The h Bar

12:31 AM

@AaronStevens Somewhat. I could learn more detailed information about it. But from what I do know black body has several definitions.

@AaronStevens But I always love to hear all points of view. So if there's some things you know, I'd love to hear them. :)

@ScientistSmithYT I don't know of more than the one main definition of black body, though I'm sure you can find it worded different ways.

I know that air when ionized with high enough voltages is the black body. But some other forms of evidence seem to say no.

@JMac Yes, the main two forms I know of are black body and black body radation. But the problem is that when I've looked at several definitions of them the definitions don't say what either really are. It's like the sources are telling me that a magnetic field is made of invisible photons. When really it isn't.

@JMac I wanted to thank you for your input. Not many people take questions like this as one that anyone can answer. So I'd like to take a moment and thank you for giving some information. It really helps. :)

@ScientistSmithYT To quote wikipedia "A black body or blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. " It's a hypothetical object that absorbs all the electromagnetic radiation that hits it (though it can still emit radiation).

If I ever ask a question anyone can help answer it. Even if it's just a little hint or a guess. Because that's where some answers are found. So if I ask a question it's everyones ballgame. :)

@JMac So a blackbody is a theoretical physical object that emits electromagnetic radation through the ionization of air via electricity.

So then that would mean the transfer is 100 percent efficient.

@JMac So we know the radation must be in the form of electromagnetic radiation. But what wavelength? And how do we measure it?

12:47 AM

@ScientistSmithYT Not really... The ionization of air doesn't have anything to do with the definition of a black body. What's important about a black body is that it absorbs all radiation that hits it. All objects radiate based on their temperature, and "black body radiation" is what you get when the object is not reflecting any of the radiation that hits it. It's extremely useful, because you can approximate the radiation from a large body as the blackbody radiation times an emissivity factor.

@JMac So for my example: Welding. Striking an electrical arc of hot plasma approximately 6,500 Degrees Fahrenheit. How would I measure the radiation off of the arc? Or how would I calculate it and the dose absorbed in human skin?

Semiclassical

In principle, a black body emits radiation of all wavelengths. But not all of those wavelengths are emitted with equal intensity: the Planck distribution dictates that distribution.

In particular, there's a peak in the Planck distribution, i.e. a wavelength where the intensity is highest. But that wavelength turns out to be inversely proportional to temperature.

That means that one can infer the temperature of an object from its radiation spectrum. (so long as one can approximate it as an ideal black body)

This is known as Wien's displacement law. Wikipedia has a bunch of applications: en.wikipedia.org/wiki/Wien%27s_displacement_law

@ScientistSmithYT I don't know how it changes with temperature; but I just found in quick searching that one source claims the emissivity of an arc at ~25,000 °F is approximately 0.012, so quite a bit less than a real blackbody.

@Semiclassical Yeah I looked at those. And from what I understand the waves have a peak and the Planck Distribution formula calculates for the average radiation emision.

But I don't think I quite understand that entirely.

@ScientistSmithYT Honestly, the place I found the emissivity was a welding related paper. You might be able to find studies on the expected radiation from welding arcs. If you are curious about the safety I'm sure someone has done the math somewhere.

12:57 AM

@JMac Did it say 0.012 CPM?

@JMac Could you give me the link to the paper you looked at?

@ScientistSmithYT CPM? It's a dimensionless number that means it emits 1.2% of the net radiation that a true black body would emit. I already closed the random paper I found. Googling "emissivity of welding plasma" should find it.

@JMac Oh ok. CPM as in Counts Per Minute.

I found a few articles on it. But I have no clue how to convert the emisitivity to CPM. I looking to convert the emisitivity to radiation terms in CPM. So I could then determine what dose of radiation does what to the human body. Specifically the skin and finding a correlation between skin burn signs to the dose of radiation.

@JMac I've got to go for now. I have a meeting to attend, but I'll do some more looking into those articles. Then I'll come back and talk about what I've found and if it's right.

@ScientistSmithYT I don't think you would expect much ionizing radiation FYI.

dmckee

1:16 AM

@EmilioPisanty Say congradulations from me. It's a big step.

I've only been in that place a fwe time, so I don't know if my advice is much good.

Try to relax and be yourself because you want a good cultural match nearly as much as you want oh-lord-a-job-any-job-at-all.

Do what research you can on the target department before you head out.

Come prepared for the teaching sample (if any).

I'm at a loss.

Emilio Pisanty

1:51 AM

@dmckee teaching sample seems to be rolled into the seminar

(which only adds to the pressure on that)

@dmckee yeah, I think that's the goal

@dmckee and thanks =)

5:44 AM

@dmckee have you seen DavidZ around?

