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Old 29-10-2019, 02:09 PM
morls (Stephen)
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What is a photon?

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'?
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Old 29-10-2019, 02:26 PM
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Short answer

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

Alex
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Old 29-10-2019, 02:43 PM
morls (Stephen)
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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?
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Old 29-10-2019, 03:57 PM
gary
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Quote:
Originally Posted by morls View Post
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?
Hi Stephen,

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

See https://en.wikipedia.org/wiki/Wave%E...rticle_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/...nario-CERN.pdf

Last edited by gary; 29-10-2019 at 04:14 PM.
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Old 29-10-2019, 04:37 PM
morls (Stephen)
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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!




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!

Last edited by RB; 30-10-2019 at 01:17 PM.
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Old 29-10-2019, 05:03 PM
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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.

Quote:
Originally Posted by morls View Post
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.
M
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!


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Old 29-10-2019, 05:34 PM
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Originally Posted by morls View Post
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!
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 :-

Quote:
Originally Posted by Feynman
In this subject we have, of course, the difficulty that the quantum mechanical behavior of things is quite strange. Nobody has an everyday experience to lean on to get a rough, intuitive idea of what will happen. So there are two ways of presenting the subject: We could either describe what can happen in a rather rough physical way, telling you more or less what happens without giving the precise laws of everything; or we could, on the other hand, give the precise laws in their abstract form. But, then because of the abstractions, you wouldn’t know what they were all about, physically. The latter method is unsatisfactory because it is completely abstract, and the first way leaves an uncomfortable feeling because one doesn’t know exactly what is true and what is false. We are not sure how to overcome this difficulty.
Graphic :-
https://en.wikipedia.org/wiki/Wave_f...rAnimation.gif
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Old 29-10-2019, 07:02 PM
morls (Stephen)
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Thanks Gary.

Quote:
Originally Posted by gary View Post
Undergraduate level lecture by Richard Feynman where he discuses probability amplitudes wrt the double split experiment -
http://www.feynmanlectures.caltech.edu/III_03.html
It's going to take me a while to get through this.
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Old 29-10-2019, 10:21 PM
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It looks like something similar to Schrodinger's Cat theory.
Because one does not know exactly, what is true or false.
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Old 30-10-2019, 08:21 AM
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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.
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Old 30-10-2019, 09:16 AM
morls (Stephen)
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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?
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Old 30-10-2019, 10:38 AM
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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.
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Old 30-10-2019, 12:12 PM
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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.

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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.
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Old 30-10-2019, 12:32 PM
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Thanks Ray, I forgot about data sent through fibre optics, and there is also 3D image projection with holography.
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Old 30-10-2019, 01:42 PM
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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)
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Old 30-10-2019, 02:08 PM
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Quote:
Originally Posted by morls View Post
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?
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.
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Old 30-10-2019, 02:11 PM
gary
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Quote:
Originally Posted by bojan View Post
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)
There you go. Wonderful.
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Old 30-10-2019, 02:35 PM
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optic fibre cables . Optic sound cables. We can use light to transmit data.
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.
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Old 30-10-2019, 04:15 PM
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Quote:
Originally Posted by gary View Post
....
Gluons are in that class and they help "glue" atoms together....
Small correction: not atoms, but quarks in nucleons (neutrons and protons).
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Old 30-10-2019, 04:31 PM
gary
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Small correction: not atoms, but quarks in nucleons (neutrons and protons).
Thanks Bojan!
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