Quantum Mechanics

Professor_Fink

onedoubleo wrote: »

Sense of smell I can kinda get but how Bird Navigation?

Apparently they use an entangled state of two spins to sense magnetic fields.

onedoubleo

Professor_Fink wrote: »

Apparently they use an entangled state of two spins to sense magnetic fields.

That is the most awesome thing I have heard in a while. Im procrastinating from a QM exam tomorrow but I am defiantly including this in it now.
Thanks for answering so quickly too.

Azelfafage

Professor_Fink wrote: »

'In search of Schroedinger's cat' by John Gribbin, as it is a pretty good pop-science account of quantum mechanics.

Poor Heisenburg.

He was using an analogy to explain quantum physics.

He never meant to refer to a real cat.

He always regretted "the cat".

During WW2 allied propaganda unfairly pillord Schrodinger:

"Germans believe that cats in gas chambers and Jews in gas chambers do not exist just because Germans cannot see them."

Unfair.

DeValera invited Schrodinger to Dublin.

DIAS is the result:

http://www.dias.ie/lang/en/commun/ambassador_visit.html

Azelfafage

Nobody pilloried me for swapping the words Heisenberg and Schrodinger.

I am disappointed.

.

Azelfafage

Carl Sagan said:

"We are equidistant from the stars and the atoms.".

Thus:
A cat is too small to be used to gravitationally lens a distant galaxy.

Equally:
A cat is too big to demonstrate the wonders of Quantum Mechanics.

.

Professor_Fink

Azelfafage wrote: »

Equally:
A cat is too big to demonstrate the wonders of Quantum Mechanics.

Again: It is only ineraction with the environment that matters, not system size. There is no reason to believe cats cannot be put into a superposition if you can isolate them from the environment.

maninasia

If you can reconstitute a photon or atom or molecule with quantum effects you will eventually be able to reconstitute a cat. It is logical,just scaled up. The division of quantum world with physical world is probably just in our heads.

Professor_Fink

maninasia wrote: »

If you can reconstitute a photon or atom or molecule with quantum effects you will eventually be able to reconstitute a cat. It is logical,just scaled up. The division of quantum world with physical world is probably just in our heads.

That isn't quite right. The division between the quantum and classical worlds is simply down to interaction with the environment. In this sense, if you could reverse the process then you would recover quantum effects. However, when photons carry away information, you can never catch up with them in order to actually reverse the process, so after a very brief period, the necessary information is propogating away from your lab faster than you can travel to catch it.

millenniumlady

Azelfafage wrote: »

Carl Sagan said:
A cat is too big to demonstrate the wonders of Quantum Mechanics.
.

Imho one of the most important messages to take from the Schrodinger's Cat thought experiment is that a quantum effect can directly influence the macro world. It directly maps quantum indeterminancy to "our-world" indeterminancy. Thus it presents an interesting toy case for considering elements of the different theories that exist.

I do suppose that people dwell on the cat too much For me it's like when I really like a song that gets played on the radio too much and in the end I don't really know how I feel about it anymore!

millenniumlady

Apologies for the misquote in the previous post! I just noticed. Clearly nobody would ever attribute such a quote to Carl Sagan! Just a chopping error on my part

del88

Scientists Teleport Info 10 Miles..........interesting stuff


http://news.discovery.com/tech/teleportation-quantum-mechanics.html

del88

A quantum phisics experiment animated for all to understand

http://www.metacafe.com/watch/149846/quantum_experiment/

Dr. Feelgood

del88 wrote: »

A quantum phisics experiment animated for all to understand

http://www.metacafe.com/watch/149846/quantum_experiment/

Why do they say "The observer collapsed the wave function simply by observing." Wouldn't it be clearer if they said that it was the act of measuring weather or not the electrons pass through the slits which causes the wave function to collapse.

The way they put it gives the impression that what collapses the wave function is the presence of a conscious entity. From what I gather on the subject this is the cause of much confusion and results in lots of crackpot theories of how consciousness is linked with this experiment.

