# Absolute Energy

1. Sep 3, 2009

### Zman

Energy is considered a relative quantity.

Is there any measure of energy that is absolute?

If temperature is measured by measuring the frequency of the emitted photons then observers with different velocities would measure different temperatures.

But a body at absolute zero would not emit photons and therefore all observers would agree upon its temperature. Is this a case of energy that is absolute?

Or is zero energy (zero temperature) a meaningless statement?

2. Sep 3, 2009

### Bob_for_short

Yes, zero energy (vacuum) is the absolute thing. Vacuum is an invariant thing. It is the same in all reference frames. In the theory it is also so.

I am not sure that the temperature depends on the reference frame in a simple way because the body radiation becomes anisotropic there (more complicated).

3. Sep 3, 2009

### atyy

In GR, the stress-energy tensor and cosmological constant (which may be considered a form of energy) are absolute. Observations indicate that our universe may be modelled by a solution of the equations of GR. However, GR also expects these energies to be associated with matter, and some aspects of the relation between GR and our current theories of matter are not understood. Look up "dark matter", and "dark energy".

4. Sep 3, 2009

### Bob S

Bob for short is correct in stating that zero temperature (energy is correct. Also, triple points of liquids (e.g., water) determine both a temperature and pressure scale. An absolute frequency scale can be determined by experimenters setting up in their own laboratory a measurement of hyperfine structure line of hydrogen (1420 MHz), and other atomic transition frequencies, and therefore determine doppler shifts, etc. of observed astronomical atomic transitions.

5. Sep 3, 2009

### atyy

Temperature is not absolute. Only dX/dT is observable.

Same with vacuum energy in the absence of general relativity.

6. Sep 5, 2009

### Bob S

Why can't an absolute temperature scale be determined by absolute zero, and the temperature of the triple point of water, for example?

7. Sep 5, 2009

### atyy

Yes, I think you are right, and certainly I'm confused. As usually defined by the engine efficiency or dS/dE, temperature can't be negative or energy will not be conserved. What I'm not sure about is that there is some freedom in defining the relation between efficiency and temperature, and I'm not sure how free that is.

8. Sep 5, 2009

### Staff: Mentor

There are a few different formulations of relativistic thermodynamics out there, and I don't think there is a general concensus on which is the most useful.