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Q & A: twins and cosmos

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Most recent answer: 10/22/2007
Q:
Well it is a bit an idea and i just wanted see what would you think of it. According to the twins paradox if one of the two twins gets on a rocket that travels in exactly same trajectory without changing it in some time hi will be back on the earth. And what a shock will be for him when he discovers that his brother has been already dead. And here comes my conclusions- 1st- If you travel with some speed(doesnít matter what) through space at some point of your journey you will find your self back at the point you started. That means the Universe should be at least sphere-like. But in fact is this actually possible? As we know the Universe is constantly growing. That means everything in the Universe isnít just moving around each other but they are moving out of there prime positions. But if the Universe is constantly growing, even in each direction, and it is sphere-like it is possible that experiment to happen in the real life. But if anything of the previous conditions isnít performed it would be impossible to make it. 2nd- I find strange that theory that states that the Universe is on the surface of a sphere. I think that the Universe it self is a sphere-like and the main question is not what is deep into the core of the sphere(and i think there are more galaxies :) ) but what is around it. 3rd- I would like to tell more about the efect of traveling with greater speed - younger for longer. I think that our bodies will function normal and it will not help. otherwise why the clock will be an exception. I think i almoste understand the idea but some more explanation will be useful :) Iím looking forward for your answers and congratulations for the great idea :)
- Mitko (age 16)
Bulgaria
A:
1. Good question but we don't know the answer. Actually, let's rephrase it a bit. It doesn't mean anything to 'travel at a speed' because our relativistic world doesn't have any physical speed measuring effects. You can take two objects traveling at some speed with respect to each other and ask if, without either one feeling any acceleration, they end up meeting again. So that's what I take your question to mean. But I still don't know the answer.
The universe is close to 'flat', just in between 'open' and 'closed'. The answer would be yes in a closed universe and no in an open or flat universe. There's no experimental way right now to tell if the universe is just barely closed.
2. When people say the universe may be like a sphere (the surface of a solid ball), they don't mean that the universe is two-dimensional. They mean like the 3-D surface of a 4-D ball. The direction 'into' or 'out of' the sphere has no physical meaning, there's no there there.
3.You're right that relativistic effects don't change how long you would experience life. All your clocks- your wristwatch, your heart, your thoughts, etc. - proceed in synchrony with each other. What does depend on your path through space-time is how much you get to see of the aging of others. As you say, that can be very different for the two twins. Who knows which one is better off?

Mike W.

(published on 10/22/2007)

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