I recently discussed some the problems inherent to time travel in works of fiction. Most notably, the paradoxes contained in the Terminator and Back to the Future movies. But now I want to take a step back and look at time itself.
Does “time” even exist?
The standard notion of time is that it is a fourth dimension just as fundamental and intrinsic to the universe as the three spatial dimensions of length, width, and height.
But if we take a step back, one could argue that time is just an abstraction of the human imagination. It could just be a convenient construct that we use to explain relationships between things in our universe. Like motion, time may not exist without the objects that we use as a frame of reference for measuring it.
Time is scientifically defined as a relationship to the rate of decay of a radioactive caesium atom. It is also often linked with the speed of light, which is often seen as a “cosmic speed limit”.
But is this view of time really necessary? Is time truly a fundamental property of our universe?
“What we observe is not nature itself, but nature exposed to our method of questioning”
- Werner Heisenberg, father of quantum physics.
A lonely whatchamatron
Imagine a universe that is completely empty save for one indivisible particle, let’s call it a “Whatchamatron”. There are no other objects in this universe besides this whatchamatron. The whatchamatron still has a length, width, and height. It still has mass (although it probably doesn’t have energy, otherwise, it would be emitting energy and we’d have another object in the universe to compare it against). These characteristics would seem to be intrinsic. Even isolated all by itself, the whatchamatron still has them, even though their meanings in such a universe are trivial.
But our whatchamatron does not (cannot) experience movement. It cannot have speed, rotation, momentum, or even orientation. These words are meaningless in a universe that contains only the single whatchamatron. There is nothing to measure any of these characteristics against.
But what about time? Does the universe in which our hypothetical whatchamatron exists have - as an intrinsic characteristic - time? What would we measure it against? There is no caesium in this universe for us to use to define a second. There is not even light. And this particle, itself, does not decay, because by decaying, it would create another object that we would be able to use as a frame of reference for measuring motion.
The whatchamatron’s universe is not expanding or shrinking. It has no discernable origin or termination. No measurable age. The whatchamatron is immortal, and its universe never changes.
As far as it is concerned, no moment in its existence is any different than any other moment of its existence.
For the whatchamatron, there is no time.
Time is decay
So, time is meaningless without change.
Without entropy.
For us, time only exists because our universe is in a constant state of decay ever since the Big Bang. Electrons and photons dart from place to place, elements decay into other elements, stars are born and explode, planets form and are shattered, gases expand and contract, galaxies fly apart from each other, our own bodies are unstable and self-destructive. Everything is changing relative to everything else. The components of our universe are constantly being rearranged. We perceive this phenomenon as “time”.
But that begs the question: if all the motion in the universe were to suddenly come to a stop, would time, itself, be frozen?
Or would time still be moving forward, and just all the objects in the universe are stationary, as would be the case if time were an intrinsic property of the universe?
How would we measure the passage of time if everything in the universe became stationary? Our definition of a “second” would have no meaning. The caesium would stop decaying, no longer giving off radiation. The radiation it had given off would be frozen in place, with no transitions between its hyperfine levels even taking place.
Relationships of time to the orbit of the earth or the speed of light in a vacuum would be equally meaningless, as the earth would not be moving in its orbit, and light would not be traveling at all (so any observer would be blind anyway).
So can time be separated from the intrinsic qualities of space? Does it really stand alone from length, width, and height as a fundamental fourth dimension of reality? If everything in the universe – every particle, every bit of energy, every force, every quantum string – were to be repositioned to where it was a “year” ago, would we, in effect have traveled through time?
Would we be back in the past?
Or would we just be in a future that is identical to the past?
How could we tell the difference?
“The distinction between past, present, and future is only a stubbornly persistent illusion.”
- Albert Einstein, inventor of relativity.
How to time travel
If we accept that time does not exist, then “time travel” becomes significantly simplified and does not violate any laws of physics, causality, or logic. Traveling through time is as easy as rearranging everything in the universe to a state that it was in previously. A process that a friend of mine compared to "backing up and restoring a hard drive".
But it's a bit more complicated. All the matter, all the energy, and every force in the universe would need to be placed exactly where it was at whatever point in “time” you wish to re-visit.
Not as easy as you were hoping, is it?
Well it gets worse.
In order for artificial time travel to be performed, some one would need a device as powerful (or more so) than all the energy and mass in the universe combined!
In order to prevent itself from being rearranged, the device would also need to exist outside of our universe. Of course, the laws of conservation of mass and energy prohibit us from removing matter or energy from the universe. It is impossible. Even black holes retain all the mass and energy that is absorbed by them. So any machine that we build within this universe would not be able to function for the purpose of traveling through time. Nor would any person be able to remove him or herself from the universe in order to observe the travel.
Everything would have to be reset.
And not just our physical bodies. But also our minds. Our thoughts, memories, and emotions would all have to be reset as well, since they exist as bioelectrical impulses in the brain.
Sorry Doc Brown, as cool as that flux capacitor looks plugged into that DeLorean, it probably won't work. We can still make flying DeLoreans though!
So our universe could be getting reset on a regular basis, but we would never know it. We would experience each moment thinking that it was the first and only time that the moment had ever come to pass.
But we could have done this a million times. There would be no way for us to know. No way for us to observe or measure the reliving of an event.
There would be no déjà vu.
We would have only one memory of the repeated event because in order to repeat, the entire universe - including our own memories - would have to revert back to the state it was in at that moment. That means that only the memories and thoughts that already existed at that moment can exist in the repeat.
We can only distinguish different moments in time based on the differences between those moments. We only experience time as change.
And without change, there is no time.
And before you start to claim that Einstein’s equations “prove” that time exists, consider yet another quotation from the man himself:
“As far as the laws of mathematics refer to reality, they are not certain; as far as they are certain, they do not refer to reality.”
- Albert Einstein, inventor of relativity.