Without a sense of time, leading us from cradle to grave, our lives would make little sense. But on the most fundamental level, physicists aren’t sure whether the sort of time we experience exists at all.

This is the topic of the first episode of our new podcast series, Great Mysteries of Physics . Hosted by me, Miriam Frankel, science editor at The Conversation, and supported by FQxI , the Foundational Questions Institute, we talk to three researchers about the nature of time.

Scientists long assumed that time is absolute and universal – the same for everyone, everywhere, and existing independently of us. It is still treated in this way in quantum mechanics, which rules the microcosmos of atoms and particles. But Albert Einstein’s theories of relativity, which apply to nature on large scales, showed that time is relative rather than absolute – it can speed up or slow down depending on how fast you are travelling, for example. Time is also interwoven with space into “space time”.

Einstein’s theories enabled scientists to picture the universe in a new way: as a static, four dimensional block, with three spatial dimensions (height, width and depth) and time as a fourth. This block contains all of space and time simultaneously – and time doesn’t flow. There’s no special now in the block – what appears to be the present to one observer, is simply the past to another.

But if that’s true, then why is our experience of time moving from past to future so strong? One answer is that entropy, a measure of disorder, is always increasing in the universe. When you run the numbers, explains Sean Carroll, a physicist at Johns Hopkins University in the US, it turns out that the early universe had very low entropy. “[The universe] was very, very organised and non-random and it’s been sort of relaxing and getting more random and more disorganised ever since.” This is likely to create an arrow of time for human observers.

We don’t know why the universe started out with such low entropy, however. Carroll suggests it it may be because we are part of a multiverse containing many different universes. In such a world, some universes would, statistically speaking, have to start out with low entropy.

Emily Adlam, a philosopher of physics at the Rotman Institute of philosophy at the University of Western Ontario in Canada, on the other hand, believes the mystery of why our universe started with low entropy is a problem that ultimately stems from the fact that physics is riddled with assumptions about the time.

“I personally am very much on the side that says time does not flow,” she explains. “This is kind of an illusion that comes from the way in which we happen to be embedded in the world”. Her hunch is that, on the most fundamental level, everything happens all at once – even if it doesn’t appear that way to us.

Adlam argues the best way to understand time would be to remove it entirely from our theories of nature – to strip it out of the equations. Interestingly, when physicists try to unite general relativity with quantum mechanics into a “quantum gravity” theory of everything, time often disappears from the equations.

Experiments could also help shed light on the nature of time, helping to test various combinations of quantum mechanics and general relativity. Natalia Ares, an engineer at the University of Oxford, believes that studying the thermodynamics (the science of heat and work) of clocks may help . “By understanding clocks as machines, there are things that we can understand better about what the limits of timekeeping,” she argues.

You can also listen to Great Mysteries of Physics via any of the apps listed above, our RSS feed , or find out how else to listen here . You can also read a transcript of the episode here .

Natalia Ares receives relevant funding from the Royal Society, EPSRC, the Foundational Questions Institute Fund, a donor advised fund of Silicon Valley Community Foundation, and from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant Agreement No. 948932). She is CSO of a start-up company called QuantrolOx, which focuses on developing machine learning for quantum device control. Some of Emily Adlam's research was made possible through the support of the ID 61466 grant from the John Templeton Foundation, as part of the “The Quantum Information Structure of Spacetime (QISS)” Project (qiss.fr). The opinions she expressed are her own and do not necessarily reflect the views of the John Templeton Foundation. Sean Carroll has received funding from the Foundational Questions Institute, the US National Science Foundation, the US Department of Energy, the Sloan Foundation, the Packard Foundation and the Guggenheim Foundation.