gravity and cooling

[lemur]

Imagine a large cloud of hot gas. If parts of the cloud cool faster than others, those parts should densify relative to their surroundings, correct? Then, if there is no external border constraining the cloud, it could expand freely due to its own heat/pressure, right? So if the condensing parts of the cloud expel more heat due to friction among the particles, couldn't this cause the surrounding gas to warm up and expand more while the condensed particles grow colder and form a heat-sink for surrounding (warmer) particles? In this way, couldn't a body of matter form from the growing disparity between condensation and expanding surroundings and eventually condense to fusion-pressure with sufficient mass? If so, why would gravity have to be something more than a tendency for matter to condense as it loses heat?

[lemur]

Is this thread not even worth responding to? I thought it was an interesting concept.

[imatfaal]

I think - and I really don't know for sure - that temperature in a cloud of gas as you describe will tend to an equilibrium of homogeneity. But this cannot be universal otherwise no universe.

"So if the condensing parts of the cloud expel more heat due to friction among the particles"

I think what would happen is that heat would flow from the hot to the cold rather than the cold getting colder.

"If so, why would gravity have to be something more than a tendency for matter to condense as it loses heat? "

because it acts on two bodies without reference to their heat loss.

 

There is bound to be some critical separation after which two areas of a gas cloud being to act as distinct entities. The search for anisotropic remains of the early universe is an important one - our models move from an expanding but smooth to a lumpy universe, and it seems these might well be the result of quantum fluctuations magnified by the inflationary phase. With greater resolution we can now observe, use, quantify anisotropies in the CMBR.

[swansont]

If so, why would gravity have to be something more than a tendency for matter to condense as it loses heat?

 

Do things actually condense in a way that mimics gravity, in the absence of gravity?

[lemur]

I think - and I really don't know for sure - that temperature in a cloud of gas as you describe will tend to an equilibrium of homogeneity. But this cannot be universal otherwise no universe.

I think what would happen is that heat would flow from the hot to the cold rather than the cold getting colder.

because it acts on two bodies without reference to their heat loss.

That's the way I would look at it in a relatively small cloud, but if you were dealing with a much larger gas-system, I wonder if very large pockets begin condensing, if the heat can convect as easily to surrounding areas that are warmer to start with. The ironic aspect of this scenario, which I'm questioning even as I think about it, is whether gas could condense due to relative cooling while at the same time generating heat due to friction caused by the condensation.

There is bound to be some critical separation after which two areas of a gas cloud being to act as distinct entities. The search for anisotropic remains of the early universe is an important one - our models move from an expanding but smooth to a lumpy universe, and it seems these might well be the result of quantum fluctuations magnified by the inflationary phase. With greater resolution we can now observe, use, quantify anisotropies in the CMBR.

Thanks for this info though I'm not sure what I could really do with it.

 

Do things actually condense in a way that mimics gravity, in the absence of gravity?

The part I wonder about is what would prevent the cloud from dissipating completely if there was no force gravity to hold it together. Assuming electrostatics were sufficient, then I can imagine the condensation generating heat that is not sufficient to prevent further condensation and thus it radiates out into the surrounding gas. I'm relying on the idea that energy in a gas cloud dissipates among colliding particles so if condensation is usurping the ability of particles to expand away from each other, they would contract due to friction among the particles.

 

 

[swansont]

The part I wonder about is what would prevent the cloud from dissipating completely if there was no force gravity to hold it together. Assuming electrostatics were sufficient, then I can imagine the condensation generating heat that is not sufficient to prevent further condensation and thus it radiates out into the surrounding gas. I'm relying on the idea that energy in a gas cloud dissipates among colliding particles so if condensation is usurping the ability of particles to expand away from each other, they would contract due to friction among the particles.

 

Without gravity you would have only diffusion. Why would there be condensation without it?

[lemur]

Without gravity you would have only diffusion. Why would there be condensation without it?

Say there's a giant expanding cloud with a certain amount of energy. That energy would be dissipating as kinetic heat, volumetric expansion, radiation, and also friction among various currents within the cloud, right? So is it possible for the cloud to continue expanding indefinitely while its energy is being dissipated in other forms than expansion? If the particles were freely in motion, unobstructed, in a vacuum I would say yes. But since there is friction and energy-loss via radiation, I think no. Therefore, as warmer areas of the cloud expand more, I would think they would exert more energy (pressure) on the cooler areas. Thus, as the cooler areas condensed more, they additional heat generated by friction could radiate out into the hotter areas causing increasing temperature/density differential. What's confusing me now is that I would expect entropy of heat from the warmer parts to the cooler ones, but if the pressure from outside the cool area was great, I don't see why the condensation couldn't squeeze heat out of the compressing gas thus perpetuating the condensation process.

