The Dangerous Irrelevance of String Theory
math.columbia.edu
math.columbia.edu
I smelled the stink early on when a project at the LHC I was joining found 6 (plus or minus 4!) possible supersymmetry events in the two years of PBs of LHC data(!) in their specific search. I made the bet against supersymmetry and went for a less hyped field, and now that their funding is drying up, I'm happily taking almost double (or more) the salary compared to other graduate students since I'm in another field. Haven't regretted since.
The hype behind string theory and HEP in general needs to die. There are so many interesting problems worthy of investigation that are not High Energy or Quantum Gravity. Yes, few things beat the fun math of HEP and string theory (although there are close contenders in the condensed matter world), few things--except perhaps validation in experiment...you know, the entire point of science in the first place.
Inconceivably large number of possible Universes cited as evidence because our own might well be in there somewhere ?
A casual passing mention that there are not 4 but 10 dimensions, and the fact that we can't see them is not proof that the theory is wrong but simply dismissed because they might be very well hidden.
To the casual observer that I am it looks like formula-tweaking to fit some mathematical framework better, but with no regard with compatibility with reality.
This sounds conspiratorial. The people who are studying string theory, should the theory not pan out, will find work other in other areas. It's not like majoring in basket-weaving-studies, where there is no value. String theory has a deep mathematical framework, and such mathematics , as well as the superior critical thinking and intellect required to learn string theory and thus signaled, are invaluable for all sorts of professions, some of which pay well: computer science, hedge fund/quant trading, philosophy, teaching physics and math, engineering, etc.
The hype behind string theory and HEP in general needs to die. There are so many interesting problems worthy of investigation that are not High Energy or Quantum Gravity. Yes, few things beat the fun math of HEP and string theory (although there are close contenders in the condensed matter world), few things--except perhaps validation in experiment...you know, the entire point of science in the first place.
HEP concerns the universe and reality itself. It's a pretty big deal considering the implications.
It needn't be. It's a plausible survival strategy, given the incentives (economic or otherwise). This doesn't require anyone to consciously decide on this strategy, let alone do so collectively. Evolution ensures that those who just-so-happen to follow this behaviour (by whatever reasoning, which may be very well intentioned) will survive; those who don't will move elsewhere.
This is the standard party line which faculty will feed you when trying to recruit you. What you eventually grow up to realize is:
the superior critical thinking and intellect required to learn string theory and thus signaled: What is actually signals is delusional arrogance and/or cluelessness, leading to 5+ years spent specializing on a topic perfectly well known to offer negligible career opportunities. A prospective employer looking at this argument will simply ask: if you are so intellectually superior, why are you not sticking with string theory instead of trying to mix with us lowbrows?
invaluable for all sorts of professions, some of which pay well: computer science: There is very little overlap between the mathematics used in theoretical physics and computer science, beyond what you will already have picked up as an undergrad.
hedge fund/quant trading: No. There was a time when physicists in general were considered good quant material. These days there is a lot more competition, and employers have wised up. Look at job offers for such positions, and you will see that those which still mention physics as an adequate educational background will now tend to say experimental physics.
The reason is, again, negligible overlap between the mathematics used by HEP theorists and experimenters. The latter, to the extent that they are not building hardware (in which case they are essentially electrical and/or mechanical engineers) are effectively statisticians doing big data work. If you spent years programming computer clusters to look for statistical anomalies in petabytes of LHC data, you can make a good case for having transferable skills useful in finance. If you spent years using Mathematica to explore differential geometry in 11 dimensions, not so much.
philosophy: A booming industry?
teaching physics and math: This is the real one. Of all the people I've known who went for a Ph.D. in theoretical physics, most ended up teaching math & physics in high school. This is rarely seen as a good outcome. They spent their youth subject to high stress and uncertainty in return for very little money, usually doing not what they may have dreamed of doing but what was considered likely to lead to publication within the time frame of a Ph.D. program and, of course, consistent with their advisor's interests. And for that, they were rewarded with a low-paying job requiring intense interpersonal interactions, pretty much the opposite of the ideal for the kind of personality which is attracted to highly theoretical endeavors. Those who are not miserable have learned to cope by focusing on other aspects of their life than work.
engineering, etc.: I can honestly say that I have never known a Ph.D. in theoretical physics who went on to become an engineer. Experimental physicists, sure, lots; they are really engineers with an unusual specialization. But not theorists. I guess you can find them if you look hard enough, but again: the overlap is negligible. If you've been doing string theory, the "switch" essentially amounts to throwing away what you learned in grad school and falling back on your undergraduate core curriculum.
Sounds at least tangentially relevant to deep learning.
https://www.youtube.com/watch?v=Q8ccXzM3x8A
tl;dr - Nobody theorized strings. They were trying to solve the scattering problem of w-bosons and came up with a solution. The interpretation of the solution is strings.
