Hey everyone, Stephen Hawking is throwing a party, and we’re all invited! One catch: Stephen Hawking is dead, and the party was in 2009. Still, the invitation stands.
What if you threw a party and nobody came, but that’s exactly what you expected? That’s precisely what famed astrophysicist Stephen Hawking did on June 28, 2009. He rented a space at Cambridge University and got balloons, decorations, and, of course, the champagne. Then he sat in the empty room for a few hours and left.
Only then did he send out the invitation.
Stephen Hawking’s time travel party was held for our future descendants should they ever crack the mystery of time travel, one of the biggest puzzles faced by modern physics.
Hawking was actually experimenting. The thing is, our current knowledge of physics does not strictly forbid time travel.
Indeed, in some cases, it’s explicitly allowed. Take the general theory of relativity, for example. This theory provides our modern understanding of gravity as the bending and curving of space and time in response to the presence of matter and energy, with that curving dictating the motion of matter.
General relativity provides several scenarios that enable time travel into the past; it’s just a matter of finding the right curvature of spacetime where you are always traveling forwards but end up in your own past, a structure known as a closed timelike curve (the “timelike” is jargon talk for always traveling slower than the speed of light).
One scenario is an infinitely long cylinder, which rotates fast enough to allow you to travel a corkscrew path around it and end up in the past. Another is the creation of a wormhole where the ends are not synchronized to the same time, allowing you to travel down one end and exit into the past.
But there are no infinitely long cylinders in the universe, and wormholes require the presence of exotic matter with negative mass to stabilize themselves, and negative mass doesn’t seem to exist.
These are just two examples; every time we try to concoct a permissible time travel setup in general relativity, we find some random rule of the universe preventing us from realizing it. And yet, no hard and fast rule outright forbids time travel, so we’re a bit stuck.
Hawking, like many other physicists, believed that a resolution to the dilemma would come in the form of an advanced theory of gravity, one we currently do not yet know.
And so if time travel is possible, then our descendants will one day figure it out, and they can read about the invitation to Hawking’s party in the historical records and go visit him.
Since nobody showed up to Stephen Hawking’s time travel party, we have a few possible conclusions:
1. Time travel into the past is not allowed. Our future descendants will realize a deeper understanding of the laws of physics and, in that deeper understanding, discover the fundamental reason why time travel into the past is forbidden. They will read about Hawking’s invitation and sigh regretfully that they could not make it.
2. Time travel into the past is allowed, but we only figure it out so far into the future that Hawking’s invitation is lost in the historical record. Who knows, maybe some ancient Sumerian philosopher also hosted a time travelers’ party thousands of years ago, writing down the invitation on clay tablets spread throughout the land, but none of them survived to the present day.
3. Time travel into the past is allowed, but we never figure it out. Maybe humanity destroys itself, armed with an advanced understanding of nature. Maybe our human intellect isn’t quite powerful enough to discern the deeper mysteries of the universe. Maybe we go on to other intellectual pursuits and never bother taking a deeper look.
4. Time travel into the past is allowed, but there are rules. Maybe it’s exceptionally complicated or energy-intensive to build a time machine, and we can send fleeting signals into the past, but not entire persons. Or maybe you can only travel as far back as the creation of the time machine, and you cannot access earlier epochs.
As experiments go, Hawking’s party wasn’t a total failure. If someone from the future had shown up, it would’ve been a rather big deal. But even failed experiments can teach us many things. In this case, the mystery of time travel will likely remain unsolved for quite some…time.
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