It's a lot more logical to see how an external measuring device could disrupt the experiment as opposed to an observer simply observing it.

maninasia

Professor_Fink wrote: »

That isn't quite right. The division between the quantum and classical worlds is simply down to interaction with the environment. In this sense, if you could reverse the process then you would recover quantum effects. However, when photons carry away information, you can never catch up with them in order to actually reverse the process, so after a very brief period, the necessary information is propogating away from your lab faster than you can travel to catch it.

You could if you travelled faster than the speed of light

take everything

http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser

Hi.
I'm another amateur so apologies if the questions are basic.

I've looked at the Young double slit experiment and have a reasonable understanding of it.

But then i see this ingenious experiment in this video and i'm blown away so i've got a few questions.

Basic description:

Light hits D0 and causes an interference pattern (because D0 is set up to not know if the light came through slit A or slit - fair enough.

The "idler"(entangled photon) of that light goes into the rest of the system hitting one of the other detectors.
If it hits either D3 or D4 the "which-path" is known.
If its idler hits D1 or D2 the which-path is unknown.
This is determined randomly (via the beam splitters).

If it hits D0/D1 this doesn't seem to be a problem.
But if it hits D3/D4, this affects the pattern at D0 (if i understand it correctly)

But of course in this specific experiment these "which-path known" D3/D4 detections are delayed- they happen after the (interference pattern) event happens at D0.

I may not have fully absorbed it so bear with me but my questions are:

1. Preliminary query:
Apart from anything else, i'm amazed that it's not the detector itself (but the "knowing" conferred by a given detector) that changes the interference pattern to banding.
This experiment establishes this obviously (but was this experiment actually important in establishing this or was all this well known prior to it).

Main queries:

1. If detection at D3/D4 occurs (even after the D0 event as in this experiment), does that knowing mean there will be a band pattern on D0 when you look at it (because the photons that hit D3/D4 and D0 are entangled).

2. Does this mean the later event (D3/D4 "which path" detection) influences the earlier D0 (band pattern) event.
That's mad so i suspect i've gone off track here
Can someone put me straight on this.

3. If you check D0 first (before its idler hits D3/D4), what do you get at D0? An interference pattern presumably (because at that point in time, you don't know its "which path" yet)?
In this scenario, does this mean the "which path" of its idler will also be destined to be unknown? ie that it influences the "idler" to go to D0/D1?
I think i'm confused (^that can't be right)- hopefully someone can help.

Is it kind of like the light knows (before it goes near any system that threatens to reveal it's path) what pattern to show, according to whether there is revelation/non-revelation later.

Professor_Fink

take everything wrote: »

1. Preliminary query:
Apart from anything else, i'm amazed that it's not the detector itself (but the "knowing" conferred by a given detector) that changes the interference pattern to banding.
This experiment establishes this obviously (but was this experiment actually important in establishing this or was all this well known prior to it).

It's nothing to do with "knowing", and the last part of the video is very misleading. What's really happening here is that you have a photon which can go through one of two slits. So it is either in mode A or mode B. It then undergoes downconversion, so you produce an entangled state of two photons in two different modes. the easiest way to keep track of this is simply to keep track of the state of the photon. Initially it is in a state 1/sqrt(2)(A+B). After downconversion you now have two photons. If the original photon was in mode A, then these will be in modes a and a', and if it was in mode B, these will be in modes b and b'. So the overall state of the system immediately after entanglement is 1/sqrt(2)(aa'+bb').

Let's focus on the 'idler' photon (I've never heard that term before and I doubt it's widely accepted). If this is in mode a' then it will pass through potentially 2 beamsplitters (the mirrors are unimportant, they are just used to change the direction of the photon). This will leave the photon in a state (1/sqrt(2) D4 + 1/2 D2 + 1/2 D1), where I have used the detector labels to label the state that will make that detector click. Similarly, if the photon is in mode b, then the photon will be in a state (1/sqrt(2) D3 + 1/2 D2 - 1/2 D1). Note there is a minus sign here, rather than the plus in the previous case. This is a feature of beamsplitters due to the fact that they introduce a unitary transformation on the photons (the operation is reversible).

