Why a physicist wants to build a particle collider on the moon

As we probe deeper into the innermost workings of the universe, our particle physics experiments have become ever more complex. In order to reveal the secrets of the tiniest subatomic particles, physicists must make colliders and detectors as cold as possible, remove as much air as possible, and keep them as still as possible to get reliable results.

So at least one physicist is asking: What if we just skipped all that and set up our particle physics experiments on the moon?

Related: 5 Strange, cool things we've recently learned about the moon

A proposal published to the preprint database arXiv earlier this year argues that the moon is actually a pretty decent place to do high-energy physics.

First, it's cold. Very cold. With no atmosphere and no water, there's nothing to transport the heat of sunlight from one place to another. At night, with the sun below the horizon, temperatures dip to minus 100 degrees Fahrenheit (minus 73 degrees Celsius) — in the range of typical cryogenic setups on Earth. In the daytime, things get a little bit hotter, reaching more than 100 F (38 C). But as the ice tucked away in the shadows of lunar craters proves, all you need to cool yourself down is a little bit of shade. Again, with no air or water, areas out of direct sunlight are blissfully cold.

Physicists need those cold temperatures for a few reasons. In accelerators, cold temperatures ensure that the superconducting magnets — used to fling the particles inside the accelerator to nearly the speed of light — don't melt themselves. Second, the hotter a detector, the more noise you have to deal with in trying to tease out the tiny signals from subatomic particles. (More heat equals more molecules vibrating, which equals more noise.) 

Besides the chilly temperatures, the fact that the moon has no atmosphere is also a major boon. Physicists have to pull all the air out of their accelerators and detectors — wouldn't want your near-the-speed-of-light particles to slam into a wandering nitrogen molecule before you even get started. But the moon has a vacuum 10 times better than anything physicists have manufactured in their experiments. And it does it naturally, without any effort at all.

Lastly, because of tidal locking — meaning our satellite body takes the same amount of time to rotate about its axis (its rotational period) as it does to orbit Earth — the moon keeps the same face pointed toward Earth at all times. This means a lunar particle beam could be pointed back toward a detecting laboratory on Earth, taking advantage of the long distance without having to work very hard to align the setup. 

Lunar neutrino factory

Perhaps, the most promising use of a lunar physics experiment would be as a source of neutrinos. Neutrinos are ghostly, nimble little particles that have no electric charge and hardly any mass at all. This enables them to flit through normal matter without hardly ever noticing — hundreds of billions of neutrinos are passing through your body right now, and you can't feel a thing.

Related: The 18 biggest unsolved mysteries in physics

Needless to say, neutrinos are hard to study and understand. They're made in copious quantities in nuclear reactions, so all it would take would be to stick a nuclear power plant on the moon and let it rip. The neutrinos that it produces would race to Earth, where we could pick them up and study them.

One aggravating and mysterious property of neutrinos is that they're capable of changing types (or "flavors" in the physics jargon) as they fly. By having a long distance separating neutrino generation and detection, we give more neutrinos a chance to "change flavors," and we can better understand this behavior. The moon makes a perfect source: It's far enough away that we could get long distances, but close enough that we could capture neutrinos in sufficient quantities to actually study (and presumably also troubleshoot the facility if something goes wrong).

Who needs Earth anyway?

Neutrinos aren't the only thing a facility on the moon could shoot at Earth. Even our most powerful particle colliders can't come close to the energies that nature is capable of generating to launch particles (and if we're being accurate, we can't even come close to a billionth of those energies). Every second of every day, high-energy particles come screeching into our atmosphere, knocking over a few molecules and releasing a shower of particle byproducts before hitting the ground. 

Known as cosmic rays, these particles come from some of the most energetic sources in the universe (think supernovas), but they are poorly understood. So what we could really use is a cosmic ray gun - something that manufactures them somewhere else and blasts them into our atmosphere so we can study them. How about … the moon? A facility on the moon could produce high-energy particles in great quantity, shoot them at our atmosphere, and let us observe the resulting showers from the ground, helping us better understand this high-energy side of the universe.

But why stop there? Why not just put the detectors on the moon too? A complete particle physics experiment, with source, accelerator and detector on the moon offers several advantages over Earth-based systems. The number-one bottleneck here is the need for a highly-controlled vacuum, which constricts Earth experiments to be relatively compact.

But on the moon, you get a vacuum for free. And that vacuum is much, much better than the one used in particle collider experiments. You could build your facility as big as your heart's content, without once having to invest in a single air pump. That's quite the advantage.

I suppose there's the minor technical challenge of actually getting there and building sophisticated experiments on the moon, but once that's solved physics could see a big, lunar-based boost.

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Originally published on Live Science.

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