
Einstein’s Thought Experiments - peterthehacker
http://backreaction.blogspot.com/2020/07/einsteins-greatest-legacy-thought.html
======
segfaultbuserr
> _Well, the demon needs to have information about the motion of the atoms,
> otherwise it does not know when to open the door. The interesting thing
> about Maxwell’s demon is that it tells us entropy is somehow the opposite of
> information, you can use information to decrease entropy._

It's the first connection between the laws of physics and the process of
computing, although it's a modern reinterpretation (the original
interpretation only argued measurements by the demon requires energy). An
interesting implication is: all computers in this universe must generate heat
because information is lost in the logic circuit. However, if we can construct
the logic circuit in such a way that all information is preserved at the
output (i.e. reversible) [0], this computer only needs an infinitesimal amount
of energy to operate.

Also a fun fact: Do you know that the term "daemon" for background processes
was originally named after Maxwell's demon? [1]

[0]
[https://en.wikipedia.org/wiki/Reversible_computing](https://en.wikipedia.org/wiki/Reversible_computing)

[1]
[https://en.wikipedia.org/wiki/Daemon_(computing)#Terminology](https://en.wikipedia.org/wiki/Daemon_\(computing\)#Terminology)

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bawana
Sabine has made a name for herself being a contrarian.

[https://www.youtube.com/watch?v=sZI8-9i0SOo](https://www.youtube.com/watch?v=sZI8-9i0SOo)

[https://blogs.scientificamerican.com/cross-
check/physicist-s...](https://blogs.scientificamerican.com/cross-
check/physicist-sabine-hossenfelder-fears-theorists-lacking-data-may-succumb-
to-wishful-thinking/)

She'll even take your money to answer your questions if you dont like physics
forums

[https://www.aps.org/publications/apsnews/201611/hossenfelder...](https://www.aps.org/publications/apsnews/201611/hossenfelder.cfm)

I dont mean to sound critical, but her statements remind me of Lord Kelvin's
statement

"There is nothing new to be discovered in physics now. All that remains is
more and more precise measurement"

Personally, I think theoretical physics should never be discussed in the
absence of the specific data it is trying to address. So much confusion
happens when the details are not understood first before the grand theory is
proposed. Theories too often are presented as gifts from the heavens and
without the corpus to which they speak.

~~~
dnautics
> "There is nothing new to be discovered in physics now. All that remains is
> more and more precise measurement"

It's not being critical, it's just a mischaracterization. I think that Sabine
very much thinks there are new things to be discovered in physics, and she is
taking an opinionated stance about what is the right way to get to them.

> I think theoretical physics should never be discussed in the absence of the
> specific data it is trying to address.

That's exactly what Sabine says.

------
reeboo
"You see, Einstein had a problem with it because it seems to conflict with the
speed of light limit in Special Relativity. We know today that this is not the
case, quantum mechanics does not conflict with Special Relativity because no
useful information can be sent between entangled particles." \-- So how does
quantum communication work? I thought it leveraged entanglement to convey
information such that it became untamperable.

~~~
modzu
Note it is only a theorem. From the wiki:

"The theorem is built on the basic presumption that the laws of quantum
mechanics hold."

They may not.

~~~
Strilanc
Theorems are the highest standard of proof we have, so it's a bit strange to
describe a result as "only a theorem".

But yes, the theorem does assume the postulates of quantum mechanics as
preconditions. It's not possible to make any statement about reality that
doesn't have this kind of "if it works this way" postulate.

~~~
modzu
yes, i regret using "only"; i meant "only" that theorems are still subject to
experimental verification and also a single theorem can support multiple
interpretations.

------
sidcool
I did not get the point: "No useful information can be sent in quantum
entanglement". Has it been found how entanglement actually works?

~~~
thereisnospork
An attempt at lay terms: imagine particles are tiny hands, and they can switch
between being left hands and right hands. So to 'entangle' two particles is to
make them shake. This means we now know that one particle is a lefty and the
other a righty (two lefts can't shake each other) but because they are so
small we don't know which is which. Now (gently) separate the two particles
and place each in a box. It is presently unknown which is lefty, and which is
righty so at this point each particle is considered to be in a superposition
of both lefty and righty. A la Schrodingers cat each particle '''hasn't
decided''' if it is a left or right.

All QE states is that when we look in one box and see a right hand we know the
other box contains a left hand - regardless of the distance between them.
There is no provision or mechanism for shaking one particle and having a
change occur with the other (information transfer).

~~~
sidcool
That's a good explanation. However, since the particles are in superposition,
the act of observation must be conveyed to the other particle, right?

~~~
thereisnospork
To be clear I'm not a physicist, but I'll try not to get out of my depth:

I would say that the unobserved hand is and always has been (post
entanglement) in the handedness dictated by viewing the other and the
superposition merely refers to the indeterminably of that state -- which is
still indeterminable from the POV of the persons with the unobserved hand[0]
despite the first hand being observed. The superposition of both particles
however has collapsed from the POV of the observers.

[0] Until they get a phonecall at less then or equal to the speed of light
from the observers of the observed hand.

You can also imagine this is a pair of entangled Schrodinger's cats - one
alive and one dead. On observing the live one there isn't a force or
mechanism[1] by which the other one is instantly killed; it was always dead.

[1]If there is, then it transcends space _and_ time such that the cat dies in
the past and so appears to have always been dead in the present.

------
drewcoo
The thought experiment did not originate with Einstein. The name
gedankenexperiment predates him and Maxwell's use of the concept. So the
article also misreports the origination with Maxwell. Maybe they should have
skipped the couching in history if they didn't know the history or even want
to look it up.

And now that I'm reading the article's comments, other readers have also made
those points.

This article is useful as a tackle dummy for skeptics but not much else.

~~~
mellosouls
Thought experiments probably go back as far as thinking.

Plato's Cave and Zeno's paradoxes are famous ancient ones; the article is
rather misleading in its association of them with recent times.

~~~
pravus
This was my thought as well. I have no idea why a special term is ascribed to
physicists for just thinking logically.

