January 1928: Dirac equation unifies quantum mechanics and special relativity

102 pointsposted a day ago
by thunderbong

57 Comments

magicalhippo

a day ago

For those enjoying the history of early quantum mechanics, I've been following Dr. Jorge S. Diaz on YouTube[1].

He has a great video series going on the people, experiments and discoveries that lead to quantum mechanics.

The videos are very accessible, but he does go into some details like various key derivations and such. Well worth watching for casual physics fans IMHO.

[1]: https://www.youtube.com/@jkzero/

CamperBob2

18 hours ago

Diaz is awesome. For me, he manages to strike just the right balance between making the topic interesting in a historical/human-interest sense and including enough technical detail to cultivate actual insight. Both his Franck-Hertz and Stern-Gerlach videos are the best I've run across on YouTube (and I see the latter has a part 2 now, so that'll be next on my watchlist.)

Hugely underrated YouTuber.

itishappy

17 hours ago

I also just love his energy. He's unapologetically excited about the stuff that excites him, and even if I don't share all his interest, it makes his videos an absolute joy to watch.

teleforce

a day ago

> Yet the technique employed to make the theory useful — renormalization — repulsed Dirac because he found it mathematically ugly.

Perhaps if he had used quaternion the solution will not be mathematically ugly or can even be beautiful [1].

[1] A quaternion formulation of the Dirac equation:

https://mauritssilvis.nl/research/publications/silvis-rug10....

elashri

a day ago

Dirac was not working in vaccum . Klein-Jordan equation was the simplest and the most obvious extension of Schrodinger equation in relativistic manner.

So historically, Dirac was focused on correcting the Klein-Gordon equation, which had issues with negative probabilities and describing electron behavior. His goal was to find a relativistic equation that resolved these problems while maintaining consistency with his own matrix mechanics formulation of quantum mechanics.

By extending his matrix mechanics formalism, Dirac derived an equation that not only addressed the issues with the Klein-Gordon equation but also predicted the existence of antimatter. I would argue that Dirac's approach was consistent with his established framework, and while he found renormalization mathematically unsatisfactory, it does not diminish the validity of his method in deriving the Dirac equation. I doubt he focused on any elegant solutions, he was actually quite happy working with matrix mechanics framework.

jesuslop

a day ago

Bohr was a big shot, Nobel prized establishment authority. In Weimberg QFT book he recalls a fragment of Dirac's memoirs:

"I remember once when I was in Copenhagen, that Bohr asked me what I was working on and I told him I was trying to get a satisfactory relativistic theory of the electron, and Bohr said 'But Klein and Gordon have already done that!' That answer first rather disturbed me. Bohr seemed quite satisfied by Klein's solution, but I was not because of the negative probabilities that it led to. I just kept on with it, worrying about getting a theory which would have only positive probabilities."

phkahler

21 hours ago

Is there a relationship between the negative probabilities of Klein and the negative energy of Dirac? Did his formulation just move the problem? If so, does it imply anything? Like are probability and energy related?

elashri

21 hours ago

Klein-Jordan equation does have both problems, negative probabilities and energies. Dirac equation solved negative probabilities and now predicts positive probabilities for both positive and negative energy states. But the negative energies problem still exists and Dirac used different interpenetration to explain them and did not get rid of them (which we knew later that this was the correct things to do). So he came with the famous negative energy solutions interpreted as antiparticles.

superposeur

19 hours ago

It’s worth mentioning that, brilliant as Dirac’s “sea of filled negative energy states” picture was, no one believes that interpretation now. The Dirac equation is better seen as the classical equation of motion for the Grassmann-valued electron field (just as Maxwell’s equations are the classical eom for photon field). There are only positive-energy states (=quantized excitations of the field). I do think popular accounts should begin mentioning this in order not to keep reinforcing the old Dirac sea interpretation.

