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> “Or traveling at speeds that exceed the sound barrier without a sonic boom.” Physics noob here so be gentle. I initially thought that the sonic boom is cause

> “Or traveling at speeds that exceed the sound barrier without a sonic boom.” Physics noob here so be gentle. I initially thought that the sonic boom is caused by the compression of the sound of the aircraft's own engines, but then realised that couldn't be correct as meteorites can also create sonic booms and they have no engines. The boom is caused by the air being compressed in front of the object? Is the loudness of the engines irrelevent to the intensity of the boom, it's all due to speed? So these sonic boom-less aircraft - if they're real - must be doing something to the air in front of it?

yetihehe 6y ago
Yes, NASA is working on reducing sonic booms, but there is still long way to go. Typically that reduction is achieved through change in aerodynamic shape so that sonic waves created by aircraft are modified/nullified. UFO's from reports typically look very unomptimised aerodynamically.
wazoox 6y ago
Yes, for instance they could use magnetohydrodynamic drives[1] to create a bubble of quasi-void around the aircraft. In the 70s Jean-Pierre Petit did some demonstrations of flying-saucer like models. 1: https://en.wikipedia.org/wiki/Magnetohydrodynamic_drive https://en.wikipedia.org/wiki/Magnetohydrodynamic_drive
- Smooth-Weather 6y ago
  Yes indeed, Jean-Pierre Petit has a YouTube channel where he talks about multiple subjects. In a 2020 video, he made a small video describing the links between UFO sightings and MHD: https://youtu.be/56fGzcp6bIU?t=65 https://youtu.be/56fGzcp6bIU?t=65 (Subtitles available). For people interested, here is a video presentation in English about applications of MHD technology for aircraft: https://www.youtube.com/watch?v=hVN7qf3DlX0 https://www.youtube.com/watch?v=hVN7qf3DlX0 And here is a conference paper about the same subject: https://januscosmologicalmodel.com/pdf/MHD/2009-Bremen-aerospace.pdf https://januscosmologicalmodel.com/pdf/MHD/2009-Bremen-aeros...
jerf 6y ago
Anything that travels faster than the speed of sound yields a sonic boom. This includes the crack of a whip, for instance. Obviously that one is smaller than what a jet produces. You can play some games with which direction the energy of the boom goes and build planes that tend to send their boom more upwards than downwards, but you can't eliminate it going in all directions. I have a loooot of issues with "UFOs are indeed alien crafts!", but certainly one of the bigger ones is the sheer stack of absurdities required to believe them, if you put them all in a pile at once and look at them seriously, and think about what they would mean if true. It isn't just that they have engines capable of absurd accelerations. It's that they also seem to use them in ways that under other circumstances we'd call "lacking purpose", i.e., they're just cruising along in one direction at twice the speed of sound, then all of a sudden go "oh, crap! I'm going the wrong way!" and slam themselves 120 degrees in another direction in a fraction of a second. It's that they also require "inertial dampers" in addition to the pile of other technologies of dubious physical possibility. We have to believe in ships that don't just move in absurd ways but also control the air around them to not be the noisiest damned things on planet Earth. It's that we also have to believe these apparently advanced aliens tend to crash surprisingly often, and that they don't want to be seen but just sort of accidentally get seen a little bit. We also have to believe in aliens that are staggeringly more advanced than humans but don't just, you know, do whatever it is they want to do because they are indeed that much more advanced, but are dicking around for decades at a time, apparently engaging in "negotiations" with our government despite their being so much more advanced that they should have no particular need to "negotiate" about anything. It's not just the nature of the claims themselves... it's that if you take the claims seriously, they imply further absurdities that really don't make a lot of sense. Even if you take the technology claims on faith, the whole "UFOs are aliens!" tend to imply an alien species that has no intelligent motivations. Any two or three of them maybe I could harmonize into a plausible theory, but the whole pile is insane. ("Aliens" also have this really interesting characteristic where they just happen to have capabilities awfully close to what someone in 1950 might have considered to be the apex of possible physical technology, but rather bizarrely don't much resemble what the apex of physical technology looks like in 2021. For instance, the whole "aliens are genetically experimenting on humanity!" is rather silly if you consider an alien race who has computing power on their smallest scout ship exceeding everything humanity has ever possessed. Why the hell would you run around "experimenting" on humans when you can just compute the answer in a few seconds? Why would you "experiment" on humanity when odds are you can simply build any life form you like directly, without even needing to base it on some existing life form? And why would you be dicking around with life at all when you can build AIs thousands of times better for some purpose than any amount of human experimentation will yield? The whole memeset is very suspiciously stuck in a very particular period of time, and not reacting to 70 years of technological progress at all. See also our hand-held drones; increasingly real tech is running beyond what these "aliens" seem to posses, if we ignore all the inertial control and such. Or our existing surveillance tech; why would they need to watch us with crash-prone enormous spacecraft when they could just loft a couple dozen satellites into orbit and watch us at will, centuries before humans could even understand what that means?) If UFOs are aliens, we have as much evidence that they are dumb as a bag of bricks as we do that they are hyperadvanced superintelligences, honestly.
pansa2 6y ago
> sonic boom-less aircraft Here’s a theoretical example: https://en.wikipedia.org/wiki/Busemann_biplane https://en.wikipedia.org/wiki/Busemann_biplane
Grazester 6y ago
What are the chances that these aircrafts are not really traveling at the speed of light instead creating their own small holes and traveling through them?
danparsonson 6y ago
I think you basically have the right idea. An aircraft in flight is continually emitting sound (and remember that sound is air compression) - the sound of the engines, but also the sound generated just by moving through air; open your car window while on a highway for an demonstration of that, and yes as you say, the volume increases with speed relative to the air. At low speeds, engine noise will dominate, while presumably there will be a speed above which 'wind noise' is louder; this is certainly true on a motorbike for example. These sound waves interfere with each other and if they're travelling at the same speed as (or slower than) the emitter, a lot of constructive interference occurs - i.e. the sound gets louder. Note this effect still occurs beyond Mach 1.0 - there's a nice animation here[0] that shows why. As you say - a sonic boom-less aircraft has to somehow move the air out of the way quietly. [0] https://physics.stackexchange.com/a/176446 https://physics.stackexchange.com/a/176446
danaliv 6y ago
> The boom is caused by the air being compressed in front of the object? Is the loudness of the engines irrelevent to the intensity of the boom, it's all due to speed? Yes, that’s it exactly. The part I would add is that after the air gets compressed, it rebounds, which is a wave. This wave emanates at the speed of sound. Ok fine, but what does that mean? At subsonic speeds, each new pressure/sound wave emanating from the object is contained within the expanding sphere of the wave that came before it. Think of ripples in a pond, with each new ripple slightly to the left, but still completely “contained” by the one before it. The two ripples expand and never touch. (We’re pretending our pond has no edges here and that the waves don’t reflect.) At supersonic speeds, the center of a new wave is outside the expanding previous wave. This has significant consequences for how the two waves interact. Subsonically, they didn’t interact at all. But supersonically, each new wave is expanding outside the previous wave, which means that pretty quickly they collide. And when they collide, they interfere. This interference is what goes boom.

