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Old Jun 21st 2019, 06:51 AM   #21
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Originally Posted by neila9876 View Post
@no one:
How big is universe? What's outside universe?
We can only see out to about 13.5 billion light years. If there is anything further out we probably won't ever be able to see beyond the singularity of the Big Bang. (And I can't imaging that we'll even see that.)

As to what's outside, if the Universe is singular (that is to say that there are no other universes) then I would say that there is nothing outside. We can't really imagine "nothing" but that doesn't mean it can't exist. However everyone probably has their own ideas about it.

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Old Jun 21st 2019, 09:48 AM   #22
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Originally Posted by topsquark View Post
That has to do with something I mentioned. The photon doesn't "see" that it has moved any distance. The Fitzgerald contraction shrinks the distance to 0.
I don't see why that's the case. Your local clock is still ticking away normally...

Length contraction applies to observations of light emitters, not space in general. An observer travelling at c relative to some stationary rest frame will still travel through space like any other observer, but any observed objects moving at relative speed c (like all photons) will have their lengths measured to be zero.

I'm sure I have holes in my understanding (especially since I never studied GR), but I'm pretty sure there's no special status obtained by travelling at the speed of light. Sure, you're moving very fast and the Universe around you would look very odd indeed from your window, but you still have to wait 8 minutes to travel that 1AU to get to the Earth from the Sun.

------
EDIT: I've been doing some further reading... it seems I'm definitely wrong here! Length contraction can apply to distances...

Last edited by benit13; Jun 25th 2019 at 08:52 AM.
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Old Jun 21st 2019, 01:40 PM   #23
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interstellar travel

Even massive objects like spaceships and people experience length contraction in direction of travel, this theoretically makes interstellar travel possible if you have enough energy. here is a link https://www.physicsforums.com/thread...travel.370573/
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Old Jun 21st 2019, 04:55 PM   #24
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@no one:
"light emitter","massive object", etc, are specific demonstration of 3D "physical" space.
One of the premises of traditional 3D space SR is v can not be c. When v = c, it's a very special situation.
Viewing us from the viewpoint of photon and viewing photon from the viewpoint of us are not the same events.
if you are accelerated to light speed, all (energy in ) cosmos will have been consumed by you, of course you can travel "interstellar"...Photon can travel in light speed because it has different physical structure with us...
When I see such threads as "how big is universe", "what's outside universe",etc,elsewhere, I reply: with only 3D space thought, someone might think to die and not able to understand it.

Last edited by neila9876; Jun 21st 2019 at 05:22 PM. Reason: detailed
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Old Jun 25th 2019, 09:02 AM   #25
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Originally Posted by wavesearcher View Post
Even massive objects like spaceships and people experience length contraction in direction of travel, this theoretically makes interstellar travel possible if you have enough energy. here is a link https://www.physicsforums.com/thread...travel.370573/
Unfortunately, although the traveller (potentially) gets to zip around the universe very quickly and see lots of things, they can't "quickly" go back to Earth and relay all their findings because all of the people on Earth will have to wait an extremely long time to hear from you.
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