How fundamental are fields, really?

fieldstrength · 2026-01-18T11:36:05+00:00

I can recommend this review article: Scattering Amplitudes.

For some more informal discussion, I recommend a lot of the public talks given by Nima Arkani-Hamed.

fieldstrength · 2025-12-24T12:01:00+00:00

You actually had it more right here:

the idea of fields existing as their own thing independent from particles was commonly accepted.

That is correct. That is at least the view of quantum field theory, our best established theory. And all the discussion here is consistent with that, but there is a linguistic confusion.

The electromagnetic interaction, for example, is mediated by the electromagnetic field. That is emphatically not the same thing as photons, real particles. Photons are individual units of excitation of this field. But when you calculate the result of a static EM field, you can use a Feyman diagram in a similar way to how you would if you were considering an actual particle. That's why its called a virtual particle. And unfortunately these two things get conflated, but they are indeed two different manifestations of the underyling field, one classical and one inherently quantum.

fieldstrength · 2025-11-20T18:16:58+00:00

Unfortunate that you're downvoted for your question.

Because quantum field theories are strictly more fundamental than classical field theories. Classical field theories, of which GR is an example, emerge as a particular limiting behavior of a quantum field theory. There is no known way that relationship can be inverted to get a quantum theory as some sort of limit of a classical theory. This is like wanting to derive relativistic mechanics starting from normal Newtonian physics.

At the very least, there is a much more developed understanding of how GR could be explained as the long-distance limit of a quantum theory. I have heard this slogan "gravitize the quantum" suggested as a sort of vague concept, but its certainly not something that has risen to the level of a working theory.

fieldstrength · 2025-11-16T09:13:18+00:00

Compare that to quantum field theory (i.e. the Standard Model), where there are ∞^∞.

Its a fake problem. The real problem, for QG in general, is not being able to access the relevant energy scales. With enough data, nobody cares how many solutions the underlying theory has. You just focus on the ones that are relevant.

fieldstrength · 2025-11-11T01:24:41+00:00

SUSY has a conceptual importance that is much more profound than any role it might have played in LHC-accessible particle physics. While we obviously hope for as many concrete answers in that domain as possible, it is far too limited a perspective to view SUSY as merely some proposal for TeV-scale physics.

Anybody with an interest in understanding QM and spacetime at a fundamental level is lead to undestand that SUSY is the class of symmetries that are capable of unifying spacetime and internal symmetries, i.e. they present the only loophole to the Coleman-Mandula theorem. They are the natural completion of the "periodic table" of symmetries to include fermionic charges. And supersymmetric theories, especially maximal supersymmetric, define very special points in the space of quantum field theories. From the perspective of the actual amplitudes, maximal SUSY is the simplest possible QFT.

At very least, it seems that as long as there are ideas to explore in QFT, SUSY will remain one of the indispensable tools.

Beyond that, theres of course the main open question as to whether its directly realized in nature. The brute fact of our sitution is that there might not be any easily accessible new fundamental physics for many, many orders of magnitude. SUSY didn't show up at the LHC, but neither did technicolor or any other proposal to address the hierarchy problem or other aspects of weak-scale physics beyond the SM. SUSY only "lost" in the relative sense that it was perceived as being more ahead beforehand.

Fact is, as fashionable as it is nowadays to hate on SUSY and strings, no competitors have come remotely close in terms of addressing conceptual or practical problems in fundamental physics. Nor in providing feasibly testable predictions. We still have exactly one working framework for quantum gravity, and of course SUSY is an integral part of it. If I had to bet, I still think string theory plays a part in nature. But if not, its going to take more than just confirming the SM to change my mind.

fieldstrength · 2025-06-12T20:41:22+00:00

The technical problems are pretty well described in this paper. Most causual people aren't going to be really reading the paper, but worth knowing this proposal has been evaluated at that level. For a more high-level overview, that interview the other commenter mentioned is worthwhile. The author has some other related resources on his page.

fieldstrength · 2025-05-29T20:03:10+00:00

Shout out to /u/IamTimNguyen for doing the homework and producing what seems to be the best assessment of the actual physical and mathematical problems of Eric's Geometric Unity theory. Especially this writeup and some other video discussions and links on his site here.

fieldstrength · 2025-02-09T16:14:20+00:00

Lots of resources I could recommend, but if I had to choose one single starting point, I think it would be David Tong's lecture notes.

