# CM and Relativity



## MarkW (Feb 16, 2015)

Some years ago I was traveling on a plane, and was listening to CM on one of those plastic tube earpiece thingies they used to have that plugged into the armrest. And I wondered if all the planes had an onboard tape system, or whether the various music channels were picked up from a relay of broadcast links (this was before satellites were used for these purposes), and that led me to wonder what the effect of the jet's velocity was . . .

And of course that led me to wonder what you'd hear in a spacecraft going at relativistic velocities listening to a ground-based CM station. Would the Doppler effect change the tempo of what you were listening to? The key? The interval relationships? Would the music be recognizable?

Any physicists out there?


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## Heck148 (Oct 27, 2016)

It's possible that there is a rekstistic effect, but the velocities of an airliner relative to "c" are a very tiny percentage..yes, the plane will contract a tiny amount in length, etc, but would these changes even be detectable??
Now, a spaceship traveling at 90% of c, that would be quite different....


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## MatthewWeflen (Jan 24, 2019)

I do believe plane audio/video systems are all on board. As such, they would be experiencing whatever time dilation you were, and as such, relative to you there would be no difference.

This stack exchange thread provides formulae (about a third of the way down) for calculating the red shift of radio waves at given velocities. Long story short, even the fastest rockets we've ever made wouldn't induce a perceptible shift. But if you were to accelerate to a significant fraction of the speed of light, things would be different for sure. It would all sound like Celibidache, or even Cobra if things got worse.

https://space.stackexchange.com/que...pacecraft-red-shift-the-further-away-they-are


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## MarkW (Feb 16, 2015)

Thanks guys. I understand that airliner velocities are insignificant. But paragraph two specifically asks about a very fast spacecraft going at relativistic speeds. What do I hear?


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## Guest002 (Feb 19, 2020)

MarkW said:


> Thanks guys. I understand that airliner velocities are insignificant. But paragraph two specifically asks about a very fast spacecraft going at relativistic speeds. What do I hear?


If you were flying away from a ground-based radio station at something approaching the speed of light, it would be, relativistically-speaking, the same as if the ground-based radio station were flying away from a stationary you.

And we know what happens to light from a galaxy as it flies away from us at relativistic speeds: it red-shifts. Radio waves being no different in electro-magnetic terms than light waves, the radio broadcast would also be red-shifted, so its frequency would drop. But the information content of the radio wave wouldn't alter as its frequency dropped, so your music wouldn't slow down or drop in pitch.


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## Manxfeeder (Oct 19, 2010)

dizwell said:


> . But the information content of the radio wave wouldn't alter as its frequency dropped, so your music wouldn't slow down or drop in pitch.


I don't know why, but that's somehow reassuring.


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## MarkW (Feb 16, 2015)

Does it matter if it's AM or FM?


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## Guest002 (Feb 19, 2020)

No. Both are forms of modulation of the a carrier wave. Both carrier wave and its modulation are travelling at the speed of light. The frequency you tune to receive them will need looking at, but once you've built a wide band filter that can cope with the Doppler-related frequency shift, you'll be able to receive the signal -with its modulation of either type- at full speed (ignoring minor details such as dispersion of the signal over an ever-widening area, for example).


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## Heck148 (Oct 27, 2016)

MarkW said:


> Thanks guys. I understand that airliner velocities are insignificant. But paragraph two specifically asks about a very fast spacecraft going at relativistic speeds. What do I hear?


You would probably hear it as you are used to hearing it, since the time/velocity change would be uniform for everyone in the spacecraft....it would differ from what one heard at low velocity, or rest...but you would not be aware of it...


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## Phil loves classical (Feb 8, 2017)

Dizwell is right. The music information is modulated and independent of frequency of the signal. The challenge is only to receive the signal, which is the hard part. You need a larger antenna system the further from the source the spacecraft is, and need to keep monitoring the spacecraft info like the distance, velocity, etc. to receive the signal. Eventually if the spacecraft is too far, the signal becomes too weak and unrealiable, or too expensive to do it.


