**Ask a monkey a physics question thread**

[OngBonga]

I would have answers which correspond with my idea of the universe. I would say that a spaceship that has infinite time and infinite fuel travelling in a straight line will eventually reach the same point in space again, provided in can stand the pressure of the big crunch - big bang in the centre of the universe, which I doubt.

[MadMojoMonkey]

where most of my confusion arises is how the ant on the balloon can travel in a constant direction and reach a spot he was before while a man in a magic-fast spaceship (presumably) cannot.

Don't be so positive about that spaceship. Space contraction and time dilation could give him a serious advantage.

As you move closer and closer to the speed of light, distances in your direction of travel are contracted. The universe becomes pancaked in your direction of travel. Also, clocks outside your ship tick more slowly. So the rate of the universe's expansion would be slowed as well.

Now to create a ship that can hold a constant acceleration indefinitely and see if it can accelerate enough to overcome the universe's accelerating expansion.

unless the balloon is constantly inflating. then the ant never reaches the same point twice.

that makes sense, but i guess im confused on how that doesn't make the balloon surface infinite.

How can that be confusing? It's just a balloon that is inflating. Still finite at any finite future time.
(We must assume this balloon is made of some material with infinite stretchiness and infinitesimal thickness.)

i mean, if there is always a new point in spacetime, that means there are infinite points in spacetime, which means spacetime is infinite right?

There are always more numbers between 0 and 1 than can ever be listed, therefore the distance between 0 and 1 is infinite, right?

No.

[wufwugy]

Don't be so positive about that spaceship. Space contraction and time dilation could give him a serious advantage.

As you move closer and closer to the speed of light, distances in your direction of travel are contracted. The universe becomes pancaked in your direction of travel. Also, clocks outside your ship tick more slowly. So the rate of the universe's expansion would be slowed as well.

Now to create a ship that can hold a constant acceleration indefinitely and see if it can accelerate enough to overcome the universe's accelerating expansion.

it's not reasonable to think that something with infinite speed and 3d travel facilities can't reach every point in space at any given time?

How can that be confusing? It's just a balloon that is inflating. Still finite at any finite future time.
(We must assume this balloon is made of some material with infinite stretchiness and infinitesimal thickness.)

so for something to be infinite, it has to be infinite at any one point in time?

There are always more numbers between 0 and 1 than can ever be listed, therefore the distance between 0 and 1 is infinite, right?

it's intuitive. what i think i dont understand is this: there are an infinite many numbers between 0 and 1. this suggests to me that something that travels to each of those numbers can never reach them all. from his perspective, he is traveling infinitely and reaching new places. if so, why can't we say that the distance he is traveling is infinite?

[MadMojoMonkey]

it's not reasonable to think that something with infinite speed and 3d travel facilities can't reach every point in space at any given time?

I don't see how any macroscopic thing can be in all the places at one time. Even with particles, we're talking about their wave function being non-zero over any volume, but it's so damn near to zero over most of the universe. "Most" being an understatement of universal proportions. The probability of finding an electron which is bound to an atom further than 1 nm from that nucleus is nearly infinitesimal. It drops off exponentially. While it is non-zero everywhere, it is ridiculously small even a few atomic radii in distance from the parent nucleus.

so for something to be infinite, it has to be infinite at any one point in time?

I'm saying the balloon is finite in surface area at the beginning of our little thought experiment. Its rate of expansion is always finite. Therefore, it can only be of finite surface area after any finite amount of time passes.

I would say that the only way this balloon can have infinite surface area is if an infinite amount of time passes. When I use infinite here, I mean the non-number version if infinity which is a concept of unendingness.

I'm basically calling BS on your notion that the balloon is infinite at any "real" time, but not on your notion that the idea of infinity is not completely discarded from this balloon's state in some arbitrarily distant future.

it's intuitive. what i think i dont understand is this: there are an infinite many numbers between 0 and 1. this suggests to me that something that travels to each of those numbers can never reach them all. from his perspective, he is traveling infinitely and reaching new places. if so, why can't we say that the distance he is traveling is infinite?

... but the distance between the numbers is infinitesimal.

So, sure, you can say he's traveled infinite distance, but does it hold significant meaning?

There are many uses of infinity. Even mathematically, we acknowledge that not all infinities are equal. Sometimes we treat infinity like a number, especially when doing integrals, but there are so many other examples.

The various uses of infinities are appropriate in their own contexts.

[MadMojoMonkey]

I would have answers which correspond with my idea of the universe. I would say that a spaceship that has infinite time and infinite fuel travelling in a straight line will eventually reach the same point in space again

I don't see any good evidence for or against this. It's not like we can point a telescope in some direction and see the Earth really faint in the distance.

Yet.

Figure out how to test this and earn yourself a Nobel Prize.

provided in can stand the pressure of the big crunch - big bang in the centre of the universe, which I doubt.

Not the center of the universe. The whole universe.

The story goes that the universe has stopped expanding and then contracts again. It's not moving back through itself; it's contracting.

When the Big Crunch happens, it will happen everywhere, all at once. Just like the Big Bang. It didn't happen at the "center" so much as it WAS the universe. The entire universe was the entire Big Bang.

The Big Bang isn't something that happened to the universe any more than being a toddler is something that happened to you.

(Bear with me on this one, despite it's flaws, I think you'll like it
Where did your toddler take place? Was it in your head? Your belly? No. It was happening everywhere, to all of you. Did you expand into something else as you got bigger? No, you expanded to a bigger you.

(Obv. this is a very loose metaphor, please don't take it any further than this.)

[OngBonga]

Figure out how to test this and earn yourself a Nobel Prize.

I would get a great deal more pleasure turning down a nobel prize than accepting one. Look, I don't even use a capital letter for nobel. Fuck you alfred.

I have no idea really. I mean the torus is such a great model, and corresponds perfectly with our ideas of conservation of energy, and equilibrium... it's so perfect. Well, nearly. The expansion of the universe appears to be increasing, which I can't get my head around in any model really. There must be a reason for this, and I think it's geometry.