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This question already has an answer here:

I was having a discussion with a friend about supersonic flights.

Concord


It was then when he proposed an idea on an interesting experiment that can be done during a supersonic flight.

The experiment goes like this.

  • You board a supersonic jet along with your friend. The flight is cruising at Mach 1 above Amazon rainforests, or some other place with comparatively low air currents. You have somehow got the permission to do the experiment which involves getting out of the plane.

  • You both take all the safety measures and stand on both the ends of the aeroplane with yourself near the nose and your friend near the tail, both facing in the direction of the aeroplane's motion.

  • Your friend yells out something at you, but you can't hear it, because it's Mach 1.

  • You ask the pilot to slow down the plane, and the plane gradually decelerates. What happens now?

  • Your friend's sound catches up with you, and you hear the message!

  • Now, you yell out something at your friend.

  • You speed up the plane, and your friend catches up with the message.

  • Now, something remarkable happens when you do this. Your friend hears the message in reverse!!

  • Now the most interesting part:

If you keep on accelerating and decelerating the plane about the speed of sound, you can make your message to be heard multiple times by your friend!!

It is weird, but is such an observation plausible?

UPDATE: Most of the comments posted here are about the rewinding part. In fact, that is only a part of the question. The real question emphasizes on the idea, of receiving a signal by overtaking it.

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marked as duplicate by Kyle Kanos, AccidentalFourierTransform, Jon Custer, John Rennie, M. Enns Nov 29 '17 at 20:46

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ what-if.xkcd.com/37 $\endgroup$ – Sanchises Nov 25 '17 at 12:20
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    $\begingroup$ Possible duplicate of Sounds of supersonic objects $\endgroup$ – Kyle Kanos Nov 25 '17 at 13:05
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    $\begingroup$ Hi KRISHNANAND. Please remove the bolding and italicising-- it is unnecessary, annoying, and extremely childish. Bold and italics are meant to emphasise some words; dressing all of them defeats its purpose. Furthermore, the picture of a plane adds nothing to the question. Consider removing it as well. Thank you for your collaboration. $\endgroup$ – AccidentalFourierTransform Nov 25 '17 at 17:34
  • $\begingroup$ @AccidentalFourierTransform I think you are right; the extensive italics may not suit the interests of the public. Edited my post. $\endgroup$ – Krishnanand J Nov 25 '17 at 17:41
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    $\begingroup$ @KyleKanos: This is not a duplicate, because it is not about a stationary target receiving sound in reverse from a moving source, unlike the linked question. $\endgroup$ – user21820 Nov 30 '17 at 8:39
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Isn't it wonderful? Hearing the same message again and again without any effort from your part!

No it is not possible, obviously, because energy cannot come from nowhere. We did not even need to think about the specific details in your question.

Your friend yells out something at you, but you can't hear it, because it's Mach 1.

You cannot hear it, but not because of what you think. At Mach 1, everything your friend does (including moving his/her vocal chords) will result in little sonic booms around him/her. The vibrations cannot propagate faster (in terms of group velocity) than the speed of sound in air, which implies that you will not hear it. Also, those wavefronts will dissipate if the plane remains exactly at the speed of sound.

You ask the pilot to slow down the plane, and the plane gradually decelerates.

Your friend's sound catches up with you, and you hear the message!

Totally false! You will hear none of what your friend said during the time the plane was at Mach 1, and merely hear some of the sonic booms that had not yet dissipated, because all their wavefronts will simply accumulate just in front of your friend once the pilot starts decelerating. You will start hearing only what your friend says after the plane has started decelerating, and you will hear it at roughly the same rate it is spoken, but shifted slightly according to the net change in plane's velocity between the time it left your friend's mouth and the time it reaches your ear. [This clarification is in response to a comment. I had treated this effect as small since the deceleration was stated to be gradual.]