yetihehe 6y ago

Yes, NASA is working on reducing sonic booms, but there is still long way to go. Typically that reduction is achieved through change in aerodynamic shape so that sonic waves created by aircraft are modified/nullified. UFO's from reports typically look very unomptimised aerodynamically.

wazoox 6y ago

Yes, for instance they could use magnetohydrodynamic drives[1] to create a bubble of quasi-void around the aircraft. In the 70s Jean-Pierre Petit did some demonstrations of flying-saucer like models. 1: https://en.wikipedia.org/wiki/Magnetohydrodynamic_drive https://en.wikipedia.org/wiki/Magnetohydrodynamic_drive

Smooth-Weather 6y ago
Yes indeed, Jean-Pierre Petit has a YouTube channel where he talks about multiple subjects. In a 2020 video, he made a small video describing the links between UFO sightings and MHD: https://youtu.be/56fGzcp6bIU?t=65 https://youtu.be/56fGzcp6bIU?t=65 (Subtitles available). For people interested, here is a video presentation in English about applications of MHD technology for aircraft: https://www.youtube.com/watch?v=hVN7qf3DlX0 https://www.youtube.com/watch?v=hVN7qf3DlX0 And here is a conference paper about the same subject: https://januscosmologicalmodel.com/pdf/MHD/2009-Bremen-aerospace.pdf https://januscosmologicalmodel.com/pdf/MHD/2009-Bremen-aeros...

