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   Author  Topic: various physics questions  (Read 17022 times)
towr
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Re: various physics questions  
« Reply #25 on: Jul 2nd, 2008, 12:46am »
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on Jul 1st, 2008, 6:51pm, BenVitale wrote:
According to decoherence theory, even a stray photon "counts" as an observer. Any particle interacting with an object in a mixed state will collapse the superposition and put the object into a pure state.
That's also what you see in experiments.
 
Quote:
The idea is called "decoherence," and it is the reason we don't see quantum-mechanical effects [often] on the macroscopic level. All the billions of particles interacting with the system -- atoms of air, infrared photons, etc. -- all serve to keep macroscopic objects like cats out of quantum superpositions.
Makes sense to me Smiley
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Re: various physics questions  
« Reply #26 on: Jul 2nd, 2008, 4:25am »
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on Jul 2nd, 2008, 12:44am, towr wrote:

But shouldn't there be a me in both rooms?
Otherwise I'd expect to end up in a universe with exceedingly less people (and matter in general). Or is it a non-physical me that's making the path through the continuum of adjacent universes? (Which has it's own set of problems).

Yes, but you experience only one path.  The "other you" experiences the other path.  So you might be under the impression that whatever you observe collapses to a pure state, while in fact the superposition still exists.
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Re: various physics questions  
« Reply #27 on: Jul 2nd, 2008, 8:41am »
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Thanks towr for answering. I thought my last physics posts were being ignored.
 
What about "superfluids"?
 
We can observe macroscopic quantum mechanical effects with superfluids. Take a bucket of liquid helium and cool it down to within a few degrees of absolute zero, it undergoes a phase transition called Bose-Einstein condensation. All the atoms hurry to enter the same quantum state, because that minimizes their total energy. So you wind up with a bucket full of atoms that have all agreed to be in the same state.  
 
And if you try to use it like a normal fluid notice it flows without viscosity. That's right, it flows without resistance through even the very smallest pores in your container, and through even the smallest pipettes. Why? Because all the atoms are already in their lowest energy state. Since they're all doing the same thing, though, the walls of a pipette can't smack them around too much -- you can't smack around one, you have to smack around every single last trillion of them.
 
Superfluids also will only permit certain values of angular momentum, e.g. 3 or 5 or 7 rotations per second. Even if you spin the bucket at 4 rotations per second from now to eternity, the helium atoms won't care. Angular momentum is quantized for their collective quantum state, and they'll only rotate at 3, 5, or 7, and never, ever at 4.
 
You can basically consider a bucketful of liquid helium to be like one giant macroscopic quantum object.
 
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Re: various physics questions  
« Reply #28 on: Jul 2nd, 2008, 2:22pm »
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That also fascinates me.  Makes you feel that there is something very wrong in your understanding of the world and the truth is there at you fingertips.
 
But you say 3, 5 or 7.  Does it extend to 1 and -1, or is it an approximation?  Is zero a valid angular speed?
 
And is there a phenomenon of superposition of different speeds?
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Re: various physics questions  
« Reply #29 on: Jul 2nd, 2008, 9:49pm »
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on Jul 2nd, 2008, 2:22pm, Grimbal wrote:
...
 
But you say 3, 5 or 7.  Does it extend to 1 and -1, or is it an approximation?  Is zero a valid angular speed?

 
Yes, 1 and 0 are valid values for angular momentum. Usually we talk about the absolute value, but whenever direction matters we see negative spins like -1 along with positive ones.  
 
 
Quote:

And is there a phenomenon of superposition of different speeds?

 
Yes! It might surprise you to know that any quantum state can be written as a superposition of different speeds. But the same is true if the word speed is replaced by "Energy", "Position", or any other physical variable you can think of!
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Re: various physics questions  
« Reply #30 on: Jul 3rd, 2008, 12:01pm »
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I need your help to figure this thing out.
 
Why?
 
Because I got ridiculed by my physics prof. [he is an experimental physicist] for showing an interest in this thought experiment.
 
