# The Naked Scientists Forum

### Author Topic: What is the lowest possible frequency for light?  (Read 4676 times)

#### jeffreyH

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##### What is the lowest possible frequency for light?
« on: 13/08/2015 01:53:06 »
Apart from just saying zero, is there a lowest frequency for a photon below which it is not possible to go?

#### alancalverd

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##### Re: What is the lowest possible frequency for light?
« Reply #1 on: 13/08/2015 07:21:25 »
No. The electromagnetic spectrum is continuous, but quantum effects are less obvious, and wave models more appropriate, at lower frequencies. I can't think of a quantum transition below about 0.1 eV, say 25THz, but there's no discontinuity of the wave properties.

#### lightarrow

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##### Re: What is the lowest possible frequency for light?
« Reply #2 on: 13/08/2015 13:13:56 »
Apart from just saying zero, is there a lowest frequency for a photon below which it is not possible to go?
If you really mean "light", that is the visible part of the EM spectrum, its range is 430–790 THz:
https://en.wikipedia.org/wiki/Visible_spectrum
so the minimum is 430 THz.
Or you intended "electromagnetic radiation"? IR, radio waves, etc, are not "light".

--
lightarrow

#### PmbPhy

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##### Re: What is the lowest possible frequency for light?
« Reply #3 on: 13/08/2015 20:24:58 »
Quote from: jeffreyH
Apart from just saying zero, is there a lowest frequency for a photon below which it is not possible to go?
Why do you refer to it as "light" in the subject heading and here as "a photon"? They don't mean the same thing.

If you're referring to photons then there is none. Suppose you had a photon traveling in the +x direction in the inertial frame S which you thought had the lowest possible frequency. Transform to a frame of reference that is moving in the -x-direction. In this frame the photon is red shifted meaning that the frequency is lower than it was in S. Therefore your assumption that there was a lowest frequency is wrong.

#### timey

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##### Re: What is the lowest possible frequency for light?
« Reply #4 on: 13/08/2015 21:12:14 »
Unless of course time has stopped, in which case your photon isnt going anywhere and there will be no redshift.

#### scotty stull

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##### Re: What is the lowest possible frequency for light?
« Reply #5 on: 13/08/2015 21:26:07 »
One cycle from the source that produced the electromagnetic pulse and the source that produced this " one pulse " is disabled forever.

#### jeffreyH

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##### Re: What is the lowest possible frequency for light?
« Reply #6 on: 14/08/2015 00:42:17 »
Quote from: jeffreyH
Apart from just saying zero, is there a lowest frequency for a photon below which it is not possible to go?
Why do you refer to it as "light" in the subject heading and here as "a photon"? They don't mean the same thing.

If you're referring to photons then there is none. Suppose you had a photon traveling in the +x direction in the inertial frame S which you thought had the lowest possible frequency. Transform to a frame of reference that is moving in the -x-direction. In this frame the photon is red shifted meaning that the frequency is lower than it was in S. Therefore your assumption that there was a lowest frequency is wrong.

Typo Pete I meant photon. I was thinking about light in particular before I posted this. I am thinking of locally emitted photons. So that the frequency can be detected in the local frame. Just like we can produce x-rays. Except I want the other end of the spectrum. I am assuming that zero frequency is out as that would have zero energy and I am also assuming that there must be a non-zero frequency below which it is not possible to produce a photon. Think in terms of a quantization of frequency. The Planck frequency you might say.

#### PmbPhy

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##### Re: What is the lowest possible frequency for light?
« Reply #7 on: 14/08/2015 02:51:17 »
Quote from: jeffreyH
Typo Pete I meant photon.
So when you meant "photon" you instead wrote "light"? Okay.

Quote from: jeffreyH
I was thinking about light in particular before I posted this. I am thinking of locally emitted photons. So that the frequency can be detected in the local frame. Just like we can produce x-rays. Except I want the other end of the spectrum. I am assuming that zero frequency is out as that would have zero energy and I am also assuming that there must be a non-zero frequency below which it is not possible to produce a photon. Think in terms of a quantization of frequency. The Planck frequency you might say.
Frankly I always ignore conversations about Planck units.

My answer still stands. There's no lower bound to the frequency of a photon. However there may be practical problems detecting such photons.

#### evan_au

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##### Re: What is the lowest possible frequency for light?
« Reply #8 on: 14/08/2015 12:21:09 »
Quote from: alancalverd
I can't think of a quantum transition below about 0.1 eV
How about the 21cm Hydrogen line, with an energy around 5μeV?
"Hyperfine levels" sounds like a quantized transition to me...

Quote from: jeffreyH
is there a lowest frequency for a photon below which it is not possible to go?
Quantum theory talks about quantisation of the energy of photons confined to a box of certain dimensions. Wavelengths beyond a certain size don't fit in the box.

Could you consider the universe to be a box of very large dimensions? This would define a maximum wavelength of around 14 billion light years...

Of course the universe is expanding, which will stretch the lower limit to longer wavelengths over time...

Quote
Apart from just saying zero...
A constant electric or magnetic field (zero frequency) does not radiate energy (ie photons) when observed in the same inertial frame of reference.

