Just a mental meandering today, do sound waves ever disappear? I understand that they diminish with distance, but is there ever a point where they disappear altogether?
Or are there (very) feint waves still hanging around in the universe, from when the dinosaurs were stomping around in the forest and or when Noah bitched at the umpire of the Palestinian U18 Reserves about the free kick that was awarded against him.
A sound wave has no particle feature, it only exists as the traveling pressure front… which is really variations in speed of the gas particles.
By quantum theory, the velocity of the gas particles can only change to specific energy levels. Since a sound wave spreads out, it must spread out eventually to be so low intensity , that it can’t have enough energy density to cause the quantum energy change in the nearby molecule that would be required .
IF I understand Isilder correctly (and please correct me if I’m not), then what you mean is, “Theoretically, the energy never really reaches zero, but it does reach a point where it goes below one Planck unit, which is effectively zero.”
If that’s correct, then please explain how light is different than sound. I noticed that you qualified your response by pointing out that sound “has no particle feature”, whereas photons are indeed particles. But I don’t see how that would make a difference. If the energy yielded by a photon was dependent on it being a particle, then my ability to see a given light source would be very iffy.
Consider a nebula, quasar, or something else that is a gazillion lightyears away, but is visible to my supertelescope. If I am looking at the light waves emitted by that source, then they propagate infinitely in all directions, just as sound waves do. But if my telescope is dependent on seeing individual photons, isn’t their number something short of infinite? In which case, after traveling such a long distance, my telescope might miss them if the telescope were located an inch to the right or left?
If the argument I made in the above paragraph is correct, then both light and sound “only exist as a traveling pressure front”, in which case, I ask: Given how far the light has traveled, and how low its energy level must be by now, can we calculate how close it must be to passing below the Planck limit?
I’m not sure I understand what you’re getting at here; you seem to be thinking of things in very strange terms. Your ability to see a light source is due to photons hitting your retina. I’m not sure what you mean by “the energy yielded by a photon.”
Sound waves and light waves are not remotely similar. Sound is caused by atoms bumping into one another, creating waves of compression that travel through matter. Light is photons, and it’s also waves propagated (transversely) in the electromagnetic field.
Photons tend to travel indefinitely until they bump into something, like some matter (say your telescope, or your eye) or they get trapped in something like a black hole. Photons don’t lose energy as they travel. (OK, yes, there is stuff like red/blue-shifting. Ignore that for now. Just think of photons as little buckets of a set amount of energy that fly through space at exactly the speed of light until they are stopped by something.)
Sound waves don’t. Not even remotely. As Isilder said, once they diminish to the point where they can not actuate motion in the next bunch of atoms, they cease propagating. Sound waves diminish because they spread out through their transmission medium, so the further away you are, the less sound energy gets to you. (They also diminish because of friction. Every time an atom bumps into an atom in front of it, some of this kinetic energy is converted to heat. The next atom therefore bumps a little less hard into the subsequent one, and so on.) Eventually, the atoms are bumping so softly that their motion is indistinguishable from the normal random motion of the matter. And of course sound waves cease propagating if there is no matter in front of them to propagate through. In space, nobody can hear you scream.
The reason that light gets dimmer the farther we are away from it is also because it spreads out. (Shine a flashlight at the wall, now move the flashlight further away.) Therefore the farther you are from the light source, the fewer photons will be pointed in your precise direction. Additionally, there’s all sorts of stuff like gas, other planets, brighter nearby light sources, and stuff that makes it harder to see stuff far away.
The number of photons in the universe is probably finite, yes. We can be reasonably certain that we will see photons from things we know about because they tend to travel (more or less) in a straight line. So if we look at your quasar and see some photons from it, we will probably see some photons from it again if we look again.
Light is NOT a “traveling pressure front” in any sense of that phrase. The medium of transmission is the electromagnetic field; photons are not exerting “pressure” on this field. They are transverse waves, perpendicular to the direction of travel, like waves on the ocean. (OK, ocean waves are a terrible analogy for a lot of reasons, but they give you a good visual idea of what we’re talking about.)
At some point, light from distant objects becomes indistinguishable from the random noise around it. But this has nothing to do with pressure fronts. The energy of individual photons doesn’t change (much) as they travel, only the number of photons that we can detect.
Yes - large, but certainly not infinite.
Yes: a telescope an inch, foot, or mile away would be receiving different photons - but still large numbers of them, and telling much the same story.
Okay, let’s presume that the quasar emits a finite number of photons. If I can see the quasar, it is because the quasar emitted a photon towards me, and it will presumably emit another photon in the same direction an instant later.
Now, suppose I move one inch to the left. Do I still see that quasar? If I do, would you say that it is because the quasar emitted another photon in a slightly different direction. Can I see it if I moved only a half inch? A millimeter? How can I see it in all these places if the number of photons is finite? Haven’t I proven that the visibility results from light’s wave action, and not from the finite photons?
No - you’ve simply proved that enough photons are arriving.
I think there was some element of some story I read (I think it was in “C” by Tom McCarthy) that had something like that as part of its bits … but about one character’s thinking something about radio waves from thoughts or something lasting forever …
Also though, beyond sound waves diminishing to the point that they literally and completely stop propagating is the fact that sound waves cancel each other out long before then, correct?
Question: what is the minimum density for sound to propagate?
Of course there is no sound in a vacuum but how far away from a vacuum does it need to be? Or is that a silly question?
Also - the cosmic background radiation remnant of the Big Bang has been described as “the sound” or “noise” of the Big Bang … how accurate is that description vs its being a metaphor?