2

7:30 AM

@frt132 hi :-)

@frt132 Did you want to ask about quantum field theory?

7:42 AM

@JohnRennie Hi! I was going to know more about those questions I asked about QFT, did some search but did not find obvious results.

@frt132 can I suggest an analogy? Bear in mind that like all analogies this one has limitations and shouldn't be taken too literally.

@JohnRennie Sorry, I want to know one thing, thank you! Why is Fourier transformation leading to the uncertainty principle? Is that because all matter are regarded to have particle-wave duality so they are treated as wave in this case which leads to the uncertainty principle

Wave-particle duality is a misleading concept and the world of physics would be better off if the phrase had never been coined.

Yes, I read somewhere probably it is a legacy of 18th century classic understanding

Quantum objects like electrons, protons, etc are neither waves nor particles. They are described by wave functions that can under the appropriate circumstances behave like waves or behave like particles.

But "behaves like" isn't the same as "is".

7:49 AM

Got it, the questions for QFT are really pressing for me, but I don't want to cost your time on the basics, I will watch all the PBS space time on the subject

and come back later : )

Quantum mechanics tells us how to describe a particle. It doesn't tell us what a particle "really is". Indeed most physicists would take the view that asking what a particle "really is" is a meaningless question best left to philosophers and other people with too much free time :-)

@frt132 if you want I can give you a simple analogy for how QFT works ...

Oh, yeah, sure

Consider a swimming pool that is still - no waves on its surface.

If you put your hand into the pool and waggle it then you can create a wave that travels across the pool.

The harder you waggle your hand the larger the amplitude of the wave.

But this is a quantum pool, and what you find is that you can't just create a wave of any amplitude. What you find is that is you waggle your hand to gently no wave is created.

Then as you waggle harder suddenly you find you've created a wave.

In QFT the wave is what we mean by a particle, so you've just created a particle.

but there's different magnitude of the wave in real life

If you waggle even harder the wave doesn't change until suddenly it doubles in amplitude. What just happened is you created a second particle so your wave now corresponds to two particles.

@frt132 in a real swimming pool the wave amplitude can be anything you want.

7:56 AM

> this is a quantum pool

But with a quantum field this is not the case. Well, not with massive quantum fields, but let's skp gently over this for now

The point is that when you try to transfer energy to a quantum field you'll find the energy gets transferred in quanta and each quantum creates one particle.

Cool

also the 'wave' doesn't have exact location?

@frt132 correct.

Though we can combine waves to create a wave that is localised. This is how we describe particle like interactions.

Since this is an invaluable opportunity, can I ask you something about some other thing?

I read in Stephen Hawking's Grand Design about an experiment. It was an experiment done by French doctors who perform open brain surgeries to remove tumors. They identify tumors in part by using electrodes to stimulate the patients' brain to see if the patients feel anything. And the patients actually reported to feel the subject feeling of 'intention' to move the hand etc when electrolly stimulated on the brain. I also read about drugs like DMT that make people 'see things'. etc.

Yes, you're welcome to ask. I'm in the chat room every day, so while debatably invaluable this is certainly not a unique opportunity :-)

@frt132 OK ... I must admin I'm not sure how physics comes into this ...

8:03 AM

There's more correlation I see about that, but It feels more and more to me that all the 'consciousness', be it attention, or ambition, or knowledge or logic or emotion etc are all from the somewhat deterministic and somewhat random evolution of everything. Does this aspect of 'determinism' somehow affect your philosophy about life?

I suspect most physicists take a pragmatic view. Since we will probably never know exactly how consciousness and the real world (whatever that is) are related we tend to ignore the question and just get on with studying what we can know. This is certainly my approach. I tend to get impatient with what I see as pointless debates.

Right, I am curious about this as I am having some difficulties with my own 'goal of life' thing, and thought that maybe knowing more about physics could help, thank you very much for the answer though.

philosophy.stackexchange.com/…

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:-)

Thank you, honestly, I couldn't find much that might interest/inspire me there. Like Stephen Hawking said, phylosophy is kind of dead?

yeah, so is their chat room

8:12 AM

@frt132 speaking as one who is nearer the end of his life than the beginning, life rarely turns out as you expect or plan, so worrying about it is of limited use :-)

Will you consider cryogenics?

As a young adult, I guess I would definitely consider that : p, lol

Have myself frozen? Only if my descendants wanted to preserve me for future consumption, and I suspect I'd be a bit tough.

As for me, I kind of feel that I would want to regain consciousness someday to know more about the world in the future or the universe or the things we haven't discovered yet, even if the probability would be like 0.0001%. Probably the thought will change too though.

8:30 AM

everything changes

Plus ça change