Cosmology and our View of the World
The Nature of Time &Time Machines,
Lead: Rob Holt & Vassiliy Vorotnikov
Summary by Becky Noyes
The ninth class started with a few rhetorical statements about the nature of time. The presenters pointed out that time is all around, but once it’s gone, it’s gone. According to Stephen Hawking, there are three Arrows of Time. The psychological arrow points in the direction that we think. The thermodynamic arrow points in the direction of entropy. The cosmological arrow points in the direction of the expansion of the universe. Currently, the thermodynamic and cosmological arrows are pointing in the same direction.
The presenters next discussed Einstein’s Theory of Special Relativity using one of his famous thought experiments. The postulate of Special Relativity states that the speed of light is always constant, so distance and time must change. Suppose that there is a train traveling the speed of light next to a clock tower. Person A steps onto the train at exactly noon leaving Person B standing by the clock tower. The light traveling away from the clock with the image of “noon” on it will travel at the same speed as the train itself. Therefore, from the vantage point of Person B, time for Person A on the train will stop.
The presenters used the Global Positioning System, or GPS, as an everyday example that illustrates the widespread presence of Einstein’s theory of relativity. According to Dr. Möbius, GPS uses “precise time measurements to figure out where you are” on the earth. Since different places on the earth travel at different speeds and are at different distances from the Earth’s center, time is the factor that is affected, which causes minute differences that are magnified over the surface of the earth. To make GPS extremely accurate, the programmers had to take Einstein’s relativity laws into account, which tests their correctness every time we use it.
The presenters talked about space-time, a unification of the three-dimensional universe with time. According to Rob, just as the surface of a balloon is a two-dimensional object that expands into three dimensions, so is the universe a three-dimensional object that is expanding into four (the fourth being time). He drew some space-time diagrams to prove this point.
The first part of the diagram shows that time is traveling in one direction. A light cone is a measure of things that can affect each other. Light has a finite speed, and nothing can affect something else at faster than that speed. Therefore, at any particular instant, nothing can instantaneously affect something else. The longer time elapsed from that instant, the more events can be affected.
A wormhole is a shortcut through time and space. The presenters used an example to describe wormholes. Supposing that a sheet of paper represents space-time, it takes a finite time to travel between two points on the sheet. If the sheet is bent so that the points are almost touching, traveling directly between the two points through the air would be a shortcut through space and time. This is a wormhole.
The presenters briefly discussed a theory about particles and antiparticles, which can spontaneously come into existence because the particle travels forward in time and the antiparticle travels backwards in time. The laws of physics are still followed.
The most heated discussions of the time class involved time travel, which is fascinating to humans because, so far, it’s impossible. There are two types of time travel: consistent and alternative. Consistent time travel is a solid loop; the future completes the past. The movie “Bill and Ted’s Excellent Adventure” and the book Harry Potter: Prisoner of Azkaban are examples of consistent time travel. Another example is the painter paradox, in which a time-traveler brings a catalogue of beautiful paintings with him to the time period when the painter is alive and painting. The time-traveler realizes that the painter is terrible, but the time-traveler leaves the catalogue behind, and the painter replicates the catalogue paintings. This sort of self-fulfilling prophesy is also evident in the story of Oedipus Rex. Alternative time travel is what most people think of when they imagine time travel. It is the scenario in which you can go back in time and change events. The Grandfather Paradox is a thought experiment about alternative time travel. Supposing that a time-traveler could ever travel back in time to a point where they could meet their grandfather before he had kids and kill him, would the time-traveler suddenly not exist? If the grandfather died before he had kids, the time traveler’s parents would never have been born, and therefore the time-traveler would never have been born. In that case, the grandfather would never have died, because the time traveler wouldn’t have gone back in time to kill him, and the paradox continues. Note that, according to consistent time travel, it would be impossible to kill your grandfather in the past because the future completes the past. The movie trilogy “Back to the Future” demonstrates alternative time travel.
There are other theories about time travel. David Deutch’s many-worlds theory states that if a time-traveler goes back in time, they will create parallel universes based on their decisions. At this point in the discussion, a student raised a paradox he had thought about. He said that since the elements in your body used to be owned by other places (trees, rocks, oxygen in Malaysia) it would be impossible to go back in time and meet yourself. Since the atoms would be duplicated, going back in time is a challenge to conservation laws. The presenter countered this argument with a reference to consistent time-travel. If the future determines the past, and time is a cohesive unit, then the atoms wouldn’t be duplicated, because the time-traveling was accounted for in the “past.” A student raised another counterargument that suggested that if singular atoms were taken from around the world, their absences wouldn’t be noticed, and the conservation laws of physics would be maintained. However, to assume that one person can time-travel is to assume that thousands of people can simultaneously time travel, which means that many more atoms would have to “go missing” and their absence might not go unnoticed. Also, Professor Möbius dispelled this argument on foundation by pointing out that the atoms would still be breaking the conservation laws regardless of whether or not we noticed their absence in other places.
The discussion progressed further to our conception of time. Many students said that the past is “real”, because we have memory of it, but the future isn’t, because we have no memory of it. Accordingly, “now” doesn’t exist; because the present is too fleeting, time feels consciousness dependent. The past is the only thing that’s real. Once it’s done, it can’t be retrieved. This brought up an interesting point. If our bad experiences change and produce who we are, then even if we could go back in time to fix them, it would be a bad idea. The discussion moved to a theory that if your body can’t travel, maybe your mental being can. This was described in the TV show “Quantum Leap”, in which the main character could only travel within his life span.
Stephen Hawking theorized in his “chronology protection conjecture” that time travel into the past would be impossible because time machines would likely self-destruct as soon as they were activated due to the physics limitations; a more feasible option is time travel into the future. For example, the Russian astronaut who has spent more than 400 days in space has traveled through time more than 13 milliseconds. Also, comatose people wake up and effectively travel into their own future, although this is no real time travel, because they still aged.
A student mentioned a book by Julian Barber in which he made the argument that time is change. Since change, not time, is what we perceive, perhaps time is nothing. This class left many students second-guessing their prior notions about time.