Quantum frogs and jumping to conclusions

[carla]

[SOLVED] Quantum frogs and jumping to conclusions

What does quantum mean exactly? What is a 'quantum leap'? And this urban myth going around about quantum matter changing upon observation (as though it knows it is being observed and therefore plays little tricks on the observer), what is this really about?
Thanks...

 

[mathman]

There is a book "Alice in Quantumland" (a lot of fun to read) by Robert Gilmore, which presents a good description of the subject for the general reader. It also includes the observation that nobody really understands quantum mechanics.

 

[FZ+]

What does quantum mean exactly?

A discrete amount of something.

What is a 'quantum leap'?

A radical change in thinking. Usually scientific/technological.

And this urban myth going around about quantum matter changing upon observation (as though it knows it is being observed and therefore plays little tricks on the observer), what is this really about?

The uncertainty principle, which is a conclusion of QM. Basically, it states that (a) position and momentum, or time and energy, are not entirely separate concepts. and (b) there is no absolute value for each of them and (c) The degree of uncertainty is governed by Uncertainty in P * Uncerrtainty in Q > half h-bar. Note: This uncertainty is not just an error in measurement, but a statement of the actual nature of stuff.

Based on this, we get the idea that every thing is expressed as a wave equation of probability, which collapses down to a specific particle on observation. Until it is observed, the nature of the entity is indeterminate. Hence the schrodinger's cat paradox, when we expand this concept to larger size objects, and conclude the cat is neither exactly dead nor alive, until we observe it and collapse it's wave function.

Several solutions exist. One is that the particles don't really do that, but Hidden Variables give it the impression of doing so. Numerous experiments (like the Aspect experiment) have disproven the so-called EPR paradox. Another is that our consciousness somehow is significant, and collapses the waveform. The Universe is Participatory. This produces some philosophical nastiness as to whether the cat is conscious, and whether we are necessary etc etc. Another is that Many Worlds exist so all the possibilities are simultaneously real. This however has little evidence. There may almost certainly be others.

A demonstration of the problem is easy to arrange - get 3 polarised filters. They, as you may recall, allow light through in only one alignment. If you put two of the filters aligned at right angles to each other, as predicted, no light goes through. The only light that gets through the first is cut out by the second filter. But if you put another filter in between the two aligned at 45 degrees to each, some light does in fact penetrate! This is completely contrary to classical theory. The quantum explanation is that some how, the act of passing through the filters introduced uncertainty to the light, giving it a probability of penetrating the other filter.

EDIT: Fixed HEP formula. Thanks Jcsd

 

[jcsd]

A quantum more specifically is the smallest amount of energy a system can gain or lose, though it has come to mean the smallest possible amount of any quantity.

A quantum leap or jump is the transmission from one stationary state of an atom or molecule accompanied by the admission or absorbtion of energy.

Properties are undefined until you measure them and the act of measuring properties can also change them.

Also for FZ+, in the most precise form of the Hesienburg Uncertainity principle the unceratinity in a pair of complemtary variables is equal to half h-bar.

 

[zoobyshoe]

Heisenberg realized there was no
way to observe an electron in
orbit without interfering with it.
There was no way to measure it's
position or speed, not even with
the most sensitive measuring tool
available: light. Imagine looking
through the best possible micro-
scope at a atom and trying to see
what an electron is doing by means
of the light bouncing off the
electron up into the lens:

"The electron apparently doesn't
like the new turn of events. It is
having a rough time of it. We are
not leaving it in peace any more.
We are not just looking at it, we
are hurling huge boulders of ener-
gy at it and it is being badly
knocked about. What sort of scien-
tific experiment is this? It is
certainly far from delicate. Sup-
pose we do manage to see the electron and note its position? It is an empty victory. The very fact that we see it means we have
scored a direct hit with a photon.
The electron is a very light par-
ticle unable to withstand a
particle of light. It is badly
jolted by the impact. In observing
the electron's position we give it
a jolt which alters its velocity. We defeat our own object."

