Do we really travel through time with the speed of light?
Here is another explanation.  In Newtonian physics lots of common quantities are represented by vectors: position, momentum, force, the magnetic potential, etc.  Other important quantities are represented by scalars: time, energy, power, the electric potential, etc.<p>One of the deep insights of special relativity is that these scalar and vector quantities are actually <i>unified</i> into a single, new entity called a &quot;four-vector&quot;.  Each four-vector has three spatial components and one temporal component.  So in the two lists I wrote above, each scalar quantity gets paired with the corresponding vector quantity: time &amp; position; energy &amp; momentum; power &amp; force; the electric &amp; magnetic potentials.  In Newtonian physics a rotation in three-dimensional space will transform one component of your position vector into another (say, some of your x-component becomes a bit of y-component).  In special relativity observers moving at different velocities are related by a similar kind of transformation of their four-vectors --- a bit of the time component mixes with the spatial component and vice versa.<p>But what happens if you do this with velocity?  Velocity is a vector.  It&#x27;s corresponding temporal quantity is a bit weird: change in time per change in time.  But it&#x27;s a little more subtle than that, because it&#x27;s actually the change in <i>coordinate</i> time per change in <i>proper</i> time.  Basically this is the ratio between how fast you observe a clock tick in an observer&#x27;s reference frame relative to how fast they see it tick within that reference frame.<p>But the really weird thing about this particular four-vector is that it <i>always</i> has a magnitude of exactly the speed of light.  No matter how fast you go, the magnitude of your four-velocity does not actually get larger.  All you do by going faster is just mix some of the time component of your four-velocity into the spatial components.  (This is why the clocks of moving observers tick more slowly.)  And when you slow down, you just mix some of the spatial components into the temporal component.  And when you&#x27;re at rest the spatial components are all at zero, so the only component of your four-velocity is in the temporal direction.  So at rest your four-velocity points directly in the future with a magnitude equal to the speed of light.  This is what is meant by the statement that we travel through time at the speed of light.