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I read in this article:

  1. Antimatter And here we are: at the very end of the feasibility spectrum into the fantastic. Antimatter is made of antiparticles, which have the same mass as particles of ordinary matter but opposite charge and spin.

Antimatter also has the highest energy density of any known substance. And if used as fuel, it could provide by far the most efficient propulsion system, with up to 40% of the fuel’s mass-energy being converted directly into thrust (compared with 1% for fusion, the next most efficient).

In 2006 the NASA Institute for Advanced Concepts (NIAC) funded a team Gerald Smith of Positronics Research, designing an antimatter-powered spaceship. They calculated just 10 thousandths of a gram of antimatter would be enough to send a ship to Mars in 45 days.

In this context, won't it be better to speak of potential energy density instead of energy density? Protons have a much higher energy density than the electrons and positrons mentioned in the article. It's only after two protons (deuterons) have fused, the energy released (potential), is, according to the citation below 1% of the mass/energy of both protons. The energy released when an electron and a positron annihilate is 40% which obviously should be 100%, but in the context of the rocket 60% is lost into whatever.

More general, wouldn't it be better to speak of potential energy density or does it depends on the context we're using it in. E.g. in thermal processes in classical thermodynamics, assuming continuous materials, solid, liquid, or gas? Can elementary particles have an energy density? If points of them exist (which I don't think because somehow it is a logical impossibility). And if that's the case can you make an average of an ensemble?

The more I think about it, the less obvious it becomes.

Deschele Schilder
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  • In thermodynamics and hydrodynamics energy density is the correct term, because some of the internal energy density comes from kinetic energy. For example, in an ideal gas all of the energy is kinetic – Ofek Gillon Dec 27 '19 at 09:40

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In this excerpt, neither is really correct. Antimatter has exactly as much energy density, potential or otherwise, as regular matter. What the text means is that it has the most practically accessible energy density of any material. What matters is not how much energy it has, but how much of it we can use.

And to answer your question in general: no, it isn't. When we speak of energy density, we're usually taking a macroscopic view of a system like a fluid. Taking a macroscopic view means that we don't know and don't care what kind of energy that is: in most cases, it will be a mix of kinetic and potential$^1$, but the point is that for macroscopic processes we choose to average it all out and include everything under the single variable "energy density".

$^1$ And when quantum mechanics comes into the picture, it's not so easy to tell the two apart.

Javier
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