Heisenberg's Uncertainty Principle position and momentum

[jevillan]

Homework Statement

Heisenberg's uncertainty principle states that you cannot simultaneously know the position and momentum of an object. In this video link, a laser is aimed through a slot and projected onto a wall. As the slot gets narrower, the projection also gets narrower. Eventually, when the slot reaches a width of a hundredth of an inch, the projection gets WIDER instead of narrower. Can someone explain to me why this is, using the uncertainty principle?

Homework Equations

The Attempt at a Solution

i have no idea how the uncertainty principle relates to this laser beam example... :S

 

[Disconnected]

Huh, I've never seen it explained like that. What you're seeing is diffraction.
He is simply explaining diffraction in terms of the uncertainty principle.

He explains it quite well. If you lock a photon into being in a very small location, the uncertainty in x is very small. This means that the uncertainty in p must be large. but wait, lasers shoot light! Light always has the same speed, so how can the uncertainty in p be high? Does the mass change? But photons don't have mass!

Well, momentum is a vector, right? So it has direction. As the speed and mass is known, the uncertainty has to manifest itself as an uncertainty in the direction in which the photon travels. Hence instead of just having parallel light, you have light with lots of directions.

Make sense?

Personally, I would have thought about it the other way around, that diffraction agrees with the HUP, rather then thinking that it causes it. But I am not the chocolatiest cookie in the bag.

 

[phinds]

Heisenberg's uncertainty principle states that you cannot simultaneously know the position and momentum of an object.

Just to be sure you're clear, you have stated the HUP as though it applied to objects in general which is ridiculous. It applies only to subatomic particles. Figuring those things for a bowling ball has nothing to do with the HUP.

 

[darkxponent]

@phindsUB applies to all particles. even balls can be practically pose HUB at high momentum.

@jevillan: the experiment that this physicist is explaning is diffraction. it has nothing to do with HUB. it is simple diffraction. it can be explaned with the help of classical optics.

This Physicist is tryin to xplain HUB to a nursery student as he is giving a non practical xample. becoz in physics relating any xperiment with theory must verify the xperiment mathematically with the theory and optics verifies the diffraction mathematically. So its simple diffraction not HUB

One more VERY IMPORTANT POINT:

DIFFRACRION IS THE ONLY PHENOMINA OF LIGHT THAT CANT BE XPLAINED WITH THE PARTICLE PICTURE OF LIGHT
IT CAN ONLY BE XPLAINED WITH WAVE PICTURE(WAVE OPTICS).

AND HUB is applied to particles not waves
so if we agree with this physicist. we are accepting that particle picture of light can xplain diffraction

this means that wave picture of light is of no use...which means LIGHT IS A PARTICLE!

now u can clearly see why it can't be HUB...it is a result of pure wave property of light

AND this xplanation is nothing but ********...this physicist should first revise his basic concepts of physics and dual nature of matter and then jump to HUB

thats it
thank you

 

[phinds]

@phindsUB applies to all particles. even balls can be practically pose HUB at high momentum.

Hm ... I did not think that was the case, but in any event in practical terms it is hardly comparable in any reasonable way. By this I mean that for a single subatomic particle if you measure the position with very high accuracy, the momentum measurement according to the HUP is necessarily very low accuracy. On the other hand, if you measure the position of a bowling ball to a very high accuracy, you can still also measure, simultaneously, the momentum to a very high accuracy.

Do you not agree with this or is your statement (which DID say "high momentum") saying that it's not true for a bowling ball traveling at a substantial portion of the speed of light? I'm thinking of a bowling ball in more normal circumstances so it's possible that we are talking about two different things really.

 

[Clever-Name]

@darkxponent, is it really that difficult to put the e infront of experiment? It's mind-numbing seeing that every other word.

Now you also state that HUP is applied to particles and not waves, but you need to realize that particles ARE waves. And to say Light is a particle is also misleading. It is a photon, which is a wave-packet. It may have particle and wavelike properties but to call it one or the other is just misleading.

The physicist is Walter Lewin, a professor at MIT. While I'm not sure I would agree with his method of teaching HUP, to state that he needs to

first revise his basic concepts of physics and dual nature of matter

is unbelievably ignorant; Walter Lewin is a pedogogical god of teaching fundamental physics.

