I read popular physics: Giant Atoms

Someone went and got me a subscription to Scientific American. So I’ve been looking at these popular physics articles, marveling at what they look like post-PhD. I will continue doing this until I get bored.

Today, I will look at an article in the February issue, titled “Giant Atoms”, by Charlie Wood. It’s a short article about the MAGIS-100 experiment, which I was previously unaware of.

I got two main points from the article:

  • The MAGIS-100 experiment drops atoms down a 100 meter vacuum tube, creating “room-length” atom waves.
  • The experiment will eventually be sensitive to gravitational waves, and new forces that interact with dark matter.

The funny thing about these popular articles, is there’s often a mismatch between the article’s hook, and what would be considered scientifically exciting. The hook is giant atom waves, but from a scientific standpoint the real goal is clearly dark matter and gravitational waves.

Not to say scientists wouldn’t get excited about the giant atom waves. But if you want to market the project to grant providers, you emphasize the potential results, and not just the fancy machines they’d be paying for. Indeed, when I retrieved a more scholarly article, the emphasis was on the results. Any talk of giant atom waves was tucked away in the references.

Why would they want to look for gravitational waves when LIGO already found them a few years back? Gravitational waves are like light waves: they come in various frequencies. One telescope might be good at detecting infrared light, another at detecting X-rays. Likewise LIGO and MAGIS are sensitive to different frequencies of gravitational waves, and might be used to study different phenomena. It sounds like MAGIS would be good for looking at cosmological sources—the gravitational equivalent of cosmological background radiation. But the experiment probably won’t be sensitive enough to see much until the planned upgrade to a 1 km shaft.

There are also multiple experiments looking for dark matter. Dark matter is likely made up of a new kind of particle, and it’s reasonable to guess that it also comes with a new force. But we don’t know how strong this force would be, and if it’s very weak then experiments would have a hard time detecting it. And just as different experiments are sensitive to different frequencies of gravitational waves, different experiments are sensitive to different masses of the dark matter particles.

And what’s up with those atomic waves? In a certain sense, this is trivial, because everything can be described as a wave, and that’s quantum field theory. If a particle wave spans a large space, that just means there is a large uncertainty in its position. Imagine the headlines: “Scientists more uncertain of atom’s location than ever”. Or alternatively, “Scientists have very precisely measured an atom’s momentum”.

But I’m sure that what they mean is that these waves are coherent over large distances. That means if you brought the two sides of the atom back together, then they would experience quantum interference.

I looked at one of the cited papers about atom interferometry. If I understand correctly, they’re using lasers to put trapped sodium atoms into a superposition of two states. One state travels faster than the other, so it separates into parts (the distance between the two parts being the “size” of the atom wave). When the parts recombine, the final state is determined by quantum interference, which depends on the phase difference between the two states. So basically this is an elaborate way to precisely measure the phase of atoms, which should be very sensitive to any external forces.

Hats off to the researchers working on this.

Another neat physics article in this issue is “The enigma of aerodynamic lift”, discussing different non-technical explanations of how airplane wings work. Although, I’m afraid that this isn’t a real scientific mystery, more of a pedagogical and philosophical one!


  1. suttkus says

    “Giant Atoms” reminded me of a Discover magazine April fools article from many years ago, about the discovery of a subatomic particle the size of a bowling ball. : – )

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