I was watching an XKCD “What-If” video recently and Randal off-handedly mentions the title fact as a given. Upon a further Google search I see explanations about why sound moves faster in liquids than gasses but nothing for my specific question. Is there an intuitive explanation for that fact or is it just one of those weird observable facts with no clear explanation
Ahhh in terms of fluid flow? I mean you can move a glass of water as fast as you want (don’t @ me physicists) if the glass is moving at that speed.
Assuming this is flow.
Let’s think about a 1 metre long pipe full of water. Let’s make the water start flowing though this pipe by pushing through the water at the start of the pipe. Now as water enters the pipe, water at the other end of the pipe exits. But there’s a time delay between the first new water that enters the pipe and the first water to leave the pipe. (I can explain this further if needed) That time delay is the time it takes the speed of sound in water to travel through the 1m pipe.
Now let’s consider water moving faster than the speed of sound in water entering the pipe. It’s moving so fast through the pipe that it actually reaches the end of the pipe before the water at the end of the pipe has had enough time to move out of the way. Here flow is effectively dead and the water isn’t really flowing though the pipe anymore, it’s just being pushed through by whatever is moving the water in the first place.
At the end of the day it’s semantics, you could define flow as being “water moving through a pipe” and in that case yeah, water can “flow” at whatever speed you want if you push it hard enough. Typically though, flow means a steady and consistent stream of fluid. And at supersonic speeds, that stream is anything but steady and consistent.
Does this mean that a drop of water can’t move through air faster than the speed of sound in air?
Or that a drop of water can’t move faster than the speed of sound in water anywhere?
I’m pretty sure they are talking about the 2nd one
How about firing a compressed air canon to attempt a sonic boom
Sound is transferred through a medium literally as a wave. When you get right down to the core of it, the wave requires movement within the medium to transmit.
So it might help to conceptualize it not as “Liquid cannot move faster than the speed of sound in it’s medium” but more like “The speed of sound in a liquid medium is defined by the speed at which energy can propagate in that system, which includes motion.”
So the medium is like a car made of liquid and the speed of sound is a passenger?
No, medium is the speed limit and the sound is a car that drives legaly.
What’s updog?
Nothing dog, whats up with you?
So you’re saying I should view the speed of sound in a medium like the speed of light in a vacuum? That it’s the “speed-limit” of how a wave propagates and so trying to exceed it is impossible for a physical wave?
Sort of. The speed of light in a vacuum is the speed of causality, nothing can go faster than the maximum speed at which one part of the universe can effect another.
It is possible for fluids to move faster than the speed of sound in the fluid around it, such as the exhaust products of a supersonic jet engine, but in these cases not all of the fluid is operating like a wave. The core of the jet experiences a laminar flow where all of the fluid is moving in the same direction and at roughly the same speed, like a laser instead of a flashlight. At the boundaries of this laminar flow exists a turbulent region where the fluid interacts with the surrounding medium and is slowed to subsonic speeds.
I don’t know how this works, but wouldn’t the exhaust of the jet be moving at or slower than the speed of sound relative to the other particles in the exhaust. Sure, compared to other particles it’s moving faster, but that doesn’t really matter if we’re only looking at the exhaust. It’s not doing anything differently, though the exhaust and outside particles will have to interact at the boundary.
Precisely. It’s those boundary areas where the jet and the medium interact where it gets complicated.
Badically. “Liquid/fluid” and “gas” don’t necessarily mean the same thing scientifically as they do colloquially, they’re actually very close to the same thing.
Fluid dynamics covers the study of liquids, gasses, and plasmas because they’re effectively the same.
Note that the speed of sound isn’t a constant across various media. Just like the speed of light isn’t a constant on different media. The “speed of light” we usually refer to is specifically in a vacuum. Light travelling through a media like water or a prism actually changes speed, however slight.
The same happens to sound. The speed of sound at altitude is different from sea level for instance, because of the atmospheric pressure difference. And sound doesn’t propagate at all in a vacuum because it requires the wave to move molecules, which don’t exist in a vacuum.
Light travelling through a media like water or a prism actually changes speed, however slight.