The Strange Story Of The Quantum
by Banesh Hoffman
1947 Dover Publications NY
P.147

So it is not correct to character-
ize the situation as one of tricks
being played on the observer.
Heisenberg was pointing out that,
with the means available any
attempt to observe the phenomenon
will change the phenomenon and
what we see will be a misrepre-
sentation of its usual position
and/or speed, whatever that may
be.

 

[Chi Meson]

Originally posted by carla
And this urban myth going around about quantum matter changing upon observation (as though it knows it is being observed and therefore plays little tricks on the observer), what is this really about?
Thanks...

I won't even mention Heisenburg or Schroedinger. Oops.

It is true that you can NOT make any observation of any thing without interacting with it. In the most obvious form, you can not tell how heavy something is without lifting it.

But even looking at something is interacting with it. You can not see something (like a passing car) unless light has struck it and subsequently bounced off toward your eye. When light hit that car, a change to that car occurred. It got a tiny bit hotter. The car even recoiled an extremely tiny bit due to the momentum of the photon. With large objects this tiny change is not noticeable.

But with single particles, the change of momentum and energy is very significant when they are struck by photons or other particles.

The point is, no matter what, if you wnat to observe something, you must hit it with a particle, and that collision will change it.

 

[carla]

Originally posted by mathman
There is a book "Alice in Quantumland" (a lot of fun to read) by Robert Gilmore, which presents a good description of the subject for the general reader. It also includes the observation that nobody really understands quantum mechanics.

Sounds good. Thanks...

 

[pop676]

i wouldn't recommend alice in quantumland if allegories and the like arent ur thing or if u just don't understand quantum mechanics. if u do understand it good but some of the things in the book are relatively vague compared to the actually principles and laws. the footnotes do help so read them!

 

[carla]

Originally posted by FZ+
A discrete amount of something.


A radical change in thinking. Usually scientific/technological.


The uncertainty principle, which is a conclusion of QM. Basically, it states that (a) position and momentum, or time and energy, are not entirely separate concepts. and (b) there is no absolute value for each of them and (c) The degree of uncertainty is governed by Uncertainty in P * Uncerrtainty in Q > half h-bar. Note: This uncertainty is not just an error in measurement, but a statement of the actual nature of stuff.

Based on this, we get the idea that every thing is expressed as a wave equation of probability, which collapses down to a specific particle on observation. Until it is observed, the nature of the entity is indeterminate. Hence the schrodinger's cat paradox, when we expand this concept to larger size objects, and conclude the cat is neither exactly dead nor alive, until we observe it and collapse it's wave function.

Several solutions exist. One is that the particles don't really do that, but Hidden Variables give it the impression of doing so. Numerous experiments (like the Aspect experiment) have disproven the so-called EPR paradox. Another is that our consciousness somehow is significant, and collapses the waveform. The Universe is Participatory. This produces some philosophical nastiness as to whether the cat is conscious, and whether we are necessary etc etc. Another is that Many Worlds exist so all the possibilities are simultaneously real. This however has little evidence. There may almost certainly be others.

A demonstration of the problem is easy to arrange - get 3 polarised filters. They, as you may recall, allow light through in only one alignment. If you put two of the filters aligned at right angles to each other, as predicted, no light goes through. The only light that gets through the first is cut out by the second filter. But if you put another filter in between the two aligned at 45 degrees to each, some light does in fact penetrate! This is completely contrary to classical theory. The quantum explanation is that some how, the act of passing through the filters introduced uncertainty to the light, giving it a probability of penetrating the other filter.

EDIT: Fixed HEP formula. Thanks Jcsd

Thankyou very much for that informative and thoughtful response FZ!

I can not profess to understand everything you have said but that's ok and all part of my own learning process.

Regards the filter and light problem, does the introduction of the third filter act as some sort of reflector off the the other two filters thereby allowing light through? Light bending?

