Undergrad Question about electromagnetics (waves and particles)

[samy4408]

I saw that we can talk about the light as particles (photons ) or as an electromagnetic wave , the question is that do we represent other electromagnetic waves (like microwaves or radio waves ) as particles (like we do with light ) ?

 

[PeroK]

I saw that we can talk about the light as particles (photons ) or as an electromagnetic wave , the question is that do we represent other electromagnetic waves (like microwaves or radio waves ) as particles (like we do with light ) ?

Yes. Quantum Electrodynamics (the quantum mechanical theory of light) applies to the entire electromagnetic spectrum.

In general there is no physical distinction between visible light and light that our eyes do not detect, such as radio waves, microwaves, x-rays, infrared and ultraviolet radiation.

 

[Baluncore]

I saw that we can talk about the light as particles (photons ) or as an electromagnetic wave , the question is that do we represent other electromagnetic waves (like microwaves or radio waves ) as particles (like we do with light ) ?

We use the model that works best for us at the time.
"(like microwaves or radio waves )". The fact that we call them waves is a clue that the photon model is not as useful as the wave model.

Consider a 100 MHz 1 watt transmitter.
Photon energy; E = h⋅freq; where Planck constant, h = 6.62607015×10^-34 J / Hz.
E = 6.626e-34 * 1e8 = 6.626e-26 joule per photon.
1 watt is 1 joule per second. That is 15.1×10²⁴ photons per second.
Counting photons at radio frequencies is going to be quite difficult.

 

[sophiecentaur]

Counting photons at radio frequencies is going to be quite difficult.

Once you get to visible light, individual photons can be fairly easily measured with photo sensors and people claim that individual photons can be seen by a fully dark adapted human eye as tiny flashes of light 'in the pitch dark'. (Never saw it myself and I wonder whether it's just random noise on the retina.)
Go up a bit higher in frequency and individual γ photons are easily heard with the clicks and crackles of a Geiger counter from a very low intensity Gamma Ray source.

 

[Albrecht]

We use the model that works best for us at the time.
"(like microwaves or radio waves )". The fact that we call them waves is a clue that the photon model is not as useful as the wave model.

Consider a 100 MHz 1 watt transmitter.
Photon energy; E = h⋅freq; where Planck constant, h = 6.62607015×10^-34 J / Hz.
E = 6.626e-34 * 1e8 = 6.626e-26 joule per photon.
1 watt is 1 joule per second. That is 15.1×10²⁴ photons per second.
Counting photons at radio frequencies is going to be quite difficult.

That is a great response. When I asked for a specific property of photons as particles some time ago, I got the response that photons are clearly waves and not particles and that I am stating nonsense by asking for particles; and my access to the forum was blocked. What has changed?

 

[Baluncore]

What has changed?

You asked the right question. It was answered by a pragmatic engineer. I half expected to be punished for having strayed from the Engineering into the Physics part of the forums.

It is important that while exploring Physics for the first time, you have faith, and avoid questioning the fundamental paradigms.

The application of the particle tools and the wave tools overlap. Use what works best in your application. If you insist on defining a hard boundary, you must be wrong in some way.

 

[Herman Trivilino]

For the wave properties to appear you need large numbers of photons.

 

[Herman Trivilino]

I got the response that photons are clearly waves and not particles

You must have misunderstood what you saw or you remember it incorrectly. Photons are particles. If you have a large collection of photons then the wave properties can appear.

 

[sophiecentaur]

Photons are particles.

There's a lot more to that statement than you are implying. Just what do you mean when you use the word 'particle'? First thing is that you cannot treat a photon as a little bullet; its position and 'extent' have no meaning so a flash of light is not a shower of photons (in the way people understand what a shower is).

It's a lot easier if we limit our mental picture of a photon as a packet of energy. That's all we can be really sure of from experiment.

Using the word Photon in an argument or description (at least at this level of things) will, in no way, make the argument any more substantial or demonstrate better understanding. Second order partial differential equations are hard so it's forgivable to try to avoid the (wave) description of many optical phenomena but you cannot get away from them by trying for the photon approach. PDE's also sneak into describing what a photon will do.

 

[Maarten Havinga]

I would say photons are better described by their wavefunction (complex and real part of it) and are therefore wave packets. Only on measurement they become pointlike particles, by some unknown process observed as wavefunction collapse.

