Is time real, or is it just an illusion? In our latest video, we meet Jim Al-Khalili, physicist and author of On Time, at the Royal Observatory Greenwich, the home of the Prime Meridian, to explore one of the deepest mysteries in science.

From space-time at the heart of Albert Einstein’s theory of relativity to the arrow of time, entropy and the origins of the universe, Al-Khalili explains why physicists remain divided over the true nature of time.

Could the past, present and future all exist at once? And is time travel really possible? Join us for a mind-bending journey through one of physics’ greatest puzzles.

Ghostly particles called neutrinos pose some of the biggest mysteries of modern physics, but researchers thought they could get a handle on them by corralling them into a laser beam. Two new analyses show that neutrinos are more slippery than that, making the proposed neutrino laser design impossible.

Researchers have been studying neutrinos since the 1940s. They are among the most abundant particles in the universe, but much about them remains unknown. Notably, they are extremely light, mere ghosts compared with more massive particles like neutrons, yet their exact mass is unclear.��

In 2025,  at the University of Manchester in the UK and  at the Massachusetts Institute of Technology (MIT) suggested a novel and surprising way to gain clarity –  using thousands of extremely cold radioactive atoms to create a laser beam of neutrinos.

, also at MIT, heard a lecture about the idea and immediately worried about it being too good to be true. He and his colleagues have now confirmed that hunch with two rigorous mathematical investigations.

Neutrinos are produced when radioactive atoms undergo nuclear decay. Jones and Formaggio theorised that if many such decaying atoms were pushed into a quantum phase of matter called a “Bose-Einstein condensate” (BEC), where they all share a quantum state, then their respective neutrino emissions would be amplified, forming a laser-like beam. Creating such a BEC would require making thousands of radioactive atoms extremely cold so they could behave quantumly, which is a big technical challenge.��

But Ketterle, who received the Nobel prize for creating some of the first ever BECs in the 1990s, and his team uncovered a more fundamental obstacle.

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The key to the neutrino laser proposal was a memory effect: when an atom in the BEC emitted a neutrino, it would be more likely to continue emitting more neutrinos in the same direction, thus pushing them into a beam, because the quantum state that all the ultracold atoms share would retain a trace of that first emission. Ketterle and his colleagues showed that this memory, although present, would be about 10,000 billion times too brief to affect the neutrinos as intended. Even more troublingly, the team uncovered that the memory would actually have the opposite effect from that intended, which Ketterle calls an anti-memory.��

“If I am an atom and I have emitted a neutrino, I am not allowed to [immediately] emit a neutrino again,” he says. This effect’s origin is subtle, stemming from neutrinos being a type of particle called fermions, which fundamentally behave differently than particles of light that our ideas about lasers tend to be built upon.��

We’ve spotted the strongest microwave laser in the known universe

Colliding galaxies can create a beam of focused microwave radiation known as a maser, and astronomers have discovered the brightest one ever seen

“I think these papers sharpen where the real difficulty lies,” says  at Queen’s University in Canada. “For nuclear-scale energies [pertaining to atoms’ decay], these requirements become extraordinarily demanding.” 

He says the new analysis does not categorically rule out every possible way to build a neutrino laser, but shows that the most conventional scenario where each atom emits one neutrino cannot work. If each atom emitted two neutrinos at a time, the analysis may be different, he says. “I think the interesting scientific question now becomes more precise rather than disappearing: what kinds of nuclear or neutrino processes, if any, avoid the limitations they have identified?” says Leach. In his view, this question is most likely to be definitively answered through experiments.��

Formaggio and Jones didn’t respond �鶹��ý’s request for comment.

For Ketterle, the lesson of the neutrino laser that couldn’t be is that science can correct itself even when doing so means abandoning inspiring and creative ideas. Getting clarity required hours of discussion, including with Jones and Formaggio. But that is ultimately how physics ought to work, he says.

Researchers may have spotted the first signs of a dark matter particle. If the result is confirmed, it could be one of the most monumental discoveries in the history of physics.

Dark matter makes up about 85 per cent of matter in the universe, but for decades physicists have been unable to determine what it itself is made of. Hypotheses have ranged from different sorts of particles and forces to strangely behaving gravity and everything in between – but the leading idea has always been that of the weakly interacting massive particle, or WIMP. As we have built more and more powerful detectors to search for WIMPs, though, their failure to show up has led many to doubt their existence and move on to more exotic ideas.

That doubt may soon come to an end. A new analysis of data from the LUX-ZEPLIN (LZ) experiment in South Dakota has revealed a single particle that may be the first WIMP ever detected. “This result is certainly an exciting one, because it provides some potential positive hint for what dark matter could be, not just what dark matter could not be,” says at Brown University in Rhode Island, who wasn’t part of the analysis.

LZ looks for dark matter using a 7-tonne tank of liquid xenon, surrounded by several layers of shielding and buried more than a kilometre down to keep out other types of particles. When an outside particle hits a xenon atom, it creates a tiny burst of light that is measured by sensitive detectors surrounding the tank. Researchers can then use that light to reconstruct the path of the incoming particle and the energy level of the interaction.

In the search for WIMPs, we generally look for events with energies below about 30 kiloelectronvolts (keV), says LZ spokesperson , also at Brown University. That assumes the simplest sort of interaction, in which the WIMP is essentially bouncing off an individual nucleon in the xenon nucleus. Having not found any hints of WIMPs in that energy range, the LZ team decided to reanalyse the first 220 days of data from the detector, searching for events with higher energies.

That is where they found their dark matter particle candidate, at about 248 keV. “You must never make an assumption that nature is going to do something the easy way,” says Gaitskell. “Nobody can accuse our universe of making the simplest and most natural-seeming choices.”

Such a high-energy event can’t have come from the most simple type of interaction between a WIMP and a nucleon – instead, it must have happened through a more complicated coupling between the WIMP and the entire xenon nucleus. If that is the case, the WIMP must have a mass more than 200 times the mass of a proton.

“It’s not just that they’re seeing something, but that there’s a hint at what the underlying mechanism might be,” says at the University of California, Berkeley, who wasn’t involved in the analysis. “If this all stands up, our learning curve is going to be pretty steep from now on.”

Actually detecting a dark matter particle could reveal some of the universe’s most closely held secrets, including information about the early universe that is otherwise nearly impossible to determine, and drastically shake up our standard model of particle physics. But as Haxton says, this detection is not yet certain.

In particle physics, the statistical threshold for a finding to go from an intriguing hint to a solid discovery is a number called 5 sigma, which means there is about a 1-in-3.5 million chance that a signal like this would show up as a fluke rather than a true sign of dark matter. Right now, this detection from LZ sits at 2.6 sigma, which means that there is about a 1-in-200 chance it could appear as a fluke.

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That might still seem fairly secure, but in the search for dark matter it is far from enough. “In science we sit down and we do so many damned experiments, I’m afraid you come across a 1-in-100 event fairly often,” says Gaitskell.

“Before one can declare victory, you need a few more data points, but now they have something to aim for, they know where to look,” says Haxton. The good news is that this analysis only covered about one-third of the data that LZ has already taken, and other dark matter detectors around the world have their own data, as well. A few more events in the same energy range could bump the discovery of WIMPs up to 5 sigma – and change our understanding of physics and the universe forever.