Here is a good way to make progress in science: take something that is so "utterly obvious" that to question it would be "contrary to common sense" - and ask what would happen if it were wrong.

The Earth cannot possibly be a sphere. If it were, people on the bottom would fall off. Heavier objects always fall faster than light ones. Women are less intelligent than men. Animal species are fixed and never change. Atoms are by definition indivisible. The rate of passage of time is absolute and the same for everyone. There are more odd numbers than there are even numbers. Nothing can be in two places at once. Since space can have no end, the universe must be infinite.

I leave it to the reader to determine what has happened to each of these "absolute truths." I will concentrate on a different one: time travel is impossible.

Here I am referring to travel into the past. We travel into the future with every passing second. Usually we all travel at the same rate. However, I know at least two ways that I can travel into the future faster than you. The first is if I climb into a spaceship and fly away, very fast, at close to the speed of light, and then turn around and return the same way. When I get back to Earth, you will have aged more than I. If I choose my speed correctly, I can arrange it so that while I have aged only one year, upon my return you will have aged ten years, or fifty years, or have been dead and buried for five hundred years.

This sounds strange, but it has been solid science since 1905 when Albert Einstein published the first papers on relativity. Physicists have to allow for this effect if they want to get the right answers when they calculate the lifetimes of subatomic particles.

I also can move faster into the future if I place myself in a very powerful gravitational field. Assuming I can find a way to survive weighing tens or hundreds of times as much as normal, when I come back I will have aged less than you. This again is solid physics, and has been so since 1916. It is supported by experiments, although not of course on humans.

Travel into the past presents a much trickier problem. The argument showing that backward travel in time is logically impossible involves changing history, and is usually known as the "grandfather paradox." Suppose that I could build a time machine. Then I could use it to go back and kill my own grandfather, before my father was conceived. Then I myself would not exist. Therefore I could not go back and kill my grandfather.

However, the grandfather paradox proves only that there must be restrictions on any possible time machines. Suppose, for instance, that with any time machine you could not go farther back than to the time at which it was built. Or suppose that a time machine could not be used to go to the relatively recent past, but only to times so long ago that nothing we did there survived to affect the present day. Possibly changes to the past have a tendency to "damp down" over succeeding generations (the counter-argument, that changes made to the past might amplify over time, can be found in Ray Bradbury's story, "A Sound of Thunder").

Kip Thorne, a well-known physicist at CalTech, has devised a way to make a time machine of the first type. It employs a theoretical feature of space and time known as a "wormhole," something permitted by the equations of relativity but not so far encountered in the real world. (Thorne, incidentally, prefers to call the grandfather paradox the "matricide paradox." When I first read that term it suggested an alternative paradox in which a man goes back in time, marries his mother, and becomes his own father.) Thorne's time machine using a wormhole requires what he terms "exotic material," which is a substance with properties far different from ordinary matter. For one thing, an exotic material must have negative total energy.

The physicist Frank Tipler used relativity theory to devise a form of time machine of the second type, permitting travel far into the past. Again, he needed something which today we can see no way to build: a gigantic cylinder of ultra-dense matter, rotating so fast that its outer surface moves at half the speed of light. It sounds impossible, or at least highly unlikely, but Tipler, deliberately tongue in cheek, ended his paper thus: "The demonstration that no possible combination of known substances, known forms of machinery, and known forms of force can be united in a practicable machine by which men shall [travel back in time], seems to the writer to be as complete as it is possible for the demonstration of any physical fact to be."

Except for the words that Tipler put in brackets, this statement is identical to one published by the leading American astronomer Simon Newcomb, in 1906, in a paper "proving" the impossibility of a heavier-than-air flying machine. Note that this was three years after the Wright brothers flew at Kitty Hawk.

Let me end with a poem about time travel, time machines, and the grandfather paradox. It is a sonnet, with something peculiar about its structure. I invite you to discover that oddity:

This lever sets the pace. It lets me say
How fast the years must run their backward flight.
Each blink of light, a speeding night or day,
Takes me to worlds long vanished from our sight.
I have a choice. One path that I can take
Makes present future, calls from ages gone
Each vanished dawn. The other branch will make
My children's children old as I look on
And tell me how the human race must die.
Could I endure such sights? Future fears
Have bad effects on me. I'll choose to fly
Into the quiet past, back sixty years.
Now I can make my test. A single blow
Ends Grandpa's life. Good Lord! Where did I go?

Give it another read-through. Do you see the unusual feature?

(If not, take a look at the first letter of each line.)

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