Whether you see a photon from that quasar, and how many of them you see, depends on how long you keep looking in that direction. The bigger your telescope, and the longer you look at it, the more photons you’ll detect. If you have a small telescope and look for only a short time, it is in fact quite plausible that you won’t catch any photons at all.
Actually, if you go outside and look up at the night sky with your naked eyes, you won’t see any quasars. Because even the optically brightest quasar is magnitude 12.9, far too faint to be seen by the naked eye. (12.9 magnitude is about 600 times fainter the faintest star you can see on a clear moonless night, away from the city.) In other words, not enough photons from this quasar are entering your eyes for your retina to respond. The human eye is only about 1/4 inch diameter, even when fully dilated, and the retina needs about a hundred photons within about 1/50 of a second to respond.
A telescope is basically a bucket for collecting photons; it collects all photons that land on the mirror and reflect them all into your eye. So you are no longer limited by the 1/4-inch aperture of your eye. If you point an 8-inch (aperture) telescope at this quasar, you should be able to see it fairly easily, because all the photons from the quasar that land in the 8-inch aperture is being redirected into your eye.
No, you’ve proven that you don’t understand how mindbogglingly many photons a stellar object is transmitting, the good old argument from incredulity. You realize that if you look at some seemingly dark patch of sky there are objects there that are too faint for you to see, right? And that these can be seen if you have a large telescope that gathers light over a larger surface, or by using a more sensitive detector than the human eye. Those objects are sending out so many photons your eye will be bombarded with tens of them every second from any possible angle if they are just slightly too dim to observe.
Oh, and
It’s easy to see the source of the confusion, here. The light we see is not, in any sense, “the sound of the Universe”, or anything like that. However, what we see has patterns to it, and those patterns were in fact caused by sound waves traveling through the very early Universe, back when it was much denser. It’s sort of like that demonstration with sand in a tube with a sound playing: You’re not actually seeing the sound; you’re seeing the effects of the sound.
Sound wave is a pressure wave and can only propagate through matter. And when matter is compressed, some of the energy goes into heat - no material is perfectly elastic. So in every real-world situation, sound waves eventually dissipate into heat.
Electromagnetic waves can propagate through perfect vacuum with no loss of energy. And space is mostly a perfect vacuum.
My apologies for being unclear. Above, when I said things like “what I can see”, what I meant was “what the great telescopes can see, including radio telescopes, which are also photon-sensitive.”
Yeah, I’ll concede that possibility. BUT
Wikipedia refers to an object called GN-z11, which is 32 billion lightyears away. My calculations are that a sphere with a radius of 32 billion light years will have a surface area of 10^60 square millimeters. Therefore, if the telescope can see GN-z11 from one position, and also from one millimeter away, I can presume that GN-z11 is putting out at least 10^60 photons at a time.
And if the telescope can see it from one micron away, then it is putting out at least 10^66 photons, but at some point this game gets silly. My point is this: Given that a photon has no mass, but it does exist, is it easier to accept that such mindbogglingly large numbers are produced? Or would Occam’s Razor suggest that we should be looking at waves instead of particles?
First of all, what do you mean by “one millimeter away”? Telescopes are bigger than one millimeter.
And second, what do you mean by “at a time”? At what time? Detecting that galaxy took weeks of observation.
In other words, if you divide that sphere into areas the size of the Hubble Space Telescope instead of square millimeters, then that’ll give you a count of the photons that galaxy produces in several weeks.
Although the question might not have been framed perfectly on the first attempt, it is fun to think about some of the mind-boggling stuff that happens in the universe in this kind of way. This is reminding me of one of Randall Munroe’s best what-if’s, which includes this snippet:
Which of the following would be brighter, in terms of the amount of energy delivered to your retina:
(a) A supernova, seen from as far away as the Sun is from the Earth
(b) The detonation of a hydrogen bomb pressed against your eyeball?
The supernova is brighter… by nine orders of magnitude
from Lethal Netrinos
So let me further illustrate my ignorance!
“Sound” (as a physical rather than as a perceptual entity) is vibration (or pressure waves) transmitting through particles of some mass of some density. Yes?
That much that good but here is where I fear I am possibly asking a question without meaning …
Is the CMBR ripples the result of ripples in space or spacetime itself or in the electromagnetic background noise that without the “sound” of the Big Bang would have been evenly distributed or perhaps distributed more according to the early flux quantum foam popping in and out?
Thank you for the patience.
I’m not sure precisely what you’re asking, but when I said that those ripples were the result of sound, I meant it literally: Those were compression waves carried by the particles of matter in the Universe. There were also ripples in spacetime (AKA gravitational waves) going on at the same time, but those are mostly manifested through variations in the polarization of the CMB, not in variations of its intensity. And the time we’re seeing in the CMB is long after quantum fluctuations were particularly relevant.
I’m sure Chronos will chip in if I’m wrong, but I don’t think acoustic waves have any bearing on the presence of CMBR per se. The CMBR originates from the time when the universe ceased to be opaque to photons.
Acoustic waves are releveant to the anisotropy of the CMBR (but perhaps that’s what you mean by ripples?) but also to distributions that we see in the universe in general, not just the CMBR.
Wiki has a good account here, read down to “Cosmic sound”.
I apologize that my limited understanding limits my ability to ask the question clearly - the compression waves went through the particles of matter. So that showing up as a wave pattern in matter I’d get. But the pattern is of the microwave background radiation, not of the matter. How did compression waves (“sound”), which by definition travel through particles of mass, result in a macroscopic wave pattern in the electromagnetic spectrum? Is it that by causing a pattern in the early soup of mass what was then emitted followed that pattern? But then why is that pattern not detectable in the mass of the universe as well?