Ask a physicist anything. (3)

[Wiccan_Child]

Does that mean that the temp required to freeze water would be different at sea level compared to Mt. Everest due to air pressure differences?

Yep! Water boils at 68°C on Mt. Everest, rather than 100°C as at sea level.

 

[Chesterton]

Yep! Water boils at 68°C on Mt. Everest, rather than 100°C as at sea level.

But he asked about freezing, not boiling. Same principle?

 

[Wiccan_Child]

But he asked about freezing, not boiling. Same principle?

Same principle. The temperature at which a substance goes from gas to liquid, gas to solid, liquid to solid, liquid to gas, solid to gas, solid to liquid, depends on the pressure. The temperature at which water goes from liquid to solid is lower - with less air pressing down on it, the water can more readily be in its liquid state, thus requires less heat to stay liquid.

 

[Chesterton]

At what temperatures does water freeze on Mt. Everest and at sea level?

 

[Wiccan_Child]

At what temperatures does water freeze on Mt. Everest and at sea level?

At sea level, water freezes when it goes below 0°C, by definition. On the tip of Mt. Everest, it freezes when it goes below about -1°C.

 

[Frumious Bandersnatch]

At sea level, water freezes when it goes below 0°C, by definition. On the tip of Mt. Everest, it freezes when it goes below about -1°C.

Actually I think it freezes at about 1 C on Mt Everest. Water is one of the few (maybe the only?) substances whose melting point decreases with external pressure because the solid is less dense than the liquid.

Added in Edit: Below the triple point the sublimation temerature decreases with temperature but that is at 4.58 torr well below the air pressure on the summit of Mt. Everest.

Another Edit, since the triple point of water is only 0.0098 C I will change my statement above to say from looking at the phase diagram that the freezing point on the summit of Mt. Everest will be between 0 and 0.0098 C but I don't know exactly where.

 

[Soul Searcher]

Interesting .. So there is a large difference in the boiling point but only a minor difference in the freezing point? I would not have expected that.

 

[Steffenfield]

So to just understand this correctly....

I can make ice without changing any outside temperature just by applying a tremendous amount of pressure on say a cup of water?

 

[Wiccan_Child]

So to just understand this correctly....

I can make ice without changing any outside temperature just by applying a tremendous amount of pressure on say a cup of water?

Essentially yes. At 20°C, you need pressure of 10[sup]7[/sup] Pa - that's 100 times atmospheric pressure at sea level, so good luck squeezing that with your hands

 

[Supreme]

My question is this: what would happen if a human being were to go into a Black Hole?

 

[Wiccan_Child]

My question is this: what would happen if a human being were to go into a Black Hole?

'Spaghettification' is the operative term Tidal forces stretch you, as your feet experience much stronger gravity than your head (assuming you go in feet first), and you get stretched out. This will almost certainly kill you!

That said, if the black hole is enormous, it's actually a shallower gradient, so you could, in principle, survive the descent. I wouldn't like to know what you'd find in there, though...

 

[Gracchus]

'Spaghettification' is the operative term Tidal forces stretch you, as your feet experience much stronger gravity than your head (assuming you go in feet first), and you get stretched out. This will almost certainly kill you!

But wouldn't you contract in the direction of motion? Wouldn't that shortening offset the tidal forces?
And as you accelerated toward the hole wouldn't you become more "wave-like", a la DeBroglie? What effect would that have on tidal forces?
And when a black hole "eats" it becomes a quasar, so wouldn't your mass be converted to energy? Does anything actually enter a black hole after it forms? Does the mass that enters a black hole manifest as energy on this side of the horizon? Could it be like e^ix = cos(x) + i*sin(x), where cos(x) is the mass on this side and sin(x) is the energy on this side, and there really is no other side?
We already know that we look out, in all directions, at a smaller past universe. So why couldn't the universe be a black hole that is inside out?

"Inquiring minds want to know!"

