Simple concepts that are taken for granted can be quite complicated after closer scrutiny. One example is the concept of time. Most scientists agree that the brain is basically hard-wired to deal with time. Temporal considerations are essential to understand motion and other real-life concerns such as cause and effect.
Dictionary Definition of Time
“Time is a non-spatial continuum in which events occur in apparently irreversible succession from the past through the present to the future.”
The dictionary uses the terms past, present, and future so that seems like a good start for time travel considerations. Anyway, in the movie Back to the Future the DeLorean control panel on Doc's time machine definitely had three different settings (see image). But check it out! The dictionary seems to suggest that going back in time might be possible by defining time as merely “apparently irreversible.” That suggests that some evidence must exist to prevent the definer from unequivocally stating that time is not reversible. Perhaps they saw Back to the Future too? Anyone who saw that movie would be aware of the history-altering concerns that time travel presents. In fact, it's been suggested that in a world of time travel, the hardest profession would be history.
Dimensionality of Time
Also in the dictionary definition, the term “non-spatial” distinguishes time from space. It's generally believed that the universe has three spatial dimensions (x, y, z) and one time dimension, commonly referred to as the fourth dimension. Why does space gets three dimensions but time only one? It doesn't seem fair does it? For time travel considerations it might be reasonable to consider at least two dimensions for time:
- backward time (into the past) and
- forward time (into the future).
Perhaps even a third dimension for time could be present time but because time is a “continuum,” it always has to be going somewhere. Going into the past or into the future seem reasonable but going into the present seems a little bizarre. But maybe that's OK. If forward or backward time stopped, wherever it stopped would be the present. Therefore, present time would simply be the time until either forward or backward time started-up again. So, time has three dimensions (backward, present, and forward) and is dimensionally equivalent to space with its three dimensions (x, y, z). Hooray! All's fair in the universe! Based on this crystal-clear logic, for this exploration of the concept of time, the dictionary definition for time must be altered :
“Time is a non-spatial continuum in which events occur in reversible succession into the past, into the future, or during the present.”
Physics of Time
In physics, there are basically two ways to describe how things work:
- classical mechanics which works fine for familiar, "normal" things like people, cars, baseballs, etc. and
- relativity which is necessary to explain super-galactically huge, sub-atomically small, and especially really fast (speed of light) things.
Classical mechanics and relativity differ primarily because of time which is constant for normal things, but actually goes more slowly or faster depending on an object's velocity relative to the speed of light and the strength of gravitational fields, for relativistic things. Time that varies because of an object's velocity relative to the speed of light (relativistic time) cannot apply to the dimension of present time because present time is only for objects that are frozen in the present and therefore have no velocity. But relativistic time should be relevant to the other two time dimensions (backward into the past and forward into the future).
Universal Definition of Time
Space and time are inseparable for relativistic things and this property is commonly referred to as the space-time continuum. So, to include relativistic things, the definition of time needs more tweaking:
“Time is a space-related continuum in which events occur in reversible succession at a rate that depends on an object's velocity relative to the speed of light or the strength of gravitational fields.”
Note that the time definition has changed from " ..a non-spatial continuum..." to "...a space-related continuum..." It allows for time itself to proceed forward into the future or backward into the past. But what about people and objects? Can they travel forward into the future or backward into the past?
Consequences of Physics for Time Travel
Physicist Stephen Hawking, one of the most creative and brilliant thinkers in the world, published his Chronology Protection Conjecture in a 1992 paper which strongly implies that time travel is impossible according to the laws of physics at all but submicroscopic levels. This is very interesting. Apparently, there's a chance that really small things can do it. Submicroscopic refers to the atomic level where relativistic effects are significant and interesting properties like quantum entanglement could allow for communication between spatially-separated, quantum-entangled particles. But even at this level, no one has reliably suggested how communication between temporally-separated, quantum-entangled particles might occur, let alone time travel itself. In Hawking's own words:
“It seems that there is a Chronology Protection Agency which prevents the appearance of closed timelike curves and so makes the universe safe for historians.”
Whether or not this will stand the test of time remains to be seen.
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