Quantum frogs and jumping to conclusions

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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

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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+

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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 neccessary 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
 
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jcsd

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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.
 
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6,171
1,275
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.
 
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Chi Meson

Science Advisor
Homework Helper
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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 wouldnt recommend alice in quantumland if allegories and the like arent ur thing or if u just dont 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 neccessary 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 doesnt 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 im 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 thats quite a crucial difference in the wording
 

FZ+

1,550
2
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.
 
197
1
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+

1,550
2
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.
 
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 with out 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+

1,550
2
Exactly Chemicalsuperfreak... The problem all stemmed from Heisenberg's initial analogy given for the process, and Einstein's opposition to HUP.
 
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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?
 

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