Stephen Hawking’s death in March incited us all take a moment and think about the famed physicist’s impact on the scientific world, and the myriad ways his research affected the way we think about the universe. As it turns out, he wasn’t exactly done. Hawking’s final paper was finally published Wednesday, in the Journal of High Energy Physics, and while it’s not exactly the science-shattering work many outlets are reporting it to be, it still puts a pretty interesting, Hawking-esque spin on one of theoretical physics’ most discussed concepts: the multiverse.
The idea that multiple parallel universes exist originates out of inflation, the incredibly rapid expansion of the universe right after the Big Bang, over repeated bursts at speeds faster than light. Many scientists think during these bursts, the smallest blips in energy at the quantum level swelled into larger pockets of space-time—effectively entire individual universes which are possibly, conceivably found everywhere, within an ever-expanding larger multiverse that houses them.
Subscribing to that view essentially means assuming that, if the multiverse continues to inflate, individual universes are being created ad infinitum. For some, that’s a tough pill to swallow. And you can count Hawking and his co-author, Thomas Hertog from the University of Leuven in Belgium, among those skeptics.
So Hawking and Hertog created a framework for a simpler model of the multiverse that limits how many new universes could form, and ensures they adhere to the same laws of physics as our known universe. As opposed to older theories of multiverse that called for universes empty and full, volatile and boring, dead in an instant or with long lives ahead of them, these would be truer to the layman’s conception of parallel universes.
The new paper is sort of an update of the “no-boundary” proposal, something Hawking and American physicist James Hartle worked on in the 1980s. Using new mathematics derived from string theory that weren’t available in the 80s, Hawking and Hertog reach the conclusion our own universe is compatible with this idea, and that our multiverse is smaller than what we might expect from eternal inflation.
“Our model fits in nicely with the theory of inflation that says our universe underwent a very rapid period of expansion in its earliest stages,” says Hertog. “But our model goes radically against the prevailing extrapolation of inflation that led to a multiverse.”
It’s a pretty neat idea! It’s just not as exceptional as one outside the field might think at first glance. For one, it remains a theoretical paper; there’s no real way to test it out or make any sort of observations of this cosmology. In practical terms, it’s not practical at all. The original “no-boundary” proposal is speculative, and by extension so are these latest conclusions.
“The main conclusion of the paper is a conjecture and not proven mathematically,” says Avi Loeb, a theoretical physicist at Harvard University and a noted skeptic of inflation theory. “It is a stimulating but not revolutionary paper.”
Moreover, it’s not exactly a bombshell of a framework. “This paper is rather the culmination of a line of research we had been pursuing for a number of years driven to a large extent by the problems associated with the multiverse,” says Hertog. It’s not a flashbulb epiphany that just appeared in the authors’ brains in an instant, but rather an example of the slow burn of theoretical physics.
Like Loeb, Andrei Linde, a theoretical physics based at Stanford University and a pioneer of inflation theory, thinks it’s important to frame the findings as conjecture, not a final statement. But he does say they “may still be very significant, and, as many prolific statements made by Hawking, it may initiate productive work in this direction. This is an extremely complicated field of research, so it is very important to know Stephen Hawking’s thoughts on that.”
Loeb also finds it encouraging that the paper tampers down on the multiverse theory’s “problematic” suggestion that everything that can happen will happen an infinite number of times. “This theory is not falsifiable, because everything is possible,” he says. “The virtue of traditional physics is that its theories could be falsified by experiments. Science is a learning experience. If we give up on the possibility of falsifying our ideas, then we will not learn anything from experiments.”
But of course, Hawking and Hertog’s theory is also not falsifiable.
Instead, the excitement over this paper might really just be the fact that it’s Hawking’s last paper. The paper was long-available to the physics community to read and discuss, and only was only submitted to JHEP and accepted for publication on April 20. This new spike in interest is really just among the public, eager to see the last thing Hawking wrote. That certainly doesn’t diminish its impact, but it would be a bit foolish to make it out to be larger than it is.
It’s a bummer Hawking is no longer around to deliver something more exceptional. “Of course I have enormously enjoyed my collaboration with Hawking,” Hertog. “But I am sad Stephen is no longer with us today to celebrate the publication of this paper and to participate in our future adventures in cosmology.” He’s certainly far from the only one.