codethief

13 hours ago

> no one believes that interpretation now

I know of at least one (tenured) person that does, at least to some degree: Felix Fenster at Regensburg University. When I met him years ago, he said taking the Dirac Sea interpretation seriously was what caused him to come up with his own program for a theory of quantum gravity, called Causal Fermion Systems[0]. I haven't looked into his theory in detail but I did find a reference to the Dirac sea[1]:

> In order to obtain a causal fermion system, we first have to choose a Hilbert space. The space of negative-energy solutions of the Dirac equation (i.e. the Dirac sea) turns out to be a good choice. […] As a side remark, it is worth noting that the Dirac sea vacuum is to be seen as an effective model describing a particular minimizing causal fermion system. It is one particular physical system that we can describe as a minimizing causal fermion system. But we should really only think of it as an effective description, in the sense that it describes only the macroscopic structure of spacetime, whereas its microscopic structure on the Planck scale is essentially unknown. […] The idea of the Dirac Sea did, however, play an important role in the conception of the causal fermion systems framework, and most of the existing literature is written with that point of view in mind. A more detailed motivation for why it is a natural starting point can be found here[2].

[0]: https://causal-fermion-system.com/

[1]: https://causal-fermion-system.com/intro-phys/

[2]: https://causal-fermion-system.com/theory/physics/why-dirac-s...

cornel_io

a day ago

That reformulation doesn't let you avoid renormalization, does it?

JPLeRouzic

a day ago

> Quaternions

I know nothing of physics, but it seems to me that rotation fingerprints are everywhere in physics. Is this just me or is there something more tangible in this remark?

nimish

20 hours ago

Rotations and spin are deeply tied into the geometrical nature of a space. It's not just you. It's core to understanding the nature of matter itself.

Cartan had only just invented spinors as an object in themselves (ignoring clifford) so a lot of the physics stuff was done in parallel or even without the knowledge the mathematicians had.

Ono-Sendai

a day ago

It's not just you. Dirac fields are constantly rotating. In fact the solutions are called spinors. (e.g. things that spin). There are a lot of rotations at the quantum level. It's also why complex numbers show up a lot in q.m.

ValentinA23

a day ago

I've been trying to get an intuitive understanding of why multiplying by e^ix leads to a rotation in the complex plane, without going into Taylor series (too algebraic, not enough geometric). I tried to find a way to calculate the value of e in a rotational setting, maybe there is a way to reinterpret compound interests as compound rotation. Any insight ?

itishappy

19 hours ago

Euler's formula is a specific case of the exponential map from Lie theory. This means e^x can be used with all sorts of interesting x types, and it often has surprisingly intuitive behavior! When x is a real number you get continuous growth. When x is a purely imaginary number you get continuous rotation. When x is complex you get continuous growth and rotation. When x is a matrix you get a continuous linear transformation (growth, rotation, and shear). What's the similarity here? Euler's formula treats it's argument as a transformation which gets continuously applied in infinitesimal amounts. This also explains the formula for calculating the value of e:

    e = lim (1 + 1 / n) ^ n where (x -> infinity)
https://en.wikipedia.org/wiki/Exponential_map_(Lie_theory)

https://en.wikipedia.org/wiki/Matrix_exponential

https://www.youtube.com/watch?v=O85OWBJ2ayo

ColinHayhurst

19 hours ago

Complex numbers and (Pauli/Dirac) matrices not required if you use Geometric Algebra. I highly recommend the book by Doran and Lasenby [0], or you can get the details from their papers, notably [1].

[0] Geometric Algebra for Physicists, CUP, 2003

[1] https://arxiv.org/abs/quant-ph/0509178

ValentinA23

19 hours ago

https://deferentialgeometry.org/papers/Doran,%20Lasenby%20-%...

page 28, equation 2.36. Thanks a lot I'll take a dive into this

Note: my inquiry was motivated by this:

https://blog.revolutionanalytics.com/2014/01/the-fourier-tra...

