# Thought Experiment: The flow of time at different parts of the univers

1. Nov 26, 2013

### Shayne T

Hey all,

Just a thought experiement here. With time dilation caused by gravity and relative velocity being a confirmed phenomena, do you think it's possible that other worlds that are extremely far away from each other, and are moving at a very high speeds relative to one another with different vastly differing gravitational fields are able to exist and play out their whole existence, relative say to only a years amount of time on another world. I am wording this question terribly, but its a lot like the twin paradox where one travels close to C realitive to his twin, and experiences a years time himself, but its like 10 years for his stationary twin back on earth. Basically, im asking if there are large enough velocity and gravity difference between planets for this to happen on a large scale (ex: after 1 year on earth, a planet on the other side of the universe experiences say a century, or millennium even.

It seems to me that these time difference between planets do exist without a doubt, because we know time dilation in real, and we also know everything in the universe is moving at varying speeds to one another. I just want to know to what magnitude this would be possible.

Cheers!

2. Nov 26, 2013

### Staff: Mentor

It's important to be clear about what, exactly, is "confirmed". What is confirmed is that, *if* two observers have a common time reference, then the one deeper in a gravity well, or the one who is moving, will experience less elapsed time. But the qualifier, having a common time reference, is crucial.

In the case of gravity, both observers need to be stationary in the same gravity field; i.e., they must be at rest relative to each other. The static nature of the gravity field then provides the common time reference.

In the case of relative motion, the two observers must meet up before and after the experiment, so the readings on their clocks can be directly compared. The direct comparison provides the common time reference.

The reason these conditions are important is that they are violated in the other thought experiment you pose; see below.

This statement has no meaning as given, because there is no common time reference; the observers can't meet up and compare clocks, and they are not at rest in the same gravity field.

The first statement does not follow from the second; see above.

3. Nov 26, 2013

### yuiop

Consider this related scenario (just for fun). Imagine a solar system similar to ours was ejected out of our galaxy, billions of years ago. Eventually life evolves on both systems and then eventually the life forms start developing technologies. To us the 'alien' system seems to evolve slowly and to the aliens we appears to evolve slowly and we will appear underdeveloped and primitive to them. Eventually both solar systems develop space travel and rocket technologies that can travel close to the speed of light and consider invading the other system, (This equal state of development is from the point of view of an observer that sees both systems receding at equal speed from him). All else being equal, the invading side will be at a disadvantage. Lets say the the aliens decide to invade us. While they travel towards us at relativistic speeds, perhaps 100 years pass for the invading space fleet, while 2000 years pass for us. That gives us 2000 years to develop our weapon and defence technologies and will have all the resources of a solar system, while the invading fleet has less time develop and only the resources they carry with them, so they might appear fairly primitive to us by the time they arrive. Of course there are so many other random factors involved that might favour the evolution of one system over the other, that all else will almost certainly not be equal.

4. Nov 26, 2013

### Staff: Mentor

Yes, in this scenario, we would expect the invading aliens to have experienced significantly less elapsed time, both for the reason you give *and* for the reason that their solar system was ejected out of our galaxy, meaning it must have had a significant velocity relative to us. In other words, it's like the standard twin paradox, with the aliens playing the role of the traveling twin. But note that we know this because we have a common time reference: the aliens eventually arrive back at Earth and we can compare clocks.

5. Nov 26, 2013

### Shayne T

why do you need to compare times in order for it to be true? i understand it would be undeniable evidence, but cant we just assume with the knowledge we have about time dilation, that two planets exist in different rates of time, if we know without a doubt that they are traveling at relativistic velocities to one and other? And be able to calculate the factor of difference using certain equations that are out there dealing with time dilation?

6. Nov 26, 2013

### yuiop

While they are great distances apart, it all comes down to who makes the comparison. Each planet will consider time to evolve more slowly on the other planet.

7. Nov 26, 2013

### WannabeNewton

Kinematical time dilation and relative 3-velocity only have meaning locally. You can get away with not caring about this in SR for mathematical reasons but the same mathematical reasons will prevent such an attitude in GR.

8. Nov 26, 2013

### Shayne T

And a comparison from an observer on planet B will be just as true as planet A's observation? I just can't wrap my head around how both can be true, but they are. I understand how they both are completely true. But yet contradictory because they both see the other as the "slower" ones.

I guess i just need a better understanding about relativity and other related topics that are relevant in this situation. The universe is just so, crazy!

9. Nov 26, 2013

### Staff: Mentor

You've misstated things a bit. You need to compare times for the answer to be *invariant*. If you can't compare times, each observer can certainly *calculate* an "elapsed time" for the other, based on some notion of simultaneity; but the answer won't be invariant, i.e., different observers won't agree on it. In relativity, that's a good signal that whatever you are looking at isn't really meaningful physically.

We can make such calculations, but the answers will be observer-dependent, because in general "time dilation" is observer-dependent. The conditions I gave in my earlier post are the conditions for there to be a notion of "time dilation" that is *not* observer-dependent.

10. Nov 26, 2013

### Staff: Mentor

They are "true" only in the observer-dependent sense. They are not "true" in any absolute or invariant sense, precisely because they are observer-dependent.

11. Nov 26, 2013

### bahamagreen

So Enrico Fermi's question, "Where is everybody?", has an answer... they have been working on it, but they are doing so real real slowly, the further away from us the more slowly they are progressing.
The fastest development in the whole universe is happening right here on the Earth (and surrounding local galactic clusters)...

12. Nov 27, 2013

### Staff: Mentor

No, that's not correct. Remember that there was an additional condition required for the aliens to have experienced significantly less elapsed time: they had to be ejected from this galaxy at a high speed.

Aliens who develop on other planets that remain within this galaxy in an ordinary solar system, like the Earth, would "develop" just as fast as us--more precisely, when the arrived here, they would only have experienced a few hundred or at most a few thousand less years of elapsed time, depending on how far away in this galaxy they came from. But there is no reason why they couldn't be millions of years ahead of us in technological development before they start, so that a few hundred or thousand years' difference is negligible by comparison.

(Aliens who develop in other galaxies could also "develop" just as fast as us, but their travel time to get here would be longer--aliens from the Andromeda galaxy would take at least 2 million years to get here. It's harder to argue that that much difference in elapsed time wouldn't be significant--though not impossible.)