Strangelets won't destroy the Earth, but are still spooky as hell

This “strange matter” as it would come to be called would quickly convert all normal matter on Earth to strange matter like itself in mere moments, a kind of particle zombie apocalypse powered by exponential growth that would leave the Earth and all life on it reduced to a relatively tiny ball of quarks about 100 meters across.

Much like the eschatological fear of a microscopic black hole dragging our planet and all of us into the void, the panic over strangelets was full of hyperbole and anti-scientific misinformation, but also very real fears of stepping into a dangerous unknown.

As we approach similar fears about new technologies and an uncertain future, there are lessons to be learned from the strangelet panic and how physicists managed to educate a distrustful public and advance the cause of human understanding.

Strange matter

To understand strange matter, you need to know about quarks. 

Quarks are the elementary particles that make up protons and neutrons, and they come in six flavors: up, down, charm, strange, top and bottom. Pretty much all matter is made of up and down quarks, with two ups and a down forming a proton and two downs and an up forming a neutron.

While larger particles can be made from other quarks, these are highly unstable due to their mass and almost immediately decay into lighter particles. Neutrons, and especially protons, are many orders of magnitude more stable (a proton will decay naturally in around 10³⁴ years, while outside the nucleus a neutron will decay in about 15 minutes) and as a result, protons have long been considered the base state of all large matter. 

The Strange Matter Hypothesis, however, says that we have this all wrong.

“Basically, strange matter is a form of matter where up, down, and strange quarks are equally represented,” Dr. Don Lincoln, an experimental particle physicist at Fermi National Accelerator Laboratory (Fermilab) who produces science videos on Fermilab's YouTube channel and who was a part of the international teams that discovered the top quark in 1995 and the Higgs Boson in 2012, told IE last month over Zoom. “And when you have matter—theoretically, of course—in that configuration, it's the lowest energy state.”

Since everything in the universe is trying to get to its lowest possible energy state (nature is fundamentally lazy), this presents a problem for us, in theory, since normal matter is in a higher energy state and will react dangerously when it comes into contact with strange matter. 

“When other matter touches strange matter, that matter will then be brought into the lower energy state,” Lincoln said, “transforming itself from a proton or neutron or whatever, into this strange state. This is a theoretical claim that hasn't been experimentally validated and may well not be correct, but this is the idea behind the worries concerning strange matter.”

This newly converted strange matter would then go on to convert other matter around it into strange matter like itself, and that matter will convert more matter still, until eventually there is no more normal matter left to convert.

“So it's like the Midas touch,” Lincoln said. “Midas could reach out and turn things to gold with a touch, strange matter can reach out and touch and make other strange matter. That's the idea.”

While this might sound rather far-fetched, the idea of matter-converting “quark nuggets” made up of up, down, and strange quarks was first theorized in 1984 by Dr. Edward Whitten, a Fields Medal-winning theoretical mathematician and physicist at Princeton University. 

“Edward Witten is one of the most respected theorists around, so it's at least theoretically credible,” Lincoln said. “Is it really credible? That's a different thing.“

Strange matter hypothesis

Whitten’s theory is actually pretty straightforward.

At the center of the second densest object in the universe, a neutron star, the pressure is so intense that the neutrons in the core effectively dissolve into “quark matter”, a soup of quarks and gluons no longer bound together into discrete particles. 

This quark-gluon plasma is still under immense pressure, and it attempts to alleviate this pressure on a quantum level by taking the energy created by the pressure in the core and converting some of its down quarks into strange quarks.

The resulting quark matter, containing up, down, and strange quarks in equal measure, which we call strange matter, is now in an even lower energy state than a normal atomic nucleus.

Additional research by Edward Farhi and R. L. Jaffe further refined the Strange Matter Hypothesis, leading to the concept of the strangelet that we have today: a clump of strange matter large enough to interact with, and including two important theoretical properties. The first is that since it is at the lowest energy state possible, a strangelet can prompt surrounding matter to convert into strange matter, quickly transforming large amounts of normal matter into strange matter.

