Do we have a complete knowledge of the spectrum of light?

[m_robertson]

The question is pretty simple really, do we have a complete knowledge on the different wavelengths of light? Or are there possibly other sources of light out there that are unknown to us?

 

[sophiecentaur]

What do you mean by that question? The EM spectrum is a continuum and does not consist of individual wavelengths or frequencies. Any measurement system will have limited resolution and a wavelength will be specified within the range implied by the resolving power of the system. There are no 'undiscovered' gaps in the spectrum. However, there are sources that have not been 'discovered or studied in detail - weird chemical compounds perhaps and they will have their own characteristic spectra - consisting of a range of different wavelengths at different levels.

 

[Blackberg]

Not sure what you would consider to be "complete knowledge". As for sources, "moving electrical charges" covers them all.

 

[m_robertson]

What I mean is the different wavelengths of light. We have an understanding of the light spectrum from gamma waves all the way to radio waves, but is that the complete and entire picture? Or could there be things beyond gamma waves and radio waves which we have yet to detect with modern instruments?

 

[DrClaude]

What I mean is the different wavelengths of light. We have an understanding of the light spectrum from gamma waves all the way to radio waves, but is that the complete and entire picture? Or could there be things beyond gamma waves and radio waves which we have yet to detect with modern instruments?

You need to realize that "radio waves" and "gamma rays" are arbitrary names given to the same phenomenon. There can be something "beyond gamma rays" or "below radio waves" only if we decide to give electromagnetic radiation above or below a certain wavelength a special name. Otherwise, you have electromagnetic radiation, with different wavelengths/frequencies.

Currently, by definition, radio waves encompass the entire long-wavelength part of the spectrum, so the limit there would be a wavelength the size of the universe. At the other extreme, what happens to EM radiation of a wavelength comparable to the Planck length, nobody knows.

 

[mfb]

The most high-energetic photons observed at particle accelerators had en energy of more than 1 TeV, or about a million times the typical energy for "normal" gamma ray sources. The limit is just given by the collision energy in the LHC, for individual photons there is nothing fundamentally new expected until you reach the Planck scale, and they are all called gamma rays.

The other direction - low frequency - is quite boring I think, you get it from thermal radiation but it is too low-energetic to do anything interesting.

 

[blue_leaf77]

The question is pretty simple really, do we have a complete knowledge on the different wavelengths of light? Or are there possibly other sources of light out there that are unknown to us?

If I were allowed to interpret this question as "is there a region in the EM spectrum whose interaction properties with materials is still not well studied?", then I would say terahertz region, this covers frequencies between far infrared and microwave. It's not trivial to generate radiation within terahertz due to the fact that this region sets the working limit of conventional electronics.

 

[sophiecentaur]

If I were allowed to interpret this question as "is there a region in the EM spectrum whose interaction properties with materials is still not well studied?", then I would say terahertz region, this covers frequencies between far infrared and microwave. It's not trivial to generate radiation within terahertz due to the fact that this region sets the working limit of conventional electronics.

We could soon find ourselves in the classification game - ionising frequencies, 'photon' frequencies and good old RF, which electronics can deal with. All the same basic stuff though.

 

[Khashishi]

... you get it from thermal radiation...

Be careful how you use that term.

 

[mfb]

Be careful how you use that term.

Where is the problem?

 

[m_robertson]

You need to realize that "radio waves" and "gamma rays" are arbitrary names given to the same phenomenon. There can be something "beyond gamma rays" or "below radio waves" only if we decide to give electromagnetic radiation above or below a certain wavelength a special name. Otherwise, you have electromagnetic radiation, with different wavelengths/frequencies.

Currently, by definition, radio waves encompass the entire long-wavelength part of the spectrum, so the limit there would be a wavelength the size of the universe. At the other extreme, what happens to EM radiation of a wavelength comparable to the Planck length, nobody knows.

This is what I was getting at. So it's just down to a matter of terminology? The fact that we don't know what happens to EM radiation at the Planck scale is the answer I was looking for, so there are elements to light that aren't understood?

 

[mfb]

I would not call it "light", in the same way the sun is not a large speck of dust, but we don't know what happens at the Planck scale, yes. That applies to all of physics, electromagnetic radiation is not special here.

 

[Jon Richfield]

This is what I was getting at. So it's just down to a matter of terminology? The fact that we don't know what happens to EM radiation at the Planck scale is the answer I was looking for, so there are elements to light that aren't understood?

I am a bit uncomfortable with "...we don't know what happens to EM radiation at the Planck scale..." We DO know that as photons' frequencies rise, so does their energy. This implies that we cannot even in principle generate photons beyond a certain frequency. Imagine converting the energy in a gram of bulk matter into a single photon (good luck trying!) Imagine converting the energy content of a planet or a sun into a single photon (potentially immortal, right?)
Now, there are all sorts of interesting implications to being able to generate such a photon, even if only in DEEEEP space, and I don't know at what level of gross energy content we would exceed the Planck limit, but I also don't know how far this side of the Planck limit we could expect behaviour inconsistent with the behaviour of plain vanilla gamma photons.

Remember, the (necessarily fuzzy) boundaries between different regions of the extended spectrum are not as arbitrary as they look; they reflect differences between the behaviour of photons at different frequencies. Eg, blue light is called that because of the degrees to which various visual pigments absorb it. UV light is defined by its effects on orbital electrons, gamma on atomic nuclei, IR because of the way it wobbles flexible molecules with suitable distributions of charges, MW because of how it rotates molecules etc. It seems to me that the question is not so much the light being different as its frequency increases beyond the gamma limit, as the question of what sort of material effect we could expect to observe from such near- ( or trans-?) Planck limited light frequencies.