ColinHayhurst

19 hours ago

p281 for Dirac equation. But I suggest you start at least from the beginning of Chapter 8. Earlier, obviously if you don't know Geometric Algebra. It's worth it; many examples but one is that the four Maxwell equations are expressed as one compact equation with geometric intuition.

sparky_z

18 hours ago

My favorite intuitive explanation was actually written by science fiction author, Greg Egan. It takes the exact approach you're asking for, reinterpreting compound interest in a 2d rotational context on the complex plane, and doesn't use more than high school math:

https://gregegan.net/FOUNDATIONS/04/found04.html#s2

Fig. 7 is the money shot.

dboreham

20 hours ago

IANAM but I'd go with "it's implicit in how complex numbers are defined". Complex numbers are a thing made up by humans (as are negative numbers), and we got to define i as "up the y-axis". Once you do that, and note that a rotation is therefore cos angle plus i sin angle, add in that e^something is an eigenfuncion of differentiation, and you're pretty much there.

Fwiw I think it's Maclaurin series for this.

Edit: obviously should be j not i.

Ono-Sendai

12 hours ago

The first thing to understand is that multiplying a complex number by i rotates the complex number by 90 degrees counter-clockwise around the origin. For example, 1 * i = i (e.g. 1 + 0i is mapped to 0 + 1i). And i*i = -1 (e.g. 0 + 1i is mapped to (-1 + 0i) and so on. e^ix is a continuous generalisation of this discrete rotation, as I understand it.

empiricus

20 hours ago

One possibility: take the unit circle, and a vertical line tangent to the circle at (1,0). Then e^ix takes that line and wraps it around the circle. This

cjfd

a day ago

Of course. The solutions of the Dirac equations live in space and space has rotation symmetry. These solution have to transform in some way under rotations.

>"Months later, Schrödinger — inspired by Louis de Broglie’s idea that matter behaves like a wave — proposed an entirely different, but mathematically equivalent, formulation of particle behavior based on the better-known mathematics of waves."

[...]

More surprising results unfurled when Dirac extended his equation to describe an electron interacting with an electromagnetic field. Experimentalists had confirmed that the electron’s intrinsic angular momentum, or spin, was equal to 1/2, but theoreticians couldn’t figure out how to properly incorporate it into their theories. With his new equation, Dirac had found, almost as an afterthought, that the spin emerged naturally.

[...]

The Dirac equation was simple and elegant, yet dense with implications. Perhaps its most profound feature was that,

instead of producing two components for negative and positive spin states, it produced four: a negative and positive spin state for each of two particles with positive and negative energy states."

Related:

https://en.wikipedia.org/wiki/Chirality

https://en.wikipedia.org/wiki/Angular_momentum

https://en.wikipedia.org/wiki/Noether%27s_theorem

hdivider

a day ago

If our society were sane, rational, advanced, the headlines would be all about scientific and technological progress. The fusion power breakthrough of 2022 by Lawrence Livermore National Lab would still dominate the news. Large corporations would compete to create the first Star Trek replicator (at least for organic matter, food, etc) by advancements in nanofabrication. Politicians would debate R&D topics and strategy, figuring out which path leads to greater broad-sector economic progress.

One can dream. :) Instead, we have a society almost entirely dependent on many kinds of technology, and yet very few understand any of it, nor care to. Wonder how long this trend can persist until some sort of phase transition appears on the horizon.

ggu7hgfk8j

a day ago

We aren't spherical philosophers in a vacuum. We are emotional animals trying our best. This fact requires constant consideration and management lest it all come crumbling down.

winwang

a day ago

Alright, so we're spherical cow-philosophers... (jk, I like your point!)

hdivider

8 hours ago

Spherical bearded philosophers. You forgot the bearded part.

guerrilla

a day ago

> trying our best

I strongly question this part. Most people just want comfort. More is never enough for them.

bckr

a day ago

Don’t worry, we can create a priesthood caste with secret knowledge of technology whose purpose is to guide the human race toward a brighter tomorrow!

