What physics thinks about Now

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Einstein was worried that physics didn’t have any way to identify in spacetime where Now is: that special feeling we take for granted now… and now… and again now. When solid-state physicist David Mermin told colleagues he was going to write about Now, they assumed he was either going to show that it’s an illusion or write about “chauvinism of the present moment” (Mermin 2014, p. 422).

Nows are personal things. I experience my fingers dancing around a keyboard now in way that, by the time you have found this text, you can’t. Mermin’s more general insight is that theories are used by people with particular experiences and beliefs and in a particular context. The user of a theory is implicitly an ingredient in its predictions and could reach different conclusions if they were in a different context. This is simultaneously deep and obvious. If I’m calculating the probability that the outcome of a throw of two fair dice will be double-six, my sums will be different to those of someone who knows that one of the dice is actually loaded.

Even though it’s possible to draw out a spacetime diagram of a sequence of Nows – potentially all the Nows of someone’s life – and no Now seems special therein, the user of the diagram knows roughly where Now is for them and is likely interested in some Nows more than others. The ability to theorise spacetime in such a way that no Now is special does not imply that no Now is special for users of the theory.

One interesting puzzle is, if I’m having a cup of tea with you, is my Now in the same location of spacetime as yours, or can I be, to all intents and purposes, conversing with a zombie whose experience of Now is spatially and temporally elsewhere, maybe an hour later down the road, doing the shopping.

Mermin (2014, p. 423) tackles a relevant puzzle. Consider two twins, Alice and Bob, who begin with their experiences of Now in sync. What happens to their Nows when one zooms off near lightspeed and returns a few years later? Mermin argues that all that is needed is the principle from relativity that someone’s personal time keeps pace with their reading of a watch, wherever they are in spacetime. Here is the principle applied to Alice and Bob:

“When they are together at home, their Nows coincide. Then Alice flies off to a nearby star at 80% of the speed of light, turns around and flies back home to Bob at the same speed. Relativity requires that if Bob’s watch has advanced ten years in the meantime, Alice’s has advanced only six. But because each of their present moments has advanced in step with the watch each is carrying, the moment of their reunion continues to be Now for them both.”

So, Mermin concludes, physics – and in particular relativity – does actually have something to say about Now. And reassuringly, if a loved one dashes off near lightspeed, when they return their Now returns to sync with yours too (assuming you were synced at the outset).


Mermin, N. D. (2014). Physics: QBism puts the scientist back into science. Nature, 507(7493), 421–423.

Conventional versus non-conventional facets of reality

One of the slipperiest concepts in social theorising is that of social construction. Another potentially more productive way to look at social reality is in terms of what properties of the world are conventional rather than non-conventional. A great example comes from the physics of time. I’ll briefly introduce this and then sketch out parallels with theorising sex/gender.

The duration of a second is conventional. One definition is that a second is

‘the duration of 9192631770 periods of radiation produced by transition of an electron between two hyperfine levels of a Caesium-133 atom.’

It is conventional in the sense that it could have been otherwise and in fact the definition of a second is changing. As Bradley Dowden explains:

‘conventions are up to us to freely adopt and are not objective features of the external world that we are forced to accept if we seek the truth.’

A non-conventional feature of time is the transitivity of events in the same frame of reference. That means if an event

A precedes B, and
B precedes C, then
A precedes C.

Let’s give this distinction a go for sex/gender. Phenotypic sex category at birth has a clear definition in terms of genitals which, for most people, coincides with sex chromosomes. The category is highly reliable and is predictive of much, including sexual reproductive roles – but only for cisgendered and heterosexual people, and even then it is imperfect (due to menopause, for instance). Sex category loses its predictive power for LGBTQ+ people.

The male/female dichotomy could have been otherwise. For instance it is a fact that many adults have a gender identity that differs from the phenotypic sex category they were assigned at birth and some people are nonbinary, i.e., neither a man nor a woman. In terms of social roles, there is much greater variation than two categories (see, e.g., twinks and bears). A binary category tethered to sexual reproductive role is not the only way to carve nature at its joints.

However, there are non-conventional facts about reproductive biology that determine, for instance, whether someone is able to become pregnant. Katrina Karkazis argues that sex/gender category should be detached from the specifics of what bodily organs we have:

‘It is long overdue that we understand sex not as an essential property of individuals but as a set of biological traits and social factors that become important only in specific contexts, such as medicine, and even then complexity persists. If we are concerned with certain cancers, for example, knowing whether someone has a prostate or ovaries is what’s important, not their “sex” per se. If reproduction is the interest, what matters is whether one produces sperm or eggs, whether one has a uterus, a vaginal opening, and so on.’

Where in spacetime is our phenomenal experience of now?

