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Don't see how quantum entanglement contradicts causality, surely it is just another aspect of causality,

Quantum entanglement contradicts relativity, not causality ?

 

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18 minutes ago, vlaiv said:

Still don't get it, how is causality associated with quantum entanglement different from "naive causality". We still naively believe that measurement at A causes correlated value of measurement B. As shown above, both A can cause B and at "the same time" (no not real time, figure of speech) - B causes A.

This is actually deeper than paradox would suggest. As each A and B measurement interact with environment thus leak information into environment via decoherence, or become entangled with environment on some level - which one becomes entangled first? There is no first one - A is first and B is first - depending on your reference frame. This entanglement propagates in environment, but again which one is "first" - or if we accept that both are first, we need to accept that on some level reality can depend on reference system (beyond things that we know depend on reference system like length, time and so), or that we live in predetermined universe.

Naive causality is simply that events are linked in the normal cause and effect chain where a past event can potentially have caused an event if a light ray can connect them.

This is not the case with quantum entanglement the measurement events are correlated without the possibility of them being connected by a light ray.

The point about spacelike separated events is that they can have any time separation you like! We can't define which caused which or appeal to local hidden variables to explain the correlation but it exists.

Regards Andrew 

Edited by andrew s

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And also , referring to previous statements.

Be aware that mass/length etc are a function of time and although they may change for different observers as you are always in your "local" time your mass & length will never actually change.

Slightly different when accelerating, but at a constant speed they will not change.

Hence although it is said you cannot travel faster than the speed of light and light "takes" @ 4 years to get to the nearest star this does not prohibit you from travelling to the nearest star in a shorter period, say a month for example.

 

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1 minute ago, Kev M said:

Hence although it is said you cannot travel faster than the speed of light and light "takes" @ 4 years to get to the nearest star this does not prohibit you from travelling to the nearest star in a shorter period, say a month for example.

 

Can you explain how?

Regards Andrew 

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10 minutes ago, Kev M said:

Don't see how quantum entanglement contradicts causality, surely it is just another aspect of causality,

Quantum entanglement contradicts relativity, not causality ?

 

Well, neither :D

When we talk about entanglement and relativity - we think of a problem phrased "nothing moves faster than speed of light".

Then we naively assume - measurement at A instantaneously "collapses" wavefunction at B, or "causes" wavefunction collapse at B.

So it might seem that there is something traveling faster than the speed of light - information that A has been measured "causes" state of wavefunction at B. It looks like some sort of information traveled faster than the speed of light - but that is not the case.

It has been shown that you can't transfer any sort of information / do communication via entanglement. It is also wrong to assume wave function collapse due to measurement and impart "causal" to measurements of A and B. Measurement of A does not cause result of measurement at B.

They are indeed correlated, and when you throw time in to the mix, and particularly different order of events then your run into "paradox" if you think in terms of causal.

My concern is that correlation together with different order of events points to something that we discard at the moment. I'm still trying to wrap my head around if it indeed means determinism of the sorts or there is "a way out" - different explanation.

6 minutes ago, Kev M said:

Hence although it is said you cannot travel faster than the speed of light and light "takes" @ 4 years to get to the nearest star this does not prohibit you from travelling to the nearest star in a shorter period, say a month for example.

Depends on reference system measured in.

If I'm on earth and I expect to get a message from you that you arrived to nearest star, I'd be silly to expect it sooner than 8 years from your departure :D

 

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5 minutes ago, andrew s said:

Can you explain how?

Regards Andrew 

For a person on the Earth, the trip would take at least 4 years, but, due to time dilation/length contraction, a person on the ship can experience an arbitrarily small duration of time for the trip.

Edited by George Jones
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4 minutes ago, andrew s said:

Can you explain how?

Regards Andrew 

Person in the space ship might experience shorter travel time. In our reference system star is at 4 ly from earth. For someone traveling fast, due to length contraction distance is less and us / star system is moving in opposite direction with the same speed so it will take less time for star to arrive to that observer in their reference frame.

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While we are at it, I might be missing something but we now accept decoherence as explanation for measurement problem, except it is not full explanation as far as I can tell.

It nicely explains how "complex" superposition reduces to "ensemble" superposition (terms in quotes are just my mental explanation, don't know proper terms for those), but it still does not explain how a single value is picked out from "ensemble" nor why does system continue to evolve from that particular measured value.

We still seem to lack "ensemble" superposition -> single measured value transition, and explanation for it?

 

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This is so much fun.  Actually this discussion has cleared up a few lingering questions from articles in Scientific American.  Got to remember  the region I live in has some math issues, pie are not square, pie are round, cornbread are square!  Einstein’s equations  seem to indicate that mass varies according to velocity if it does then  variation does occur with point of view.

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3 minutes ago, George Jones said:

For a person on the Earth, the trip would take at least 4 years, but, due to time dilation/length contraction, a person on the ship can experience an arbitrarily small duration of time for the trip.

Exactly and this is the basic problem, we tend to reference everything back to our own local time on earth.

We cannot actually measure the speed of light , only the apparent time taken between the start and finish of our measurement, this increases linearly with distance so gives an untrue reading.

The fastest tool we have for measuring anything is a light beam so "unexpectedly" if we use light to measure itself it will always come back with the same value.

