Hello, and welcome to Lost in Space-Time, the physics newsletter that's all about finding new particles of information, wherever in the cosmos they lurk. This month, we ask whether muons could be the key to physics beyond the standard model of particle physics, plus we look at a new idea to explain dark energy, ask how gravity escapes from a black hole when nothing is supposed to and explore whether we'll ever find a magnetic north without a south.
Could muons set physics in a spin?
A few years back, in the aftermath of the discovery of the Higgs boson in 2012, New Scientist ran a special issue entitled “Crunch time for physics: What's next?”. I’m not sure how I feel when I say we could run it almost unchanged today.
The premise back then was that, with the Higgs found, the picture of particles and their interactions given by the standard model of particle physics was complete, while on larger scales the standard model of cosmology was giving us a well-tested picture of the workings of the universe as a whole. Yet taken together, these two models left gaping holes in our understanding . For instance, to square with observations, the standard model of cosmology needed to invent dark matter and dark energy – entities for which the standard model of particle physics provided no explanation.
Then there were questions like why the four forces of nature had such different strengths, with gravity out on a limb, or why indeed the cosmos exists at all as we see it, given the standard model of particle physics predicts matter and antimatter should have annihilated in the first fraction of a second.
To answer these questions, we argued back then, physics was in desperate need of either a surprising new experimental revelation, or a new theoretical impulse. The same remains true almost a decade on. But – wishful thinking it might be – I do wonder now whether we’re seeing the first glimmers of unknown physics that could move things along. And intriguingly, for the first time in a very long time, it seems to be experimentalists, not theorists, who are making the running.
In last month’s newsletter, I highlighted the persistent anomalies perhaps indicating the indirect influence of unknown massive particles that are being seen at LHCb, one of the four big experiments at CERN’s Large Hadron Collider (LHC). And in the 12 February issue of the magazine, we had an interview with Alex Keshavarzi , a researcher at the Muon g-2 experiment at Fermilab near Chicago, Illinois. For a while now, this experiment has been making measurements of a quantity known as the anomalous magnetic dipole moment of the muon that don’t square with standard model predictions. That too could indicate the ghostly influence of unknown particles popping out of the quantum vacuum that fills empty space and quickly disappearing again.
The Muon g-2 experiment at Fermilab is probing anomalous properties of muons CREDIT: Reidar Hahn/Fermilab
Muons are intriguing particles. They were discovered in cosmic rays in 1936, and soon found to be particles that were exactly the same as electrons, only about 200 times heavier. That famously led the physics Nobel laureate Isidor Rabi to quip “who ordered that?” – what use had nature for a copycat particle that lived for just microseconds and apparently had no role to play in the construction of matter?
Looking back, the discovery of the muon was the first step towards the establishment of the standard model, with its characteristic three “generations” of particles that differ only by their mass. As a historical aside, to back up my throwaway comment just now about theorists usually making the running: the muon remains the only particle of the standard model to have been discovered before theorists predicted its existence. To readers who often ask me why fundamental physicists put so much trust in wild theoretical speculation, there’s your answer.
It seems unlikely that whatever is causing the LHCb and Muon g-2 anomalies, if indeed anything is causing them, is the same thing. But then again, we simply don’t know: theorists are playing catch-up in both cases, asking what sort of new particles and interactions might explain them.
If I am going to draw a lesson from the developing muon story, it’s that after a largely fruitless decade since we ran that special issue, we really have got to look everywhere for evidence of new physics. Back then, a lot of hopes were pinned on the hugely energetic collisions of the LHC creating new and unknown massive particles. Experiments like Muon g-2 are much subtler affairs, relying on highly precise measurements of specific particle properties to tease out the unknown. It’s too early to say which approach, if either, is most likely to bear fruit. But if the Muon g-2 approach does so, given the paucity of new physics that has emerged from the LHC, that might make the case for building new, bigger and better high-energy particle smashers even more difficult to make.
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A fun paper appeared on the arXiv preprint server last month. Entitled “Casimir cosmology”, it’s from Ulf Leonhardt at the Weizmann Institute of Science in Israel – a name that’s familiar to me from his earlier work on “invisibility cloaks” made out of metamaterials.
This new paper is about a potential solution to a problem I mentioned in the previous section – the nature of the dark energy that seems to be acting against the effects of gravity to accelerate the expansion of the universe. One very obvious potential guise for dark energy is a sort of vacuum energy, the cumulative pushing effect of “virtual” particles that quantum field theory tells us are constantly popping up and disappearing again in empty space. But when you try to work out the strength of this vacuum energy, you come up with a figure 120 orders of magnitude higher than the observed strength of dark energy – a mismatch that has been dubbed “the worst prediction in physics”.
