Diffraction

[DParlevliet]

How does Quantum mechanics explain diffraction?

If light goes throught a single slit it is diffracted, the waves bent in the slit. In classical waves this is caused by the carrier, which consists of particles which radiate spherical. But a EM-wave has no carrier. So why does the wave bends in exactly the same way as classical waves?

[swansont]

E-M waves are still waves.

[Strange]

How does Quantum mechanics explain diffraction?

Feynman's lectures on QED explain this: http://vega.org.uk/video/subseries/8

Also available as a book.

 

 

 

If light goes throught a single slit it is diffracted, the waves bent in the slit. In classical waves this is caused by the carrier, which consists of particles which radiate spherical. But a EM-wave has no carrier. So why does the wave bends in exactly the same way as classical waves?

I don't think it is anything to do with the medium (is it?)

Edited October 21, 2013 by Strange

[DParlevliet]

Formula of E-M waves does not predict diffraction

 

Feynman is a method of calculation, based on the assumption of diffraction. It does not explain why.

 

EM-waves has no medium

[swansont]

Formula of E-M waves does not predict diffraction

 

 

What?

 

If you mean the wave equation doesn't, then technically yes, that's true: the wave equation is for free space. But that's not what I said. Diffraction is a prediction of wave behavior when the wave is incident on an obstacle.

[DParlevliet]

That is what I wanted to know.

What is the formula which predict this?

On what physical effect is that based?

[swansont]

It stems from Huygens principle

 

http://www.math.ubc.ca/~cass/courses/m309-03a/m309-projects/krzak/

[Strange]

Feynman is a method of calculation, based on the assumption of diffraction. It does not explain why.

 

Feynman has something to say about that in his lectures. I'm not sure "why" is really in the scope of physics. If you can answer "why" at one level, you can always just ask "why" again about the answer. It is a never-ending rabbit hole.

[DParlevliet]

It stems from Huygens principle

 

http://www.math.ubc.ca/~cass/courses/m309-03a/m309-projects/krzak/

Huygens is calculated on spherical radiating particles of the medium. But QM waves are no supposed to have a medium.

[Strange]

Huygens is calculated on spherical radiating particles of the medium. But QM waves are no supposed to have a medium.

It doesn't say anything about the medium: http://www.mathpages.com/home/kmath242/kmath242.htm

 

(LIke QED, it is a mathematical abstraction that works.)

Edited October 21, 2013 by Strange

[swansont]

Huygens principle doesn't rely on a medium to work. Its existence in discussion is likely an artifact of the age, when a medium was assumed.

[DParlevliet]

Huygens principle doesn't rely on a medium to work. Its existence in discussion is likely an artifact of the age, when a medium was assumed.

 

So it is calculated with a medium. But when it was proven that there was no medium, the way he prooved and calculated it is wrong. Of course the outcome is still right, but now it is only a description, not an explanation.

 

For classical waves with a medium Huygens principle is still right and calculated right (an interference pattern from sferical; radiating particles)

[swansont]

 

So it is calculated with a medium. But when it was proven that there was no medium, the way he prooved and calculated it is wrong. Of course the outcome is still right, but now it is only a description, not an explanation.

 

For classical waves with a medium Huygens principle is still right and calculated right (an interference pattern from sferical; radiating particles)

 

The principle still applies. It did/does not depend on there being a medium.

[Strange]

 

So it is calculated with a medium. But when it was proven that there was no medium, the way he prooved and calculated it is wrong.

 

It works. That's all you can ask of physics, really.

[DParlevliet]

Several Wiki:

"In 1678, Huygens^[1] proposed that every point to which a luminous disturbance reaches becomes a source of a spherical wave"

 

"According to the wave theory of light proposed by Christiaan Huygens, light travelled in the form of longitudinal waves via an omnipresent, perfectly elastic medium having zero density, called aether"

 

QM: "Huygens' principle can be seen as a consequence of the isotropy of space—all directions in space are equal. Any disturbance created in a sufficiently small region of isotropic space (or in an isotropic medium) propagates from that region in all radial directions. The waves created by this disturbance, in turn, create disturbances in other regions, and so on. The superposition of all the waves results in the observed pattern of wave propagation"

 

"Einstein himself noted that his own model which replaced these theories could itself be thought of as an aether, as it implied that the empty space between objects had its own physical properties"

 

So Huygens certainly calculated his principle on the existance of a medium. Even the QM explanation implies some medium. In all cases not a classical medium of matter is ment, like the earlier ether.