My physics prof told me that the whole Schroedinger's cat thing is something of a magnet, intentionally absurd, a metaphysical nonsense, that I should not waste my time on this, and do real physics instead.
 
But then, I mentioned the GRW Theory (Ghirardi, Rimini and Weber)
 
http://philosophy.rutgers.edu/FACSTAFF/BIOS/PAPERS/LOEWER/loewer-schroed ingers-cat.pdf
 
He got very emotional. I don't need to repeat word for word what he said to me in class, in front of everybody. In summary he belittled me for this whole Schroedinger's cat business.
 
I'm really surprised why a physics prof would get all uptied over this theoretical physics experiment.
 
There are theoretical physicists who don't think that this is a stupid idea, they don't think that it is a waste of time. So for now I keep my mouth shut in class about this cat thing.
 
I have this forum to discuss about this experiment.
 
 
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Re: various physics questions  
« Reply #31 on: Jul 3rd, 2008, 12:13pm »
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on Jul 3rd, 2008, 12:01pm, BenVitale wrote:

He got very emotional. I don't need to repeat word for word what he said to me in class, in front of everybody. In summary he belittled me for this whole Schroedinger's cat business.
 
I'm really surprised why a physics prof would get all uptied over this theoretical physics experiment.

Perhaps he is a cat lover who believes that you are trying to run before you can walk.  
 
By the way, for some reason Physics seems to be a magnet for cranks and crackpots with tunnel vision.
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Re: various physics questions  
« Reply #32 on: Jul 3rd, 2008, 12:56pm »
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Perhaps he is a cat lover ...

 
As a matter of fact, he has a cat, because sometimes he comes to class with cat hair on his clothes.
 
Quote:

you are trying to run before you can walk.  

 
True. I got fascinated by the Elegant Universe, Hawking's string theory on NOVA (PBS).
 
String theory is another thing my physics prof likes to ridicule.
 
Is it true or false that :
 
(a) Quantum theory is highly counter-intuitive
(b) many physicists have trouble with it
 
ThudanBlunder, what do you think of the different perspectives of this experiment?
 
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Re: various physics questions  
« Reply #33 on: Jul 3rd, 2008, 1:38pm »
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In the Scientific American's article "The Nature of Space and Time"
 
http://zebu.uoregon.edu/~imamura/123/lecture-7/hawking.html
 
an excerpt
 
Quote:

[Let] us consider the Schroedinger's cat thought experiment. It describes the plight of a cat in a box, where (let us say) a photon is emitted which encounters a half-silvered mirror, and the transmitted part of the photon's wave function encounters a detector which, if it detects the photon, automatically fires a gun, killing the cat. If it fails to detect the photon, then the cat is alive and well. (I know Stephen does not approve of mistreating cats, even in a thought experiment!) The wave function of the system is a superposition of these two possibilities....But why does our perception not allow us to perceive macroscopic superpositions, of states such as these, and not just the macroscopic alternatives "cat is dead" and "cat is alive"?...  
 
I am suggesting that something goes wrong with superpositions of the alternative space-time geometries that would occur when general relativity begins to become involved. Perhaps a superposition of two different geometries is unstable and decays into one of the two alternatives. For example, the geometries might be the space-times of a live cat, or a dead one. I call this decay into one or the other alternative objective reduction, which I like as a name because it has an appropriately nice acronym (OR). How does the Planck length 10-33 centimeter relate to this? Nature's criterion for determining when two geometries are significantly different would depend upon the Planck scale, and this fixes the timescale in which the reduction into different alternatives occurs.

 
 
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Re: various physics questions  
« Reply #34 on: Jul 3rd, 2008, 6:48pm »
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on Jul 3rd, 2008, 12:56pm, BenVitale wrote:

String theory is another thing my physics prof likes to ridicule.

In this case his derision is more understandable.
 
on Jul 3rd, 2008, 12:56pm, BenVitale wrote:

True. I got fascinated by the Elegant Universe, Hawking's string theory on NOVA (PBS).

