What is a photon?

[morls]

Sorry for the rudimentary question, but I need to freshen up my understanding of some basics here. As the title says, what is a photon? I understand that it has no mass. I understand that it travels at the speed of light. Is it a packet of energy? Is it a unit of measurement, like 'gram' or 'volts'?

[xelasnave]

Short answer

https://en.wikipedia.org/wiki/Photon

Alex

[morls]

Thanks Alex, but now I have more questions...
If a photon is a type of elementary particle, how can it have frequency? I can understand the idea that a stream of photons may have a frequency of striking a surface, but how can a single, massless particle have frequency?

[gary]

Hi Stephen,

Through a mind-boggling attribute called wave-particle duality.

See https://en.wikipedia.org/wiki/Wave%E2%80%93particle_duality

There is a classical experiment called the double-slit experiment that
demonstrates it dramatically.

See https://en.wikipedia.org/wiki/Double-slit_experiment

It is an experiment that is relatively easy to do and is staple of
high school physics classes.
See :- https://indico.cern.ch/event/193928/contributions/1472192/attachments/280321/391984/SCeTGo-Scenario-CERN.pdf

[morls]

Thanks Gary.

There's something I'm still not understanding about all this.

In the "basic" double-slit experiment described on the wikipedia linked to above, it's a coherent light source - single-phase, single frequency, in effect, a stream of photons emitted at a steady rate, yes? I imagine this would be a very narrow beam, i.e. with a diameter of 1 photon. This light source is directed at a surface with two slits in it.

Here's where I'm stuck:

Are the two slits narrower than the diameter of the coherent light stream (i.e. is 1 photon hitting both slits simultaneously)? In the video "Simulation of a particle wave function" (on the right hand size of the same webpage referred to above) this appears to be the case.


These must be very small slits, each less than half the diameter of a (massless) photon when allowing for the material between them!


:shrug: :shrug: :shrug:

The image I have in my mind of these photons is of a stream of ping-pong balls being fired out of a nerf gun. I think my classical training is showing!

[Sunfish]

I have had heard it said that if you think you understand quantum effects you are probably wrong, as it does not make any sense to have spooky action at a distance. So we miss which is pretty weird.

I think one of the interesting things about the experiment is that the single photon appears to travel through both slots simultaneously and interfere with itself to create the classic sum over history diffraction pattern . A calculation of all the possible single particle projections through the slits produces the same diffraction pattern . Spooky and strange quantum probability proves wave particle duality as a kind of analogue I think. So not really ping pong balls . Way stranger at that scale.

[gary]

Hi Stephen,

If you try and imagine the balls not as discrete balls but as probability
curves - what are termed "probability amplitudes" - you are part of the way there. :)

Undergraduate level lecture by Richard Feynman where he discuses
probability amplitudes wrt the double split experiment -
http://www.feynmanlectures.caltech.edu/III_03.html

Don't make the mistake of feeling somehow inadequate if you
can't visualize it. Nobody can. Even he warns :-



Graphic :-
https://en.wikipedia.org/wiki/Wave_function#/media/File:QuantumHarmonicOscillatorAnima tion.gif

[morls]

Thanks Gary.


It's going to take me a while to get through this.

[billdan]

It looks like something similar to Schrodinger's Cat theory.
Because one does not know exactly, what is true or false.

[Sunfish]

Uncertainty for sure is a big factor. Known unknowns ? ha. . But then if you are a scientist and can understand and calculate the uncertainty you can make remarkable things happen. Quantum computing being just the latest of a long line of devices and actions relying on the understanding of quantum uncertainty.

Truly amazing.

Even a factor in the ability of birds to navigate in earths magnetic field using the quantum effect of molecules in their eyes.

[morls]

I agree, it's amazing.
While I'm working may way through the Feynman lecture, might I pose a related question?
Are photons considered 'messenger' particles in quantum mechanics? If so, is it correct to take this to mean they transmit information?

[billdan]

We capture photons with our cameras at night from distant galaxies, so in this context they do transmit information.
As to whether we can modulate individual photons and transmit information like we do with Radio / TV or microwave communications I don't know.

[Sunfish]

optic fibre cables . Optic sound cables. We can use light to transmit data. I am sure we can use light to project data , and lasers to do all sorts of things. As for Paul Davies idea that there is some inherent organising information in light and gravity I think that is not really a science question.

[billdan]

Thanks Ray, I forgot about data sent through fibre optics, and there is also 3D image projection with holography.