Actually, via simple reasoning it should have been obvious that you will not hear most of what your friend said before the deceleration, because otherwise your friend could speak an arbitrarily long message, but it is impossible for you to hear an arbitrarily long message in a finite span of time (the plane can decelerate to zero velocity in finite time).

Now, you yell out something at your friend facing forward, realising that if you had faced backwards, your message would get ultrasonic for your friend.

This makes no sense. Whether you are facing forward or not, the vibrations in the air that you produce will propagate outwards in roughly the same manner if the plane is travelling at less than Mach 1. The speed of propagation is the speed of sound in air, so it has nothing to do with the direction you are facing. You are not throwing a ball. You are making vibrations in the air.

As explained earlier, if you are facing backward, your friend hears it at the same rate it was spoken. If you are facing forward, you friend will also hear it similarly just muffled because you are blocking.

Now, you speed up the plane, and your friend catches up with the message. Now, something remarkable happens when you do this. Your friend hears the message in reverse!! If you keep on accelerating and decelerating the plane about the speed of sound, you can make your message to be heard multiple times by your friend!!

Even if you look at the 'forward-facing' part of the spherical wavefront, so that you friend can catch up with it later, note that the longer you wait the further the distance it has to travel and the lower the intensity. Also, once your friend catches up with it, it 'breaks' around him/her and will be gone, so he/she will not hear it multiple times.

In sum, your friend will hear what you say the first time from the 'backward-facing' part of the wavefront, which should arrive no matter whether you face forward or backward, and will hear what you say when the plane is at less than Mach 1, rather softly and in reverse the second time from the 'forward-facing' part of the wavefront, but only if you are facing forward when you said it. Also, he/she will not hear anything you say facing forward while at Mach 1 or higher, because the wavefront will 'break' against yourself and dissipate.

UPDATE: Most of the comments posted here are about the rewinding part. In fact, that is only a part of the question. The real question emphasizes on the idea, of receiving a signal by overtaking it. The energy loss due to the sound propagation can be easily solved by using a hollow straight tube, between the friends.

If you tightly seal one end of your tube to the mouth of the source and the other end to the ear of the target, then clearly you have a stationary medium relative to them through which sound will travel normally. You are effectively putting their mouth and ear back inside the plane, where as you know you can yell from the front to the back with no problem.

If there is a gap at both ends, obviously if the gap is not too small the air will flow through the tube at roughly the same velocity as outside the tube, only slightly lower. So the effects will be pretty much the same as having no tube. In particular, the 'forward-facing' wavefront when you are at less than Mach 1 travels forward, not backward into the tube, so it will not help your friend hear it.

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    $\begingroup$ I had mentioned "without any effort from your part". It does not apply to the airplane. $\endgroup$ – Krishnanand J Nov 25 '17 at 16:03
  • $\begingroup$ @KRISHNANANDJ: The random downvoters notwithstanding, I am sure that your 'thought experiment' is incredibly flawed for the reasons I gave in my answer, even if you think the plane can supply the energy you need. The fact is that it does not, because the plane does not supply any energy to produce the sound waves forming your utterance, so my underlying objection that you cannot get energy from nowhere is correct. $\endgroup$ – user21820 Nov 25 '17 at 16:16
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    $\begingroup$ Okay, but what about the forward facing part? I agree to the fact that sound propagation has no connection with your velocity, but I believe that the second observer perceives it to be ultrasonic, because the relative velocity of the sound according to him would have an ultrasonic magnitude. $\endgroup$ – Krishnanand J Nov 25 '17 at 17:05
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    $\begingroup$ @KRISHNANANDJ: You simply forgot that the source is moving at zero relative speed with respect to the target. The vibrations are not produced in the air with the same frequencies as when the source is stationary with respect to the air. The other answer about reversing music is irrelevant to your question for the same reason, as is the XKCD situation. $\endgroup$ – user21820 Nov 25 '17 at 17:11
  • $\begingroup$ That didn't occur to me. I will update my post. Thanks for pointing that out. $\endgroup$ – Krishnanand J Nov 25 '17 at 17:16
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The possibility of receiving a message or music in reverse was discussed by Lord Rayleigh, in his Theory of Sound (1896). If the source is moving toward the observer at twice the speed of sound, a musical piece emitted by that source would be heard in correct time and tune, but backwards.