So substituting these back into the equation 1/sqrt(2)(aa'+bb') for the entire state of the system, we obtain 1/sqrt(2)(a(1/sqrt(2) D4 + 1/2 D2 + 1/2 D1)+b(1/sqrt(2) D3 + 1/2 D2 - 1/2 D1)). We can rearrange this to get (1/2)(1/sqrt(2))(a+b)D2 + (1/2)(1/sqrt(2))(a-b)D1 + 1/2 aD4 + 1/2 bD3. Note that this is simply the state of the system, and has nothing to do with consciousness or any other such sillyness. From this we can immediately read off what will happen. 2 of the for terms contain both a and b. These are the terms which can give rise to an interference pattern (actually they give rise to two different patterns due to a phase shift of -1), and these are also the terms that will make either D2 or D1 click. The other terms (corresponding to clicks of D3 or D4) only contain either a or b on its own, and so do not cause interference.

Note, we have not yet said which photon is measured first! The pattern doesn't depend on which photon is detected first, but this is a feature of correlations in the state, and has nothing to do with us making observations of the system.

How can this be? Well, lets assume the location of the photon at D0 is measured first. How well this conforms to one of each of the patters (whether interference patterns or single mode patterns) will determine the probability of the corresponding detector clicking. This is simply a conditional probability, nothing more.

So this has nothing to do with consciousness, or being human, etc. and trying to spin it as such is total nonsense.

take everything wrote: »

Main queries:

1. If detection at D3/D4 occurs (even after the D0 event as in this experiment), does that knowing mean there will be a band pattern on D0 when you look at it (because the photons that hit D3/D4 and D0 are entangled).

Since there is only one photon, you don't actually see a pattern. Looking at the state above it should be clear that that the location of the photon should be consistent with no interference (the single mode case). Note however that this just determines a probability distribution over possible locations for it to be detected at. There is no interference vs no-interference measurement for a single photon. You need to measure and compare the statistics for many photons.

take everything wrote: »

2. Does this mean the later event (D3/D4 "which path" detection) influences the earlier D0 (band pattern) event.
That's mad so i suspect i've gone off track here
Can someone put me straight on this.

Nope, it just means the results have to be consistent with one another. There is no causal relationship.

take everything wrote: »

3. If you check D0 first (before its idler hits D3/D4), what do you get at D0? An interference pattern presumably (because at that point in time, you don't know its "which path" yet)?
In this scenario, does this mean the "which path" of its idler will also be destined to be unknown? ie that it influences the "idler" to go to D0/D1?
I think i'm confused (^that can't be right)- hopefully someone can help.

As I mentioned above, there is no single photon measurement that will tell you whether there is interference of no interference. You simply get a photon with a probability distribution over possible locations that is consistent with a particular pattern. For D3/D4 it is consistent with the respective single mode pattern.

take everything wrote: »

Is it kind of like the light knows (before it goes near any system that threatens to reveal it's path) what pattern to show, according to whether there is revelation/non-revelation later.

The light doesn't know anything. These are just correlations that are induced by the optical elements. There is really no mystery here. It only looks mysterious to laymen because they generally don't know any quantum optics.

Professor_Fink

maninasia wrote: »

You could if you travelled faster than the speed of light

Which you can't do.

maninasia

I don't like the word 'can't' for something that's not fully understood. Plus you say the photons 'propagate away' at the speed of light, can you bounce the photons back again to get the info?

Professor_Fink

maninasia wrote: »

I don't like the word 'can't' for something that's not fully understood.

It is fully understood. No-signalling is a feature quantum mechanics, relativity and electromagnetism. It's one of the most well established and supported principles in all of physics. Saying it's not fully understood is choosing to ignore hundreds of years of progress in our understanding of the universe.

Plus you say the photons 'propagate away' at the speed of light, can you bounce the photons back again to get the info?

