Before going in deep, you should know some basic concepts:
- In accordance to Einstein’s Theory of Relativity, When an object moves through space relative to us its clock run slow as compared to ours(due to time dilation). That is, the speed of its motion through time slows down.
- Time is the fourth dimension, just like our other three spatial dimensions.
Einstein proclaimed “all objects
in the Universe are travelling 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 emphasised
this as the reason that relativistic effects are so unfamiliar in the everyday
world. All of this is equally 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 generalised sense that is equal to the speed of
light.
To understand this perspective,
let’s imagine rather impractical automobile that rapidly attains its cruising
speed of 100 miles per hour and stick to this speed, no more, no less, until it
is shut off and rolls to a halt. As the distance between start and finish lines
is 10 miles, the car should cover this distance with one tenth of an hour i.e.
6 minutes. Tyler is asked to drive the car and Misty records the data. Results
are recorded from dozens of test drive, although most of the time it was 6
minutes but last few are good deal longer: 6.5, 7, 7.5 minutes. Misty is unable
to explain such anomalously long times, he consults to Tyler and asks him about
last few runs. Tyler has a simple explanation. He tells Misty that as track
runs from east to west, as it got later in the day, the sun was glaring into
his view. During the last three runs it was so bad that he drove from one end
of the track to the other at a slight angle. The north south and east west
directions are two independent spatial dimensions in which a car can move.
Tyler’s explanation illustrates that even though car was travelling at 100miles
per hour on each and every run, during the last few runs it shared this speed
between two dimensions and hence appeared to be going slower than 100 miles per
hour on each and every run. Whereas in
the initial runs car was travelling in east west direction and all of its
motion was confined to one dimension.
Note, just like the impractical
single speed car discussed above, speed can be shared between the different
dimensions, in the same manner object’s motion is also shared between different
dimensions – Three Space and One Time. If an object is sitting still (relative
to us) and consequently doesn’t move through space at all, then in analogy 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. And here’s the leap: Most of our motion is through time
dimension and not through space. And this is the reason that relativistic
effects are so unfamiliar in the everyday world.
If an object does move through
space, this means that some of the previous motion through time must be
diverted. Just like the car travelling at an angle, this sharing of motion
implies that the object will travel more slowly through time than its stationery
counterparts, since some of its motion is now being used to move through space.
That is, its clock will take more slowly if it moves through space. This is
exactly what we found earlier. We now see that the 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.
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 any object can possibly achieve. This is analogous to our
car being test driven directly in the North South direction. Something
travelling at light speed through space will have no speed left for motion
through time. Thus light doesn’t get old. A photon that emerged from the Big Bang
is of same age today as it was then. There is no passage of time at light
speed.
I think now you might be having a
pretty good understanding of this spacetime thing.