I can see you a few years down the line failing your final exams but being offered a full professorship on the spot. LOL
 
on Jul 3rd, 2008, 12:56pm, BenVitale wrote:
Is it true or false that :
 
(a) Quantum theory is highly counter-intuitive
(b) many physicists have trouble with it
 
ThudanBlunder, what do you think of the different perspectives of this experiment?

(a) True, as our intuition stems from classical logic.
(b) Hardly true. They grew up with it and they realize it's the only game in town.
 
What experiment? Oh, you mean this ongoing one that we are all unwittingly part of? Roll Eyes  
What a pity we can never observe any results without affecting them. Grin
 
Now, what do you think of my theory?  Cool
 
By the way, did you know that Heisenberg was once stopped by a traffic cop while out driving? The cop said, 'Have you any idea how fast you were travelling?' Heisenberg replied, 'No, but I know exactly where I am.'
 
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Re: various physics questions  
« Reply #35 on: Jul 4th, 2008, 12:31am »
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on Jul 3rd, 2008, 6:48pm, ThudanBlunder wrote:
(a) True, because our intuition stems from classical logic.
Not even that. Classic logic is also counter-intuitive to most people, and has to be learned. Which just makes the clash with QM all the more aggravating.
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Re: various physics questions  
« Reply #36 on: Jul 7th, 2008, 5:45am »
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Here's some disturbing news:
http://www.nature.com/news/2008/080702/full/454008a.html
 
What they claim they can do there is undo measurements (of a certain kind). Why is that disturbing? Well, take a pair of entangled photons, say. Now if you measure one, the other will be in the exact same state; no matter how far apart they are. But what if you can undo a measurement? That means you can repeat observations until you see a measurement you like, which will then also be identical at the other end. All you need is a bit of good timing, and you can send information at instantaneous speed.
Or so, at least, goes my understanding of the phenomenon.
(I suppose it's possible some other mechanism will interfere with this scheme; it almost certainly has to.)
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Re: various physics questions  
« Reply #37 on: Jul 7th, 2008, 8:25am »
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They don't actually undo a measurement.  They do a partial measurement that is inconclusive.  It only hints at one result.  Then they do the opposite measurement, equally inconclusive, that hints at the opposite result.  They end up now knowing anything about the quantum state.
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Re: various physics questions  
« Reply #38 on: Jul 7th, 2008, 9:04am »
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Of course, the measurement isn't always inconclusive. And when it's not they can't undo it either. But when it's inconclusive, they can repeat it, if they want to, until it isn't.
I suppose the obstacle is that the probability is equal for it to conclude on the first or second measurement.
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Re: various physics questions  
« Reply #39 on: Jul 7th, 2008, 11:23am »
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towr,
 
do you have the whole article?
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Re: various physics questions  
« Reply #40 on: Jul 7th, 2008, 11:44am »
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on Jul 7th, 2008, 11:23am, BenVitale wrote:
towr,
 
do you have the whole article?
Yes, I'll send you a link via the board's PM system.
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Re: various physics questions  
« Reply #41 on: Jul 7th, 2008, 3:13pm »
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Thanks for the link to MIT theoretical physics and for the Schroedinger's cat article.  
 
That's very cool!
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Re: various physics questions  
« Reply #42 on: Jul 11th, 2008, 11:16am »
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Experimental vs. Theoretical physics debate.
 
Heisenberg stated, "What we observe is not nature itself but nature exposed to our method of questioning."  
 
So theory in part constructs reality.
 
What can we say about equipments used in experiments? Don't they determine the results? Couldn't we say that equipments construct results?
 
The act of studying an event can change it.
 
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Re: various physics questions  
« Reply #43 on: Jul 11th, 2008, 1:17pm »
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Has anyone read "The Final Theory"
 
http://www.thefinaltheory.com/?gclid=CL2jicXSuJQCFSUqagodJnXvTQ
 
 
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Re: various physics questions  
« Reply #44 on: Jul 11th, 2008, 1:26pm »
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on Jul 11th, 2008, 11:16am, BenVitale wrote:
Heisenberg stated, "What we observe is not nature itself but nature exposed to our method of questioning."  
 