 

 

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4 minutes ago, Michael Kieth Adams said:

This is so much fun.  Actually this discussion has cleared up a few lingering questions from articles in Scientific American.  Got to remember  the region I live in has some math issues, pie are not square, pie are round, cornbread are square!  Einstein’s equations  seem to indicate that mass varies according to velocity if it does then  variation does occur with point of view.

This just shows how much difficulty there is around this, if we assume mass varies with speed according to an observer , then which observer do we use ?

If we use all the observers in the universe then the mass would be every value from zero to infinity ( well almost infinity ).

Therefore mass is either

A-Constant

or

B-Non-existent

 

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6 minutes ago, vlaiv said:

While we are at it, I might be missing something but we now accept decoherence as explanation for measurement problem, except it is not full explanation as far as I can tell.

It nicely explains how "complex" superposition reduces to "ensemble" superposition (terms in quotes are just my mental explanation, don't know proper terms for those), but it still does not explain how a single value is picked out from "ensemble" nor why does system continue to evolve from that particular measured value.

We still seem to lack "ensemble" superposition -> single measured value transition, and explanation for it?

 

I agree. I think decoherence explains why we see the classical world emerging with the classical states position and velocity. However, it does not resolve the measurement problem.

Regards Andrew 

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15 minutes ago, Kev M said:

Exactly and this is the basic problem, we tend to reference everything back to our own local time on earth.

We cannot actually measure the speed of light , only the apparent time taken between the start and finish of our measurement, this increases linearly with distance so gives an untrue reading.

The fastest tool we have for measuring anything is a light beam so "unexpectedly" if we use light to measure itself it will always come back with the same value.

 

 

Just as well then we don't have to as it has been defined as a particular value! 

Regards Andrew 

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1 minute ago, andrew s said:

I agree. I think decoherence explains why we see the classical world emerging with the classical states position and velocity. However, it does not resolve the measurement problem.

Regards Andrew 

Some time ago I started thinking about it along following lines, but haven't progressed too far - mostly because lack of understanding / knowledge on the topic.

Somewhere I've read fairly interesting explanation that goes in line with my way of thinking about it.

It goes something like this in trying to "explain that there is no measurement problem at all":

Suppose we have an electron with spin being in some nice complex superposition - and it enters measurement apparatus - decoherence happens and electron is now in 1/2 spin up + 1/2 spin down. There is actual measurement, but instead saying electron is now spin down, we need to actually say:

System has evolved into 1/2 electron up / registered up + 1/2 electron down / registered down state and it further evolves into 1/2 electron up / registered up / scientist noted up + 1/2 electron down / registered down / scientist noted down state.

Now question becomes why in reality we experience only "one branch" of this, or rather why do we only remember one branch of this.

My way of thinking is that decoherence also "leads" to different meaning of probability - somehow. Before decoherence we have propensity type of probability, while after decoherence we have ensemble type of probability. Above example fits Bayesian type of probability - we can use probabilities from this "complex state" 1/2 measured up + 1/2 measured down + prior knowledge to discard one branch and "recalculate" probabilities based on the branch we know happened (this does not explain how we know what happened - or rather why we know of certain branch and not the other).

But point is - when we have "complex" superposition state actual interpretation of probability is different than what we have after decoherence - decoherence makes reality either / or - somehow one definite answer by some mechanism, and inherent lack of knowledge (even in principle) what real situation is so we can only calculate as 50% chance of one and 50% chance of other.

Something along those lines.

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I think what you have basically said is that by measuring an object we change it.  Point of view.  Maybe mass is just variable.  Ask schrodinger’s cat.

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9 hours ago, George Jones said:

For a person on the Earth, the trip would take at least 4 years, but, due to time dilation/length contraction, a person on the ship can experience an arbitrarily small duration of time for the trip.

Yes, of course,  first degree brain fade. Too much wine before posting. 

Regards Andrew 

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@vlaiv I think you are articulating different approaches to interpreting QM. One extreme being Many Worlds where all branches occur but we only experience one and a Block type world where all already exists and we just move through it.

I don't think any form of strong determinism is scientifically testable.

Regards Andrew 

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1 hour ago, andrew s said:

@vlaiv I think you are articulating different approaches to interpreting QM. One extreme being Many Worlds where all branches occur but we only experience one and a Block type world where all already exists and we just move through it.

I don't think any form of strong determinism is scientifically testable.

Regards Andrew 

Good point.

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This has been a lot of fun.  I am amazed that a tired old fat man who knows a little science can spark such wonderful discussions and I am more amazed at how much of these comments I actually understand.  I am beginning to think however  that at least part of everything is the parts of the equation we don’t know.  We talk about mass when 90% or more is missing.  We are bailing out the same boat when we speak of dark energy.  Now that we have studied some other solar systems we should be beginning to have some idea of the mass of planets left over from dead stars.  If I’ve got it right, the universe has not been around long enough for that to be the source of all dark matter, but it should make up some of it.  We should have some idea by now how much.  We know about how much dark matter there should be, we know the effects of dark energy, how powerful it must be, how long it has been acting.  There is so much that we don’t know.  We have pride of our knowledge but we need to remember, a very few years ago liquid water was rare, now we find it all over the place.  We haven’t seen it yet but we are pretty sure there is a ninth planet.  We know about how big it must be and roughly where but that’s about it. This is a very exciting time to be alive.  Thank you all so much,Mike.

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