Some time ago there was a discussion of the minimum wavelength for “light”. As I recall, there was more-or-less consensus on the Planck length. Anyway, my question is what is the maximum wavelength for light (and, I guess, the minimum frequency)?
If you’re looking for the maximum wavelength for light, why does your Thread Title call for the minimum? Or are you really asking for the minimum and you misspoke?
The minimum and the maximum for light are really matters of definition. Visible light is usually held to run from circa 400 nm to 700 nm. the response of the eye doesn’t abruptly cut off, so there’s no definite rule. Ultraviolet extends to shorter wavelengths, down to somewhere around 150 nm. Below that you have X-rays and Gamma Rays, whose ranges overlap, and are distinguished by their method of generation. Some people would still call this 'light". At longer wavelengths you have infrared light, which goes from circa 700 to several microns (how many, again, depending upon who’s doing the defining), beyond which are microwaves, radio waves, millimeter waves, centimeters waves, etc. It’s all electromagnetic waves and electromagnetic radiation. I have no idea what the shortest wavelength measured is (although it is inversely proportional to the energy of the photon) nor the longest, but there is no definite shortest or longest wavelength I know of. Cosmic rays and muonic transitions can have phenomenally high energies.
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I don’t know anything directly linking this to the Planck length. as Chronos has noted, it’s an overused measure.
There isn’t any theoretical maximum for wavelength that I know of. Anything falling into a black hole will become infinitely redshifted as it passes through the event horizon, which means any light emitted will tend to infinitely long wavelengths. The same goes for things far enough away for the expansion of the universe, where the farther a galaxy is from us, the more it becomes redshifted.
I think some of the confusion in this thread is coming from your use of the word “light” to mean electromagnetic radiation in general. The minimum and maximum wavelengths for electromagnetic radiation are thought to be the Planck length and the width of the universe, respectively.
We tend to divide up the electromagnetic spectrum into groups based on general properties of that group, though the electromagnetic spectrum is continuous and the dividing lines between groups is rather arbitrary.
“Light” is generally everything past the top of the microwave band (300 GHz) to the beginning of the X-ray band (30 petahertz). Like I said though, these divisions are arbitrary, and there’s nothing significantly different between 299 GHz, which is in the microwave band, and 301 GHz which is in the far infrared light band.
Light is also further divided into infrared, visible, and ultraviolet, with the visible portion being 400–790 THz. Again, this is an arbitrary designation, because as CalMeacham already mentioned, the eye’s response with respect to frequency doesn’t stop abruptly.
I see no reason a wavelength needs to be limited by the boundries of the universe. One may not be able to get a complete phase, but the wavelength should be definable beyond that.
Could one broadcast a modulated signal at optical frequencies; effectively treating light like radio waves for purposes of communication?
Sure. It is done all the time. TV remote controls modulate infrared light to send pulse codes to your television. Fiber optic systems use light to transmit signals.
You could do it with visible light as easily as you could do it with infrared, but it would be a bit annoying for your eyes.
The easiest way to make a light “transmitter” is to use your signal to modulate the current through an LED. Your “receiver” would then be based around a photodiode or phototransistor.
There is no reason to believe that light cannot have a wavelength shorter than the Planck length. If light has a minimum wavelength, then it’s not an unreasonable guess that it might be somewhere in the general vicinity of the Planck length, but it might be that the minimum is some other length entirely, or it might be that there is no minimum length. We just don’t know enough to say.
I’m trying to figure out what a wavelength shorter than the Planck length would be like. Suppose it was a third of a Plank length. That means three waves would fit in that physical minimum unit of length. How might one distinguish between that and a wave whose wavelength was a tenth of a Planck length? Wouldn’t they be indistinguishable?
It is a common misconception that the Planck length is the smallest measurable distance, or the grid-scale on which the world is quantized, or various such things. The Planck length is not actually known to have any particular physical significance; it’s just the length that falls out of the relations between other constants. It’s not unreasonable to guess that this will turn out to have some significance in itself, but if there is any, it is as of yet unknown.