Stephen Hawking
answered on 22 Nov 2018:
last edited 26 Nov 2018 8:31 pm

Hi 699mnpd42,

Probably not 🙁

If we look at the maths, there’s a term in relativity that looks a little like this:

Time = 1 ÷ (1- (v÷c)) (Time dilation equation)

Where v = velocity, c=the speed of light

This means as we go faster the slower we appear to the universe. Once we hit the speed of light (v = c), it would be like time had stopped! If we could somehow go faster than the speed of light (v>c) it would be the same as going backwards in time (the equation becomes negative).

Here’s the problem. We know from Einstein’s famous equation (E=MC^2), that anything with mass is limited to travelling less than the speed of light (v<c). Humans have this horrible feature, we're made of stuff and stuff has mass.

There are a few cool tricks in quantum mechanics though. For example, a positron can be treated as an electron travelling backwards in time! This helps when predicting particle behaviour, but we don't have any anti-particle humans to test that on yet.

So unfortunately, we probably won't be able to go backwards in time, but knowing this lets us test the limits of physics in our Universe!

I’m surprised that Stephen didn’t mention his own “Chronology Protection Conjecture”, where he argued “It seems that there is a Chronology Protection Agency which prevents the appearance of closed timelike curves and so makes the universe safe for historians.”

“Closed timelike curves” are constructions in general relativity where you can travel round a particular trajectory and find yourself back at the same place AND TIME that you started from. As you’ve taken some time to complete the circuit, this effectively requires time travel. General Relativity doesn’t seem to forbid such things, and there are particular constructions – for example, travelling close to a very long, very massive cylinder – which seem to allow them. Stephen’s conjecture is that none of these methods for creating closed timelike curves actually works in practice. It is only a conjecture, though – that’s maths-speak for “we can’t prove this!”

@Rosalind Franklin. The scenario described also requires you to be pretty close to a black hole. The conjecture is a mathematical description of not being able to travel backwards in time. I really like how Stephen concludes this paper:

“There is also strong experimental evidence in favour of the conjecture from the fact that we have not been invaded by hordes of tourists from the future.”

Well, technically, we all travel through time, as otherwise everything would happen at once!
But I suspect that’s not what you mean, I have no idea if it may be possible to create something that allows you to chose how fast and in what direction you travel through time, that kind of physics came a long time after me.

Interestingly, have a look at how different cultures view time: in Europe it’s common for us to think of the future as being ahead of us, but there are places where people believe that the future is behind us, as you can’t see behind you! What do you think?

## Comments

anon-195096commented on :hmm…

Rosalind Franklincommented on :I’m surprised that Stephen didn’t mention his own “Chronology Protection Conjecture”, where he argued “It seems that there is a Chronology Protection Agency which prevents the appearance of closed timelike curves and so makes the universe safe for historians.”

“Closed timelike curves” are constructions in general relativity where you can travel round a particular trajectory and find yourself back at the same place AND TIME that you started from. As you’ve taken some time to complete the circuit, this effectively requires time travel. General Relativity doesn’t seem to forbid such things, and there are particular constructions – for example, travelling close to a very long, very massive cylinder – which seem to allow them. Stephen’s conjecture is that none of these methods for creating closed timelike curves actually works in practice. It is only a conjecture, though – that’s maths-speak for “we can’t prove this!”

Stephen Hawkingcommented on :@Rosalind Franklin. The scenario described also requires you to be pretty close to a black hole. The conjecture is a mathematical description of not being able to travel backwards in time. I really like how Stephen concludes this paper:

“There is also strong experimental evidence in favour of the conjecture from the fact that we have not been invaded by hordes of tourists from the future.”

Mary Somervillecommented on :Well, technically, we all travel through time, as otherwise everything would happen at once!

But I suspect that’s not what you mean, I have no idea if it may be possible to create something that allows you to chose how fast and in what direction you travel through time, that kind of physics came a long time after me.

Interestingly, have a look at how different cultures view time: in Europe it’s common for us to think of the future as being ahead of us, but there are places where people believe that the future is behind us, as you can’t see behind you! What do you think?