Exploring Quantum Physics: Basic Questions and Misconceptions

[MyQuest]

Hi Learned Members,

I am not a regular physics student but a curious one. I have read some books written for laymen by physicsts - (like In search of Schrodinger's cat). I do not understand few basic things below. Maybe I am not even framing the questions correctly. Please excuse me if so.

1. Was quantum physics invented simply because of the assumed failure of our ability to view particles like electron (because the photon knocks-off the electron)?

2. What is the practical use of uncertainity principle? It simply seems to justify uncertainity. Was this ever used/proved by experiment? Or is this simply a rule arrived because we do not have instruments to track an electron?

3. Are we not simply inventing theories just because we do not have instruments to observe the behaviour of sub-atomic particles like electrons?

4. And, because of the above, every other person on Earth tries to justify everything - (spoon bending, time travel, multiple worlds mentioned in some religions, sci-fi movies and so on) and point to Quantum Theory as proof. What is really possible according to Quantum Physics?

5. Has any of the theoratical surmises/predictions(Quantum Physics) been proved conclusively by experiments? If so, could you give the names of the experiments?

I do not mean to offend anyone. I understand fully that I am half-baked. Could someone please enlighten me?

 

[Bill_K]

Was quantum physics invented simply because of the assumed failure of our ability to view particles like electron (because the photon knocks-off the electron)?

Historically, quantum mechanics was invented to explain why atoms do not collapse.

What is the practical use of uncertainity principle? It simply seems to justify uncertainity.

Very little, if any. I'm amazed at how much attention the Heisenberg uncertainty principle gets on this forum. It is a very small aspect of quantum mechanics. Most textbooks mention it on page one and go on from there.

Has any of the theoratical surmises/predictions(Quantum Physics) been proved conclusively by experiments? If so, could you give the names of the experiments?

The atomic bomb. Seriously, almost everything in physics and chemistry, certainly everything involving the properties of materials, has its foundation in quantum mechanics.

 

[DrChinese]

5. Has any of the theoratical surmises/predictions(Quantum Physics) been proved conclusively by experiments? If so, could you give the names of the experiments?

Welcome to PhysicsForums, MyQuest!

Yes, there have been tens of thousands of experiments using quantum theory as their basis. All say the same thing: QM is rock solid. There are many derived elements from QM, and these have made QM useful over the past 80+ years of its existence. So far, there have been no errors found in it (in the normal sense of that phrase).

The Heisenberg uncertainty principle (HUP) is itself very well tested. Due to a number of ingenious experiments, it is clear that the HUP is not a result of experimental limits. It is a fundamental element of the universe we occupy.

 

[honolulu_boy]

Hi,

1. Was quantum physics invented simply because of the assumed failure of our ability to view particles like electron (because the photon knocks-off the electron)?

3. Are we not simply inventing theories just because we do not have instruments to observe the behaviour of sub-atomic particles like electrons?

I ask myself the same question and the fact is that I don't know what type of sensors were used to measure the temperature of blackbody and the precision of some experimentations as the "electron diffraction". Sometimes, I have the impression that there was no other choice and quantum mechanics is just a question of Scientific pragmatism to deal with the world of particles.

 

[BruceW]

I'll try to explain the questions to the best of my ability. I have completed an undergraduate degree in physics, so hopefully I can at least explain the basics ;)

1) Quantum mechanics came about because classical physics failed to explain certain phenomena (for example photoelectric effect).
2) The uncertainty principle is a fundamental law of our universe. It doesn't matter how good your equipment is, you can't break this law. Many experiments have been done that agree with this, so we can assume that it is true. The double-slit experiment is a well-known practical example of this principle.
3) Again, the accuracy of our instruments has nothing to do with it. The everyday notions of momentum, position, e.t.c. take on a whole new meaning at the quantum level.
4) Problems with non-local effects and the many worlds interpretation are both controversial issues. It seems that almost everyone you ask gives a different answer.
5) Try looking up physics journals, probably half the stuff you find will be experiments that agree with Quantum physics. Probably the 2 most famous examples are the photoelectric effect and the double-slit experiment.

 

[honolulu_boy]

Hi,

The double-slit experiment is a well-known practical example of this principle.

But in the case of the double-slit experiment do we really know what the electron really do or we are just looking after the interference figure ?

 

[honolulu_boy]

I mean does the weak measurements we do only give partial information and aren't limited to statistical inferences ?

 

[BruceW]

You're right, the double-slit experiment is not usually used as an example of how the uncertainty principle works. Sorry for causing unnecessary confusion.
I should have used the single-slit experiment as example instead. Here's the explanation of how the single-slit experiment works:
So, a single electron is fired at the slit. This electron's wavefunction is initially represented by a plane wave (with exactly known momentum). When the electron goes through the slit, its wavefunction is now represented by a much smaller spread in position eigenstates, and as such, it now must have a larger spread in momentum eigenstates (due to Uncertainty principle). And therefore, when the electron comes out of the slit, it won't be traveling in the same direction anymore, which is why electrons are diffracted by a small enough slit.

 

[MyQuest]

Thanks for all replies!

I will note and remember that Quantum Mechanics played a big role in understanding properties of materials.