Not after the fact. It needs to already be set-up before hand, which is equivalent to essentially decoupling the system from the environment. There are already much better ways of doing this (bang-bang decoupling, refocusing, decoherence free subspaces, etc.).

maninasia

Professor Fink, I think you are falling into a trap again. I'm no expert on quantum mechanics but as I have said before there is nothing to stop another revolution of thought down the line, it's early day in our understanding of what's possible. You say hundreds of years of scientific progress has brought us to this but time and again previous axioms have been revised and re-written. We don't have the energies to fully explore the quantum world at present and the one 'below' the quantum world.


The speed of light limitation always seems abitrary to me ...not a scientific conclusion I know but I am suspicious of abitrary limits such as this. What about warped space? How come the speed of light can vary up to a maximum? Why that particular maximum? Speed of light in a vacuum, what is a vacuum anyway?
Also isn't it possible to jump through space almost instantly if space is warped enough.

Professor_Fink

maninasia wrote: »

Professor Fink, I think you are falling into a trap again. I'm no expert on quantum mechanics but as I have said before there is nothing to stop another revolution of thought down the line, it's early day in our understanding of what's possible. You say hundreds of years of scientific progress has brought us to this but time and again previous axioms have been revised and re-written. We don't have the energies to fully explore the quantum world at present and the one 'below' the quantum world.

All of modern physics obeys a correspondence principle, where it corresponds to the previous physics within the well studied regimes. Any new physics will also have to obey such a correspondence principle what is currently know. Just because we don't yet know everything doesn't mean we don't know anything.

maninasia wrote: »

The speed of light limitation always seems abitrary to me ...not a scientific conclusion I know but I am suspicious of abitrary limits such as this. What about warped space? How come the speed of light can vary up to a maximum? Why that particular maximum? Speed of light in a vacuum, what is a vacuum anyway?

It's not arbitrary. A vacuum is the ground state of the field in quantum field theory. It is the speed of light in a vacuum (c) that is the limit. It is possible for particles travelling through air (for example) to travel faster than light does in air.

maninasia wrote: »

Also isn't it possible to jump through space almost instantly if space is warped enough.

No. You move through space at a speed strictly less than c, you simply reduce the space between the two points.

maninasia

The correspondance principle doesn't mean that major paradigm shifts don't occur, they do, especially when we look at things at levels further down. Surely you aren't going to argue that are no more levels beyond what we understand now i.e. string theory, information theory of universe, multiverse. We have not reached anywhere close to the Planck energy so cannot know exactly what is going on at that level yet alone have 'discovered' the higgs boson yet. There is also no grand unified theory. Seems to me there is an awful lot yet to learn.


You mentioned it was impossible to get the information from photons back but obviously that is not the case if you could jump through space. I well understand that the principle of jumping through space is different than travelling faster the speed of light through space, yet the end effect is that you would travel faster than the speed of light. Woods for trees?

Professor_Fink

maninasia wrote: »

The correspondance principle doesn't mean that major paradigm shifts don't occur, they do, especially when we look at things at levels further down. Surely you aren't going to argue that are no more levels beyond what we understand now i.e. string theory, information theory of universe, multiverse. We have not reached anywhere close to the Planck energy so cannot know exactly what is going on at that level yet alone have 'discovered' the higgs boson yet. There is also no grand unified theory. Seems to me there is an awful lot yet to learn.

You're just saying words now. String theory is a quantum theory. It does not in any way overturn quantum mechanics. "Information theory of the universe" is not a real theory. I can only assume you mean the attempt to reformulate the postulates that give us modern physics into more information theoretic terms. It's a rather incomplete way of reformulating the postulates which gives rise to exactly the same physics, further 'no signalling' is one of the first postulates taken in such an approach. The multiverse is simply a consequence of quantum mechanics. None of these give rise to signalling theories. There isn't a whole lot of point in me engaging further with this, since you seem to be just sticking in buzzwords and don't quite understand the what they mean.

