h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Entry: The Limits of Quantum Mechanics: The Einstein Podolsky Rosen Paradox - A569126
Author: Freewheelin' Franklin (aka Hell) - U171578

That has been something I wrote a long time ago. Now I've given it a final polish. Comments can only help.

HELL

"Einstein, Podolsky and Rosen suggested that it must be known beforehand who is going to be who, there must be something in G that tells the S who is going to be H and who is going to be F. The result should be pre-determined, and does not happen by chance"

Do you mean "something in B that tells the S....."?

Pongo

Exactly. (I will just change that!) One point for you Pongo. Does that mean you understood the EPR?

Great!

HELL

Understood as far as anyone can Do Quantum Scientists even fully understand it? My head starts to hurt whenever I look into Quantum mechanics

Pongo

That's it?

Hei

I dont know a lot about quantum mechanics, but I enjoyed reading this entry. Fascinating stuff.

What Im not sure that I understood is the following;

"The result was that entangled particles do indeed behave as quantum mechanics propose. The particles do not have a predetermined fate.
But at the same time it was also shown that there is no 'spooky action at a distance'. Turning one particles around will not affect the second particle. It's just not known, and there's no way to figure out."

Does this imply that they found that each of the particles had no predetermined fate - but if one of the components turned into H - the other one had to become F? Would that not be counted as a 'spooky action at a distance'?

And what precisely is IT in 'It's just not known, and there's no way to figure out'?

Ohwell - maybe this is yet another of those things that Ill never will be able to twist my mind around... ...

On a pedantic note; Shouldnt it be 'Turning one particle into' - not 'Turning one particles into' in the same paragraph? (I know Im throwing stones in a glass house here - my own written English isnt as up to date as I would wish it to be, and I might be wrong )

(I havent commented on anything in PR before - so sorry if Im doing this all wrong - but trying, and asking is the only way to learn... )

AneRikke

Hi.

Welcome to PR!

The spooky action at a distance would be both 'daughter' particles communicating. Like if I turn one around the other one would immediately feel it.

It's more like that both have no idea, but when they decide they know which way to go.

It is not known HOW they do it, but there is no spooky action at a distance. And there is NO way to figure out.

yes that IS confusing.

HELL

Interesting entry, Hell!

I may have comments; I may not. I want to think about it!



PS: Regards to Anna and the babies!

Hellooooo! ZSF!

Long time no see!

Anna and I have been quite busy in the last months (moving house etc.). But we're slowly coming back to life. She'll be back to hootoo soon, I hope.

That quantum mechanics EPR stuff may not allow spooky actions at a distance, but it IS spooky by itself...

See you around!

HELL

I haven't read it yet, but before i do, i'll point out that the table half way down is too wide, and forces me to scroll horizontally. Please fix it!

Now to read it and comment properly...

Tango

Ok, i've now read it. And i'm confused (not suprising...). You say that people have done experiments and the particles act as Quantum Mechanics say they should. I thought the whole problem with it was that as soon as you observed them, they resolved into real particles, rather than the in "flux" particles or whatever the term is. So there is no way to tell, experimentally, whether the particles decided when you opened the box, or if they were set from the beginning (and i don't mean it was set which would eventually change to what, but they were already in there final forms).

Can you explain how these experiments work?

Tango

Removed on request

Hello Tango.

I have no idea about what table you are talking about

Maybe you mean the ASCII thing. This will probably be changed in the edited version - or even replaced by a nifty picture... Who knows. In the meantime you will have to scroll horizontally. (I fortunately have a 15'' monitor and don't have to )

OK now to your other comments, Tango: OK, it *IS* a confusing topic... But take it like it is written in the Entry: The particles have *no idea* how they are until they are measured. Maybe you are right and it should be useful to introduce one simple 'gedanken' experiment to the Entry to elucidate how it works.