Minkowski (1908) posited a structure of four dimensional (4D) spacetime that treats time as another dimension just like the three dimensions of space. One way to think about spacetime is that everything that has happened, is currently experienced by someone as happening, or is ever going to happen, has a location in this 4D structure. Past, present, and future exist in spacetime as a frozen block. This realist interpretation of Minkowski spacetime is known as eternalism or the block universe.

Assuming that conscious experience is real and part of the physical world, there is an interesting problem to solve of where it is, in particular where now is, and how we experience the passage of time. Even those who argue that the experience of time is an illusion have to explain what enables the illusion (Prosser, 2012).

Consider a 2D toy model of spacetime: one dimension of time, one dimension of space. You could imagine this as a model of a universe in which it is only possible to jump up and down and not move in any other direction. Also suppose there are only three points in this spacetime. The figure below illustrates three example scenarios of where phenomenal experience could be. The black points denote where someone is and the orange blobs denote where their conscious experience of now is occurring.

Scenario a consists of a multitude of experiences scattered across spacetime. In this toy universe, there are three experiences at three nows. It doesn’t seem to work, at least not according to my experience of the world. As I type this, I see my fingers dance across the keyboard. There is movement. But in scenario a, each of the three individual experiences is frozen. Another way to think of the universe of a is as an animated flip book which, in the example above, has three pages. The problem is that there is nobody and nothing to flip the book to bring the animation to life. There is no way to travel between the nows.

I first spotted scenario b in a chapter by Natalja Deng (2019, p. 12). The idea is that conscious experience is temporally extended across spacetime rather than located at multiple individual points in spacetime. Using the flip book analogy, imagine threading a string through the pages of the book, passing through the points where and when someone having an experience is located. That piece of string is where conscious experience is. So in our universe, conscious experience would be some kind of 4D structure. This is compatible with the view of objects (such as us) as 4D spacetime “worms”. It is very appealing as it allows information flow across time, though it’s unclear to me why there would be an experience of the passage of time, rather than simultaneous experience of all times past, present, and future. The now is rather over-extended.

Scenario c is another logical possibility that I haven’t yet found in the literature, illustrating that consciousness does not have to be spatially or temporally close to the action being experienced. The idea is that the information required for temporal experience is accumulated over time, along the spacetime worm, until finally you get a burst of experience covering your whole life at the moment you die. Or perhaps this happens at the very end of time down one end of the block universe. In any case, according to this possibility, if you are having a conscious experience then either time has ended or – at this end of the universe – you are dead (but worry not, most of your spacetime worm is still alive). Note the absence of orange left of the final blob: for most of your existence you are living as a zombie. Scenario c again doesn’t explain how the burst is experienced as passage of time.

So much for that exercise!

There are very many other ways to think about the passage of time. One is to reject the notion of a block universe. Physicist Avshalom Elitzur really doesn’t like it at all (Falk, 2016):

“I’m sick and tired of this block universe. I don’t think that next Thursday has the same footing as this Thursday. The future does not exist. It does not! Ontologically, it’s not there.”

Another is to keep the block universe but conjecture the existence of a moving spotlight in which particular regions of spacetime “glow” and become experienced (Skow, 2009). I don’t understand the details of this yet, e.g., it seems to require another dimension of time to index what regions glow, and consequently another problem to solve.

Yet another is to conceive of the universe as a growing block, so the past and present exist; however, the future doesn’t yet (Perović, 2021, reviews different versions of the theory). Time passes as the universe grows and “now” is at the newly created edge of the block. This seems compatible with Muller and Maguire’s (2016) theory that the universe expands in 4D by Hubble expansion:

“We postulate that the increase in space is accompanied by an increase in time, by the creation of new moments of time. […T]he future does not yet exist; we are not moving into the future, but the future is being constantly created. The moment we experience as ‘now’ can be identified as the instant of new time that was just created.”


Deng, N. (2019). One Thing After Another: Why the Passage of Time Is Not an Illusion. In The Illusions of Time (pp. 3–15). Springer International Publishing.

Falk, D. (July 19, 2016). A Debate Over the Physics of Time. Quanta Magazine.

Minkowski (1908). Space and time. Lecture given at the 80th Meeting of the Natural Scientists in Cologne on September 21, 1908.

Muller, R. A., & Maguire, S. (2016). Now, and the Flow of Time. arXiv.

Perović, K. (2021). Three Varieties of Growing Block Theory. Erkenntnis, 86, 623–645.

Prosser, S. (2012). Why Does Time Seem to Pass? Philosophy and Phenomenological Research85, 92–116.

Skow, B. (2009). Relativity and the Moving SpotlightJournal of Philosophy106, 666–678.