[neilep (OP)]

Dear Watchers Of The Old TV Series Quantum Leap !


As a sheepy I of course deal with entanglement every day which I thought would stand me in good stead to understand this. I even fired up my old Amstrad CPC 464  in the hope it would help but three weeks in and it's still booting !!







Take a look at Boris and Nathan. There they are minding their own business happily being all quanty and stuff billions of light years apart.






Nathan and Boris Being All Quanty Just Moments Ago.

Apparently they can communicate with each other instantaneously !! Even though they are zillions of miles apart !!..what's that all about then ?

How can they communicate faster than light ? and what is it that is linking them together ?

If couriers employed this into their delivery process we'd all be very happy !


whajafink ?


hugs



neil
xxxxxxx

[Halc]

As a sheepy I of course deal with entanglement every day

Such is the plight of the nappy...

Apparently they can communicate with each other instantaneously !!

They cannot. No message (say the sheep prevailed over the wolves in the world cup today) can be sent at faster than light, no matter how quanty the stuff is in their possession.

How can they communicate faster than light ?

But they haven't. No demonstration of such communication or faster-than-light cause->effect has ever been made.

All that entanglement means is that if they both measure their entangled things in the same way at any time, the results will be found to be correlated. That's never been shown to be communication.

[evan_au]

As I understand it, if two photons are entangled, their state is linked. There are devices that can produce (a small fraction of) entangled photons.

A typical quantum state that is entangled is their polarisation: say Vertical (V) or Horizontal (H, like the pupil of a sheep's eye).
- So if one sheep sees a H photon, the other sheep (potentially an alien sheep in a paddock on the other side of the galaxy) will also see a H photon.
- But there is a 50% random chance that it could be H or V, so that conveys no information to our very observant sheep
- But let's say our fancy photon factory is on Earth, and we want to send entangled photons to Jupiter.
      - A sheep on Earth could observe one of the entangled photons and declare it H (within 20ms)
      - The sheep near Jupiter would observe the other photon about 43 minutes later (ie not simultaneous), and also declare it H
      - But the sheep on Earth can't take advantage of that fact (eg for secret sheepie communications) unless he encrypts his message with the observed H/V, and sends the encrypted message to the other sheep. And the fastest way to send a message (as far as we know) is by light, which will arrive at Jupiter about 43 minutes later.

So:
1) The entangled photons don't have to be detected simultaneously (but they could be detected simultaneously if the photon factory was set up exactly half-way between Boris and Nathan)
2) The entangled photons themselves can't be used to send information faster than light, because the H or V state of any photon is completely random
3) To use entangled photons for communications, you need to send extra information via another channel, which (as far as we know) can't travel faster than light.

[Eternal Student]

Hi Mr. Sheepy,  hope you are well today.

    Non-locality is where you should have gone with this.  Entanglement causes concerns about locality and the nature of reality.   Some of Nathan's properties aren't just local to Nathan or in some cases aren't local to Nathan at all.  Some of Nathan's properties can be measured over at Boris' location if you want to, which is reasonably interesting.  More interestingly, the entangled properties were not determined until after measurement (which could have been done at either location).
    Sadly it doesn't really allow communication instantaneously        If we start deliberately changing something about Nathan over at Nathan's location then Boris is under no obligation to undergo the same change.

and what is it that is linking them together ?

    They are quantum mechanical objects.  We could re-define what it means for two objects to be entangled as follows:  Two objects are entangled    if and only if    The correlation between some of their properties is stronger than would be possible for non-quantum macroscopic objects.
    There isn't necessarily any substance linking the two objects together.  Explaining "why" they seem to be linked depends on your interpretation of quantum mechanics.  I'll skip all of that since it's probably in many other threads and popular science articles already.   An equally important question is why more objects aren't linked or entangled in this way - this seems to be a consequence of coherence (*see note below).  Another topic that will fill many Pop Sci articles.

Here's a short YT video featuring Sabine Hossenfelder who says most of this stuff more eloquently than I could (takes about 7 minutes):

Best Wishes.
[Seems that someone else has posted something while I was writing this].

*LATE EDITING:   I meant decoherence but ended up writing coherence.  Coherence is rarely used as a term in quantum mechanics.

[evan_au]

An equally important question is why more objects aren't linked or entangled in this way

Sean Carrol (Mindscape Podcast) is fond of describing any interaction between two objects as entangling their quantum states - when you take a measurement, the state of the measurement system is entangled with the state of the object measured (and vice-versa)...

It's just that the complex quantum state of the universe is unknowable, so if something (eg the spin of an electron) becomes entangled with the universe, the state of the electron also becomes unknowable.

The surprising thing is that we can manage to go through a sequence of steps that will put some object into a known quantum state - and then entangle it with another object. That's why quantum computing is so hard - things get too easily entangled with the universe, and that's what's holding us back...
https://www.preposterousuniverse.com/podcast/

[Eternal Student]

Hi Evan-au,
   Thanks for the link to Sean Carroll's podcasts.  One of the things that puts me off those podcasts is their length.  For example,  1 hour 28 minutes for a Podcast with Robert Sapolsky.  Are you strongly recommending these?  It's also difficult to listen to one of these while typing something completely different on a computer.
Best Wishes.

