The following is an extract from our Lost in Space-Time newsletter. Each month, we hand over the keyboard to a physicist or mathematician to tell you about fascinating ideas from their corner of the universe. You can sign up for Lost in Space-Time here.
Does your workday ever include picking fights about whether empty space is actually empty or whether time is an illusion? Has a co-worker ever told you that you’ve caused them emotional pain by discussing concepts from quantum field theory? Welcome to the life of a żěè¶ĚĘÓƵ physics reporter.
Trying to keep up with the most recent developments in physics is a never-ending exercise in reconfiguring your sense of how the world works. You’re constantly racing to catch up with a wave that is coming to dissolve all of your preconceived notions about the nature of physical reality, destroying assumptions as though they were sandcastles on the beach. This is exhilarating work, and this past year was no exception.
Advertisement
Over the course of the past 12 months, I found myself speaking to researchers who are trying to uncover whether we live in a multiverse, hunting for exotic particles in unassuming chunks of metal, discovering that seemingly fundamental notions like cause preceding effect can sometimes be up for debate, working to discover what exactly time is and trying to understand what happens when you take a darling of classical physics like entropy and plunge it into the quantum realm. “Oh wow,” I said on myriad Zoom calls, often followed by “I know this will sound like nonsense but” to my editors afterwards. Now I want to give you a taste of everything physicists learned about the ins and outs of our reality in 2024 – and everything that still remains mysterious.
First, we have to talk about the multiverse. The idea that our world is just one of many originated in the 1950s with physicist Hugh Everett. He had a problem with quantum mechanics, and he realised that he could make peace with the theory if he recast it as a set of facts about an ever-expanding set of universes. In Everett’s view, if a quantum object starts off in a state where it can have several different properties – like a quantum analogue of being purple or pink – but you cannot tell which colour it has unless you interact with the object, that is because each colour state exists in a separate universe. When you do interact with the object and find out what colour it is, you are effectively discovering whether you live in the “purple object universe” or the “pink object universe”, both of which are real and exist in parallel.
In April, I reported on an extension of this idea that suggests the multiverse is actually much, much bigger than we thought. A team of researchers argued that many more universes arise when you determine the object’s colour. This multitude of multiverses stem from additional interactions – between you and the surface the object is sitting on, for instance, or between the object and the sun whose light makes its colour visible.
The multiverse was back on my desk in September when a different team of researchers used computer simulations – and the idea that quantum objects must have well-defined records of past behaviour – to assess whether the many worlds hypothesis can be reconciled with our daily experience of physics. Their calculations, though not without critics, showed that our reality does seem to be compatible with a quantum multiverse.
The multiverse raises a whole swath of dilemmas about cause and effect because now every possible outcome actually takes place – but most of them play out in a different universe. But you don’t have to explode our reality into myriad parallel strands to find causality troubling. For the past decade, physicists have been debating whether “A causes B” and “B causes A” must be mutually exclusive in the quantum realm. For a while, it really looked like an answer in the negative would win out. The triumph for so-called indefinite causality – where both scenarios are true and indistinguishable – appeared to come with a 2017 experiment that involved seemingly causality-defying particles of light. Physicists ran with the idea so much so that, for example, in July I wrote an article simply titled “Quantum computers may work better when they ignore causality”.
But in September, two physicists threw down the gauntlet and presented a mathematical argument for all those past studies being cases of misinterpretation. They appealed to the structure of space-time, the fabric of our reality, and insights from quantum information theory, ultimately asserting that even in the quantum world, either “A causes B” or “B causes A” and nothing in between. Is this the last word on causality? Has it been saved from the relentless “well, actually” of quantum theory? Several physicists told me that only future experiments will be able to tell.
Other quantum physics experiments were more generous in their revelations in 2024. In March, researchers completed an experiment 30 years in the making and saw a glimpse of something that looked an awful lot like a particle of gravity. They spotted it in a chunk of a very cold semiconductor that was also exposed to a very strong magnetic field. Physicists have been searching for hypothetical particles of gravity for decades and finding them would advance the yet-unsurmountable task of developing a quantum theory of gravity.
In November, a similar experiment turned up a particle that is massless when it moves in one direction but has a mass if it makes a 90-degree turn and starts moving perpendicularly. Both particles were technically “quasiparticles”, which means that they do not exist by themselves. Instead, they emerge from a collection of other particles that have been corralled together by some quantum effect. This is a well-known occurrence in so-called condensed matter physics, but the new denizens of the quantum realm that it keeps producing continue to be fascinating.
One thing that the quantum realm has not been hospitable to recently, however, is the laws of thermodynamics. Thermodynamics dates to the 1800s, when physicists and engineers tried to understand the laws governing heat, energy and temperature in order to leverage these rules to build better engines. One notable insight from this era is that changes in entropy, or the measure of disorder, control much of what can happen in a system that is trying to get warmer, colder or more energetic.
But there are different definitions for what it means for entropy to increase – and it turns out that they don’t match when quantum rules are in play. A mathematical study published in October found that three definitions of entropy, which have previously been considered equivalent, can produce different results in the quantum realm. This is bad news for anyone who has been trying to make thermodynamics quantum, which includes researchers who are trying to harness quantum effects to make better batteries as well as many who are building quantum computers. The physicists I spoke to simply said that all of thermodynamics for quantum objects now must be re-examined. (I guess I have that to look forward to in 2025.)
And before we completely run out of it, I also have to tell you about what’s been going with time. As I reported in May, it may be a quantum illusion. It may feel real, but it could actually be an emergent property, like those quasiparticles that cannot exist by themselves. Building on an idea from the 1980s, a team of physicists mathematically proved that many physics equations that we rely on to understand the world would still hold if time existed only as a consequence of all objects secretly being quantum entangled – inextricably linked with some clock. This gets complicated because many objects can count as clocks and quantum entanglement is only well defined for some objects, but researchers believe that this idea should be tested in future experiments. It also raised lots of discussion in żěè¶ĚĘÓƵ’s New York newsroom, where many of us already felt that time is fake, especially when working on a tight deadline.
Whether time is real or not, we have, just like in a physics experiment, chosen to arrange our lives along a number line and to the beat of a clock’s ticks. As a result, it is undeniable that 2025 is upon us. What might it bring in terms of mind-blowing physics? I honestly have no idea, but I am sure I can expect a lot more “oh wow” moments in my future.
