Nothing is stationary, Fixed points in space, The possible end of time

[Person012345]

Here's my question... what happens if you have two ships travelling near light speed heading towards each other, just far away enough to miss, how does time dilation affect their relative speed to each other?

I'm not entirely sure what you mean by the question. Do you mean how people on each ship measure the speed at which the other ship is approaching?

And another question. If you fire a laser from Earth and at the same time a ship next to Earth but travelling near-lightspeed in the opposite direction of the laser shoots a different laser in the same direction as the first laser, how does it work that this laser would leave the ship at C relative to the ship, and yet also travel at the same speed relative to Earth as the other laser? Would it?

Yes. Why does this work? It just does. Further explanation is mathematical, see special relativity.

Edit: Wait, I may have misread this one - It travels at c relative to everything. It doesn't matter what frame of reference you're in, you will measure the speed to be c relative to you. Hence the dilation of time.

If they were both pointed at the same target (ignoring the fact that the ship would have presumably had to pass through the target to achieve it's speed and trajectory) would they hit it at the same time?

If they were fired from the same distance, yes.

This is actually the very thing that causes time dilation.

Edited April 29, 2013 by Person012345

[Gniuz]

It came from a point in a way (just an ever increasing denseness that would have got to a point that it suddenly expanded) but the expansion we see today is moving in every direction and cannot be given a point. Also as time is the fourth dimension we work with and everything before the big bang is irrelevant to what happened afterwards, the second it was expanding that whole idea or picture we think of as the central point disappears.

I'm not a theoretical physicist but have read many books on the subject, its just human nature to think of an expansion coming from one single point. In truth I don't think anyone knows 100%. It was thought that the expansion should be slowing but in fact the opposite is happening.

I'm more curious about what are we expanding into?

G.

[Person012345]

And if the ship was traveling at c, it's equal to say that everything is moving past the ship at c. What this would mean is that there could not possibly be movement in any direction other than that movement in that one direction, because doing so would exceed c (or would cause things not to be moving at c past you). This means all outside things would be absolutely and totally stationary to you relative to each other, the light would never leave the earth, molecules and atoms could not vibrate, no movement could possibly be observed to occur apart from the linear c speed motion. However an observer on earth would still be observing their own atoms to vibrate and their life to carry on. The only conclusion is that time has stopped in that situation.

I think.

Edited April 29, 2013 by Person012345

[Stochasty]

Yeah, that doesn't really explain much. To someone at the target, would the lasers hit at the same time? To someone on Earth? To the people in the ship? Would someone on Earth see the ship travelling at near lightspeed in one direction and the laser travelling at lightspeed in the other? Or would one or both appear slower as to prevent the total difference being more than the speed of light?

As I said - depends on whose doing the measuring. You asked "what if the people on the ship fired a laser at the same time as the people on the Earth?" At the same time according to whom? To the people on the Earth or the people on the ship? If the former, then the Earth will see the two beams strike the target simultaneously. If the latter, then the people on the ship will see the same. Unless the ship is somehow passing directly through the Earth at the moment the laser is fired, the two are mutually exclusive, so it really does matter when I ask "who is doing the measuring." Regarding the people at the target, the answer depends on how the target is moving relative to both the Earth and the ship.

How do we have light coming or going from 2 directions simultaneously appearing to be at C?

Because that is the property of light (and other massless fields) in this Universe. Everyone everywhere will always measure light to be travelling at C. As for how it's possible - I can't explain that without going into the math. Suffice it to say that time dilation and length contraction conspire to make it work.

I'm not a theoretical physicist.

I am.

I'm more curious about what are we expanding into?

Nothing. Or, possibly, "into the Universe itself" if you'd prefer to think of it that way. Don't think of the expansion of the Universe like an explosion. Think of it like stretching a fabric - except there's no one at the edges pulling, because there is no edge - so far as we know.

[Holo]

The balloon metaphor is good on this. To an ant living on the surface of the balloon, there is no "central point" the balloon is expanding from.

[squeaky024]

Yeah the universe expanded from a singularity, but because that was the entire universe, you could say that the big bang was everywhere and everywhen, as it is still going

[Stochasty]

The universe expanded from a singularity.

Not true. This is a common misconception. There's no actual "bang" in the Big Bang theory; we don't know what happened to start the expansion. We only know that the Universe was in an inflationary phase, and that after some amount of growth settled down into the Universe we see today. The length of that inflationary phase (subject to certain constraints) and the extent of the Universe prior to it are unknown.

[qeveren]

Couldn't you say that the direction that points toward the center of the expansion of the universe is called "the past"? Ie. if the stretching of the skin of the balloon is the metric expansion of space, then the increase in radius of the balloon is the progression of time.

[Person012345]

Couldn't you say that the direction that points toward the center of the expansion of the universe is called "the past"? Ie. if the stretching of the skin of the balloon is the metric expansion of space, then the increase in radius of the balloon is the progression of time.

Perhaps but that's not what most people mean when they say "the centre of the universe".

[ROFLCopter64bit]

First one is true. Everything is relative.

First one contradicts the second, and vice-versa.

Third one is unknown. We just need to sit and wait.

[Stochasty]

That's a pretty sophisticated way of looking at things, qeveren, - it's known as the cosmological arrow of time - but saying that the direction of the center is "the past" doesn't really work. You should remember that the balloon analogy is just an analogy; it's not a representation of the actual state of affairs.

