# The Naked Scientists Forum

### Author Topic: How does special relativity apply in space?  (Read 1096 times)

#### Mark Wilson

• Guest
##### How does special relativity apply in space?
« on: 15/10/2011 23:30:02 »
Mark Wilson  asked the Naked Scientists:

Naked Scientists,

I have a question regarding time and relativity.

As I understand it, Einstein tells us that there is no fixed point in space and therefore time can only be measured relative to the observer.  Let's say a rocket with an atomic clock aboard blasts off the Earth and heads toward the Andromeda Galaxy at half the speed of light.  A second atomic clock remains here on Earth with someone to observe it.  The clock aboard the spaceship should go slower than the one on Earth because of its greater speed relative to the observer on Earth.

My question is one that I've never heard anyone ask before.  It has to do with the definition of the word 'relative'.  Let's says the rocket has a human passenger who is observing the clock onboard the spaceship.  This passenger looks out the window and sees the Earth moving away at half the speed of light relative to the spaceship.  The clock on Earth should go slower than the one on the spaceship because of its greater speed relative to the observer on the spaceship.

Both scenarios must be correct but how can they be?

Mark Wilson

San Diego, California

What do you think?
« Last Edit: 15/10/2011 23:30:02 by _system »

#### yor_on

• Naked Science Forum GOD!
• Posts: 11993
• Thanked: 4 times
• (Ah, yes:) *a table is always good to hide under*
##### How does special relativity apply in space?
« Reply #1 on: 16/10/2011 00:18:30 »
That one is only true in a uniform motion. To see that one you need to ask yourself if there will be a greater energy relative a higher uniform motion, if you think there will be, then you have a definition of 'energy relative motion'.

When it comes to 'time dilations' and who is 'aging' relative whom, then that 'energy' will play a role for that motion. A simpler way is to assume that for any uniform motion there has to be a preexisting acceleration. Then you use that one to define a 'speed' relative some origin.

When it comes to two 'uniformly moving' objects, where you don't know the acceleration for any of them, or where one of them is totally 'unknown' then you can't define a 'time dilation. A 'time dilation' can only be confirmed if you have a common origin, like Earth, and then leave it to come back later. That will give you a definition of a first 'common time /frame of reference' that you then can use to compare, for example the 'twins experiments' biological age, relative each other.

You have to remember that a 'time dilation' only will exist in the comparison. Neither of those 'twins' will at any point find themselves 'aging' faster or slower, as measured in their own frames of reference. And if they each have one million heartbeats to spend, relative their own wrist watch, then that will be their defined amount, no matter what they do, or go.

« Last Edit: 16/10/2011 00:23:03 by yor_on »

#### MikeS

• Neilep Level Member
• Posts: 1044
##### How does special relativity apply in space?
« Reply #2 on: 16/10/2011 07:59:54 »
Both clocks appear to slow from the opposite reference frames and both objects are in relative motion but only one object (the rocket) is in motion relative to all else.  (Yes, I know this is an over simplification)

The observer on the rocket would observe all other observable clocks in the universe to be running slow.  He could deduce from that, he was the one travelling at relativistic speed.  The observer on the Earth only observes the clock in the rocket to be running slow and could deduce from that it was the rocket that was travelling at relativistic speed.

So time actually has slowed for the rocket and its passenger relative to the rest of the universe.  From the perspective of the passenger he would notice time passing normally in his local time frame.

#### The Naked Scientists Forum

##### How does special relativity apply in space?
« Reply #2 on: 16/10/2011 07:59:54 »