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Quantum Mechanics

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KaraCleary
875850.  Fri Jan 06, 2012 11:11 am Reply with quote

I completely agree with both of you. Which is why I want to know how observation is defined, in order to be better able to find what exactly the basis for this idea is.

Quote:
I guess I'm with you Bob - I don't agree with the Boss Man (jumping jack) that a "sound" only exists when it is heard.

PDR


I had received the impression that this argument was more about how 'sound' is described - in terms of the sound waves becoming a sound only when heard by someone? Maybe I'm wrong. Formal background in physics/metaphysics, anyone?

Maybe, like wave particle duality, it's not something we can understand. The two slit experiment, for example, is incomprehensible as the wavefunction is in R^(3/2), at least before it becomes normalized.

 
Posital
875901.  Fri Jan 06, 2012 3:14 pm Reply with quote

It's about whether something actually exists when there is no one to perceive it.

I'm beginning to suspect that this is the case - and is the only way to make sense of the strange quantum effects we can produce.

As for walking through walls - if you travel fast enough you should be able to quantum tunnel through a wall - or penetrate it as an x-ray (whichever viewpoint you prefer) with near 100% success.

 
KaraCleary
875933.  Fri Jan 06, 2012 5:07 pm Reply with quote

Posital wrote:
As for walking through walls - if you travel fast enough you should be able to quantum tunnel through a wall - or penetrate it as an x-ray (whichever viewpoint you prefer) with near 100% success.


Shame my wavelength is far to long and unwieldy. Hm. As for the tree falling, I believe it was mentioned on a QI episode, but I can't remember which series.

 
soup
875935.  Fri Jan 06, 2012 5:15 pm Reply with quote

KaraCleary wrote:
I had received the impression that this argument was more about how 'sound' is described - in terms of the sound waves becoming a sound only when heard by someone? Maybe I'm wrong. Formal background in physics/metaphysics, anyone?


I don't have a formal background in physics,but.

Having watched the episode a few times it seems Jack (Mr Lloyd) was saying that pressure variations in the air do exist but these variations are not called sounds until someone hears them. So very much a case of "when are these variations called sounds", not that pressure variations did not occur in the air.

 
KaraCleary
875939.  Fri Jan 06, 2012 5:35 pm Reply with quote

Quote:
I don't have a formal background in physics,but.

Having watched the episode a few times it seems Jack (Mr Lloyd) was saying that pressure variations in the air do exist but these variations are not called sounds until someone hears them. So very much a case of "when are these variations called sounds", not that pressure variations did not occur in the air.


That was indeed what I thought. Not questioning reality as such, then. That is the realm of cats and metaphysics.


Last edited by KaraCleary on Sat Jan 07, 2012 5:19 am; edited 1 time in total

 
CB27
875977.  Fri Jan 06, 2012 10:06 pm Reply with quote

My personal one man tent camp of thought is that there's "sound", there are "sound waves" and there is "perception of sound".

Sound waves are the vibrations of gases, liquids and solids which create a sort of bridge to carry and perhaps even augment the sound from it's origin.

In some ways this can already be described as sound, but is not the end of the story, which I guess is where JJ was coming from. What I think he was referring to is the "perception of sound", because if the origin of the sound is too far away from you, the sound waves can dissipate to a degree where you cannot perceive the sound any more. Likewise, there are some frequencies some people can't hear, as well as other animals. I guess JJ thinks that the perception of sound is what matters.

Some people, and I think Stephen sort of alluded to this on the show, think that the sound waves themselves are sound. However, this can also present a problem. If you beat a drum in a vacuum, you create the origin of the sound, but there is nothing to carry the sound waves, but is that enough to say there is no sound?

 
KaraCleary
876003.  Sat Jan 07, 2012 5:25 am Reply with quote

Oh, how confusing... You see, I am firmly of the opinion that the generation of sound waves is enough to mean there has been a sound. That is true, to me, even in a vacuum... isn't it?

According the The Free Dictionary, a sound wave is: A wave of compression and rarefaction, by which sound is propagated in an elastic medium such as air.

So according to this, in a vacuum the sound doesn't exist. Which seems more or less reasonable, but is also very annoying.

 
CB27
876118.  Sat Jan 07, 2012 5:19 pm Reply with quote

Now you know why I think sound is broken into the three elements, the generation, the waves, and the perception.

 
Posital
876239.  Sun Jan 08, 2012 1:22 pm Reply with quote

Perhaps there's a fourth (or more) - hearing and then perception - and possibily - interpretation - and even - emotional resonance.

My body hear the noise (like a trace on an oscilloscope - converted from pressure waves).
I perceive it (it's a kind of crunching noise).
I interpret it (the gears are sounding a little funny needs fixing).
I respond emotionally (Waah - more wonga to keep the jalopy going - general pain - my poor motor - general sadness - loss of holiday - etc).

