Einstein’s Thought Experiments | Hacker News Reader

Einstein’s Thought Experiments

backreaction.blogspot.com

> 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

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

Sabine has made a name for herself being a contrarian.

https://www.youtube.com/watch?v=sZI8-9i0SOo

https://blogs.scientificamerican.com/cross-check/physicist-s...

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

https://www.aps.org/publications/apsnews/201611/hossenfelder...

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.

- your initial comments are only ad-hominem

- i don't think kelvin's statement represents her views

- Being against a new collider does not mean one takes the position of Kelvin, or is anti-science.

She doesn't argue against investment in novel science. Her argument is that a larger collider is the wrong kind of investment to make, based on what we've already observed at the LHC

> "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.

"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.

Quantum entanglement can't be used to send information without also sending some photons or other signal between two locations. That's one problem Einstein had with entanglement- at the time, it looked like entanglement might let you send information "at a distance" without transmitting anything. It turns out that entanglement is real but it conspires to prevent any entangled correlations from actually conveying classical information on their own.[0]

The idea behind quantum cryptography is to send photons from A to B that are specially prepared so that any attempt to read those photons in flight would be detected. You use these photons to pick a shared key, and then communicate between A and B using that key via a classical channel. If you detect someone snooping on the initial key exchange, you can toss that key and try again.

[0] https://en.wikipedia.org/wiki/No-communication_theorem

Quantum communication isn't communication in the sense that you can decide what to send. You have a shared quantum state due to entanglement and you can guarantee that both sides see the same information when they observe the state but it's more like putting two identical messages into sealed envelopes and sending them off to be opened later (with the caveat that because the message hasn't been determined until it is observed you can prevent anyone from knowing what it is ahead of time).

Much more detailed explanation here: https://www.forbes.com/sites/chadorzel/2016/05/04/the-real-r...

But you _can_ decide what to send! It's just that the sending is never faster than the speed of light because it always involves the transportation of some physical stuff. In the case of superdense coding, you have to send one half of an entangled particle-pair which can't be done faster than the speed of light. The benefit is that you can send 2 bits of classical information by transporting only one (entangled) qubit.

In case of quantum teleportation, you need to send two bits of classical information via whatever mechanism you want to use which again can't be done faster than the speed of light. The benefit is that you can transport the quantum-state of the sender's qubit.

In superdense coding you send one quantum bit ahead of time and hang onto it’s entangled partner. Then later you send one classical but which, when combined with the quantum bit you shared earlier, conveys two classical bits of your choice.

You don't send any classical bits in superdense coding. You start out with a pair of entangled qubits in the state |00>+|11>. Then, if Alice has the first qubit and Bob has the second, by manipulating the first qubit Alice can perform 4 actions:

1. Nothing, which leaves the combined state unchanged: |00>+|11>.

2. Flip the sign of the base state |11>, thereby changing the combined state to |00>-|11>.

3. Flip the first bit which changes the combined state to |10>+|01>.

4. Perform choice 3 and flip the sign of base state |10>, which changes the combined state to -|10>+|01>.

Now notice that all 4 possible combined states are orthogonal to each other. But we reached each orthogonal state by manipulating only Alice's qubit. When Bob receives Alice's qubit he can put the 2 qubits together and see which state the combined system is in. You wouldn't be able to do this by sending a classical bit as that can't participate in entanglement. And entanglement is needed to access 4 different orthogonal states via manipulating just one qubit.

You're right---I forgot the protocol. Thanks for the correction!

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.

This isn't a very fair assessment. All evidence we've gathered so far seems to indicate that they do. Of course you're technically right to say that "they may not", but that's true of any physical theory, so it's not particularly meaningful.

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.

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.

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

If you are familiar with probability theory, think of it like this: whatever you do with an entangled particle, it will never change other entangled particles' marginal probability, only the joint.

That's very helpful thanks, I do understand what that means but I didn't know I should/could think of entanglement like that.

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).

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

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.

We have a simple mathematical model (quantum mechanics) that predicts the results of experiments involving entanglement. Using that mathematical model you can prove the no communication theorem [1]. As far as we can tell, we know how entanglement works.

1: https://en.wikipedia.org/wiki/No-communication_theorem

In a way, entanglement is inevitable if you superposition is possible.

For example: imagine particle decayed into two other particles. For conservation of momentum to hold, those two particles must go in opposite direction.

So if you measure direction of one particle, you instantly know direction of the other one. Otherwise you would vitness universe in invalid state.

Yea it does.

Try this explaination: http://www.felderbooks.com/papers/bell.html

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.

Thank you, I read the title and was trying to figure out what this person was talking about. There are references to thought experiments going back to the Ancient Greece and Rome.

It's pretty obvious that thought experiments have existed for a long time ("If I kill him, I can mate with the females"). I think what the blog post is saying is that thought experiments applied to physics became popular with Einstein.

This is also wrong. Everyone from Aristotle to Newton and onward to Einstein were doing thought experiments in physics the entire time.

Ibn Sina proposed the famous "Floating man" thought experiment a thousand years ago

You couldn't have... you know...?

https://en.wikipedia.org/wiki/Floating_man

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.

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

Reply on news.ycombinator.com