# Is it possible to hear the past?

From this Stack Exchange Physics Post, I am certain that it is possible to view the past.

But then this interesting question came to me.

Is it possible to hear the past?

Ok, you might say, "Well, we are hearing the past, aren't we?" and go onto immense details about the speed of sound against the difference and stuff.

But what if I wanted to hear the sound of a place extremely close by days, weeks, months or even years back?

Would that be possible?

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Could you please be more specific? Surely every time we hear something, we hear the past. We hear what happened $t=s/v$ time earlier where $s$ is the distance of the source and $v$ is the speed of sound. For example, thunder is heard when the actual event, the lightning, is already several seconds in the past. It's increasingly hard for sound to propagate over too long distances $s$ because there's not enough medium for sound to propagate (sound goes around equator in 2 hr) - no air outside the earth etc; and because the sound weakens as it propagates. But in principle, we may hear the past. – Luboš Motl Sep 23 '12 at 7:04
@LubošMotl I mean like, days, weeks, months or even years back, from the exact same or similar location. Check my edit. – think123 Sep 23 '12 at 9:06

## 3 Answers

Luboš' comment really answers your question, but to more specifically address your comment:

The big difference between sound and light is that sound requires a medium to travel in while light does not. In fact light travels best in a vacuum where by definition there is no medium. The reason we can see back 13.5 billion years is because the light has been travelling through an almost perfect vacuum so there's nothing to impede it.

The problem with anything travelling through a medium is that no medium is perfectly elastic and there are always energy losses due to viscous damping. This is why when you strike a bell it may ring for a while but won't ring for anything like 13.5 billion years. You could ring the bell and wait for the sound wave to travel right round the Earth. You'd hear the sound about 120,000 seconds later, so this would allow you to hear a day and a bit into the past. However, unless it was an extraordinarily loud bell the sound would have decayed to below thermal noise in that time so you wouldn't be able to hear it.

This isn't really an answer to your question, but I mention it because it's cool: cosmic events like supernovae form shock waves when they hit ares of concentrated interstellar gas, and this is arguably a form of sound. In that case you can hear sound from many years ago, though you'd need an awfully big ear!

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i think it's possible. since sounds waves never die and it keep moving in the earth forever, maybe we can hear the sound of people who was in the room? but not with our ears maybe using the new technology. i see it scary somehow. hearing someone's voice and he's dead, but it's cool!

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Sound waves can diminish to below audible levels. – Kyle Kanos Mar 15 at 17:10
Also sound waves lose their coherence, becoming ever more randomized. At some point we regard the molecular vibrations as heat, and not sound. So the OP should ask is "what is the limit for recovery of an initial sound as coherence is lost?" – Peter Diehr Mar 15 at 17:17

Yes, this has been possible since the invention of the phonograph.

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## protected by Qmechanic♦Mar 15 at 16:47

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