# Atomic clock used in spaceships

1. Mar 8, 2012

### AnilML

Hi,
Not sure how stupid my question might be. Could anyone please clarify me, why the atomic clock in a space ship is taken as a reference while explaining a special theory of relativity?. I always wonder how 2 people one outside the space ship and the other inside it feels about a time in 2 different manner. and this problem exists only when I carry an atomic clock inside a spaceship (for the reference of a time). If i carry an analog clock(run by a battery), both the people inside and outside the space ship can see the same time right.?

2. Mar 8, 2012

### bcrowell

Staff Emeritus
No, it doesn't matter what kind of clock you use. In real-world experiments such as the Hafele-Keating experiment http://en.wikipedia.org/wiki/Hafele-Keating_experiment , they use atomic clocks because the relativistic effects are small, so you need a very accurate clock in order to detect them.

It's only because the effect is the same on all clocks that we can consider time dilation as an effect on time rather than an effect on the specific internal mechanisms of some type of clock.

3. Mar 8, 2012

### ghwellsjr

And it's not being inside or outside a spaceship that is the issue, it's one clock traveling at some speed in relation to another clock. We just like to put the people in the spaceships so that they can breathe and have a means of propulsion so that the clocks can get up to a speed difference between them. So sometimes we talk about people in two spaceships traveling and sometimes we talk about people in just one spaceship while the people outside remain on earth where, of course, breathing is not a problem and where they have their own clock. Remember, two clocks are involved and when there is a speed difference between them, meaning they are either getting closer together or getting farther apart, then each will determine that the others clock is ticking slower than their own.

4. Mar 8, 2012

### AnilML

Thanks a lot.. a lot... But my little brain thinks, ( analog clock ) the mechanism which moves the needle of a clock will always be same in all the frame of references. so the 2 guys one in spaceship and other stands outside must see the same needle movement of a clock. And why should a person assumes atomic clock is the accurate one.. Instead of thinking about time dilation and space contraction..etc, one can simply thinks the way we have understood what the light is might be wrong. Or one can simply think atomic clock is not accurate. right ?

I ll be eagerly waiting for your clarification sir. Thanks a lot again..

5. Mar 8, 2012

### ghwellsjr

Are you saying the analog clock is accurate while the atomic clock in the same circumstance is inaccurate?

6. Mar 8, 2012

### phyzguy

Your intuition is wrong. As bcrowell said, all clocks slow down because time itself slows down. This has been verified by many, many experiments.

7. Mar 9, 2012

### Staff: Mentor

The mechanism that moves the needle is the same in all frames of reference - what's different is the speed at which all mechanisms, including that one, appear to operate when viewed from different frames of reference. Use a mechanical clock operated by a pendulum, and the moving observer will measure a different speed for the pendulum, and hence a different slower tick rate for the clock, than will the observer at rest relative to the pendulum.

8. Mar 9, 2012

### ghwellsjr

Einstein said a balance clock is preferred. A pendulum clock is dependent on gravity and not good.

9. Mar 11, 2012

### sunil83_nie

Hello friends, I have small doubt here. what the time actually is ? in relativity experiments the differences found in measurements only, I feel the time is something other than physical quantity it has only positive direction, right? If we consider the time as a fourth dimension why it is not possible for us to go in negative direction? please clarify this , Thanks

10. Mar 11, 2012

### DaveC426913

Spacetime is comprised of three space-like dimensions and one time-like dimension. One of the characteristics of time-like dimensions is that we can only move in one direction through it.