<p>We actually can get to high enough temperatures to do nuclear chemistry experiments outside of particle accelerators. Plasmas have achieved <a href="https://www.reddit.com/r/fusion/comments/1go2b78/us_firm_sets_fusion_plasma_temperature_record_at/?rdt=53734#:%7E:text=US%20firm%20sets%20fusion%20plasma,for%2024%20hours%20%3A%20r%2Ffusion" rel="noreferrer">temperatures of almost 400 million degrees</a> and we can observe nuclear fusion happening at those temperatures (and of course fission happens at very low temperatures anyway so we don't need high temperatures for that). Particle accelerators can get to much higher temperatures - <a href="https://www.theverge.com/2012/8/15/3244513/cern-scientist-hottest-man-made-temperature" rel="noreferrer">5.5 trillion kelvins</a> - and they still work as perfectly good models, they just have much less reactant to work with than experimental fusion reactors. We can also observe stars via spectroscopy and do a sort of  vicarious &quot;experiment&quot; by observing their spectral lines over their life cycle but I wouldn't really count that.</p>
<p>We <em>can't</em> do the experiment to test string theory and verify AdS/CFT yet, however. Evidence for string theory will come in many forms, including Lorentz symmetry maintenance down to the Planck scale, the existence of a graviton (which also pops up at the Planck scale), and a certain set of quantum gravitational effects (which also only become significant at the Planck scale). We don't have any way of getting to Planck energy/distance scales yet so we just have to trust in whatever quantum theory of gravity we each subscribe to until we get evidence one way or another. String theory is, mathematically/theoretically valid, but we've never observed any of its quirks because they only show up at much higher energies than we can get to; fusion and nuclear chemistry on the other hand we can directly experiment with.</p>

CC BY-SA 4.0
2024-11-17T12:56:14.270Z
I'm just wondering: Yes, we can generate the *temperatures* inside stars, and it's not too difficult -- however, the nuclear physics inside stars are obviously also determined by the pressure, and *that* is something that is a bit elusive, or we'd have fusion reactors now. So taking the question a bit broader as "How can we understand nuclear chemistry at **the conditions** inside a star?", it is less easily answered, perhaps. (Sure, one can infer; maybe higher pressure simply means more of the same of what we see at lower pressure; but that would need some explanation.)
Peter - Reinstate Monica
CC BY-SA 4.0
2024-11-17T16:11:06.497Z
@Peter-ReinstateMonica:  for nuclear reactions the pressure only influences the rate at which the reactions happen.  At higher pressures collisions become more common.  We measure the fraction of collisions that result in each reaction, then use modelling to compute the bulk effects and temperature/pressure equilibrium
Ross Millikan
CC BY-SA 4.0
2024-11-18T12:17:48.237Z
@Peter-ReinstateMonica making nuclear fusion happen is a solved problem. What's not solved is "make it happen at scale without putting in more energy then we get out". And yes, we've reached >1 aparently but again not at scale and not for the full system...
kruemi
CC BY-SA 4.0
2024-11-18T17:22:29.630Z
Those are not the fusion experiments that we used to figure out stars - those experiments were more of the top secret nuclear weapon type, and at the time it was hard for astrophysicists to obtain the relevant reaction cross sections.
AtmosphericPrisonEscape
CC BY-SA 4.0
2024-11-16T23:21:59.703Z
2024-11-16T23:21:59.703Z
controlgroup
<p>I really want to know two things:</p>
<ol>
<li><p>Is it actually true that we can't model nuclear reactions at millions of degrees, because we CAN in accelerators? My understanding is that we can't, but I'm always terrified that my understanding is wrong.</p>
</li>
<li><p>If we are only confident about our understanding of nuclear chemistry because the mathematical models fit observations, why doesn't that also apply to string theory predicting quantum field theory by projecting it onto a lower number dimensions via the ADS/CFT correspondence?</p>
</li>
</ol>
<p>Or to phrase it as a contrapositive, people say string theory can't be disproven because the energies are too high. Why does that not also apply to nuclear chemistry?</p>
<p>The title field wasn't long enough to allow me to ask that complete question in the title. I will happily accept just an answer to the first part. If no one here is familiar with the ADS/CFT correspondence, then please ignore it instead of voting to close just because it is also about physics. To me, it's all just one amazing reality.</p>

CC BY-SA 4.0
2024-11-16T23:47:28.093Z
Not sure I understand the first question.  Nuclear physics experiments usually take place at energies far higher than in stars. It is the low energies in stars that lead to remaining uncertainties.
ProfRob
CC BY-SA 4.0
2024-11-17T00:49:58.523Z
@ProfRob Yeah, but I didn't know they could DO very high energy experiments. That's the resolution of my question — my own ignorance. I prefer answers that show I didn't know what I was talking about, because they make me less stupid, and because science returns to the state of not having any contradictions.
Miss Understands
CC BY-SA 4.0
2024-11-17T18:31:54.033Z
Related: https://physics.stackexchange.com/q/551563/123208
PM 2Ring
<star><nucleosynthesis><astrochemistry>
Star power: How can we understand nuclear chemistry at millions of degrees when we can't do that experiment? Because the math predicts observations?
Miss Understands
# Star power: How can we understand nuclear chemistry at millions of degrees when we can't do that experiment? Because the math predicts observations?

