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 โ€œchauvinism of the present momentโ€ (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 Bobโ€™s watch has advanced ten years in the meantime, Aliceโ€™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), 421โ€“423.