The second important theoretical property of strange matter to note is that as a strangelet grows larger, it grows more stable. So, a sufficiently large strangelet would not need the high-pressure crucible of the core of a neutron star to prevent the strange quarks in strange matter from decaying.

This is the core of the theory, and while there are those who are concerned about strange star collisions sending strangelet particles shooting through the universe like apocalyptic shrapnel until they hit something made of normal matter (namely us), it was the lead-up to the beginning of operations at the RHIC in the late 1990s that actually brought the fear of strangelets home to Earth.

Why people freaked out

When accelerators like RHIC and LHC smash particles together, they do so by accelerating separate streams of atomic nuclei to significant fractions of the speed of light using radio waves and magnets, and then redirecting the streams onto a collision course with each other.

The resulting high-energy collisions don’t just break apart the atomic nucleus, they overcome the strong nuclear force holding protons and neutrons together, sending all manner of quarks and other elementary particles flying as special sensors watch from the sidelines.

It’s these particles that physicists are most interested in, and it’s how we discovered all of the various quarks and particles that make up the standard model of particle physics, currently the most successful theoretical framework ever devised for understanding the universe.

Of course, this also means that you’re going to have a lot of up, down, and strange quarks coming into high-energy collisions with each other, giving rise to the fear among some that these particle accelerators might produce the exact right conditions for a strangelet to form. At which point, we’re off to the apocalypse.

“It's there,” Lincoln said of the mix of up, down, and strange quarks produced by particle collisions, ”it happens all the time. And as far as mixing up, down, and strange quarks, there is a particle called a lambda. A lambda particle is already a particle with an up, down, and a strange quirk in it. And that's not dangerous. What’s dangerous, according to the strangelet theory, is that you don't just have an up, down, and a strange quark, but you've got a lot of similar things together, bound together, like a super nucleus of sorts. So you need lots of up, down, and strange quarks together. 

“During collisions, E=mc² says that energy will make matter, so strange quarks are made in the process. And there's this tight cauldron of just quarks running around with strange quarks in the mix, and so the theory goes that if you get a fluctuation of a lot of strange quarks in the same place, then this fireball transition into strange matter would happen. And, since strange matter is supposedly in a lower energy state, it's stable. And if it's stable, it persists.”

So if the underlying theory is at least credible, and there’s more than enough material for strangelets to possibly form, why shouldn’t we worry about this?

Strangelets and other fears

In order to assuage the concerns of the public and scientific community alike, the director of the Brookhaven National Laboratory commissioned a report in 1999 to consider fundamental safety concerns about the particle collider, including the formation of strangelets.

On that question specifically, the report addressed the four major concerns about the potential for strangelets to form and found that these concerns were unfounded.

First, it noted that despite its theoretical possibility, there is no evidence anywhere in the universe that strange matter actually exists. 

Second, while strange matter grows more stable as a strangelet grows larger, the reverse is also true, namely that a tiny strangelet, like those that a particle collider could produce, would be inherently unstable and would decay before it could ever interact with anything. 

Third, heavy particle colliders like RHIC would be too high-energy to ever produce a stable strangelet. 

Finally, the most theoretically stable strangelet was overwhelmingly likely to have a positive electrical charge, so it would be strongly repelled by the positively charged atomic nucleus, preventing the kind of interaction necessary for a runaway chain reaction.

The high-energy collisions at LHC meanwhile, are simply too “hot” for strangelets to coalesce, CERN has argued, making it even less likely for a strangelet to form there than at RHIC.

But a single report on a single particle collider, and explanations with phrases like “this makes it even less likely that strangelets could form” isn’t exactly the ironclad assurance that many people will be looking for, especially those who don’t trust scientific experts.

“What we really need is a safety explanation that transcends the specific danger,” Lincoln said. “And it turns out, luckily, we know how to do that. And the way we do that is by turning to nature and saying ‘Do collisions of the kind that occur at Brookhaven or in the LHC occur in nature already?’ If they do, and they haven't caused some catastrophic damage, then we're okay.”

Looking to cosmic rays — high-energy particles shot through space by supernovae, the relativistic jets of black holes, or any number of explosive phenomena in the universe — we find just that sort of experiment in nature, Lincoln said.

