Can the human eye detect just a single photon?

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Discussion Overview

The discussion centers on whether the human eye can detect a single photon, exploring the underlying mechanisms of photon detection in the retina, the physiological thresholds for perception, and the nature of photons themselves. It includes theoretical considerations, physiological explanations, and anecdotal observations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants question the definition of a photon, suggesting it may not be accurately described as a single wave cycle of an electromagnetic sine wave.
  • One participant explains the biochemical process in the retina when a photon is absorbed by rhodopsin, detailing how multiple photons are needed for a signal to be sent to the brain.
  • Another participant expresses skepticism about the ability to perceive light with just 6 photons, suggesting that even under ideal conditions, perception may be uncertain.
  • There is a mention of a threshold for perception defined by the observer's ability to correctly identify stimuli only half the time, indicating a level of uncertainty in detection.
  • One participant references a Physics FAQ from 1996, implying that there may be established discussions on this topic that have not been cited in the thread.
  • Another participant introduces the idea of cosmic rays and their detection, questioning how this relates to the perception of high-energy photons.
  • There is a claim that individual photons of visible light can be perceived after prolonged darkness, although this is debated among participants.
  • One participant notes that while single photons can be detected using specialized equipment like photo-multipliers, this does not necessarily translate to human perception.

Areas of Agreement / Disagreement

Participants express differing views on the nature of photons and the human eye's ability to detect them. There is no consensus on whether a single photon can be perceived, and multiple competing perspectives are present throughout the discussion.

Contextual Notes

Some claims about photon perception depend on specific conditions such as total darkness and the physiological state of the eye. The discussion also touches on the ambiguity surrounding the definition of a photon and its implications for perception.

jaydnul
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First off, is a photon just one wave cycle of an EM sine wave (2∏). If so, could the human eye detect just a single photon by itself?
 
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When the photon encounters the rhodopsin molecule in the retina, the rhodopsin absorbs the energy of the photon and a cis-double-bond is temporarily converted into a single bond which then rotates 180°. The double bond reforms with the molecule now in the trans configuration - the shape is changed from curved to straight. The new shape no longer fits its binding site and an unstable linkage, subsequent shape changes, and ultimate breakdown lead to movements that are detected by the nerve cell, which depolarizes its membrane and initiates a signal.

The retina has 10 layers of neural processing in force prior to a signal exiting the optic nerve. A single photon signal from a receptor is not enough to forward ahead to the brain. If the retina is totally unbleached (fully dark adapted after about 30 minutes of total darkness) then the threshold for seeing light is about 6 photons arriving very close to the same place and time in the retina... (spatio-temporal summation) that is enough to make it through processing that results in a signal you will see as a brief point of light.
 
I would think that with 6 photons, and under those ideal conditions, you would still be wondering to yourself "did I see something there"? lol.

Though, I'm not very familiar with human physiology to be of more help.
 
First off, is a photon just one wave cycle of an EM sine wave (2∏).
No. In fact, there hardly is a notion of a "single photon".

If so, could the human eye detect just a single photon by itself?
That depends on the energy of the photon, which can range from zero to somewhere about infinity.
 
Matterwave said:
I would think that with 6 photons, and under those ideal conditions, you would still be wondering to yourself "did I see something there"? lol.

Though, I'm not very familiar with human physiology to be of more help.

Actually, you are on the right track... the doubt about whether something was seen is part of the measurement. In all kinds of discrimination tests like this, the threshold is defined as just when the observer is correct only half the time.
 
A.T. said:
There is a related phenomenon with cosmic rays. Apparently single sub-atomic particles can be perceived, although the mechanism is not clear.
http://en.wikipedia.org/wiki/Cosmic_ray_visual_phenomena

I'm not sure how this translates to single high-energy photons.

I'm not even sure how this is on-topic in this thread!

Zz.
 
lundyjb said:
First off, is a photon just one wave cycle of an EM sine wave (2∏). If so, could the human eye detect just a single photon by itself?

This is a very popular view of what a photon must be. Unfortunately it just doesn't make sense. One 'clincher' argument is that a single photon of a Long Wave Radio signal would occupy more than 1km of space and yet it represents less than a millionth millionth of the energy of a single photon of visible light. Don't go looking for a 'size' for a photon in conventional terms.

People claim that single photons of visible light can be perceived after hours of time spent in total darkness, when the eye is fully dark-adapted. I think this is based on using a (measured) low level of light and the rate of 'random' flashes seen correlates with the illumination level. That implies that the individual photons are, in fact perceived.

It is certainly possible to detect individual photons of light using a photo-multiplier, so it's is not that unlikely.
 

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