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Author Topic: Why can radio waves pass through a solid wall but light cannot?  (Read 18030 times)

Offline Geezer

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Visible light is composed of photons. I understand that radio transmissions are composed of lower energy photons. A sheet of drywall can block visible light photons, but it has almost no effect on radio frequency photons. Can anyone help me understand why?
« Last Edit: 11/08/2009 22:14:33 by chris »


 

Offline daveshorts

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It is basically to do with whether an object can absorb energy at the frequency of the light. Visible light has a frequency in a range that atoms and molecules in solids interact with very strongly. There are lots of electrons which can absorb the energy of a visible light photon, so most things are opaque.

Where as radio waves have a very low frequency and to absorb them efficiently you need charges which can move at this frequency. Because this is low it means that to absorb significant amounts of energy the charges have to be able to move a long way - energy = force * distance if something is vibrating 1014 times a second this distance is going to be a lot shorter than 100 000 times a second, hence large pieces of conductor are very good at absorbing radio waves.

This is a pre-quantum mechanical way of thinking about it, but it all converts nicely into quantum mechanics if you do enough maths. A large piece of metal will have lots of energy levels in the radio part of the spectrum so it can absorb radio waves well, a lump of plaster doesn't.
 

lyner

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Metals have very mobile electrons, corresponding to very low energy transitions (movement at DC is possible, even) and little energy loss. This gives rise to reflection, as opposed to absorption of em waves.
It is important to bear in mind that the transitions are not between discrete levels but between and within bands because of the dense packing in solids. This means that the effects tend to be broad band and uniform. The H atom is not a good model to start with to understand what's going on here.
 

Offline Geezer

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Thanks to Dave and Sophie for taking time to answer my question. My training is in EE rather than Physics, so I tend to adopt a "wave" mindset rather than a "particle" mindset. Waves seem to work well in many situations, as do particles in others. There is the "duality" compromise, but that seems to be something of a "cop out".

These questions suggest (at least to me) that we are slightly clueless about the true nature of space. If space has no properties, then particles reign supreme. If space has properties then, perhaps, particles are merely a manifestation of energy interacting with space.

If we lack an adequate model to explain what space is, or is not, how do we expect to explain concepts like "Dark Matter" and "Dark Energy".

Based on almost zero scientific evidence on my part, I suspect we may be missing something rather important here.
 

lyner

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There is the "duality" compromise, but that seems to be something of a "cop out".
Hardly "a copout". It just shows that some things just can't be explained or understood using past ideas. As I keep pointing out, people are obsessed with  knowing what things 'really are'.  No chance. You can only do your best and plod in the right general direction. That, btw, is eminently worthwhile!
 

Offline Geezer

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Plodding may be OK for some, but I am running out of light years. I think it is important that we develop models that help us understand the nature of space.

Perhaps I should re-phrase my question:

Is space "something" or "nothing"?

If space is "nothing", then our understanding of interactions between objects in space cannot rely on space and must rely on something else, for example, particles.

If space is "something", then our understanding of interactions between objects in space can be influenced by space itself.

If we don't have a model that explains the behavior of space, how can we explain our universe?

 

lyner

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I think that no one looks on space as being "nothing"? The idea of a finite Universe precludes that!

Why should you find duality so unsatisfactory? Like you, I see that waves give a good model which predicts what will happen very well very often. The description using photons always consists of enormous fudges. I haven't come across a feasible idea of 'size' of the region of influence of one of these 'objects'. But, for many purposes, the little bullet is a good, arm waving, idea to work with - so I don't reject it.
Don't feel that you are missing out on things because you won't live long enough to know 'the truth'. It's half full - not half empty! Every new thing you find out about or  every problem solved is good.
 

Offline Geezer

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Would it be legitimate to describe EM radiation, including light, as energy propagating through space; the quantization effect being a function of space itself? No zero mass particles required!

I have a suspicion (based on not much!) that we are not actually going to find dark matter, at least not enough to account for the observed effects.

Interaction between matter and space produces the force we know as gravity. (I like to think of this as space pushing matter towards other matter rather than matter attracting other matter.)

Now, suppose for a moment that space, like most of us, works better when it's not too busy! (Sorry for the poor analogy). What I mean is that our measurements of gravity (the push effect of space) are based on our observations of objects in our general vicinity. But this is a "busy" space. There is a lot of matter in it. Could this large amount of matter/mass somehow attenuate the overall push effect?

As we move into interstellar space, there is much less matter and space is presumably much "smoother" (less distorted), so the push effect might be substantially greater.

At this point, the math defeats me, but there would probably be a "mass per unit volume" effect on gravitational force.
 

Offline Weldon02

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lyner

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Would it be legitimate to describe EM radiation, including light, as energy propagating through space; the quantization effect being a function of space itself? No zero mass particles required!

As a very sketchy description, it is not without some validity. I would agree that the particle idea (as opposed to the idea of quantisation of energy) seems an unnecessary step in understanding.
You have ignored the  electromagnetic bit, though and that must be included in any serious model, surely.
 

