# Relative time in space

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1. Oct 17, 2015

### mileymo

What speed does a ship travelling through deep space need to travel at for 1 day on the ship to equal 1 day on earth?

2. Oct 17, 2015

### BvU

Any speed will do. But both will observe the other guy's clock is running slow !

3. Oct 17, 2015

### SlowThinker

I think it's 11km/s in the nearby Solar system, up to some 500km/s outside the Solar system, and even bit faster outside the Milky Way.

4. Oct 17, 2015

### Vitro

If by 1 day you mean 24 hours measured by a clock on Earth and 24 hours measured by a clock on the ship, then they represent the same amount of time regardless of the speed of the ship relative to Earth. During the 24h measured by a clock on Earth a person on Earth would have aged 1 day, during the 24h measured by a clock on the ship a person on the ship would also have aged 1 day.

5. Oct 17, 2015

### Staff: Mentor

If the ship and the Earth are moving relative to each other, then there is no invariant way of matching up their "rates of time flow". So this question doesn't have a well-defined answer.

6. Oct 17, 2015

### Staff: Mentor

How are you coming up with these numbers?

7. Oct 18, 2015

### mileymo

What I mean is, if you travel at 87% the speed of light, then 1 day on that ship is equal to 2 days on earth. So, is there a lesser percentage of the speed of light at which time would be equal for both the ship and the people on earth?

8. Oct 18, 2015

### BvU

That's only for an observer on earth ! The guy in the ship thinks it's the other way around !

In your line of reasoning: zero. Stay put on earth.

9. Oct 18, 2015

### mileymo

Thanks. Much appreciated!

10. Oct 18, 2015

### Staff: Mentor

There is - relative speed zero, meaning that the ship is not moving relative to the earth.
You can calculate this from the time dilation formula, which google will find pretty quickly - try "relativity time dilation".