Of course, even for an observer in the same frame of reference, you need to turn on the "constant" electric or magnetic field, which represents a frequency which will radiate photons. I guess the slowest you could turn on an electric or magnetic field on Earth would be about 5 billion years - or if you start from Michael Faradays experiments with electromagnetism, a century or so.

However, I vaguely recall hearing that a constant electric or magnetic field, when viewed from a different frame of reference, does emit photons? [Did I read this correctly?] Does this mean that even zero frequency cannot be considered zero frequency for all observers?

#### PmbPhy

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##### Re: What is the lowest possible frequency for light?
« Reply #9 on: 14/08/2015 18:36:21 »
Quote from: evan_au
I guess the slowest you could turn on an electric or magnetic field on Earth would be about 5 billion years -
Not really. An electric field is on or off depending on whether it exists or it doesn't. You can therefore change the magnitude or direction at any rate you desire.

#### alancalverd

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##### Re: What is the lowest possible frequency for light?
« Reply #10 on: 14/08/2015 22:40:56 »
Quote from: alancalverd
I can't think of a quantum transition below about 0.1 eV
How about the 21cm Hydrogen line, with an energy around 5μeV?

I stand corrected. Must engage brain.

#### jeffreyH

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##### Re: What is the lowest possible frequency for light?
« Reply #11 on: 15/08/2015 12:01:33 »
What I was wondering is how the longest wavelength photon would interact with the shortest wavelength gravitational wave.

http://www.tapir.caltech.edu/~teviet/Waves/gwave_spectrum.html

I have no idea how you could determine the wavelength relationship from the acceleration and mass of an object generating the gravitational waves. Actually detecting such long wavelength photons would also be a difficulty.

#### PmbPhy

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##### Re: What is the lowest possible frequency for light?
« Reply #12 on: 15/08/2015 12:14:15 »
What I was wondering is how the longest wavelength photon would interact with the shortest wavelength gravitational wave.

http://www.tapir.caltech.edu/~teviet/Waves/gwave_spectrum.html

I have no idea how you could determine the wavelength relationship from the acceleration and mass of an object generating the gravitational waves. Actually detecting such long wavelength photons would also be a difficulty.
In what way are you referring to gravitational waves interacting with light waves?

#### evan_au

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##### Re: What is the lowest possible frequency for light?
« Reply #13 on: 15/08/2015 12:33:50 »
If you had a long-wavelength coherent source, and it crossed paths with a gravitational wave, the stretching of space would cause a slight phase shift in the long-wavelength source.

We don't currently know of any long-wavelength coherent sources, we don't have antennas big enough to detect photons of wavelength much larger than the Earth, and small phase changes in a very low frequency are very hard to detect. So this doesn't seem like a very promising approach.

However, we do have a good alternative: pulsars produce wideband bursts (radio to optical and X-Rays), but at a very low frequency: 1 to 1000 times per second (pulse wavelengths of 105 to 108m=larger than the Earth). Because it is a wideband signal, we can still detect it despite its enormous pulse wavelength.

Gravitational waves will cause a phase shift in the arrival time of these pulses. In principle, this could be detectable, if you observed long enough. This is the basis of the Pulsar Timing technique described on the graph (link above).

#### jeffreyH

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##### Re: What is the lowest possible frequency for light?
« Reply #14 on: 15/08/2015 12:55:11 »
What I was wondering is how the longest wavelength photon would interact with the shortest wavelength gravitational wave.

http://www.tapir.caltech.edu/~teviet/Waves/gwave_spectrum.html

I have no idea how you could determine the wavelength relationship from the acceleration and mass of an object generating the gravitational waves. Actually detecting such long wavelength photons would also be a difficulty.
In what way are you referring to gravitational waves interacting with light waves?

That's the bit I don't know. There should be an effect but what would it be?

#### jeffreyH

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##### Re: What is the lowest possible frequency for light?
« Reply #15 on: 15/08/2015 12:59:03 »
What if the wavelength of the photons equalled the wavelength of the gravitational wave?

#### PmbPhy

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##### Re: What is the lowest possible frequency for light?
« Reply #16 on: 15/08/2015 13:19:09 »
Quote from: jeffreyH
That's the bit I don't know. There should be an effect but what would it be?
The effect would be extremely difficult, if not a practical impossibility, to detect. A gravitational wave is basically a time varying metric, i.e. a "ripple" in spacetime curvature. As light passes through it will undergo changes in wavelength at a rate at which the gravitational wave oscillates. Therefore the light may or may not come through it with an altered frequency.

#### jeffreyH

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##### Re: What is the lowest possible frequency for light?
« Reply #17 on: 15/08/2015 13:35:08 »
Quote from: jeffreyH
That's the bit I don't know. There should be an effect but what would it be?
The effect would be extremely difficult, if not a practical impossibility, to detect. A gravitational wave is basically a time varying metric, i.e. a "ripple" in spacetime curvature. As light passes through it will undergo changes in wavelength at a rate at which the gravitational wave oscillates. Therefore the light may or may not come through it with an altered frequency.

I agree. Not a very practical proposition.

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##### Re: What is the lowest possible frequency for light?
« Reply #17 on: 15/08/2015 13:35:08 »