A new telescope can detect a candle across the Atlantic

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Discussion Overview

The discussion revolves around the capabilities of a new telescope that reportedly can detect a candle across the Atlantic. Participants explore the underlying physics, including the nature of photons, electromagnetic fields, and the implications of quantum mechanics in this context. The conversation touches on theoretical aspects, practical applications, and the relationship between classical and quantum descriptions of light.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant calculates the photon density at the telescope and raises questions about the area covered by a photon and the nature of photon wavetrains.
  • Another participant suggests that photons can be treated as classical electromagnetic waves until they reach the detector, which is typically small and requires quantum mechanical considerations for accurate measurement.
  • A different viewpoint argues against the need to 'pretend' photons are classical waves, stating that the electromagnetic field can be measured directly and that Bohm's interpretation suggests no collapse of the field into particles.
  • One participant emphasizes that classical equations can effectively describe systems involving multiple photons, although quantum corrections may apply, and that the interpretation of these equations is a mathematical rather than philosophical issue.
  • A participant expresses frustration over the discrepancy between classical calculations and observed phenomena in the photoelectric effect, questioning how energy can trigger atomic excitation almost instantaneously despite classical predictions of much longer timescales.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between classical and quantum descriptions of light, with some advocating for the utility of classical models in practical applications, while others challenge this perspective based on quantum mechanics. The discussion remains unresolved regarding the implications of these differing interpretations.

Contextual Notes

Participants highlight limitations in classical calculations when applied to quantum phenomena, such as the photoelectric effect, and note that assumptions about the nature of photons and electromagnetic fields may vary among interpretations.

Who May Find This Useful

This discussion may be of interest to those studying optics, quantum mechanics, and the interplay between classical and quantum theories in the context of light and electromagnetic fields.

map19
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Publicity often says that a new telescope can detect a candle across the Atlantic, etc.
Well, a candlepower is defined as 1/683 Watts at 540 X 10^9Hz.
This gives photon energy at 2.2 eV.
One candlepower produces 4.2 x 10^15 photons/s radiated isotropically.
At 3,000km the area of the sphere is about 10^14m^2
So the photon density at the telescope is 4.2 x 10^15/10^14 or about 4 photons per sq meter per second.
Two things to note:
The associated electromagnetic field would be very small, but not zero.
A second is a long time in quantum matters.
My questions are: How do you demonstrate the area covered by a photon at the telescope mirror ?
How long is the photon wavetrain ?
(I can’t believe I’m asking this) does the field collapse to the photon particle ?
Is this like the photoelectric effect where a classical calc shows that 10 minutes or so would be required to dislodge the electron, where in fact it occurs in about 10^-9s.
 
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The "practical" answer is that you can "pretend" that your photon is a classical EM wave up until it hits the actual detector (which in real-life is always a farily small device; usually just microns in size); i.e the mirror/antenna or whatever you are using (depends on the wavelength) to focus the radiation is just designed using standard optics/engineering and only the bolometer (or whatever type of detector is used) needs to be designed using QM.
 


No need to 'pretend' you can measure the electric and magnetic fields to show it is an EM field.
Bohm has demonstrated that there is no 'collapse' and his ontological description of boson fields does not imply boson particles, such as photons.
for an electron to be raised to an excited state by the field by interaction, the whole system - field plus atom - enters a new state in which the quantum of energy has gone into the atom. Somehow the quantum of energy has been gleaned from the whole field.
The field has continuous coverage and you can not specify that it is in packets before it reaches the atom.
That's why I gave the example of the photoelectric effect.
 


That is not what I meant.

My point was that even for a single photons the classical EM equations seems to work well when designing antennas and optics. Note that this is not a "trivial" result; a full QM (QED)description of e.g. a waveguide will give you a result that is in general NOT identical to what you expect from classical physics.
However, for a number state it turns out that the classical equations and QM agree (although there might be some higher order corrections; I don't remember the details), which is why we can use our normal "toolbox" of techniques and equations for just about everything except the detector itself.

What interpretation you use is irrelevant; that the classical equations are a good approximation (or in some cases identical) to the full QM model is a mathematical -not a philosophical- result.
 


Agreed. But this still leaves me frustrated.
I want to know why, let's use the photoelectric effect, energy from the field can appear within the orbit of an atom and trigger excitation within a nanosecond, when the classical calc says that 10 minutes of continuous exposure would be required at the quoted field strength.
And, in general, if you quote photons only, see QED by Feynman, how long is the wavetrain. it has to be at least a few cycles.
 

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