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Physics, Astronomy & Cosmology / Have you ever seen a FIREBALL?

« on: 16/03/2023 02:46:27 »

The night sky phenomenon (not the explosion type).

I got to see one, 12.37 am (approx), June 27th, last year.  Over the east of England.

I couldn't get over it, went back inside the house and immediately reported it to UK meteor network (with all the details I could recall - it was over in a second or two).  Even better, it wasn't just me.  A couple of hundred people reported it to the site and then some even submitted CCTV footage of it.  The idea being, the site staff wanted to find out where it touched down.

I took a tour up mount Teide on Tenerife - the EU's highest mountain apparently - in 2019 and seem to recall seeing a few shooting stars.  They were nothing compared to what I saw last year, though.

They're supposed to be things you get to see 'once in a lifetime' if lucky.  Apparently.

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Physics, Astronomy & Cosmology / Re: Do you change the spectrum of radiation inside an oven if you change its shape?

« on: 12/03/2023 18:11:55 »

As (almost) always, the answer is "it depends".
The emission line from a laser can be subject to "mode hopping" because, as its temperature changes, the wavelengths reinforced by resonance also chance. The Fabry Perot resonator depends critically on the size and shape of the cavity.

With a black body cavity, the light can't know how far way the other side is, and the spectrum depends on the temperature.

If I get a box with mirror walls and put a police car in it with the lights flashing, that light is still blue.

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Physics, Astronomy & Cosmology / Re: Do you change the spectrum of radiation inside an oven if you change its shape?

« on: 12/03/2023 14:13:44 »

Blackbody radiation is usually considered (theoretically modelled and suitable equations derived) by looking at the radiation that can exist inside a cavity or an "oven".    The interior of the oven can support various modes of radiation within it,  each mode has a particular frequency,   those frequencies ultimately assumed to be caused (generated by) charged particles oscillating at that frequency in the walls of the oven.

The effect of standing wave to radiation spectrum is only significant when the oven walls are highly reflective. Otherwise, the effect of standing wave, and shape of the oven, would be miniscule.
Candle soot is highly absorbtive/emissive, even when it's only applied to a flat surface. It absorbs light and reemit electromagnetic radiation close to black body spectrum.

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Physics, Astronomy & Cosmology / Re: Do you change the spectrum of radiation inside an oven if you change its shape?

« on: 11/03/2023 22:32:49 »

So Planck was wrong after all.

No, he was right, which is why he didn't say what shape or size the cavity was; he knew it didn't matter as long as it was black.

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Physics, Astronomy & Cosmology / Re: Do you change the spectrum of radiation inside an oven if you change its shape?

« on: 11/03/2023 20:33:17 »

Best Wishes.

LATE EDITING:  Someone added the tag "microwave oven" to this thread.  I have reduced that to "oven".   Microwave ovens are not a good example of blackbody radiation existing inside the cavity and are only relevant when properties like standing waves were being discussed.

Thanks for Correcting it & providing an Explanation for the same.

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Physics, Astronomy & Cosmology / Re: Do you change the spectrum of radiation inside an oven if you change its shape?

« on: 11/03/2023 17:45:41 »

So Planck was wrong after all.

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Physics, Astronomy & Cosmology / Re: Do you change the spectrum of radiation inside an oven if you change its shape?

« on: 11/03/2023 11:53:23 »

if we look inside an ideal carbon spherical shell* at red heat, the tiniest surface imperfection will alter the Planck resonance spectrum by adding or subtracting wavelengths equal to arbitrary multiples of the height of the imperfection,

No, it won't.
That's why (or because, depending on how you look at it) they use graphite; it's black.

You can not get cavity resonances if the walls are black.

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Physics, Astronomy & Cosmology / Re: Do you change the spectrum of radiation inside an oven if you change its shape?

« on: 10/03/2023 21:57:51 »

Multiple modes can be put to good use by making the second cavity tunable.
- This allows selection of one mode amongst many possibilities
- By selecting different modes in different devices, you can have many different devices transmitting on the same optical fiber, increasing the fiber capacity by a factor of 10 to 100: "Wavelength Division Multiplexing"
- If the independent wavelengths are closer together, then better control over wavelength accuracy and stability is needed (ie more cost)
https://en.wikipedia.org/wiki/Wavelength-division_multiplexing

Today, Erbium-Doped Fiber Amplifiers (EDFA) are used to amplify a wide range of wavelengths (other exotic elements like Thulium, Praseodymium and Ytterbium are also usable).
- This is effectively a laser without a cavity, and it is able to amplify all wavelengths in a Wavelength Division Multiplexing system.
- See EDFA at https://en.wikipedia.org/wiki/Optical_amplifier

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Physics, Astronomy & Cosmology / Re: Do you change the spectrum of radiation inside an oven if you change its shape?

