Natural Science Forum / Physics / Relativity / June 2005

The next paragraph gives the relevant passage from Brian Greene.
Notice he is simply discussing how time can be seen as a
dimension in a conceptual sense and for the sake of illustration.
I add the comment of what possible deeper mechanisms may be
involved. Do not assume that there is no mystery of how time can
be affected by motion, etc. Without knowing the causal mechanisms,
it is natural for people to find it a stretch of the imagination to
know how time can slow down in fast moving objects. This is the
source of the endless debates raging in this newsgroup about
relativity since its inception. In case you are convinced there
is no mystery. Pls. explain the causal mechanisms.

Brian Greene stated:

"Since this view proclaims that space and time are simply
different examples of dimensions, can we speak of an object's
speed through time in a manner resembling the concept of its
speed through space? We can.

A big clue for how to do this comes from a central piece of
information we have already encountered. When an object moves
through space relative to us, its clock runs slow compared to
ours. That is, the speed of its motion through time slows down.
Here's the leap: Einstein proclaimed that all objects in the
universe are always traveling through spacetime at one fixed
speed - that of light. This is a strange idea; we are used to the
notion that objects travel at speeds considerably less than that
of light. We have repeatedly emphasized this as the reason
relativistic effects are so unfamiliar in the everyday world. All
of this is true. We are presently talking about an object's
combined speed through all four dimensions -three space and one
time - and it is the object's speed in this generalized sense that
is equal to that of light. To understand this more fully and to
reveal its importance, we note that like the impractical
single-speed car discussed above, this one fixed speed can be
shared between the different dimensions-different space and time
dimensions, that is. If an object is sitting still (relative to
us) and consequently does not move through space at all, then in
analo gy to the first runs of the car, all of the object's motion
is used to travel through one dimension-in this case, the time
dimension. Moreover, all objects that are at rest relative to us
and to each other move through time-they age-at exactly the same
rate or speed. If an object does move through space, however,
this means that some of the previous motion through time must be
diverted. Like the car traveling at an angle, this sharing of
motion implies that the object will travel more slowly through t
ime than its stationary counterparts, since some o f its motion
is now being used to move through space. That is, its clock will
tick more slowly if it moves through space. This is exactly what
we found earlier We now see that time slows down when an object
moves relative to us because this diverts some of its motion
through time into motion through space. The speed of an object
through space is thus merely a reflection of how much of its
motion through time is diverted.

We also see that this framework immediately incorporates the fact
that there is a limit to an object's spatial velocity: the
maximum speed through space occurs if all of an object's motion
through time is diverted to motion through space. This occurs
when all of its previous light-speed motion through time is
diverted to light-speed motion through space. But having used up
all of its motion through time, this is the fastest speed through
space that the object-any object-can possibly achieve. This is
analogo us to our car being test-driven directly in the
north-south direction. just as the car will have no speed left
for motion in the east-west dimension, something traveling at
light speed through space will have no speed left for motion
through time. Thus light does not get old; a photon that emerged
from the big bang is the same age today as it was then. There is
no passage of time at light speed."