[Geezer (OP)]

Would it be possible to run a version of the double-slit experiment http://en.wikipedia.org/wiki/Double-slit_experiment using much lower frequencies than visible RF, perhaps in the microwave range?

If so, would we expect to observe the same results that we observe with visible RF?

[yor_on]

" The way to show wave nature of light is with interference experiments (ie double slit). This would actually be much easier to do with radio waves, given their longer wavelength.

The particle nature you can show via quantization, as in the photoelectric effect. So you'd need an absorber of radio waves. It would be very difficult to perform, especially with the cosmic microwave background radiation cuddling  everything up.

The second is defined as a certain number of oscillations of a particular transition in a cesium atom. So if you stimulate that transition, and with very sensitive detectors show that the radiation emitted is quantized, then you're done."

This document describes experiments done with light and microwaves and how it brings the students to the conclusion that both are the same differing only in wavelength/frequency. Light and Electromagnetic Waves Teaching in Engineering Education

And here is another experiment with interference patterns contra 'particles' that to me seem too point to Feynman's 'many paths' ( That light goes all possible 'ways' simultaneously but with different probability, 4-real to explain why it acts like it does. single photons two ways.

[Geezer (OP)]

This would actually be much easier to do with radio waves, given their longer wavelength.

Cool! Has anyone actually done it?

[lightarrow]

Would it be possible to run a version of the double-slit experiment http://en.wikipedia.org/wiki/Double-slit_experiment using much lower frequencies than visible RF, perhaps in the microwave range?
If so, would we expect to observe the same results that we observe with visible RF?

Yes. Yes.
http://phoenix.phys.clemson.edu/labs/224/diffraction/index.html
http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/measuring-the-speed-of-light/

[yor_on]

Ah, lightarrow found them

When I looked I didn't find any such experiment with really long wavelengths, I thought it might be due to that as you increase the wavelength also will increase the distance between the fringes? (fringe=One of the light or dark bands produced by the interference and diffraction of light).

But I did find this proposal which seems really interesting.
A Cosmic-Scale Double-Slit Experiment

Searching I found this really nice description of waves too for those of you, like me that like to have a little  'something' to look at when discussing.

As for my comment yesterday about my last link, shoving support for 'many paths'. The support is in 'Shock # 1:'  being presented there, not in 'Shock # 2:' that can be discussed as it is not exactly true as presented.

"It is a widespread misunderstanding that, when two slits are open but a detector is added to the experiment to determine which slit a photon has passed through, then the interference pattern no longer forms and the experimental apparatus yields two simple patterns, one from each slit, superposed without interference. Such a result would be obtained only if the results of two experiments were superposed in which either one or the other slit is closed. However, there are many other methods to determine whether a photon passed through a slit, for instance by placing an atom at the position of each slit and monitoring whether one of these atoms is influenced by a photon passing it. In general in such experiments the interference pattern will be changed but not be completely wiped out. Interesting experiments of this latter kind have been performed with photons[7] and with neutrons.[8]"

Double-slit_experiment

[Geezer (OP)]

Thanks for the refs Yor_on and Lightarrow. I'll look at them next year []

[LeeE]

I actually remember doing a practical microwave diffraction experiment at school - we used what looked like a section of birdcage i.e. vertical steel bars spaced about 1 cm apart for our diffraction grating.  We also used a lens made out of paraffin wax for microwave refraction too, iirc.

[Soul Surfer]

Interference and diffraction effects happen for all electromagnetic radiations at all frequencies.  However if you are talking about observing the non intuitive quantum effects these are only observable for frequencies at which individual quanta can be detected which requires at least infra red frequencies although the quantum effects would still occur at low frequencies.

[Geezer (OP)]

Interference and diffraction effects happen for all electromagnetic radiations at all frequencies.  However if you are talking about observing the non intuitive quantum effects these are only observable for frequencies at which individual quanta can be detected which requires at least infra red frequencies although the quantum effects would still occur at low frequencies.

Oh! Yes, I was referring to particle like behaviour. Is this limitation simply because we don't have photon detectors that work at lower frequencies?

[Soul Surfer]

You must remember that the energy of an electromagnetic photon is planck's constant multiplied by the frequency of the photon (h x nu) so as the frequencies get lower the energy of individual photons get smaller.  this makes it more difficult to detect low frequency photons individually.

The non intuitive quantum process involves observing that individual photons passing through the apparatus are subject to interference and diffraction that is an individual photon is always in some way an extended object.

I am surprised that so many people are fixated in some way on this experiment because quantum mechanics is clearly a non local process.  In some ways I think that the converse experiment with matter is far more surprising. 

ALL material objects like electrons have a wavelength that depends on their momentum in the frame of reference of the experiment  that is the slower the particles move, the longer their wavelength.  They are therefore also subject to interference and diffraction and individual electrons passing through an apparatus will generate interference patterns. 