« on: 10/03/2023 16:21:10 »

Part of your practical problem would be the amplitude range as well as the frequency range of your black body radiation. Evan's lasers at least restrict the frequency range to near-negligible so it's not inconceivable that you could look for missing lines in their uniformly high intensity spectrum but if we look inside an ideal carbon spherical shell* at red heat, the tiniest surface imperfection will alter the Planck resonance spectrum by adding or subtracting wavelengths equal to arbitrary multiples of the height of the imperfection, and all against the background of every other surface element doing its bit!


*mathematicians and theoretical physicists may like cubes, but engineers ask awkward questions about what happens in the corners, so physicists prefer to use spherical shells.

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Physics, Astronomy & Cosmology / Re: Do you change the spectrum of radiation inside an oven if you change its shape?

« on: 10/03/2023 13:36:57 »

To a good approximation*, an electron in the wall of the oven which drops to a slightly lower energy level (and emits a long wave photon as a consequence) does not know what shape and size the oven is.

Waveguides are often silver plated or even lined with superconductors to improve the sharpness of their frequency response.
A black body is  (pretty much definitively) not.

The analogy isn't  getting a resonance from a flute; it is trying to play a tune on an exhaust silencer. (I might pay good money to watch someone try to do that.)

If the wall opposite absorbs all the radiation which hits it, there's no way for the radiation to "know" how far away it is.

It's not a good approximation if one of the walls is a good mirror.
I did some pointless research on this as a student. You can change the half life of fluorescence by putting the emitter near a mirror.
But you don't make black bodies out of mirrors so it's beside the point.

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Physics, Astronomy & Cosmology / Re: Do you change the spectrum of radiation inside an oven if you change its shape?

« on: 10/03/2023 11:52:53 »

A waveguide is effectively a long, thin oven designed to support one particular mode and frequency. It can obviously support harmonics but these are usually filtered out or not generated in the first instance.

Much the same applies to wind instruments. A flute extracts a fairly pure sine wave from the exciting edge tone, whereas a conical-bore instrument like a saxophone carries a lot more overtones.

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Physics, Astronomy & Cosmology / Re: Do you change the spectrum of radiation inside an oven if you change its shape?

« on: 10/03/2023 11:45:01 »

I'm not sure it's possible to tell.
I think we are looking at the idea that wavelengths comparable with the length of the side of the box will be perturbed by the size and shape of the box.


If I get a metal box (like a microwave oven) and put a tungsten light bulb in it. the light doesn't get out.
If I make a small (say 1mm) hole in the wall, the light gets out and I can measure its spectrum. The box hardly makes a difference to that.
I can, also measure the infra red from the lamp.

In principle, the bulb also emits microwave and radio wave radiation.
But I can't see them- not because the box changes the spectrum of the lamp, but because the long wavelengths will not pass through the small hole.
The longer than about 1mm wavelengths are filtered out

So, I can only observe the spectrum of what's happening in the box through a filter which I know removes anything with a wavelength longer than 1mm.
If I make the hole bigger (comparable with the size of the box) in order to look at wavelengths comparable with the size of the box then...
it isn't a box any more.

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Physics, Astronomy & Cosmology / Re: Do you change the spectrum of radiation inside an oven if you change its shape?

« on: 10/03/2023 08:04:10 »

Lasers have a 1-dimensional cavity, for which there are several modes.
- Cheap semiconductor laser pointers continually jump between these modes
- More expensive lasers for telecommunications often have a second optical cavity coupled to the first; only one mode is able to resonate in both cavities, reducing interference due to the different speed of light of different modes when traversing an optical fiber.

This video shows mode changes every few seconds in a green laser.

This page shows a typical laser spectrum, averaged over time - but in practice, not all of these resonances are there at the same time.
https://www.researchgate.net/figure/The-laser-spectrum-at-t-6-nsec-Axial-cavity-modes-are-visible-in-the-laser-spectrum_fig4_321257044

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Physics, Astronomy & Cosmology / Re: Do you change the spectrum of radiation inside an oven if you change its shape?

