# How fast is a relativistic velocity?

1. Aug 5, 2004

### whydoyouwanttoknow

How fast do you have to be going before you would be said to have a relativistic velocity?

2. Aug 5, 2004

### jcsd

Obviously it's subjective, but 0.1c perhaps or even 0.5c?

3. Aug 5, 2004

Staff Emeritus
The dilations happen at all speeds, even 3 mph. The question is when does the size of the dilations become of interest to you?

4. Aug 5, 2004

selfAdjoint is right, but I say at 0.1c is where is comes into considerable effect.

5. Aug 5, 2004

### whydoyouwanttoknow

So your average GPS sat. is going at a relativistic speed because if you didn't take time dilation into account they'd give you the wrong position? But for the rest of us who cares that our car is going 60kph?

6. Aug 5, 2004

### robphy

Calculate $$\gamma=\frac{1}{\sqrt{1 - \left(\frac{v}{c}\right) ^2}}$$.
When your instruments can distinguish $\gamma$ from 1.0 [for example, with a very accurate clock], then you may consider the problem to require relativistic considerations.

7. Aug 5, 2004

### Staff: Mentor

Yep. So even saying .1C is incomplete: it depends on the situation.