Hey it's professor Dave, let's talk about
special relativity.
Remember before how we said that the
Newtonian paradigm breaks down in the
realm of the very small and the very
fast? We learned about quantum mechanics
for the small, but what about the fast?
To answer this, let's recall that in
classical physics we learned about
relative motion.
Galileo developed the concept of an
inertial reference frame, stating that
measurements of velocity depend on the
reference frame you adopt. But given that
it was the 17th century, there were
limitations as to the experiments he
could perform. When young Einstein got
his hands on Galilean relativity, he gave
it a huge facelift. The part that stayed
the same was the importance of assigning
inertial reference frames, which we
pretend are motionless while everything
else is moving relative to that frame,
which could be the earth, or a person, or a
train. Each inertial reference frame has
its own set of axes and a clock, whether
real or imaginary, to measure time. But
Einstein wondered what would happen if
you assigned an object moving very close
to the speed of light as the inertial
reference frame, or even a beam of light
itself. What would happen then? As it
turns out, some pretty strange things.
Special relativity is comprised of just
two postulates, which at first glance
seem very simple, but we quickly realize
that in order for them to be true we
have to completely restructure our
understanding of space and time. First
let's recall that an inertial reference
frame is one where no acceleration is
taking place. It must have some constant
velocity, including zero if at rest, and
we typically approximate the earth as an
inertial reference frame. The first
postulate states that the laws of
physics are the same in every inertial
reference frame. Whether you are standing
still on earth or traveling with
constant velocity in a car or plane or spaceship,
the same laws of physics always apply.
But the speed of light, represented by
the letter c, is a law of physics. It is a
constant that is used in numerous
equations, so the second postulate states
that the speed of light in a vacuum will
be the same in every inertial reference
frame. Before we brush this statement
aside, let's understand how incredible it
is. If you are standing still on the
ground and you see a car go by at 100
kilometers an hour, you will measure the
car's speed as being 100 kilometers an
hour. But if you are in another car going
90 kilometers an hour on the same road
and that first car passes you, you will
measure the car's speed as being 10
kilometers an hour, because it is only
moving 10 kilometers an hour with
respect to the inertial reference frame
of your car. The car has a different
speed depending on which inertial
reference frame you adopt, just like
Galileo said. But Einstein said that
light doesn't work this way. If you are
standing still on earth, you will measure
the speed of light as being 300 million
meters per second. If you are in a plane
you will reach the same conclusion.
If you are in an ultra-fast spaceship
moving 299 million meters per second, you
will still measure the speed of light as
being 300 million meters per second. No
matter what you do, it's the same.
Experiments have verified this, because
when comparing two objects emitting
light, one stationary and one in motion,
they always yield the same value for the
speed of light. But how can this be possible?
How is c always the same and why can't
we catch up to it? It's not just because
we don't have the technology to go so
fast, it's because the speed of light is
a fundamental law of physics. It is the
universal speed limit. The problem arises
when we now have to try to account for
these different reference frames. With
the earlier example, the person on the
ground and the person in the slower car
measure different speeds for the faster car,
and this agrees with our everyday
experience. But in order for the person
on the ground and the person in the
spaceship moving near the speed of light
to measure the same speed for light, they
must be experiencing time in different
ways. This is the first incredible
conclusion we can derive from special
relativity. Time is not some rigid
detached parameter as Newton envisioned.
It does not flow at an absolute rate.
Time is relative.
It flows at different rates for
different observers. As much as this
sounds like science fiction, special
relativity has been verified
experimentally countless times, and to
remarkable degrees of precision, so this
is no scam. This is how the universe
works. Let's move forward and learn all
about special relativity.
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