Thought experiments are conceptual experiments used to illustrate scientific principles through logic rather than experimentation. Some key thought experiments discussed in the document include Galileo demonstrating that heavier objects do not necessarily fall faster, Einstein envisioning chasing a beam of light and considering the implications of the constancy of the speed of light, and Einstein's thought experiment about an observer in a closed box unable to determine if they are accelerating or under the influence of gravity. Thought experiments have been used since ancient Greece and can illustrate, attack, support, or help understand scientific concepts.

1. Thought Experiments
John Ashmead
Sunday, April 1, 2012

2. “I will a little tink”
– Well-known Swabian
Sunday, April 1, 2012

3. 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

4. Galileo
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…
Sunday, April 1, 2012

5. Einstein
Chasing light
Equivalence principle
Clock in a box
Sunday, April 1, 2012

6. 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

7. Chasing the light
http://www.pitt.edu/~jdnorton/Goodies/
Chasing_the_light/index.html
Sunday, April 1, 2012

8. 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

9. 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
Sunday, April 1, 2012
relation

10. 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

11. 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

12. Other thought experiments
Einstein Rosen Podolsky
Maxwell’s demon
Newton’s Apple
Black hole information paradox
…
Sunday, April 1, 2012

13. Scientific methods
Observation
Experiment
Computer simulations
Big data
Sunday, April 1, 2012

14. Relativity & madness
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
June 24th , 1969 of my own free will in order to arrive at
Donald Crowhurst a definition of simultaneity."
threw himself off his
trimaran in despair "You can't do THAT!" Crowhurst, an
at relativity electrical engineer, protested to his
journal. "I thought, 'the swindler.' "
From there he descended into madness.
Sunday, April 1, 2012

15. References
Einstein - The Meaning of Relativity
Ohanian - Einstein’s Mistakes
Isaacson - Einstein
Sorenson - Thought Experiments
Usual googling…
Sunday, April 1, 2012