In March 1955, Einstein wrote to the widow of his oldest friend, Michele Besso: “Now he has departed from this strange world a little ahead of me. That means nothing. People like us, who believe in physics, know that the distinction between past, present and future is only a stubbornly persistent illusion.”

Like most people, you probably accept that the present moment exists across the entire universe; you believe the past used to exist but does so no longer and you’re sure that the future is a misty realm of possibilities, out of which some definite reality will crystallize when eventually the future becomes the present.

For more than one hundred years many physicists have disagreed with you, believing that there is nothing special about ‘the present moment’ and that future and past are equally real. This view, known as the block universe, is a consequence of special relativity.

Imagine that it’s 4011 AD and humanity has spread out to the stars. We survived global warming, the return of the ice and the end of oil; somehow we muddled through it all and on this warm, clear summer’s evening you’re relaxing, looking up at the beauties of the night sky.

Moonlight illuminates a pleasant landscape and you wonder what your friend Julie is doing up at the moon base right now. You look at your watch, 9.55 pm, and you imagine her wearily glancing at her wall clock showing the same time, her shift about to end. Of course, you can’t know for certain. Any signal from the moon will take just over a second to get to earth. No matter: you’re sure she’s doing something definite right now and in just over a second you could confirm exactly what.

So what exactly does it mean to talk about what’s happening somewhere else ‘right now’? It means that when, at some future time, the evidence reaches you, (a light image, a radio signal), you will factor in the light-delay to retrodict exactly what was happening ‘over there’ at the time you’re currently calling ‘right now’. That is the precise and unambiguous operational definition (although it gets complicated if you accelerate with respect to the ‘somewhere else’ while the light signal is still in transit).

Enough of Julie. Your gaze turns to the constellation of Cygnus and its brightest star, Deneb. The Deneb mission has just arrived after 1,600 years of star flight. The media have been profiling the crew for days, and you’re particularly taken by Amanda, the communications officer. The videos taken all those years ago, before the mission launched, show a warm, attractive woman sparkling with the idealism you’ve always found so appealing. Amanda must be getting ready for her first broadcast back to earth, scheduled for 10 pm this very evening. You think of her looking at the clock a little nervously as the seconds tick down to ten. No need to rush to the TV: her message won’t reach the Earth for another 1,400 years.

Assuming the Deneb crew adjusted their clocks to compensate for time-dilation on their trip, and assuming they are stationary with respect to you at this moment you are not wrong. Your clock and their clock both read exactly 10 pm. It’s getting a little chilly so you take a brisk stroll down the path, walking towards Cygnus.

And everything changes.

As you walk, the correct answer to the question: “What is Amanda doing exactly now?” is that it’s a little more than four minutes past ten, she’s just finished her report and is sipping a relaxing coffee.

You spin around and walk back. This act rings the changes once again. For Amanda out at Deneb, at this precise moment of yours as you walk back to the house it’s coming up to four minutes to ten and she’s nervously preparing her piece to camera. Simply by walking the other way, what Amanda is doing ‘right now’ has moved almost nine minutes into the past!

You’ve just encountered the relativity of simultaneity. People at different locations in relative motion don’t agree about what time it is in the universe. The effect is normally small, but for high velocities and/or very large distances the effects become very significant.

* A spacecraft in the Andromeda Galaxy cruising away from earth at a fixed 240 km/sec in 2011 asks the question: ‘What’s happening on earth in the Milky Way galaxy at the present moment?’ The answer is that Jesus is being born. The spacecraft comes to a halt relative to the Milky Way. Two and a half million years later the spacecraft’s AI system will be able to see Jesus’ birth through its powerful telescope and will say: “Yes, that event happened when I was cruising, back in 2011”.

* A missile at the distance of the moon (~380,000 km) and travelling towards us at 240 km/sec considers clocks on earth ‘now’ to be one millisecond ahead of our own measure of ‘now’. This may not seem much to you and me, but it’s an eternity to earth-bound defense computers. The missile’s “now on earth” is millions of computer instructions ahead of where we earth-bound inhabitants consider our programs to be at.

For very distant creatures, a small change in speed can have very large effects. Brian Greene in ‘The Fabric of the Cosmos’ (page 134) considers an alien in a galaxy ten billion light years away, at the edge of the visible universe. Simply by ambulating towards or away from us at 10 mph, the alien’s view of what is happening ‘right now’ on earth swings from 149 years in the past to 149 years in the future.

For distant entities moving relative to us, what’s happening here in their now can be far in our past, or even more disconcertingly, far in our future. There are no paradoxes because in the jargon, there is a space-like separation between them and us and we can’t communicate faster than light. But what does it mean for the reality of the past and future?

If, by strolling towards Cygnus at 10 pm, it turns out that Amanda is, ‘right now’, looking at her clock reading 10.04 and 24 seconds then her future must already exist, mustn’t it? Suppose you get in your car and drive at 55 mph in the direction of Cygnus: her ‘now’ is an hour ahead of yours. But if you stop the car a minute later, you and distant Amanda will agree that now the time is exactly the same. How could both these facts be true if her ‘now’ is unique?

And if she, in her turn, walks towards earth inside her starship at exactly 10 pm and notes that ‘right now’ it’s just gone four minutes past 10 back on earth (even though you think it’s just 10 pm) what’s the status of your past and future?

It’s the present moment for you as you read this. It’s equally the present moment for me as I write it (and also, I consider, for you). All ‘present moments’ are equally ‘present’; all moments are equivalent sub specie aeternitatis – under the aspect of eternity, in our block universe.
—
Notes
A common argument against the block universe is that it appears inconsistent with ‘free will’. But if the laws of physics apply to all the atoms in a human body, then ‘free will’ is in a straightjacket; it’s an epiphenomenon reflecting our lack of conscious knowledge as to how we really work. If, on the other hand, ‘free will’ is the untrammelled freedom to do absolutely whatever, then at some point the person exercising ‘free will’ must violate the laws of physics … for which there is absolutely zero evidence. Think about it.

Another argument against the block universe is that maybe special relativity is just wrong. But if it were, radar and your cellphone GPS would give the wrong results while high-velocity machines like particle accelerators would simply not work at all. Relativity is amply confirmed by practical experience.

The calculation discussed here is simply the Lorentz transformation for time. The difference in the two versions of ‘now’ = relative-speed times distance/speed-of-light squared. In symbols, Δt = vd/c2. It will all become clear if you review the excellent “Spacetime Diagrams Tutorial” cited below, a document which was prepared for Open University students in the UK learning about spacetime for the first time.
—
Further reading
1. ‘The Fabric of the Cosmos’, Brian Greene, (Penguin, 2004) p.134.
2. ‘Spacetime Diagrams Tutorial’, R. S. Zimmer (PDF, 10 pages).