Quantum Basic Questions!

Reply Mon 6 Jun, 2011 07:44 am
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?

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Reply Mon 6 Jun, 2011 08:22 am
1. I believe the first impetus that drove the theory of quantum mechanics was in trying to derive a theory for how light works. (See Einstein's theory of "corpuscular light.")

2. Currently there are projects for creating a quantum computer, which can solve problems difficult to perform on classic computers.

3. No, they were (are) deriving theories to describe the physical universe, and then using experiments to support or disprove those theories.

4. I'm sure that people use all kinds of things to support crazy theories.

5. Double-slit experiment. Also, most of modern chemistry is based on the knowledge of how electrons fit into "orbits."

Reply Mon 6 Jun, 2011 11:09 am
1. QM was created because of the problems of particle physics. Sub-atomic particles don’t follow the rules of macroscopic particles. It was what was called the “Ultraviolet Catastrophe” that is generally regarded as the basic problem QM was needed to “fix”. Also known as the Blackbody Radiation Problem it regards how a perfectly emitting body would emit radiation. Classical Physics said the black body at thermal equilibrium would emit energy with infinite power. QM was developed and said it was more like E = T exp 4.
QM was right and rocked our world! There are too many other examples to list.

2. The uncertainty principle is an attribute of nature. So as to defending its existence – it is a real phenomenon -- we have little choice to accept it even if it doesn’t do anything some would consider “practical”. The Casimir–Polder force is an example of the uncertainty principle at work.

3. Theories attempt to make sense of observations to the point that we can make practical predictions about future events.

4. Justifying something is a rather vague measure as, among other things, it is totally subjective. In science the established fields have agreed on methods and measures to move together in a meaningful way. What the rest of the world does isn’t “science’s” business and it likes it that way.

5.Two Scientists in 1971 Cluaser and Freedman did the seminal experiment to answer that question. They determined (many times confirmed) that to the limit of statistical predictability the predictions of QM are correct to within 3% (their limit).

A.While QM seems to be an easy target because of the many non intuitive (read:way -- strange) features. This is not QM it is nature itself. In reality QM in the form of QED, for example, is almost unimaginably more precise than any other human construct. The QM theoretical magnetic moment of the neutron compared to the actual measured magnetic moment is correct out to TWENTY decimal places! It is the most exact theory that has ever been created.
Reply Mon 6 Jun, 2011 12:52 pm
I'd forgotten about the black body radiation issue. Thanks for reminding me.
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Reply Mon 6 Jun, 2011 02:56 pm
Quantun physics was driven by Newtonian physics not working with atomic particles with so low a mass.
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Reply Tue 7 Jun, 2011 07:03 am
Thanks for the replies!

I will note and remember that Quantum Physics is the base of modern chemistry.

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.
Reply Tue 7 Jun, 2011 07:48 am
MyQuest wrote:
Is it not possible to observe an electron(or any other sub-atomic particle like it) in a double slit without disturbing it?

They've tried, and the very act of observation affects the outcome of the experiment.

MyQuest wrote:
It this a limitation of our instruments/methods of observation or strangeness of quantum world?

Strangeness of the quantum world.

MyQuest wrote:
If this is how quantum world behaves, how did we conclude so?

The math predicted this behavior, and the physicists scratched their heads and thought, "that can't be right." So they ran the experiment, and the math was right.
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Reply Wed 8 Jun, 2011 12:11 pm
When the object observed is altered by the observation means it cannot be observed or alternatively it means the observation is interacting with the object so the input of the observation must be included in the equation. It is like a hidden camera situation. When a person doesn't know of the camera's existence then he acts normally. But if he knows there a hidden camera he would ham it up.
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Reply Wed 8 Jun, 2011 12:27 pm
The concept of changing something by observing it (physically taking a measurement) is not unique to the QM world in any way. When you use an air gauge to get the air pressure in a tire you let out some of the air so now the tire cannot have the pressure you just read off on the gauge. Put a thermometer in the hot oil in the pot on the stove and the oil’s temperature is reduced. In the macroscopic world these small changes are usually unimportant, not so in QM.

Interestingly, in the world of research of the very cold it is often far easier to create a colder temperature than to create a measuring device that can measure the temperature while effecting it the least, so a meaningful value is given.
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Reply Wed 8 Jun, 2011 03:35 pm
I find a lot of the philosophy students asking highly complex physics type questions when they skipped high school physics and want to tackle post graduate physics.
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