He answers "But is it falsifiable?" with "Of course it is! If special relativity is false, so is string theory." Which misses the point completely. Is it actually falsifiable?
The reason String Theory first became so popular in the mid-1980's was because of its prediction of a massless chargeless spin-2 particle (the graviton) which behaves like gravity in a quantum setting. Surely this could be called a proven observation of String Theory?
Barry explains this quite succinctly in The Big Bang Theory: "I'm not a string theorist; I'm a string realist".
This led to the tongue-in-cheek saying that string theory post-dicted the existence of gravity at all---since it isn't built into the foundational assumptions of the theory at all (which is more than a lot of the other models, such as the "gluing" you propose, can say). And since string theory is guaranteed to be quantum-mechanical, a lot of interest developed towards understanding it as a theory of quantum gravity (most other models fail to account for how GR can be made compatible with quantization and how the renormalization works sensibly).
Gravity isn't the same as mass. It's related, but not equivalent.
inertia is conveyed by the higgs field.
gravity is conveyed by the gravitational field.
we know that those fields exist, because we can observe them.
if the fields exist, the exchange particles (higgs boson and graviton) have to exist, too - or our theory is wrong.
thats kind of circular reasoning, but we cant derive physics from first principles anyway.
We probably could derive physics from first principles if we knew them.
if you can provide a detector the size of the solar system, proving/disproving its existence will be simple.
we could derive anything from first principles if we knew them. theres nothing probable about that. thats the whole point of first principles. do you want me to call you captain obvious?
Mass is essentially rest energy by another name. The presence of a non-zero Higgs field gives certain elementary particles that would otherwise be massless such a non-zero rest energy.
In contrast, compound 'particles' (hadrons, atoms, chairs and tables, ...) only gain a miniscule amount of mass from the Higgs mechanism: They are bound states of interacting constituents that are whizzing around, generating ripples in various quantum fields (sometimes described as clouds of virtual particles), with the biggest contribution by ripples in the field of the strong nuclear force.
Now, in quantum theory, any field comes with associated particles, and for the Higgs field these are the Higgs bosons, and for the gravitational field these are the (conjectured) gravitons.
While gravitons would be intimately related to how gravity works at the quantum level, the relation of Higgs bosons to inertia is rather incidental.
The Higgs explains why fundamental particles have mass in the "F=ma" Newton's-Laws-Of-Motion sense. But you can formulate classical mechanics without gravity at all. The m in this equation is sometimes referred to as the inertial mass.
F_gravity = G M m / r^2 on its face looks like it is talking about the same inertial mass (or, by adjusting G, something proportional to the inertial mass). But, this is an assumption---a body could equally well have a gravitational mass independent of its inertial mass.
If inertial and gravitational masses are proportional to one another then when you calculate the acceleration due to gravity you can cancel the m from both sides, and you find that all bodies fall the same way (even light, which has m=0 and might cause concern that the cancellation isn't valid). This is a repercussion of inertial = gravitational mass. Einstein promoted this observation to the Equivalence Principle (and people do experimental searches for violations of this assumption).
The Higgs gives particles inertial mass. But the Higgs doesn't cause things to fall "down" (assuming you can define "down" without a gravitational field). You need the gravitons to communicate the gravitational attraction, which is independent of the existence of mass.
If so, has any actual string-theoretical mathematical relationship between gravitons and Higgs bosons been developed that would explain general relativity? Or is it stuck at "in string theory, something like 'gravitons' could exist"?
Forces have to be conveyed by something. If you want to push someone, you have to physically touch them. you have to REACH them. you cant just push the air, across the room, and expect them to feel the push.
this is true on the microscopic level as well. when particles exert gravity on each other, they do so at a distance. but something has to cross this distance. it is not ordinarily obvious how the sun traps our planet in its gravitational field - at a distance. there has to be something that exchanges the gravitational force between sun and earth. we call this thing the graviton. it is important to notice that something that does the "job" of the graviton HAS TO exist. maybe its not a particle. but something causes the exchange of gravitation and thats what we're looking for.
you know that "in space, no one can hear you scream". thats because there is no air that could make the sound waves travel.
in a similar way, without the graviton, there would be no "medium" that conveys the gravitational force.
the problem with detecting the graviton is that it is very weak. we would have to build really expensive machines to "observe" it.
explaining this theoretically is not difficult. we have plenty of "theories". the problem is confirming them with experiments.
It is well known that QM is incomplete, so forcing forces to be quantum might not be the right way forward at all.
Strings are not quanta to begin with. They behave somewhat similarly at lower energies at most.
Forces are essentially a model too, and are compatible with both wave, quantum and string physics. Each substantially different.
heres your "well, actually" cookie.
For example, lets suppose some mass is moving at a constant inertial velocity through empty space. To me it seems that your reasoning would require gravitons to communicate to the mass that there are not other masses nearby and so to 'tell it' that 'straight line' for it means to go in a euclidian straight line. Otherwise, how does it know?