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## MatthewWeflen (Jan 24, 2019)

Planning a trip out of the solar system sounds pretty good lately.


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## MarkW (Feb 16, 2015)

In other words (I think), even though the Doppler shifted carrier wave elongates, along with the encoded signal, which by rights should cause the tempo to slow and the key to drop, the time dilation I would experience as I zipped along would affect what I perceive proportionally? (And yes, I have really fancy tuner.  )


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## BachIsBest (Feb 17, 2018)

MarkW said:


> In other words (I think), even though the Doppler shifted carrier wave elongates, along with the encoded signal, which by rights should cause the tempo to slow and the key to drop, the time dilation I would experience as I zipped along would affect what I perceive proportionally? (And yes, I have really fancy tuner.  )


No. The time dilation you experience is more or less irrelevant here. As said before, you need to modify your equipment but the information will stay the same and will be perceived at full speed. Do higher frequency radio stations play faster music? Of course not!


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## MarkW (Feb 16, 2015)

BachIsBest said:


> No. The time dilation you experience is more or less irrelevant here. As said before, you need to modify your equipment but the information will stay the same and will be perceived at full speed. Do higher frequency radio stations play faster music? Of course not!


I don't mind being a doofus, but in this case, the signal is already encoded before the frequency shifts. If I have this right (which admittedly I may not), when the frequency is red-shifted, the wave length increases, so that even though the signal is received at c, we get fewer "wave peaks" per unit time, so the encoded information arrives more slowly. (I don't mind if I am wrong. I just want to understand. )


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## SONNET CLV (May 31, 2014)

MarkW said:


> Some years ago I was traveling on a plane, and was listening to CM on one of those plastic tube earpiece thingies they used to have that plugged into the armrest. And I wondered if all the planes had an onboard tape system, or whether the various music channels were picked up from a relay of broadcast links (this was before satellites were used for these purposes), and that led me to wonder what the effect of the jet's velocity was . . .
> 
> And of course that led me to wonder what you'd hear in a spacecraft going at relativistic velocities listening to a ground-based CM station. Would the Doppler effect change the tempo of what you were listening to? The key? The interval relationships? Would the music be recognizable?
> 
> Any physicists out there?


Too bad Einstein's not around to fiddle with this question, but my own limited understanding of E=mc² and other such relativistically relevant (or even irrelevant) letter/number equations suggests that if you were on a spaceship moving at the speed of light you would likely hear the music before it even started to play … or is it that the music you hear would be music that was not yet written for another thousand light years … or is it simply that you'd hear the music backwards … or maybe you wouldn't hear the music at all because the music is listening to you instead … alas! This explains why I never became a physicist.


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## BachIsBest (Feb 17, 2018)

MarkW said:


> I don't mind being a doofus, but in this case, the signal is already encoded before the frequency shifts. If I have this right (which admittedly I may not), when the frequency is red-shifted, the wave length increases, so that even though the signal is received at c, we get fewer "wave peaks" per unit time, so the encoded information arrives more slowly. (I don't mind if I am wrong. I just want to understand. )


The frequency is changed. However, this merely affects what frequency you need to set your transmitter at. The frequency of the light waves you receive is not related to the frequency of the sound waves your radio produces.


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## BachIsBest (Feb 17, 2018)

SONNET CLV said:


> Too bad Einstein's not around to fiddle with this question, but my own limited understanding of E=mc² and other such relativistically relevant (or even irrelevant) letter/number equations suggests that if you were on a spaceship moving at the speed of light you would likely hear the music before it even started to play … or is it that the music you hear would be music that was not yet written for another thousand light years … or is it simply that you'd hear the music backwards … or maybe you wouldn't hear the music at all because the music is listening to you instead … alas! This explains why I never became a physicist.


