# Is anything in the universe stationary?

According to this thread, a ship leaving earth is moving and the earth is ‘stationary’. Now given that eart is moving aroung the sun, the sun around the center of the galaxy, and the galaxy (presumably) through space, two questions:

1. Is anything really stationary? and…
2. Since the earth is moving a direction, isn’t it possibly that a ship moving away from that direction at the same speed wouldn’t age (along with it’s occupants) at all ? Or if it moved away from that direction at a speed slower than than earth is hurtling through space, might the people on the earth be the ones who don’t age?

It’s kinda difficult to explain, but neither the ship nor the earth is stationary. The confusion deals with frames of reference.

Picture this: you’re on a bus in outer space, with no acceleration. You can’t tell whether you’re moving or not. Now a star goes flying by the window. Is the star moving, or are you?

Relativity tells us that there’s no right answer, and it’s in fact impossible to determine. Now if either object were accelerating, you’d be able to point it out. But you can’t tell which object is moving if both are going at constant velocity.

There is a particular point I feel I should make. Velocity is different from speed. Speed is a scalar quantity, meaning that it only has a size associated with it. Velocity is a vector quantity, meaning that it has a size and a direction. So a car moving 60 mph northbound is going a different velocity from one moving 60 mph westbound. The point of this was to highlight the fact that changes in direction are accelerations (not that I think you’re confused about that, but I could see somebody being confused).

So is anything stationary? No one knows. The only thing we can say is that some objects are not accelerating.

To your other question: the aging effects are due to the fact that the ship is moving at near the speed of light, not due to the relative motion of the ship and the planet. A ship moving at near light-speed in an otherwise empty universe would still suffer the same effects.

Say you are riding on a bus going 40 mph and you throw a ball out of the back window at what would have been 40mph had you been standing on the surface of the earth. The ball thrown from the bus would be moving at zero mph. If you threw thw ball at 30 mph it would effectively go backwards from the point you threw it even though it was accelerating.
So, since the earth is moving and you launch a rocket in the opposite direction, wouldn’t this be the same thing?

I’m not sure that I understand your question. I think the answer is yes, but I’m gonna wait and see if someone else can answer it.

I’ll try to clarify by re-stating.
Say you are riding on a bus going 50 mph and you throw a ball out of the back window at what would have been 40mph had you been standing on the surface of the earth. The ball thrown from the bus would be moving at -10 mph in relation to the thrower (as seen from the earth, the place the earth twin would be), even though it was accelerating. We know that the ball is accelerating because were a twin to be in the ball at the time of throwing, he would feel the G force.The twin riding in the ball, even though it is accelerating, is traveling at a lesser velocity the earth twin. This should cause the earth twin to age more slowly - not the ball (accelerating) twin.
So, since the earth is already moving at a constant velocity (like the bus) and you launch a rocket (like the ball) in the opposite direction, wouldn’t this be the same thing?

[quote]

1. Is anything really stationary? [/quote
Yeah. My cat.

Yes, and that’s why the twin paradox isn’t really a paradox after all.

Yes. I am. And the rest of the universe revolves around me.

That must come in handy for changing lightbulbs.

On a seriouse note Re: ultrafilter’s last responce - now I’m really confused!

As evidenced by my attrocious spelling…

Preview, preview, preview…

I can only speak from my own area, so theologically there’s no problem with this. Other than the fact that my music teacher used to insist the universe was centred on his desk, but frankly I like Spoofe better. I’d be interested to know if it just revolves of its own accord or if you have to maintain a kind of hula-hoop wiggle to keep it going.

Man enters newsagent’s and approaches salesgirl.
“Miss, do you keep stationery?”
“No, I wiggle about a bit!”

HahahahahHAAAAAAA.

The state of being stationary is a meaningless concept in the world of relativity because there is no preferred reference frame. Things are either moving with respect to some other object or not.

I’ve noticed this anomaly in the physics of the universe myself. Interestingly, if you loudly explain the laws just before throwing the ball, it immediately comes back and hits you in the face. A glitch I guess. They’ll fix it in the next patch.

I see why you were confused; I misread your post. I’m going to see if someone else can field this one, cause I’m not sure how clearly I can explain it. Dr. Lao, care to try?

warmgun:

In your example, only the the twin the the ball undergoes any acceleration (CHANGE in velocity). Hence, he/she would be the only one to experience the slowing of time. Going faster/slower than another entity has no time implication; only changes in velocity as stated above by ultrafilter.

–the stretch

Obviously I’m a little out of my league here, but from what I understand this does not seem true.
The examples of time dilation usually go something like this:
“If twin ‘A’ gets on a rocketship and travels at 10% c for ‘x’ # of years, when he/she gets back they will be ‘so and so’ years younger than the twin (B) that stays behind.”
This implies that going faster does matter. Otherwise the time factor wouldn’t matter. If you could accelerate to, say, 20% c in 10 minutes (and live - hey, this is a mind experiment!) and stop in another 10 that’s all you you would need to do to cause the extended time dilation - no years in space needed. Right?
Further, What about the experiments where atomic clocks are put on supersonic jets and flown around the world a couple of times to prove TD. Those jets are not in a constant state of acceleration. They ‘quickly’ reach cruising speed and just maintain it for a while. According to your statement above, as soon as they reached the end of the acceleration curve all effects of TD would stop.

Changes in velocity are only used to determine whose velocity is changing. Relativistic effects are based solely on how fast one is traveling. I think that you need the math in order to resolve this one, and I’m not familiar enough to post it here.

I appreciate your input. But if this true, why wouldn’t my example hold true?

Where is Chronos when you need him???

Sorry, my first post was distracted and misleading. Ultrafilter was simply too polite to say so. (I made it all day without turning CNN on; alas, I couldn’t resist this evening…)

Without getting into the math, I am somewhat confident we can state the following:

• I am either demonstrably accelerating or at rest in my reference frame. (disregading gravity) (with respect to the OP, all unaccelerating objects have equal claim to being stationary - if SPOOFE is not accelerating, then I agree with him.)

• If an object is moving with respect to my reference frame, its clock appears runs slower than mine. (My clock also appears to run slow from its perspective)

• Should something in my reference frame go accelerating out of it and then back in, like our twin-in-the-ball, its clock appears to run slow during its motion, and its clock is BEHIND my clock when it returns to my frame. The object cannot “decelerate” out and come back in with its clock ahead. If I sit “still,” any object’s change in velocity out of my unaccelerating reference frame is an acceleration and will result in the object’s clock being behind if it rejoins.

Sigh. I hope this helps more than my earlier attempt!

–the stretch

Alright, I’d like to clarify something ultrafilter said earlier. Acceleration involes velocity and since velocity has direction (its difference between speed) the jet flying around the world is constantly changing Velocity, it does reach a cruising speed and stay at that speed but it’s velocity is constantly changing, which is why it is always accelerating.

As an example take the moon. The moon can be said to orbiting the earth or constantly falling(accelerating) toward the earth, but since it also has a tangential aspect of momentum it falls around the earth.

I’d like to point out that my entire explanation makes perfect sense to me although I don’t think it will help anyone else, I’ve never thought of myself very good at explaining these types of things