 

[pop676]

no it is half silvered on one side so as to let light only come through one side and its angled

 

[pop676]

also the uncertainty principle makes it clear that it penetrates and doesn't because the photons must take both paths but since we are lookin at it we can only use the probability of light passing through. it goes back to the whole electron thing and the superpostion of states

 

[carla]

What Zoobyshoe and Chi Meson said seem to make the most sense. Analogy that comes to mind. Someone decides to find out how much sealife there is in very deep oceans not usually reached by light. The introduction of light kills of the life because this is life not adapted to light (as we know it) and therefore the quantity equals zero although it is known that sea-life exists via discovery using other methods..

hey...I'm not a scientist so I'm trying to grasp the problem with layperson understanding.

 

[pop676]

thats understandable everyone learns differently and meson did describe it very well better than i did i think oh well another question answered at least by someone

 

[pop676]

if u have any more question ud like answered id be more than happy to try and explain it

 

[pop676]

carla uve got mail

 

[carla]

Originally posted by pop676
if u have any more question ud like answered id be more than happy to try and explain it

Thanks pop...it's hard to respond to everyone's posts but I do read them all and I read yours too.

 

[pop676]

sry I am just really bored and i need sumtin to do

 

[carla]

Reflection was the wrong wrong wrong term to use. I meant, refraction, I think. That is light BENDING via all possible avenues open to it to allow it to travel through!

 

[pop676]

yes that's quite a crucial difference in the wording

 

[FZ+]

Regards the filter and light problem, does the introduction of the third filter act as some sort of reflector off the the other two filters thereby allowing light through? Light bending?

Not so easy. If it did so in the classical sense, it would mean that all of the photons would go through since on the other side of the first filter, they would be all the same and bent the same way. But experiments show that only about 25% of the photons that passed the first filter were outputted.

 

[Hydr0matic]

Originally posted by FZ+
But if you put another filter in between the two aligned at 45 degrees to each, some light does in fact penetrate! This is completely contrary to classical theory.

Is this correct ? I thought classical theory could explain this perfectly. The third filter in between changes the polarization of the light. This can easily be tested by turning the second (last) filter 90° compared to the third one in the middle. If the polarization indeed is changed, no light will go through, but if the polarization stays the same some light should pass through.

If this isn't the prediction of classical theory, then what is ?

 

[FZ+]

Is this correct ? I thought classical theory could explain this perfectly. The third filter in between changes the polarization of the light. This can easily be tested by turning the second (last) filter 90° compared to the third one in the middle. If the polarization indeed is changed, no light will go through, but if the polarization stays the same some light should pass through.

I may be wrong in this, but I believe that by classical theories, either all of the photons should go through or none of the light should go through. Since the photons coming out of the first filter are in terms of polarisation identical, they should all be changed in the same way by the second polariser. However, only some of them change, and we have a probability of changing the polarisation instead of a definite yes/no.

 

[Chemicalsuperfreak]

Originally posted by carla
What Zoobyshoe and Chi Meson said seem to make the most sense. Analogy that comes to mind. Someone decides to find out how much sealife there is in very deep oceans not usually reached by light. The introduction of light kills of the life because this is life not adapted to light (as we know it) and therefore the quantity equals zero although it is known that sea-life exists via discovery using other methods..

hey...I'm not a scientist so I'm trying to grasp the problem with layperson understanding.

The problem is that Zoobyshoe and Chi Meson are wrong. They state probably the most common misunderstanding of any scientific principle.

At the quantum level, things are really weird, and in my opinion cannot really be understood without quite a bit of mathematics and physics classes. The Heisenburg Principle doesn't say that you can't determine a particles postition and momentum because any observation changes it.

What the HUP says is that if a particles momentum is known, its postition is fundamentally and intrinsically indiscrete, it's not only that we can't know it, but that a specific postition doesn't exist. And vice versa. Again, this is hard to understand viewing it from the classical world that we live in. But HUP means a lot more than the inability to accurately measure particles.

 

[FZ+]

Exactly Chemicalsuperfreak... The problem all stemmed from Heisenberg's initial analogy given for the process, and Einstein's opposition to HUP.

 

[zoobyshoe]

Originally posted by FZ+
The problem all stemmed from Heisenberg's initial analogy given for the process, and Einstein's opposition to HUP.