Unfortunately, it is impossible for massive objects to reach the speed of light. You could ask 'what if' questions (accepting the fact that these lead to possibly contradictory results as you have broken the rules) but in this case, things become truly stupid. In a reference frame travelling at the speed of light, the entire universe should (of course because this is impossible there could be some debate on the matter) be a point (as in it all literally collapses to a point). This makes the entire question very irrelevant. The original question was what happens when the speed gets close-ish to the speed of light which is theoretically possible.

If you wanted to hear the music before it played you would have to be travelling faster than the speed of light as, after you collapsed the universe to a point going the speed of light, if you went faster the universe would re-expand but in the other direction. i.e., all dimensions would be flipped, _including time_.


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## KenOC (Mar 7, 2011)

It seems to me that, if you were moving away from the music source, the frequency of the signal would be red-shifted downwards. Then any signal received from that frame of reference would have an increased distance between peaks and valleys (from your point of view) and, if received, would necessarily be at a lower frequency and the music would last longer. If you were approaching the signal source, the opposite would occur.

Off the top of my head, of course.


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## DavidA (Dec 14, 2012)

Brings me in mind of a thought experiment we mused todo at university called the Twin Paradox. In physics, the twin paradox is a thought experiment in special relativity involving identical twins, one of whom makes a journey into space in a high-speed rocket and returns home to find that the twin who remained on Earth has aged more.


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## Guest002 (Feb 19, 2020)

MarkW said:


> I don't mind being a doofus, but in this case, the signal is already encoded before the frequency shifts. If I have this right (which admittedly I may not), when the frequency is red-shifted, the wave length increases, so that even though the signal is received at c, we get fewer "wave peaks" per unit time, so the encoded information arrives more slowly. (I don't mind if I am wrong. I just want to understand. )


"Fewer wave peaks per unit of time" is the very definition of a lowering of frequency, so that sentence is a bit tautological.

But anyway: as to your underlying question.

Does Radio 3 (broadcasting at 92.6MHz) play music more slowly than Classic FM (broadcasting at 101.9MHz)?

[Pick any other two classic music stations you're familiar with if you're not living in the UK!]

If you agree that it does not play the music more slowly, can you then see why lowering the frequency of the carrier wave doesn't alter the rate at which the carrier wave is modulated?

If you can't then perhaps it's because (like most of us, I think!), you don't quite understand how FM works. With FM, we encode information in a carrier wave by varying the instantaneous frequency of the wave. And the instantaneous frequency is the difference between the frequency of the carrier and its center frequency. And that difference is proportional to the modulating signal. So, the carrier signal will drop in frequency because of red shift. And the centre frequency will drop equivalently in frequency, because of the red shift. And the proportional difference between the carrier frequency and the modulating signal will thus remain constant, since both frequencies drop by an equivalent amount due to the same red shift applying to both. And therefore, you will receive the same signal at a lower frequency as at a higher one.

And that's precisely the same reason why Mozart or Bach sounds identical whether you listen to it on Radio 3 or Classic FM (except that the announcers sound more intelligent on Radio 3).


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## Guest002 (Feb 19, 2020)

KenOC said:


> It seems to me that, if you were moving away from the music source, the frequency of the signal would be red-shifted downwards. Then any signal received from that frame of reference would have an increased distance between peaks and valleys (from your point of view) and, if received, would necessarily be at a lower frequency and the music would last longer. If you were approaching the signal source, the opposite would occur.
> 
> Off the top of my head, of course.


Again, that's not how FM works. Simplified explanation coming up! Here's an FM signal:









FM works by adjusting the distance between peaks and valleys all the time!

Your music isn't the reception of a peak or a trough of the wave form. It's the reception of the difference between a series of peaks/troughs and the base frequency of the carrier wave. So as your carrier wave frequency lessens due to red shift, so the wave gets spaced out by the same amount -and therefore the difference between the peaks/troughs and the base frequency doesn't alter.

