How to Build a Time Machine Book

A very dear friend of mine bought this for me - and she was concerned that I might have read it already. I've read a couple of Davies' books - but not this one. I can hardly remember what the others were called now - but they weren't called How to Build a Time Machine of that I'm quite certain.

One of the others was also about time and also gave a rather involved discussion on why zapping off at the speed of light is even better than Oil of Olay if you are after younger looking skin. I've never understood this, never read an explanation that makes any sort of sense. I've now read Davies' explanation in two books and I'm still none the wiser. I accept it happens - most of these guys have Physics degrees and tell me in all ponderous seriousness that it is so - all the same, it would be nice if just once someone would actually say what actually happens in a way that is both clear and comprehensible.

I once read an explanation that said imagine you are looking up from earth at a rocket that is speeding away from you at close to the speed of light. You also have to imagine that this is a special glass rocket, so you can see inside. There is a clock inside the rocket that measures time, but not your standard clock. Rather one that works by bouncing light rays off two parallel mirrors. Light travels at a fixed rate, so bouncing light off mirrors sounds as good a way to measure time as any other. The problem is that the person on the rocket sees the light bounce straight up and down - but you, standing on earth, see the light bouncing at an angle - a series of V shapes - which takes into account the velocity of the rocket. Light travelling in V shapes is going to take longer than light travelling straight up and down - so time will seem to go slower on the rocket ship than on earth. One is supposed to say, "It's all relative" at this point - and so I will.

The problem is that it is relative - and so if you have the same clock ticking away beside you and I am on the rocket ship it will look, to me, as if your clock is making V shapes too. Both of us will think the other person's clock is running slow. Davies points out that this is, in fact, the case, but then says the reason why the person in the rocket is ageing so much slower than the person left on earth has something to do with accelerating to light speed and then decelerating back down again. He doesn't explain what it has to do with this or how it works - but he is not alone - no one ever seems to explain it. Like I said, I've always been left to take it on trust and to regret having never finished my physics degree.

I'm not going to tell you how to make a time machine - you can buy the book for that if you are interested. All I will say is that if you think you are going to be the next Dr Who or even The Master, you are probably going to be a bit disappointed.

Now, one of the reasons why we know time travel isn't something that takes off in a big way in the future is because we don't get visits from our descendents, or, at least, I haven't had any yet. You would think that if time travel ever does become possible in the future and if the problem is purely one of technology, then when it is 'invented' that at least one of your many great, great (etc) grandkids would pop by - if only for some help with a history essay.

The fact they don't and haven't is a pretty good argument - pretty well conclusive - that we don't ever invent time travel. However, this new atom smasher in Europe is proposing to change people's minds on even this.

One of the things Davies says you might need to be able to go backwards in time is a wormhole and that would also require a way of inflating wormholes that are thought to exist at the quantum level. Our brand new atom smasher opening today in Europe might be just the thing for this and therefore flicking that particular switch might give all your future relatives a way to pop down and see you just in time for Christmas.

Look, I've very strong doubts that you are going to need to set any extra places for Christmas dinner - you can call me a sceptic if you like.

This is where I become completely confused. And I quote, "Time travel must not be confused with the equally fascinating (and equally speculative) topic of time reversal." Hmm. So, why not? Well, Davies doesn't really explain. How can you possibly go back in time without somehow reversing it? This is my big problem with time and time travel altogether.

You see, on the grandest scale time isn't just something that tells you when your boiled egg will be runny or rock hard - it is a dimension of the universe and as such it is the thing that space expands into. (I recommend you pause and think about that for a second - so I'm going to say it again - time is that which space expands into).

I used to wonder what it would be like to stand at the edge of the universe - I don't have to wonder anymore. I'm always already there. And so are you. The universe has been expanding for 13 billion years (give or take) and we are right now at the edge of that expansion. When you look around the night sky you see billions of stars (well, if you have a good enough telescope) and each one of those stars has one thing in common - they are all living in the past relative to you.

Given that the universe is expanding into time I can't for the life of me work out what it would mean to go back in time. That would seem to mean - well, time reversal. And time reversal would seem to mean somehow squeezing the universe back to where it was. If you want to go back to 1950 the meaning of going back to 1950 is to have the universe the same size and shape as it was then. I don't know how much energy that would take - to squeeze the universe back into 1950 - but I'm guessing that 'a lot' isn't really going to cover it. And given most of us talk about the impossibility of 'getting the toothpaste back into the tube' it would seem somewhat ambitious to suddenly give up on the toothpaste and move on to squeezing the entire universe back to 1950.

People don't like to hear that things are impossible - and that is one of the nice things about being a person, I guess. But just because Dr Who has his Tardis is no reason to suppose that in the future everyone gets to have one of their own. I feel like I'm spoiling the party, but until someone can explain more of the physics of time travel than is done here - or at least how going back in time isn't the same as reversing time - then I'm going to have to assume this is all just so much wishful thinking, if not arrant nonsense.

All the same, Davies does make it clear that time travel - particularly backwards time travel - is problematic at best and this is probably the major value of his book.