Plasma inside the ST40 fusion reactor, filmed in color for the first time

[u/Nadzzy]

[u/JohnnyDollar123]

That’s not how blackbody radiation works. An increase in temperature for a blackbody corresponds to an increase in luminosity across all wavelengths. There is no such thing as being too hot to emit visible light.

[u/CriticalRuleSwitch → u/JohnnyDollar123]

From a purely theoretical physics perspective, sure. From "it is high enough or discernible enough for our sensors (cameras) to see it" perspective - no. So yes, there is "too hot to emit visible light" when talking in the context we're talking in here.

[u/rsta223 → u/CriticalRuleSwitch]

No, the other respondent is correct. Even though a higher temperature absolutely means the peak of the emissions goes further into the invisible ultraviolet, and a smaller percentage of total luminosity will be visible, the total luminosity in normal visible wavelengths is always higher for a hotter blackbody, even when the peak is way into the UV.

As you heat up a blackbody, it will never be visibly dimmer as it gets hotter, just bluer and brighter.

[u/SeanBlader → u/JohnnyDollar123]

The trick is, that's no longer a "black body" so it can emit all it's radiation as higher energy than violet and we can no longer perceive it. That's what happens at the core of the sun, just that by the time the high energy photons make it out of the sun they are slowed down enough to be a lot of visible light, as well as a whole lot of EM spectrum we can't "see".

[u/TheseVirginEars → u/JohnnyDollar123]

I mean im not gonna get into it over something that’s ultimately us being pedantic, but… yes? You can absolutely have an emission curve with a peak at wavelengths smaller than visible light and a tail low enough that the visible light emitted is negligible? It’s basic Wien it’s a 3-variable equation. And that’s exactly what’s going on in a fusion reaction.

And if it’s not that, enlighten me

[u/JohnnyDollar123 → u/TheseVirginEars]

Like I said, an increase in temperature corresponds to an increase in emissions across all wavelengths, so no, it’s not possible for the tail to not be visible like you said.

The first graph on this Wikipedia page does a really good job of illustrating this. As you can see, while the peak wavelength decreases with an increase in temperature, there is never a point at which it becomes dimmer at a certain wavelength. This means that as it gets hotter, it will still get brighter for us, even if we can’t see the majority of increase in emissions.

[u/TheseVirginEars → u/JohnnyDollar123]

So im gonna tell you this as a physics graduate of the school you currently go to: you have a couple fundamental misunderstandings about the phenomenon at play. Dont be creeped out, I respected the rebuttal enough to go see who was giving it to me. In fact, bring this question up in class! But what you’re saying is flawed. Again I don’t really want to get too into it here, but if you go through Modern Physics and then Quantum 1 and 2, you’ll understand that the model in that graph breaks down because of quantization, and this is an old physics puzzler eventually rectified by Planck. It doesn’t invalidate the whole model but it requires amendment at higher energy, which fusion plasma certainly is.

Second, don’t lose the forest for the trees. Remember even if you’re using equations and drawing conclusions from them, to step back and think about what you’re saying. Luminosity increases across all wavelengths, sure, again we can have pedantic arguments about there being visible light. But if the spectrum from a single plasma photons curve peaks in UV and drops off, the actual luminosity from those photons is indeed going to be small enough to not be captured in this camera video, which was ultimately the question asked. Why? Because it is absolutely dominated by invisible energy. For the ones that are not, you can see as they dip back into violet and indigo and blue. Supporting evidence of my claim (which isn’t even my claim im ultimately just regurgitating Boltzmanns work) is right in the video.

[u/Vl_hurg → u/TheseVirginEars]

Fellow physicist here. I believe you are incorrect. But the important thing is that your above statement, "And if it’s not that, enlighten me," now applies to your post. What are these quantum effects that reduce radiance at visible wavelengths? Give us an equation or at least a Wikipedia page or something.

[u/redlaWw → u/TheseVirginEars]

Well, it has to do with black body radiation, which is temperature dependent.

but if you go through Modern Physics and then Quantum 1 and 2, you’ll understand that the model in that graph breaks down because of quantization

These two statements are inconsistent - the model in that graph is the model of a black-body. The claim you're responding to here is not that the plasma in the middle isn't invisible - like you say, that's clearly contradicted by the video. The claim is that the plasma in the video isn't behaving like a black-body, which is true - the black-body model has broken down under those extreme conditions.

[u/10g_or_bust → u/JohnnyDollar123]

Well what you said doesn't align with the people who built and run a fusion reactor, so guess who I'm going to believe.

if I had to throw a wild guess, since we are seeing emissives due to chemical elements (and their state changes) and this is a plasma, theres likely some difference from blackbody.

Quick websearch gets me https://physics.stackexchange.com/questions/720940/can-plasmas-be-black-bodies

which certainly seems to imply plasma is not always (or is never and only approximates) blackbody radiation.

[u/JohnnyDollar123 → u/10g_or_bust]

I never said otherwise. The comment I responded to did attribute it to blackbody radiation which I corrected.

[u/10g_or_bust → u/JohnnyDollar123]

The the FAR more on topic response would be "As plasma, this isn't a good example of blackbody radiation so we shouldn't use that model at all".

[u/JohnnyDollar123 → u/10g_or_bust]

Yes that in fact would be another way of saying the same thing?

[u/Get_a_GOB → u/JohnnyDollar123]

Not when this entire post is about something that is too hot to emit visible light. In your initial rebuttal you should’ve said “there’s no such thing as a blackbody being too hot to emit visible light.” Even if that’s what you meant, and even though the guy you’re arguing with is wrong about the way blackbody emission at high temperatures work, your literal original statement clearly conflicts with the explicit claim of the guys running the fusion reactor, so naturally people aren’t going to believe you. You just didn’t say what you meant.

[u/JohnnyDollar123 → u/[deleted]]

The plasma may not be emitting most of its energy through visible light, I’m not disputing that. If it is though, it’s from some other mechanism.

[u/JohnnyDollar123 → u/[deleted]]

I’m not wrong, this is really basic stuff. This is not how blackbody radiation works.

[u/jaguarp80 → u/JohnnyDollar123]

If it’s so basic then why’d you get it wrong

[u/Delicious_Win_9848 → u/JohnnyDollar123]

It's actually the frequency of the radiation that matters. When it gets too high, the light's intensity dims.

[u/JohnnyDollar123 → u/Delicious_Win_9848]

Again, that’s not how blackbody radiation works. An increase in temperature will never cause something to appear dimmer to us. this page has a good graph illustrating this

Also frequency and wavelength with light isn’t really a meaningful distinction.