1: https://en.wikipedia.org/wiki/Magnetohydrodynamic_drive

For people interested, here is a video presentation in English about applications of MHD technology for aircraft: https://www.youtube.com/watch?v=hVN7qf3DlX0

And here is a conference paper about the same subject: https://januscosmologicalmodel.com/pdf/MHD/2009-Bremen-aeros...

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

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.

Here’s a theoretical example: https://en.wikipedia.org/wiki/Busemann_biplane

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.

The only claims I'm really interested in are the ones where you have multiple credible, on the record, eyewitnesses claiming multi-modal detection of phenomena exhibiting behavior that is clearly beyond current human capabilities, such as the 2004 Nimitz incidents.

Once you consider the possibility of non-human intelligences being involved it also becomes very hazardous to attempt to understand the motives behind their actions. For all we know our entire reality could be constructed in an alien laboratory designed to measure the development of our individual and collective intelligence by observing our reactions to strange events.

That said I do admit to having a lot of trouble trying to understand why craft that are apparently capable of hypersonic travel would spend hours moseying down the California coast from Catalina to Guadalupe islands. I mean if they were trying not to be noticed why not avoid the Nimitz CSG completely ? And if they wanted to be noticed wouldn't they have done something a little more dramatic than floating along at 100 knots (they didn't exhibit any spectacular behavior until an attempt was made to actually intercept them) ?

> > 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.

> That said I do admit to having a lot of trouble trying to understand why craft that are apparently capable of hypersonic travel would spend hours moseying down the California coast from Catalina to Guadalupe islands. I mean if they were trying not to be noticed why not avoid the Nimitz CSG completely ? And if they wanted to be noticed wouldn't they have done something a little more dramatic than floating along at 100 knots (they didn't exhibit any spectacular behavior until an attempt was made to actually intercept them) ?

These at least are pretty easy to put together: Teenage aliens out for a joyride.

This is more my point than the true combination of every possible claim. It's more, go beyond arguing about whether something did or did not happen, and consider, what does it imply if it did? People are often too bedazzled by the immediate arguments around what may have happened to consider this argument, but it's a very powerful, simple logical tool that can be useful in resolving those questions.

You tend to get rather silly answers for a lot of this stuff if you take it seriously. Especially when you consider what tech looks like in 2021 and what feasible tech looks like in another 50-100 years, to say nothing of thousands. Aliens with interstellar travel tech but apparently no computers to speak of, no internet, no protein folding projects, no awareness of how useful satellites can be rather than literally flying down, apparent ignorance of how much of the universe isn't on planet earth (there is almost no conceivable resource-based argument for them to be here because everything is more abundant out there with no crazy animals running around bothering you) etc. the whole thing just comes off very silly.

One of the things you don't get crazy answers for is classified, but fully human craft. It just there are humans (military or whatever) with capabilities we don't know. That is not a terribly crazy theory; if you look at the past at how long things stayed classified and extrapolate into the present time, it isn't completely out of the question that the military may have some things that are fully 20-30 years ahead of civilian tech, in very particular branches of development. (I don't think it's plausible that they are uniformly ahead of civilian development by that amount, but it's very plausible to have spikes in very certain areas. We have very significant evidence of the NSA having similar spikes in crypto math.) Combine the possibility of real sightings, with a real experiment to see how people react (we have the entire term "trial balloon" for that, because it's a thing we know humans do), and just a bit of misinterpretation of what's happening by the people watching it, and it's not hard to see this as a very significant possibility.

(In fact, if you study what the military has done to its own people at times, it's not even slightly hard to believe this was a deliberate test of something. Even ignoring the unproveable rumors or what other militaries have done, and skipping the unethical stuff, we know militaries will test and trial things on their own people on a routine basis. Even things as simply as the drills not really labelled "drills" until later are examples.)

And I think people lean way too hard on "but aliens could be incomprehensible!". They are bounded by living in the same universe as us, needing to survive to accomplish any other goal, needing energy and mass resources which are exclusionary (if X is using them Y doesn't get to, and all the subsequent game theory that implies), etc. That doesn't mean that we'll understand them to the n'th degree but it does put some bounds on how "cloud cuckoo land" they can be before it's just absurd.

If the craft involved in the Nimitz incident were human then I would be even more puzzled by their behavior because I think I have a pretty good idea of human motivations and behavior. If they're "alien" (by which I don't necessarily mean extraterrestrial, just unknown non-human entities) then at least I can fall back on the argument that I may not be able to understand their motivations and behavior.

Also there are many obstacles to explaining them as secret US technology. For one thing a number of knowledgeable people have indicated that no the US military doesn't test secret technology on completely uninformed participants. That's just not the way it's done. Also if it was secret US technology why did they let it leak ? Surely they would have made sure that people like Fravor and Day signed NDA's. On the other hand if the leak was intentional (ie. to send some kind of message to adversaries) then why wait 13 years before making it public ? Basically it just doesn't add up.

> That doesn't mean that we'll understand them to the n'th degree but it does put some bounds on how "cloud cuckoo land" they can be before it's just absurd.

I think here you're vastly underestimating the potential weirdness of things we don't know about.

Or that's what they want you to think. (/s)