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Nothing seems more intuitive than the passage of time.  However, special relativity violates this intuition, and raises a major philosophical problem.

Table of Contents

Newtonian Time
Nothing seems more intuitive than the passage of time.  The usual paradigm is the Newtonian one1.  Time marches forward relentlessly.  It is absolute and the same everywhere.  It has no beginning and no end.  It is linear and infinite.  There is a special time, now, that is distinct from all other times.  Events in the past, at times earlier than now, cannot be changed.  Events in the future, at times later than now, are unknown and cannot be predicted.  The only open questions about Newtonian time are: how does time pass, and how fast does it pass?  The standard answer to the first question is that the passage of time is a primitive feature of the universe, one that cannot be analyzed any further.  The second question is generally considered incoherent, as a rate of time passage implies its passage relative to another standard, but it is not clear what this other standard is2.

One of the remarkable features of the equations of Newtonian physics is that they are symmetrical with respect to time3.  Thus, in a Newtonian universe, a video played in reverse ought to look much like one played in the correct direction.  This is true for simple systems such as orbiting celestial bodies, but is obviously not true for any even mildly complex system, such as a glass falling off a table and shattering.  The only law of physics that includes a preferred direction of time is the second law of thermodynamics, which states that the amount of order in any physical system will decrease as time progresses.  Thus the falling glass moves from an intact, ordered state to a shattered, disordered state.

As Stephen Hawking explained very well in his book A Brief History of Time, the second law of thermodynamics is due solely to a statistical fluke4.  For some reason, the universe of the past was in a highly ordered state.  It moves at random from one state to another, but because there are so many more disordered states than there are ordered states, it invariably moves to a more disordered one.  Despite its simplicity, the fact remains that the second law is the only law of physics that specifically deals with a past and future with respect to time.

Relativistic Time
Einstein’s theory of special relativity, published in 1915, shattered the Newtonian paradigm5.  It predicted the following counterintuitive features of time, all of which have all been demonstrated experimentally:

  • Space and time are one and the same, in that “distance” in space-time can be clearly measured.  To accelerate from one velocity to another is simply to rotate one’s orientation in space-time.
  • Time does not proceed at the same rate for everyone.  Time for an object moving relative to an observer will slow down relative to the time for the observer6.
  • Perhaps most strange of all, “now” is not the same for everyone.  What is happening “now” on Mars will not be the same as what is happening there “now” for someone in a space ship moving relative to the Earth.  This is known a “relativity of simultaneity”7.

The second item listed above leads to a famous paradox in relativity theory called the “twin paradox”8.  In this scenario, one twin is sent off in a space ship at a significant fraction of the speed of light, while the other remains on earth.  At some point the twin in the space ship turns around and speeds home.  Because time will have passed slower for the twin in the space ship, he will be younger on his return to earth than his brother.

The Big Problem
The third effect of relativity on time listed above, the relativity of simultaneity, is a big problem for the philosophy of time.  As mentioned above, according to the Newtonian paradigm, “now” is a special time.  But if it is special, it should be unique: there cannot be two “nows”.  However, if that is the case, then in the Mars scenario described above, which “now” is the correct one on Mars, the one that corresponds to now on earth, or the one that corresponds to now in the space ship?

There seem to be only two possible solutions to this problem, and neither one is very appealing.  The first is to assume that there is a particular velocity, or “reference frame” in the jargon of physics, that is somehow special.  It might happen to be the case that the velocity of the space ship was the special one, in which “now” on Mars as viewed from the ship would be the correct one.  What we would then perceive as happening now on Mars might actually be a solidified part of the past, or might not have happened yet!  The big problem with this solution is that it flies in the face of the most basic assumption in physics, that the laws of physics are the same everywhere and always.  It would especially violate the philosophical basis of the special theory of relativity itself, which Einstein developed after assuming that there were no such preferential reference frames.  Ironically, if this solution were true, special relativity and the laws of physics would still appear to be the same everywhere and for every inertial reference frame.  However, in reality, one would be preferred.

The other solution is that time does not pass at all9!  In this scenario, time would be as fixed as space is, in the future as well as in the past.  Despite the apparent randomness of quantum theory10, the future would be as fixed and deterministic as the past, although it would appear to us to be random.  The passage of time would be purely psychological; for some reason we experience only a single point in time, and for some reason we appear to move inexorably forward along the dimension of time.  The obvious problem with this solution is that it is so counterintuitive.  If time does not really flow, why does it seem so certain to us that it does?  Despite this fact, most physicists actually believe in the second solution rather than the first11.

Time seems to be easy to understand intuitively, but special relativity has shown us that it is not what it seems.  The predictions of that theory, while strange, seem palatable.  However, the implications for the philosophy of time are grave.  Either we must accept the existence of a kludgy preferred inertial reference frame with respect to the passage of time, or time does not pass at all.  Neither solution is acceptable.  The passage of time is another subject that falls under those in philosophy that may never be fully explicated.

End Notes

  1. For more information see Rynasiewicz, Robert, “Newton’s Views on Space, Time, and Motion”, The Stanford Encyclopedia of Philosophy (Winter 2012 Edition), Edward N. Zalta (ed.), URL = <http://plato.stanford.edu/archives/win2012/entries/newton-stm/>.
  2. Davies, Paul, “That Mysterious Flow”, Scientific American, Vol. 287, No. 2 (August, 2002).
  3. This is also true of most of the equations in modern physics theories.
  4. Hawking, Stephen, A Brief History of Time, Bantam Books, New York, NY (1988).
  5. For a simple summary of the special theory of relativity, see my essay on the great discoveries of 1879-1931, in particular the section on Special Relativity.
  6. Einstein’s general theory of relativity predicted that time would also slow down in a gravitational field.  This prediction has also been verified experimentally.
  7. “Relativity of simultaneity”, Wikipedia, URL = <http://en.wikipedia.org/wiki/Relativity_of_simultaneity>.
  8. For a more complete description of the twin paradox, see “Twin paradox”, Wikipedia, URL=<http://en.wikipedia.org/wiki/Twin_paradox>.
  9. Callender, Craig, “Is Time an Illusion?”, Scientific American, Vol. 302, No. 6 (June, 2010).
  10. For more on quantum theory and randomness, see my essays on Quantum Theory and Free Will.
  11. Kirchmann, Daniel M., “Einsteins Fault”, URL=<http://www.tardyon.de/mink.htm>.

November, 2013

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