What was Heisenberg's initial
analogy, and how did Einstein's
opposition contribute to the
common misunderstanding of HUP?

 

[Hydr0matic]

Originally posted by FZ+
I may be wrong in this, but I believe that by classical theories, either all of the photons should go through or none of the light should go through. Since the photons coming out of the first filter are in terms of polarisation identical, they should all be changed in the same way by the second polariser. However, only some of them change, and we have a probability of changing the polarisation instead of a definite yes/no.

But this is practically tha same as the QM interpretation right ? The classical interpretation should be that of simple vectortransformation. Split the vector of each wave hitting the polarizer into two perpendicular components, one of which is aligned with the polarizer.

Your statement that all lightwaves should be changed the same way by the second polarizer is correct, they are. The classical answer lies in realizing that all waves pass through, each one with a lower amplitude than before. Think waves, not photons.

 

[Hydr0matic]

Originally posted by zoobyshoe
What was Heisenberg's initial
analogy...

http://www.aip.org/history/heisenberg/p08b.htm

 

[zoobyshoe]

Originally posted by Chemicalsuperfreak
The problem is that Zoobyshoe and Chi Meson are wrong. They state probably the most common misunderstanding of any scientific principle.

Zoobyshoe is not wrong.
What I wrote was a simplified but
accurate paraphrase of the prob-
lems Heisenberg described one
would encounter when trying to
determine the probabiities of
both the location and momentum
of an electron in orbit via a
microscope. I did not claim this
to be an explanation of The
Heisenberg Uncertainty Principle.
Hesenberg said what I said he
said:
Heisenberg's Physics and Philosophy
Address:http://www.marxists.org/reference/subject/philosophy/works/ge/heisenb3.htm

My goal was to explain the origin
of the "Urban Legend" about quanta
"playing tricks" on the observer.
I believe Heisenbergs concerns
about how observation and measure-
ment may change what is observed
and measured are the primary
source of any such legend. If you
read the linked paper you will see
it is an issue he raises in more
than one situation. I am not aware
to what extent these concerns
informed his final formal state-
ment of The Uncertainty Principle.

 

[Chi Meson]

Originally posted by Chemicalsuperfreak
The problem is that Zoobyshoe and Chi Meson are wrong. They state probably the most common misunderstanding of any scientific principle.

Um, I disagree. I am aware of the extreme simplification of the condition, but what I said is quite correct.

ALso what I said does not go against what C'freak said, and what I said does not indicate a misunderstanding of the principle (In fact I was not even referring to HUP).

What I said was what I believe to be the most basic answer to the original question: It remains true that you can not observe anything (in classical or quantum terms)about a single particle without interacting with it, and interacting with a particle will change the condition of the particle.

Please let me know, specifically, what is wrong about this.

BTW, I just remembered what could be an exception to this: the analogy of the shoe: if you put a left shoe in one box and a right shoe in another box, but didn't know which was which, as soon as you looked in one box (say its the left shoe) then you immediately know that the other box contains the right shoe "without interacting with it." I think this is discussed in the book "in search of Schroedinger's cat"

 

[selfAdjoint]

There is nothing wrong with the statement that to observe a particle you must interact with it and thus change it. What is wrong is to state that this is what the uncertainty principle is about. The U.P. as was said, does not concern your observation but basic existence. It says that where the photon frequency is perfectly known (for example), its location is truly undefined, not just unknown.

The problems of interacting field theory are real problems, but they are at a different level from uncertainty.

 

[CJames]

Numerous experiments (like the Aspect experiment) have disproven the so-called EPR paradox.

Wait, what? Are you saying that quantum entanglement has been disproven? No more quantum teleportation? Oh man my novel's completely ruined...

 

[jcsd]

Originally posted by CJames
Wait, what? Are you saying that quantum entanglement has been disproven? No more quantum teleportation? Oh man my novel's completely ruined...

No the Aspect experiment PROVES quantum entanglement.

 

[CJames]

Cool. Can I get a link?

