# Is there an upper limit to e/m frequency?

1. Sep 26, 2010

### Greylorn

Wondering if, since the Planck constant limits the amount of energy that can be transferred as a function of time (at least that's been my interpretation), can it be used to define a high end limit to electromagnetic radiation frequency?

And is this even an interesting question?

2. Sep 27, 2010

### xepma

Moving to higher and higher frequencies implies you severely increase the energy scale you are working at. At low energies you can simply apply classical electromagnetism. But at some point this description doesn't suffices. You will notice that quantum effects will come into play and the theoretical framework of classical electrodynamics seizes to apply. On a theoretical level, it gets replaced by quantum electrodynamics (QED). In this theory the coupling constant is energy dependent (it is said to be "running") and quantum mechanics severely alters the behavior of the electric and magnetic field. The Lamb shift is probably the best known experimental signature of QED.

At some energy scale, QED also starts to break down and unification with other forces kicks in. Quantum electrodynamics unifies with the weak nuclear forces, and both are replaced by the single theory of the electroweak force. Here, you can no longer talk about photons and the electromagnetic field alone. Continuing the process of increasing the energy scale will, very likely, eventually lead to unification with the strong force as well -- although we do not have singled out a specific theory in this regime. Beyond these energyscales it all becomes very speculative.

Particle physics completely resolves around trying to understand what happens at the large frequency scales. It's been a very interesting question for over 100 years!

3. Sep 27, 2010

### haael

The maximum upper frequency of electromagnetic waves is equivalent to spacetime quantization, with all the bad consequences, namely violation of Lorenz invariance. What happens when you have a "maximal" photon and you try a blue Doppler shift on it?

4. Sep 27, 2010

### ueit

Special relativity allows you to choose a reference frame in which the frequency is as low or as high as you want. There is no limit to it. As far as we can tell, spacetime is continuous so planck length does not impose a limit on wavelength.

5. Sep 30, 2010

### Greylorn

If I understand you correctly, are you implying that we can adjust an observation by pretending that we are observing it from a different reference frame?

If I'm observing a high-frequency e/m wave on planet earth, why would the Planck length not limit the frequency?

6. Sep 30, 2010

### Greylorn

I'd say, we can't get there from here.

Suppose we consider spacetime quantization as a possibility. Is violation of Lorenz invariance that bad a thing?

7. Sep 30, 2010

### haael

It's supported by all observations in the first place.

8. Sep 30, 2010

### Greylorn

Thank you. Your answer put a lot of stuff together. Now, I'm especially curious about what happens at the the transition points between classical and quantum electrodynamics, and the frequency where the transition between Q.E.D. and the electroweak force occurs. It seems to me that for a determination of a general e/m model, understanding what happens at these transition points is essential. ??

9. Oct 1, 2010

### Greylorn

Got that! But, we know that our understanding of the universe is imperfect. A few mysteries remain unresolved, and we are confident that they will eventually be resolved.

The history of science is replete with instances where a longstanding concept did not work out. Is it possible that despite its excellent practical applications, the Lorentz transformation is such a concept? For example, how does it affect quantization of energy?

I did not understand the derivation of the Lorentz transformation when first exposed to it in school decades ago, and after your comment I looked it up and checked it out. At first read, the derivation looks like mathematical sleight of mind. But I never made a nickel from my expertise in theoretical math, so the problem is clearly my own. I shall re-study the derivation until I understand it, then come back on this. May be a few weeks.

In the interim, any thoughts you have are welcome. And afterward as well!

10. Oct 1, 2010

### haael

Still if you want to claim a theory that violates Lorentz invariance it has to be supported by experiments. Current observations do not show any violations of it, which sets some lower bounds of energy scales where the violation may occur.

It is used to explain Doppler shifts.

I personally don't like theories that deny relativity. I know we need some idea to move forward, but I doubt this is a good way.
Quantum physicists don't like relativity and continuous space, since it is not renormalizable. Alas, we should not throw away a proven physical theory, just because some odd mathematical method doesn't work on it.

11. Oct 1, 2010

### Upisoft

Like it or not there is already such observation. I'm talking about cosmic background radiation (CBR). Observers having relativistic velocities will observe much higher directional anisotropy than us. Simply said, it is possible to have an observer seeing light from the direction they are going to, as the CBR can be blueshifted from microwave spectrum to visible spectrum.

Thus there is *preferred* inertial frame of reference. It is the frame of reference where CBR is most close to isotropy.

