Is Wavelength the Only Measure of Light's Size?

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Discussion Overview

The discussion centers around the concept of whether light has a size, dimensions, or occupies space, particularly in relation to its wavelength. Participants explore the implications of measuring wavelength and its connection to the physical properties of light, as well as drawing parallels to sound waves.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants assert that light does have a size and dimensions, while others question this notion, suggesting that a single photon does not have a well-defined size due to quantum mechanics.
  • One participant mentions that the position operator for modes of the electromagnetic field lacks an eigenvalue, implying that the spatial localization of a single photon is not well-defined.
  • It is proposed that while a single photon may be considered to have an infinite size, spatial localization occurs when large numbers of photons are involved.
  • There is a philosophical inquiry into whether measuring wavelength is equivalent to measuring light itself, with suggestions that it relates to reproducible measurements of physical properties rather than light's intrinsic characteristics.
  • Participants discuss the analogy of sound, noting that while wavelength provides information about the distance between peaks, it does not encompass the full characterization of a wave, including its intensity or direction.

Areas of Agreement / Disagreement

Participants express differing views on whether light has a size, with some asserting it does and others arguing against it. The discussion remains unresolved regarding the implications of measuring wavelength and its relationship to the concept of size in light.

Contextual Notes

The discussion highlights limitations in defining size and spatial extent in the context of quantum mechanics and the measurement of waves, indicating that assumptions about the nature of light and sound may vary among participants.

swerdna
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Does light have a size? In other words, does it have dimensions and occupy space?

ETA - Is measuring the wavelength of light actually measuring light?
 
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Yes to all of the above.

Claude.
 
Claude Bile said:
Yes to all of the above.

Claude.
Thanks - It’s hard to imaging being able to measure the size of something that has no mass and travels at around 300,000 kilometres per second! Can the size of sound also be measured by its wavelength?
 
swerdna said:
Does light have a size? In other words, does it have dimensions and occupy space?

ETA - Is measuring the wavelength of light actually measuring light?

In terms of quantum mechanics, the position operator for modes of the electromagnetic field does not have an eigenvalue. In english, that means that the spatial localization of a single photon is not well-defined.

So, a single photon has no size. Technically, it has an infinite size- a single photon is everywhere becasue the momentum (wavelength) is precisely specified.

Obvously, there is spatial localization to light- we illuminate small parts of space all the time. But that's becasue we are using large numbers of photons and the momentum is no longer so well specified for the group.

As for measuring the wavelength 'actually' measuring light, that's kind of a philosophical question rather than a science question. We can reproducibly make a measurement of *something*- the angle of diffraction off a grating, energy by a photdetector, a frequency (or difference frequency), things that are tied to physical properties of light. Did you have a specific question about a specific result?
 
swerdna said:
Can the size of sound also be measured by its wavelength?
Wavelength tells you the distance between successive peaks, it does not tell you anything about the spatial extent of the wave, the intensity of the wave, the direction it is moving in...

You can see where I'm getting at here, wavelength is one piece of the puzzle in terms of characterising a wave, but it is by no means the full story.

Claude.
 

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