# How fast is a relativistic velocity?

#### whydoyouwanttoknow

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

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#### jcsd

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

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

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

#### whydoyouwanttoknow

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

#### robphy

Homework Helper
Gold Member
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.

#### russ_watters

Mentor
whydoyouwanttoknow said:
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?
Yep. So even saying .1C is incomplete: it depends on the situation.