 

[jcsd]

Here's a thread on Bell's inequality (which the Aspect experiment set out to prove):

https://www.physicsforums.com/showthread.php?s=&threadid=4884

 

[FZ+]

Originally posted by CJames
Wait, what? Are you saying that quantum entanglement has been disproven? No more quantum teleportation? Oh man my novel's completely ruined...

No... I mean the paradox is disproven - ie. Einstein et al's conclusion that QM cannot be a complete description of the world because it allow quantum entanglement etc. (Though some people still dispute the conclusiveness of the Aspect experiment.)

 

[selfAdjoint]

Exactly right. John Bell, who was a partisan of Bohm's hidden variable theory, devised his inequalities in the hopes of supporting that theory and atttacking quantum mechanics. See his book Speakable and Unspeakable in QM. But in fact quantum mechanics turned out to explain the effect and the Aspect and subsequent experiments demonstrated that the Bell correlations really happen. As you say, some quantum attackers haven't given up and are nit-picking the experiments.

QM says that what subsists between the entangled particles is a correlation between attributes, not a causal relationship. This knocks message sending on the head. I think the Bell-Aspect story is the greatest demonstration of the quantum nature of the world since the two slit-one electron experiment described in Feynmann's textbook.

 

[Chi Meson]

Originally posted by selfAdjoint
There is nothing wrong with the statement that to observe a particle you must interact with it and thus change it. What is wrong is to state that this is what the uncertainty principle is about.

I totally agree. And that's not what I said. It's like a "Three's Company" episode: it's all just a big misunderstanding!

Getting beyond it: SelfAdjoint said:

"This knocks message sending on the head."

Which way? Does this business allow for using quantum weirdness to send information faster than light, or dies this prove it impossible. Or does it have nothing to do with this?

As I recall, single pieces of data can be determined instantaneously (such as that shoe analogy) but it was believed that you couldn't string such data into "information." (No hyper-c phone calls).

 

[selfAdjoint]

message sending FTL is impossible with the quantum understanding of the Bell-Aspect results. Suppose you have one of a pair of entangled particles. You want to change its state to send a bit to the other station, where the other particle will change its state and be recorded.

But you can't measure the state of your particle, because that will destroy the entanglement and with it your hopes of a link. So you would just have to trust that your particle is still in its entangled state. But somebody, or even some random event, could have "measured" the particle at the other end. That would have destroyed the entanglement too. So you're up the creek either way. You might have a link or not, but you'll never know for sure.

When all of this is developed in the math of the theory, it is known as Eberhardt's theorem. Quantum entanglement gives enhanced correlation, but it doesn't give cause, and you can't use it to send messages.

 

[quantumdude]

Originally posted by carla
What does quantum mean exactly?

It corresponds to the concept of discreteness, as opposed to continuity. For example, in classical mechanics the energy of a particle bound in a potential is a continuous function of the state variables. Upon closer inspection of microscopic systems however, we see that there this is not the case. Only certain allowed energy levels are occupied. For instance, in order for an atomic electron to be promoted from a lower energy state to the next higher energy state, it cannot do so via a continuous gaining of energy. Rather, it must absorb a quantum of energy.

What is a 'quantum leap'?

A quantum leap is a stroke of genius on par with the one(s) that led to quantum mechanics.

And this urban myth going around about quantum matter changing upon observation (as though it knows it is being observed and therefore plays little tricks on the observer), what is this really about?
Thanks...

This is due to a popular misconception that "observation" is somehow related to "cognizance". When a physicist speaks of "observations" in QM, it means only that the system interacts with the detection apparatus. No mind or consciousness is implied or needed.

 

[quantumdude]

Originally posted by FZ+
A demonstration of the problem is easy to arrange - get 3 polarised filters. They, as you may recall, allow light through in only one alignment. If you put two of the filters aligned at right angles to each other, as predicted, no light goes through. The only light that gets through the first is cut out by the second filter. But if you put another filter in between the two aligned at 45 degrees to each, some light does in fact penetrate! This is completely contrary to classical theory.

No, classical electrodynamics accounts for it just fine. What is completely contrary to the classical theory is when you do a similar experiment using particles and Stern-Gerlach filters.