Leonhardt’s contention is that this mismatch is overblown, something he bases on insights from a part of physics he is very familiar with, the Casimir effect. This is an utterly weird, practical effect of vacuum energy – basically, you can use it to do mechanical work, for example pushing two mirrors together, thereby apparently getting something from nothing . (It’s not actually something from nothing, of course: the whole point is that even empty space has a non-zero vacuum energy, so this doesn’t violate conservation of energy.)
When theorists do calculations to explain the size of Casimir effects in the lab, they use a technique called “renormalisation” to cancel out the effects of different particles and fit the observations to the theory. Leonhardt says that if you take the same approach, not on the nanoscales at which the Casimir effect works, but on cosmological scales, you can perform a similar trick and so explain dark energy. Not only that, but the “amount” of dark energy you get is just right also to explain the Hubble tension, a mismatch between observed and expected rates of the universe’s expansion that has got many cosmologists hot under the collar at the moment.
It’s an idea that is in many ways so simple I’m surprised no one’s had it before – which might be the best reason not to believe it. Leonhardt is not a cosmologist, of course, but he does have a strong theoretical track record, and I’ll be interested to see where, if anywhere, this idea goes.
How does gravity escape from a black hole?
Time to answer a reader question now. Charles Todd asks: if nothing can escape a black hole, how does its own force of gravity get out? Good one, Charles. Well, the answer according to our current understanding of physics is that there’s nothing associated with gravity that can escape.
That goes back to a fundamental divide between gravity and the other forces of nature that explains why we have one standard model for particle physics, and one for cosmology. The other three forces of nature – electromagnetism and the strong and weak forces – all have a description in terms of the quantum field theories that underlie the standard model of particle physics. Gravity doesn’t – according to the general theory of relativity , it is a “fictitious” force generated by the warping of space and time by matter. Ergo, there is no equivalent of photons of light to be swallowed by a black hole.
Nothing escapes from a black hole - except gravity, apparently CREDIT: Shutterstock/REDPIXEL.PL
That said, the holy grail of a unified physics would be to achieve a quantum description of gravity – in which case hypothetical particles known as gravitons would play a part in transmitting the force. Like photons of light, gravitons are presumed by most theorists to be massless (although some disagree and think they might have a small mass). Either way, you are left with the conundrum of how gravitons would escape from a black hole.
And that is a truly intriguing question. The existence of black holes was only predicted by general relativity, and many theorists, Einstein included, were intensely sceptical that anything like them could exist in reality . But we now have a mountain of evidence to say that they – or something very like them – do exist. We also know by now that general relativity is a supremely well-tested theory, so any quantum theory of gravity would have to largely reproduce its predictions. What such a theory truly looks like, and so how it can square the circle posed by this question, is an unknown.
Magnetic monopoles are particles with a magnetic “charge”. Just as particles can come with positive or negative electric charge, a monopole would have positive or negative magnetic charge – the equivalent of a magnetic north or south pole on its own. The non-existence of natural magnetic monopoles appears to be a fact, but there’s nothing in the underlying equations of electromagnetism that suggests it should be so.
It’s a story I’ve been following for years. Over a decade ago, I edited a feature on one unexpected place where something looking like monopoles was turning up – within solid crystals – and I wrote about the search for "natural" monopoles at the LHC back in 2014 . (I love, by the way, some of the explanations theorists come up with for the monopole no-show – particularly, as mentioned in that feature, the idea that the period of breakneck cosmic expansion at the big bang known as inflation might have diluted the natural density of monopoles to “one in the entire volume of the visible universe”.)
The latest null result doesn’t exclude the possibility of monopoles existing, but it says that if they do exist, they must be at least 70 times more massive than the proton. That strikes me as rather generous, as the Higgs boson is over 130 times the mass of the proton, and you would have thought if there were monopoles in the same sort of energy space, we would have got wind of them by now. But what do I know – as they say, the search goes on.
also in new scientist
1. There’s still time to sign up for the one-day masterclass I’m hosting on 26 March at the British Library in London on “Frontiers of Cosmology”. It will feature six talks from top UK-based researchers on topics from the big bang to quantum gravity, plus the chance to pose your own questions. Well worth coming along to.
2. Even more immediately, if you’re anywhere around Manchester, UK, Saturday 12 to Monday 14 March are the dates for our festival of science, New Scientist Live. Alex Keshavarzi from the Muon g-2 experiment is just one of the main researchers talking about their work, plus there’s a wealth of interactive experiences suitable for people of all ages. You can also join us virtually.
3. It’s not fundamental physics, but our regular columnist Chanda Prescod-Weinstein considered the overlooked role of Earth-observing satellites in all our lives last month – as ever, well worth a gander.
That’s it for now. Thank you for reading! If you have any comments or questions, you can let me know by emailing me at lostinspacetime@newscientist.com and I’ll try to answer them in an upcoming newsletter. If you know someone who might enjoy Lost in Space-Time, please forward it on. If you haven’t yet, you can sign up to get it in your inbox every week here.
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