What would you say about this:

Imagine you are sending two entangled photons (one is going to have a vertical polarisation the other one a horizontal, you just don't know which one) to points A and B. A and B set their detectors so that both detect horizontal polarizations. So far so good. Each time A detects a signal he knows B will detect no signal. Nothing special. Now - imagine they set their polarizers to 45 degrees. In other words, there's a fifty-fifty for each photon to be horizontal or vertical. There are now three new options: a) Detect one horizontal b) detect one vertical and c) NO detection at all. When a) and b) compare their results they will find out that WHEN they BOTH detect photons they will be complementary:

A: X100XXX0X0XXX1XXX
B: X01XXXX111XXX0XXX

The result:

R: -11----1-1---1---

(where 1 indicates positive for entanglement)

Now imagine there is a third guy in between the source and - say - A. This guy (C) is able to read out all photons and to determine their polarisations. He is also able to send them on immediately to avoid any synchronization double chacks between A and B. C sends out the photons heading for the recipient, let's say A, with the polarization pattern he figured out:

A should receive : X100XXX0X0XXX1XXX
C reads and sends: 11001000001111011
A detects (50:50): 0X100XX010XX11XX1
B receives : X01XXXX111XXX0XXX

A and B compare : --0----101---1---

The two zeroes indicate that the particles re-sent were not entangled. The message is corrupted. C read the whole message and knew how the photons must look like. If the *particles* knew who they are they would arrive at A at least with a correct pattern (note that C and B are entangled) and C would have succeeded in intercepting the message.

clearer now? Should I add this?

The point is: There is no way to know what the particles are going to do. Not even they know. The particles, or photons in our example, are subject to chance. (Be that governed by hidden variables or non-locality)

HELL

PS: If I were C I'd try to intercept both arms and send entangled noise to A and B... So they think their line is safe but only receive garbled nonsense.

The four lines above are best viewed when the characters are aligned.

Like here (sorry):

A : X100XXX0X0XXX1XXX (should receive, as seen above)
C : 11001000001111011 (reads all and sends right pattern to A)
A : 0X100XX010XX11XX1 (detects this, with the 50:50 polarizer)
B : X01XXXX111XXX0XXX (receives this, as before)

R : --0----101---1--- (zeros indicate disentangled photons)

Ok, maybe it isn't a table, but whatever it is it has a fixed width, and shouldn't.

Ok, nice explanation, apart from the fact that that wasn't what i asked...

There is no way to know whether they are entangled, or whether the whole quantumn mechanics thing is wrong, and they start off being whatever they are when they are seen. That means you can't just check if they match, and know they are entangled.

Tango

Fixed widths... well... it's going to go away in the edited version anyways. And I think a schematic is particularly illustrating. I'll leave it in, even though you'll have to scroll horizontally, Tango. I apologise for the inconvenience.

But back to QM...

That was what you asked, Tango. Maybe it's my explanation that was to complicated. That IS the way to show that they ARE indeed entangled. And: Quantum mechanics works.

You see.. If they were NOT entangled none of the above would work. But it does work.

BTW, you don't need the whole cryptography stuff to show it works.

If you just have an 'entangled line' between A and B with their 50:50 detectors they will always detect the complementary message.

If the line is not 'entangled' the 50:50 detectors would result in a garbled noise.

Clearer now?

HELL

No, not at all. How does that experiment show that the photons haven't been in a set orientation from the moment they were released? Isn't this the whole point? This is a version of the shrodingers (sp?) cat thing, isn't it? There is no way to know whether the cat is dead or alive without opening the box, so you don't know if the cat only decides (regardless of whether its decision is predetermined) when you open it, or if it decided as soon as the event that could have killed it happened.

Tango

Tango

Maybe I can help, maybe not!
The way I understand it the Photon exists in both states untill it is observed, at which point it decides on which polarization it's going to have. This is hard to get your head around because this menas in the case of Schrodinger's cat, the cat is both dead and alive, untill observed.

Pongo

Yes, Pongo, thanks. Tango: It is a different thing from saying the cat is *either* dead *or* alive, and it's just you that don't know it. In quantum mechanics the cas *IS* both. Totally dead AND totally alive. It will be decided which at the moment you open the box.

It's like in one of the Dilbert books. He made a test for cancer and gets a letter from the doctor. He doesn't know. He could be ill he could be healthy. It will be decided when he opens the letter - It's an absurd example, but anyways.

It's the same with the frigging photons. If you have entanglement and a 50:50 detector both messages will be complementary (the have decided at the very moment of detection). If you don't have entanglement (the photons have decided before detection) both messages will be random.

Clearer?

HELL