# Slowing time on a plane

1. Apr 10, 2006

### kuahji

I'm not sure if this has been asked/discussed before or not. I tried to look but couldn't find a thread, so if there is one please direct me to it.

In this book I'm reading it states that if two planes start off at say the US Naval Observatory, if one goes eastward & the other westward, time will move slower on the plane going eastward. Both planes travel at the same speeds, same height, etc. The reason the book states is because the plane traveling westward goes against the rotation of the Earth. If the US Navel Observatory is the reference frame I don't understand why time one way is slower than the other. If the reference frame was in outer space I could see why (you'd see the plane going eastward at say 100kph + the speed of the earth's rotation).

Perhaps I'm over thinking this one or perhaps I don't understand as much as I thought I did. Any thoughts?

2. Apr 10, 2006

### scott1

I havn't herd anything about that before.

3. Apr 11, 2006

### Garth

The US Naval Observatory is not an inertial reference frame, it is on a rotating Earth.

Garth

4. Apr 11, 2006

### yogi

Go to google and search on Hafele & Keating

5. Apr 11, 2006

### pervect

Staff Emeritus
I'd have to dig a bit for supporting references, but your book has it right. To make the observation more specific by pinning down the details, when the two planes next meet (are in the same point in space-time) the westward moving one will have more elapsed time on its clock.

What I don't quite understand is what you don't understand.

6. Apr 11, 2006

### kuahji

Einstein for dummies... I'm reading it on the side from classes for fun.

I thought there could be no abosulte at rest reference frames... You could say the universe is rotating around the Earth.

Basically, I thought it didn't matter if the intial reference frame was on Earth. Because in theory you could say the Earth is moving around the Sun, & the sun around a black hole (in theory).

I'm not at that level yet to understand the math. I'm sure when I catch up the ideas will click better. I just thought from what I read you could say the Earth is standing still & the universe is rotating around the Earth. Then in theory the planes that are flying there would be no time dilation. The Earth's rotation is decelerating b/c of the pull from the moon, but if it wasn't we couldn't say the Earth was rotating correct? I think now that I slept on it, it's making a bit more sense. Just have to wait for the math to catch up.

This is what was confusing me. If I was standing on Earth & two people walked 2km from me (on East & one West) at the same speeds. I shouldn't be able to tell the time dilation even with the most accurate equipement, right (b/c everything is in uniform motion)? But, if I was in space orbiting the planet, one would have time dilation or do they both have the dilation in both reference frames? That is what is confusing me.

7. Apr 11, 2006

### Janus

Staff Emeritus

http://www.bartleby.com/173/23.html

8. Apr 11, 2006

### pervect

Staff Emeritus
Basically, the simple answer to your problem is that rotation is absolute. You can tell by a variety of experiments, including a ring laser gyro, that the Earth is rotating, without reference to any other object.

So it is false to say that you can think of the Earth as standing still and the []universe is rotating about it.

It is correct to say that you cannot know what velocity the Earth has without reference to some other object. The same cannot be said for rotation, however.

This is often sumarized by saying that rotation is absolute, but velocity is relative.

Last edited: Apr 11, 2006
9. Apr 11, 2006

### kuahji

Thanks for all the help. It makes sense to me now. :)