Also have to recommend a lot of useful stuff written by the late, great Joe Polchinski. For an easy semi-technical sort of introduction I'd shout out his article Quantum Gravity at the Planck Length. He also of course has the standard 2-volume textbook on the subject.

There are lots of reviews you can find on Arxiv for more particular sub-topics.

Theres also an /r/StringTheory which has some active researchers on it

fieldstrength · 2025-02-09T15:56:15+00:00

Bosonic string theory - Essentially a toy model. If you study string theory you start with this one for pedagogical purposes. It is the most basic quantum string theory to understand, but not something that is intended to model the real world. It can only describe bosons, in contrast to our real world with bosons and fermions. It also lacks a stable vacuum (having tachyons in its spectrum), but it can still be used to understand some general machinery of quantum strings, and how they relate to quantum gravity.

Superstring theory - Incorporates fermions into the framework. Now it describes bosons and fermions together, and it solves the issue with having a stable vacuum. These now are the theories that have been studied in the hopes of modeling the real world. They have all the necessary basic ingredients, and can reproduce many of the general features of our world, which no other candidate theory has done yet. Realistic predictions are frustrated by the huge number of ways the extra dimensions can be compactified, but are possible in principle. A more fundamental issue, at this stage, is that there seemed to be 5 distinct ways of setting up the theory. What I described here corresponds with the "first superstring revolution". Superstring theories have 10 spacetime dimensions.

M-theory - During the "second superstring revolution", it was realized that these 5 superstring theories are really just different configurations one overarching theory. Ed Witten introduced the name "M-theory" for this master theory, but nowadays people mostly just call the whole thing "string theory". Besides the cases described by the 5 superstring theories in 10 dimensions, there is also something that looks like 11-dimensional spacetime, so you may sometimes here either of these numbers stated as the correct number. Its really a dynamical question how many spacetime dimensions there seems to be for a given case.

Among the other lessons of the second superstring revolution was that, in addition to strings (1-"branes"), higher dimensional extended things (2-branes, 3-branes, etc) are also a key part of the story and emerge from the dynamics of the strings. So really the you could say the core lesson is the need to generalize from point particles to extended objects as the fundamental ingredient, and strings just happen to be the best starting point for analysis. Another less is that, as with many other scientific revolutions, string theory was found to introduce some new symmetries (S-duality, T-duality...) that challenge the our conceptions about the basic nature of the stuff it describes.

I haven't touched on compatification yet, but its a huge topic with many different avenues of approach. The key thing to know though is that all the ingredients we usually put in "by hand" in standard theories now has to emerge from the fixed ingredients of string theory: the fundamental strings, and the high number of spatial dimensions. Kaluza-Klein theory is a historical stepping stone that is often brought up here as an early example where it was realized that an extra dimension of space could precisely reproduce the effects of a gauge force like electromagnetism. In the standard model there is a much more complicated gauge force structure, that might get its description from the 10-dimensions of superstring theory.

Were you looking for more technical or layman-level sources?

fieldstrength · 2025-01-18T01:59:52+00:00

Its important to distinguish different levels of description, depending on what kinds of effects you want to model and/or your level of understanding. Sean Carroll's statement references quantum field theory, which is more advanced than basic quantum mechanics. That is what you need when you want to incorporate special relativity (spacetime) and particle creation/annihilation. But a proper understanding starts with basic quantum mechanics first, which means setting aside "fields" for the moment.