 

[Supreme]

'Spaghettification' is the operative term Tidal forces stretch you, as your feet experience much stronger gravity than your head (assuming you go in feet first), and you get stretched out. This will almost certainly kill you!

That said, if the black hole is enormous, it's actually a shallower gradient, so you could, in principle, survive the descent. I wouldn't like to know what you'd find in there, though...

Thanks!

 

[Wiccan_Child]

But wouldn't you contract in the direction of motion? Wouldn't that shortening offset the tidal forces?

You'd stretch out, because the force pulling your feet would be greater than the force pulling your head in. Your feet would accelerate towards the singularity faster than your head, so your entire body would stretch out.

And as you accelerated toward the hole wouldn't you become more "wave-like", a la DeBroglie? What effect would that have on tidal forces?

Meh, I'm not a big fan of waves Particles, all the way down!

And when a black hole "eats" it becomes a quasar, so wouldn't your mass be converted to energy?

Maybe, if you get caught in the accretion disk and don't actually hit the singularity.

Does anything actually enter a black hole after it forms?

Sure. It's a big, black vacuum! Things enter a black hole just as easily as asteroids hit Jupiter.

Does the mass that enters a black hole manifest as energy on this side of the horizon?

It manifests as a bigger black hole. Black holes have only three properties: mass, spin, and charge. So whatever falls into a black hole can only affect those three properties (and the various phenomena that come from them).

Could it be like e^ix = cos(x) + i*sin(x), where cos(x) is the mass on this side and sin(x) is the energy on this side, and there really is no other side?
We already know that we look out, in all directions, at a smaller past universe. So why couldn't the universe be a black hole that is inside out?

Because... an 'inside out' black hole doesn't make much sense. The universe could be a black hole.

There's a delicious idea I once read. Basically, black holes create universes within them, and the properties of the universe are derived from the previous universe, and the black hole. So, in a cosmic version of natural selection, the multiverse becomes disproportionately filled with black hole spawning universes: the majority of universes are going to be those that just so happen to be good at making black holes, since they themselves come from a black-hole-forming universe.

This is one possible way for a multiverse to exist, and it explains away any 'fine tuning' arguments people might have. Fun!

 

[Assyrian]

I came across a calculation once that if you had a black hole the mass of the universe the event horizon would have a radius of about 13.7 billion light years

 

[Wiccan_Child]

I came across a calculation once that if you had a black hole the mass of the universe the event horizon would have a radius of about 13.7 billion light years

That's suspiciously close to the age of the universe...

Mass of the universe M = 10[sup]55[/sup] kg
Schwarzschild radius r = 1.48 x 10[sup]-27[/sup] m/kg x M = 1.48 x 10[sup]28[/sup] m

So the event horizon of that black hole would have a radius of 1.56 trillion light years

 

[Assyrian]

Is that including dark matter? What made me suspicious was that it fits the observable universe now, but it would not have in the past in an expanding universe, though the mass of the universe would have been the same. Hey it's a beautiful hypothesis lets not spoil it with some inconvenient facts.

 

[Wiccan_Child]

Is that including dark matter? What made me suspicious was that it fits the observable universe now, but it would not have in the past in an expanding universe, though the mass of the universe would have been the same. Hey it's a beautiful hypothesis lets not spoil it with some inconvenient facts.

Well, the mass of the universe is hard to calculate, with estimates ranging by ten orders of magnitude. If dark matter isn't counted in those calculations, well, that just means the radius is about two orders of magnitude bigger again!

 

[Assyrian]

Isn't the Schwarzschild radius proportional to mass?

 

[Wiccan_Child]

Isn't the Schwarzschild radius proportional to mass?

Aye. And since Dark Matter is in total 100 times heavier than normal matter, it's 100 times the Schwarzschild radius.

EDIT: Woops, I tell a lie, dark matter is only 5 times as heavy as normal matter. Though if dark energy counts, it's 20 times heavier.