All the cool kids are talking about dark energy and dark matter. Why? Because these are things that current physical models don't predict. The hope is that understanding one or both of these concepts will unveil a new world where everyone gets cookies and is born understanding physical concepts like tensors. OK, maybe not everyone will get a cookie.
But dark matter and energy are not the only things that current physical models don't predict. The Standard Model, which describes the behavior of fundamental particles and forces, has a number of physical constants. These constants are measured, and we know them accurately. But there is no theory that predicts why these constants have the values they do.
What can we do about this? One option is to measure these constants under as many different conditions as possible and test to see if they are, indeed, constant. The focus of this effort has mainly been on change with respect to time. Researchers compare astronomical measurements, made on very distant objects, to lab measurements. To within measurement uncertainty, the fundamental constants have not changed in the last few billion years.
The only way to really check for variations in fundamental constants is to push those uncertainties back by coming up with more sensitive techniques. This is exactly the goal of a group of researchers in the Netherlands. You'll note that I say "goal"—the research is ongoing. Creating new measurement techniques is a long and complicated procedure.
We like fountains
If you ask physicists to name the most sensitive and accurate measurements we can make, most would answer "time." We can measure the second to an accuracy of one part in 1015. And this measurement is the inspiration behind the approach of the scientists.
To obtain this accuracy, we first cool a beam of atoms to very, very low temperatures and shoot them upward in a vacuum. The atoms slowly (because they are cold) creep upward, stall out thanks to gravity, and fall back down. Time is measured by setting the atom's internal state on the way up and reading the atom's internal state on the way down. The atom's state changes in a very precise way with time, so the interval can be measured with mind-boggling precision.
Measurements on atomic fountains are certainly ideal for high precision. But for the purposes of measuring fundamental constants, even these measurements are not sensitive enough. Any changes to the fundamental constants will be extremely tiny, and they'll produce correspondingly minuscule changes in the properties of individual atoms. In the end, the Universe wins.
But this may not hold for molecules. Molecules have symmetries (or a lack of symmetry) and internal charge distributions, which can exert some pretty strong internal forces. In a sense, these forces amplify the influence of small changes in fundamental constants.
Think of it like this. Currently, we treat the electron like a point charge. But maybe the electron is more like a tiny rod with one end more negative than the other (called an "electric dipole moment"). At present, the electric dipole moment of the electron (a fundamental constant) has a value of zero. In a molecule, electrons can end up unevenly distributed between different atoms, generating large electric fields within the molecule. If the electron has a non-zero dipole moment, it will respond to that field, and that can be determined by careful measurements of the molecular properties.
Bring me a molecular fountain
The solution is to replace the atoms in the fountain with molecules. But that's not an easy thing to do. To make a fountain, the molecules have to be cold. Not only so that they are moving relatively slowly, but also so that the distribution of speeds is relatively narrow. So the first task is to cool the molecules.
The way this is done with atoms doesn't work very well with molecules. To cool atoms, you shine lasers on them. The color of the laser light is tuned such that if the atom is moving into the light, it absorbs energy in the form of a photon and re-emits a photon in a random direction. The double kick that the atom receives (once on absorption and once on emission) slows it slightly. After emitting a photon, the atom is available to absorb another one, so it can repeatedly absorb and emit photons, slowing down as it does. Once the atom slows below a certain speed, it no longer absorbs any light and stops cooling.
This cycle is closed: as long as its energy is high enough, the atom always returns to a state where it can absorb and emit another photon. Molecules, however, are much more complicated, and it's difficult to close the cycle. What usually happens is that the molecule will almost return to the ground state, retaining just a small amount of energy. That leaves the atoms vibrating relative to one another. Repeated absorption and emission has the effect of slowing the molecule's translational motion, but it leaves it more jiggly than a five-year-old on a sugar diet with extra sugar and caffeine thrown in for good measure. In other words, externally cool, internally very hot.
Strictly speaking, you've achieved your goal in that the molecules are moving very slowly. But every time the molecules collide, some of the vibrational energy is expended to give them a kick. All that stored energy results in a rapid heating and expansion. Your fountain explodes.
Not all molecules
This all means that cooling molecules in the same way that we cool atoms is not a great solution. But some molecules are arranged in such a way that their electrons tend to congregate at one end, complaining about the proton bullies on the other side. Overall, the molecule is electrically neutral, but up close, it's not—it has its own local electric field. These molecules can be cooled because they will respond to an external electric field.
The researchers applied this idea to a supersonic jet of ammonia molecules. The jet shoots up a tube that has an electric field pushing the ammonia back down the tube. At the end of the tube, the ammonia is slowed such that its forward motion is a bit slower than the rate of expansion of the ammonia jet. At this point, further cooling is pointless because the cloud will continue to expand rapidly.
To overcome the expansion and cool the ammonia further, the researchers designed fields that trap the ammonia in little pots. Essentially, motion sideways is gently slowed until the molecules are driven gently back into the center of the pot. The pot itself travels along the tube, slowing as it goes, so the forward motion of the ammonia cloud is also slowed. By the time the ammonia falls out of the pot, its forward motion is down to under 2m/s and the cloud's rate of expansion is down to about 50cm/s.
The researchers demonstrated that this is good enough to use it as a molecular fountain. They ionized the ammonia and detected the ions at the top and bottom of the fountain.
Slow and steady
And now for the fundamental constant measurements, right? Not yet. Every measurement has two aspects to it. One is the sensitivity and precision of an individual measurement, and the second is the accumulation of many measurements to overcome the limitations of individual measurements.
At this point, the slow ammonia molecules will allow for improved sensitivity for individual measurements, but accumulating large piles of data is going to be tedious. The fountain fires a cloud of ammonia twice per second, and for the longest delay times (the highest fountain), the researchers only detect a falling ion once every five seconds. In other words, the fountain has to shoot ten times before a falling molecular ion is detected. It will be a long, slow process to gather statistics.
But that's a bit beside the point. All of these techniques start out performing really poorly, and they only get better with time. I suspect we will start seeing results from this apparatus before 2017 is out. The first results will be demonstrations of high-precision measurements on the properties of ammonia (or some other molecule). After that, the researchers will start measuring fundamental constants in earnest.
Physical Review Letters, 2016, DOI: 10.1103/PhysRevLett.117.253201