« on: 10/03/2023 06:51:48 »

The only way radiation of those frequencies could have got into the cavity would be when the oven door was opened

If you are looking at the spectrum excited by the cavity magnetron, it spans a range of about 10MHz (it jumps around between different modes inside the magnetron).
- So there are a number of modes that might be excited by the magnetron
- To minimise uneven heating due to standing waves, microwave ovens often have a rotating platform for the food, or else a mode stirrer (like a metal fan, but usually out of sight)

https://www.researchgate.net/figure/Magnetron-spectrum-without-PLL-control-and-with-full-heater-power-68-W_fig4_3074637

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Physics, Astronomy & Cosmology / Re: Do you change the spectrum of radiation inside an oven if you change its shape?

« on: 09/03/2023 23:33:49 »

Someone else is trying real hard to bake their cake on in here...

https://www.thenakedscientists.com/forum/index.php?topic=66414.0

(thou it's a very long cake, supposedly with no cherries atop)

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Physics, Astronomy & Cosmology / Re: Do you change the spectrum of radiation inside an oven if you change its shape?

« on: 09/03/2023 07:15:45 »

One of the boundary conditions that must be satisfied is that ψ = 0 at the walls of the oven  ...since these waves do not continue through the walls

I understand that historically, black body radiation was calibrated with a block of graphite (already pretty black, visually), with a sphere carved out in the center, and a hole bored through from the outside, so you could observe the radiation inside the cavity.
- You heat up the graphite block to a certain temperature (presumably with no oxygen present), and measure the spectrum of the radiation visible at the small hole.

Because graphite is a fairly effective black body, it actually absorbs any radiation impinging on the surface of the sphere
- getting hotter in the process
- and generating black-body radiation back into the cavity
- You measure the spectrum after this has settled down to an equilibrium

Because the graphite is absorptive, you don't need "ψ = 0 at the walls", since the radiation acts as if it is propagating off to infinity
- When in fact it gets absorbed within a few mm of the wall
- So there are no standing waves in the cavity
- This is quite unlike a metal-walled microwave oven, where the E field is zero at the conductive walls, and standing waves are definitely present (causing uneven heating of the food)

So I suggest that a reflective metal oven is a much worse representation of black body radiation than a cavity carved in graphite.

See the diagram here:
https://www.physics.utoronto.ca/~phy293lab/experiments/blackbody.pdf

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That CAN'T be true! / Re: How can academia tolerate such error?

« on: 28/02/2023 12:37:57 »

Internal combustion engine=ice. I am sorry for any confusion, I should not be using abbreviations that may be misunderstood. PS, as explained to ZerO I haven't fully emerged from my annual hibernation phase.

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Physics, Astronomy & Cosmology / Re: How Do We Know The Universe is 13.8B yrs Old If We Can Only See The Observable ?

« on: 26/02/2023 10:02:43 »

This recent release from the James Webb Space Telescope identified some galaxies that seem to be as big as the Milky Way, but with red shifts that suggest that they reached this size only 500-800 million years after the Big Bang.
- Current theories of galaxy formation can't account for how a galaxy would grow so big, so fast.

https://www.popularmechanics.com/space/deep-space/a43026293/jwst-discovers-impossible-galaxies/

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Physics, Astronomy & Cosmology / Re: How Do We Know The Universe is 13.8B yrs Old If We Can Only See The Observable ?

« on: 25/02/2023 15:32:25 »

How can we know it is 13.8 billion years old if all we can see is the observable Universe ?

The age was not computed by looking as far as we can see. Hubble's constant of about 70 km/sec/Mpc was measured nearly a century ago, long before they were looking at things a significant percentage of the distance to the edge of the observable universe. The age can be computed directly from just that one constant.

There's about 3e19 km in a Mpc, so 70 km/sec/Mpc is the same as 2.3e-18 km/sec/km which, cancelling the km part, is 2.3e-18 sec^-1
The reciprocal of that is 4.35e17 seconds which is 13.8 billion years.

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Geology, Palaeontology & Archaeology / Re: Why do insects fly?

« on: 20/02/2023 20:24:49 »

Apparently, flowering plants evolved about 130 million years ago.

But flying insects reached their maximum size about 300 million years ago, supported by higher oxygen levels than today.
Some had wingspan of up to 70cm. Some were insectivores (like dragonflies today), but there was obviously something nutritious at the base of the food pyramid...
https://news.ucsc.edu/2012/06/giant-insects.html

Maybe your hydrogen car economy could be supported by solar-powered home electrolysis, storing hydrogen and driving fuel cells in the evening?