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Watsisname
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18 Nov 2020 03:23

midtskogen wrote:
Source of the post This could probably be effectively visualised in an animation

Always wanted to do something like that, so why not. :) I'll put up three different scenarios for comparison.
For starters let's generate a typical Mach cone. A constant-frequency source passes by at 1.5 times the speed of sound. Theory predicts the Mach cone angle should be about 41.8 degrees, and that looks like what we get:

Image

Intuitively the observer (black star) first sees a sudden burst of sound, which then decreases in frequency. 

Next let's have the source accelerate from rest, so that it breaks the sound barrier as it passes the observer:

Image

This case is dramatically different, since the observer detects sound from the subsonic regime first, so they hear frequencies that first increase, then decrease.

 Finally here's something that's at least qualitatively more similar to a meteor transitioning from supersonic to subsonic. 

Image

This seems closer to the first example, and I think matches your intuitions pretty well.
 
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18 Nov 2020 04:53

Wow.  You beat me to it.   :D
Last edited by midtskogen on 18 Nov 2020 15:05, edited 1 time in total.
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18 Nov 2020 05:21

There is one thing these visualisations tell us, which I didn't realise.  The delay between the sonic transition and the initial sound can not trivially be used to calculate the distance, as indicated by the waves emitted before the transition (red) and the waves emitted after (blue), unless you're almost directly below (which might be unknown).  You are likely to hear the compressed sound of the meteor before it goes subsonic.  That didn't occur to me.  But since "old" fronts fade quickly, it will diminish in intensity quicker, I believe.  The same thing if the meteor impacts before going subsonic.
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18 Nov 2020 15:06

I edited the post above with a visualisation of a more realistic deceleration.  Units on the axes are in km and the animation is in real time.
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18 Nov 2020 17:19

Beautiful! I love how we did this completely independently. :D And yours does clearly show how observers who are not very close will hear the sound from the supersonic flight first, which wasn't obvious.

I also learned something interesting by doing this. I always imagined that if a plane suddenly accelerated through the sound barrier, then the sonic boom would be most powerful for observers directly below the transition point. But not so. An observer directly below will hear the upshift and then downshift of frequencies, with a perhaps substantial increase in sound intensity, but the actual sonic boom propagates more forwards, and the worst of it strikes the ground well ahead of the transition point.  I never knew this. The sonic boom itself also has a very interesting shape when the object is accelerating through the sound barrier.

Another interesting thing to consider is the effect of variation of speed of sound with altitude (or more precisely, with temperature.) It might be very important for meteor entries, as they pass through a very wide range of temperatures.
 
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18 Nov 2020 21:55

Watsisname wrote:
Source of the post It might be very important for meteor entries, as they pass through a very wide range of temperatures.

Yes, the speed of sound is lower at higher altitudes.  And the winds are higher also, which can alter the propagation significantly.  I did not attempt to model that in any way, just assuming constant speed at all altitudes, which gives nice circles.  Otherwise it quickly gets a lot more complex to draw.  Luckily for this simulation, these effects are minimal for the initial waves, as the meteor drops into the temperate and calm troposhere amazingly fast.  And it doesn't affect much the purpose of this visualisation anyway.
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19 Nov 2020 05:32

I made another visualisation where the meteor fragments into three pieces: a few grams, a few kilos and above 100 kilos.  Of course, the smalles fragment wont make much sound, but it's interesting to see the differences in deceleration.
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19 Nov 2020 22:29

midtskogen wrote:
There is a similar sound heard in the Mythbuster cement truck explosion, which I'm not sure how to explain.  The guy referred explicitly to the sound of that explosion saying that it was exactly like that.  The pitch drop in the Mythbuster explosion sounds faster, but in the Swedish videos I suspect that we miss the very first bit as the audio is saturated, so we only hear the slower, continued pitch drop, not so much the more powerful "Mythbuster" sound.

I've always imagined the pitch changes of whizzing bullets as doppler effects, but perhaps there are other factors as well.  For bullets it could certainly be a change in rotation speed, but a spinning rock of this size surely doesn't have a spin frequency matching the sound frequency.  Still, perhaps high sound frequencies can be produced by lower rotation periods due to funny air displacements around the rock.