[swansont]

Say there's a giant expanding cloud with a certain amount of energy. That energy would be dissipating as kinetic heat, volumetric expansion, radiation, and also friction among various currents within the cloud, right? So is it possible for the cloud to continue expanding indefinitely while its energy is being dissipated in other forms than expansion? If the particles were freely in motion, unobstructed, in a vacuum I would say yes. But since there is friction and energy-loss via radiation, I think no. Therefore, as warmer areas of the cloud expand more, I would think they would exert more energy (pressure) on the cooler areas. Thus, as the cooler areas condensed more, they additional heat generated by friction could radiate out into the hotter areas causing increasing temperature/density differential. What's confusing me now is that I would expect entropy of heat from the warmer parts to the cooler ones, but if the pressure from outside the cool area was great, I don't see why the condensation couldn't squeeze heat out of the compressing gas thus perpetuating the condensation process.

 

Why would there be condensation? You've introduced it into your scenario, but without gravity, why is it there? There's no mechanism for it.

 

Generated heat will not cause the hot areas to heat up at the expense of cooler areas; that violates the second law of thermodynamics. Heat flows spontaneously from hot to cold, making temperature differentials decrease. They will not spontaneously increase.

[lemur]

Why would there be condensation? You've introduced it into your scenario, but without gravity, why is it there? There's no mechanism for it.

 

Generated heat will not cause the hot areas to heat up at the expense of cooler areas; that violates the second law of thermodynamics. Heat flows spontaneously from hot to cold, making temperature differentials decrease. They will not spontaneously increase.

Why couldn't the collective force of the expanding hot areas be sufficient to cause compression of the cooling areas? The entropy of heat is what I'm struggling with - i.e. if there is some way the heat could radiate away from the cooling/condensing area into the expanding hot surroundings. Maybe this could be facilitated by the inertia of the expansion as a whole, i.e. creating a vacuum force in the center as the outer portions of the cloud collectively move away from the center?

 

edit: upon re-reading this, the creation of a vacuum in the center due to expansion contradicts the idea that pressure could be the cause for compression in the center. I wonder if it would make a difference if the particles of the cloud were subject to some other strong attractive force, such as nuclear force, i.e. if the amount of heat-energy was so high that the particles were all completely ionized.

Edited May 20, 2011 by lemur

[swansont]

More heat always flows away from the hot areas to the cold than in the opposite direction.

[lemur]

More heat always flows away from the hot areas to the cold than in the opposite direction.

True, but what about in a rapidly expanding system? Doesn't inertia of expansion have some effect on centripetal heat transfer? I.e. couldn't the expanding outer part of a cloud have such high pressure that it could be driving both the expansion of the outer edge as well as compressing the inner core?

[swansont]

True, but what about in a rapidly expanding system? Doesn't inertia of expansion have some effect on centripetal heat transfer? I.e. couldn't the expanding outer part of a cloud have such high pressure that it could be driving both the expansion of the outer edge as well as compressing the inner core?

 

No. If it's a spontaneous system, more heat always flows away from the hot areas to the cold than in the opposite direction.

 

You have changed condensation to compression, and those two terms are not synonyms, and now have a specific scenario of a low-pressure interior and high-pressure exterior, without explaining how you got that.

[lemur]

No. If it's a spontaneous system, more heat always flows away from the hot areas to the cold than in the opposite direction.

But what about the expansion? In terms of radiation, there would be redshift centripetally and blue-shift outward. Plus, I think the outward motion of the particles would also create more pressure and therefore heat in the outward direction than centripetally. I'm not sure how these outward-oriented energy-effects would combine with the entropy-effect that causes the hot areas to want to transfer their heat to the cooler areas.

 

You have changed condensation to compression, and those two terms are not synonyms, and now have a specific scenario of a low-pressure interior and high-pressure exterior, without explaining how you got that.

Good point, which I'm not sure about. However, "condensation" referred to the effect of the cooler interior losing pressure as it cools while "compression" referred to the hotter exterior exerting centripetal pressure. I guess I'm just not sure what would hold the cooler parts together besides gravity. On the other hand, I don't see how a core of particles that are radiating away energy and thereby cooling can expand as forcefully as their surroundings, if those are absorbing the radiation from the cooling core. Could a large expanding cloud of gas simply continue expanding indefinitely if it had a limited amount of energy? If its internal friction exceeded its capacity to expand, wouldn't it have to maintain a certain amount of cohesion in various areas even while its overall volume was growing?

 

 

[swansont]

But what about the expansion? In terms of radiation, there would be redshift centripetally and blue-shift outward. Plus, I think the outward motion of the particles would also create more pressure and therefore heat in the outward direction than centripetally. I'm not sure how these outward-oriented energy-effects would combine with the entropy-effect that causes the hot areas to want to transfer their heat to the cooler areas.

 

A redshift means the energy of the radiation is lowered. That will cause less heating, so I don't see how this helps your scenario.

[lemur]

A redshift means the energy of the radiation is lowered. That will cause less heating, so I don't see how this helps your scenario.

I know that. The redshift is occurring toward the center, because the system is expanding. So heat is radiating outward and less so inward (centripetally). This would cause the interior to cool relative to the exterior, which would be heating more due to blueshift in the outward direction.