[hamdani yusuf (OP)]

You have asserted that it can travel backwards,

Which post?

Your post # 420 above.

In this post, I didn't assert that it can travel backwards in time. You only made that conclusion because you assumed that light was a point particle.

Pixel size has no influence on the distribution pattern, only on your perception of it.

It certainly has. Especially if you take it to the extreme, where one pixel is large enough to cover the whole area behind the slit.

[alancalverd]

I repeat, I have made no assumptions on the nature of light. I have used only the known fact that it travels from source to receptor at a finite speed.

[hamdani yusuf (OP)]

I repeat, I have made no assumptions on the nature of light. I have used only the known fact that it travels from source to receptor at a finite speed.

There's an implicit assumption that the receptor has no effect on the light source.

[alancalverd]

How can it, unless it transmits some signal to the source? What evidence is there for such a signal? How does it know where the source is? Or does every receptor radiate in all directions in the hope of finding a source? Does it continue to radiate in the absence of a source?

Beware of the "observer effect" - yet another myth of bad science writing.

[hamdani yusuf (OP)]

How can it, unless it transmits some signal to the source? What evidence is there for such a signal? How does it know where the source is? Or does every receptor radiate in all directions in the hope of finding a source? Does it continue to radiate in the absence of a source?

Beware of the "observer effect" - yet another myth of bad science writing.

What is the lowest frequency generated by an EM transmitter?
Why can't the receiver also transmits some sort of radiation which ends up being sensed by the transmitter?

[alancalverd]

There is no minimum frequency, though obviously f = 0 will not launch an em wave.

We receive emr from sources at the edge of the observable universe. Your question assumes that we launched our "request"  signal 14 billion years before the Big Bang, in the full knowledge of where we and each specific target would be, 28 billion years later. Including, of course, the precise date of the invention of the telescope.

[hamdani yusuf (OP)]

There is no minimum frequency, though obviously f = 0 will not launch an em wave.

It makes 0 the minimum frequency.

[hamdani yusuf (OP)]

We receive emr from sources at the edge of the observable universe. Your question assumes that we launched our "request"  signal 14 billion years before the Big Bang, in the full knowledge of where we and each specific target would be, 28 billion years later. Including, of course, the precise date of the invention of the telescope.

No. It's your assumptions.
No knowledge is required for electromagnetic interactions to occur. EM radiation spreads to all direction. It interferes (edit: superposes) with radiation from all other sources. When they are in the line of sight of each other, energy exchange can happen.

[alancalverd]

EM radiation spreads to all direction.

A surprising statement from a man who experiments with lasers.

It interferes with radiation from all other sources.

My x-ray measurements are unaffected by ambient light.

When they are in the line of sight of each other, energy exchange can happen.

Only if they remain in line of sight whilst the photons are in transit.

[hamdani yusuf (OP)]

A surprising statement from a man who experiments with lasers.

Radiation to other directions are cancelled due to destructive interference.

[hamdani yusuf (OP)]

My x-ray measurements are unaffected by ambient light.

That's because your detector isn't sensitive to ambient light.

[hamdani yusuf (OP)]

Only if they remain in line of sight whilst the photons are in transit.

Why won't they?

[alancalverd]

And we all thought it was something to do with Brewster mirrors.

[alancalverd]

That's because your detector isn't sensitive to ambient light.

But you said that all em radiation interferes with all the rest, i.e. nothing to do with the detector.

[hamdani yusuf (OP)]

That's because your detector isn't sensitive to ambient light.

But you said that all em radiation interferes with all the rest, i.e. nothing to do with the detector.

The more general term is superposition.
Interference should be reserved for a special case of superposition where spatial cancellation of the waves occurs. Although it can also mean different things in radio communications.

[alancalverd]

Now there's a fun thing. When we use two photon energies, as in dual-energy x-ray analysis (DEXA) there is no evidence of superposition: the result is independent of whether we vary the generating potential with time or use a broad spectrum and analyse the received spectrum continuously.

Radio interference, on the other hand, is definitely a matter of superposition.

[hamdani yusuf (OP)]

Now there's a fun thing. When we use two photon energies, as in dual-energy x-ray analysis (DEXA) there is no evidence of superposition: the result is independent of whether we vary the generating potential with time or use a broad spectrum and analyse the received spectrum continuously.

Radio interference, on the other hand, is definitely a matter of superposition.

Have you tried two radiation sources with similar frequency? Do they show something like beat phenomenon in sound?

[hamdani yusuf (OP)]

My conclusion thus far regarding the title is because we can't make a transmitter with less than one electron, and we can't make a receiver with less than one electron either.

[alancalverd]

Have you tried two radiation sources with similar frequency? Do they show something like beat phenomenon in sound?

Obviously possible, and frequently encountered in radio reception, but your original statement referred to all em radiation, not similar frequencies mixed in a receiver. I have no evidence of an x-ray fluorescent screen responding at all to 100 kHz radiation.

[alancalverd]

My conclusion thus far regarding the title is because we can't make a transmitter with less than one electron, and we can't make a receiver with less than one electron either.

The receiver is irrelevant. And if you accelerate a single electron continuously, say with a sinusoidal electric field, you won't generate quantised photons, just a continuous em wave.