"Center" implies that there is a single point from which the expansion originated; this isn't the case. If you trace back the world lines of points in the Universe, they don't converge to point within the timeframe of which we have knowledge. Even if you extrapolate back inflation into the past beyond the realm for which we have evidence, the metric becomes singular only in the infinite past - and there is no reason to expect that our understanding of the laws of physics holds in that regime. Furthermore, to our best measurements, the global curvature of the Universe is consistent with the Universe being spatially flat, which means there's no reason to suspect that the Universe is closed or finite, which would mean that the physical size of the Universe doesn't change no matter how you stretch or compress it.

[WestAir]

That's a pretty sophisticated way of looking at things, qeveren, - it's known as the cosmological arrow of time - but saying that the direction of the center is "the past" doesn't really work. You should remember that the balloon analogy is just an analogy; it's not a representation of the actual state of affairs.

"Center" implies that there is a single point from which the expansion originated; this isn't the case. If you trace back the world lines of points in the Universe, they don't converge to point within the timeframe of which we have knowledge. Even if you extrapolate back inflation into the past beyond the realm for which we have evidence, the metric becomes singular only in the infinite past - and there is no reason to expect that our understanding of the laws of physics holds in that regime. Furthermore, to our best measurements, the global curvature of the Universe is consistent with the Universe being spatially flat, which means there's no reason to suspect that the Universe is closed or finite, which would mean that the physical size of the Universe doesn't change no matter how you stretch or compress it.

So the expansion of space is forcing the Universe to literally "expand" into... itself? As in, an infinitely sized Universe is expanding to become even more infinite? That no matter how far back in time we look, up to but not including infinity, the "infant" Universe a quark time after the Big Bang is still infinite in physical size?

[Person012345]

So the expansion of space is forcing the Universe to literally "expand" into... itself? As in, an infinitely sized Universe is expanding to become even more infinite? That no matter how far back in time we look, up to but not including infinity, the "infant" Universe a quark time after the Big Bang is still infinite in physical size?

*planck time.

Infinity is complicated. Really infinity makes no sense and it's stupid that any real thing exists with an infinite value, but it does. The only way to really wrap your head around it is mathematically, brains aren't really built to understand infinite things in a physical way.

[Person012345]

Although even mathematically I think we have a fair bit of trouble understanding infinities at all.

Whoever programmed the universe left a lot of bugs in. That's my theory.

[Holo]

So the expansion of space is forcing the Universe to literally "expand" into... itself? As in, an infinitely sized Universe is expanding to become even more infinite? That no matter how far back in time we look, up to but not including infinity, the "infant" Universe a quark time after the Big Bang is still infinite in physical size?

I know nothing about this, but I think the universe is finite but curved, so you'll eventually end back up where you started if you travel far enough. So the universe goes from being finite to being finite but larger.

[comicbstudeo]

essentially it is mpossible for something to be stationary as it is all relative. if i am in a space ship in orbit it will look like the earth is moving not me

[Person012345]

I know nothing about this, but I think the universe is finite but curved, so you'll eventually end back up where you started if you travel far enough. So the universe goes from being finite to being finite but larger.

Our observations would indicate that it's flat and infinite in spacial extent.

[Nikolai]

essentially it is mpossible for something to be stationary as it is all relative. if i am in a space ship in orbit it will look like the earth is moving not me

You're right in basic principle, but not in this particular example. Being in orbit around something involves an acceleration, and accelerations are not symmetrical.

[Stochasty]

So the expansion of space is forcing the Universe to literally "expand" into... itself? As in, an infinitely sized Universe is expanding to become even more infinite? That no matter how far back in time we look, up to but not including infinity, the "infant" Universe a quark time after the Big Bang is still infinite in physical size?

Yep. This is pretty much the case. Infinity times any non-zero finite constant is infinity, so no matter how much you stretch or compress the Universe it's still infinite. When cosmologists talk about the size of the Universe, they talk about the scale factor of a given epoch compared to the scale factor today - that is, the ratio of the distance between two points today compared to that distance in the other epoch - rather than trying to actually discuss the physical size of the Universe (which, even if the Universe happens to be closed and finite, is unknown and really, really big).

I don't know of any cosmologists who believe that the Universe really is infinite in size, but it's big enough that it looks infinite to us so far as we have been able to measure. To get a handle on how big this is, imagine standing on another planet and trying to measure its radius by measuring how quickly objects of known size sink below the horizon as they move away from you relative to their apparent angular size - except that when you try this, you find that objects never sink below the horizon at all, no matter how far away they get (ie., the horizon is literally the vanishing point). That's basically the state that we're in with our measurements regarding the global curvature of spacetime.

[P-Square]

As the universe continues to expand it will and is creating new stars, planets and solar systems therefore the end is only the beginning.

I was thinking earlier today and I thought, how do you define something as stationary? For example, when your car is parked; your speedometer reads 0 kph/mph. But actually because the earth is rotating, technically you're not stationary. Even the earth isin't stationary since it rotates and it also revolves around the sun. And if the big bang theory is true, the sun isin't stationary either since the force from the big bang is propelling all of existence outwards from the original point where the big bang occurred.

Which also leads me to another point, the only fixed point in space is the center of the universe, since everything else is moving.

And it lead me again to another point. If all of existence is moving away from each other gradually, that would mean in the FAR future you will not be able to determine if time is actually passing. Since you can determine time when events are happening. And because everything is moving outwards, all the matter in existence will be extremely spread apart that you won't be able to see anything but the black sea of space. You can't see any stars because they would be SO far that the number of lightyears to determine how far they are from you will be incomprehensible by any type of artificial or living intelligence. Therefore no observable events are taking place.

well, my philosophical side got the best of me. lol. Anyways, what do you think? Also, please try to prove me wrong.