Diffraction CAN in fact be unified to both the particle and wave nature of light if you assume that light is a PHOTON and not a 'particle' or 'wave', it is both! Thus allowing wave-like properties AND particle-like properties as I have already stated.

Diffraction can also be easily defined as Interference, which is what was observed in the video as the light spot began to get wider, there were two bands of destructive interference on either side. Feynman said

"no-one has ever been able to define the difference between interference and diffraction satisfactorily. It is just a question of usage, and there is no specific, important physical difference between them

edit - Removed a link to a paper, I read it and laughed at some of the things they were saying.

Also, here's a link to something ZapperZ wrote about this very topic. He basically verbatim explains what Walter Lewin has:
http://physicsandphysicists.blogspot.com/2006/11/misconception-of-heisenberg-uncertainty.html

 

[darkxponent]

cant understand why people here are talking like children
why do you people read only theory in physics...to comment on any of these topics u must also know *HOW THAT THEORY WAS DERIVED*

i would like to xplain one thing
Light wasnt given dual nature by birth...physicist gave it dual nature(why?) -becoz not all properties of light can be xplained by wave properties and not all can be xplained by particle properies...so to make it easier physicists assumed that sometimes light is wave and sometimes it is particle

for example: for studing diffraction physicists called it wave and used all wave equations
and for studing atomic spectra they called it particle(photons)

and what light really is this is still not clear but we can study all properties of light(some thru particle some thru wave and some thru both)

even Einstien said in 1951 that "in 50 years of research i could not understant what is light"(i don't remember the original statement)

 

[Clever-Name]

Ok, I don't disagree with anything you are saying. That doesn't nullify anything of what I have previously said. However, to strictly state light is EITHER a wave OR a particle is WRONG. It is neither. We can quantify certain observations as wave-like and particle-like, but inherently light is too complex for our feeble minds to understand. Light is a 'quantum-particle' if you will; as are electrons, protons, neutrinos, etc. etc. They behave in a quantum sense. To put classical definitions on things is only a ploy to make it easier to understand.

And one more thing, dark, you say people are talking like children but all I have seen is well-mannered debate. You on the otherhand willingly spell things wrong and seem to be getting quite flustered when someone disagrees with you. Rather ironic when the only thing childish here are the format of your posts.

 

[darkxponent]

Ok, I don't disagree with anything you are saying. That doesn't nullify anything of what I have previously said. However, to strictly state light is EITHER a wave OR a particle is WRONG. It is neither. We can quantify certain observations as wave-like and particle-like, but inherently light is too complex for our feeble minds to understand. Light is a 'quantum-particle' if you will; as are electrons, protons, neutrinos, etc. etc. They behave in a quantum sense. To put classical definitions on things is only a ploy to make it easier to understand.

And one more thing, dark, you say people are talking like children but all I have seen is well-mannered debate. You on the otherhand willingly spell things wrong and seem to be getting quite flustered when someone disagrees with you. Rather ironic when the only thing childish here are the format of your posts.

okay i m just new to this physics site(infact any blogging site) so sorry if i am not followin the diisciplines of this website.

but i assure u i m not new to physics...and i do not comment on anything just for the sake of commenting

back to topic: LIGHT : there is no word wave like and particle like...see in xperiments we have to be mathematicall as well for the sake of quantitative analysis of the xperiment and for this we must consider light as pure particle or pure wave. (we can't take it as both)

regarding to what i commented on diffraction xperiment: i stick to my statement "only wave optics can xplain diffraction mathematically" and in real physics there must to be mathematical proof for a theory otherwise the theory remains physics fiction not physics...as maths is real logic and if something is not logicall it is not physics

the story told by Walter Lewin may sound interesting but it is not real physics(at least for me)

i will only and only believe it if and only if there is a mathematical proof

i don't believe peaple just becoz they have big names...i believe only those things which sound logically irrespective of who says it

 

[Clever-Name]

Since you are new I will forgive your ignorance. But you need to realize the more regular posters on this website (myself included) are educated in Physics. For my case I am half way through an undergrad degree in Theoretical Physics. For many others we have PhDs, Professors, grad students, engineers, and many other students. This board is filled with people who know what they are talking about, so don't assume everyone else is 'childish' just because you might not agree with their point of view. People are usually kind enough to state 'I am not an expert in ____ field' before making a comment on the point.