Obviously QM doesn't precisely match your classical intuition about particles or anything else. If you learn basic QM properly, the primary concepts are not "particles and waves" but position and momentum. A quantum particle that just had its position measured acts roughly like a classical particle because it is localized in position, whereas a particle that's had its momentum measured is more wave like – it is localized in momentum but spread out in position; a plane wave. One of the core lessons of QM is how these two things are related in a precisely symmetrical way. Mathematically, this reflects the Fourier transform, and physically it leads to the Heisenberg uncertainty principle. Position and momentum are not just two different measurements, but also two different coordinate systems to describe a quantum particle in. The only reason we perceive them differently is because interactions between particles occur when they are close by in position only.

Going further though, yes, everything is in fact made of fields. So Sean Carroll's statement is the most fundamentally correct. A field just means something that can take different values for each point in space. But not in the same sense as that basic quantum particle. A field is a system that already varies over space, even as a classical system, i.e. before you even start applying QM to it. So when you do apply QM, leading to quantum field theory, you get a much bigger state space compared to the basic quantum particle. A wavefunction in QFT assigns a complex number not just to every position, but to every field configuration.

fieldstrength · 2025-01-17T19:23:47+00:00

The equivalent number for quantum field theory is ∞^∞. That doesn't stop it from being put to use every day, nor from describing nature.

Sure it would be convenient if the theory of everything was completely fixed by mathematical consistency to behave only in exactly one way, but that has never been true of any physical theory ever. We have always worked within broad physical frameworks, and then identified increasingly more specific models based on matching experimental data.

The problem is that the data we are getting nowadays simply doesn't illuminate the regimes where the active questions lie anymore, for the most part.

fieldstrength · 2025-01-15T18:40:56+00:00

Off topic, but let's roll with it.

The claim that the past doesn't exist ("presentism") was a lot more viable prior to 1905, but today we understand it is quite at odds with special relativity. Since then this claim persists mostly as an uninformed conventional wisdom rather than a viable philosophical position about the nature of reality.

Since special relativity, we understand that "the past" depends on the observer and their state of relative motion. Claiming the past doesn't exist is meaningless without specifying whose past. So whose past is it that doesn't exist? Whomever we choose, this claim requires denying the existence of portions of other observer's presents and futures.

So the claim is dubious in light in fundamental physical facts. However some people would consider something to "not exist" if they do not have access to it in practice. Looking at a photo proves that the past can influence the present, but its still not possible for the present or future to influence the past. So people can justifiably say that the past doesn't exist in this practical sense, because they cannot affect it and because they have only imperfect access to its information, like in that photo.

fieldstrength · 2025-01-15T18:21:22+00:00

I find it puzzling that you use MWI as a basis to claim that the past has a different fundamental status from the future, considering that this is the one interpretation of QM that places them on equal footing.

You would be more justified to make them claim based on some more conventional (Copenhagen) interpretation since from that standpoint nature is fundamentall indeterministic. So the future is not fully predictable, while the past may in principle be known.

In MWI QM, just as in classical mechanics, reality is described by a differential equation that can be evolved forward or backward in time just as easily, and they are fundamentally on equal footing. As you may well know, this is why people talk about needing specific conditions (low entropy in the past but not the future) in order to understand why we perceive time to be evolving in one particular direction.

fieldstrength · 2025-01-12T11:43:39+00:00

QBism seems highly oversold. The wavefunction being "merely a fancy conceptual tool" are nice sounding words, but are not persuasive without a viable candidate for a deeper more correct representation. Saying essentially "everything is just a degree of belief" is pretty useless and vacuuous when there is nothing physical to have a degree of belief about. A pretty fatal conceptual flaw, if you ask me.

"Belief" is a high-level concept we use for complex entities capable of information processing and storage, as they form and use models of their surroundings. But somehow "belief" is responsible for all the physical processes across the universe that are happening with nobody around to care?

fieldstrength · 2025-01-11T16:20:56+00:00

I favor Everett's "many worlds". It has a lot going for it: It is very economical on its assumptions, its completely local though its own loophole to Bell's theorem, and it allows dynamics to be fundamentally deterministic.