So theory in part constructs reality.
That is one way to phrase it, but you could more realistically say theory structures the way we look at reality.
Reality is as it is, regardless of theory.  
 
Quote:
What can we say about equipments used in experiments? Don't they determine the results?
They influence the result, certainly. But if they in themselves determined the result, than science would merely be make-believe.
Would there no longer be atoms if we didn't have electron microscopes to see them with? Surely they must always have been there (well, for some 14 billion years), regardless of whether we could observe them. That we don't know something exists hasn't any bearing on whether it does.
 
Quote:
Couldn't we say that equipments construct results?
Up to a point. It's a system, of what is measured and what measures; and who interprets it.
Without the reality check of what is actually there to be measured, the whole exercise would be rather pointless.
 
Quote:
The act of studying an event can change it.
Only if the event involves the study (like how a study of working condition affects the workers being studied). However, you can't, say, change WWII by studying it, since it's all in the past. Whatever was, was. You can certainly change how we see it, what we know of it, you can change it epistemologically; but ontologically it stays the same.
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Re: various physics questions  
« Reply #45 on: Jul 11th, 2008, 1:53pm »
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on Jul 11th, 2008, 1:17pm, BenVitale wrote:
Has anyone read "The Final Theory"
http://www.thefinaltheory.com/?gclid=CL2jicXSuJQCFSUqagodJnXvTQ

Probably not, and with good reason.
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Re: various physics questions  
« Reply #46 on: Jul 11th, 2008, 2:48pm »
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on Jul 11th, 2008, 1:53pm, ThudanBlunder wrote:

Probably not, and with good reason.

 
I haven't read it. I'm just curious to know whether or not has read it. I just don't have enough time to read any books unrelated to my studies.
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Re: various physics questions  
« Reply #47 on: Jul 11th, 2008, 2:51pm »
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on Jul 11th, 2008, 1:26pm, towr wrote:

 
 
They influence the result, certainly. But if they in themselves determined the result, than science would merely be make-believe.
Would there no longer be atoms if we didn't have electron microscopes to see them with? Surely they must always have been there (well, for some 14 billion years), regardless of whether we could observe them. That we don't know something exists hasn't any bearing on whether it does.
 
Up to a point. It's a system, of what is measured and what measures; and who interprets it.
Without the reality check of what is actually there to be measured, the whole exercise would be rather pointless.
 
.

 
Yes, I agree with you.  
 
Take, for example, our telescopes. From the early telescopes to the sophisticated and powerful ones of today, haven't they shaped and reshaped our theories about the size of the universe?
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Re: various physics questions  
« Reply #48 on: Jul 11th, 2008, 3:07pm »
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on Jul 11th, 2008, 2:48pm, BenVitale wrote:

I haven't read it. I'm just curious to know whether or not has read it. I just don't have enough time to read any books unrelated to my studies.

According to this guy, obviously a crank, all of modern physics is wrong. But despair not! Buy his book and you can be put right.
I gave up after I read this:
 
Q: Light slows as it passes through water or
     glass, causing it to bend, but how can it
     return to light-speed on its own once it exits?
 
A: This is impossible in today's science. No object in nature
can speed up of its own accord after being slowed. A bullet
doesn't spontaneously speed up after it is slowed by passing
through a wooden block, so how does a photon of light
mysteriously return to its original speed once it exits a glass
block?

 
Now correct me if I am wrong, but as photons have zero rest mass, they don't need a force to accelerate them.
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Re: various physics questions  
« Reply #49 on: Jul 12th, 2008, 4:32am »
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on Jul 11th, 2008, 3:07pm, ThudanBlunder wrote:
Now correct me if I am wrong, but as photons have zero rest mass, they don't need a force to accelerate them.
Not to mention it is exactly the way waves behave, so it's explained by the wave-character of the photon.
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