12. Oct 1, 2010

### haael

God, I know that physical objects set preferred frames. Even Earth has its own frame that we all live in.

But this doesn't violate Lorentz invariance, just as the existence of the prime meridian doesn't stop the Earth from being a sphere.

13. Oct 1, 2010

### Upisoft

In other words it is completely true in a universe without matter (edit: well also energy and whatever other physical things can exist) i.e. no observers. I agree with that.

14. Oct 1, 2010

### Greylorn

I'm with you completely on this. I like relativity theory, but gave up plans to pursue post-graduate physics upon encountering quantum mechanics. No problems with the data, even at the theoretical level. IMO there is something fundamentally wrong with QM at the mathematical level, and at the level of interpretation. I'm not smart enough to figure out what, but am working on the possibility that time, which is a common element of both theories, is quantized. I'm just not good enough to work the math.

15. Oct 1, 2010

### Bob S

For any photon energy E=hν, it is possible to "multiply" the photon frequency ν by a factor 4γ2 by backscattering the photon beam off an approaching electron beam of energy γmc2. See Eq (2) in

http://www.docser.com/High-energy-photon-beam-production-with-laser-Compton-backscattering-2185553 [Broken]

So if anyone claims to have the maximum frequency photon beam, backscatter it off an electron beam of energy γmc2 and multiply the frequency by a factor 4γ2. In principle, there is no limit to this. Or is there?

Bob S

[Postnote] Detailed examination of the eq(2) in the above ref shows that in the high energy limit, the backscattered photon energy cannot exceed the energy of the electron beam, Eelectron = γmc2, as might be expected.

Last edited by a moderator: May 5, 2017
16. Oct 2, 2010

### Greylorn

That should be "easy" to determine. Has anyone tried backscattering the backscattered beam, ad infinitum until a frequency limit is reached?

If previous posts to this thread are correct, a sufficient number of cascaded photon backscatterings will produce matter. Creating matter from light could be a worthy experiment.

Last edited by a moderator: May 5, 2017
17. Oct 4, 2010

### mheslep

I doubt 'easy' is appropriate, given measurement (detection) difficulties at higher and higher freqs.

18. Oct 5, 2010

### haael

Not quite.

It is known since long ago, that equations may be symmetric but its solutions not. The canonical example is $$x^2 - 1 = 0$$. This equation is invariant under the transformation: $$x \arrow -x$$, but none of its solutions is.

That means, the equations of the Universe may be Lorentz-invariant, but the actual state of matter may not.
It is also possible that a ground state of equations be non-symetric, then we talk about a spontaneously broken symmetry.

Of course, the matter in the Universe still may be symmetric. For instance: if the Universe was uniformly filled with gas, then it would be translation-symmetric. Also, a single particle could be quantum "smeared" all over the Universe, so that a probability of finding it is equal at each point.

Last edited: Oct 5, 2010
19. Oct 6, 2010

### ueit

Because there is no absolute reference frame, the frequency is not objectively defined. The same photon appears to have different frequency for different observers. I see no reason why for a certain observer a photon could not have a wavelength smaller than Planck length. Do you know of a physical theory that forbids it?

20. Oct 6, 2010

### granpa

that sort of thing is why its hard to combine relativity and quantum mechanics

21. Oct 6, 2010

### Bob S

I agree. The reason that photon Compton backscattering on relativistic electron beams multiplies the photon energy by 4γ2 is because the photon has different frequencies in different reference frames. The Compton backscatter technique has successfully been used on electron beams up to about 45 GeV (γ = 88,000). As pointed out earlier, the maximum backscattered photon energy is the electron beam energy in the observer's reference frame. This is the only limit I am aware of.

Bob S

22. Oct 7, 2010

### Greylorn

Nope! Would a photon with such a small wavelength be capable of transferring its energy, nondestructively, to an observing instrument?

23. Oct 7, 2010

### Bob S

This is an interesting statement. The shortest wavelength that can be nondestructively deflected I believe is via Bragg diffraction on crystal planes: nλ = 2d sin θ. I seem to recall doing 411-KeV gold Au-198 photons (0.03 Angstroms) once.

How could "transferring its energy" be "nondestructive"? Are Compton scattering or the photoelectric effect nondestructive?

Bob S

24. Oct 8, 2010

### ueit

1. I do not see the relevance of the question.
2. Assuming that the question is relevant, what is the difference between a wavelength of 0.999 Planck and a 1.001 Planck one from the point of view of the "nondestructive transfer"?