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For over a century, scientists have thought of time as a fundamental property of the universe. But the more researchers seek to understand time\u2019s ultimate nature and the mechanics behind its emergence, the more questions they have. What does gravity have to do with time? How about quantum phenomena? Are there mysterious particles that travel faster than light\u2014and therefore mess with time? Any of these concepts might one day prove to be key ingredients that forged the phenomenon we call time, or at least might have a deep, inseparable connection with it. But whether through mathematics or observation, time seems to play with the laws of physics, producing more questions than it answers. Here are some significant ideas scientists are juggling when it comes to this most mysterious concept.<\/p>\n
Time\u2019s Arrow May Emerge From Gravity<\/p>\n
Time has an arrow: It only ever seems to move in one direction. The future<\/a> is always unknown to us, while the past forever remains locked and inaccessible. And yet, the vast majority of the laws of physics don\u2019t seem to care about the direction of time at all. The equations that govern everything\u2014from subatomic particles to the orbits of planets\u2014can\u2019t discern between forward and backward motion in time.<\/p>\n So, where does the arrow of time<\/a> come from? A growing group of physicists believe that it originates in the force of gravity itself. But to get there requires a radical reshaping of Einstein\u2019s General Theory of Relativity. The new idea is called Shape Dynamics, and it just may be our future theory of gravity<\/a>.<\/p>\n For over a century most physicists have believed that the key to time\u2019s arrow lies in the concept of entropy, which very roughly is a measure of a system\u2019s disorder. The second law of thermodynamics states that in closed systems entropy always goes up, meaning that systems always go from ordered to disordered. Not only does this law govern physical systems, but it also jibes with everyday experience: it\u2019s much harder to clean a room than it is to make it messy.<\/p>\n But this connection raises an annoying problem. For entropy to generate an arrow of time, our universe had to start in an exceptionally low-entropy, highly ordered state. This strikes most physicists as a contrived, tacked-on assumption that doesn\u2019t mesh with our understanding of a messy, chaotic Big Bang, our best theory for how our universe began.<\/p>\n In 2014, physicist Julian Barbour, PhD, and his colleagues proposed an intriguing solution to this apparent paradox: gravity. But not gravity as we know it through the general theory of relativity, which says that space and time bend under the influence of matter and energy. Instead, he recast Einstein\u2019s famous equations using another language, called Shape Dynamics. Shape Dynamics focuses on the relationships between objects, rather than the spacetime, the unified fabric of space and time, that they sit in. Using this framework, Barbour found that if you take a random collection of particles and let them interact through their mutual gravity, an arrow of time naturally emerges.<\/p>\n As the particles interact, they build more complex arrangements with increasing entropy, but not before passing through a period of low-entropy, highly ordered organization. During the experiment, this all seemed to happen naturally. Even though gravity doesn\u2019t care about the flow of time, an arrow organically emerged through the gravitational dynamics of the particles.<\/p>\n But Barbour\u2019s model was highly simplistic. He pretended the universe was nothing more than a collection of particles that only interacted through the single force of gravity. The real universe is much richer than that, with many different kinds of particles interacting through the four forces of nature we know today.<\/p>\n So the approach to an idea like Barbour\u2019s is with skeptical curiosity: poking at it from all directions to see if it holds up to scrutiny.<\/p>\n But so far, nobody has been able to use Shape Dynamics to build a fully realistic portrait of the evolution of the universe to the same level of detail that we understand the cosmos through General Relativity. However, even though the theory is decades old, relatively few researchers around the world are working on it, so we shouldn\u2019t expect huge advances\u2014yet. Still, it\u2019s a fascinating idea: we experience the flow of time because it\u2019s a natural outcome of the basic laws of physics. And that makes the idea of Shape Dynamics worth pursuing further.<\/p>\n Time Could Be Flowing in Reverse<\/p>\n Physicists still wonder why\u2014or, perhaps, if\u2014the arrow of time functions in one direction. In classical physics, equations function without regard for time\u2019s directionality, or its preference for moving into the future or past. One way to comprehend this mathematical disregard for directionality is to look at a video of a swinging pendulum. Whether the video plays forward or in reverse, the pendulum\u2019s motion remains the same, and time\u2019s directionality<\/a> is indistinguishable. And yet, time as we experience it clearly moves in one direction. So, researchers continue to search for the reason why time behaves asymmetrically.<\/p>\n Over more than two years, a team of physicists in England pursued this question theoretically, seeking to mathematically derive time\u2019s arrow in open quantum systems. In a paper<\/a> published in January 2025 in the journal Scientific Reports, they present results that indicate two arrows of time<\/a> exist in these systems, moving in opposite directions. This discovery could urge scientists to reconsider how time\u2019s arrow impacts thermodynamics, quantum mechanics, and even the universe\u2019s creation.