Free energy will promulgate a forward leap in human progress akin to the discovery of fire. It will bring the dawn of an entirely new civilization -- one based on freedom and abundance. -Sterling Allan

Of course, when Sterling Allan talks about free energy, he's talking about natural energy from sources like wind and solar, not the violating-the-laws-of-thermodynamics type of energy.

There is, of course, no such thing as truly free energy, or energy that we can take out of nothing and use for something, which is why perpetual motion machines not only don't work, but are physically impossible. (Although it is amusing to try to get as close as possible.)

But as many of you have noted about dark energy, there is a non-zero amount of energy that seems to be inherent to space itself. As the Universe expands, it appears to create more space, and hence, more energy.

Now, the energy density is tiny. So tiny that we didn't even discover the existence of dark energy until 1998, and if you were to compare it to the energy stored in, say, the mass of a human body, you would have to spread a human being out over the entire inner solar system (to fill a sphere the size of the orbit of Mars) just to get the same density as dark energy, which is about two protons per cubic meter.

Now, while energy is ill-defined in general relativity, we understand energy and momentum well enough (as well as more complicated properties of metric spaces, which I will not go into) to know that dark energy should have the following properties based on our current observations:

1. It should have a constant energy density everywhere in space.
2. It should be impossible to add to or take away from that energy density.
3. That energy density should also remain constant throughout time.

However, a recent paper has come out on the arXiv (and was discussed over on Cosmic Variance earlier today) that seeks to test that first assumption: is dark energy a constant everywhere in space?

Using a hypothetical improvement on a technique called atom interferometry (illustration above), they are proposing that changes in dark energy density could lead to changes in atomic motions, and could hypothetically exert a force on atoms.

Now, there are all sorts of reasons to believe that dark energy doesn't exert a force on atoms. Namely, the following big ones:

1. Dark energy, as far as we can tell, affects the expansion of space and nothing else, meaning it shouldn't exert a force on atoms.
2. You can only exert a force (whether you're dark energy or not) if your field changes from point-to-point. On the other hand, dark energy is observed to be a constant everywhere in space.

But, it isn't like we have a better proposal out there to try to perform some laboratory test on dark energy. Does it couple to matter? We don't think so, but we haven't tested it sufficiently to know for sure. Is it a constant everywhere in space? We think so, but we don't know if it clumps (even a little) around masses like the Earth or the Sun.

There is so much we don't understand about dark energy, and how the Universe's expansion on the largest scales relates back to what we can observe in a laboratory on Earth that this is possibly the most exciting prospect to come out concerning dark energy all year!

So do I think this is likely to produce anything new? No, probably not. There are many good reasons to believe that we know what we're talking about, but that doesn't really matter. What matters is that -- in order to know anything for sure -- we need to do the experiment. This idea deserves to get a little bit of a buzz, if for no other reason than we need to throw ideas around about dark energy, and be open to the notion that what we're seeing is so bizarre it could really turn our view of the Universe on its head.

But there still isn't any truly free energy out there, not even if dark energy does change from point-to-point. Which is too bad... because if there was some, I could then move to phase 3. I'll keep dreaming.