“We know that most of the cosmic rays from space are such that the collisions with the atmosphere are much lower than LHC energies. However, there are ultra-high energy cosmic rays that have energies that transcend these by a fair bit, and this transformed energy is equivalent to being hit head-on.” 

One such cosmic event demonstrates how the power of many of these events can dwarf anything humans can produce in a particle accelerator.

“On October 15, 1991, there was a cosmic ray that smashed into Earth's atmosphere, which had the highest energy ever recorded, and it's so high that they call it the Oh My God particle,” Lincoln said. “It was 3.2 x 10²⁰ electron-volts, so if you transform that into the LHC equivalents, then that collision energy is about 60 times higher than the energy of the LHC.”

Not bad, but one event could be an anomaly. For more certainty, you’ll need a lot more data points. Fortunately, the universe is literally full of them.

“Look at collisions that are a little bit lower, ones that are like 20% of [the Oh My God particle event]. We've seen hundreds of collisions that are 20% of that energy. So in that range, those are 25 times more energetic than LHC collisions.”

And that’s just since we started looking for these high-energy collisions in the atmosphere. The Earth has been here for 4.5 billion years and has been getting pelted with these particles the entire time, so if a strangelet was going to form from a high-energy collision, it’s had billions of years to turn the planet into goo long before we turned on the LHC. 

“And when you look at the Earth, it’s not that big,” Lincoln added. “Take a look at Jupiter, it’s much larger. Look at the sun. If this were a problem, if this strangelet-production thing were indeed dangerous, we would look out in space and we would see stars transforming themselves into strange stars, or disappearing into black holes [from microscopic black holes forming from high-energy collisions], or whatever. And we don't see that. So, from this, we can conclude fairly definitively that no matter what thing you can possibly imagine, it can't be dangerous because nature has already done the experiment for us.”

The final word on strangelets?

Ultimately, the question of strangelets and the threat (or non-threat) they pose might have a much simpler answer, and it’s one that has a lot more experimental evidence in its favor: they don’t exist.

The entire idea of strangelets rests on the theory that strange matter is the lowest energy state matter can take. In fact, the strange matter hypothesis argues that, in the end, all matter will eventually decay into strange matter, though it will be many times the life of the universe before that would happen.

What if this premise is simply wrong? Pull on this hard enough and the entire strangelet idea unravels very quickly.

“That would be my explanation for why we don't see [strange matter],” Lincoln said. “Either it doesn't form at all, or it's not the lowest energy state, because we don't see matter transitioning to that state in nuclear reactions in very heavy nuclei in heavy ion collisions.

“There has been no evidence that strangelets have been created, but if they have, they don't persist long enough to give an experimental signature. So it's one of those two things: it's not made or it's not the lowest energy state.”

If it’s not the lowest energy state matter can take, then this zombie matter has no bite, and if it never forms, then there just isn’t any reason to fear it. That might seem like a neat bow to tie around a controversy that even made its way into legal journals, but reality is never that simple.

Understandably, people will always pull back from what they don’t understand, and the apocalyptic will always have its appeal, whether scientific or otherwise.

“What's happening here is people are taking scientifically credible ideas, adding a little bit of pop culture understanding, and throwing in a little bit of apocalyptic science fiction that's been written over the past 100 years, mixing it together in a heady brew, and coming up with something to fear,” Lincoln said.

It’s taken a concerted effort from physicists to successfully educate not just the public but policymakers that the concern about strangelets, microscopic black holes, monopoles, and other dangers didn’t match the science and that their fears were unfounded—a narrow needle to thread when you’re dealing with a public that is often distrustful from the outset whenever an expert arrives on the scene to explain away their concerns.

Is that fear warranted? Are particle physicists playing fast and loose with the planet’s future for reasons that many, if not most, will rarely hear about, much less understand? In order to move science forward, these questions have to be answered whenever they arise.

“I think it is absolutely fair;” Lincoln said, “not only is it fair, it is imperative that the scientific community answer these questions, and there are answers. When you are exploring something new, be it colliders or genetic technology or any sort of thing that we don't know, it is reasonable to be asked, ‘But is it safe?’ And people have a right to expect an answer.”