thought experiments

Thought ExperimentsJohn Ashmead

Sunday, April 1, 2012

“I will a little tink”– Well-known Swabian


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


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


Einstein

Chasing light

Equivalence principle

Clock in a box


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."


Chasing the light

http://www.pitt.edu/~jdnorton/Goodies/Chasing_the_light/index.html






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, …


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


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.


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!


Other thought experiments

Einstein Rosen Podolsky

Maxwell’s demon

Newton’s Apple

Black hole information paradox

…


Scientific methods

Observation

Experiment

Computer simulations

Big data


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.


References

Einstein - The Meaning of Relativity

Ohanian - Einstein’s Mistakes

Isaacson - Einstein

Sorenson - Thought Experiments

Usual googling…


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Technology

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bygeneral relativity

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