How physics moves from wild ideas to actual experiments

Neutrinos are some of nature’s most elusive particles. One hundred trillion fly through your body every second, but each one has only a tiny chance of jostling one of your atoms, a consequence of the incredible weakness of the weak nuclear force that governs neutrino interactions. That tiny chance means that reliably detecting neutrinos takes many more atoms than are in your body. To spot neutrinos colliding with atoms in the atmosphere, experiments have buried 1,000 tons of heavy water, woven cameras through a cubic kilometer of Antarctic ice, and planned to deploy 200,000 antennas.

In a field full of ambitious plans, a recent proposal by Steven Prohira, an assistant professor at the University of Kansas, is especially strange. Prohira suggests that instead of using antennas, we could detect the tell-tale signs of atmospheric neutrinos by wiring up a forest of trees. His suggestion may turn out to be impossible, but it could also be an important breakthrough. To find out which it is, he'll need to walk a long path, refining prototypes and demonstrating his idea’s merits.

Prohira’s goal is to detect so-called ultra-high-energy neutrinos. Each one of these tiny particles carries more than fifty million times the energy released by uranium during nuclear fission. Their origins are not fully understood, but they are expected to be produced by some of the most powerful events in the Universe, from collapsing stars and pulsars to the volatile environments around the massive black holes at the centers of galaxies. If we could detect these particles more reliably, we could learn more about these extreme astronomical events.

Other experiments, like a project called GRAND, plan to build antennas to detect these neutrinos, watching for radio signals that come from their reactions with our atmosphere. However, finding places to place these antennas can be a challenge. Motivated by this experiment, Prohira dug up old studies by the US Army that suggested an alternative: instead of antennas, use trees. By wrapping a wire around each tree, army researchers found that the trees were sensitive to radio waves, which they hoped to use to receive radio signals in the jungle. Prohira argues that the same trick could be useful for neutrino detection.

Crackpot or legit science?

People suggest wacky ideas every day. Should we trust this one?

At first, you might be a bit suspicious. Prohira’s paper is cautious on the science but extremely optimistic in other ways. He describes the proposal as a way to help conserve the Earth’s forests and even suggests that “a forest detector could also motivate the large-scale reforesting of land, to grow a neutrino detector for future generations.”

Prohira is not a crackpot, though. He has a track record of research in detecting neutrinos via radio waves in more conventional experiments, and he even received an $800,000 MacArthur genius grant a few years ago to support his work.

More generally, studying particles from outer space often demands audacious proposals, especially ones that make use of the natural world. Professor Albrecht Karle works on the IceCube experiment, an array of cameras that detect neutrinos whizzing through a cubic kilometer of Antarctic ice.

“In astroparticle physics, where we often cannot build the entire experiment in a laboratory, we have to resort to nature to help us, to provide an environment that can be used to build a detector. For example, in many parts of astroparticle physics, we are using the atmosphere as a medium, or the ocean, or the ice, or we go deep underground because we need a shield because we cannot construct an artificial shield. There are even ideas to go into space for extremely energetic neutrinos, to build detectors on Jupiter's moon Europa.”

Such uses of nature are common in the field. India’s GRAPES experiments were designed to measure muons, but they have to filter out anything that’s not a muon to do so. As Professor Sunil Gupta of the Tata Institute explained, the best way to do that was with dirt from a nearby hill.

"The only way we know you can make a muon detector work is by filtering out other radiation [...] so what we decided is that we'll make a civil structure, and we'll dump three meters of soil on top of that, so those three meters of soil could act as a filter," he said.

The long road to an experiment

While Prohira’s idea isn't ridiculous, it's still just an idea (and one among many). Prohira’s paper describing the idea was uploaded to arXiv.org, a pre-print server, in January. Physicists use pre-print servers to give access to their work before it's submitted to a scientific journal. That gives other physicists time to comment on the work and suggest revisions. In the meantime, the journal will send the work out to a few selected reviewers, who are asked to judge both whether the paper is likely to be correct and whether it is of sufficient interest to the community.

