Doctor Emmett Brown's Seven Arrows of Time
- Daily Strange
- May 19, 2021
- 9 min read
Did you know physicists can find no scientific law that says it is impossible for time to run backwards? Think about that for a moment. Wouldn’t it be strange to see Marty McFly or Doc Brown grow younger, a broken-down DeLorean reassemble, and have memories of future events as well as the past? It could really give a person a feeling of DejaVu, and in the case of Back to the Future Part II that is a very understandable reaction. Time is an elusive concept. If someone were to ask you the question “What is time?” you could say it’s what your Timex measures. But upon closer examination, this thing we call time becomes a puzzling mystery indeed. It cannot be tasted, heard, or seen, yet time is something we always seem to never have enough of. Even when time hangs heavy, we eventually run out of it. Time is the one thing that affects everyone. We live with it on a day-to-day basis but barely understand it.
Time can be viewed in two different ways. First, there is the contemporary idea that time is linear. If we could take a picture of the linear time we would see a straight line starting from the beginning of time and extending infinitely into the future, with every event ever to take place in the past, present, and future resting somewhere along the time-line. However, in ancient times, mankind viewed time differently. They believed it was cyclic in nature. This concept of cyclic time can be thought of by imagining a circular clock face with the beginning of time being represented by the number one and the end of time by the number twelve. Then, when the hands of the clock reach number one again, time begins anew.
This concept of time led man to believe that historical events were destined to repeat themselves in each new cycle. That would mean there would be another rise and fall of Rome, another civil war between North and South, and even another Hitler. Only recently has mankind come to think of time as linear and even though scientists have yet to fully comprehend the nature of time, they have deduced seven arrows of time which keep things going in the right direction. The first of these arrows are referred to as psychological time. In other words, it’s the only arrow of time that’s all in our minds. Confused? Let’s approach it in another direction, then.
Scientists have coined the term “psychological time” to mean that we all perceive time as always moving forward. In Back to the Future Part II, we see Doc and Marty travel from the future to 1985, and back again to 1955, which might lead us to the conclusion that time is running backwards. But this is not so. Although our two-time tinkerers have discovered a way of traveling through time, they always perceive events as moving forward in time, never backward.
But in fiction, a writer can give a character the ability to remember future events as well as the past. One such example would be the White Queen in Alice In Wonderland, who tells Alice, “It’s a poor sort of memory that only works backwards.” As strange as that sounds, the queen had a point. Everything, including people, is made up of atoms and molecules that obey the time-reversible rules of Newton and Einstein.
Now, the mathematical equations within the Laws of Physics do not distinguish between past or future, which means there are always two solutions to these equations. One of the solutions sends these atoms and molecules into the future and the other sends them into the past. If this is actually the case, then why don’t we have memories of things yet to come, since all events along the timeline, whether they are in the future or the past, are fixed and unchanging? “Perhaps clairvoyants would claim that dual ability,” states Dr. Tony Rothman in his article.
The Seven Arrows of Time, published in the February 1987 edition of Discover, “but for the rest of us, when memory works at all, it’s sadly — or mercifully — limited to events that have already taken place. That’s the first arrow of time.” Our next arrow is the electromagnetic arrow. A light, X rays, radio waves, and ultraviolet and infrared radiation are all composed of electromagnetic waves that travel into the future, not the past. For example, if we look up at the sun we will see it in the position it occupied just over eight minutes previously because it takes that long for the light to travel from the sun to the earth. If this arrow did not exist it would be possible for somebody from the year 1990 to send radio transmissions into the past and communicate with someone from the year 1955.
We all know that doesn’t happen, and so it seems this second arrow of time would be easily understood. But this is not the case. Physicist James Clark Maxwell successfully described all radiation as a mixture of oscillating electric and magnetic fields in his laws of electrodynamics. As is the case with the basic laws of physics, those of electrodynamics do not distinguish between past or future. Although the mathematics of Maxwell’s theory allows electromagnetic waves to travel into the future or the past, scientists discount the idea of waves going backward in time. Yet scientists don’t disregard this kind of reverse motion because it is impossible, only because it is highly, highly improbable. To illustrate this point, picture a stone being thrown into a pond. As soon as the rock hits the water, ripples are created that expand to the edges of the pond.
Now, if these ripples were to flow in reverse they would first appear at the edges of the pond and then contract inward to the point where the stone hit the water. The reason nobody has ever observed such an event is because it is as improbable as a broken down DeLorean suddenly resembling. That’s our second arrow.
The third arrow of time. Quantum mechanics is the theory that deals with the behavior of matter on the atomic scale. It concerns itself only with predicting what will happen when atomic particles interact. Since these microscopic bits of matter mean little or nothing to most of us, we will have to use an object we are all familiar with to try and illustrate the idea behind this theory.
Let’s take a coin, for example. If we flip the coin into the air, quantum mechanics states that the coin is fifty percent heads and fifty percent tails until the moment we uncover it, whereupon it becomes wholly heads or wholly tails. Yet if this process worked in reverse of the coin would still be fifty percent heads and fifty percent tails after you uncovered it. In other words, the head and tail of the coin would have merged. The coin would be lying there and we would see both sides of the coin simultaneously. But quantum mechanics provides no way for this reverse behavior to occur. The fact that such things don’t happen gives us time’s third arrow.