[bojan]

Laws of thermodynamics:
Information means lowering entropy, to lower entropy you need energy.. therefore to send information you need energy...
Photon carries energy, so it can be used for exchange of information, just like any other particle.

Photon rest mass is zero, but because it moves (with light speed, and it is light speed because the rest mass is zero) it carries certain amount of energy, depending on it's frequency (or wavelength)

[gary]

Hi Stephen,

The term "messenger particles" is used to describe a class of sub-atomic particles.

There are the four fundamental forces of nature, namely :-
* Gravity
* The weak force
* Electromagnetism
* The strong force

Messenger particles, what are termed "gauge bosons", bring about these
forces between other particles like protons, neutrons and so on. So they
act as intermediaries.

Gluons are in that class and they help "glue" atoms together.

Photons are also messenger particles which are associated with the
electromagnetic force.

The term "messenger" here is distinct from the word "information" when
it used in areas such as photonics.

That's about as much as I know but here is a wonderful introduction
to the topic of sub-atomic particles including messenger particles :-
https://www.britannica.com/science/subatomic-particle

As an electrical engineer, when engineer's use the term "information"
we are typically referring to the field of Information Theory which has
its footing in mathematics and statistics.
See https://en.wikipedia.org/wiki/Information_theory

When physicists refer to "information" I believe they are generally
referring to "physical information" such as the state of a system, for
example, the state of a quantum system.
See https://en.wikipedia.org/wiki/Physical_information

There will be cross-over between the two in areas such as quantum
cryptography but as far as the field of study called "Information Theory"
goes it is essentially contained purely in mathematics without having
to refer to the physical world, despite it having immense practicality in
the modern technological world.

[gary]

There you go. Wonderful.

[gary]

The basic active element of the Internet are optical fibres using quantum well lasers.

My former boss (I have worked for myself for a long time now) was an
Australian quantum semiconductor device physicist and he was the one who
first did the experiment to observe the quantum well effect that then
in turn made the quantum well laser possible.

He was one of the smartest people I've ever known and he did these things
and other significant achievements well before I met him. Alas I am just
a humble engineer and by comparison I have no expertise or involvement in these areas. :)

The lasers are also used in every CD, DVD, Blu-Ray player and barcode reader ever made.

[bojan]

Small correction: not atoms, but quarks in nucleons (neutrons and protons).

[gary]

Thanks Bojan! :thumbsup:

[doppler]

Another interesting thing about photons is they make everything look "solid". Even though the atoms that make up everything that we see are mostly empty space, the void is filled with the electromagnetic fields generated by the electrons within the atom. These affect light waves as they move through materials, preventing them from passing straight through unimpeded. Switching to radiation of shorter wavelength, like X-rays or gamma rays, allows even relatively dense materials to become transparent.

[morls]

Very helpful information, thanks everyone

[bgilbert]

The Nobel Laureate, Willis Lamb junior Wrote a paper titled "Anti photon", where he suggested the word ''photon" should be banned from the lexicon. I agree with Lamb, but for stronger reasons.
Maxwell's Equations can explain almost everything that the photon is normally used for.

[bojan]

Well.... it is true for practical purposes (RF etc) but it is not enough. QED explains the rest.
And Standard model goes even deeper....

[bgilbert]

If its good enough for RF, then Its good enough for for all other forms of electromagnetic radiation, they are the same. RF engineers don't use the concept of the photon and do pretty well, Wi-Fi , 5G, VDSL and radio astronomy etc.

[bojan]

Yes, but that is not the whole story.

We are talking here physics, not RF engineering.
(BTW, I am RF engineer by profession).

[Wavytone]

As both physicist (first degree) and electrical engineer (second degree).. a photon is both, yet neither provides a complete description.

The wave description explains many phenomena nicely such as diffraction, superposition (interference), coherence (lasers), maxwells equations, antenna theory and ties in nicely with quantum mechanics.

But there are other aspects that are explicable as particle, but not wave...
- Slit experiments with single photons show that when one is detected (ie arrives at a sensor) the entire wave function collapses, ie on conversion of the photon to an electron, the wave ceases to exist. The wave is physically distributed over macroscopic distances (two slits) yet when absorbed at a sensor resulting in an electron (essentially a very small particle) the entire wave collapses. In this respect the photon behaves as a particle. Yet, when there are two slits, the photon must pass through one, or other to reach the sensor, yet somehow it "knows" the presence of the other slit. With multiple photons in succession, the two-slit interference pattern pattern is produced.
- The photo-electric effect of metals is explicable using Einsteins particle representation;
- A photon also has measurable momentum when it strikes a macroscopic object, which is very much a particle behaviour.

