Special Relativity. It’s been the bane of space explorers, futurists and science fiction authors since Albert Einstein first proposed it in 1905. For those of us who dream of humans one-day becoming an interstellar species, this scientific fact is like a wet blanket. Luckily, there are a few theoretical concepts that have been proposed that indicate that Faster-Than-Light (FTL) travel might still be possible someday.
A popular example is the idea of a wormhole: a speculative structure that links two distant points in space time that would enable interstellar space travel. Recently, a team of Ivy League scientists conducted a study that indicated how “traversable wormholes” could actually be a reality. The bad news is that their results indicate that these wormholes aren’t exactly shortcuts, and could be the cosmic equivalent of “taking the long way”!
Originally, the theory of wormholes was proposed as a possible solution to the field equations of Einstein’s Theory of General Relativity (GR). Shortly after Einstein published the theory in 1915, German physicists Karl Schwarzschild found a possible solution that not only predicted the existence of black holes, but of corridors connecting them.
Unfortunately, Schwarzschild found that any wormhole connecting two black holes would collapse too quickly for anything to cross from one end to the other. The only way they could be traversable would be if they were stabilized by the existence of exotic matter with negative energy density. Daniel Jafferis, the Thomas D. Cabot Associate Professor of Physics at Harvard University, had a different take.
As he described his analysis during the 2019 April meeting of the American Physical Society in Denver, Colorado:
“The prospect of traversable wormhole configurations has long been a source of fascination. I will describe the first examples that are consistent in a UV completable theory of gravity, involving no exotic matter. The configuration involves a direct connection between the two ends of the wormhole. I will also discuss its implications for quantum information in gravity, the black hole information paradox, and its relation to quantum teleportation.”
For the purposes of this study, Jafferis examined the work performed by Einstein and Nathan Rosen in 1935. Looking to expand upon the work of Schwarszchild and other scientists seeking solutions to GR, they proposed the possible existence of “bridges” between two distant points in space time (known as “Einstein–Rosen bridges” or “wormholes”) that could theoretically allow for matter and objects to pass between them.
By 2013, this theory was used by theoretical physicists Leonard Susskind and Juan Maldacena as a possible resolution for GR and “quantum entanglement”. Known as the ER=EPR conjecture, this theory suggests that wormholes are why an elementary particles state can become entangled with that of a partner, even if they are separated by billions of light years.
It was from here that Jafferis developed his theory, postulating that wormholes could actually be traversed by light particles (aka. photons). To test this, Jafferis conducted an analysis with the assistance with Ping Gao and Aron Wall (a Harvard graduate student and Stanford University research scientist, respectively).
What they found was that while it is theoretically possible fir light to traverse a wormhole, they are not exactly the cosmic shortcut we were all hoping for them to be. As Jafferis explained in an AIP press statement, “It takes longer to get through these wormholes than to go directly, so they are not very useful for space travel.”
Basically, the results of their analysis showed that a direct connection between black holes is shorter than that of a wormhole connection. While this certainly sounds like bad news to people who are excited by the prospect of interstellar (and intergalactic) travel someday, the good news is that this theory provides some new insight into the realm of quantum mechanics.
“The real import of this work is in its relation to the black hole information problem and the connections between gravity and quantum mechanics,” said Jafferis. The “problem” he refers to is known as the Black Hole Information Paradox, something that astrophysicists have been struggling with since 1975, when Stephen Hawking discovered that black holes have a temperature and slowly leak radiation (aka. Hawking radiation).
This paradox relates to how black holes are able to preserve any information that passes into them. Even though any matter accreted onto their surface would compressed to the point of singularity, the matter’s quantum state at the time of its compression would be preserved thanks to time dilation (it becomes frozen in time).
But if black holes lose mass in the form of radiation and eventually evaporate, this information will eventually be lost. By developing a theory through which light can travel through a black hole, this study could represent a means of resolving this paradox. Rather than radiation from black holes representing a loss of mass-energy, it could be that Hawking Radiation is actually coming from another region of space time.
It may also help scientists who are attempting to develop a theory that unifies gravity with quantum mechanics (aka. quantum gravity, or a “Theory of Everything”). This is due to the fact that Jafferis used quantum field theory tools to postulate the existence of traversable black holes, thus doing away with the need for exotic particles and negative mass (which appear inconsistent with quantum gravity). As Jafferis explained:
“It gives a causal probe of regions that would otherwise have been behind a horizon, a window to the experience of an observer inside a spacetime, that is accessible from the outside. I think it will teach us deep things about the gauge/gravity correspondence, quantum gravity, and even perhaps a new way to formulate quantum mechanics.”
As always, breakthroughs in theoretical physics can be a two-edged sword, giving with one hand and taking away with the other. So while this study may have thrown more cold water on the dream of FTL travel, it could very well help us unlock some of the Universe’s deeper mysteries. Who knows? Maybe some of that knowledge will allow us to find a way around this stumbling block known as Special Relativity!