Can Photons Travel Through Time?

Of all the things science fiction has portrayed, time travel is by far the coolest. Sadly, reality and the laws of physics that come with it have basically eliminated all hope of a wormhole opening up and allowing us to travel through time… unless you’re a photon. Luke Butcher of the University of Cambridge has written a paper, describing a potential form of a wormhole that could be held open long enough to allow a photon to pass through. The paper has been submitted to the journal Physical Review D and has been published on arXiv.org in an open access format.

Wormholes were first suggested by Albert Einstein and Nathan Rosen in 1935. Essentially, it’s a hypothetical whirlpool-like passageway that would allow a traveler to escape the confines of the space-time continuum. While a wormhole could lead to a parallel Universe, it could also bend around and lead back to a different point of space and time in our current Universe. However, wormholes are regarded as remarkably unstable and would not hold open long enough to be used for travel purposes. 

Physicists then began to wonder if something could be used to strengthen the wormhole and keep it open. In 1988, a team from Caltech suggested that negative energy appeared to fit the bill. As positive energy would attract matter and pull the wormhole closed, negative energy would have the opposite effect and repel matter, holding it open.

Those researchers at the time began to explore the idea of using Casimir energy as the source of negative energy to stabilize the wormhole. In a vacuum (like space) parallel smooth plates that are near one another undergo quantum effects which trap energy (either positive or negative, depending on the circumstance) between them. If the wormhole vortex got started and negative energy was added to the center, it would brace the hole open and retard the collapse.

However, there was still one more problem: the wormhole itself would be tiny. When depicted in science fiction, wormholes are spacious openings that allow travelers in large spaceships to pass through with ease. In reality, even if a wormhole existed, humans still wouldn’t be able to get through because the passageway simply wouldn’t be wide enough. Butcher reasoned that while humans couldn’t pass through a wormhole, photons might be able to.

Butcher ran some new calculations, building off of previous research. His new design should be able to hold a wormhole open, but it would have to be very long and very narrow. There is also no good way to get the negative energy into the correct location, as the renormalized Casimir energy-density would be zero. “Nonetheless, the negative Casimir energy does allow the wormhole to collapse extremely slowly, its lifetime growing without bound as the throat-length is increased,” Butcher wrote in the paper. “We find that the throat closes slowly enough that its central region can be safely traversed by a pulse of light.”

Even if Butcher is completely correct in his calculations and he was able to hold a wormhole open long enough for a photon to pass through, that doesn't mean that your lifelong dream of going back in time and meeting Cleopatra is about to come true just yet. This paper only deals with holding the passage open, and doesn’t cover what would happen to the person—or photon—once inside.