Then there are small groups of photons - solitons - which are important for high data transmission rates in optical fibres.

Morls... yes photons carry information. While the pure theory is taught in physics, information theory is an electrical engineering topic not taught in science degrees.

Direction, polarisation, energy (frequency/wavelength), relative phase and group velocity, periodic changes in brightness and the waveform or the timing between pulses (eg eclipsing binary stars, Cepheid variable stars, and pulsars), the rise and decay rates of big pulses (supernovae), the Doppler shift (rotating stars and binaries) as well as the gravitational redshift (distant galaxies) all convey information.

Frequency-shifts or relative-phase shifts can convey information (for example a laser beam through an optical fibre can be used to sense mechanical strain or vibrations along the length of the figure and the time-of-flight can locate the position along the fibre).

There is the frequency distribution (spectrum) of the observed light, that also conveys information.

And lastly to warp your mind, 2-dimensional Fourier transforms can be used to do real-time filtering of images, and holograms can be used to model 3-D objects.

Direction is useful in astronomy - for example the bending of starlight by gravity first predicted by Einsteing, verified at an eclipse in 1919, and subsequently the gravitational lensing of distant galaxies by foreground ones.

For an example in which direction alone is used to encode 10 bits see https://www.technologyreview.com/s/602454/single-photon-carries-10-bits-of-information/

These all make the binary pulses (0 or 1) used to encode data on CD's and DVD's look rather primitive compared to what could really be stored on a macroscopic object if several of the above techniques could be combined reliably.

[bgilbert]

Moris,

did you take the advice of Alex, and look at; 'Photon' on Wiki.
It might be more productive than listening to some of us sparring.
At all cost maintain your curiosity and even skepticism and don't be
"bullied by erudite sycophants to the standard model. Progress is often
achieved by overturning existing well established theories

[bgilbert]

Max Plank and Albert Einstein had a serious disagreement about the nature of "the light quanta". Plank insisted quantization only occurred in 'matter', at the point of creation or detection of the radiated field. Moreover, the radiated field behaved entirely according to Maxwell’s equations, and therefore, was not quantized. Einstein was not convinced, and continued with the notion that the field itself was quantized.

The term "photon" was later adopted for the "light quanta" by the scientist, Gilbert N Lewis. It took some years before the notion of the "photon " was finally accepted by the mainstream scientists, but Max Plank was not one of them. The relationship between Max and Albert was never the same after this disagreement.


It is interesting to note, that Niels Bohr, who later was to be an adversary of Albert’s, resisted the notion of Albert’s "photon", for quite some time, before finally acquiescing.

Towards the end of Albert’s life he was said to have stated;

"All the fifty years of conscious brooding have brought me no closer to answer the question, “What are light quanta?” Of course today every rascal thinks he knows the answer, but he is deluding himself."
— Albert Einstein

[bgilbert]

The laser was developed in spite of quantum mechanics (QM), not as a result of QM.
H R Townes, the inventor of the laser, happened to mention to Niels Bohr what he was working on, Niels replied that it could not possibly work because of the uncertainty principle. Thankfully Townes ignored Niels Bohr.
Neils Bohr also told Hanbury Brown that his proposal to measure the diameter of stars using intensity correlations could not possibly work, because of the uncertainty principle. Hanbury ignored Niels and spent 17 years at Narrabri successfully measuring star diameters.
Twenty years later an absurd theory of bunching, based on erroneous measurements of arrival statistics for laser light was developed, to explain the Hanbury Brown Twiss effect. This effect was also thought to explain the operation of the laser.
QM has now incorporated a QM interpretation of the Hanbury Brown Twiss effect into its vocabulary.

Barry

[bgilbert]

I thought adding this link may invigorate this thread?


Willis Lamb junior is a Nobel Laureate for his experimental measurement of the "Lamb shift". This measurement is one the two most precise measurements in physics

https://jumpshare.com/v/cxGki8ocqtHqajtpknCA

Barry

[strongmanmike]

Fascinating paper...

Mike

[bgilbert]

G'day all

I stumbled on this article, and was shocked and amazed to find the author is a kindred spirit.

https://jumpshare.com/v/0paoiqJZG45O0GdDHdWQ

Could it be that the day's of the absurd "photon", are numbered, and the obscene billions of dollars spent on the "quantum computer" can be diverted to more practical research?

Barry

[bgilbert]

What is the difference in the length of a thermal photon from the sun and a coherent photon from a laser?

Barry