Maybe someone could write a foundational science fiction novel about this.

codethief

13 hours ago

Influx by Daniel Suarez comes to mind (though it has a slightly different spin).

koolala

21 hours ago

Anathem by Neal Stephenson?

aeonik

19 hours ago

Foundation Series by Isaac Asimov

bckr

20 hours ago

I’ll have to check that one out

koolala

19 hours ago

It's my favorite utopian science book!

mikhailfranco

a day ago

The LLNL fusion result was not a breakthrough. The fusion output was about 1% of the energy input. The exaggerated press release was just a PR ploy to get support for continued DoE funding, which was expiring at the end of 2022.

orwin

a day ago

And while we talk about fusion, even when the energy output surpass the energy input and the reaction is stable enough, how to you harness the energy? Because the reaction happen within a vacuum, the only way is to capture expelled neutrons and make electricity from it somehow.

antonvs

a day ago

> The fusion power breakthrough of 2022 by Lawrence Livermore National Lab would still dominate the news.

If our society were sane, rational, advanced, it would recognize that that "breakthrough" was a minor, arbitrary improvement in reaction efficiency, that realistically brings us no closer to commercially viable fusion power, and doesn't prove anything about the possibility of that.

That reaction still consumed something like 100 times the power that it produced, and the "power" that it produced was just heat energy, which would still entail losses when converted into usable form.

On top of that, the nature of the Livermore reaction is not one that's even intended or suitable for commercial power production.

At this point, we simply don't even know whether controlled, commercially viable fusion will ever be able to produce more power than it consumes. There's no guarantee that it will.

If you're not aware of what I'm referring to, this article is a starting point: "Why the nuclear fusion ‘net energy gain’ is more hype than breakthrough": https://whyy.org/segments/why-the-nuclear-fusion-net-energy-...

While this might all seem like an irrelevant aside to the point being made above, it's relevant because it shows how pervasive misinformation is, even when coming from supposedly scientific sources.

elashri

a day ago

The announcement was correct and precise. I am not sure what misinformation you are describing here.

Regarding your 100 more energy claim. It overlooks key facts about the NIF breakthrough. The fusion reaction itself achieved net energy gain, producing 3.15 MJ compared to 2.05 MJ of input laser energy - far from consuming "100 times the power it produced." While the total facility power usage was indeed higher due to laser inefficiencies, this misses the crucial scientific achievement. This was basically humanity's first controlled fusion reaction producing more energy than was directly input to the fuel. Dismissing this as a "minor, arbitrary improvement" understates its significance. This wasn't just about efficiency metrics - it demonstrated fusion ignition was possible, a fundamental physics milestone that had eluded scientists for decades. Though challenges remain for commercial fusion power, the breakthrough proved a critical theoretical concept that many thought impossible. Many critics before that were referring to this point as the reason why it isn't worth it to keep researching. And they were proved wrong.

Trying to redefine the announcement and experiment result to mean something else so that you can attack is a dishonest behavior.

roelschroeven

a day ago

Nobody ever doubted that fusion ignition was physically possible. It happens in stars all the time, and people have achieved it in thermonuclear weapons.

This was the first time fusion ignition was achieved in a laboratory setting, i.e. in a controlled fashion. Is that seen as a fundamental physics milestone? To me it seems more an incremental engineering achievement.

antonvs

a day ago

> The announcement was correct and precise.

"The" announcement? There were several announcements, with varying degrees of scientific rigor.

Here's one typical example: https://www.llnl.gov/article/49301/shot-ages-fusion-ignition...

Quote: "...achieved fusion ignition — creating more energy from fusion reactions than the energy used to start the process."

That is not "correct and precise." In fact, without any mention of the additional context that at least 300 MJ of power was used to produce 3.15 MJ of not directly usable heat energy, it's incorrect, imprecise, and misleading at best.

It's also misleading because it doesn't tell you that NIF's definition of "ignition" is significantly different, in essential respects, from the term's use in other fusion contexts. For example, ignition at NIF doesn't mean that a self-sustaining reaction has been achieved. As such, the use of this term at all is dubious. It has no fundamental meaning here, it's just a name being used for an arbitrarily defined efficiency target.