[Sensei]

How does Quantum mechanics explain diffraction?

If light goes throught a single slit it is diffracted, the waves bent in the slit. In classical waves this is caused by the carrier, which consists of particles which radiate spherical. But a EM-wave has no carrier. So why does the wave bends in exactly the same way as classical waves?

 

Slit for us is smooth and thin, but after large zooming it's more like tunnel with mountains (this can be seen in any microscope).

Photon traveling close to the edge of slit is colliding with one or more "mountains" and is reflected.

 

Similar thing happens when you shine laser pointer on wall- regardless of angle at which you look at wall, you see laser dot. How? Photons colliding with microscopic impurities in the wall are reflected in the all directions. From one steady beam of photons, they're spread everywhere.

 

Modern military airplanes and u-boats are using similar impurities on the body to be undetectable by radio and sound waves. They are reflecting waves in the all directions, instead of like smooth body sending them back to radar.

[DParlevliet]

Slit for us is smooth and thin, but after large zooming it's more like tunnel with mountains (this can be seen in any microscope).

Photon traveling close to the edge of slit is colliding with one or more "mountains" and is reflected.

 

 

 

I don't think so. Otherwise the diffraction would depend on the roughness of the edge. When you make is very smooth, there will be hardly diffraction. Also only a small part of the wave, at the slit edges, does diffract while the main part in the middle would pass without diffraction.

According Huygens principle and calculation (and light reacts in that way) the whole surface of the slit diffracts, not only the edges.

[swansont]

I don't think so. Otherwise the diffraction would depend on the roughness of the edge.

I agree

 

Also only a small part of the wave, at the slit edges, does diffract while the main part in the middle would pass without diffraction.

Are you agreeing with this, or with Huygens?

[DParlevliet]

Of course I agree with Huygens. But as referenced above, Huygens calculated his principle with a medium (ether) in which every point becomes a source of a spherical wave. Later on it was proven that no ether exist. But if a QM wave acts exactly according Huygens principle, then also in vacuum every point acts as a source of a spherical wave. Why? that was the question of this topic.

[swansont]

Of course I agree with Huygens. But as referenced above, Huygens calculated his principle with a medium (ether) in which every point becomes a source of a spherical wave. Later on it was proven that no ether exist. But if a QM wave acts exactly according Huygens principle, then also in vacuum every point acts as a source of a spherical wave. Why? that was the question of this topic.

No, that wasn't the question you originally asked. The answer to that one remains Huygens' principle, which still does not depend on there being a medium. It's a property of the waves, not of the medium.

[Strange]

Note that the mathematics associated with Huygens' principle does not include any parameters relating to the medium. So even if he conceived of it happening in a medium, he did not need to take that medium into account.

 

It is almost as if the medium isn't there.

[DParlevliet]

No, that wasn't the question you originally asked. The answer to that one remains Huygens' principle, which still does not depend on there being a medium. It's a property of the waves, not of the medium.

 

See again my wiki quote: "According to the wave theory of light proposed by Christiaan Huygens, light travelled in the form of longitudinal waves via an omnipresent, perfectly elastic medium having zero density, called aether"

 

The formule of QM waves does not include diffraction.

[Strange]

The formule of QM waves does not include diffraction.

 

No, but like many things it uses a different route to get the same result. (Rather like GR doesn't treat gravity as a force but Newton does.)

 

Either way, there is no need for a medium.

[swansont]

See again my wiki quote: "According to the wave theory of light proposed by Christiaan Huygens, light travelled in the form of longitudinal waves via an omnipresent, perfectly elastic medium having zero density, called aether"

Mentions ether ≠ relies on ether

 

The formule of QM waves does not include diffraction.

You say this as if it means something. Diffraction is an effect, which you determine by applying boundary conditions to the wave equation, just like the rest of quantum mechanics.

[DParlevliet]

You say this as if it means something. Diffraction is an effect, which you determine by applying boundary conditions to the wave equation, just like the rest of quantum mechanics.

 

Which wave equation? How are bounderies applied in this equation?