Tipler on Time Travel

I was pleased and honored to get this comment from Prof Frank Tipler regarding a recent article on the theory and practice of Time Travel posted in this space. 

For those of you who may not know, I exploited an idea proposed by Tipler in his 1974 paper, “Rotating Cylinders and the Possibility of Global Causality Violation” as the explanation for the faster than light drive in my space opera SUPERLUMINARY.

I was not aware that he was listening to my Red Pill Religion podcasts. Small world! 

The words below are his:


Let me add some physics to your discussion about time travel. In 1974, I published the first article in a major physics journal (Physical Review D) to suggest that it might be possible to make a time machine. My original title was “Constructing a Time Machine,” but the referees thought this title too radical, so I changed it to the title that Larry Niven “stole” (his word) for his time travel story in ALL THE MYRIAD WAYS.

Alas, within a few years I was able to prove several theorems, which I published in Physical Review Letters (1976) and Annals of Physics (1977), showing that any attempt to actually construct a time machine in a universe subject to classical general relativity would create a singularity (something intrinsically infinite, something not subject to any laws of physics), that would destroy the time machine. One of these theorems was rediscovered a decade later by Steve Hawking. So Niven’s proposal that the universe — at least if it is classical — would stop any attempt to construct a time machine has been confirmed mathematically.

But what about quantum mechanics? My friend David Deutsch, famous for inventing the quantum computer, made the proposal you discuss, that the Grandfather Paradox can be avoided by having you kill your grandfather in a parallel universe. (The parallel universes do exist, by the way. Steve Hawking once told me that he considered the Many Worlds version of quantum mechanics “trivially true.” He meant that only an ignoramus would deny the existence of the parallel universes. I agree.)

But I disagree with David that switching to a parallel universe will solve the problem. The difficulty? No one, not even David, has been able to prove that “unitarity” holds in a quantum multiverse that allows going back in time, and I think there is good evidence that it does not hold in such a multiverse.

“Unitarity” is the quantum form of “cause and effect.” But it is a very strong form of cause and effect, because it says “cause and effect” can be understood not only as working from past to future (the usual form) but also from future to past. Thus, we can understand that what is happening today is the result of it being required to happen so that some future event will certainly happen. Teleology is alive and well in modern physics! Note the parallel to your discussion of the series of future time travellers.

Unitarity is a mathematical consequence of energy conservation. Thus, if unitarity does not hold, then energy is not conserved. The converse is not true — it is possible for unitarity to hold and still energy is not conserved. However, energy non-conservation due to unitarity violation amplifies. Stanford physics professor Leonard Susskind has pointed out that if unitarity is just a tinny bit violated, then were you to turn on your microwave oven, so much energy would be created out of nothing that the Earth would be blown apart!

So, no unitarity violation. Once again, note the parallel to your discussion of the series of time travelling civilizations trying to correct the errors of “previous” time travellers. The point is, all of these science fiction stories have analogues in quantum mechanical microphysics. John Campbell was on to something when he changed his magazine title from “Astounding” to “Analog.”

You correctly point out that time travel, were it to actually exist, would have to be limited to just visiting, not interfering with, the past. But we will be able to visit the past — visit the dinosaurs, say — in virtual reality in the far future anyway. I have written an entire book, THE PHYSICS OF IMMORTALITY (Doubleday, 1994) showing exactly how this will work as a matter of physics. Bottom line: we will all be resurrected into virtual reality in the far future, never to die again. Notice a parallel with your “red pill religion” columns?

It gets better. I mentioned above that a “singularity” is an entity that is intrinsically infinite and not subject to the laws of physics. In fact, Fred Hoyle created his Steady State Theory (now called “Inflation Theory”) in an attempt to eliminate this entity which, in Hoyle’s words, “is not only not subject to the known laws of physics, but not subject to any conceivable [logically possible] laws of physics.”

A singularity inside space-time would be a mathematical disaster, but if it is on the edge of space-time, it would constitute a “supernatural being” (outside of “nature” — “physics” in Greek) which created the entire universe out of nothing (on the edge of space-time, a singularity mathematically would determine everything than happens in space-time — create— and outside the singularity and the nature it determines, there is nothing — no time, no space, no matter, no nothing).

In fact, three distinct (in one natural topology) singularities exist, which are really just one singularity (in another natural topology. I have discussed these three-in-one singularities in detail in my book THE PHYSICS OF CHRISTIANITY (Doubleday, 2007). One of these singularities, the Big Bang or Initial Singularity, is obtained using an argument that is essentially the same as St. Thomas Aquinas’ Second Way argument, except that one does not have to establish that the sequence of causes cannot be infinite. The sequence IS infinite, but modern mathematics allows us to use the entire sequence to define the Uncaused First Cause, the Initial Singularity. Details in PHYSICS OF CHRISTIANITY.

Frank J. Tipler
Professor of Mathematical Physics
Tulane University