An Example of a "Learning Process" Journal (using the 2 colored box format)

"Time Travel in Einstein's Universe" by Richard Gott (Chap. 1, pp. 3-32)

The book consists of 5 chapters:

1. Dreaming
2. Travel to the Future
3. Travel to the Past
4. Beginning of the Universe
5. Report from the Future.

This chapter is largely an introduction that mentions how we have been interested in time travel, largely through literature. The following books are mentioned:

1. The Time Machine by H. G. Wells (1895)
2. Back to the Future [movie] (1985)
3. Timescape by Gregory Benford (1980)
4. Bill and Ted's Excellent Adventure [movie] (1989)
5. Somewhere in Time [movie] (1980) {Christopher Reeve and Jane Seymour}
6. All You Zombies by Robert Heinlein (1959)
7. Orion by Ben Bova (1984)
8. Contact by Carl Sagan (1985)
9. Star Trek

There are also a few isolated items of interest, which help flesh-out the story.

• Minkowski was Einstein's mathematics professor.
• The idea of an object's "worldline" in 4-dimensions (3 space + time) with time the vertical dimension.
• The earth's worldline is a helix over a period of one year.
• Since each object has a worldline, and since every subatomic particle is an object as well as every distinct collection of particles, there are truly an infinite number of world lines, many (all?) of which are interacting with one another.
• Manyworlds interpretation: the universe contains not just one world history but many in parallel. David Deutsch at Oxford favors this approach. Richard Feynman also contributed to this view.
• The universe may have many worldlines, some of which may involve time travel, but such worldlines must be "self-consistent" in the sense that if one witnesses the past, one cannot actually change the past. This is considered the "conservative" interpretation. It is favored by Gott as well as other physicists such as Igor Novikov (Copenhagen) and Kip Thorne (Caltech).
• A worldline that is a circle implies time repeating itself. Such a worldline is called a "jinn" by Novikov (from 1001 Arabian Nights). "Novikov has noted that in the case of a macroscopic jinni like this the outside world must always expend energy to repair any wear and tear (entropy) it has accumulated so it can be returned exactly to its original condition as it completes its loop. [p. 23]
• "Permissible in theory, macroscopic jinn are improbable. ... According to quantum mechanics, if one has enough energy, one can always make a macroscopic object spontaneously appear (along with associated antiparticles, which have equal mass but opposite electric charge) - it's just extremely unlikely." [p. 23]
• "Why are physicists like me interested in time travel? ... it's because we want to test the boundaries of the laws of physics." [p. 29]

This is one of the books I bought at Audrey's while in Edmonton for the NCTM conference. It appears to be well written and easy to follow, so I will give it a try.

It is easy enough to grasp the idea of a worldline for a single object, but the idea of imagining all possible worldlines and their interactions is a bit boggling. It is bad enough to consider all possible worldlines for objects on earth, but expanding this to include all objects in the universe is a much larger mental game.

David Deutsch's book, The Fabric of Reality, is one of my favorite books, even though I have only read about half of it. I must return to this and finish it.

The conservative approach seems to me to involve the very paradox it tries to avoid. If one could actually travel to the past, then one must have an effect (simply by being there), and this must alter the history (i.e. worldlines of many objects).

This first chapter sets the scene for the remainder of the book. It demonstrates that we have long had an interest in the idea of time travel, both in literature as well as in physics. The next two chapters will focus on the physics of time travel, first to the future, then to the past.

Science Index