Time Travel: How Big Is The Universe — Really?

4 min readFeb 4, 2022

September 23, 2011. It made the news within just minutes: Neutrinos emitted from Geneva, Switzerland, had been allegedly observed to reach Gran Sasso, Italy, 60 nanoseconds earlier than light. The announcement was of course made with all the necessary scientific rigour and distance, namely, asking for help in the hunt for possible errors in the measurement process. And rightly so: a few months later, in 2012, it was attributed to a flaw in the electronic systems of the detector.

Most of the physicists, during this period of heated debate, were very pragmatically expecting this (not really exciting) outcome, because it is widely accepted or assumed that no matter, no information can travel faster than light. I actually felt the same — even though I must confess I had a minute sparkle of hope that the measurement was correct. Here’s why.

Time travel

I pointed out in a previous post that faster-than-light travel is equivalent to time travel. In terms used by physicists, this is because in this case the departure and arrival events are spacelike-separated, pardon my French, and hence their order is relative to whomever is observing. If you travel faster than light, then some people will actually see you go back in time.

Go back in time. The dream has been in the collective mind for a long time. Think of H.G. Well’s famous 19th-century novel, up to all 20th century science-fiction shows à la Star Trek. Who wouldn’t like to see dinosaurs, to spot the first human who successfully kindled a fire, to solve the Neanderthal enigma, to have a debate with Socrates or to go for a walk with Einstein on the Princeton campus.

Yet, this was not the reason I was excited. It is much bigger than that really.

Two visions of the world

Many people see time travel as a paradox because of the incompatibility between causality and free will. More precisely, if you search for the “grandfather’s paradox” in your favorite search engine, you will find that there seems to be an incompatibility between:

on the one hand free will (being 60 years in the past, it would be within your power to prevent your grandparents from meeting each other)
and on the other hand the fixity of the past (you exist, hence you were born).

Yet, it would be premature to conclude to an impossibility. There actually are at least two plausible accounts for time travel that elegantly work around this apparent inconsistency.

The Many-Worlds Interpretation (Hugh Everett, David Deutsch, …)

In this view, somebody who goes back in time actually will find themselves in a parallel universe, that is, with a different chain of events. Maybe they will travel to 1800, prevent Napoléon from selling Louisiana, and we would write all our scientific papers in French (hint: watch Sliders, it’s a nice series). With this interpretation of time travel, there is no way to then go back to your original universe. This view was introduced in a famous paper by David Deutsch in 1991 [1], in the continuation of Hugh Everett’s many worlds interpretation of quantum physics.

The Self-Consistent Interpretation (Igor Novikov, …)

In this view, often attributed to Novikov [2], the timeline is self-consistent, that is, grandfather’s paradoxes — strange loops — are automatically excluded by nature. The timeline is a mathematical fix point. What you will do after going back in time (from your perspective) is exactly what you have already done and that caused all events since this point in time (from an external perspective). The probability of your doing something different and causing any paradox is zero.

Time (Travel) Will Tell

The scientific methodology aims at first building theories that predict Nature’s behavior, and then experimenting to select the right theories. In this case, the experiment to decide which of these two theories is true is faster-than-light travel. If we ever manage to time travel, it will tell us how big our universe really is.

Should David Deutsch be correct, this would lead to an unprecedented Copernician revolution in Mankind’s history. The new world was America. It is now Mars and the Big Bang. It could become even bigger by several orders of magnitude and would force us to rethink our whole approach of what Reality means. Should Novikov be right, it would also be a revolution that could force us to rethink Quantum Physics (and perhaps even make Einstein happy). People who regularly share philosophical dinners with me might guess that I am secretly hoping for the latter interpretation.

The adventure is already continuing. While I was writing this article, there was already new food for thought on neutrinos in a paper published by Robert Ehrlich last December [3].

Exciting moments. And it’s not just about physics. It’s about being open to new ideas.

References

[1] Quantum physics near closed timelike lines, David Deutsch, Physical Review D 44, 3197, November 1991. http://dx.doi.org/10.1103/PhysRevD.44.3197

[2] Cauchy problem in spacetimes with closed timelike curves, John Friedman, Michael S. Morris, Igor D. Novikov, Fernando Echeverria, Gunnar Klinkhammer, Kip S. Thorne, and Ulvi Yurtsever, Physical Review D 42, 1915, September 1990. http://dx.doi.org/10.1103/PhysRevD.42.1915

[3] Six observations consistent with the electron neutrino being a tachyon with mass…, Robert Ehrlich, Astroparticle Physics, December 2014. http://arxiv.org/abs/1408.2804