Colloquially, but not in actuality. Light still travels at the same speed technically, but it bounces off particles which makes it take a longer path so it takes longer to get from one point to another, but it’s speed is still constant.
but it bounces off particles which makes it take a longer path
If I get the explanation on Wikipedia right, it’s not the photon taking a longer path, but the photon is absorbed by electons and re-emitted after a short delay. This effect is what decreases the effective speed of light in a transparent medium.
In exotic materials like Bose–Einstein condensates near absolute zero, the effective speed of light may be only a few metres per second. However, this represents absorption and re-radiation delay between atoms, as do all slower-than-c speeds in material substances. As an extreme example of light “slowing” in matter, two independent teams of physicists claimed to bring light to a “complete standstill” by passing it through a Bose–Einstein condensate of the element rubidium. The popular description of light being “stopped” in these experiments refers only to light being stored in the excited states of atoms, then re-emitted at an arbitrarily later time, as stimulated by a second laser pulse. During the time it had “stopped”, it had ceased to be light. This type of behaviour is generally microscopically true of all transparent media which “slow” the speed of light.
“Liquid/fluid” and “gas” don’t necessarily mean the same thing scientifically as they do colloquially, they’re actually very close to the same thing.
Both, liquids and gases, are fluids. The main difference is that liquid phases have a free surface, e.g. the level of water in a glas, whereas gases don’t. Their surface is equal to the surface of their compartment.
Just to add because nobody mentioned that yet… But you can always push stuff faster than the speed of sound, it will just stop being a liquid, and probably explode, but there’s no law saying the material can’t go faster.
You already got some answers, but I thought of something you might find interesting: if you had a multiple kilometers long pole in a vacuum and pushed on it, the push itself would propagate at the speed of sound!
Meaning the other end wouldn’t really move immediately, but it would instead take multiple seconds or even minutes if the pole is large enough. If it’s made of oak and is 9 km long, it would take around 3 seconds (the speed of sound in oak is around 3 km/s IIRC).
I think this was experimentally shown by a Youtuber. The speed of sound is a slight oversimplification since there are multiple types (https://www.engineeringtoolbox.com/sound-speed-solids-d_713.html)
I think Randall mentioned this at one point but I never really understood it. Is it something like on a molecular level they’re still taking some time to push in to each other? It’s so damn trippy. At what point is my long pole going to have a delay when I push it? It sounds unreal
Basically, when you push something, you push molecules, those in turn push the other molecules etc., that’s what it is.
The delay is there every time, it’s just usually really fast, the speed of sound in solid mediums is much bigger than the speed of sound in air.
Do incompressible materials therefore have extremely high speed of sound?
Yes. Nothing is truly incompressible. The speed of sound can be viewed as a measure of how much a material can squish on the atomic level before the next atoms move.
Nothing is truly incompressible.
Exactly. One usually speaks of quasi-incompressibility when the resistance against compression (bulk modulus K) is much greater than the resistance against shear (shear modulus G), which is often the case for liquids such as water (strictly, fluids like water don’t have any resistance against static shear deformation, i.e. G = 0).
However, water has a lower bulk modulus (K =2 GPa) than e.g. steel (K = 160 GPa), which is considered a compressive material, as its shear modulus and bulk modulus are in the same order of magnitude (G = 81 GPa).
So, sound is vibration, right? And it’s going to vibrate as fast as it can, at the fastest rate that liquid can move. So it’s not that the rate of movement comes from the speed of sound; if anything, it’s the reverse.
it’s going to vibrate as fast as it can, at the fastest rate that liquid can move.
Not quite logical unless there’s a reason why it should vibrate as fast as it can
Imagine the liquid as a road, and the sound travelling through it as a runner on said road. Now, sound is faster than the road, because road isn’t moving anywhere. But if we replace the road with one of those airport speedwalking walking pads (kinda like escalators but flat on ground), now the ground is moving, but as luck would have it, the runner is now moving even faster. The more you speed up the pad, the faster the runner moves, even if the runner themselves has not increased their speed.
Weird stuff, but it does make sense. :)
Use a better search like Bing or duckduckgo next time. googol sucks and was never any good. Quit using ignorant garbage.