Before attempting to explain the fourth arrow of time, it is important to note that nuclear reactions have no time preference. What that means is nuclear reactions don’t distinguish between forward and backward time. It’s kind of like the molecular structure of lead being altered so that it is turned into gold. Well, the same nuclear reaction can work in reverse so the gold can change back into the lead. So if a certain type of nuclear decay is possible, its inverse or opposite will be possible too.
Alternate timeline anyone? Not if it's created by a rich punk named Biff Tannen right? In the picture above, Doc Brown attempts to explain how small events in history can create big problems in time. In the years to come, scientists may crack the mysteries associated with the concept of time. But for now, the only thing keeping us from complete bewilderment concerning the nature of time is time's seven arrows which, fortunately, keep all events timely.
A simple way to understand this phenomenon is to look at the number 3. Let’s say that particle 3 is created when we combine particles 1 and 2. Now, according to theoretical law, when particle 3 decays we will again have particles 1 and 2. And this will always be the case. But then we have the kaon, which, like other particles, can be positively or negatively charged or it could be neutral, with no charge at all. Now the problem with the neutral kaon, which is created in the laboratory and does not exist in nature, is that it likes to defy theoretical law. Ninety-nine percent of the time it will obey the law and decay into three particles. Yet one percent of the time this neutral upstart decays into two, proving that it is not time-reversible.
In order to examine the next three arrows of time, we must leave the microscopic world and plunge into the world of nature’s grandest phenomena. Next on our agenda is the black hole. It is a massive star that has collapsed into an object possessing an intense gravitational field so strong that even light cannot escape from it. Yet to examine the fifth arrow we also need to bring the black hole’s theoretical counterpart into the picture. And that object is referred to as a white hole. Now, according to theory, a black hole has such an insatiable appetite that anything which enters into its gravitational field gets gobbled up. But its counterpart, the white hole, works in reverse. If this is true, then somewhere out in the universe there is a white hole spitting out everything the black hole has consumed.
Now scientists tend to agree that the universe distinguishes between forward and backward time. If it didn’t, though, there would be as many white holes as black. Although a few black holes have been detected, such as the one in the binary star system Cygnus X-1, no one has ever been able to observe a white hole. Black holes, to physicists, are elegant and simple objects, yet, as Dr. Tony Rothman states, “Their opposites are troublesome and incomprehensible —especially because of a feature called the singularity. “The singularity, in a black hole, is the point where all the collapsed matter has fallen, where the density is infinite, and where the known laws of physics break down altogether. Since light cannot escape the black hole, its singularity is hidden from the external world. But everything escapes the white hole, light included, so its singularity is visible to even the most casual spacefarer. And because physical laws have gone haywire at the singularity, time there may go forward or backward; TV sets may pop into existence; so, too, may politicians.
Scientists have found such behavior so implausible that they have proposed what we might call the Black Power Hypothesis, which forbids white holes altogether. If that hypothesis is correct, then nature provides another arrow of time by allowing black holes to exist but not their time-reversed counterparts.” As we journey now toward time’s sixth arrow, we encounter the expanding universe.
When the Big Bang occurred, the initial blast flung matter outward. Eventually, this matter began to cool and resulted in the formation of the galaxies, and their constituent parts, such as stars and planets. However, if the universe progressed backward in time it would also contract until the galaxies collided with one another and everything, including us, fried in a shower of radiation.
Most scientists believe the expansion of the universe will continue indefinitely. Yet others, such as cosmologist John A. Wheeler, feel the expansion will come to an end one day and the universe will collapse, eventually ending in a Big Crunch.
The question then, as Dr. Tony Rothman puts it, is, “Will time begin to run backward at the turnaround? Will broken watches reassemble themselves? Will the dead rise? If time runs forward in an expanding universe and backward in a collapsing one, then presumably at the moment of recollapse it runs in both directions at once, with watches shattering and assembling simultaneously.” Because of this paradox, most cosmologists have come to the conclusion that in a collapsing universe, time cannot run backward, thus giving us our sixth arrow of time.
Finally, we come to our seventh and last arrow of time: entropy. A simple way to approach entropy is to think of it as an increasing state of disorder. Take, for example, a jigsaw puzzle. In its completed state, it is a highly ordered system. Now, if you throw it on the floor, it will scatter into many pieces, bringing disorder to the system, and thus increasing entropy.
According to the second law of thermodynamics, entropy will always increase. To prove this law. Dr. Tony Rothman gives us this example. “If a gust of wind blows a speaker’s notes away, they will usually fall to the ground out of order. That’s because there’s only one correct order, but many wrong ones — and so chance favors an increase in entropy, or disorder, in all but the entropy reversed world. The fact that entropy always increases in our world,” states Dr. Rothman, “constitutes the seventh arrow of time.”
Still, confused? Don’t feel too bad. Confusion tends to lead to greater understanding, and in the case of time, your understanding of it is likely greater than you realize, provided you view it as something more than what your Timex measures. Still, the implications of Doc Brown’s seven arrows of time are enough to make anyone’s head spin. Broken watches reassembling, having memories of future events as well as the past, and the backward flow of time are all events that do not in any way transgress the laws of physics but certainly disrupt our own sensibilities. Yet nature as a whole seems to resist this concept of reversible time by throwing a few kinks into the works such as disallowing the black holes time-reversed counterpart, the white hole. So when all is said and done, time travelers Marty McFly and Doc Brown, regardless of their adventures through time, still perceive time as moving forward, which, of course, always leads us Back to the Future.
- Written By John S. Davis -
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