I'll tell you what: Feel free to laugh your ass off at me when someone demonstrates superluminal communication.

maninasia wrote: »

You mentioned it was impossible to get the information from photons back but obviously that is not the case if you could jump through space. I well understand that the principle of jumping through space is different than travelling faster the speed of light through space, yet the end effect is that you would travel faster than the speed of light. Woods for trees?

What principle of jumping through space? There is no such thing. Even spacetime manifolds with Einstein-Rosen bridges require timelike travel between points.

You seem to be suggesting that we say nothing definitive about physics, and equivocate on everything. While it is technically true that you can't formally prove anything in phyics, this is entirely unhelpful and a rather pointless conversation to be having. There is absolutely no evidence for superluminal signalling, and enormous evidence against it. Ask any (real) physicist and you'll get the same answer.

maninasia

If Einstein-Rosen bridges exist or can be created then yes jumping through space is entirely possible. There is nothing definite about physics, there is only definition to a certain level that we can probe. I'm sure you can understand that point quite well.

Professor_Fink

maninasia wrote: »

If Einstein-Rosen bridges exist or can be created then yes jumping through space is entirely possible.

That is complete and utter nonsense. All massive particles are restricted to timelike paths on the manifold, independent of its topology. A bridge is simply provides an alternate path between two locations.

maninasia

You like to hide behind the specific scientific jargon but the fact remains, if you travelled between two points in space by jumping extradimensionally you would obviously get there before the photon which is travelling in a straight line, this refutes your argument that you could not get the information back!

A bridge provides an alternative path between two locations, a shorter and quicker path (wood for the trees again).

You also say 'real physicists' don't believe in these things..but some of the leading physicists in the world back these ideas, Stephen Hawkings, Thorne etc.

Professor_Fink

maninasia wrote: »

You like to hide behind the specific scientific jargon but the fact remains, if you travelled between two points in space by jumping extradimensionally you would obviously get there before the photon which is travelling in a straight line, this refutes your argument that you could not get the information back!

There really is no point in me engaging with this, though I can hardly help myself. The path you travel on is what is called a geodesic. These -are- straight lines. A photon ahead of you traveling in the same direction covers exactly the same distance, but simply does it quicker than you. Either you have know idea what you are talking about, or you haven't bothered to think it through.

It's not hiding behind jargon, it's being accurate.

maninasia wrote: »

You also say 'real physicists' don't believe in these things..but some of the leading physicists in the world back these ideas, Stephen Hawkings, Thorne etc.

That's total crap. Neither Hawking nor Thorne advocate(d) signalling theories. I challenge you to provide a single shred of evidence that any of them suggested superluminal signalling was possible.

maninasia

Ok here is the scenario. Photons are shot off ahead of me from Earth to Mars. I cannot get the information from the photons back because they are travelling in a straight line ahead of me at the speed of light in a vacuum. Assuming I could travel very close to the speed of light I could still never get them back.

However, if I have access to a wormhole that I can stabilise and send myself or at least some two way sensors that connects Earth and Mars I can get the information from Mars as the photons arrive and get the information back.

I have travelled 'faster than the speed of light' by using another method, by warping space.

I never said they said superluminal signalling was possible, but they were both heavily involved in conjecturing the existence of wormholes.

Professor_Fink

maninasia wrote: »

I have travelled 'faster than the speed of light' by using another method, by warping space.

First, as I have stressed, this is not travelling faster than light. You are simply taking an alternate, shorter path. The problem then arises that any photons ahead of you on the path you travel will escape you, since they too travel the shorter path, but at a greater velocity than you.

maninasia

Whatever you want to call it the end result is the same I have travelled faster than the photon travelling at the speed of light and arrived there ahead of it.

Professor_Fink

maninasia wrote: »

Whatever you want to call it the end result is the same I have travelled faster than the photon travelling at the speed of light and arrived there ahead of it.

No, you can travel faster than a photon taking a longer route than you. Any photon taking the same route as you gets there before you.

Further, you don't need wormholes to get somewhere before a photon which takes a longer route: just set up two almost parallel mirrors so that a photon bounces backward and forward three hundred million times before escaping. You could beat that photon by walking.