To take a really daft example, in that picture above, you could interpret the 'compacted together' bit of the signal as a 1, and the 'spaced out' bits as a zero, right?

Well, now red shift the signal:









So now things are spaced out more and the frequency has dropped -but you can still identify a 'compacted together' bit and a 'spaced out' bit, right? So you can still interpret a 1 and a 0. In other words, your eyes can perceive a variance from the 'ordinary' signal, no matter whether that 'ordinary signal' is tight and high frequency or stretched out and low frequency. FM is the same: the music component is discernible as a 'variance from the ordinary' -and if the 'ordinary' is stretched out, it doesn't alter the radio's ability to discern a variance from it. And the variance will be perceived as a quantity that is identical to the way it was originally encoded, because it and the signal it's a variance from have both been stretched by equivalent amounts.


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## Phil loves classical (Feb 8, 2017)

Keep in mind, they can't use FM or AM for deep space travel. The power required to transmit at those low frequencies would just be phenomenal. Then at increasingly higher frequencies, the Doppler effect becomes less, until it wouldn't not noticeable. This is only for analog signals. In reality, just as with satellite radio, they use digital signals, which have information stored within each packet to render the information accurately.


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## millionrainbows (Jun 23, 2012)

So it wouldn't turn Karajan into Celibidache...


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## Guest002 (Feb 19, 2020)

Phil loves classical said:


> Keep in mind, they can't use FM or AM for deep space travel. The power required to transmit at those low frequencies would just be phenomenal. Then at increasingly higher frequencies, the Doppler effect becomes less, until it wouldn't not noticeable. This is only for analog signals. In reality, just as with satellite radio, they use digital signals, which have information stored within each packet to render the information accurately.


Yes, I think we all understand that we're talking very hypothetically! Unless you happen to have an engine in your shed that can get you to ~0.9c!

The original question, remember, was just about how we'd hear a ground-based radio station if travelling at relativistic speeds. It's about how Radio 3 would sound to Captain Kirk if he was passing by, not whether Radio 3 should switch to packet-based deep space network transmission


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## MarkW (Feb 16, 2015)

KenOC said:


> It seems to me that, if you were moving away from the music source, the frequency of the signal would be red-shifted downwards. Then any signal received from that frame of reference would have an increased distance between peaks and valleys (from your point of view) and, if received, would necessarily be at a lower frequency and the music would last longer. If you were approaching the signal source, the opposite would occur.
> 
> Off the top of my head, of course.


That's kind of what I was thinking and where I was heading, but I'm only a layperson.


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## MarkW (Feb 16, 2015)

dizwell said:


> Again, that's not how FM works. Simplified explanation coming up! Here's an FM signal:
> 
> View attachment 132451
> 
> ...


How about AM, which is based on modulating the amplitude of an individual wave, rather than measuring variance from a reference frequency>


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## Guest002 (Feb 19, 2020)

MarkW said:


> How about AM, which is based on modulating the amplitude of an individual wave, rather than measuring variance from a reference frequency>


Same deal: it doesn't alter a thing. AM encodes the signal as a variance in amplitude of the carrier signal, with the amplitude being proportional to the signal being encoded. Stretch the wave (i.e., lower its frequency) and it doesn't change the variance in amplitude against the carrier signal, though that carrier signal is now at a lower frequency than it was before.

I suppose basically the confusion here is that radio waves aren't sound waves. The wave that the radio receives is not like the sound wave your ear drum receives. The wave being received is not the music (whereas the sound compressions and decompressions received by your eardrum are, indeed, a direct representation of the music). So, yes, if we slowed down the rate of compressions and decompressions in a sound wave, the pitch would change and things would sound weird. Or like Ferneyhough.

But if the frequency of a radio signal varies -well, the radio isn't converting the radio wave itself into sound. It's converting the change of instantaneous frequency (FM) or amplitude (AM) against a common reference frequency. So, if that frequency decreases, it doesn't change the relative proportionality of the instantaneous frequency or amplitude. Therefore, the music encoded by the radio wave is returned unaffected in either case.


