Quantum Mechanics -- What's the Deal?

[IchabodPlain]

Ok, I have never taken a physics class, and readily able to plead ignorance whenever the subject comes up. However, between the recent article about building a larger QM structure, and the new movie Repo Men which contains a narrative by Jude Law on the scientist, cat, and poison filled box, I am ready for some answers here.

So what's with the parts of quantum mechanics that claims objects can be in two places/states at the same time? As I've understood it, when these phenomena are observed, the object can only be at one place/state at that time.

Frankly, the concept baffles me, and also strikes me as stupid. Then again, since I know nothing about physics, so I may be the stupid one here. Anyway, could some of the fine forum members here put the E in JREF for me?

 

[Trent Wray]

I almost HATE to do this to you, but here is a link to a video that gives a semi-decent explanation of one of the mind-blowing aspects of Quantum Physics, shown by the double slit experiment:



Now, there is a tinge of "woo" in it as it gives a bit too much credence to the effect an observer has in the experiment (imo), but if you watch the video and follow up with more in-depth studies you'll find that even minus the "woo" ---- quantum mechanics and the many implications of the various aspects will likely mind**** you. The video will also explain a little about superposition.

I think the video is taught from an angle akin to that movie, "What the **** do we know?" or whatever, which has woo all throughout it IIRC. It's why I hate to use the video link as an intro, but it's still pretty good to get a basic premise imo.

When a scientist refers to something as "spooky action at a distance", it can only mean good things

Anyway, I'd recommend starting with the double-slit experiment and if that doesn't make the hair on the back of your neck stand on end, then you're a better man than me LOL. After that, head towards quantum entanglement if you like or perhaps quantum tunneling. But don't skip Schroedingers Cat or Heisenberg's Uncertainty Principle by any means

Just my 2 cents

p.s. --- I ashamedly don't know how to embed a video in a post or I would have edit: fixed! thanks UncaYimmy!

 

[lomiller]

So what's with the parts of quantum mechanics that claims objects can be in two places/states at the same time? As I've understood it, when these phenomena are observed, the object can only be at one place/state at that time.

If you know the place exactly, then the state (or at least momentum) is completely indeterminate, if you know the momentum exactly then the location is completely indeterminate.

Personally the phenomenon that always seemed strangest to me is tunneling. Because there is always a some uncertainty in the location of a particle, if you fire it against a barrier, at the point where the particle encounters the barrier the uncertainty in it’s position has a chance to place it on the other side of the barrier in which case it will continue to move as if the barrier didn’t exists.

 

[Ziggurat]

Personally the phenomenon that always seemed strangest to me is tunneling. Because there is always a some uncertainty in the location of a particle, if you fire it against a barrier, at the point where the particle encounters the barrier the uncertainty in it’s position has a chance to place it on the other side of the barrier in which case it will continue to move as if the barrier didn’t exists.

You don't need the uncertainty principle to explain tunneling. In fact, it can screw you up if you try to do so. Using the uncertainty principle suggests that the width of your barrier is the primary determinant of tunneling amplitudes, but the barrier height matters just as much. Tunneling is a result of the Schrodinger equation having solutions for energies less than the potential. This means that particles are not strictly confined to classically allowed regions, and it applies even in cases where the uncertainty principle doesn't require penetration into classically forbidden regions.

 

[Dancing David]

Ok, I have never taken a physics class, and readily able to plead ignorance whenever the subject comes up. However, between the recent article about building a larger QM structure, and the new movie Repo Men which contains a narrative by Jude Law on the scientist, cat, and poison filled box, I am ready for some answers here.

So what's with the parts of quantum mechanics that claims objects can be in two places/states at the same time? As I've understood it, when these phenomena are observed, the object can only be at one place/state at that time.

Frankly, the concept baffles me, and also strikes me as stupid. Then again, since I know nothing about physics, so I may be the stupid one here. Anyway, could some of the fine forum members here put the E in JREF for me?

Hi there,

There are plenty of good resources to get started.

But the first thing to understand is scale, in the case of Schroedinger's cat, that is a macro-scale object. It is not subject to the 'super position' of the Copehagen Interpretation.

QM is about the behavior of very very samll things.

This is a good one, although it is more about string theory.
http://www.pbs.org/wgbh/nova/elegant/program.html

Wikepedia isn't that bad either.