This in no way changes the fact that you cannot catch up with a photon which takes the same path as you.

maninasia

Talk about wood for the trees again. If you can create wormholes you automatically can communicate at faster than light speeds. There's no point to take a bus when you can take a plane .

Professor_Fink

maninasia wrote: »

If you can create wormholes you automatically can communicate at faster than light speeds.

So you've totally ignored everything I've said, and come back to the start. Fine. You're totally incorrect but I'm not going to waste my time having the same argument a second time.

Dr. Feelgood

Professor_Fink, would you mind commenting on a post I made earlier in this thread? It's in relation the the infamous double slit experiment.

Dr. Feelgood wrote: »

Why do they say "The observer collapsed the wave function simply by observing." Wouldn't it be clearer if they said that it was the act of measuring weather or not the electrons pass through the slits which causes the wave function to collapse.

The way they put it gives the impression that what collapses the wave function is the presence of a conscious entity. From what I gather on the subject this is the cause of much confusion and results in lots of crackpot theories of how consciousness is linked with this experiment.

It's a lot more logical to see how an external measuring device could disrupt the experiment as opposed to an observer simply observing it.

Professor_Fink

Dr. Feelgood wrote: »

Why do they say "The observer collapsed the wave function simply by observing." Wouldn't it be clearer if they said that it was the act of measuring weather or not the electrons pass through the slits which causes the wave function to collapse.

While I have indeed heard this said, it's not particularly accurate (as I guess you have noticed). What you suggest is a bit more accurate, but neither is the full story. What is really happening is that if the path of the particle passing through the slit becomes correlated with an external system (whether this be by measurement, by unitary evolution i.e. purely quantum evolution without measurement, or some other mechanism) interference is no longer possible.

This is easy enough to see: Interference occurs when you can get to the same final state (the same state of the entire universe) by going two or more different routes. However, if the route taken is correlated with some system, then the end state is correlated with the route taken. Since different routes then have different end states, interference cannot occur.

As you point out, this is nothing to do with consciousness. However, it isn't even to do with the collapse of the wave function, as you get the same results either way. This is what gives rise to the many worlds and Copenhagen interpretations of quantum mechanics. Only the latter has wave function collapse, yet the mathematical predictions are identical, so it is impossible to even tell if wave function collapse actually happens.

MoogPoo

Hi, I'm a bit confused about Hermitian operators, commutativity, simultaneous eigenfunctions and Heisenbergs uncertainty and how they are related so is this right. Could someone help with all this please?

I get commutative operators and the properties of commutativity and can prove whether 2 operators commute. If they commute does that mean you can get simultaneous eigenfunctions? Is that the same as they can be measured simultaneously with certainty? Also if 2 operators dont commute, then youve to use Heisenbergs uncertainty so they can be measured simultaneously. Is that right?

Finally, I read that if the expectation value is real using a certain operator then that operator must be Hermitian? Is there a proof of this or is it obvious? Sorry just a bit confused with all this, havent put all the pieces fully together. Thanks

maninasia

Professor_Fink wrote: »

So you've totally ignored everything I've said, and come back to the start. Fine. You're totally incorrect but I'm not going to waste my time having the same argument a second time.

Why am I incorrect when I stated that 'communication at faster than light speeds is possible with wormholes'?
If you can warp and bend space there is no single route.

maninasia

Professor_Fink wrote: »

First, as I have stressed, this is not travelling faster than light. You are simply taking an alternate, shorter path. The problem then arises that any photons ahead of you on the path you travel will escape you, since they too travel the shorter path, but at a greater velocity than you.

That's not a problem if you create another wormhole with a shorter path.
What is speed...distance/time. I have cut the distance with my wormhole..the end result is the same.

ZorbaTehZ

MoogPoo wrote: »

Finally, I read that if the expectation value is real using a certain operator then that operator must be Hermitian?

