thought experiments
DESCRIPTION
Role of thought experiments in physics: Galileo, Einstein, others. What they are good for, how they evolve into real experiments.TRANSCRIPT
Thought ExperimentsJohn Ashmead
Sunday, April 1, 2012
“I will a little tink”– Well-known Swabian
Sunday, April 1, 2012
Definition
Go back to Pre-Socratics; long history
Range from ‘scientific parable’ to ‘experimental plan’
Used to illustrate, to attack, to support, & simply to understand
Sunday, April 1, 2012
Salviati: If we take two bodies whose natural speeds are different, it is clear that on uniting the two, the more rapid one will be partly retarded by the slower, and the slower will be somewhat hastened by the swifter. Do you not agree with me in this opinion?
Simplicio: You are unquestionably right.
Salviati: But if this is true, and if a large stone moves with a speed of, say, eight, while a smaller stone moves with a speed of four, then when they are united, the system will move with a speed of less than eight. Yet the two stones tied together make a stone larger than that which before moved with a speed of eight: hence the heavier body now moves with less speed than the lighter, an effect which is contrary to your supposition. Thus you see how, from the assumption that the heavier body moves faster than the lighter one, I can infer that the heavier body moves more slowly…
Galileo
Sunday, April 1, 2012
Einstein
Chasing light
Equivalence principle
Clock in a box
Sunday, April 1, 2012
Young Einstein"...a paradox upon which I had already hit at the age of sixteen: If I pursue a beam of light with the velocity c (velocity of light in a vacuum), I should observe such a beam of light as an electromagnetic field at rest though spatially oscillating. …
From the very beginning it appeared to me intuitively clear that, judged from the standpoint of such an observer, everything would have to happen according to the same laws as for an observer who, relative to the earth, was at rest. …
One sees in this paradox the germ of the special relativity theory is already contained."
Sunday, April 1, 2012
Chasing the light
http://www.pitt.edu/~jdnorton/Goodies/Chasing_the_light/index.html
Sunday, April 1, 2012
Equivalence principle
You can’t tell the difference between gravity & acceleration
With conservation of energy, the foundation of general relativity
Implies red-shift, curvature of light by gravity, time dilation, …
Sunday, April 1, 2012
Clock in a box
Open the shutter, briefly. Photon escapes with known uncertainty in time.
Now, weigh the box. Now we know the energy!
But this violates the energy/time uncertainty relation
Sunday, April 1, 2012
It was a real shock for Bohr...who, at first, could not think of a solution. For the entire evening he was extremely agitated, and he continued passing from one scientist to another, seeking to persuade them that it could not be the case, that it would have been the end of physics if Einstein were right; but he couldn't come up with any way to resolve the paradox. I will never forget the image of the two antagonists as they left the club: Einstein, with his tall and commanding figure, who walked tranquilly, with a mildly ironic smile, and Bohr who trotted along beside him, full of excitement…The morning after saw the triumph of Bohr.
Sunday, April 1, 2012
Chain of uncertainties
But to weigh the box we must measure its height
But to do this, we have to apply an impulse
And therefore there is an uncertainty in its height
And, therefore (by general relativity) in its time!
Sunday, April 1, 2012
Other thought experiments
Einstein Rosen Podolsky
Maxwell’s demon
Newton’s Apple
Black hole information paradox
…
Sunday, April 1, 2012
Scientific methods
Observation
Experiment
Computer simulations
Big data
Sunday, April 1, 2012
Relativity & madness
June 24th , 1969 Donald Crowhurst threw himself off his trimaran in despair at relativity
During the voyage, an around-the-globe sailboat race, Crowhurst had been reading Einstein's book "Relativity: The Special and the General Theory."
[constancy of the speed of light] "is in reality neither a supposition nor a hypothesis about the physical nature of light, but a stipulation which I can make of my own free will in order to arrive at a definition of simultaneity."
"You can't do THAT!" Crowhurst, an electrical engineer, protested to his journal. "I thought, 'the swindler.' " From there he descended into madness.
Sunday, April 1, 2012
References
Einstein - The Meaning of Relativity
Ohanian - Einstein’s Mistakes
Isaacson - Einstein
Sorenson - Thought Experiments
Usual googling…
Sunday, April 1, 2012