Einstein’s gravity

Newton’s law of gravity would appear to describe an invisible,
almost magical, force that acts between all objects however far
apart they are (although it does become much weaker with
distance) and no matter what lies between them, even empty space.
We therefore say that the force of gravity requires no ‘medium’ (or
‘stuff’) to act through. Einstein gave a much deeper explanation
than this. He claimed that gravity does not act directly on an object
but on space itself, causing it to warp. This warping, or curving, of
space then causes objects within it to behave in a different way than
they would if the space they were in was not warped. Confused?
Let us take a step back and see how Einstein came to this seemingly
unnecessarily abstruse interpretation.
Have you ever had a ride in one of those amusement park
simulators? You take your seat along with a few other passenger
inside a closed capsule and watch a short film of a futuristic chase
scene. The capsule feels like it is really accelerating, braking,
whizzing roundsharp corners, riding bumps, climbing and falling.
In fact, suspending your disbelief is surprisingly easy. The
principle used in these rides is known as Einstein’s principle of
equivalence and is so simple that it can be stated in one word:
g-force (or is that two words?). Einstein realized that the force
you feel when accelerating (probably felt most clearly when on a
plane speeding along the runway just prior to take off) and the force of gravity are equivalent to each other. In fact, we say that
the acceleration of the plane which pushes us back against the seat
is providing a g-force. The ‘g’ stands for gravity and is, in fact, a
quantity with the units of acceleration not force. So an acceleration
of one ‘g’ would be equal to the acceleration a body undergoes
when falling.
At first glance this appears to be rather far-fetched. After all,
the force pushing you back in your seat is to do with motion and
acceleration whereas the force of gravity acts even when you are
standing still (by keeping you stuck to the ground). But think a
little about how the simulator ride actually works. How is it that
you get the sensation of acceleration even if you look away from
the convincing images on the screen? After all, the simulator is
not moving anywhere, it just tips and rocks about on its stand.
All it needs to do to give the impression of forward acceleration,
say at one ‘g’, is to tip back so that you and your seat are facing
upwards. We are so used to the sensation we feel when we lie on
our backs in bed at night that we forget about the pull of Earth’s
gravity forcing our heads down into the pillow. In fact, this force
which we usually take for granted is equivalent to the force which
pushes us back in our seats if we were in a car accelerating from
nought to sixty miles per hour in just over two and a half seconds!
This is why it is so easy to fool the brain into thinking that
the gravitational force we are really feeling in the simulator is an
acceleration force. In the same way, when our simulator ride stops
so suddenly that we feel ourselves being thrown forwards, all that
is happening is that the simulator is tipping us forwards and letting
gravity do the rest.
Another example that demonstrates the principle of equivalence
at work is the flip side of the simulator example, namely
using acceleration to simulate gravity. This is the most common
example that is used when the subject is taught. Imagine you are
strapped in to your seat in a real rocket awaiting countdown for
lift-off. Your seat is such that you are facing upwards towards the
top (front) of the rocket. Imagine, further, that you are so relaxed
and laid back about your trip that you drop off to sleep—not very
likely, I know. When you wake up, and before you have a chance to look out of the window, the principle of equivalence would say
that you will not be able to distinguish between the sensation you
would feel if the rocket were still on the launch pad with gravity
forcing you down into your seat, and the sensation you would feel
if the rocket had left Earth long ago and was now out in space
accelerating at a constant one ‘g’. In fact, if you continue to resist
the temptation of looking out the window to check whether
it is the empty blackness of space or the familiar surroundings
of the rocket launch site staring back at you, you would not be
able to find any experiment that you could carry out inside the
rocket that would allow you to guess where you were2. By experiments
I mean anything fromsimple observations, such as studying
the swing of a pendulum or watching a ball fall, to sophisticated
measurements involving laser beams and mirrors; basically any
experiment which could distinguish between the behaviour of objects
undergoing an acceleration of one ‘g’ and the effect of Earth’s
gravity.
Finally the suspense is too much and you look outside to see
that you are indeed accelerating through space. However, all those
physics experiments have worn you out so you get back into your
seat and go to sleep. When you wake up you feel weightless. You
are glad you remembered to strap yourself in or you would have
floated off and bumped your head on the instrument panel. Now
you are faced with another puzzle if you don’t look outside. You
see, you could either be drifting in space at a constant velocity
with the rocket engines shut off, which would surely account for
the sensation of weightlessness, or you could be falling through
the Earth’s atmosphere and in danger of imminent death if you
don’t take control of the rocket quickly. You see, when you are
falling freely through Earth’s gravitational field you experience
weightlessness, as though the pull of Earth’s gravity has been
switched off.