 

[Furcifer]

Because there is always a some uncertainty in the location of a particle, if you fire it against a barrier, at the point where the particle encounters the barrier the uncertainty in it’s position has a chance to place it on the other side of the barrier in which case it will continue to move as if the barrier didn’t exists.

It's best not to think of it as a particle at the barrier because it's behaving as a wave. It's only beyond the barrier it can be detected as a particle. (and before of course)

 

[Molinaro]

Another variant of the double split experiment not mentioned in the video above has the detector only being turned on after the single electron is fired, and before it reaches the slits. And that still causes the interference pattern to disappear.

 

[W.D.Clinger]

So what's with the parts of quantum mechanics that claims objects can be in two places/states at the same time? As I've understood it, when these phenomena are observed, the object can only be at one place/state at that time.

When you flip a fair coin, and the coin is still in the air, you can't say whether it will come up heads or tails. All you can say is there's a 50% chance it will come up heads, and a 50% chance it will come up tails. You won't know which until the coin interacts with the ground and comes to rest in one of the two states: heads or tails.

Quantum mechanics seems to be telling us that many observable properties of objects are like that. Until the object interacts with something else (in one of the ways that physicists describe as an observation), the property exists as a probability distribution over the possible outcomes of observation, not as a single outcome.

 

[RussDill]

When you flip a fair coin, and the coin is still in the air, you can't say whether it will come up heads or tails. All you can say is there's a 50% chance it will come up heads, and a 50% chance it will come up tails. You won't know which until the coin interacts with the ground and comes to rest in one of the two states: heads or tails.

Quantum mechanics seems to be telling us that many observable properties of objects are like that. Until the object interacts with something else (in one of the ways that physicists describe as an observation), the property exists as a probability distribution over the possible outcomes of observation, not as a single outcome.

I like this, but it really doesn't give the impression of the depth of the effect. The coin flip we just don't have all the info. I don't think many people really understand the full scale until they come across and understand the consequences of Bell's inequalities.

 

[PixyMisa]

Read about the Elitzur-Vaidman quantum bomb tester. It can tell working bombs from duds by blowing up two thirds of the working bombs. The remaining bombs are all positively identified as good or bad.

It started out as one of those wacky thought experiments, but was actually built and successfully tested not long after.

 

[blobru]

Read about the Elitzur-Vaidman quantum bomb tester. It can tell working bombs from duds by blowing up two thirds of the working bombs. The remaining bombs are all positively identified as good or bad.

It started out as one of those wacky thought experiments, but was actually built and successfully tested not long after.

QM is kewl!

Btw, if anyone's interested, the math for that's the sum of a geometric series (which use a neat trick to work out):

From PM's wiki link: with each round of tests, 1/2 the working bombs blow up, 1/4 are identified, and 1/4 have to be retested (and 1/4 of that 1/4 are identified, 1/2 blow up, 1/4 retested, and so on). So,

ratio_identified_working_bombs = 1/4 + 1/4² + 1/4³...

[multiply everything by 1/4]

1/4 ratio_identified_working_bombs = 1/4² + 1/4³ + 1/4⁴...

[subtract top sum from bottom; everything cancels except]

3/4 ratio_identified_working_bombs = 1/4

ratio_identified_working_bombs = 4/12 = 1/3 (& 2/3 blown up, as quoted above).

 

[Roboramma]

My personal favourite exposition QM here: http://research.microsoft.com/apps/tools/tuva/
Specifically, the sixth lecture in that series of lectures by Richard Feynman, but I personally recommend viewing them all.

 

[PixyMisa]

QM is kewl!

I love the Elitzur-Vaidman quantum bomb tester for three reasons:

1. It's a proven practical use for some of the weirdest aspects of QM.

2. It solves a problem that has no answer in classical physics.

3. Both the name and the explanation sound like something out of a golden age SF novel. You can just hear our lantern-jawed scientist-hero saying to his mousy lab assistant (who will later be revealed as stunningly beautiful when she removes her glasses and lets her hair down)

Stella, we have to get these bombs tested before the alien invasion force arrives. Let's see. We'll need a Mach-Zehnder interferometer and single-photon light source - we can use the one from the quantum beer cooler we built for Mack's farewell party...