jerf 6y ago

Anything that travels faster than the speed of sound yields a sonic boom. This includes the crack of a whip, for instance. Obviously that one is smaller than what a jet produces. You can play some games with which direction the energy of the boom goes and build planes that tend to send their boom more upwards than downwards, but you can't eliminate it going in all directions. I have a loooot of issues with "UFOs are indeed alien crafts!", but certainly one of the bigger ones is the sheer stack of absurdities required to believe them, if you put them all in a pile at once and look at them seriously, and think about what they would mean if true. It isn't just that they have engines capable of absurd accelerations. It's that they also seem to use them in ways that under other circumstances we'd call "lacking purpose", i.e., they're just cruising along in one direction at twice the speed of sound, then all of a sudden go "oh, crap! I'm going the wrong way!" and slam themselves 120 degrees in another direction in a fraction of a second. It's that they also require "inertial dampers" in addition to the pile of other technologies of dubious physical possibility. We have to believe in ships that don't just move in absurd ways but also control the air around them to not be the noisiest damned things on planet Earth. It's that we also have to believe these apparently advanced aliens tend to crash surprisingly often, and that they don't want to be seen but just sort of accidentally get seen a little bit. We also have to believe in aliens that are staggeringly more advanced than humans but don't just, you know, do whatever it is they want to do because they are indeed that much more advanced, but are dicking around for decades at a time, apparently engaging in "negotiations" with our government despite their being so much more advanced that they should have no particular need to "negotiate" about anything. It's not just the nature of the claims themselves... it's that if you take the claims seriously, they imply further absurdities that really don't make a lot of sense. Even if you take the technology claims on faith, the whole "UFOs are aliens!" tend to imply an alien species that has no intelligent motivations. Any two or three of them maybe I could harmonize into a plausible theory, but the whole pile is insane. ("Aliens" also have this really interesting characteristic where they just happen to have capabilities awfully close to what someone in 1950 might have considered to be the apex of possible physical technology, but rather bizarrely don't much resemble what the apex of physical technology looks like in 2021. For instance, the whole "aliens are genetically experimenting on humanity!" is rather silly if you consider an alien race who has computing power on their smallest scout ship exceeding everything humanity has ever possessed. Why the hell would you run around "experimenting" on humans when you can just compute the answer in a few seconds? Why would you "experiment" on humanity when odds are you can simply build any life form you like directly, without even needing to base it on some existing life form? And why would you be dicking around with life at all when you can build AIs thousands of times better for some purpose than any amount of human experimentation will yield? The whole memeset is very suspiciously stuck in a very particular period of time, and not reacting to 70 years of technological progress at all. See also our hand-held drones; increasingly real tech is running beyond what these "aliens" seem to posses, if we ignore all the inertial control and such. Or our existing surveillance tech; why would they need to watch us with crash-prone enormous spacecraft when they could just loft a couple dozen satellites into orbit and watch us at will, centuries before humans could even understand what that means?) If UFOs are aliens, we have as much evidence that they are dumb as a bag of bricks as we do that they are hyperadvanced superintelligences, honestly.

pansa2 6y ago

> sonic boom-less aircraft Here’s a theoretical example: https://en.wikipedia.org/wiki/Busemann_biplane https://en.wikipedia.org/wiki/Busemann_biplane

Grazester 6y ago

What are the chances that these aircrafts are not really traveling at the speed of light instead creating their own small holes and traveling through them?

danparsonson 6y ago

I think you basically have the right idea. An aircraft in flight is continually emitting sound (and remember that sound is air compression) - the sound of the engines, but also the sound generated just by moving through air; open your car window while on a highway for an demonstration of that, and yes as you say, the volume increases with speed relative to the air. At low speeds, engine noise will dominate, while presumably there will be a speed above which 'wind noise' is louder; this is certainly true on a motorbike for example. These sound waves interfere with each other and if they're travelling at the same speed as (or slower than) the emitter, a lot of constructive interference occurs - i.e. the sound gets louder. Note this effect still occurs beyond Mach 1.0 - there's a nice animation here[0] that shows why. As you say - a sonic boom-less aircraft has to somehow move the air out of the way quietly. [0] https://physics.stackexchange.com/a/176446 https://physics.stackexchange.com/a/176446

danaliv 6y ago

> The boom is caused by the air being compressed in front of the object? Is the loudness of the engines irrelevent to the intensity of the boom, it's all due to speed? Yes, that’s it exactly. The part I would add is that after the air gets compressed, it rebounds, which is a wave. This wave emanates at the speed of sound. Ok fine, but what does that mean? At subsonic speeds, each new pressure/sound wave emanating from the object is contained within the expanding sphere of the wave that came before it. Think of ripples in a pond, with each new ripple slightly to the left, but still completely “contained” by the one before it. The two ripples expand and never touch. (We’re pretending our pond has no edges here and that the waves don’t reflect.) At supersonic speeds, the center of a new wave is outside the expanding previous wave. This has significant consequences for how the two waves interact. Subsonically, they didn’t interact at all. But supersonically, each new wave is expanding outside the previous wave, which means that pretty quickly they collide. And when they collide, they interfere. This interference is what goes boom.