Relativistic Hydrostatic Pressure in Constant Gravitational Field

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Discussion Overview

The discussion revolves around the behavior of pressure in a perfect fluid subjected to a constant gravitational field when the fluid is in a relativistic state due to high temperature. Participants explore how the standard hydrostatic pressure equation may change under these conditions.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant states that for a perfect fluid in a constant gravitational field, the pressure at depth z is given by P = ρgz, where ρ is the fluid density.
  • Another participant questions the classification of the fluid as a perfect fluid, suggesting that it may not be appropriate since perfect fluids do not conduct heat.
  • A later reply proposes a modified pressure equation, P = k_B Tρ_0/m + ρgz, indicating that the relativistic nature of the fluid affects the pressure calculation, with ρ being adjusted by a factor of γ (Lorentz factor).
  • One participant acknowledges a misunderstanding regarding the properties of perfect fluids, suggesting that it is possible to consider a perfect fluid at a certain temperature for calculations related to energy density and pressure.
  • Another participant agrees that while perfect fluids do not conduct heat, it is reasonable to analyze them under certain temperature conditions as proposed in the discussion.

Areas of Agreement / Disagreement

Participants express differing views on the classification of the fluid as a perfect fluid and whether it can be analyzed under relativistic conditions. There is no consensus on the implications of temperature on the behavior of the fluid.

Contextual Notes

Participants have not fully resolved the implications of temperature on the properties of perfect fluids, and there are assumptions regarding the definitions of perfect fluids and relativistic effects that remain unaddressed.

antoinebret
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Hi everybody,

When I have a perfect fluid within a constant gravitational field g, the pressure at depth z simply reads P=\rho g z, where \rho is the fluid density (incompresible).

If now my g is still constant, but my fluid is relativistic. Not because it's moving (everything is still) but because it is hot with k_B T \sim mc^2. What happens to P = \rho g z ?

Thanks!
 
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Perfect fluids do not contain or conduct heat, so you have some other kind of fluid, perhaps ?
 
Mentz114 said:
Perfect fluids do not contain or conduct heat
They do.
...it is hot with k_B T \sim mc^2. What happens to P = \rho g z ?
P= k_B T\rho_0/m + \rho g z with \rho=\gamma \rho_0.

You can use [ tex ] and [ /tex ] (click on the formula):
[tex]P=k_B T\rho_0/m + \rho g z[/tex]
 
Ich said:
They do.
If you're right, I have been misinformed.
 
Ok, my comment was too general.
What I meant is that it makes sense to ask about a perfect fluid with a certain temperature in the way antoinebret did. From his data you can calculate energy density and pressure and answer the question with the perfect fluid behaviour, the extreme case being a photon gas.
I agree that there's no heat conduction, though.
 

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