(Asked by: username_0 on 2024-11-16)

I really want to know two things:

1. Is it actually true that we can't model nuclear reactions at millions of degrees, because we CAN in accelerators? My understanding is that we can't, but I'm always terrified that my understanding is wrong.
2. If we are only confident about our understanding of nuclear chemistry because the mathematical models fit observations, why doesn't that also apply to string theory predicting quantum field theory by projecting it onto a lower number dimensions via the ADS/CFT correspondence?

Or to phrase it as a contrapositive, people say string theory can't be disproven because the energies are too high. Why does that not also apply to nuclear chemistry?

The title field wasn't long enough to allow me to ask that complete question in the title. I will happily accept just an answer to the first part. If no one here is familiar with the ADS/CFT correspondence, then please ignore it instead of voting to close just because it is also about physics. To me, it's all just one amazing reality.

**Tags:** star, nucleosynthesis, astrochemistry
> **Comments:**
> **username_1**: Not sure I understand the first question.  Nuclear physics experiments usually take place at energies far higher than in stars. It is the low energies in stars that lead to remaining uncertainties.  
> **username_0**: @ProfRob Yeah, but I didn't know they could DO very high energy experiments. That's the resolution of my question — my own ignorance. I prefer answers that show I didn't know what I was talking about, because they make me less stupid, and because science returns to the state of not having any contradictions.  
> **username_2**: Related: https://physics.stackexchange.com/q/551563/123208  

---

## 1 answers

### Answer by username_3 on 2024-11-16. Score: 20 | Accepted answer

We actually can get to high enough temperatures to do nuclear chemistry experiments outside of particle accelerators. Plasmas have achieved [temperatures of almost 400 million degrees](https://www.reddit.com/r/fusion/comments/1go2b78/us_firm_sets_fusion_plasma_temperature_record_at/?rdt=53734#:%7E:text=US%20firm%20sets%20fusion%20plasma,for%2024%20hours%20%3A%20r%2Ffusion) and we can observe nuclear fusion happening at those temperatures (and of course fission happens at very low temperatures anyway so we don't need high temperatures for that). Particle accelerators can get to much higher temperatures - [5.5 trillion kelvins](https://www.theverge.com/2012/8/15/3244513/cern-scientist-hottest-man-made-temperature) - and they still work as perfectly good models, they just have much less reactant to work with than experimental fusion reactors. We can also observe stars via spectroscopy and do a sort of vicarious "experiment" by observing their spectral lines over their life cycle but I wouldn't really count that.

We *can't* do the experiment to test string theory and verify AdS/CFT yet, however. Evidence for string theory will come in many forms, including Lorentz symmetry maintenance down to the Planck scale, the existence of a graviton (which also pops up at the Planck scale), and a certain set of quantum gravitational effects (which also only become significant at the Planck scale). We don't have any way of getting to Planck energy/distance scales yet so we just have to trust in whatever quantum theory of gravity we each subscribe to until we get evidence one way or another. String theory is, mathematically/theoretically valid, but we've never observed any of its quirks because they only show up at much higher energies than we can get to; fusion and nuclear chemistry on the other hand we can directly experiment with.

> **Comments:**
> **username_4**: I'm just wondering: Yes, we can generate the *temperatures* inside stars, and it's not too difficult -- however, the nuclear physics inside stars are obviously also determined by the pressure, and *that* is something that is a bit elusive, or we'd have fusion reactors now. So taking the question a bit broader as "How can we understand nuclear chemistry at **the conditions** inside a star?", it is less easily answered, perhaps. (Sure, one can infer; maybe higher pressure simply means more of the same of what we see at lower pressure; but that would need some explanation.)  
> **username_5**: @Peter-ReinstateMonica:  for nuclear reactions the pressure only influences the rate at which the reactions happen.  At higher pressures collisions become more common.  We measure the fraction of collisions that result in each reaction, then use modelling to compute the bulk effects and temperature/pressure equilibrium  
> **username_6**: @Peter-ReinstateMonica making nuclear fusion happen is a solved problem. What's not solved is "make it happen at scale without putting in more energy then we get out". And yes, we've reached >1 aparently but again not at scale and not for the full system...  
> **username_7**: Those are not the fusion experiments that we used to figure out stars - those experiments were more of the top secret nuclear weapon type, and at the time it was hard for astrophysicists to obtain the relevant reaction cross sections.  

---

