Has tomorrow already happened?

Think of your “now” as a line you draw across the cosmos. Einstein found that when you move, your line tilts. Representational Pics/iStock

Do you share the same "now" as the stranger who walks past you tonight?

It sounds like a question for philosophers, or for two friends arguing over cutting chai at midnight. It is neither. It is a physics question - and the answer, discovered a century ago and still barely known outside laboratories, is: not quite. Hidden inside that "not quite" sits the largest question you can carry down a pavement: has your tomorrow already happened?

Tonight on Marine Drive, two strangers will pass each other and never meet again. What they will not share is today. Not here - out there. Two-and-a-half-million light years away, in the galaxy of Andromeda, the farthest object your naked eye can see on a clean dark night, his "right now" and her "right now" are separated by about a week.

We grow up believing the universe keeps one clock. That "now" is a single moment stretched across everything - that somewhere there is a master present, ticking at once for stars, cities, and us. Einstein's deepest discovery was that this master clock does not exist. The universe does not have a "now". Each of us carries our own.

Think of your now as a line you draw across the cosmos. A thin slice that declares: this is what is happening everywhere at this moment. Einstein found that when you move, your line tilts.

The tilt is absurdly small. Across a room, across a city, it changes nothing you could ever notice. But a small tilt on a long enough lever becomes a large swing. Push a door gently near its hinge and the far edge sweeps through a wide arc. Andromeda sits at the far edge of a door two-and-a-half-million light years long.

So: walk, and your slice of "what is happening in Andromeda right now" shifts by a few days compared with someone standing still. Walk in opposite directions, as our two strangers do, and your todays out there drift roughly a week apart. Drive along the Sea Link and the gap stretches into weeks. Take a flight to Delhi, and your present in Andromeda slides by more than a year.

Both strangers are correct. That is the part the mind resists. Relativity does not say one of them is confused. It says the universe offers no fact of the matter about which distant moment is "now". The cosmos does not print one calendar. It prints a private edition for every traveller, tilted by the direction of their stride.

The British physicist Roger Penrose sharpened this into a famous thought experiment. Suppose, he said, a fleet is preparing to launch from Andromeda toward Earth. For the man walking toward Nariman Point, the council out there has already met. The decision has been taken. The fleet is on its way. For the woman walking toward Chowpatty, the meeting has not yet begun. The decision still hangs open.

Same pavement. Same second. For one of them, an event is already history. For the other, it has not yet happened.

Now take one honest step further, as Penrose did. If something can be "already decided" for one stroller and "still undecided" for another, in what sense is the future truly open at all? Perhaps every moment - past, present, future - simply exists, laid out like the pages of a book that is already written. In this picture we do not watch the universe happen. We move through it, page by page, and we call the turning "time". Physicists have a name for it: the block universe.

Here I owe you a physicist's honesty, the part that enthusiasm usually skips.

Nobody can see any of this. The light arriving from Andromeda tonight left it two-and-a-half-million years ago, when our ancestors were still learning to shape stone. No walker can peek at "now" out there. The paradox permits no messages, no warnings, no prophecy. Your commute, your deadlines, your dinner - none of it changes.

And physicists themselves still argue about what it means. Some insist that "now" across such distances is not a physical fact at all, only a bookkeeping convention - that asking what Andromeda is doing this instant is like asking what lies north of the North Pole. Others take the block seriously and accept its eerie conclusion: the future already exists, and free will is the feeling of reading. A hundred years on, the argument is not settled. I have learned that some questions do not end. They only deepen.

But let me tell you why I keep returning to this strange idea, and it is not the invasion fleet.

If the block universe is right - if all moments are equally real - then the past is not deleted. Your grandmother, pouring tea in a 1974 kitchen, is not erased from reality. She exists, permanently, at her coordinates in spacetime, as real there as you are here. Loss stops being annihilation and becomes geography. The people we have lost are not gone from the book. They live in earlier chapters, and the chapters do not burn.

Einstein reached for exactly this. In March 1955, his oldest friend Michele Besso died - the companion of his Bern patent-office days, the only person thanked in the 1905 relativity paper.
Einstein wrote to Besso's family and did not offer them heaven. He offered them physics: to those who believe in it, he wrote, the separation of past, present, and future is only a "stubbornly persistent illusion". A month later, Einstein was gone too.

He believed, to the end, that his friend was not nowhere. Only elsewhere - a few pages back.

So tonight watch the walkers. They think they are walking home. They are walking through different versions of "now".

Nishant Sahdev is a physicist at the University of North Carolina at Chapel Hill, US. He chases ideas that refuse to stay in laboratories for your favourite Sunday mid-day.