as distance/time, if the distances that light travels to the two observers is different it must be the case that the time the light travels is also different, therefore time is relative to each observer, it is not absolute. Identical clocks carried by different observers would record different times. Time is not independent of space, but the two are combined into the concept of space-time.

When did time start?

For Aristotle and the Greek philosophers the universe is constant – it has always existed and always will exist. Any idea of progress is countered by the argument that periodically occurring natural disasters wipe away progress and send us back to the beginning again in an endless game of snakes and ladders. C

Writers in the Judeo Christian tradition believe that God created the universe at a finite point in time – for St Augustine this was around 5000BC based on an analysis of the book of Genesis. Most scientists agree that the universe had a finite beginning, but that the time the universe began is in the range 20 billion to 10 billion years ago.

For scientists, the finite start for the universe was first scientifically proven by Edwin Hubble’s observation in 1929 that distant galaxies are moving away from us, and therefore that the universe is expanding. The luminosity of stars of various sizes is constant, therefore how they appear to us is a measure of their distance from us. The colour of something moves towards the red end of the spectrum as it moves away from us, therefore the degree of red shift tells us the speed with which it is moving away. Since distant galaxies had a red shift proportional to their distance from us Hubble could observe that distant galaxies are moving away from us faster than closer galaxies and he therefore concluded that the universe is expanding. It followed from this that at some distant point in the past, the universe must have started at a single point – in a big bang.




There have been arguments that rather than a big bang, the universe might be in an endless cycle of expansions and contractions, with the contraction phase never quite bringing all of the elements together to a single point. Hawking supports the big bang approach based on his and Penrose’s development of Penrose’s earlier work on the life of stars. This proved mathematically, based on the current known mass of the universe and the general theory of relativity, there must have been a big bang.

Ok, so that’s when the universe started, but does this mean that time itself started at the same time? Yes it does, argues Hawking. All of our theories of science are based on the idea that space-time is smooth and nearly flat. At the big bang space-time is infinitely curved, therefore our theories break down. Since an understanding of what happened before the big bang would have no application to what happens after it, and vice versa, we can say that for all intents and purposes we should cut anything that happened prior to the big bang out of our model and say that time started at the big bang.

Why is time a one-way concept?

The laws of physics don’t imply any directionality for time – moving forwards or backwards are equally valid. This however doesn’t accord with our experience. Hawking gives three reasons for time having directionality – sometimes called the ‘Arrow of Time’.

The first is the Second Law of Thermodynamics, which states that bodies move from a state of order to a state of entropy/disorder. This implies directionality as we see cups smash but not un-smash. Second is the psychological reason – we can remember things that happened in the past, but not things that are going to happen in the future, and the third, a little more obscure for me, as the Cosmological dimension – that time moves forward as the universe expands (and would presumably move backwards were the universe to contract?).

Will there be an end to time?

By tying the concept of time to the life of the universe, the question we’re really asking here is whether the universe has a finite life – will it end or continue to exist?

As noted above, Aristotle considered the universe to be constant, so there would be no end. For the Judeo Christian tradition the world will end on Judgement Day – I’m not 100% sure about this, and Hawking doesn’t discuss it..

For Hawking the question is whether the universe will continue to expand, or whether it will start to contract at some point and end back at the single point. The answer to which future we have depends on the rate of expansion. If this is beyond a critical rate then the universe will continue to expand forever. There is some uncertainty about the amount of mass in the universe, and this is relevant to the rate of expansion as a universe of greater density will require a higher critical rate of expansion, but based on what we know at the time of writing it seems likely that there will be endless expansion.

Other considerations

Hawking considers that it is possible to explain what happens during the big bang with quantum mechanics – the universe at that point is so small that quantum mechanics rather than the general theory of relativity is relevant. There’s a degree of uncertainty, and the link between quantum mechanics and relativity hasn’t yet been worked out. This is the part of the book that I struggled with the most, and I’ll need to come back to it to understand the implications.

Our score – 10/10 – as simple and engaging an explanation as is possible given the complexity of the subject matter. But very interesting.