Comment by dredmorbius | Hacker News Reader

Comment by dredmorbius | Hacker News Reader

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> This describes the case of a plane suddenly appearing in mid-air

False.

> and starting to make noise

False

> As long as it flies subsonically, i.e. sound outpaces the aircraft...

> If you're 100km away, the aircraft will reach you after 1000s...

These statements exhibit a fundamental misunderstanding of the phenomenon. It's not that the sound outpaces the aircraft. It's that light from the aircraft (reflected or transmitted, e.g., by landing / navigational lights) travels faster than sound.

When light from the aircraft reaches you, the sound is lagging behind the light.

At the height of a jet airliner (~FL30, 30,000 feet), light reaches you in 30 microseconds. At the height of a small plane, about 3,000 feet, say, it's 3 microseconds.

Sound takes 27 seconds to reach you from the jetliner, and 2.7 seconds to reach you from the small plane.

If the jetliner is flying at 600 mph (~mach 0.8, ~515 knot) the aircraft has travelled 4.6 miles (7.4 km) from the position from which its sound was emitted before that sound reaches you. The apparent position indiciated by vision and sound don't match.

If the small aircraft is travelling at 122 knots (140 mph) (cruise speed for a Cessna 172), it has travelled about 1/10 mi (0.16 km) before the sound reaches you. That's about 550 feet.

Both cases are for when the aircraft its directly overhead. The apparent difference will increase as the aircraft is closer to the horizon (arriving or departing).

Again, the visual position and apparent aural position of the aircraft are not the same.

You can determine this yourself, if you're outside and hear a jet aircraft flying at altitude. If you look to where the sound appears to be coming from you will not see the aircraft. It is going to be nearly 5 miles further along its path of travel. It can be surprisingly difficult to visually find the aircraft if you've only first heard it. If instead you're watching the sky and first see the aircraft, it will be quite some time, about 30 seconds, before the sound reaches you, and that sound will seem to be considerably far back along the aircraft's path of travel.

> It's that light from the aircraft travels faster than sound.

For all practical purposes we can say the light reaches the observer instantly, whereas the sound takes some (significant by comparison) amount of time. Over such short distances, and when comparing it to something that is so much slower (299,792,458 m/s vs 343 m/s, 874 thousand times faster), there is no point in measuring the infinitesimal time it takes light to travel the distance from the plane to the observer.

For the general case presented in the article, at the average cruising altitude and speed mentioned within it, the conclusion is that it takes sound from the plane so much longer to reach the observer than the instantaneous light from the plane, that the actual plane itself has traveled another 2.1km in that time. You are (instantly) seeing the current position of the plane but hearing the sound it emitted 2.1km ago.

Ok, you can adjust my illustrative example of O(100s-1000s) by the 30ms to account for a finite speed of light if you think that makes any difference to the argument. Let me know.

This has got to be a troll. I have trouble believing you are being serious saying things like this.

That we can calculate the distance to a lightning to great accuracy (my example in the very first sentence) by accounting for a finite speed of sound only and assuming an infinite speed of light? Yeah, I'm pretty serious that we can neglect that error of some microseconds when concerned with a process that takes many orders of magnitude more.

https://jkorpela.fi/wiio.html

> " These statements exhibit a fundamental misunderstanding of the phenomenon. It's not that the sound outpaces the aircraft. It's that light from the aircraft (reflected or transmitted, e.g., by landing / navigational lights) travels faster than sound."

Explain to me again how any of that explains why the airplane passes me (and I see it 1e-6s later which seems to be somehow super important to you) before I hear it, even though its sound is traveling towards me much faster than the plane.

You're demonstrating Wiio's Law.

Seems to cut either way.

Slight tweak: it's 30 microseconds, not 30 milliseconds. But I wholeheartedly agree with your points!

For all practical purposes it's really completely unrelated to the speed of light. Nothing would change in the argument if light travelled instantaneously. Sure, the numbers would change by O(1e-6s), but I'll admit that I wouldn't be able to tell the difference when watching an aircraft.

Precisely my point.

I'd meant to edit my post to note that for the purposes of this phenomenon, light speed is instantaneous.

Though in the more general case, the phenomenon would apply to any case in which two signals or channels travel at different rates or speeds. Light and sound are the examples most familiar to us, though other alternatives exist.

Neutrinos can tell us what is occurring at the core of the Sun with an ~8 minute delay whilst the propagation of EMR effects from the Sun's core is thought to take 10,000 to 170,000 years, as these travel through repeated collisions, absorption, and re-emission.

Diffusion processes such as smell or other chemical materials both travel more slowly than either light or sound, and at different rates for different compounds --- heavier compounds diffuse more rapidly than lighter ones. This is incorporated into the chemical signalling processes evolved by insects such as ants, in which some compounds are heavy and complex (usually for food or other valuable resources), others are light and fast (danger or alert signals). Again, for a moving or propagating phenomenon, these will move at different rates.

For more complex phenomena, you might note that there are early / rapidly-moving indicia and those which move more slowly. Again, understanding the difference between these, the rates at which they travel, and their association and interactions with the processes originating and surrounding them will assist in drawing an accurate inference of the root phenomenon.

All sensation is mediated, not direct, and that mediation has a direct effect upon sensation.

You're the one bringing up the finite speed of light that has zero relevance to the problem at hand.

Gah! Thanks, corrected in original.

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