Realistically, the term is being used to try to connect what NIF is doing, in a facility ostensibly intended for nuclear weapons research, to what fusion power research efforts are doing. It's a hype-driven word game, it's not meaningful.

Back to the quote above: it's carefully worded to sound as though it's saying something that not true. No layperson without prior knowledge of nuclear fusion issues is going to correctly understand that statement - and indeed, most of the initial press about this didn't, i.e. the journalists reporting it didn't understand what it meant, which is what the article I originally linked to was responding to.

That brings us to the main point: I didn't say anything about an announcement. I responded to someone who was talking about what our society would do if it "were sane, rational, advanced".

I'm saying that it's extremely unfortunate that our society is too scientifically illiterate to correctly report on and understand what ultimately was a somewhat routine scientific achievement, reaching a defined efficiency target that has no particular fundamental meaning in the context.

> The fusion reaction itself achieved net energy gain, producing 3.15 MJ compared to 2.05 MJ of input laser energy - far from consuming "100 times the power it produced."

It used at least 300 MJ of power to drive the lasers[1]. 300 / 3.15 = 95. But that factor of 95 would just be to reach a break even point with the heat energy produced, it's not directly usable energy.

For actual usable energy, according to a 2023 presentation at the LLNL High Energy Density Science Seminar[2], "For a power plant, gain would need to be increased ~1000x relative to current NIF performance."

None of the announcement about this so-called "ignition" event mentioned any of this, and nor did most (any?) of the mainstream press about it.

The reality here is that in order to maintain public interest in nuclear fusion, and keep getting funded, it has to be presented as though fusion power is just around the corner - "5 years!". What I was pointing out is that "if our society were sane, rational, advanced," we would not need to play such games. We would not need to continually mislead the public, we would not need to pretend that facilities being used to do nuclear weapon "stockpile stewardship" research have some relevance to fusion power, and so on.

I also found it ironic that the commenter who wanted a "sane, rational, advanced" society appeared themselves to be a victim of the misleading hype around the NIF event, saying that it should "still dominate the news." It simply wasn't that significant.

> This wasn't just about efficiency metrics - it demonstrated fusion ignition was possible, a fundamental physics milestone

This is incorrect, as explained above. "Ignition" here is a term defined by LLNL to apply to their particular weapons-oriented fusion facility. There's nothing "fundamental" about it. It's a defined target for experimental efficiency, that's all.

> ... that had eluded scientists for decades

And still does, at any facility that's trying to achieve nuclear power generation, and not just a weapons research facility blasting a pellet with 300 MJ from 192 lasers. The NIF result is simply not transferable to any other fusion scenario.

> Trying to redefine the announcement and experiment result to mean something else so that you can attack is a dishonest behavior.

It's not clear that you yet understand the full extent of the deception that you've been subjected to, so you're trying to shoot the messenger.

[1] https://ww2.aip.org/fyi/2022/national-ignition-facility-achi...

[2] https://heds-center.llnl.gov/sites/heds_center/files/2023-03... (bottom of 59th slide)

elashri

21 hours ago

As a fellow scientist, I will go and read the details from the research paper that the group published [1]. Anything else is nonsense for me. it gives a clear view on the goals, physics and what was done. Including all the details you would get that. I will quote the first paragraph from the paper summary

> In summary, the December 5, 2022 experiment on the National Ignition Facility, N221204, was the first time that fusion target gain was unambiguously achieved in the laboratory in any fusion scheme. The demonstrated level of target gain on N221204 of 1.5 times is a proof of principle that controlled laboratory fusion energy is possible

And they specifically mention that it is not overall facility-wise net gain in the next paragraph

> Notethat G_{target} > 1 does not imply net energy gain from a practical fusion energy perspective, because the energy consumed by the NIF laser facility is typically 100× larger than E_{laser}. The NIF laser architecture and target configuration was chosen to give the highest probability for fusion ignition for research purposes and was not optimized to produce net energy for fusion energy applications.

