0BRILLIANT THOUGHT EXPERIMENTS IN
PHYSICS-A CASE STUDY
A precise definition of a Thought Experiment is
hard to come by, although it can be stated that all a thought experiment asks
for is a creative and discerning faculty; no elaborate machinery associated
with traditional experimental set ups are required.
A
quintessential Thought Experiments would be The Twin Paradox, courtesy Einstein’s
genius, where he demonstrated that the twin travelling with the speed of light
will be younger than his brother after a certain time has elapsed! A string of
other such innovative thoughts (it is said that Einstein discovered E=mC2 by
‘riding on a beam of light) ultimately decimated Newtonian Physics and ushered
in a revolution called Quantum Mechanics- arguably the finest scientific
achievement of all time. Most interestingly, the doyen of Thought Experimenters,
Albert Einstein never used the term personally.
Let us now go on a voyage through scientific annals
in search of fascinating and enlightening Thought Experiments. Galileo
and falling bodies-
Aristotle, the Greek philosopher (384-322B.C) held
that heavier bodies fall faster than lighter ones and for 2000 years hence
people believed him until Galileo barged into the scene. Although his escapades
on the top of the Leaning tower of Pisa is legendary and it is impossible to
ascertain its vericability, yet the thought experiment that follows is authentic
and has the signature of his unparalleled genius.
Consider two stones of unequal weight. For ease of
reference, the heavier one is ‘H’ and the lighter one ‘L’. Both are dropped
simultaneously from the top of a tower (not necessarily ‘The Leaning Tower Of
Pisa’). As per Aristotle, H will hit ground earlier than L. The time taken by H
is recorded. Now both H and L are tied together by a mass less string and
dropped as a compound object H+L from the top of the same tower. The compound
object H+L is heavier than the heavier body H alone, so as per Aristotle’s
diktat H+L will take lesser time to hit ground than H alone.
But we have not taken the lighter body into the
picture as yet. L will slow up the heavier counterpart tied to it, acting as a
kind of brake because its own tendency is to fall at a slower rate. So the H+L
composite should fall slower than the heavier piece alone.
So finally we arrive at an absurd contradiction,
both deducible from Aristotle’s diktat-The composite body H+L is both slower
and faster than the heavy body alone simultaneously WHICH IS IMPOSSIBLE. It
proves that Aristotle’s musings about falling bodies must be wrong while at the
same time providing an obvious clue as to what the right answer must be-“Since
the composite body cant both be slower and faster than the heavy body at the
same time, both must be falling at the same rate-which is precisely what
Galileo went on to state in his version of ‘The Law of Falling Bodies.’
No
discussion on thought experiments can ever be complete without an account on
Einstein’s marvellous thought experiments which mesmerized people from all
walks of life because of their elegancy and simplicity.
Some of them were so
illuminating and obvious at the same time that listeners often pondered why
they themselves have not thought of them before! Following is an account which describes how
physical laws and there manifestations are related to the frame of reference
used to describe them.
Let us
assume that we are in a glass room. A light bulb located at the exact centre of
the room flashes periodically. The room is moving at a high but uniform speed
towards an outside observer. At the exact moment that the room reaches the
outside observer, the light bulb flashes. The physical event that has
occurred-flashing of the light bulb and emergence of light waves(or photons, as
light has dual character-courtesy Einstein’s photoelectric effect,1905) which
travel with the same speed in all directions; is the same for us and the
external observer. Can there be any difference between what we see and what the
outside observer sees?
The answer according to Newtonian Physics is ‘no’
because Newton espoused the immutability of
physical laws, i.e. to say since physical laws are universally applicable, there
manifestations must be uniform too. Einstein on the other hand begged to differ
(and thank God for that). Einstein contested that since the light from the
bulb travels away from the bulb at a constant speed, and since all the walls of
the room are equidistant from the light source; it follows that we will see the
light strike both the front and the rear wall of the glass house at the same
instant. However, for the outside observer there is an all important
difference. From the observer’s viewpoint outside the room, the front wall is
moving forward, away from the approaching light. At the same time the rear wall
is moving forward to meet it. Thus from the point of view of the outside
observer, the light will reach the rear wall before it reaches the front wall.
So what is simultaneous for us will be two distinct events for the outside
observer for the same physical event that has taken place- flashing of the
light bulb.
The elegant conclusion of the above
thought experiment flies in the face of established principles of Classical Physics,
that ‘the same physical event will appear different depending on the observer’s
frame of reference.’