At this stage, reviewers may find problems with Prohira’s idea. These may take the form of actual mistakes, such as if he made an error in his estimates of the sensitivity of the detector. But reviewers can also ask for more detail. For example, they could request a more extensive analysis of possible errors in measurements caused by the different shapes and sizes of the trees.

If Prohira’s idea makes it through to publication, the next step toward building an actual forest detector would be convincing the larger community. This kind of legwork often takes place at conferences. The International Cosmic Ray Conference is the biggest stage for the astroparticle community, with conferences every two years—the next is scheduled for 2025 in Geneva. Other more specialized conferences, like ARENA, focus specifically on attempts to detect radio waves from high-energy neutrinos. These conferences can offer an opportunity to get other scientists on board and start building a team.

That team will be crucial for the next step: testing prototypes. No matter how good an idea sounds in theory, some problems only arise during a real experiment.

An early version of the GRAPES experiment detected muons by the light they emit passing through tanks of water. To find how much water was needed, the researchers did tests, putting a detector on top of a tank and on the bottom and keeping track of how often both detectors triggered for different heights of water based on the muons that came through randomly from the atmosphere. After finding that the tanks of water would have to be too tall to fit in their underground facility, they had to find wavelength-shifting chemicals that would allow them to use shorter tanks and novel ways of dissolving these chemicals without eroding the aluminum of the tank walls.

“When you try to do something, you run into all kinds of funny challenges,” said Gupta.

The IceCube experiment has a long history of prototypes going back to early concepts that were only distantly related to the final project. The earliest, like the proposed DUMAND project in Hawaii, planned to put detectors in the ocean rather than ice. BDUNT was an intermediate stage, a project that used the depths of Lake Baikal to detect atmospheric neutrinos. While the detectors were still in liquid water, the ability to drive on the lake’s frozen surface made BDUNT’s construction easier.

In a 1988 conference, Robert March, Francis Halzen, and John G. Learned envisioned a kind of “solid state DUMAND” that would use ice instead of water to detect neutrinos. While the idea was attractive, the researchers cautioned that it would require a fair bit of luck. “In summary, this is a detector that requires a number of happy accidents to make it feasible. But if these should come to pass, it may provide the least expensive route to a truly large neutrino telescope,” they said.

In the case of the AMANDA experiment, early tests in Greenland and later tests at the South Pole began to provide these happy accidents. “It was discovered that the ice was even more exceptionally clear and has no radioactivities—absolutely quiet, so it is the darkest and quietest and purest place on Earth,” said Karle.

AMANDA was much smaller than the IceCube experiment, and theorists had already argued that to see cosmic neutrinos, the experiment would need to cover a cubic kilometer of ice. Still, the original AMANDA experiment wasn't just a prototype; if neutrinos arrived at a sufficient rate, it would spot some. In this sense, it was like the original LIGO experiment, which ran for many years in the early 2000s with only a minimal chance of detecting gravitational waves, but it provided the information needed to perform an upgrade in the 2010s that led to repeated detections. Similarly, the hope of pioneers like Halzen was that AMANDA would be able to detect cosmic neutrinos despite its prototype status.

“There was the chance that, with the knowledge at the time, one might get lucky. He certainly tried," said Karle.

Prototype experiments often follow this pattern. They're set up in the hope that they could discover something new about the Universe, but they're built to at least discover any unexpected challenges that would stop a larger experiment.

Major facilities and the National Science Foundation

For experiments that don't need huge amounts of funding, these prototypes can lead to the real thing, with scientists ratcheting up their ambition at each stage. But for the biggest experiments, the governments that provide the funding tend to want a clearer plan.

Since Prohira is based in the US, let's consider the US government. The National Science Foundation has a procedure for its biggest projects, called the Major Research Equipment and Facilities Construction program. Since 2009, it has had a “no cost overrun” policy. In the past, if a project ended up costing more than expected, the NSF could try to find additional funding. Now, projects are supposed to estimate beforehand how the cost could increase and budget extra for the risk. If the budget goes too high anyway, projects should compensate by reducing scope, shrinking the experiment until it falls under costs again.