 

[Klystron]

As stated, use the tool appropriate to the task at hand. To list some items

• The electromagnetic (EM) radiation spectrum is continuous from large to tiny wavelengths or small to large frequencies as you prefer. Wavelength and frequency are reciprocal measurements.
• Engineers often prefer the term wavelength while others use frequency, as for radio stations.
• 
  
• Wave terminology for EM is ubiquitous and useful but EM fields mathematically describe EM radiation. Radio transmissions, microwaves, light waves, X-rays are the same thing measured at different frequencies for different applications.
• 
  
• Photons (bosons) are the quanta of EM energy useful for describing EM at the smallest discrete level.
• Electrons (leptons) appear to be the elementary particle at the root of EM phenomena.
• Terminology and theories progress over time as new technology improves measurement.

To paraphrase previous comments, we mostly measure and manipulate EM radiation as fields and herd electrons as aggregate charge carriers. Articles and textbooks describe physics experiments using the equipment, theories and mathematics available in their time. Try to learn as much mathematics as you are able in order to really understand science and engineering.

 

[sophiecentaur]

Engineers often prefer the term wavelength while others use frequency, as for radio stations.

Antenna design Engineers are about the only ones, these days, to use wavelength. Plus, possibly transmission line users and some filter designers.
In the past, there was no easy way to measure Radio frequencies directly because the devices weren't available. Engineers (and amateurs) would find the wavelength of a signal by observing max's and min's on transmission lines. Nowadays, you can get a frequency counter to measure some extremely high frequencies but, by the time you get to millimetre waves and shorter, we still use wavelength and, for high energy waves (X Rays and beyond, it's common to talk in terms of Energy and electron Volts (eV). All the same stuff but the units are chosen to suit the particular users.
This table in Wikipedia is interesting and the energy of photons is shown in the right hand column.

 

[nightvidcole]

Yes, microwaves and radio waves are quantized just like the rest of the EM spectrum. That said, the amount of energy in a single photon at these low frequencies is quite miniscule - we're talking about micro-electronvolts or so! You won't be able to detect that unless your detector is very close to absolute zero temperature - thermal energy at room temperature is around 25 meV. Thus, when you receive a radio signal, it is from a very very large number of photons - and you don't observe the quantized nature of the radiation.

 

[PeroK]

That said, the amount of energy in a single photon at these low frequencies is quite miniscule -

The word is actually minuscule.

 

[Herman Trivilino]

Thus, when you receive a radio signal, it is from a very very large number of photons - and you don't observe the quantized nature of the radiation.

Unfortunately, when people hear about the em wave properties they think that's a display of the quantum wavefunction. Which of course it isn't. Electromagnetic waves are a purely classical phenomenon.

Many textbooks perpetuate this misconception in the way they compare, for example, the double-slit experiment results using light to the results using electrons. The em wave is not to photons as the wavefunction is to electrons. There's a wavefunction associated with photons, but it's not the em wave.

 

[sophiecentaur]

There's a wavefunction associated with photons, but it's not the em wave.

Are you suggesting that the basic diffraction equations associated with waves don't apply or are you just giving a warning against going too far with the analogy?

 

[Herman Trivilino]

Are you suggesting that the basic diffraction equations associated with waves don't apply or are you just giving a warning against going too far with the analogy?

The latter.

 

[tech99]

Yes, microwaves and radio waves are quantized just like the rest of the EM spectrum. That said, the amount of energy in a single photon at these low frequencies is quite miniscule - we're talking about micro-electronvolts or so! You won't be able to detect that unless your detector is very close to absolute zero temperature - thermal energy at room temperature is around 25 meV. Thus, when you receive a radio signal, it is from a very very large number of photons - and you don't observe the quantized nature of the radiation.

Do we have evidence for photons at low frequencies?

 

[Vanadium 50]

Do we have evidence for photons at low frequencies?

What does that even mean?

How low is low?
What constitutes evidence?
What properties of photons are I allowed to postulate? ("None" is not an acceptable answer as it leaves open the question of what exactly is a photon)

A better-defines question would be "what is the lowest energy photon that has been observed?"

 

[nightvidcole]

Do we have evidence for photons at low frequencies?

