67. Niels Bohr and the Barometer

The following was a question in a physics degree exam at
the University of Copenhagen: "Describe how to determine the
height of a skyscraper with a barometer."

One student replied: "You tie a long piece of string to the neck of
the barometer, then lower the barometer from the roof of the
skyscraper to the ground. The length of the string plus the length
of the barometer will equal the height of the building."
This highly original answer so incensed the examiner that the
student was failed immediately. He appealed on the grounds that
his answer was indisputably correct, and the university appointed
an independent arbiter to decide the case. The arbiter judged
that the answer was indeed correct, but did not display any
noticeable knowledge of physics. To resolve the problem it was
decided to call the student in and allow him six minutes in which
to provide a verbal answer which showed at least a minimal
familiarity with the basic principles of physics. For five minutes
the student sat in silence, forehead creased in thought. The
arbiter reminded him that time was running out, to which the
student replied that he had several extremely relevant answers,
but couldn't make up his mind which to use. On being advised
to hurry up the student replied as follows:

"Firstly, you could take the barometer up to the roof of the
skyscraper, drop it over the edge, and measure the time it
takes to reach the ground. The height of the building can then
be worked out from the formula H = 0.5g x t squared. But bad
luck on the barometer.

"Or if the sun is shining you could measure the height of the
barometer, then set it on end and measure the length of its
shadow. Then you measure the length of the skyscraper's
shadow, and thereafter it is a simple matter of proportional
arithmetic to work out the height of the skyscraper.

But if you wanted to be highly scientific about it, you could tie a
short piece of string to the barometer and swing it like a
pendulum, first at ground level and then on the roof of the
skyscraper. The height is worked out by the difference in the
gravitational restoring force T = 2 pi sqrroot (l / g).

"Or if the skyscraper has an outside emergency staircase, it
would be easier to walk up it and mark off the height of the
skyscraper in barometer lengths, then add them up. "If you
merely wanted to be boring and orthodox about it, of course,
you could use the barometer to measure the air pressure on
the roof of the skyscraper and on the ground, and convert the
difference in millibars into feet to give the height of the building.

But since we are constantly being exhorted to exercise
independence of mind and apply scientific methods, undoubtedly
the best way would be to knock on the janitor's door and say
to him 'If you would like a nice new barometer, I will give you
this one if you tell me the height of this skyscraper'."

The student was Niels Bohr, the only Dane to win the Nobel
Prize for Physics.
(I don't think this is a true Bohr quote but it sounds good)

Here are some true or almost true quotes:
Neils Bohr Quotations

* There is no quantum world. There is only an abstract physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature.

"And anyone who thinks they can talk about quantum theory without feeling dizzy hasn't yet understood the first word about it."

"Nothing exists until it is measured." (Note: In truth, things exist but exist as a probability, which does not settle into one state until measured)

"The opposite of a correct statement is a false statement. But the opposite of a profound truth may well be another profound truth."

"We all agree that your theory is crazy, but is it crazy enough?"

"How wonderful that we have met with a paradox. Now we have some hope of making progress!"