<\/p>\n In the team\u2019s framework, open quantum systems are preferable since they allow for change over time\u2014like energy dissipating into the surrounding environment, also known as entropy. This is important to the researchers\u2019 model because it serves as a proxy for the passage of time<\/a>; when you see energy dissipate from a system into its surroundings, you can conclude the direction in which time is flowing. This dissipation is also irreversible: if you spill a glass full of milk, the milk can\u2019t flow back into the glass.<\/p>\n The team expected to see just one arrow of time in their equation, Rocco says. But with the open quantum system, Rocco says they obtained \u201cslightly modified equations,\u201d which revealed two arrows of time spontaneously arising.<\/p>\n Crucially, Rocco and his team found that entropy<\/a>, described in the second law of thermodynamics, is preserved in their model. This law states that entropy, or a tendency toward a disordered state, increases over time within a system. In both directions of these two arrows of time, the systems still hurdle toward disorder as energy dissipates into its surrounding environment. The arrow can\u2019t reverse direction once it\u2019s set off, returning the milk to the proverbial glass. Rather, entropy progresses whether the arrow of time shoots off into the past or the future.<\/p>\n This twin entropy demonstrates a symmetrical nature of time in open quantum systems. From a given moment, time\u2019s arrow might have shot off in the opposing direction. \u201cThe physics is actually telling us that the opposite direction might have been equally possible,\u201d Rocco says.<\/p>\n That means the team\u2019s results could call into question how we think about the arrow of time in thermodynamics, cosmology, and quantum mechanics. For instance, Rocco mentions that some people speculate whether at the time of the Big Bang<\/a>, two universes emerged, traveling in opposite directions in time. His team\u2019s result certainly doesn\u2019t confirm this speculation, but the theoretical framework they developed could help us reassess our assumptions of time\u2019s function in the universe.<\/p>\n Time Could Have 3 Dimensions<\/p>\n Albert Einstein\u2019s special theory of relativity is one set of rules that helps us understand the universe<\/a>. This theory tells us that time and space are linked together in a four-dimensional fabric, called space-time, with three dimensions of space and one of time. Time is special because we can only move in the direction of our future<\/a>, unlike the spatial dimensions, where we are free to move wherever we like. Plus, special relativity tells us that movement in space-time is limited to no faster than the speed of light, over 186,000 miles per second.<\/p>\n But, what would the universe be like if you could travel faster than light? The physics of special relativity says it\u2019s impossible to move from below the speed of light and accelerate to reach it, let alone go faster than light. But the rules of special relativity also allow the reverse situation, where objects can start off faster than light already\u2014but then they can never slow down below that limit.<\/p>\n When researchers from the Universities of Warsaw and Oxford asked this question in their 2022 paper<\/a>, the answer they found is astounding. For observers who are already traveling faster than light, the connection between time and space is flipped. Instead of three spatial dimensions and one temporal dimension, faster than light observers see three dimensions of time<\/a> and only a single dimension of space. This means that particles follow more than one trajectory at once. In effect, they travel into more than one future simultaneously.<\/p>\n This corresponds with the very strange rules of quantum mechanics, which say that particles, which are smaller than atoms, can seem to appear in multiple locations at once. We must view particles as waves of probability that slosh around. Where the waves peak, we have a good chance of finding the particle the next time we go looking for it. And where the waves are at their minimum, we are unlikely to see that particle.<\/p>\n So, from our normal slower-than-the-speed-of-light perspective, it looks like these faster-than-light objects are acting like waves. Think about it. If you throw a baseball it only follows one path. But if the baseball hits a nearby pond, the water waves travel in multiple directions at once.<\/p>\n In fact, the concept of a particle in this perspective completely breaks down. Instead you can only talk about fields, which are waves that exist throughout all of space and time. The researchers point out that many theories in physics, especially ideas concerning the fundamental physics occurring in the extremely early universe, already employ fields that have faster-than-light properties.<\/p>\n Whether or not the individual particles\u2014known as tachyons\u2014can travel faster than light remains an open question. That\u2019s because the existence of such particles would really mess up basic concepts in the universe, like causality and the progression from past to future that we are familiar with. But quantum mechanics already stretches those concepts, so it\u2019s possible that the universe already runs on these non-intuitive rules. Perhaps we just need to make the connection.<\/p>\n Maybe Time Doesn\u2019t Even Exist<\/p>\n