Communications of the ACM
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Future Tense: The Deadline Paradox
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First, I must admit... I copied this entire piece from the April 2012 Communications I had received in the mail six months ago. It was a lifesaver. I was late on deadline, so I typed it out word for word and sent it in.
When in October 2011 I opened April 2012 and read the piece supposedly written by me, my first thought was the whole thing was an elaborate prank by someone who knew I had a professional interest in time travel. Yet it was just too good, too convincing. So I took the issue to Gillian, a physicist friend at Caltech; I have a rusty physics degree but needed expert guidance.
She read it through, smiling at the mention of her name in the text. "It's clever," she said. "Whoever wrote this knows you well. But they clearly don't understand relativity. It's easy enough to build a time machine to travel into the future. Special relativity makes it inevitable that anyone moving fast enough will shift forward through time relative to those left behind. But getting an object into the past is much more of a challenge. The engineering would be extreme."
"Extreme as in difficult? Or as in impossible?"
"Impossible for a good few thousand years, at least. We're talking about crossing interstellar space, collecting maybe 10 neutron stars, slamming them together into a cylinder, somehow preventing them from collapsing into a black hole, and rotating the whole thing at cosmically high speed. And even if this happened way into the future, they couldn't have sent your piece back to you. They could never travel back to our time. That was, in fact, the problem with the MIT time travellers' convention."
"Time travellers' convention?" I shook my head.
"Back in 2005 a group of time-travel enthusiasts and physicists held a convention at the Massachusetts Institute of Technology. The idea was to attract people from the future. But no one arrived. At least no one who would admit to having arrived. They couldn't, because a relativity time machine can't travel further back than the point it was first switched on. It doesn't make time flow backward but acts as a shortcut to a place where time runs slower. There's no reaching the deep past. Unless someone has an operating time machine right now, this story couldn't have come from the future."
"And that would mean manipulating neutron stars?"
"Unless the experiment was based on Mallett's theory works."
"Which is?"
"Ronald Mallett is a theoretical physicist at the University of Connecticut who suggested you can produce the same effect as the neutron-star cylinder (it's called frame dragging) in a lab using a spiraling light source instead of stars. Frame dragging is one of the weirder bits of general relativity. There's a small component of the gravitational pull at right angles to the main attraction. If you spin an object, that side-ways pull drags spacetime along with it, like a spoon stirring, say, molasses. Mallett reckons there'll be a measurable effect if you can get a big enough stack of rotating light beams."
Gillian knew the team working on Mallett's idea at Caltech, intellectual home of the late Nobel physicist Richard Feynman. It wasn't difficult to get access to its frame-dragging lab when the researchers were out. Like every physics experiment I've ever seen (okay, with the exception of the Large Hadron Collider) the machine looked more Tinkertoy than Doc Brown De-Lorean. The tower of laser spirals did reflect a certain majesty, admittedly, climbing up the center of a stairwell maybe 30 feet high. But the whole thing looked as if it would fall apart if you sneezed on it.
We put the copy of Communications I was sent in an envelope, with instructions for recipient(s) unknown to mail it to me, unopened. But there was no knowing what would really happen. We just had to hope whoever found it wouldn't get too curious and open it, sending the future veering off into some uncharted direction, perhaps sweeping us along with it.
A relativity time machine doesn't make time flow backward but acts as a shortcut to a place where time runs slower.
I saw the envelope fall down, down, down the tower like Alice in the rabbit hole. But it didn't disappear. That was the strange thing. It just receded endlessly, timelessly, into the distance as it fell. I ran down the stairs to wait by the base of the column. It ought to have dropped through. It should have been there on the oil-stained concrete floor. But it wasn't.
It just receded endlessly, timelessly, into the distance as it fell.
And so I received this message from the future. Who wrote it? Not me. I just copied it. And not that future me. It was already published when that other me put it in the envelope. But that's not the thing that puzzles me most.
I don't have a physicist friend named Gillian. I don't live near Caltech. I'm in Swindon, England, not far from Stonehenge. Ronald Mallett exists, but many physicists believe his theory is flawed, and it has yet to undergo a practical trial. I see only one possible explanation. According to the "many worlds" interpretation of quantum theory, there are countless parallel universes, and each quantum decision brings a switch of worldline for the universe we occupy. Maybe creating the paradox of this self-produced story pushed me into a different alternate universe, one with no Gillian. So... is the timelessness of the publishing schedule finished at last? Will I continue to exist myself? Or will I go out like a candle?
©2012 ACM 0001-0782/12/0400 $10.00
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