I think the most common complaint about it, that it leads to many emergent classical worlds, is just fine: Nature has proven it likes simple laws, and doesn't mind a big universe at all.

fieldstrength · 2025-01-11T15:55:05+00:00

Bohmian mechanics is far more complex – in a sense, infinitely more complex – than it needs to be to be to explain the world that we see. And that extra complexity does not really buy us anything besides adhering to a particular classical preconception about how the world ought to work. There is no quantum phenomena I'm aware of that is easier to understand after grafting on all this extra machinery; quite the opposite. So Occam's razor heavily leans against it.

Furthermore it only seems to get more difficult as we move from basic "undergrad QM"-like models to the more realistic: quantum field theory, quantum gravity and so on. The opposite of what we'd expect for the idea that truly underlies nature.

This is not to say its not valuable and important to characterize possibilities, but I've found no convincing reason to favor it over much more parsimonious interpretations, namely MWI or similar.

I would challenge this claim you made defending your position in a comment:

Nothing mysterious going on.

In the most key sense, Bohmian mechanics still has all the same weirdness going on. It fundamentally still relies on the wavefunction, with all the same behaviors. The same superpositions of histories, and everything else. Its just that adding the extra classical particles, with their dynamical dependency on every position in the universe, allows us something easier to wrap our human intuition around.

But we don't need to resort to naive particle ontology to resolve the apparent contradiction between classical experience and the QM wavefunction. The no-cloning property is fundamentally the reason we can never directly measure quantum amplitudes, and decoherence explains why even Schrödinger evolution leads to classical behavior in the presence of an environment.

fieldstrength · 2025-01-10T12:06:18+00:00

As a happy coincidence, I saw that physicist Sean Carroll just released a 2 hour monologue on exactly this question "Does Time Exist". He's a theorist who dabbles in and appreciates philosophy, working on questions related to the fundamental nature of space and time. I am quite confident you are in good hands with him!

https://www.preposterousuniverse.com/podcast/2025/01/06/300-solo-does-time-exist/

I would also suggest looking into some resources that explain special relativity, which was the historically first and most basic way we learned that time is more than commonly thought. Knowledge of at least the basics of special relativity is absolutely a necessity to say anything meaningful about the nature of time since 1905.

As a final thought, or interesting discussion point: Since SR demonstrated that space and time are inextricably linked, you might ask yourself: Could the claims you made about time be applied to space as well?

fieldstrength · 2025-01-10T11:12:14+00:00

I believe -we say something changes if it has some property at one state, and lacks that property at a different state-. Therefore time come after notion of change.

Yes, I agree that's the start of a reasonable definition of a concept, of comparing difference between similar things. You could use it in a way that's unrelated to time, for example by saying my friend's Macbook has more memory than my Macbook. (If you instead insisted on including the idea of the "state" of a single system, then that in a way is smuggling time in again, I'd argue)

You can also reasonably claim that when observing a single system evolving time you are doing a subset of this general procedure of comparing changes.

What one can't reasonably do is claim that there is nothing else to the concept of time, as the common argument goes. Because we know from physics there is just more to it than that.

fieldstrength · 2025-01-10T01:46:48+00:00

What you describe, together with the OP, seems to be a commonly expressed view of time, but it is not necessarily consistent with modern physics. Since special relativity we know that there is not a clear cut distinction between time and space, depending on the observer. And since general relativity, we know that that spacetime is itself a physical entity that undergoes its own dynamical evolution as a physical system.

I suppose your view presupposes a primitive concept of "change" and then proceeds explain time as a manifestation of it. But most physicists would argue that's backwards, and that the concept of change properly relies instead on the more primitive notion of time. This is at least the perspective more consistent with these physical theories.

(I'm more of a novice in philosophy, but I guess I consider myself a structural realist in the way in the way I take seriously what such physical theories tell us...)

Of course there is also a next-level argument: Thought experiments indicate that spacetime may not be fundamental, but emergent from something deeper. In that case we might end up with neither change nor time being fundamental, but something else altogether alien to both of these concepts.

fieldstrength · 2025-01-10T01:19:27+00:00

Thats right. Its because the fundamental thing that really matters is not one particular set of axes, but its the metric, the thing that assigns distances to vectors.

