What's the difference between the past and the future? Not a great deal, if you take a purely relativistic view of the universe, say George Ellis from the University of Cape Town in South Africa and Tony Rothman from Princeton University in New Jersey.
The standard spacetime diagrams used in relativity accord no special status to the past, the present or the future. That's because they assume that everything evolves from time-reversible local physics.
In fact, it is possible represent such a universe using a kind of spacetime diagram in which space and time merge into a single entity. "The universe just is: a fixed spacetime block,"say Ellis and Rothman. In this view, no instant has any special status: "All past and future times are equally present, and the present "now" is just one of an infinite number."
This kind of "block universe" has indeed been studied by various physicists in recent decades with limited impact.
Today, Ellis and Rothman introduce a significant new type of block universe. They say the character of the block changes dramatically when quantum mechanics is thrown into the mix. All of a sudden, the past and the future take on entirely different characteristics. The future is dominated by the weird laws of quantum mechanics in which objects can exist in two places at the same time and particles can be so deeply linked that they share the same existence. By contrast, the past is dominated by the unflinching certainty of classical mechanics.
What's interesting is that the transition between these states takes place largely in the present. It's almost as if the past crystallizes out of the future, in the instant we call the present. Ellis and Rothman call this model the "crystallizing block universe" and go on to explore some of its properties.
They point out, for example, that this crystallization process doesn't take place entirely in the present. In quantum mechanics the past can sometimes be delayed, for example in delayed choice experiments. This means the structure of the transition from future to past is more complex than a cursory thought might suggest.
Ellis and Rothman suggest that their model provides a straightforward solution to the problem of the origin of the arrow of time. "The arrow of time arises simply because the future does not yet exist," they say.
That's a thought-provoking but ultimately unconvincing model in its current form. But it'll be interesting to see whether Ellis and Rothman can conjure a little more substance from the idea.
What it needs, of course, are some testable predictions, things that cosmologists usually spend little time worrying about. Don't hold your breath.
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