 

[yenchin]

You're right, the double-slit experiment is not usually used as an example of how the uncertainty principle works. Sorry for causing unnecessary confusion.
I should have used the single-slit experiment as example instead. Here's the explanation of how the single-slit experiment works:
So, a single electron is fired at the slit. This electron's wavefunction is initially represented by a plane wave (with exactly known momentum). When the electron goes through the slit, its wavefunction is now represented by a much smaller spread in position eigenstates, and as such, it now must have a larger spread in momentum eigenstates (due to Uncertainty principle). And therefore, when the electron comes out of the slit, it won't be traveling in the same direction anymore, which is why electrons are diffracted by a small enough slit.

And here is the video:

 

[MyQuest]

It seems the double slit experiment was an eye-opener on the quantum world's behaviour(below).

1. Electron beams produced interference pattern - particles behaved like waves !

2. Electrons fired one at a time towards double slit created interference pattern ! Where does the other electron come from?!

3. Puzzling even more - if we try to trace the electron, this very action of observing collapsed the interference pattern and creates simple beams. It seemed the quantum world abhored observation!


This is where I got confused. Is it not possible to observe an electron(or any other sub-atomic particle like it) in a double slit without disturbing it? It this a limitation of our instruments/methods of observation or strangeness of quantum world? If this is how quantum world behaves, how did we conclude so?


Once again, thanks for all the replies.

 

[BruceW]

mathematically speaking, for double-slit interference to happen, the electron's wavefunction must be a superposition of both paths. But if we know which slit the electron went through, then its wavefunction is only due to one path.
So in an experiment, you can either: 1) Find out which slit the electron went through and not get interference. OR 2) Not find out which slit the electron went through and get interference.
You might then think "But how does our knowledge affect what the electron does?" This is a controversial issue, but most people would say the behaviour of the electron depends on the experiment we are doing.

 

[BruceW]

So the point is that it is not due to our inaccurate equipment. It is actually because of the laws of quantum mechanics.
The laws of quantum mechanics (or technically I should really say postulates) came about because they fit with the experiments that have been done. In other words, QM is a highly empirical theory, whose philosophical implications are still controversial.
As such, many of the big contributors to quantum mechanics had very different views on how useful/literal QM should be taken.
For example, Einstein explained the photoelectric effect by saying light was absorbed in discrete packets. But he felt that QM was an unsatisfactory theory, and came up with many thought experiments in an attempt to disprove it.

 

[DrChinese]

...This is where I got confused. Is it not possible to observe an electron(or any other sub-atomic particle like it) in a double slit without disturbing it? It this a limitation of our instruments/methods of observation or strangeness of quantum world? If this is how quantum world behaves, how did we conclude so?

Just to add to what BruceW said: It is NOT the effect of the apparatus in and of itself that causes the collapse. It is the overall context of the setup, as to whether knowledge is gained (or could be) or not.

Take a double slit setup using light. Place one polarizer over the left slit at angle L, another over the right slit at angle R. When L-R=0, there IS interference. When L-R=90 degrees, there is NO interference. Obviously, the presence or absence of a polarizer does not change anything unless which slit information is gained. That becomes progressively more feasible as L-R goes from 0 to 90 degrees.

 

[A. Neumaier]

4. And, because of the above, every other person on Earth tries to justify everything - (spoon bending, time travel, multiple worlds mentioned in some religions, sci-fi movies and so on) and point to Quantum Theory as proof.

This is because quantum physics
(i) has a very high and deserved reputation of being really true, and people want to attach this label to their dubious theories in order to appear more respectable to the layman;
(ii) is presented in the public as having a flavor of mystery (due to the frequent misuse of classical imagery for talking about quantum phenomena), so it looks plausible (but only to those understanding very little about the real nature of quantum mechanics) that other mysterious things are related to quantum stuff.

What is really possible according to Quantum Physics?

Nothing of what you mention under 4.

What is really possible is the stuff you see realized in current technology, such as transistors (giving rise to TV, computers and VLSI), superconducting magnets, scientific equipment to measure things with detail or accuracy one could only dream of in earlier times, etc..

 

[A. Neumaier]

3. Puzzling even more - if we try to trace the electron, this very action of observing collapsed the interference pattern and creates simple beams. It seemed the quantum world abhored observation!

The notion of an electron is something with a well-behaved mathematical description, but whose attempt at visualization plays havoc with our imagination. Taking the talk too serious (rather than the formulas) causes the (misguided) mystery part in the public view of quantum mechanics. But it can be profitably be used to create highly selling books!

This is where I got confused. Is it not possible to observe an electron(or any other sub-atomic particle like it) in a double slit without disturbing it? It this a limitation of our instruments/methods of observation or strangeness of quantum world? If this is how quantum world behaves, how did we conclude so?

It is a limitation of the simplistic concept of ''an electron being somewhere'' applied to a much more complex reality described by an "electron field".

 

[BruceW]

I agree with Neumaier in that to truly understand what quantum mechanics is, you have to do the maths. An explanation of quantum mechanics can give someone an introduction to what it is, but its true colours come out when you learn about hermitian operators, and the postulates of QM.

 

[MyQuest]

Hi All,

Thanks for all the replies. I now see that it is possible to interpret the results of double slit in many different ways because of the supposedly 'abnormal' outcome while there 'should' be a mathematical(and not mystical) explanation for the outcome. I will defenitely look into the mathematical aspect and try to understand if I can :)