stupid physics questions

[Istar]

It is known how many quarks make-up the subatomic particles; ergo, the largest calculable number must be at least the total number of quarks, not atoms. However, since the smallest measurable distance at the quantum level is the Planck Length, then the largest calculable number must, at least, be the number of Planck Lengths in the Universe.

However, the Universe is expanding. Thus, the number of Planck Lengths is increasing, and therefore this number is at best indeterminate, and possibly infinite. Hawking and Du Sauvoy both do good treatments of the mathematics involved in calculations involving infinities, and indeed a goodly chunk of higher mathematics requires that infinity be both infinite and a quantifiable.

The cardinality of infinity is inherently a paradox, but can be expressed as a concept and used to good effect.

[Dio]

Are you assuming the Planck length isn't expanding along with the universe?

[Darkflame]

If your smallest unit expands with your biggest thing - could you tell an increase at all?
Would increasing in size be then the effective same thing as C becoming slower? could you tell the difference?
[/very vague thinking]

[pogo]

Except it would take an infinite time for the universe to grow to infinite size. In other words it will never be infinitely big. No matter how long you wait it will never happen.

Infinity is an odd thing. It can be treated as a number but you have to be careful with it because it doesn't behave as other numbers. E.g. I would say it's obvious that

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1 + ∞ = ∞

and

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1 * ∞ = ∞

But then what is

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∞ – ∞

?

The answer could be one, or zero, or even infinity.

In maths infinity often arises but normally it does not appear in the final answer, or at least not in a problem where the answer is a real number such as 'the size of the universe'

[guythp]

If you were to assume that you had a method to get water to boil in the outer atmosphere and in space, and assuming you had a fuel source that... basically, I wondered if a steam rocket (if that's the right word) would work in space. In that I mean steam forced out of an outlet similarly to a rockets exhaust would have a similar propulsive effect.

I'm not the sharpest spoon in the cutlery drawer, but the thought got me wondering. If anyone with any knowledge could confirm this theoretical question, I'd be curious to find out.

[Dio]

Yes, it would. It's just every action has an equal and opposite reaction. If you throw a hammer away, you move too. Rockets are just throwing exhaust away. After all what's a hydrogen/oxygen rocket, but a jet of steam?

[pogo]

Yes, steam would work. And in theory storing it as water then boiling it would give you a lot of thrust for your stored mass. You'd still need a source of energy to boil it though, and what better than a source of energy, such as Hydrogen + Oxygen, which also generates your end product, and does so explosively so to provide maximum thrust.

The other problem with converting water to steam is it takes a lot of energy to do so. You know this if you've ever boiled some water then left it to boil dry. It takes much longer, and so much more energy, to turn it all into steam (at 100°C) than it does to boil it from 0°C to 100°C.

[guythp]

Thank you very much for clearing that one up for me! I think I need to understand the whole propulsion thing (as in; a propeller wouldn't work in space, but a rocket would) a bit more. I suppose it's more energy based. Possibly. I think. :/

[zip accident]

I think I need to understand the whole propulsion thing (as in; a propeller wouldn't work in space, but a rocket would) a bit more.

Propulsion is based on making something go one way so you go the opposite way. With a propeller it's air, and there isn't any in space, for the same reason a jet which also throws air backwards won't work in space, but with a rocket it's the burnt fuel that goes backward, and since you take the fuel with you, that still works in space.

[tufty]

and since you take the fuel with you, that still works in space.

<pedant>It's actually the *oxidiser* you're taking with you. You take the fuel with you in any case</pedant>

[zip accident]

and since you take the fuel with you, that still works in space.

<pedant>It's actually the *oxidiser* you're taking with you. You take the fuel with you in any case</pedant>

True, but if you took oxygen with you to run a propeller motor or a jet, since the air isn't there you wouldn't get much propulsion (though the exhaust would act as a weak rocket).

[Absolute_Zero]

You don't need a heat source to boil water in space... it will boil due to the vacuum and then freeze.

[edit]
http://scienceblogs.com/startswithabang ... t-happens/

[pogo]

Not much use though. Water will vaporise in space as there's no pressure. But with no pressure you get no force to drive you forward. If you try and e.g. contain the water vapour to build up pressure enough to provide a motive force then pressurised it remains liquid. So you also need to heat it to generate pressurised steam to drive yourself forward. You don't need to worry about freezing while doing this: once it's released as steam it will freeze but it's pretty easy to keep it warm up to that point: vacuum is an excellent insulator.

[venusian]

So on that blog's comment somebody asked what happens to the (hot) water's energy when it sublimates.

I've got a couple of idea: it's embedded in the bonds of the resultant crystal structures; that it's transformed into kinetic energy as the substance spreads out; there's some kind of non-visible radiation. None of these seem very convincing or rational to me. So where does it go?

[Dio]

That's the whole point of evaporation.

You have N molecules moving with a temperature (average kinetic energy, very roughly) of T. A molecule escaping from the surface is statistically likely to be one of the ones moving faster. Therefore, the average kinetic energy of the remaining molecules is lower and T drops.