If you are so set on wave optics describing this phenomena then how can you explain that this still occurs when you do the same experiment with electrons. You CAN'T explain it unless you resort to quantum mechanics, and once in the realm of QM you cannot deny that the HUP has a heavy hand in the mechanics of what's going on.

It's ok to hold your own opinion. But when the monsters of physics say something contradictory to you then you need to rethink your point of view. It's different for one person to oppose you, but when the entire physics community opposes you then you need to change.

 

[darkxponent]

If you are so set on wave optics describing this phenomena then how can you explain that this still occurs when you do the same experiment with electrons. You CAN'T explain it unless you resort to quantum mechanics, and once in the realm of QM you cannot deny that the HUP has a heavy hand in the mechanics of what's going on.

case of electron is as simple as light...during diffraction it behaves like pure wave(it is not in dual state)

It's ok to hold your own opinion. But when the monsters of physics say something contradictory to you then you need to rethink your point of view. It's different for one person to oppose you, but when the entire physics community opposes you then you need to change.

i don't se the entire physics community opposin me...there is only one person(that physicist)...and i just opposed personell opinion of that physicist not opinion of whole physics world...i do believe most of the physicist...Newton einstien-all of them: as what they said they prooved.

no one(the entire physics community) explains diffraction usin HUP

 

[Clever-Name]

By saying that the electron isn't in a dual state just flies in the face of quantum mechanics.

I have to then ask this question. What qualifies you to hold such a strong opinion in this? Do you have a degree in physics?

 

[darkxponent]

By saying that the electron isn't in a dual state just flies in the face of quantum mechanics.

oh you don't try to teach me quantum mechanics

i want to ask you one question
can you xplain what feyman meant when he said "no one really understands quantum mechanics"??

this is QM bro. every person understands it diffrently...and i know even big physicists of 20th cent have diff opinions about it...you might have heard about Einstein and bohr

regarding to my xplanation on electrons diffraction-"this is my way of understanding it"

and will you please explain me that what is wrong in understanding it this way?

I have to then ask this question. What qualifies you to hold such a strong opinion in this? Do you have a degree in physics?

do anyone need to be phd to give an opinion...i think its all about thinking logically and fundamentally

and the answer of your question i don't have a degree and i don't feel shy sayin this

i joined this website to gain some and to give some...so if you are more qualified than me and you think i am sayin something wrong than you should try to explain it logocally...all you are doing is tryin to underestimate me

 

[Clever-Name]

When Feynman said 'No-one really understand quantum mechanics' he meant that the underlying mechanism behind the WHY of what we observe is impossible to truly grasp. That's where the "Shut up and calculate" viewpoints comes into play. We can use quantum mechanics and we can get expected results based on the established theory, but the mechanism behind WHY it works that way is what people can't understand and that's why Feynman said what he said.

There's a difference in saying "This is how i understand it" and saying "no-one understands quantum mechanics" and just because no-one understands quantum mechanics doesn't mean every interpretation is equally valid.

I asked the question of your credentials because it sounds to me like you don't understand the first thing about quantum phenomena. It's hard to sound reputable on this website when you don't know the first thing about what you're blabbering about.

If you have a hard time grasping the HUP then that's something you should outright say and then myself and others can help rectify your misunderstanding. HUP is not all of quantum mechanics, people DO understand its implications and DO know how to teach it to you.

I think your explanation is wrong because you are trying to force light into one of two categories, particle or wave, in this case wave. You are trying to solely treat light as though it is a wave and thus only use wave optics to describe its motion. TBH no-one really knows what a photon is, it's one of those fuzzy "no-one understands" bits about quantum mechanics. But we do know that it is a quantum particle, not a classical particle, not a classical wave, it is a quantum particle and quantum particles need QUANTUM mechanics to describe it, not classical wave optics.

Quantum particles have wave-like and particle-like properties, but in no instance is it solely one or the other. That's one of the interesting things about quantum mechanics. You need to treat the particle as a wave-function and describe its motion with the Schrodinger Equation with conditions based on the system you're looking at. You cannot at any point collapse it to a simple wave, treat it with wave optics, then treat it again as a wave-function and continue on with your analysis.

 

[darkxponent]

okay i don't disagree you

but i said my way of understanding(about the electron) doesn't mean i just believe it blindly

i can prove it mathematically as well and you do know i can do this...and not only me many can.