So you don't have to go and claim a deception. You want to claim it wasn't significant which is your opinion but that is not what the actual scientific community in the field (who know more than you) would agree.

[1] https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.13...

antonvs

19 hours ago

> So you don't have to go and claim a deception.

I didn't claim a deception in the research paper. I've clearly stated what I'm claiming, and you've said nothing that changes any of that.

In fact, you originally didn't even mention the research paper, you said "the announcement". The deception was in every official announcement, none of which included any details of the caveat that you quoted. That deception continued, mostly unwittingly I'm sure, in all the press on the matter.

You're shifting the goalposts to try to support a point which is irrelevant to what I've been saying.

> You want to claim it wasn't significant which is your opinion but that is not what the actual scientific community in the field (who know more than you) would agree.

It's not significant with respect to commercial nuclear fusion power, which was the entire basis for all the reporting about it.

The idea that "the actual scientific community" would support your position is an unsupported claim that's easily refuted.

For example, Victor Gilinsky, a physicist who previously a commissioner for the US NRC, wrote in "What’s fueling the commercial fusion hype?"[1]:

> "Recent White House and Energy Department pronouncements on speeding up the 'commercialization' of fusion energy are so over the top as to make you wonder about the scientific competence in the upper reaches of the government."

That article discusses the NIF experiment among others, highlighting out the discrepancies between the official announcements and what the experiment actual does. It also points out that the experiment "is, in effect, a miniature (secondary) thermonuclear bomb, with the lasers playing the role of the triggering fission reactions (primary)," which helps explain "its lack of promise for civilian use."

There have been plenty of similar criticism from other scientists, including Daniel Jassby previously of Princeton Plasma Physics Lab, and M.V. Ramana at U. British Columbia.

In "Clean Energy or Weapons? What the ‘Breakthrough’ in Nuclear Fusion Really Means"[2], Ramana wrote, "without the excitement created by these hyped-up statements, it would be impossible to get funded for the decades it takes to plan and build these facilities."

Again, in a "sane, rational, advanced" society, this would not be necessary. And you, and the commenter I originally replied to, would not have had clear misapprehensions about the experiment as a result. In your case, at the very least, you appeared to believe that "ignition" was some fundamental physical phenomenon in this case, which it is not, in the context of the NIF experiment.

> As a fellow scientist

As a scientist, you should be interested in what's true.

--

[1] https://thebulletin.org/2024/02/whats-fueling-the-commercial...

[2] https://science.thewire.in/the-sciences/clean-energy-weapons...

jojobas

a day ago

NIF is first and foremost a thermonuclear weapons research facility. The "breakthrough" you're talking about doesn't bring us an inch closer to fusion power.

8bitsrule

a day ago

I think that a 'breakthrough' would be to realize that harnessing fusion outside of a solar environment is a hopeless dream that, carefully fed, is very good at provoking research grants.

_s_a_m_

a day ago

No they would not. Not all and probably most progresses are not technological. Are you living under a rock?

readthenotes1

a day ago

Some many years ago some people collected some negative traits to describe the foibles of people. Unfortunately, these negatives seem to dominate much of the news:

Pride, Greed, Lust, Anger, Gluttony, Envy, Sloth.

If we could somehow dim the influence of these human traits, we might get closer to the world you described

> Pride, Greed, Lust, Anger, Gluttony, Envy, Sloth

The greatest popular innovation of our time appears to be to have extended the above list with Falsehood, Cruelty, and Pollution.

Jensson

18 hours ago

Falsehood Cruelty and Pollution are results of the 7 sins. Cruelty is typically caused by Anger or Envy, Pollution from Gluttony and Sloth, Falsehood from Pride and Envy etc.

terminalbraid

a day ago

I assure you falsehood, cruelty, and pollution have existed long before our time.

exe34

a day ago

what are you talking about? the most important thing is to make sure senators use the correct bathroom!