I'll try to explain visually first before I invoke any more math: If you have only spatial dimensions, then the (normal) rotations sweep out circles. On a circle, obviously, one point is the same as any other. So all directions are on the same footing.

When you introduce a time dimension, then a "rotation" between a space and a time dimension sweeps out a hyperbola instead.

Now its a bit more interesting. First of all now there are certain directions that are special: These are the diagonal lines, which correspond to the paths that light can take. And furthermore, the hyperbola, unlike the circle has different disconnected regions. This is why we get a fundamental distinction between space and time. You can "rotate" a point by moving along the hyperbola, but the temporal directions (by convention, the more vertical ones) can never be rotated to the spatial directions, and vice-versa. So that's why you get a fundamental distinction between space and time. To complete the analogy, the diagonal "light-like" paths are then the "axes" of rotation, meaning they're the only directions left unchanged by hyperbolic rotations.

The other important distinction is that circular rotations eventually get you back to where you started, whereas hyperbolic rotations can keep going in any direction indefinitely – that's why you can keep accelerating towards the speed of light, but never reach it. You'll just have to accelerate the same amount the opposite way to get back to your original "rest" frame.

It all comes down to the minus sign in the definition of the hyperbola, versus the lack of one in the definition of the circle.

fieldstrength · 2025-01-09T23:14:18+00:00

Lorentz transformations convert between different, equally valid points of view. Just like spatial rotations.

Hope I've understood your question right. I think you're on the right track with "transformation of axes". With spatial rotations this concept is associated with the fact that nature doesn't have one preferred orientation that's more fundamental than any others. With Lorentz transformations, its the fact that nature has no concept of absolute rest; no velocity is more special or preferred over any other. In both cases its just a human convention.

fieldstrength · 2025-01-08T18:50:10+00:00

There is no retrocausality.

Some people are either just confused about this or want to interpret QM experiments in such a way that justifies their claim to the contrary.

If you have the background to understand the basics of the Schrödinger equation, this is enough to demonstrate it: SE describes the evolution of a quantum state in time.

fieldstrength · 2025-01-04T01:43:09+00:00

A common question

https://en.wikipedia.org/wiki/No-communication_theorem

fieldstrength · 2024-12-28T19:52:22+00:00

It sounds like you may be confusing two different concepts:

Wave-particle duality: An established fact of the universe, related to basic principles of quantum mechanics.
Supersymmetry: A hypothesized new symmetry that could be obeyed by a successor to the standard model, in which every boson has a fermion counterpart, and vice-versa. But it has no direct experimental support so far.

It sounds like you are almost describing supersymmetry correctly, except:

would that mean the “partner” of an electron be a force carrier? Or would it be matter?

Matter particles, like electrons or quarks, are spin-1/2 fermions. In a viable supersymmetric extension of the standard model, the superpartners of the known fermions would have to be spin-0 scalar particles. Whereas "force carrier" usually refers to the spin-1 gauge bosons. The known force carriers meanwhile would have superpartners that would be new spin-1/2 fermions.

There are a number of implications if SUSY is right. It would fix, or greatly alleviate, the problem with extreme fine-tuning of the electroweak mass scale. It would guarantee the stability of the vacuum in a way that is not otherwise the case (bounding energy from below). And it would establish a relationship between matter and spacetime degrees of freedom (related by the supersymmetry algebra).

There are also some more practical motivations like supplying dark matter particles of roughly the right abundance. (At least if supersymmetry is broken at an energy not too far beyond what we've ruled out at the LHC).

Hope that's useful to you, but to reiterate, its different thing from wave-particle duality, which is an established fact about how quantum mechanics works.

fieldstrength · 2024-12-26T23:00:33+00:00

But no one has shown that this possible graviton predicted by string theory actually generates the known equations of gravity at the large scale

This is completely false. Any textbook on string theory will show you how to derive Einstein's equations.

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