My fave TED link: http://www.ted.com/talks/evelyn_glennie_shows_how_to_listen.html

 
Loki
876251.  Sun Jan 08, 2012 3:09 pm Reply with quote

Thankfully I have formal education in Physics and we covered the section on quantum mechanics in a lot of detail.

The question of observation influencing the behaviour of the quantum particle is best explained with reference to the legendary twin slit experiment.
Suppose we have a setup with a source of electrons (or any other quantum particle) projecting them against a screen. We can place a shield with two slits in the way and the wave particle duality of the electron results in an interference pattern being projected on the screen. This shows us the behaviour of the particle but we don't know which of the two holes it came through.

If we place a light source of high enough frequency (photon energy defined as E = hf where h is plank's constant) then when the electron passes through one of the holes we will get a tiny flash of light in the hole. Fantastic! we now know which hole the electron passed through but the interference pattern on the screen will have changed.
This is what is meant when we talk about quantum particles changing due to the act of observation; the reason we can see the electron is because it absorbs the energy of the photon and becomes energetically excited, when it de-excites it releases that energy in the flash we see. The excitation process will have changed the momentum of the particle and so the pattern by which we study its momentum changes; thus knowledge of position and momentum being mutually exclusive.

The concept of Schrodinger's Cat is slightly different: if we know that we have a quantum system that can occupy only two states and have not observed which it is in then we do not know which it occupies. Using Schrodinger's time independent wave equation we can determine energy eigenvalues and a normalised wave function for the system (in lay terms we know the particle is in there somewhere and if it's unbiased then it's 50:50 which state it's in so the normalisation coefficients are equal). Normalisation, incidentally, is setting the total probability of finding the particle somewhere in the system to 1, if the probability is not 1 then something is fundamentally wrong with the initial premise.
So, our unobserved system has a normalised energy wave equation the fully describes the system and both states have energy so both exist (cat is both alive and dead here). When we observe which state it is in, one of those states must become 0 and normalisation forces the other to 1, so the energy redistributes and we say we have collapsed the wave equation.

Sorry this is kind of long.

 
Jenny
876312.  Sun Jan 08, 2012 8:34 pm Reply with quote

Long but interesting - thank you Loki and welcome to the forums :-)

 
Posital
876323.  Mon Jan 09, 2012 2:33 am Reply with quote

Yup - see also Quantum Superposition.

 
djgordy
876330.  Mon Jan 09, 2012 4:33 am Reply with quote

Posital wrote:
It's about whether something actually exists when there is no one to perceive it.


If something doesn't exist when nobody preceives it, how can there be an "it" to perceive?

Quote:
I'm beginning to suspect that this is the case - and is the only way to make sense of the strange quantum effects we can produce.


You may say you don't believe that things don't exist unless you percieve them but I bet you wouldn't cross a busy road at rush hour with your eyes closed.

 
dr.bob
876344.  Mon Jan 09, 2012 6:04 am Reply with quote

CB27 wrote:
If you beat a drum in a vacuum, you create the origin of the sound, but there is nothing to carry the sound waves, but is that enough to say there is no sound?


As Kara says, a sound wave is a wave of compression and rarefaction, by which sound is propagated in an elastic medium.

If you beat a drum in a vacuum, there are sound waves propagated in the elastic medium of the drum skin. This is demonstrable if you press your ear to the side of the drum, the sound will be conducted directly through the bones in your skull and you'll be able to hear the noise of the drum.

The fact that it's in a vacuum means there is no medium to carry this sounds beyond the drum. Doesn't mean the sound isn't there, though.

Loki wrote:
The concept of Schrodinger's Cat is slightly different


And much misunderstood. Schroedinger created his famous thought experiment to highlight how absurd he found this idea of superimposed wavefunctions. Clearly the cat is either dead or alive, it can't be both. The fact that we don't know which it is doesn't help the cat at all.

 
Loki
876434.  Mon Jan 09, 2012 12:13 pm Reply with quote

dr.bob wrote:

And much misunderstood. Schroedinger created his famous thought experiment to highlight how absurd he found this idea of superimposed wavefunctions. Clearly the cat is either dead or alive, it can't be both. The fact that we don't know which it is doesn't help the cat at all.


I think from the piece he wrote here http://www.tu-harburg.de/rzt/rzt/it/QM/cat.html#sect5 the intention was to demonstrate that Quantum behaviour is extremely unintuitive and when extended to the macroscopic world we are familiar with no longer makes any sense.
So it's an expression of the difference between classical and quantum physics by trying to model one in the domain of the other.

 

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