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Question about photons

by radioactive8
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radioactive8
#1
Dec19-13, 11:05 AM
P: 35
Hello
I am a physics student at the university of Athens (1st year) .
Recently i have been studying special relativity by myself and i have question that i cant understand by myself
When we speak about photons and their energy we use this formula : E=hf
where h : planks constant and f:frequency
my question : How can we speak about the energy of photons and use frequenscy .. how can a photon have its own frequency ?

and something else . Can an electron ( and in general every particle) be considered as a wave also ? (like the photons)
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Nugatory
#2
Dec19-13, 11:28 AM
Sci Advisor
Thanks
P: 3,462
Quote Quote by radioactive8 View Post
my question : How can we speak about the energy of photons and use frequenscy .. how can a photon have its own frequency ?
If you're studying special relativity, then you don't need the quantum mechanical concept of photon at all - it will do more harm than good. Instead, every time you find yourself saying "photon", say"light signal" or "flash of light" instead. Light is an electromagnetic wave, so there's no problem assigning it a frequency and a wavelength.

Can an electron ( and in general every particle) be considered as a wave also ? (like the photons)
Yes, although again this is quantum mechanics not special relativity. You can learn and understand SR without going anywhere near quantum mechanics, using just the classical concepts of light as an electromagnetic wave - and I strongly recommend doing exactly that.
radioactive8
#3
Dec19-13, 11:34 AM
P: 35
Understood and thank you ...
Actually i am taking physics courses via the internet and one of the lessons i am attending is called " introduction to relativity , nuclear physics and cosmology" so i believe i will have no problems ..

Naty1
#4
Dec19-13, 11:55 AM
P: 5,632
Question about photons

If your search for anything like 'photon energy' in these foums you'll find loads of prior discussions.

Briefly, in the standard model of particle physics relativistic quantum theories describe
'particles' as probabilistic wave functions, not like 'little balls'. When we detect such phenomena we observe them as point particles and physical action at small scales takes place in discrete steps as in multiples of 'h'. What they 'really are' between observations is less clear and you can find discussions in these forums...

The particular quantum theory for light and matter is QED. Roughly, you can think of particles as detectable excitations or a quanta of the underlying quantum fields.... Planck's constant doesn't appear in Einstein equations so you won't find anything quantum-mechanical from them.

here are a few views I like to get you started:

tomstoer[I think]
Particles appear in rare situations, namely when they are registered.
...
Marcus:
The trouble with the particle concept is that one cannot attribute a permanent existence; It only exists at the moment it is detected. The rest of the time there is a kind of spread out thing---a cloud---a wave---a field---something that is less "particular", something that cannot be detected.

One recent discussion including energy is here :
http://www.physicsforums.com/showthr...=photon+energy

There is a long and valuable discussion here...at least I learned a lot from it:

What is a particle
http://www.physicsforums.com/showthread.php?t=386051


Can an electron ( and in general every particle) be considered as a wave also ? (like the photons)
As you might infer from the above, this is really the best way to view an electron. The old view of an electron like a planet 'orbiting' a sun [nucleus] doesn't work so well. Viewing an electron as a resonant, standing, wave analogous to a vibrating string works well. In such a view, the electron is not in a specific location, it is to be found at various places in the electron 'cloud' with different probabilities. The modern term is electron 'orbital'.

Youll find some great illustrations of these in Wikipedia under ATOMIC ORBITAL. http://en.wikipedia.org/wiki/Atomic_orbital be sure to scroll down the page as there as a number of interesting illustrations.
Claude Bile
#5
Dec20-13, 06:22 AM
Sci Advisor
P: 1,474
Quote Quote by radioactive8 View Post
Hello
I am a physics student at the university of Athens (1st year) .
Recently i have been studying special relativity by myself and i have question that i cant understand by myself
When we speak about photons and their energy we use this formula : E=hf
where h : planks constant and f:frequency
my question : How can we speak about the energy of photons and use frequenscy .. how can a photon have its own frequency ?

and something else . Can an electron ( and in general every particle) be considered as a wave also ? (like the photons)
On this point; the frequency uncertainty of a photon is intrinsically linked with its position uncertainty. In other words, the better known the position of the photon is known, the larger the spread in frequencies and vice versa.

A photon with a perfectly known position will have an arbitrary frequency. Conversely, a photon with a perfectly known frequency will have an arbitrary position.

This property is linked to the linearity of waves and the Fourier transform relationships between quantities like time and frequency.

Claude.
radioactive8
#6
Dec20-13, 07:54 AM
P: 35
Quote Quote by Claude Bile View Post
On this point; the frequency uncertainty of a photon is intrinsically linked with its position uncertainty. In other words, the better known the position of the photon is known, the larger the spread in frequencies and vice versa.

A photon with a perfectly known position will have an arbitrary frequency. Conversely, a photon with a perfectly known frequency will have an arbitrary position.

This property is linked to the linearity of waves and the Fourier transform relationships between quantities like time and frequency.

Claude.
Isn't that Heisenberg uncertainty principal ? oh and
One recent discussion including energy is here :
http://www.physicsforums.com/showthr...=photon+energy

There is a long and valuable discussion here...at least I learned a lot from it:

What is a particle
http://www.physicsforums.com/showthread.php?t=386051
thank you .!!!
Naty1
#7
Dec20-13, 07:57 AM
P: 5,632
That's a really nice synopsis from Claude if you have been exposed to Fourier analysis...

If you are interested in additional insights, see this discussion

"How big is a photon."
http://www.physicsforums.com/showthread.php?t=657264

It's long and involved, but if you plow through it you'll really find out the subtelies
of 'particles'....read different/conflicting views

Here is a quote I like from a famous scientist [maybe in the second link of the prior post above]:

Carlo Rovelli:

“…we observe that if the mathematical definition of a particle appears somewhat problematic, its operational definition is clear: particles are the objects revealed by detectors, tracks in bubble chambers, or discharges of a photomultiplier…”

A particle is in some sense the smallest volume/unit in which the field or action of interest can operate….Most discussions regarding particles are contaminated with classical ideas of particles and how to rescue these ideas on the quantum level. Unfortunately this is hopeless.
.... A particle detector measures a local observable field quantity (for instance the energy of the field, or of a field component, in some region). This observable quantity is represented by an operator that in general has discrete spectrum.
It is from that last sentence that the ambiguity arises.
Naty1
#8
Dec20-13, 08:48 AM
P: 5,632
Isn't that 'Heisenberg uncertainty principal' ?
yes...the issue is what HUP 'means'.....

HUP is another quantum phenomena whose 'meaning' [interpretation] is still debated.....
Just search the that name in these forums and you will find dozens of discussions/arguments/viewpoints.
K^2
#9
Dec20-13, 07:33 PM
Sci Advisor
P: 2,470
Quote Quote by Claude Bile View Post
On this point; the frequency uncertainty of a photon is intrinsically linked with its position uncertainty. In other words, the better known the position of the photon is known, the larger the spread in frequencies and vice versa.
It's worth noting that basic QM and QFT treat this a little different. In QM, photon is a wave, which has uncertainty in momentum and position. Momentum, of course, being directly linked to its frequency.

Quantum Field Theory would say that no, a photon has an exact frequency and an exact position. But the uncertainty is in which photon you are observing. (A photon with which frequency, and at what location.) It's just semantics when you deal with a single particle, but becomes important when you consider many-particle systems.

Just thought you should know that's out there, because you might run into statements about photons with exact frequency and location on this forum or in textbooks, and it might be confusing.


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