Cecil Moore wrote:


Yes, signals traveling in opposite directions don't interfere.



Yes, signals traveling in opposite directions don't interfere.



This is a distinction with no technical value. Waves in the same
location are subject to the usual rules of linear superposition of the
fields. Whether you want to call this "interference" is simply a
philosophical choice. There is a whole gamut of results resulting from
the superposition, ranging from zero field to a maximum of all the field
magnitudes combined. The terms "destructive" and "constructive" are
sometimes used to denote the extreme cases, but those terms are not so
well defined for the more intermediate cases.

There is utterly no scientific distinction that applies to "signals
traveling in opposite directions." The mathematical results may look
special in the opposite direction case, but the same basic equations
apply in all cases.

73,
Gene
W4SZ

Gene Fuller wrote:
Cecil Moore wrote:
Yes, signals traveling in opposite directions don't interfere.

This is a distinction with no technical value. Waves in the same
location are subject to the usual rules of linear superposition of the
fields. Whether you want to call this "interference" is simply a
philosophical choice.

Not so. Here's what Eugene Hecht says: "... optical
interference corresponds to the interaction of two
or more [plane] light waves yielding a resultant
irradiance that deviates from the sum of the component
irradiances."

Superposition can occur with or without interference. If
P1 and P2 are the power densities for two plane waves:

If Ptot = P1 + P2, there is no interference because the
resultant power density does not deviate from the sum of
the component power densities.

If Ptot P1 + P2, there exists interference because
the resultant irradiance does deviate from the sum of
the component power densities.

There is utterly no scientific distinction that applies to "signals
traveling in opposite directions."

Interference only occurs when coherent, collinear waves
are traveling in the same direction. When they are
traveling in opposite directions, standing waves are
the result. Let's limit our discussion to plane waves.

The mathematical results may look
special in the opposite direction case, but the same basic equations
apply in all cases.

Yes, but boundary conditions apply. The phasors of the plane
waves traveling toward each other are rotating in opposite
directions so interference is impossible. Here is a slide
show about interference which only occurs when the waves are
traveling in the same direction.

http://astro.gmu.edu/classes/a10594/...8/l08s025.html
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:
Gene Fuller wrote:
Cecil Moore wrote:
Yes, signals traveling in opposite directions don't interfere.

This is a distinction with no technical value. Waves in the same
location are subject to the usual rules of linear superposition of the
fields. Whether you want to call this "interference" is simply a
philosophical choice.

Not so. Here's what Eugene Hecht says: "... optical
interference corresponds to the interaction of two
or more [plane] light waves yielding a resultant
irradiance that deviates from the sum of the component
irradiances."

Superposition can occur with or without interference. If
P1 and P2 are the power densities for two plane waves:

Why do you attribute such magic to the word "interference"? Do you think
that Hecht's "interaction" is any different than superposition?

What if the waves are not quite anti-parallel, say at an angle of 179
degrees? Is interference now possible?

Suppose the waves are only 1 degree from parallel. Does that negate the
interference?

Repeating: This is a distinction with no technical value.

73,
Gene
W4SZ

Gene Fuller wrote:
Why do you attribute such magic to the word "interference"? Do you think
that Hecht's "interaction" is any different than superposition?

It is not magic. "Interference" and "superposition" simply
have different definitions.

Interference is a subset of superposition, i.e. interference
cannot occur without superposition but superposition can occur
without interference. This subject is covered in every optics
text that I have ever seen, including Born and Wolf. Given two
waves of equal power densities (irradiances) if the resultant
irradiance is not equal to the sum of the two irradiances, then
interference has occurred.

What if the waves are not quite anti-parallel, say at an angle of 179
degrees? Is interference now possible?

Impossible in a transmission line which is the context.
In free space, I would guess that interference is possible
in their common direction of travel.

Suppose the waves are only 1 degree from parallel. Does that negate the
interference?

For coherent waves in free space, that would ensure interference
until the beams diverged. It should result in the usual light
and dark interference rings.

Repeating: This is a distinction with no technical value.

Maybe it would help if you published a video of you waving
your hands as you scream that assertion at the top of your
lungs? :-)
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:
Gene Fuller wrote:
Why do you attribute such magic to the word "interference"? Do you
think that Hecht's "interaction" is any different than superposition?

It is not magic. "Interference" and "superposition" simply
have different definitions.

Interference is a subset of superposition, i.e. interference
cannot occur without superposition but superposition can occur
without interference. This subject is covered in every optics
text that I have ever seen, including Born and Wolf. Given two
waves of equal power densities (irradiances) if the resultant
irradiance is not equal to the sum of the two irradiances, then
interference has occurred.

What if the waves are not quite anti-parallel, say at an angle of 179
degrees? Is interference now possible?

Impossible in a transmission line which is the context.
In free space, I would guess that interference is possible
in their common direction of travel.

Suppose the waves are only 1 degree from parallel. Does that negate
the interference?

For coherent waves in free space, that would ensure interference
until the beams diverged. It should result in the usual light
and dark interference rings.

Repeating: This is a distinction with no technical value.

Maybe it would help if you published a video of you waving
your hands as you scream that assertion at the top of your
lungs? :-)

Cecil,

Many people, myself included, treat the term "interference" in a
qualitative manner. The general meaning is that two entities somehow
interact in a noticeable way, and the result has some signature of that
interaction.

You appear to use a very precise, quantitative definition of
"interference." I do not recall ever seeing such a quantitative
definition. Could you please give us a reference or an exact quote from
some reasonably reputable source that defines "interference" in a
quantitative and unambiguous manner?

You imply that some interactions lead to "interference" and some do not.
How can the unwashed among us know when the magic occurs and when it
does not?

73,
Gene
W4SZ

Gene Fuller wrote:

...
You imply that some interactions lead to "interference" and some do not.
How can the unwashed among us know when the magic occurs and when it
does not?

73,
Gene
W4SZ

You mean if I just wash it will increase my ability to understand? D*mn
man, I would NEVER have thought it possible. Indeed, if most were to
suggest that, I would laugh. But, given it is you, ... chuckle

And please, take this as a friendly joke! (albeit a poor one) I tire of
the religiously devout crying "blasphemy" and posting stones and
pitchforks! ROFLOL

Warm regards,
JS

Gene Fuller wrote:
You appear to use a very precise, quantitative definition of
"interference." I do not recall ever seeing such a quantitative
definition. Could you please give us a reference or an exact quote from
some reasonably reputable source that defines "interference" in a
quantitative and unambiguous manner?

I've already posted what Eugene Hecht said about interference.

In the irradiance (power density) equation,
Ptot = P1 + P2 + 2*SQRT(P1*P2)cos(A)
the last term is known as the "interference term", page 388 of
"Optics" by Hecht. Here's another reference:

http://en.wikipedia.org/wiki/Interference

A Google search for "electromagnetic wave interference" yielded
1,650,000 hits.

You imply that some interactions lead to "interference" and some do not.
How can the unwashed among us know when the magic occurs and when it
does not?

If the interference term in the above irradiance (power
density) equation is not zero, then interference is present.

In the s-parameter equation, b1 = s11*a1 + s12*a2, if b1
equals zero while s11, a1, s12, and a2 are not zero, then
total destructive interference is present.

Assume we superpose two coherent, collinear voltages, V1 and V2:

If (V1+V2)^2 V1^2+V2^2, then constructive interference is
present.

If (V1+V2)^2 V1^2+V2^2, then destructive interference is
present.
--
73, Cecil http://www.w5dxp.com