Offline Geezer

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Why can radio waves pass through a solid wall but light cannot?
« Reply #10 on: 17/08/2009 02:17:04 »
As a very sketchy description, it is not without some validity. I would agree that the particle idea (as opposed to the idea of quantisation of energy) seems an unnecessary step in understanding.
You have ignored the electromagnetic bit, though and that must be included in any serious model, surely.
[/quote]

Sketchy it is, I agree. However, I think the EM aspect is a bit of a bum steer.

We use EM to transmit and receive, lets call it QTE (quantized transmitted energy) at certain frequencies, usually called RF. We (typically) don't use EM to transmit and receive QTE at visible frequencies. A flame and an eyeball are all that's required. And yet, these are both manifestations of QTE, just at different frequencies, or, if you prefer, with different frequency photons.

Therefore, the EM aspect is simply a function of how we couple energy to, and recover energy from, space with "RF" signals. It may have nothing to do with the actual energy propagation method in space. What that is, and how it works, is a far bigger mystery.

Our thinking is ingrained with terms like EM, RF etc, because they work very well to describe many practical and theoretical observations, but they may also tend to obscure deeper insight. We get comfortable with what we are good at, me included.

BTW, you are probably aware of how Very Large Array radio telescopes work. The idea is that you can synthesize an enormous "virtual" radio telescope from an array of small radio telescopes. This does call for some tricky signal synchronization and recording methods, but it does seem to work very well.

Imagine if we could do the same thing at optical frequencies. We could build an enormous optical 'scope. But that's impossible, because visible light would have to be received by radio antennae. Turns out, there actually are nano-scale antennae that can receive visible light. It will probably be years before they could actually be used in a Visible Light Radio Telescope, which is just as well, because the data storage and synchronization problems are a big nightmare, not to mention amplifiers that can operate at those frequencies, but sooner or later, it might actually work. 

 

lyner

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Why can radio waves pass through a solid wall but light cannot?
« Reply #11 on: 17/08/2009 14:57:23 »
Most detectors of light are not coherent devices so you lose a lot of information which an RF interferometer doesn't. That is a nuisance if you want to use your idea for optical measurements.
You could imagine some sort of Laser / Mixer as the basic receiving element which would produce an output at a frequency that could be then signal processed.

But you can already produce partial large optical mirrors which can give you high resolving powers. Remember - the picture you get from any interferometer is not pretty and requires 'intelligent' use of you want to get proper sense out of it. You can't beat a large circular(ish) reflector for good pictures.
 

Offline Geezer

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Why can radio waves pass through a solid wall but light cannot?
« Reply #12 on: 17/08/2009 17:36:00 »
Most detectors of light are not coherent devices so you lose a lot of information which an RF interferometer doesn't. That is a nuisance if you want to use your idea for optical measurements.
You could imagine some sort of Laser / Mixer as the basic receiving element which would produce an output at a frequency that could be then signal processed.

But you can already produce partial large optical mirrors which can give you high resolving powers. Remember - the picture you get from any interferometer is not pretty and requires 'intelligent' use of you want to get proper sense out of it. You can't beat a large circular(ish) reflector for good pictures.

Perhaps the following can "shed some additional light" (sorry!) on the subject.

http://www.optics.rochester.edu/workgroups/novotny/antenna.html
 

lyner

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Why can radio waves pass through a solid wall but light cannot?
« Reply #13 on: 17/08/2009 21:48:21 »
Looks to me a lot like hologram technology but it's a bit brief. There are microwave equivalents, I think. It's passive stuff - not active, as far as I can see.
A novel approach, tho'.
 

Offline Geezer

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Why can radio waves pass through a solid wall but light cannot?
« Reply #14 on: 18/08/2009 00:08:43 »
Yes. It doesn't give away too much.

If I understand correctly, the telescope would "simply" be a radio telescope (albeit a very high frequency radio telescope) with an antenna capable of receiving visible light. And if it can be made to work with a single radio telescope, why not an array of radio telescopes? (Assuming certain "minor" technicalities are overcome!)

I'm likely indulging in over-creative extrapolation here, and the images produced from such a device could indeed be horrible, but at least it might form a basis for a science fiction novel.
 

lyner

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Why can radio waves pass through a solid wall but light cannot?
« Reply #15 on: 18/08/2009 22:37:06 »
It's a matter of coherence. A radio receiver responds to phase and amplitude of a wave. Most light detectors just respond to energy (just amplitude) so you can't add the outputs vectorially to get a 'pattern' as with a phased radio antenna array.
There isn't really a light frequency equivalent to an antenna - which is what the paper seems to be proposing. They just seem to be playing with words and I can't see why because the technique is novel enough not to require over-egging.
 

Offline Geezer

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Why can radio waves pass through a solid wall but light cannot?
« Reply #16 on: 18/08/2009 23:38:32 »
Perhaps I got it wrong, but I thought they were proposing antennae that do actually operate at light frequency in both phase and amplitude. I agree, amplitude alone won't hack it. I'll see if I can dig up anything else.
 

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Why can radio waves pass through a solid wall but light cannot?
« Reply #16 on: 18/08/2009 23:38:32 »

 

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