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## MarkW (Feb 16, 2015)

dizwell said:


> Same deal: it doesn't alter a thing. AM encodes the signal as a variance in amplitude of the carrier signal, with the amplitude being proportional to the signal being encoded. Stretch the wave (i.e., lower its frequency) and it doesn't change the variance in amplitude against the carrier signal, though that carrier signal is now at a lower frequency than it was before.
> 
> I suppose basically the confusion here is that radio waves aren't sound waves. The wave that the radio receives is not like the sound wave your ear drum receives. The wave being received is not the music (whereas the sound compressions and decompressions received by your eardrum are, indeed, a direct representation of the music). So, yes, if we slowed down the rate of compressions and decompressions in a sound wave, the pitch would change and things would sound weird. Or like Ferneyhough.
> 
> But if the frequency of a radio signal varies -well, the radio isn't converting the radio wave itself into sound. It's converting the change of instantaneous frequency (FM) or amplitude (AM) against a common reference frequency. So, if that frequency decreases, it doesn't change the relative proportionality of the instantaneous frequency or amplitude. Therefore, the music encoded by the radio wave is returned unaffected in either case.


Thanks! .


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## Phil loves classical (Feb 8, 2017)

dizwell said:


> Yes, I think we all understand that we're talking very hypothetically! Unless you happen to have an engine in your shed that can get you to ~0.9c!
> 
> The original question, remember, was just about how we'd hear a ground-based radio station if travelling at relativistic speeds. It's about how Radio 3 would sound to Captain Kirk if he was passing by, not whether Radio 3 should switch to packet-based deep space network transmission


If there was some way the analog FM/AM station could transmit to space, then at those low frequencies the Doppler effect would be so big, you'd have to change the FM/AM tuning constantly. You could track the signal by the signal-to-noise ratio, and adjust the tuner automatically, assuming there are no rival stations with a large signal that would throw the tracking off. Hypothetically, at a good sample rate, and high signal, you wouldn't notice a thing in the music.


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## SONNET CLV (May 31, 2014)

BachIsBest said:


> Unfortunately, it is impossible for massive objects to reach the speed of light. ....


So … does this mean no Bruckner or Mahler Symphonies (or even Beethoven's Ninth!) on a spaceship? I hope this doesn't mean we will have to settle for, God forbid!, AM-Radio-length "pop" music!


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## BachIsBest (Feb 17, 2018)

SONNET CLV said:


> So … does this mean no Bruckner or Mahler Symphonies (or even Beethoven's Ninth!) on a spaceship? I hope this doesn't mean we will have to settle for, God forbid!, AM-Radio-length "pop" music!


Actually, only Cage's 4:33 can reach the speed of light.


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## Guest002 (Feb 19, 2020)

Phil loves classical said:


> If there was some way the analog FM/AM station could transmit to space, then at those low frequencies the Doppler effect would be so big, you'd have to change the FM/AM tuning constantly. You could track the signal by the signal-to-noise ratio, and adjust the tuner automatically, assuming there are no rival stations with a large signal that would throw the tracking off. Hypothetically, at a good sample rate, and high signal, you wouldn't notice a thing in the music.


Well, accounting for the Doppler shift, though not at relativistic levels, is a solved problem (witness: the Voyager probes, Cassini and so on). You're right it's a problem, but in the world of utter hypotheticals that this thread has drifted into, it's not one that can't be fixed.


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## Phil loves classical (Feb 8, 2017)

dizwell said:


> Well, accounting for the Doppler shift, though not at relativistic levels, is a solved problem (witness: the Voyager probes, Cassini and so on). You're right it's a problem, but in the world of utter hypotheticals that this thread has drifted into, it's not one that can't be fixed.


A black box fancy tuner should do the trick.


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