You're getting mixed up, it's the other way around, Hermitian operators have real eigenvalues, easy to proove as well, see here

Professor_Fink

MoogPoo wrote: »

If they commute does that mean you can get simultaneous eigenfunctions?

Yes. More specifically it means that they are simultaneously diagonalisable. If you have one or more degenerate eigenvalues (meaning two orthogonal eignevectors/eigenfunctions have the same eigenvalue) then the specific choice of vectors in that plane isn't fixed. So if you compute an eigenbasis, it's unique if and only if there is no degeneracy. That means if you compute eigenbasis for two commuting operators it is not necessarily the case that they are identical. Rather you are guaranteed that there exists some basis that is an eigenbasis for both operators.

MoogPoo wrote: »

Is that the same as they can be measured simultaneously with certainty?

Yes.

MoogPoo wrote: »

Also if 2 operators dont commute, then youve to use Heisenbergs uncertainty so they can be measured simultaneously. Is that right?

If the operators do not commute then they cannot be measured simultaneously. This means that there is some uncertainty, which can be calculated in the usual way.

MoogPoo wrote: »

Finally, I read that if the expectation value is real using a certain operator then that operator must be Hermitian? Is there a proof of this or is it obvious?

This is certainly the case. It's really easy to prove too.

Take the expectation value <X> = c, where c is real. This is short hand for <w|X|w>=c. Taking the Hermitian conjugate (transpose followed by complex conjugate) clearly leaves c invariant. Thus you have <w|X|w> = c = <w|X^\dagger|w>. As this holds independent of the vector |w>, you have <w_i|X|w_i> = <w_i|X^\dagger|w_i> for a basis |w_i>. Hence X = X^\dagger, and so X is Hermitian.

Professor_Fink

maninasia wrote: »

Why am I incorrect when I stated that 'communication at faster than light speeds is possible with wormholes'?

Did you miss my earlier post? As I said, I'm done with this.

maninasia

Ok you said the path has changed, the speed of light has not. Fine, but communication at faster than the speed of light is possible by warping space. Therefore speed of light is not a final barrier. The maximum speed of light in a vacuum could be as important in the grand scheme of things as the maximum speed of a ship on the high seas.

MoogPoo

whoops never looked at sticky. Thanks that making sense now

take everything

Hi.
This is probably a stupid question but can someone tell me:
Does Quantum Mechanics violate causality in any way.
I know the generally-accepted interpretation is that QM is indeterministic.
But isn't indeterminism still causal?
(As determinism is causal).

I suppose what i'm wondering is:
If you do a measurement in QM with the exact same settings, will the answer be exactly the same or does the randomness in QM mean the answer will always be different.
And even if it was, is this reconcilable with causality.

Sorry if this comes across as an elementary query.
It's just bugging me a bit.
Thanks.

Professor_Fink

take everything wrote: »

Hi.
This is probably a stupid question but can someone tell me:
Does Quantum Mechanics violate causality in any way?

No. Quantum mechanics is a non-signalling theory, so causality remains intact. If you view the wave-function as a whole, then quantum mechanics is also deterministic. It is simply the act of making measurements that introduces indeterminism. Given a quantum system which some initial state and some Hamiltonian (which governs how the state evolves in time), you can calculate the exact state of the system after any fixed time. Further there is always a measurement you can make which confirms the system is in this state and succeeds with probability 1.

krd

maninasia wrote: »

Ok you said the path has changed, the speed of light has not. Fine, but communication at faster than the speed of light is possible by warping space.

My understanding of a wormhole is gravity is used to shrink space. So, the light that travels through a wormhole is still travelling at c. And anything that passes through the wormhole will still have a velocity less than the speed of light.

southcentralts

I have a quick question about quantum entanglement after reading that we managed to QE two diamonds. Because they are entangled, information can theoretically be passed between them faster than the speed of light ( or so my understanding of the subject goes), in which case if one of the diamonds was launched and sent into deep space traveling at 90% the speed of light with a receiver linked to it for, say microwave signals ( I do not know if this would be used but speed of light is the important factor for my question) sent from earth. Would the diamond back on earth begin receiving messages from the future and over time, the messages would go further and further into the future, would they not?

and if possible in the answer, a yes or a no before you dive into the scientific equations and faults with my theory.

p.s. The euromillions results would be a result, but I am thinking if this did work it could be used as early warning system for natural disasters, earthquake or tsunami as only a small message would need to return - location and time of event.

maninasia

It's a good question. If the one of the pair could be activated (read) only by a specific wavelength (for instance a signature of a supernova) and then the entangled partner back on earth have an instantaneous change. This could provide an early warning system of supernova radiation advancing towards Earth at the speed of light.

