How Black Holes And Quantum Entanglement Could Send Messages Back In Time Like In "Interstellar"

The desire to go back in time to fix some grievous mistake or prevent a tragedy is near-universal. If we can’t do that, the next best thing would be to send a message back in time to warn our former selves or others. The idea is a central feature of the film Interstellar, and physicists want as much as anyone to know if it could work, but call it retrocausal communication, perhaps because it looks better on grant applications.

The theory of General Relativity demonstrated that gravity affects time, and under extreme circumstances, such as in the vicinity of a black hole, can produce closed timelike curves (CTCs). CTCs theoretically allow “chronology-violating” systems to travel backward in time so that they can interact with an earlier version of themselves.

Whether this would work in practice is an ongoing problem. Even non-physicists can spot the potential for “grandfather paradoxes”, where changing the past prevents a future in which the time travel could occur, named after the danger of a time-traveler killing their grandfather and therefore never existing. For physicists, grandfather paradoxes are just one of the issues to wrestle with, including the implications of quantum mechanics on CTCs.

If your head hurts from thinking about this too much, there’s a way out. The forces involved in getting close enough to a black hole to make a CTC are likely to be devastating for anyone formed of flesh and blood, making it unlikely a living being could time travel. However, information might be able to go where humans cannot. Cornell University’s Dr Kaiyuan Ji and Professor Mark Wilde and Professor Seth Lloyd of MIT have explored the limits on the efficiency of communication imposed by noise in CTCs.

The trio define time travel as noiseless if “past and future versions of the system are identical”. Noise is both the usual obstacle randomness poses to message transmission, and the changes made to the past by this time-reversed communication. 

Crucial to the process is quantum entanglement, because it allows changes to the state of one entangled particle to change another, even when distant in space or – in this case – time.

In the film, because the father was present when his daughter decoded the message, he knows how best to send it to communicate most efficiently. This, the authors argue, provides a mechanism by which the noisiness of the communication can be overcome, allowing a clear signal to make its way through.

“Evidently, this allows what results from the daughter’s decoding to influence the father’s encoding, which in turn influences the daughter’s decoding via the noisy P-CTC, thus forming a causal loop,” the authors write. (P-CTC here means post-selected CTC, one where an event later in time determines what happened earlier).

“However, to stress again, granted the existence of the noisy P-CTC, the formation of this loop is physically justified,” the authors write. “The father and the daughter operate in a perfectly chronology-respecting manner, including the father’s retrieving and consulting his memory of past events; the only chronology-violating element here is the noisy P-CTC, which models a given mechanism beyond their control.” 

Instead of a grandfather paradox, this is more like traveling back in time and saving your grandfather, thus allowing yourself to exist. There is a different sort of problem here; how such a loop came to be formed, since without the message backwards the conditions to send it could not have been created. Nevertheless, this is less troubling than the traditional version. The authors also add; “In the presence of a causal loop, the notion of past and future becomes elusive and can be treated flexibly.”

Having explored the mathematics of this situation the authors consider the implications for the evaporation of black holes by Hawking radiation. 

The ideas are so far outside our experience as to seem absurd, but some equally bizarre aspects, such as quantum entanglement, have been confirmed repeatedly. One explanation, albeit a minority one, for how entanglement can appear to violate physical laws against faster-than-light communication is that entangled particles send messages backwards in time to their partners. If so, perhaps retrocausal communication is not so strange.

Lloyd told New Scientist that the applications of the work may not lie in messages sent back in time, but in clearer communications into the future. “Nobody’s built an actual physical, closed time-like curve, and there are reasons to think it’s very hard to make one. But all channels are noisy,” he said.

The study has been accepted for publication in Physical Review Letters and a preprint is available via the arXiv.

[H/T: New Scientist]