How funny would it be to watch Cleopatra sing “Happy Birthday” to President Kennedy? Or how remarkable would it be to domesticate a dinosaur? And how wonderful would it be to relive the proudest moment of your life, or to save yourself from the most humiliating?
Time travel, a long-time speculation of scientists and dreamers alike, would open the door to such unlimited possibilities. However, what most fail to realize is that before we can accomplish time travel, we have to define time.
“Time,” as we are taught in school, is a simple measurement. We speak about it in both specific and abstract terms, whether it’s in exact “seconds” or merely “the other day.” In our daydreams, we often visit the “past” and “future.” However, as definitions grow more complex, the philosophy of time condenses to two main theories: that of a linear time, and that of an unreal time.
Linear or “real” time, as we are more familiar with, involves a past, present, and future. You have memories which you remember in the present; and you can project expectations to create a future. We believe there was a beginning for everything, and there will be an end for most things. According to Sir Isaac Newton, time can be described as a dimension of sequential events.
The opposing view suggests time is real to us only when we measure it; so time might not actually exist at all. In fact, time may just be a human construct. This idea dates as far back as the 5th century BC, to the Greek philosopher Antiphon. He believed “time is not a reality, but a concept or a measure.” In 1908, J. M. E. McTaggart pointed out that “since every event has the characteristic of being both present and not present, time is a self-contradictory idea.” In truth, all we can really prove is the present, while the past exists solely in our memory.
If this is the case, time travel would be a moot point.
Since plausible evidence exists on both sides, scientists continue to argue. While some believe time exists as positively as space does, physicist Julian Barbour argues that quantum equations take their truest form when expressed outside of time. Outside of time, the equations can consider every possible momentary configuration of the universe.
This is a profound thought, one that deserves a completely separate consideration.
In this way of thinking, quantum mechanics lends itself to a different sort of time travel, beyond what you might have been expecting.
Let’s say there are two particles that are so small that we think of them as electrons. In fact, they are electrons. These electrons react to one another. You might have heard of the Pauli Exclusion Principle as a series of impressive physics buzzwords, but now you’ll intimately understand why it’s so important. The Principle states that two interacting particles cannot occupy the same energy state. So, if we have two electrons, and they have to be different from one another, we will call one of them spin up, and the other spin down. Problem solved!
Now let’s make things complicated. Another great series of words you’ve probably come across is “particle-wave duality.” This refers to the idea that particles really have no set location until we observe them. Reread that sentence, because it’s not supposed to make sense the first time.
Particles, instead, are each waves, with probabilities of existing in one place or another. This mathematical wave function (an equation describing how probable it is the particle will exist where and with what energy) collapses to one, singular answer the moment the particle is observed. An example you probably know from high school is the electron cloud around a nucleus. This cloud is really a smearing of probabilities. The electron can be anywhere inside that cloud; some locations are more probable than others. When we observe an electron, we nail down exactly what spot it’s in and the smearing for that particular electron collapses to that one location.
So what does this have to do with time travel?
Well, for one, you might wish you could go back and never got interested in quantum mechanics.
For those of you who don’t know when to give up, like me, we will take our implications further. Let’s say we have those same two electrons, interacting, but we don’t observe them. We don’t know which is spin up and which is spin down. In fact, both electrons have a 50/50 chance of being spin up or spin down until they are observed, and they collapse to one or the other. So now we separate the two, far apart from one another. When we examine the electron we’ve kept close, we see it is spin up. This causes the other electron to choose spin down, though it is no longer in contact with the first particle. This is because the reaction that created our spin up also had to create the spin down, in that moment in the past. However, it didn’t manifest as such until we observed it!
What this means is, by observing the spin up electron in the present, we dictate what happened in the past. By collapsing the wave function of the spin up electron, which until that moment had remained a 50/50 probability, we also collapse that of the spin down electron. This is a phenomena which may also be familiar to you. It’s called “quantum entanglement.”
Quantum mechanics is still very counterintuitive, and its implications in other areas of science are just beginning to be explored. This might make you think, though, that when you stare at an object you’re determining that object’s past along with the past of something else across the Milky Way, entangled with the items in your living room.