Let us do a “thought experiment ” What is a “thought experiment”? An experiment carried out in thought only. It may or may not be feasible in practice, but by imagining it, one hopes to learn something useful. German word: Gedankenexperiment
A Thought Experiment: Imagine a dark, cloudy, moonless night. A power outage is happening all over the entire city. You are sitting in your 4 th floor apartment thinking and worrying about your science test tomorrow. Suddenly, a commotion downstairs. You somehow manage to find your torch and rush to the window.
A Thought Experiment (contd.) A Funny Torch: It turns on only for a moment, every 15 seconds.
Every time the torch lights up you see a man standing in the large open space in front of your building. 0 sec, at A 15 sec, at B 30 sec, at C 45 sec, at D … You have no idea what is going on.
When You mark his positions on a piece of paper. Connect point A to B, B to C, C to D, and so on, by straight lines. What do you see? A zigzag path! What do you think was going on?
A drunken man wandering around aimlessly. That was easy. One does not need an Einstein’s IQ to figure that out.
Another Thought Experiment Suppose you are sitting in a big stadium, watching a game of soccer, being played between two equally good teams. Suppose the players are invisible: nothing except the ball is visible. Open your eyes once every 15 sec. What will you see?
“He is happiest who hath power to gather wisdom from a flower.” … Mary Howitt (1799 -1888) (English poetess)
Now I want to describe a real (not a gedanken) experiment.
*Robert Brown *1827 *observed pollen grains from a flowering plant through a microscope. * to his surprise he noticed that tiny particles of the grain suspended within the fluid, moved in a haphazard fashion. Write this down
If you were Robert Brown How would you understand this observation? (Remember, you are in 1827!) Would you suspect that the pollen is alive? Would you get excited at the thought that you may have discovered the very essence of life or a latent life force in every pollen? What other experiments would you perform to test your suspicions?
This is what Mr. Brown did: *He repeated his experiment with other fine particles including the dust of igneous rocks. *He found that any fine particle suspended in water executes a similar random motion. *This phenomenon is now called Brownian Motion. Write this down.
Result of an actual experiment Particle positions were recorded at intervals of 30 sec.
Are you puzzled? Scientists in the 19 th century were puzzled by this mysterious phenomenon. Knowing what you know about how atoms/molecules are constantly moving, can you come up with an inference? Obviously, the suspended particle is not moving on its own unlike the drunkard in our 1 st thought experiment. Why then is it moving? And why in an erratic way? Think … Want a hint? Recall our 2 nd thought expt.
If you have not already guessed, here is the rational explanation for the mysterious jerky movement of tiny particles suspended in fluids, which made Mr. Brown famous:
Basic Understanding (contd.) Molecules are perpetually moving in different directions, some faster than others. As they move, they keep colliding with each other, which can possibly change their speeds and directions of motion.
Basic Understanding (contd.) Now you can very well imagine the fate of the particle unfortunate enough to be placed in the mad crowd of water molecules. The poor fellow is getting hit, at any instant, from all sides, by millions of water molecules. The end result is that its position keeps changing randomly.
Importance of Contribution *It provided a convincing evidence for the molecular theory of matter *It showed that atoms and molecules are real physical objects. *Skeptics who doubted their existence were silenced. *Brownian motion as the microscopic process responsible for diffusion on a macroscopic scale. Write this down.
Moral of the Story Even a lowly pollen grain can tell us a lot about the constitution of matter. Nothing of this would have been possible without the inquisitive mind of the scientist.
References: R. S. Bhalerao Department of Theoretical Physics TIFR, Mumbai University of Pune 12 November 2005 Einstein’s Miraculous Year edited by John Stachel, Princeton, 1998 100 Years of Brownian Motion P. Hanggi and F. Marchesoni cond-mat/0502053 Brownian Motion: Theory & Experiment K. Basu and K. Baishya Resonance Vol. 8, No. 3, 71-80 (2003)
“There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.” … Mark Twain (1835-1910)
Quiz 1. Brownian Motion 2. Einstein 3. Brown 4. Movement of pollen grains in oil/water A. Measurement of pollen grain movement B. Random movement of particles C. Bombardment of water molecules bumping into them. D. Observed Jittering Movements of pollen grains
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