The transmission of information would in this case be in the sensing of the change of the entanglement rather than encoding any specific signal.

However I think one sticking point is you need to actively read (I don't think there is a mechanism for a passive output) the entangled partner back on Earth. I've not been able to get a clear answer to this yet before Professors and their ilk start diving down the black tunnel of quantum mechanics.

An answer from another forum
'Because there is no apparent correlation until measurements are compared, the states will be observed in whatever reference frames they're observed in, and no one will agree on who observed first. But they will agree, when they compare measurements, that there was entanglement.'

So you would have to read the partner on Earth to determine it's state, but you do not know if the entangled partner in space has been read first or not. There is no signal like 'green' or 'red' that can be programmed into the entangled state, it will just give a random assortment of 'greens' and 'reds' when you read them. It's only when you communicate using slower than light speed that you can get information as to when the other observer made their observation and therefore who was actually first to observe. That's my understanding of it, could be wrong though.

maninasia

Professor_Fink wrote: »

That isn't quite right. The division between the quantum and classical worlds is simply down to interaction with the environment. In this sense, if you could reverse the process then you would recover quantum effects. However, when photons carry away information, you can never catch up with them in order to actually reverse the process, so after a very brief period, the necessary information is propogating away from your lab faster than you can travel to catch it.

This is interesting. If we could prevent the photons travelling very far in space we can catch those photons before they escape.
We could also use some type of warping of space to catch the photons and bring the information back again. In theory.

southcentralts

Thanks for your reply maninaisa.

I guess I was a bit confused by the concept of quantum entanglement as I thought once 2 diamonds were entangled, one could be manipulated and it would affect both and figured this could be used to transmit information in binary - inactive state and manipulated. So I guess I misunderstood the "manipulated" piece.

Fount the article
Walmsley and his colleagues set up an experiment that would attempt to entangle two different diamonds using phonons. They used two squares of synthetically produced diamond, each three millimeters across. A laser pulse, bisected by a beam splitter, passes through the diamonds; any photons that scatter off of the diamond to generate a phonon are funneled into a photon detector. One such photon reaching the detector signals the presence of a phonon in the diamonds.

But because of the experimental design, there is no way of knowing which diamond is vibrating. "We know that somewhere in that apparatus, there is one phonon," Walmsley says. "But we cannot tell, even in principle, whether that came from the left-hand diamond or the right-hand diamond." In quantum-mechanical terms, in fact, the phonon is not confined to either diamond. Instead the two diamonds enter an entangled state in which they share one phonon between them.

To verify the presence of entanglement, the researchers carried out a test to check that the diamonds were not acting independently. In the absence of entanglement, after all, half the laser pulses could set the left-hand diamond vibrating and the other half could act on the right-hand diamond, with no quantum correlation between the two objects. If that were the case, then the phonon would be fully confined to one diamond.

If, on the other hand, the phonon were indeed shared by the two entangled diamonds, then any detectable effect of the phonon could bear the imprint of both objects. So the researchers fired a second optical pulse into the diamonds, with the intent of de-exciting the vibration and producing a signal photon that indicates that the phonon has been removed from the system. The phonon's vibrational energy gives the optical pulse a boost, producing a photon with higher energy, or shorter wavelength, than the incoming photons and eliminating the phonon in the process.

The reread helped, I had assumed after entanglement that vibrations in one diamond would translate to the other one offering a way to communicate but they are entangled because they are both vibrating when only one should be.