stupid physics questions

[Dio]

It would not be suprising at all if sometime we'll discover something else that will turn all into "yes all we knew so far was kinda correct but was a special case of and/or there are special cases where this do not apply".

Well, that's a pretty accurate description of Newtonian / Keplerian orbital motion vs. relativity, with the poster case being relativity's description of the precession of the orbit of Mercury.

[Darkflame]

If alternative models fit the observations just as well as relativity, they should be considered with equal weight untill new evidence comes in.
Thats a big if though.

[gumboot]

Is it, then, a necessity that all matter start out in the same place so that it has the same (relative) mass added to it by the process that established its current velocity? Or is mass relative as well?

What if you fly back towards the centre of the big bang and weigh a bunch of stuff as you pass by? Will things start getting lighter? Or will you (and your measurment devices) get heavier as you do more moving around even if it's back the way you came?

What are the options for measuring mass, anyway? Some devices are inherently relative (balance scales), and others rely on a local gravitational field and a bit of elastic and some arithmetic.... I'm not sure you could make any clear determination about mass of something you find in space if you've done a lot of acceleration to get there.

[gumboot]

When, in fact, some combinations had to come up,

Why?

[Absolute_Zero]

Mass, length, time and kinetic energy are all relative.

What if you fly back towards the centre of the big bang [...]

According to conventional wisdom, you're already there, everywhere is. Space itself inflated, rather than the bang expanding out into space.

[gumboot]

Mass, length, time and kinetic energy are all relative.

This is what I suspected, but I wasn't going to try to think up test cases until somebody asserted it.

What if you fly back towards the centre of the big bang [...]

According to conventional wisdom, you're already there, everywhere is. Space itself inflated, rather than the bang expanding out into space.

So no momentum has been imparted onto any of this matter which appears to be travelling apart?

[Darkflame]

Well, depends if the universe is curved no?
Space could still expand on a flat area, and you'd at least get a center of mass...I think.
If its more like the bubble idea, then you'd just wrap around and there would be no center.

When, in fact, some combinations had to come up,

Why?

If your saying why do we exist at all, thats quite a different question to saying "why do we exist with these precise constants".
My anaology is simply that if a lottery draw takes place at then some combinations of numbers come out of it.
If a universe is created with constants then there doesnt have to be any reason for these specific constants.

Thats not the same as saying any particular constant value had to exist. Maybe a universe could exist without a light speed limit. Or maybe one that changed daily. Or a limit on relative acceleration but not top speed. Theres many imaginable things that could have happened....and many more we probably couldn't imagine.
But just because it turned out to have a light speed limit, doesn't mean there has to be a reason behind the precise value it has.

In fact, given we are only a small species evolved on one world observing one universe with one set of rules, I think its quite arrogant to say our constants are the only way it could have happened. Its simply an unknowable. We could point to changes that wouldn't have worked for us, but beyond that there isn't much we can say.
Run the universe again a thousand times over you might get a thousand different values out.

[zip accident]

Run the universe again a thousand times over you might get a thousand different values out.

I agree that you might, I suspect you also agree that there's no knowing whether we would get the same answer each time, or some more limited number than 1000 repeating at random, or even not at random.

[disciple]

So when we say no physical object can exceed the speed of light, what is this relative to? The universe has no stationary fixed point against which to measure does it?

[Absolute_Zero]

So when we say no physical object can exceed the speed of light,

Entities of nonzero rest mass cannot be accelerated to the speed of light.... would be more precise.

what is this relative to?

The speed of light as observed by any observer under any circumstances... fancifully contrived [ala thought experiment] or otherwise.

The universe has no stationary fixed point against which to measure does it?

I've struggled with this and given up... I'll simply say 'no'.

[Absolute_Zero]

So no momentum has been imparted onto any of this matter which appears to be travelling apart?

I'll leave that to someone with greater faith in the Big Bang model of what's supposed to have occurred over the preceding 13.7 billion years.

In the here-and-now, that which is red-shifting away, appears to be accelerating. A herd of elephants have taken residence in the front parlour.

[lazerwalker]

So when we say no physical object can exceed the speed of light, what is this relative to? The universe has no stationary fixed point against which to measure does it?

It's relative to *anybody*. If I'm stationary (relative to the Earth, for concreteness) and you're on a fast train, and we both measure the speed of light, we'll both get the same answer, despite the fact we're moving relative to each other.

In a similar vein, velocities don't add up in the same way when they're a significant fraction of light speed. If I'm travelling south towards you at half the speed of light (relative to the Earth again), and you're travelling north at half the speed of light, naively you'd think I'd be travelling at light speed, relative to you, however if you measured my speed I'd only be travelling at 4/5ths light speed relative to you. It's all a bit strange.

[disciple]

So when we say no physical object can exceed the speed of light, what is this relative to? The universe has no stationary fixed point against which to measure does it?

It's relative to *anybody*. If I'm stationary (relative to the Earth, for concreteness) and you're on a fast train, and we both measure the speed of light, we'll both get the same answer, despite the fact we're moving relative to each other.

Aye, which was why I was asking about physical bodies.

In a similar vein, velocities don't add up in the same way when they're a significant fraction of light speed. If I'm travelling south towards you at half the speed of light (relative to the Earth again), and you're travelling north at half the speed of light, naively you'd think I'd be travelling at light speed, relative to you, however if you measured my speed I'd only be travelling at 4/5ths light speed relative to you. It's all a bit strange.

You're not joking.

So no object can exceed the speed of light relative to any other, and when newtonian physics would indicate this is happening... well it's not really! And from what I (think I) understand so far it's because newtonian physics assumes time is a constant, which it ain't due to something or other which occurs at extreme velocities. Or something.

[pogo]

Newtonian physics is a good approximation [to relativity] at low speeds, and for a long time that's all we knew, i.e. we could not do anything to disprove Newton. The motivation for relativity was mostly to do with non-physical things like light and electro-magnetism. It just happened to have a profound effect on our understanding of dynamics too. Only after relativity did people do experiments to confirm it's effects on dynamics and so disprove Newton. Now of course we fire particles around accelerators at near light speeds almost routinely, and allow for relativistic effects in satellites, but we could not do this 100 years ago.

[Absolute_Zero]

So no object can exceed the speed of light

Sorry for the repetitious pedantry but it's quite important, no object can reach the speed of light. Hardly anyone reasonably or theoretically thinks so, but tachyons might exist.

Also to note with regard to space travel, there may be ways of effectually sidestepping the light speed issue.
http://en.wikipedia.org/wiki/Alcubierre_drive