I listened to a number of Chelyabinsk videos trying to find the same sound, but nothing like it.  Chelyabinsk came in quite shallow, though, and didn't enter the troposhere in such insane speed as in Sweden (probably well over 10 km/s at 15 km altitude!).  The pressure from that point and below must have been enormous.  It will be interesting to see what kind of rock could survive that.  But iron is unlikely.  10 tonnes of iron at 70 degree angle would just punch through the atmosphere and hit the ground supersonic.

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This is seriously great stuff!  Will you venture out to look for fragments, Mid?
About Chelyabinsk if I remember correctly, wasn't our attention fixed on a different (predicted) asteroid that was forecast to make a close pass to Earth and meanwhile this asteroid "snuck up" on us from sun-side?  Interesting coincidence (unrelated?) that it came on the exact same day as the one that was well predicted to be a close pass.
 
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19 Nov 2020 22:32

Watsisname wrote:
Beautiful! I love how we did this completely independently. :D And yours does clearly show how observers who are not very close will hear the sound from the supersonic flight first, which wasn't obvious.

I also learned something interesting by doing this. I always imagined that if a plane suddenly accelerated through the sound barrier, then the sonic boom would be most powerful for observers directly below the transition point. But not so. An observer directly below will hear the upshift and then downshift of frequencies, with a perhaps substantial increase in sound intensity, but the actual sonic boom propagates more forwards, and the worst of it strikes the ground well ahead of the transition point.  I never knew this. The sonic boom itself also has a very interesting shape when the object is accelerating through the sound barrier.

Another interesting thing to consider is the effect of variation of speed of sound with altitude (or more precisely, with temperature.) It might be very important for meteor entries, as they pass through a very wide range of temperatures.

Having grown up with the Concorde flying right over my head, I can verify that you are 100% correct.  Heard the sound well before I saw the Concorde.
 
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20 Nov 2020 01:38

A-L-E-X wrote:
Source of the post Will you venture out to look for fragments, Mid?

No, only from my desk.  There are people out in the field.  It's hard to find the time, besides there are quarantine restrictions in effect.

A-L-E-X wrote:
Source of the post About Chelyabinsk if I remember correctly, wasn't our attention fixed on a different (predicted) asteroid that was forecast to make a close pass to Earth and meanwhile this asteroid "snuck up" on us from sun-side?

It didn't "snuck up" because focus was on the close pass.  We wouldn't have detected it anyway.  The other close pass was unrelated.

A-L-E-X wrote:
Source of the post Having grown up with the Concorde flying right over my head, I can verify that you are 100% correct.  Heard the sound well before I saw the Concorde.

If you heard the sound before it passed right over you, it simply means that it was still subsonic.  I think Wats point was that the greatest sonic boom will not be heard right below the transition point, but later, well ahead of it.
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20 Nov 2020 01:56

How long after take off did the Concorde take to become supersonic?  The booms were pretty unmistakable.

About the asteroid, I heard that no one knew about it because it approached us from the "blind side" (the sun side) and so it was in the sun's glare and that's why it went undetected until it made impact.  It was amazing that the world was focused on one asteroid and a completely different one made impact.
 
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20 Nov 2020 04:54

A-L-E-X wrote:
Source of the post How long after take off did the Concorde take to become supersonic? 

I don't think it was allowed to do that over land?

A-L-E-X wrote:
Source of the post About the asteroid, I heard that no one knew about it because it approached us from the "blind side" (the sun side) and so it was in the sun's glare and that's why it went undetected until it made impact.


Yes, it was very difficult to spot before because of that.  But they are still hard to detect.
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20 Nov 2020 09:34

midtskogen wrote:
A-L-E-X wrote:
Source of the post How long after take off did the Concorde take to become supersonic? 

I don't think it was allowed to do that over land?

A-L-E-X wrote:
Source of the post About the asteroid, I heard that no one knew about it because it approached us from the "blind side" (the sun side) and so it was in the sun's glare and that's why it went undetected until it made impact.


Yes, it was very difficult to spot before because of that.  But they are still hard to detect.

