Touching The Limits Of Knowledge

Cosmology and our View of the World


The Nature of Time &Time Machines,
Rob Holt & Vassiliy Vorotnikov


Summary by Hannah Varn

Stephen Hawking noted three different arrows of time:

1) Psychological – this is the time we perceive; subjective and relative among individuals and situations
2) Thermodynamic – the direction towards which entropy tends
3) Cosmological – the direction of the expansion of the universe

The psychological and thermodynamic arrows point in the same direction, as does the cosmological arrow, for the moment. What would happen if the universe begins to contract?

Einstein developed Special Relativity in 1905. The theory relates time and energy and states that the passing of time is relative to the observer. The two principles of special relativity are:

1) Only relative to the reference frame can you determine speed
2) The speed of light (3 x 10^8 m/s) is always the same, no matter what reference frame (distance and time changes)

At the speed of light, or close to it, time dilation occurs. This means that, relative to a standard reference frame, for anything moving at the speed of light, time would appear to be going much slower. Muons normally decay very quickly, but when they are accelerated to near the speed of light they can last much longer and travel through our atmosphere and cause cancer in humans.

Einstein’s General Theory of Relativity states that the closer you get to a massive object the slower the time becomes for an outside observer. The definition given on the handout is: “Gravitation is viewed as being a manifestation of curved space and time” (Wikipedia)

GPS: The GPS system is an everyday test of Einstein’s theories. It uses satellites moving quickly through space (and further away from the massive earth) to relay messages back to earth where time is moving relatively slower. GPS is accurate to about a foot.

Time Travel: There are several theories that attempt to show how a person or thing could travel in time. One such theory examines the idea of time travel through a wormhole.

An illustration with 3-dimensional analogy of a wormhole may be found at, based on an article under "How Stuff Works".
Here space is represented by a two dimensional sheet of paper, it can be curved so that it touches itself, thus creating a much shorter distance between two points (Space is actually three dimensions, not two, and the fourth dimension is time. If one were to travel through a wormhole one would have traveled an immensely large distance in only a short length of time.

Stephen Hawking presents another conjecture involving the spontaneous formation of particles and antiparticles. In this model, mass and energy are still preserved because the particles cancel each other out at their conception and their reunion. An antiparticle would therefore be identical to its correspondent particle when viewed backwards in time.

Russian astronaut Valeri Polyakov traveled 13 milliseconds into the future during his 438-day stay on the Russian Space Station due to the station’s speed.

The light cone (from a Wikipedia article) represents the past, present and future in space-time. The observer is situated at the intersection of the two dimensions of space and time. The light cone is the measure of the things that can have affected you and the measure of things you can affect (your sphere of influence).

Hawking notes that the laws of physics must be obeyed even during time travel, because time is finite. The future completes the past and thus when traveling backwards in time, nothing could be changed. He presents the grandfather paradox, which states that you can’t go back in time and kill your grandfather because it didn’t happen (obviously, since you were born).

However, some advocates of space travel believe that the past can be altered either by creating a branched future or through the “many worlds” theory. This theory posits a different universe for every possibility anytime there is more than one thing that could happen.

Discussion on whether or not the multi-universe option would actually constitute time travel:

Professor deVries: The multi-universe theory doesn’t answer the grandfather paradox. It isn’t time travel, it’s jumping into another world altogether. This would represent a break in the causal stream. Causality and time are interlinked.

Rob: You’re not jumping into another universe; you’re creating another universe.

Professor deVries: This creates the problem of sets of time. There has to be a time within time when the new universe starts, breaks off, etc. There would have to be an absolute time in which all other times exist. That doesn’t follow.

Marty M.: This would show a sort of superiority of universes. Whichever one came first would be the most determining universe.

Professor Möbius: The bootstrap paradox says that the people in the future could bring back the blueprints for the time machine, or an art lover could bring back the catalog of a painter’s work before he had painted it. How did these things come into being in the first place?

Time needs to be a coherent whole in order to avoid these paradoxes.

Becky: All parts of one’s past were necessary to create ones present. Changing the bad things in the past would change the person one is today.

Neal: The past is an accomplished fact. We like to think about time travel because we are uncomfortable with the fact that we can’t change the past. This speaks to our ideas about responsibility and control. Each present decision quickly dissolves into the past and therefore something we can’t take back.

Is traveling back in time even physically possible? Some discussion on what this would mean in terms of the physical laws:

DJ: Everything in my body was at one point part of something else (my parents, the material world). If I go back in time, especially after only a short amount of time, I would be duplicating the matter that exists, which goes against the conservation of mass and energy (i.e. the laws of physics).

Morgan: Physical time travel would be possible because the constitution of our bodies is constantly changing. It doesn’t take energy to time travel.

Professor deVries: Mass energy would still change on a global scale. You would have to switch places with something of equal mass energy (a tuna fish, for example).

Vassiliy: Could antimatter help in the equation?

Professor Möbius: Energy is still needed to produce antimatter.

Why our obsessions with time travel? What does psychological time have to offer for the discussion?

Professor Smith: If we are not just our brains (i.e. we are composed of not just a brain, but a mind), then it is not just our body that can travel back in time. Also, we haven’t touched on Hawking’s psychological arrow of time. The past is a real thing; we can access it through our memories. We have no such capacity of viewing the future and it remains strange to us. Where and when is the present? Why does time pass slower or faster based on what we are doing? What is our obsession with time travel? Isn’t psychological time just as interesting, if not more so?

DJ: If only the mind or soul traveled back in time than there would be no alteration of the laws of conservation.

Sam: Maybe Hawking is right and you really can’t travel back in time at all.
DJ: Time travel into the future is still possible, because it hasn’t happened yet. A coma is a kind of psychological leap into the future.

Some book recommendations:

Neal: Time is dependent on movement (About Time) and time is nothing but change so without change there is no time (The End of Time). Recommends these books.

Professor Smith: Recommends also: Matter and Memory.