Even more counter-intuitive this process applies to all individual bodies like human beings (and schrodinger's cat) however as the bodies get more massive the wavelengths are exceedingly short and the effective diffraction and interference patterns undetectably small.  However this experiment has been performed using quite large complex molecules like buckyballs

[litespeed]

SS

You seem well versed in this subject. Perhaps you would comment on something that has always perplexed me. Do photons have dimensions? This perplexes me because photons vary in wave length by orders of magnitude.

My GUESS is that photons do not have dimension, but leave a variety of measurable wave lengths in the electomagnetic field as they pass.

[Soul Surfer]

To be totally pedantic quantum mechanics implies that all the particles in the universe, that is both massive particles like protons and electrons or massless like photons, exist to a certain extent throughout the entire universe, because all the integrals that define the properties of a particle extend to infinity.

The "size" of a particle could be considered as the range of distance over which it was reasonably probable that something  (another particle or an experimental structure) could interact with this particle.

This size is a function of the particle's "wavelength" when considered as a wave, that is a few wavelengths.  The precise number of wavelengths depends on how long you are prepared to spend looking for it  The longer you spend looking the "bigger" the particle gets!

remember also the wavelength of matter particles like electrons and protons gets shorter the more massive the particle is and the slower it is moving.

Electrons have a low mass and therefore have quite long wavelengths.  This is why they exist in orbitals which extend a considerable distance from the nucleus and defines how big atoms are.

Protons and neutrons  are much more massive and interacting with very much higher energies in the nucleus and that makes a nucleus small.

So for massive particles the lighter the particle and the slower it is moving the "bigger" it gets. 

For very slow moving atoms in extremely cold "Bose Einstein condensations"  atoms can become so "big" that they are in effect visible to the naked eye!


Let me point out though that this increase in "size" does not mean that the atom itself has changed in any way. it is just like Schrodinger's cat which is simultaneously alive and dead at the same time the atom exists everywhere in a large volume of space with a reasonably high degree of probability and is linked with many other atoms and cannot be distinguished from them it is a total quantum entity.

[litespeed]

SS

I must confess a particular hostility to Schrodinger's cat. I think it was a condescending analogy, at best. However, a photon traveling through space leaves behind it an unambiguous residue of wave length. I find this oddly in sync with string theory. Specifically, the wave length is perfectly measurable, but is the residual product of something that has no size; it only has resonance that is left in its wake.

For instance. We talk of electromagnetic waves that measure peak to valley in METERS.  The thing that left that wave has nothing more then an intrinsic resonance of indeterminate size, and certainly not one meter long. Further, the 'item' that left the meter sized wave in its wake does not seem to have been reduced in energy at all.

One important question is how the PHOTON is affected AFTER its residual wave has been absorbed by the appropriate antenna. The antenna can only absorb the energy if it is tuned to the resonant length frequency. Yet that frequency can only be absorbed AFTER it is created in space time once the photon has passed. Accordingly, once the wave has been created, absorbed by the antenna, the photon must subsequently dissipate. 

Sounds once again to me like "spooky action at a distance". To quote Albert. Accordingly, it seems to me that photons are firmly anchored to space time and can be destroyed and converted to electromechanical energy AFTER they have crossed the appropriate length antenna.

This sounds to me very much the converse to entangled particles.

[Soul Surfer]

I do not understand properly what you are trying to say.  You do not seem to have the correct mental image of what is going on. 

You seem to be obsessed with the concept of a point particle.

Photons don't leave electromagnetic waves in their tracks.  The electromagnetic waves ARE the photons.   In the case of radio waves vast numbers of coherent photons.  The best way of thinking of photons is as wave packets of energy which start small build to a peak and die away as they pass.  A bit like sets of waves in the ocean.

As I pointed out above all photons and particles do have a physical size and radio wave photons can be very large indeed with the tiny energy of the photon effectively spread over volumes greater than one kilometer. 

The concept of a point particle like an electron is not that the particle is a mathematical point it is just that to the current detail of our measurements the particle is structureless in that its charge behaves like a physical point but it clearly has structure in the form of its spin but the details of this are only covered by the many billions of string theories from which we have yet to refine a complete model

[lightarrow]

SS

You seem well versed in this subject. Perhaps you would comment on something that has always perplexed me. Do photons have dimensions? This perplexes me because photons vary in wave length by orders of magnitude.

My GUESS is that photons do not have dimension, but leave a variety of measurable wave lengths in the electomagnetic field as they pass.

There isn't any valid model of photon, at the moment, so it's meaningless to talk about its dimensions (meaningless in the sense I specified). No one can prove that photons are point-like and no one can disprove that they are extended objects. Maybe they don't even exist, as bodies flying from source to detector. What we know about photons is related to how a source of light or a light detector gives away/absorbs energy and momentum.
(Before you could reply about momentum I remind you that classical electromagnetic waves have momentum too).