 

[Fredrik]

So what's with the parts of quantum mechanics that claims objects can be in two places/states at the same time? As I've understood it, when these phenomena are observed, the object can only be at one place/state at that time.

It's easy to to think that that this means that the system is in one of the states at all times, but that has been proved wrong by experiments. If the system is always in a state that can be the result of an experiment, then the results of a series of experiments must satisfy mathematical relationships called Bell inequalities. QM predicts that these Bell inequalities will be violated, and experiments have confirmed that prediction.

QM is kewl!
Btw, if anyone's interested, the math for that's the sum of a geometric series (which use a neat trick to work out):

From PM's wiki link: with each round of tests, 1/2 the working bombs blow up, 1/4 are identified, and 1/4 have to be retested (and 1/4 of that 1/4 are identified, 1/2 blow up, 1/4 retested, and so on). So,

ratio_identified_working_bombs = 1/4 + 1/4² + 1/4³...

[multiply everything by 1/4]

1/4 ratio_identified_working_bombs = 1/4² + 1/4³ + 1/4⁴...

[subtract top sum from bottom; everything cancels except]

3/4 ratio_identified_working_bombs = 1/4

ratio_identified_working_bombs = 4/12 = 1/3 (& 2/3 blown up, as quoted above).

Here's that trick for an arbitrary x with |x|<1.

[latex]$1+x+x^2+\cdots+x^n=\frac{(1+x+x^2+\cdots+x^n)(1-x)}{1-x}$[/latex]

[latex]$=\frac{1-x^n}{1-x}\rightarrow\frac{1}{1-x}$[/latex] when n→∞, if |x|<1.

So

[latex]$|x|<1\implies\sum_{k=0}^\infty x^k=\frac{1}{1-x}$[/latex]

 

[trvlr2]

A March 18 article in Scientific American discusses a macro world effect observed.
Apparently, a small tab of material can be in two states at once.
It might have heard the sound of one molecule clapping, or something like that.
Weird.

 

[Trent Wray]

Read about the Elitzur-Vaidman quantum bomb tester. It can tell working bombs from duds by blowing up two thirds of the working bombs. The remaining bombs are all positively identified as good or bad.

It started out as one of those wacky thought experiments, but was actually built and successfully tested not long after.

I've noticed most of your posts are "no" or "that's wrong".

This is my personal favorite of yours thus far, because it's some awesomely cool **** you link to . Thanx for that!

 

[blobru]

QM is kewl!

I love the Elitzur-Vaidman quantum bomb tester for three reasons:

1. It's a proven practical use for some of the weirdest aspects of QM.

2. It solves a problem that has no answer in classical physics.

3. Both the name and the explanation sound like something out of a golden age SF novel. You can just hear our lantern-jawed scientist-hero saying to his mousy lab assistant (who will later be revealed as stunningly beautiful when she removes her glasses and lets her hair down)

Stella, we have to get these bombs tested before the alien invasion force arrives. Let's see. We'll need a Mach-Zehnder interferometer and single-photon light source - we can use the one from the quantum beer cooler we built for Mack's farewell party...

Stella removed her glasses and let her hair down. "And after we've saved the Earth yet again, I can think of a few more super positions we can experiment with."

The professor looked up from his slide-rule with an exasperated sigh. "Stella, you know how I feel about females and their natural urges. They're unscientific. If you didn't make the best darn coffee this side of Aldebaran, I'd have replaced you with a mechano-assistant years ago. Now let's get back to work, shall we?"

Stella's wave function collapsed into a feminine frown. As she put her hair back up and replaced her glasses, she couldn't help but wonder if being carried off by a BEM was such a bad thing after all...

​

 

[PixyMisa]

I've noticed most of your posts are "no" or "that's wrong".

You need to find a thread where people are less wrong.

 

[Trent Wray]

You need to find a thread where people are less wrong.

Beautiful response

ETA: nominated btw

 

[sol invictus]

Read about the Elitzur-Vaidman quantum bomb tester. It can tell working bombs from duds by blowing up two thirds of the working bombs. The remaining bombs are all positively identified as good or bad.

It started out as one of those wacky thought experiments, but was actually built and successfully tested not long after.

Actually with a small modification (extra interferometer arms) you can accurately sort the bombs and only blow up an arbitrarily small fraction of the good ones.

It's completely crazy...