A thermal expansion coefficient of spacetime?

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Discussion Overview

The discussion revolves around the concept of whether spacetime can be viewed as having a coefficient of thermal expansion, similar to materials like water. Participants explore the implications of temperature on spacetime, cosmological expansion, and the nature of space and time in physics.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants propose that spacetime might have a coefficient of thermal expansion analogous to the coefficient of thermal expansion (CTE) of water, questioning how density might inflect with temperature.
  • Others argue that the cosmological constant and its relation to dark energy complicate the idea of associating temperature with cosmological expansion, noting that the universe is currently cooling while expanding.
  • It is mentioned that density and pressure are temperature-dependent functions connected via equations of state, which could imply a relationship between temperature and cosmological models.
  • Some participants express skepticism about measuring a thermal expansion coefficient for spacetime, questioning the feasibility of such measurements.
  • There is a discussion about whether space possesses ponderable properties, with some suggesting that spacetime might be treated as a field, while others maintain that it lacks such properties.
  • Participants discuss the concept of temperature in relation to the vacuum of spacetime, with references to Hawking Temperature and Planck Temperature, but the definitions and implications remain unclear.
  • One participant challenges the scientific validity of discussing temperature in empty space, arguing that without a measurement method, such ideas cannot be considered scientific.
  • Another participant counters that defining temperature in the universe is possible, referencing calculations that involve volume, density, and particle interactions, although they note that temperature generally decreases with volume changes.
  • There is a question raised about the concept of 'absolute hot,' which remains unresolved in the discussion.

Areas of Agreement / Disagreement

Participants express a range of views on the relationship between temperature and spacetime, with no consensus reached on whether spacetime can be treated as having thermal properties or how such properties could be measured.

Contextual Notes

Limitations include the lack of clarity on how to measure temperature in spacetime, the dependence on definitions of temperature and properties of spacetime, and unresolved discussions about the implications of temperature changes on cosmological models.

ryan albery
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Are there any theories or thoughts that view spacetime as 'having' a coefficient of thermal expansion... analogous to the CTE of water? An inflection with density in regards to temperature?
 
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Well, since nobody knows what the cosmological constant actually is physically, such as maybe
dark energy, or not, who knows? Energy density, maybe; heat, not so much as far I can see.

If heat is transfrerred via conduction, convection and radiation, you'd need a mechanism that associates those kind of changes with varying cosmological expansion rates. But cosmological expansion doesn't seem well described temperature. Not only is the universe currently the coolest it has ever been, around 2.7 degrees C these days, expansion is currently accelerating as the universe continues to cool. And it expanded superluminally when it was really, really hot and cooling. So what expands superluminally when hot and cold and continues as it approaches absolute zero?
 
Density and pressure seem not 'in general' linearly/proportional to temperature, like an ideal gas.
 
ryan albery said:
Are there any theories or thoughts that view spacetime as 'having' a coefficient of thermal expansion... analogous to the CTE of water? An inflection with density in regards to temperature?

No. How would you measure such a thing?
 
Vanadium 50 said:
No. How would you measure such a thing?

Same way as you'd measure the expansion/contraction of the universe, only assuming spacetime is an actual thing.
 
I see no reason to treat 'space' as possessing any ponderable properties [to borrow from Einstein]. It does appear, however, it can be treated as a field. Does that constitute a 'property'? That is unclear to me.
 
I can understand how 'space' itself doesn't have any ponderable properties, but spacetime, I think even Einstein came to think of 'it' as a thing.
 
I can understand how 'space' itself doesn't have any ponderable properties, but spacetime, I think even Einstein came to think of 'it' as a thing.

Maybe our ideas of space and time, or spacetime are so far myopic. Carlo Rovelli describes some discordances:


In SR, if you move along at a constant velocity, your time ticks off at a steady pace. Not so in GR...Proper time along a path [called a worldline] is an observable but it is not 'ticks at a steady pace'...

The proper time, tau, along spacetime trajectories cannot be used as an independent variable either, as tau is a complicated non-local function of the gravitational field itself. Therefore, properly speaking, GR does not admit a description as a system evolving in terms of an observable time variable. ...

This weakening of the notion of time in classical GR is rarely emphasized: After all, in classical
GR we may disregard the full dynamical structure of the theory and consider only individual solutions of its equations of motion. A single solution of the GR equations of motion determines “a spacetime”, where a notion of proper time is associated to each timelike worldline...

But in the quantum context a single solution of the dynamical equation is like a single “trajectory” of a quantum particle: in quantum theory there are no physical individual trajectories: there are only transition probabilities between observable eigenvalues. Therefore in quantum gravity it is likely to be impossible to describe the world in terms of a spacetime, in the same sense in which the motion of a quantum electron cannot be described in terms of a single trajectory.
 
  • #10
Is there any definition for a temperature of the 'vacuum' of spacetime?
 
  • #11
I guess for some "parts' it would be the Hawking Temperature.

What's the Planck Temperature really mean?
 
  • #12
Ryan, I'm afraid that none of what you are writing makes any sense at all. I was trying to nudge you in a more scientific direction by asking how this is measured. But the idea of an expansion of a space of fixed volume does not make sense - it either has fixed volume or it does not. The idea of a temperature of an empty volume of space does not make sense either.

If you can't describe how one would go about measuring it, it's not science.
 
  • #13
Yeah, data are by the things you measure. But to call anything other than data to be 'non-scientific'; not science, I disagree.
 
  • #14
So empty spacetime can have a field (like the Higgs), contains dark energy, and transmits EM radiation and gravity, but the idea of temperature is undefined? I'm just trying to wrap my brain around that:)
 
  • #15
The average temperature of the universe is definable. To define such changes per the era your calculating the average temperature. These calculations use variations of the ideal gas laws. Those laws involve the volume, density, number of particle species (ie photons,neutrinos etc), the interactions of those particles etc.

However in all calculations the temperature drops as a result of a change in volume. Regardless of which time period your discussing. The exception to that rule is if new interactions occur from new particles coming into measurable existence. Oft termed az freezing into existence in some high energy particle physics textbooks
 
  • #16
Does the idea of 'absolute hot' make sense?
 
  • #17
Ryan, you're hijacking your own thread here. And no it doesn't.
 

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