[Soul Surfer]

I do not think that you are being very sensible light arrow.

How do you define the size of an object?

The only way I can understand it is that it is the volume in which other objects can interact with it.  If you define size in this way all particles have clearly defined sizes.  these may well vary under different experimental conditions but that is quite reasonable.  the sizes that I have been defining are those well established by experimental science and nothing to do with any theory.

[yor_on]

Soul Surfer, I didn't know why, but you got me confused when you wrote that you could see an atom in a Bose Einstein condensation.

"At extremely low temperatures or at small size scales, on the other hand, the usefulness of classical mechanics begins to wane. The crisp analogy of atoms as Ping-Pong balls begins to blur. We cannot know the exact position of each atom, which is better thought of as a blurry spot. This spot-known as a wave packet-is the region of space in which we can expect to find the atom. As a collection of atoms becomes colder, the size of each wave packet grows. As long as each wave packet is spatially separated from the others, it is possible, at least in principle, to tell atoms apart.

When the temperature becomes sufficiently low, however, each atom's wave packet begins to overlap with those of neighbouring atoms. When this happens, the atoms "Bose - condense" into the lowest possible energy state, and the wave packets coalesce into a single, macroscopic packet. The atoms undergo a quantum identity crisis: we can no longer distinguish one atom from another."

So in a way you might call that a 'super atom' but seen from another perspective you don't have 'atoms' any more. You have a geometrically constricted 'fog' without any specific atoms visible.
Bose Condensate described

As for "The electromagnetic waves ARE the photons." Maybe, in a way, just as the 'super atom' can be said to be a lot of atoms interacting under a very specific condition joining into a new inseparable state containing new properties. But there is a particle/wave duality existing that hasn't been explained, even though there are yet unproven theory's trying to do so, like string theory that you mention.
Atomic Particles, Photons and Heisenberg’s Uncertainty Principle (HUP)

----Quote from it--

"These experimental observations were in direct opposition to those expected for a wave theory of radiation. In wave theory, no threshold energy would be required for photoelectron release. A wave with low energy would simply operate long enough to contribute sufficient energy to cause the electron to be ionized. The kinetic energy of the photoelectrons would be expected to increase with the intensity of the radiation “waves”.

Another effect that the wave theory of radiation cannot explain is the transmission of the Sun’s rays through what is virtually a perfect vacuum between the star and the Earth in which there is nothing in which waves can form and carry the transmitted energy, unlike that which occurs in the oceans."

--end of quote..


Then you state "As I pointed out above all photons and particles do have a physical size and radio wave photons can be very large indeed with the tiny energy of the photon effectively spread over volumes greater than one kilometer. " Seen as a wave this is perfectly correct, as HUP comes into play.

As for what decides a photons wavelength, it's called the 'wave vector'. And the definition of that is mostly applicable to monochromatic light (Lasers)and is a generalization of a vector representation of a wave. A theoretical model in other words neither proving nor disproving it as the sole fact.

[lightarrow]

I do not think that you are being very sensible light arrow.

How do you define the size of an object?

The only way I can understand it is that it is the volume in which other objects can interact with it. 

And so how would you define a photon's size?

[litespeed]

SS - Thanks for your forbearance! I am trying to work this out, really I am

OK. Lets talk about a single, very long radio wave photon that is absorbed by a an appropriate antenna. 1) Will a single such photon have enough energy to be converted into a single, corresponding; small electric current?

If so, at some point the entire photon is converted instantaneously to something else. However, as you point out, a single photon could be miles long. So. Does it make any sense to even discuss whether the entire photon must reach the antenna first, or that it can be converted as soon as the front of the 'wave' interacts with the antenna?

[Geezer (OP)]

(HUP)
----Quote from it--


"Another effect that the wave theory of radiation cannot explain is the transmission of the Sun’s rays through what is virtually a perfect vacuum between the star and the Earth in which there is nothing in which waves can form and carry the transmitted energy, unlike that which occurs in the oceans."

Yor_on - Perhaps you can help me understand something that I seem to get stuck on all the time.

Space is described as a vacuum (not made by Electrolux of course []) and, therefore, waves cannot form in something that has no "substance". (So far so good.)

However, it also seems to be well accepted that mass/matter "distorts" space to produce gravity.

So, why would photons not just be another aspect of the distortion of space? In that case, photons might merely be energy propagating through space in some manner that we don't understand. We clearly don't know what makes gravity work, so why would photons be any different?

We also know that matter and energy are somewhat convertible, and we have vivid demonstrations that prove this. So, why would all matter not simply be energy that is somehow "locked up" in space? In other words, all matter is just another aspect of space itself.

Or did I just reinvent string theory?