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Well, there is actually a lot of effort on your part (going mach 1 outside a plane is not easy). There are two bigger problems though. The intensity of the sound is decreasing with distance as 1/d^2. So when you 'hear' your friend it is like you trying to hear him when he is a few hundred metres away. Apart from trying to detect such a low signal, there is also a supersonic boom, making it even worse.

EDIT: Since your question has also an edit: If you put a tube between the two friends, it is true you dont have a problem with sound volumen, but then you also dont have any of the effects you are looking for. This would be the same like sitting inside the airplane (since the air for communication is stationary between the two friend). Nothing weird happening inside a concorde.

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    $\begingroup$ Can one even breathe when moving supersonic relative to the air around you? I believe that breathing is a prerequisite to speaking. $\endgroup$ – kasperd Nov 25 '17 at 12:18
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    $\begingroup$ This does not answer the question. The question is not about the detectability of the signal. It's a thought experiment, and any valid answer should tell if the message is reversed or not, with an explanation. For comparison, nobody is going to send signals to an orbiting spacecraft while falling into a black hole, and given the chaos in the vicinity such signals would possibly be undetectable. But that does not make it a valid thought experiment. $\endgroup$ – sampathsris Nov 25 '17 at 13:33
  • $\begingroup$ Incidentally, the XKCD posted as a comment on the question by @Sanchises actually answers the question. $\endgroup$ – sampathsris Nov 25 '17 at 13:37
  • $\begingroup$ @Krumia I think it does. I focused on what followed "now the most interesting part:". The question you are alluding to has not even been mentioned in the text (apart from the heading) $\endgroup$ – lalala Nov 26 '17 at 15:59
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Yeah that is possible.

The moment you produce the sound, imagine that the sound travels in a perfect sphere with the point when you produced the sound as the centre. The radius of the sphere increases at a rate equal to the velocity of sound in air. You start from the centre if the circle... Traveling along with it.

Imagine the words you said as a train... The words you said first would be at the front and the ones you said last at the back. As you catch up from behind... You'll hear the sound in reverse.... Once you cross it when u slow down... It catches upto you and you hear it again... But when u speed up again you cross it from behind and hear it in reverse again.

An interesting thing is that the sonic booms would have no effect on this since we only cross mach 1 when we are behind the sound wave and since the sonic boom is in the form of backward cone... It wouldn't interfere with the original sound wave..

But as you go on the sound intensity will fade away... So it wouldn't be heard infinitely... Though we could hear it a few times.

Now all that is considering that at the exact moment when you produce the sound you are at rest...ie all the sound is made from a single source.

Else what will happen is if the sound is produced when travelling at mach 1 all the sound you produced will get superimposed cos when you say sthg... What you said the moment before is currently where you are... And so everything you say will get gobbled up into one... Cos its continuous no of spheres and not just a single sphere... A sentence would be quite long if you look at it like that

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  • $\begingroup$ +1 You should consider cleaning up your sentences. Last para reads like a message sent on a mobile phone. $\endgroup$ – Deep Nov 25 '17 at 10:52
  • $\begingroup$ Frankly, nearly everything you said is wrong. For example, you appear to agree with the asker that you can hear the same message multiple times, but you said that you understand my answer, which explained why you cannot... You need to look at how the wavefronts propagate. Just start with a single wavefront for a single pulse. It expands in a spherical shape, but each point on it will be dissipated when it hits something, including yourself if you travel fast enough to reach it. You'd have to go to multiple points to hear the pulse multiple times; on a straight line you hear at most twice only. $\endgroup$ – user21820 Nov 26 '17 at 10:05

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