Hmm not sure about the Concorde, perhaps that is correct, but I'd have to see the flight path.  There is about 100 miles of land east of JFK airport on Long Island.  But the FAA may have put those regulations in, many around here have complained about high airplane noise!
The asteroid spotting difficulty makes me wonder....how many years (decades?) are we from spotting and stopping an asteroid that would be big enough to rise to an ELE?  Of course such a thing statistically probably only happens once in 100 million years but being prepared is still important.  If we had a Shoemaker-Levy type collision like Jupiter had, I dont want to think about the amount of damage that could cause.
 
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29 Nov 2020 08:05

Possibly new physics in the CMBR

https://www.sciencealert.com/a-twist-in ... ew-physics
https://journals.aps.org/prl/abstract/1 ... 125.221301


But now scientists have discovered something peculiar about the CMB. A new measurement technique has revealed hints of a twist in the light - something that could be a sign of a violation of parity symmetry, hinting at physics outside the Standard Model.

According to the Standard Model of physics, if we were to flip the Universe as though it were a mirror reflection of itself, the laws of physics should hold firm. Subatomic interactions should occur in exactly the same way in the mirror as they do in the real Universe. This is called parity symmetry.

As far as we have been able to measure so far, there's only one fundamental interaction that breaks parity symmetry; that's the weak interaction between subatomic particles that is responsible for radioactive decay. But finding another place where parity symmetry breaks down could potentially lead us to new physics beyond the Standard Model.

And two physicists - Yuto Minami of the High Energy Accelerator Research Organisation in Japan; and Eiichiro Komatsu of the Max Planck Institute for Astrophysics in Germany and Kavli Institute for the Physics and Mathematics of the Universe in Japan - believe they have found hints of it in the polarisation angle of the CMB.

....

"If dark matter or dark energy interact with the light of the cosmic microwave background in a way that violates parity symmetry, we can find its signature in the polarisation data," Minami explained.

The problem with identifying β with any certainty is in the technology we use to detect the polarisation of the CMB. The European Space Agency's Planck satellite, which released its most up-to-date observations of the CMB in 2018, is equipped with polarisation-sensitive detectors.

But unless you know exactly how these detectors are oriented relative to the sky, it's impossible to tell whether what you're looking at is actually β, or a rotation in the detector that just looks like β.

The team's technique relies on studying a different source of polarised light, and comparing the two to extract the false signal.

"We developed a new method to determine the artificial rotation using the polarised light emitted by dust in our Milky Way," Minami said. "With this method, we have achieved a precision that is twice that of the previous work, and are finally able to measure β."

Milky Way sources of radiation are from much closer than the CMB, so they are not affected by dark matter or dark energy. Any rotation in the polarisation should, therefore, only be a result of a rotation in the detector.

The CMB is affected by both β and the artificial rotation - so if you subtract the artificial rotation observed in the Milky Way sources from the CMB observations, you should be left only with β.

Using this technique, the team determined that β is non-zero, with a 99.2 percent certainty. That seems pretty high, but it's still not quite enough to claim a discovery of new physics. For that, a confidence level of 99.99995 percent is required.

But the finding certainly demonstrates that the CMB is worth studying more closely.

"It is clear that we have not found definitive evidence for new physics yet; higher statistical significance is needed to confirm this signal," said astrophysicist Eiichiro Komatsu of the Kavli Institute for the Physics and Mathematics of the Universe.

"But we are excited because our new method finally allowed us to make this 'impossible' measurement, which may point to new physics."
https://www.sciencealert.com/a-twist-in-the-background-radiation-of-the-universe-hints-at-new-physics
 
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30 Nov 2020 01:00

Statistical significance isn't the only factor to consider, but the methodology as well, and in particular the treatment of possible influences by the foreground, such as dust within the Milky Way. That is what led to the later retraction of the claim of detecting primordial gravitational waves in the CMB by BICEP2. The statistical significance of the detection of phosphine on Venus was likewise found to be much lower than initially thought. 

So whenever there is a finding with especially extraordinary implications, like "new physics!", we should be especially cautious and await further review.

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