Just wondering whither I am wandering...

Hmmm... did someone say something about Big Bangs...?

 

[mfb]

This implies that we cannot even in principle generate photons beyond a certain frequency.

It does not - you just need a larger accelerator. Or you can simply go to a different coordinate system to get any photon energy you want.

It seems to me that the question is not so much the light being different as its frequency increases beyond the gamma limit, as the question of what sort of material effect we could expect to observe from such near- ( or trans-?) Planck limited light frequencies.

Nothing new, unless there are new, undiscovered particles. Beyond something like 1 GeV, all you get are interactions of photons with elementary particles, and the Standard Model can describe them very well. While the types of particles that can get produced in those collision still changes if you go to higher energies, from a macroscopic point of view it always looks the same: photon hits other particles and creates an electromagnetic shower (with smaller hadronic components).

 

[Jon Richfield]

It does not - you just need a larger accelerator. Or you can simply go to a different coordinate system to get any photon energy you want.

Firstly I think you missed the point. Two points in fact.
1: There is a finite amount of energy in the observable universe. Jiggle your coordinate system any way you like, you won't go beyond that (or are you betting?) I leave it to you to estimate the wavelength of the photon that would be equivalent to the mass-energy equivalent of say our closest 5 billion light years or so. Frankly I suspect that the gravitational effects of such a photon alone would present us with some intriguing problems. (Black hole on the cob, anyone?) :) That sort of thing is what I had in mind when musing about the sort of discontinuity of behaviour that might justify us in distinguishing our photon from gamma photons.

2: Even granting that we mustn't be greedy just for the sake of large scale physics, what sort of approach would you use to convert real-life diffuse sources such as condensed matter, not to mention plasma accumulations like the sun FTM, into a single photon? You would need some pretty fancy coordination, because you couldn't run after your photon, shouting "Wait, here are some tributary photons that you missed!" even if you compromised on producing TWO photons to conserve momentum, could you? That could seriously affect your options for collecting the energy from the rest of our local 10 billion-light-year diameter globe I reckon. Unless you happen to have news for me of course...

Nothing new, unless there are new, undiscovered particles. Beyond something like 1 GeV, all you get are interactions of photons with elementary particles, and the Standard Model can describe them very well. While the types of particles that can get produced in those collision still changes if you go to higher energies, from a macroscopic point of view it always looks the same: photon hits other particles and creates an electromagnetic shower (with smaller hadronic components).

Forgive me if I regard that assurance with reserve. There tend to be some very curious things that happen when one leaves known constraints; such as when we enter the scale of photons whose behaviour might be dominated by their own gravitational effects, and which will have to travel through some 5 billion light years of space that we have evacuated of particles to generate them. Something gives me a nasty feeling that something unexpected might happen, possibly even more unexpected than someone actually developing technology for creating even a mingy little 1-kg mass-equivalent photon, or even two of them. But don't hesitate to surprise me!

I realize that you could object that the government would never underwrite any such project, so that my proposal is unrealistic, and so it certainly is, but, like the original question, it is an academic concept, a gedankenexperiment if you like, and as a physicist, you will realize they they too have their place in the history of science. I, as it happens am not even equipped to carry such a gedankenexperiment to any sensible conclusion, but hoped that someone else might.

 

[mfb]

1: There is a finite amount of energy in the observable universe.

Yes, but it is orders of magnitude above the Planck energy. Like... 60 orders of magnitude.

Frankly I suspect that the gravitational effects of such a photon alone would present us with some intriguing problems. (Black hole on the cob, anyone?)

That is exactly the idea of the Planck energy (you would need this as invariant mass, a single photon alone cannot make a black hole, so you better collide two photons).

2: Even granting that we mustn't be greedy just for the sake of large scale physics, what sort of approach would you use to convert real-life diffuse sources such as condensed matter, not to mention plasma accumulations like the sun FTM, into a single photon? You would need some pretty fancy coordination, because you couldn't run after your photon, shouting "Wait, here are some tributary photons that you missed!" even if you compromised on producing TWO photons to conserve momentum, could you? That could seriously affect your options for collecting the energy from the rest of our local 10 billion-light-year diameter globe I reckon. Unless you happen to have news for me of course...

Build a massive linear accelerator for electrons, let them hit a target, some collisions will give photons close to the original electron energy. Do the same for the other side.

Forgive me if I regard that assurance with reserve. There tend to be some very curious things that happen when one leaves known constraints; such as when we enter the scale of photons whose behaviour might be dominated by their own gravitational effects, and which will have to travel through some 5 billion light years of space that we have evacuated of particles to generate them. Something gives me a nasty feeling that something unexpected might happen, possibly even more unexpected than someone actually developing technology for creating even a mingy little 1-kg mass-equivalent photon, or even two of them. But don't hesitate to surprise me!

As I said, at the Planck scale we know something new will happen. Before that, there could be something new, but we don't know. It could be very boring.

Sure, such a collider is so far out of reach that we don't need to discuss funding...

 

[Jon Richfield]

...
As I said, at the Planck scale we know something new will happen. Before that, there could be something new, but we don't know. It could be very boring...

Still, I have a totally unsupported suspicion, not even a self-respecting speculation, that if you pumped up a photon too deeply into Planck limit territory, it would split itself into more, lower-frequency photons and that would be a behaviour not to be expected of plain vanilla gamma rays. We would be justified in calling such light a new type, just as we speak of gamma ray photons, we then could speak of Planckrayons. Try getting fun dings for relieving gamma boredom in that way!

 

[Drakkith]

Since this thread appears to be straying off into speculation land, thread locked.