In a recent article I asked what scientists mean when they speak of matter warping spacetime.  We looked into the idea a bit and discovered that the scientists are describing the change in the behavior of their measurement devices:  paths and clocks behave differently in the presence of large aggregates of material.  Interestingly, the relativistic laws inscribing these changes in paths and clock behavior indicate that if one such device were to travel fast enough, it should move backward in time.  What does this really mean?

First we have to start by demystifying the notion of time itself.  For the mathematicians working in general and special relativity, time is what clocks do.  In other words, time is a measurement read off of some device.  But what are these devices doing?  It turns out, that any two bodies in motion can constitute a clock in the scientific sense.  Time itself is simply the comparison of two regular motions.  For instance, the Earth moves around the Sun.  That constitutes a clock relative to human motion throughout the year on Earth.  Likewise, the Earth also revolves such that that the sun rises and sets regularly.  Humans used these clocks for millennia to schedule daily routines and plan yearly planting cycles.  That we use divisions of sunrises such as hours and minutes as our standard for measurement is completely arbitrary to life on our planet.  

Any regular motion can be referenced with respect to our own and called a measure of time: a clock.

Since the dawn of humanity we have established the regularity of the year with respect to the day, and we have since created all sorts of gadgets that can be set act as clocks.  Pendulums were used for many years but required resetting frequently because of slowing due to friction.  Likewise, hourglasses faded in popularity because they needed reset often.  Quartz crystals vibrate regularly with electrical stimulation and so grew to great popularity as clocks with great precision.  The atomic clocks used by NASA scientists are not so different.  Scientists have simply found that certain elements, like Cesium, vibrate regularly in the laboratory and because these vibrations are so fast, extremely small increments of time can be assessed.

So time is the comparison of some motion with respect to another predictable, periodic motion.  Motion is simply the relocation of some material body.  So is it possible to time-travel?  Well, it turns out that the word travel, synonymous with change in location, is only useful in terms of orthogonal directions:  known as x, y, and z in the cartesian system. 

Application of the word travel to a measurement of an idea, such as relative motions, has no direct physical correspondence.  

One cannot physically change locations between measurements because in physics, a location can only mean where a body exists with respect to the others in the discussion.  In other words, time is not at all a location!  So no travel is possible ‘in’ time.  Unfortunately, in an attempt to simplify notion of trajectories, mathematicians have gotten in the habit of appending time measurements with location measurements into a metric called the spacetime.  This has confused the notion of time even more gravely.

By creating the set of measurements consisting of location and relative motion into an abstract idea called spacetime, wherein, it is easy to see how one might confuse time for a location.  Afterall, relocation is defined for 3 of the 4 metrics (length, width, and height).  Yet, time itself- the relative motion of two bodies- has a relative value.  Time can only specify before and after and by how many cycles or fractions thereof.  Unlike a location, which specifies three distances to other bodies, time has only a single reference to previous measurements.  This makes time a number on an axis, not an orthogonal dimension of architecture. 

 Treating time as a dimension in this traditional sense can lead to some fantastic science-fictions but each is strung by the same fatal flaws of logic:  abstractions do not have physical locations.

Without two or more locations, no travel is possible in physics.

What the mathematicians really mean when they say that bodies moving faster than light can ‘travel backward’ in time, is that their descriptive laws break down in these circumstances. Einstein himself gave a physical interpretation to this apparent relation within his mathematics: no body can exceed the speed of light. Next time we’ll look at why this must be the case.