Partially: https://www.science.org/doi/10.1126/science.adg3812

 

[pinball1970]

Do we have evidence for photons at low frequencies?

Radio telescopes? https://www.aanda.org/articles/aa/f... new and,broad range of astrophysical studies.

 

[sophiecentaur]

Radio telescopes? https://www.aanda.org/articles/aa/full_html/2013/08/aa20873-12/aa20873-12.html#:~:text=LOFAR, the LOw-Frequency ARray, is a new and,broad range of astrophysical studies.

The noise power that a radio telescope would be seeing (incoming and front end) would not be the lowest achievable. To observe photons /quanta of the 'lowest' frequency I suggest you'd need a cooled highly screened enclosure (in deep space). Inside, you would have a modulated source and a detector, all with as low noise performance that you could obtain which were minimally coupled from each other in order to observe single photons. Then the signal to noise ratio would be as low as you could get, practically.

But what would that prove? The principle of EM quantisation has already been established. The experiment would only lower the frequency limit. There are better things to do with the space programme.

 

[f95toli]

The noise power that a radio telescope would be seeing (incoming and front end) would not be the lowest achievable. To observe photons /quanta of the 'lowest' frequency I suggest you'd need a cooled highly screened enclosure (in deep space)

No need for deep space; space is actually quite warm because of all the radiation, there is no way you would be able to do any sensible experiments in such as noisy environment.

Experiments with single microwave photons (down to about 2 GHz or so) are not at all uncommon (look up circuit- or cavity quantum electrodynamics). The experiment/devices are typically cooled down using dilution refrigerators which go down to about 10 mK or so; more advanced experiments can go even lower in temperature (a few hundred microkelvin).

 

[sophiecentaur]

No need for deep space;

You're right in principle but I imagine setting up an experiment in a large setup at a low enough K temperature for 1peV (~300khz) would be a bit taxing on Earth temperatures (despite fancy refrigeration).

But would it be worth it?

 

[f95toli]

But would it be worth it?

Probably not. It is fairly straightforward to do experiments with single photons with frequencies down to hundreds of MHz, slightly lower (tens of MHz) should be possible, but lower than that starts to get tricky. But I agree that here wouldn't be much point.

Note that size is not really that much of an issue, the setups needed are not actually that large and normally fits into commercially available dilution refrigerators. Cooling such setups below 10 mK or so is now routine in hundreds of labs around the world.

 

[sophiecentaur]

Note that size is not really that much of an issue,

I was not thinking about basic difficulties in making refrigeration; I was considering the difficulty of time / phase measurement and detecting / identifying one quantum. I could easily be wrong here but wouldn't you want to identify a single event and wouldn't the apparatus need to be more than a wavelength long? That would involve a Big Fridge. I wouldn't know where to start....

 

[f95toli]

I was not thinking about basic difficulties in making refrigeration; I was considering the difficulty of time / phase measurement and detecting / identifying one quantum. I could easily be wrong here but wouldn't you want to identify a single event and wouldn't the apparatus need to be more than a wavelength long? That would involve a Big Fridge. I wouldn't know where to start....

Not at all, single microwave photon detectors can be very, very small (microns) and are typically operated in the lumped regime where they usually connected to transmission lines (say coplanar waveguides).
Many experiments are chip-based but it is also possible to build detectors using e.g., 3D cavities
See e.g.,
https://journals.aps.org/prx/abstract/10.1103/PhysRevX.10.021038

 

[sophiecentaur]

Not at all, single microwave photon detectors can be very, very small (microns) and are typically operated in the lumped regime where they usually connected to transmission lines (say coplanar waveguides).
Many experiments are chip-based but it is also possible to build detectors using e.g., 3D cavities
See e.g.,
https://journals.aps.org/prx/abstract/10.1103/PhysRevX.10.021038

How stupid of me. I was thinking that the wavelength of an em photon would have to be at all relevant when its 'position' is undefined. I was thinking in terms of a 'radio receiver' circuit and an antenna.

Thanks for putting me straight.

 

[sophiecentaur]

I was thinking in terms of a 'radio receiver' circuit and an antenna.

it is also possible to build detectors using e.g., 3D cavities

Otoh, a detector for, say 1MHz would need a pretty vast cavity or some MF receiver type of wound resonator. Then, the Q would be a problem.