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Quantum Mechanics

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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.

 
CB27
876450.  Mon Jan 09, 2012 1:19 pm Reply with quote

dr.bob wrote:
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?
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.

Exactly my point about different definitions of what "sound" is, and that it's realy various stages.

The sound waves in the drum skin are the source of the noise, but the sound waves will not travel to you through a vaccum, so you will not hear it. In once sense you can say sound exists, but in another (which I think is JJ's stance), the sound doesn't exist because it did not travel to you through the sound waves, and you were not able to perceive it.

By pressing your ear to the drum, the sound waves travel through you and you are able to hear he sound, so additional stages have been added to it, but is this enough to legitimise it as sound compared to the previous scenario?

 
Loki
876482.  Mon Jan 09, 2012 3:02 pm Reply with quote

Ok, sound! You are of course completely correct in thinking of sound as waves but you may also be interested to know that they can be treated as particles called "phonons".

A phonon is the quantised particle of a lattice vibration within a medium; in this case the drum-skin. So if you beat the drum you create a vibration in the skin but it cannot propagate beyond the edges of the drum because there is no medium to carry the vibration. This is analogous to a laser between two mirrors bouncing back and forth; outside the mirrors you can't detect it but would be mistaken to say that it is not present.

The age old debate of whether sound occurs due to vibration or the electro-chemical impulse in the ear upon stimuli is just semantics. If your tree falls then gravitational potential energy is transformed into some other form, sound if you like or a discharge of vibrations if you don't think it's sound; either way the energy is conserved.

 
Posital
876558.  Tue Jan 10, 2012 2:49 am Reply with quote

djgordy wrote:
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?
We can talk about the millions of neutrinos impacting us without perceiving them.

My x-ray vision isn't quite up to that yet...

 
dr.bob
876566.  Tue Jan 10, 2012 5:23 am Reply with quote

Loki wrote:
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.


For me, the key phrase in that article is:

"That prevents us from so naively accepting as valid a "blurred model" for representing reality."

To me, this is Schrödinger saying that, whilst the mathematical interpretation of QM allows us to model what's going on, one should be very careful when trying to translate maths into what's actually going on in reality.

After all, we can model simple harmonic motion mathematically by using imaginary numbers, but nobody's suggesting that simple harmonic motion involves anything imaginary, or that waves slip in and out of some imaginary dimension.

 
dr.bob
876567.  Tue Jan 10, 2012 5:30 am Reply with quote

CB27 wrote:
The sound waves in the drum skin are the source of the noise, but the sound waves will not travel to you through a vaccum, so you will not hear it. In once sense you can say sound exists, but in another (which I think is JJ's stance), the sound doesn't exist because it did not travel to you through the sound waves, and you were not able to perceive it.


Yeah, and one of those senses is wrong.

Just because a sound doesn't travel to your ear, doesn't mean it doesn't exist. It may not exist at a point close to your ear, but it exists elsewhere. In this case, it exists within the structure of the drum, but is prevented from travelling any further without some kind of propagating medium.

That would be a bit like me having a sealed box full of poisonous gas and claiming that the gas doesn't exist unless I open the box and allow it to travel over to me where it will kill me. That's clearly nonsense since the gas exists within the confines of the box, just as the sound exists within the confines of the drum.

CB27 wrote:
By pressing your ear to the drum, the sound waves travel through you and you are able to hear he sound, so additional stages have been added to it, but is this enough to legitimise it as sound compared to the previous scenario?


Of course, the sound is simply transmitting through a propagating medium. In this case it's the bones in your skull. This is different from, but functionally identical to, the usual case of sound waves propagating through the air.

 
PDR
876569.  Tue Jan 10, 2012 5:46 am Reply with quote

dr.bob wrote:
That would be a bit like me having a sealed box full of poisonous gas and claiming that the gas doesn't exist unless I open the box and allow it to travel over to me where it will kill me. That's clearly nonsense since the gas exists within the confines of the box, just as the sound exists within the confines of the drum.


I like that analogy, but perhaps a better one might be claiming that the gas is only poisonous when it's in a set of lungs. So the 2kg of phosgene is not poisonous because it's sealed into a box, and it only *becomes* poisonous when it's released onto animals and humans.

Whilst it would be possible to construct a definition in that way I would suggest that it's not a very useful definition, and the discrimination that it makes is pointless.

PDR

 
soup
876576.  Tue Jan 10, 2012 6:40 am Reply with quote

Loki wrote:

The age old debate <snip> is just semantics.


A lot of QI questions are based on semantics .
viz:- What is the tallest mountain?

 
PDR
876577.  Tue Jan 10, 2012 6:47 am Reply with quote

soup wrote:
viz:- What is the tallest mountain?


The one that is taller than all the others, obviously.

DUH!

PDR

 
soup
876582.  Tue Jan 10, 2012 7:30 am Reply with quote

PDR wrote:
soup wrote:
viz:- What is the tallest mountain?

The one that is taller than all the others, obviously.


Where measured from seems to be the basis of this one.
post 12699

 
WordLover
876583.  Tue Jan 10, 2012 7:39 am Reply with quote

PDR wrote:
dr.bob wrote:
That would be a bit like me having a sealed box full of poisonous gas and claiming that the gas doesn't exist unless I open the box and allow it to travel over to me where it will kill me. That's clearly nonsense since the gas exists within the confines of the box, just as the sound exists within the confines of the drum.


I like that analogy, but perhaps a better one might be claiming that the gas is only poisonous when it's in a set of lungs.

PDR, did you intend your analogy to be a false claim, like dr bob's claim about poisonous gas in a box? Because I think yours muddies the waters. A substance which is poisonous to animals of one species (if they breathe it) might be safe for another species. So poisonousness is not an inherent property of the substance. The question "is it poisonous?" needs to be understood in relation to who/what might breathe it.

PDR wrote:
So the 2kg of phosgene is not poisonous because it's sealed into a box, and it only *becomes* poisonous when it's released onto animals and humans.

Whilst it would be possible to construct a definition in that way I would suggest that it's not a very useful definition, and the discrimination that it makes is pointless.

PDR
This discrimination has a point in some circumstances. Asbestos roof tiles might be of little danger if they're securely fixed where they belong on the roof, but are dangerous if there's the danger of them coming loose, in high winds, say.

 
PDR
876586.  Tue Jan 10, 2012 8:06 am Reply with quote

This was an analogy to the assertion that "sound" only exists when vibration in a medium impinges on an eardrum (or microphone) - I would take the view that "sound" is an inherent property of vibration in a medium in the same way that toxicity is an inherent property of phosgene (whether it is currently stored in a sealed box or an open set of lungs).

I think calling it a "false claim" is taking the analogy too literally.

PDR

 
dr.bob
876602.  Tue Jan 10, 2012 9:39 am Reply with quote

PDR wrote:
Whilst it would be possible to construct a definition in that way I would suggest that it's not a very useful definition, and the discrimination that it makes is pointless.


Which neatly sums up 99% of the discussions commonly had about whether or not falling trees make a sound.

 
PDR
876606.  Tue Jan 10, 2012 9:50 am Reply with quote

I suspect that all of that sentence from the word "commonly" (inclusive) is superfluous!

:-)

PDR

 

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