Late editing:   + It has adverts in it.  Is it really worth our time listening to this?

[evan_au]

Are you strongly recommending these?

There are some episodes where the topic sounds really interesting, so I listen to those.

But I like the monthly AMA (Ask Me Anything) episodes.
- They are very long (eg 3 hours), but it is a wide-ranging opinion piece based on short answers to listener questions.

It's also difficult to listen to one of these while typing something completely different on a computer.

This is not background music - it requires attention to make any sense!
- I listen to it on walks or drives

[Eternal Student]

OK,  thanks @evan_au.
  I also appreciate your earlier reply   btw.
   It's almost a bit sad to listen to something that requires that much attention while you are walking.  You did say walking not running?  Presumably you do it for the experience not just to keep fit.
    Anyway, we're drifting off topic.    @neilep  I hope you are OK.    I'm sorry that no potential for faster online shopping delivery services has been identified by anyone in this thread so far.

Best Wishes.

[Colin2B]

  More interestingly, the entangled properties were not determined until after measurement (which could have been done at either location).

That is true even of unentangled properties, we don’t know the state until measured. However, we did know about the correlation and did put the particles into a prepared state.

  Sadly it doesn't really allow communication instantaneously        

not so sure about the sadly bit, it could cause a lot of problems if it were true.

 

We could re-define what it means for two objects to be entangled as follows:  Two objects are entangled    if and only if    The correlation between some of their properties is stronger than would be possible for non-quantum macroscopic objects.

I prefer definition used by @ halc

All that entanglement means is that if they both measure their entangled things in the same way at any time, the results will be found to be correlated.

The correlation does not have to be stronger than for non-quantum (see the eg given by @evan which could be set up classically), although in some circumstances it is. I think you are meaning the eg of the 2 qubits in the Bell theorem, where there is a significant difference between classical and quantum probability and that’s where it gets really interesting.

Coherence is rarely used as a term in quantum mechanics.

Used a surprising amount in working physics. Quantum coherence is responsible for all quantum interference phenomena and is important feature of the laser and superconductivity. I think it’s also a feature of superfluidity, but not an area I’ve looked into.

[neilep (OP)]

THANK EWE to @Halc @evan_au @Eternal Student @Colin2B

I really appreciate all your responses. It makes fascinating reading.

I just found  this website where they cite " “when two particles are inextricably linked together no matter their separation from one another. Although these entangled particles are not physically connected, they still are able to share information with each other instantaneously — seemingly breaking one of the most hard-and-fast rules of physics: that no information can be transmitted faster than the speed of light.” This the basis for the premise that quantum entanglement can enable communications faster than the speed of light"

[Halc]

I just found this website where they cite "... Although these entangled particles are not physically connected, they still are able to share information with each other instantaneously"

And there are plenty of other pop articles like that one that make similar incorrect claims.
There has never been a demonstration that they are able to share information with each other instantly. That is simply one possible way to interpret the actual science. No graduate level quantum mechanics course would teach that.

[Eternal Student]

Hi again.

@Colin2B.  Thanks for your comments.
   I'm not going to bother arguing over the small stuff.  You've taken some bits out of context but I'll just let that go.  What's more important is that the OP and other readers understand what might be happening in QM.  I'll agree that my comments may have got in the way of this.  Here's one concrete example:

 I think this statement made by me:

We could re-define what it means for two objects to be entangled as follows:  Two objects are entangled    if and only if    The correlation between some of their properties is stronger than would be possible for non-quantum macroscopic objects.

  was probably written late at night and was not intended to be a formal definition of entanglement to replace all other defintions in all situations.  It is just about the more interesting examples of entanglement where the notion of local realism is challenged.
 
Best Wishes.

[yor_on]

Yeah, locality and quantum entanglements is tricky.

http://www.quantumphysicslady.org/glossary/local-realism/

[yor_on]

Then again, just a thought. 'Locality,' when used to define your wristwatch makes it unlike any other clock. A strict local definition should be unique, if we split in Plank times, and then try to define other clocks relative ones own. The universe doesn't seem to care about that, neither does it get disturbed by LorentzFitzGerald Contractions. So what we define as 'locality' is rather questionable to me.

[evan_au]

walking not running?  Presumably you do it for the experience not just to keep fit.

A brisk walk is more my speed. Same-moment arrival is not my goal!
- Take a bit of time (1 hour maximum) to get out of the home office, soak up some winter sun, listen to some podcasts to broaden my horizons, and use some of the exercise equipment our local government has thoughtfully provided along a nearby path.

- I enjoy going to the gym for exercise, but that's out of the question while we are in lockdown. So I ride a stationary bike while watching TV; this travels even slower than my brisk walk...

[Colin2B]

You've taken some bits out of context but I'll just let that go. 

Not intentional I assure you, but bound to happen when posts are written under pressure and/or read under time constraints. Unfortunately this is not an ideal medium for discussion, you don’t get the immediate feedback of face to face and we have limited time for proof reading. Quite often I don’t reply to posts because by the time I’ve sorted the spammers and spent time proof reading my draft - “does that really say what I mean” - the thread has moved on.