# Travelling to the future (paradox)

1. Apr 17, 2012

### alexmahone

Some time ago, I watched a Stephen Hawking documentary which said that you could travel to the future if you went at a fraction of the speed of light for some amount of time.

The explanation was this: Consider an observer O at rest. Let the time traveller be T. O sees T whiz past him and concludes that T's clock is running slower than O's. So when T stops (assume that he remains in the same geographic time-zone), O knows that his own clock will be ahead of T's and so T will get the impression that he has travelled to the future.

But now consider T's frame of reference. He sees O whiz past him and concludes that O's clock is running slower than T's. So when T stops, he thinks that his own clock will be ahead of O's.

Who is right?

2. Apr 17, 2012

### ghwellsjr

They're both right but I'm sure the documentary pointed out that the traveler T has to eventually turn around and come back to observer O at which point they will both agree that T is much younger than O.

3. Apr 17, 2012

### alexmahone

Assume that T does not turn around. Instead, he whips out a phone when he alights and calls up O to ask the time. (Assume that the distance travelled by T is not much.)

4. Apr 17, 2012

### Ryan_m_b

Staff Emeritus
It's a good question that everyone seems to think of at some point. I believe the difference is that both can agree that T is at some point in an accelerating reference frame whereas O is not. I'm pretty sure the proper name for your observation is the twin paradox which is not considered a true paradox.

5. Apr 17, 2012

### ghwellsjr

If they both use the same reference frame in which they are mutually at rest, they will agree that T is younger, but that doesn't make it absolutely true because in the inertial reference frame in which T was traveling, or in other reference frames, there ages can be different. This is the issue of relativity of simultaneity.

6. Apr 28, 2012

### gopolks

Was this the doco that Hawkings said, if you built a train track around earth, and the train went 186000 miles per second, in 150 years, the people will get out the train, and would've only aged a week, and thus you have time travel.

7. Apr 28, 2012

### alexmahone

Exactly!

8. Apr 28, 2012

### gopolks

So basically what is keeping people from time travelling as someone on these boards pointed out, is just a engineering issue.

9. Apr 28, 2012

### alexmahone

Every time we hop into a vehicle or even walk, we are travelling to the future compared to someone who is at rest. However, we travel by a negligibly small amount. What's keeping us from travelling into the far future is an engineering issue.

10. Apr 28, 2012

### ghwellsjr

If the engineering issue is ever solved, then it will become an economic issue, just like the engineering issue of going to the moon was solved decades ago but the Apollo program was cut short due to economic factors and has never been funded since.

11. Apr 29, 2012

### gopolks

Its the fact that it's possible to travel to future in terms of physics, I think that should excite people, even though it wont happen.

12. Apr 29, 2012

### sweet springs

Hi.
To the future it is easy, "Rip van Winkle", but impossible is to the past.
regards.

Last edited: Apr 29, 2012
13. Apr 30, 2012

### Millenniumf

Of course, you realize that if the engineering problems were solved, what we get may be something like this:

http://www.xkcd.com/989/