I think baked into your logic is that there is something special about geodesics in the presence of masses and so you need to tell the moving mass to 'curve', but to the mass it is just going straight, even if to an external observer it appears that it is curving or even orbiting.
- whether you call it "gravitational force" or "curvature of spacetime along whose geodesics massive objects slide" - the effect has to be mediated by a "particle". for popular media, "particle" is too big a word, because people tend to think of protons or atoms. subatomic particles are just excitations of quantum fields. little blips of localized energy, of which we are only able to see the top layer.
^ this has nothing to do with general relativity. general relativity describes the macroscopic world pretty well. it generally breaks down on very small scales.
how planets move is described very well by general relativity. how they mediate the involved forces is not described at all.
edit: i just thought about that straight line statement. there seems to be a misconception that a geodesic is a "generalized straight line". That is not remotely true. Geodesics, in mathematics, are "shortest paths". While that happens to coincide with what a straight line does in a plane, generalizing that meaning in the other direction doesn't work.
In general relativity, we talk about geodesics when we mean "out of all the possible paths we can take, we are choosing the one that minimizes energy loss". That is, then, a geodesic. But a geodesic is far from a straight line in terms of movement. Its the path of least resistance in the energy picture.
If you ask "whats the difference?" - the difference is that a straight line in energy space is not a straight line in regular space. Earth, for example, is travelling along a geodesic. But it is clearly accelerated towards the sun. There is nothing "straight line" about it.
When you fall into a black hole, you travel along a geodesic. But it wont feel like a straight line to you at all.
That you happen to be travelling along a straight line in the absence of forces is just a tautological truth. Applying differential geometry to that statement just makes it way more complicated to state the obvious.
Geodesics being generalized straight lines is exactly true. Also note that they are not necessarily shortest paths: In the framework of affine connections, they are defined as autoparallels.
Earth, for example, is travelling along a geodesic. But it is clearly accelerated towards the sun.
Earth is in free fall around the sun, so accelerometers will read 0. That's the whole point of General Relativity: Geodesic motion is not a consequence of Newton's second law, but the first one.
Why are you so convinced that curved spacetime is only an appearance and that there has to be particles behind it that create the appearance? Why cannot curved spacetime be the fundamental explanation itself?
Normally this doesn't matter because gravity is so weak compared to the scale where QM effects dominate but it is a mathematical inconsistency that becomes very relevant towards the extremes of both theories, in particular: the very early universe just after the big bang and the dynamics near black holes.
You're hiding a lot of assumptions in this idea. That model of curved geodesics sounds sensible in isolation, but it's completely contrary to how all other known forces work and seems incompatible with quantization (which is again how all other known physics works). I mean sure the universe conceivably could have three fundamental forces that work via particle exchange and one that works by curved spacetime, or magic, or the hand of god. But that doesn't seem very likely, and that kind of inconsistently would go against the history of physics up to this point.
which is in no way his fault. media present it exactly that way.
My description is, from my limited understanding, just describing General Relativity, which is a totally accepted and highly verified by observation and experiment.
Where you say it is incompatible with quantization, you are assuming that gravity has any quantization to begin with, which has never been observed. You are literally 'begging the question' here, assuming that gravity quantization needs to be explained by gravitons, when neither quantization of gravity nor gravitons have ever been observed.
Grandparent is indeed describing GR. I wanted to try to give a more outside perspective; I think we forget just how weird GR is just because we're used to it. I wonder how we'd be thinking about quantum gravity if we'd discovered QM (which has been verified far more rigorously than GR) first.
> Where you say it is incompatible with quantization, you are assuming that gravity has any quantization to begin with, which has never been observed.
Sure, but as I said quantization is how all other known physics works. While indeed we haven't observed quantization of gravity, fundamentally gravity happens in the same universe as the rest of physics, so something has to give. (And continuous approximations to quantized reality are again exactly how the rest of physics works, whereas I struggle to even imagine how you could recover quantum behaviour from a continuous underlying theory - though I'd be fascinated to hear about any such efforts).
I would say that it doesn't, and that inertial motion in the absence of other masses is meaningless, and I think this is a key insight that allowed the development of GR in the first place (since any attempt to introduce absolute coordinate systems breaks causality because you can have multiple outcomes from the same boundary conditions, at least in the Hole Paradox).
I would say in addition that every other part of physics that I am aware of can be described locally, for example you can detect if you are in an magnetic field (at least in theory) even at the level of a single proton (since it has a magnetic moment). There is NO test you can do locally to detect that you moving inertially in the presence of a gravitational field. If you can't detect it locally, either there is a huge coincidence (in this case the coincidence is that inertial mass and gravitational mass are identical) or you can't expect to find any messenger particles, since why would you need to 'tell' a mass that it is in a gravitational field when it can't even detect it to begin with.
it makes no prediction at all about WHY its happening. einsteins theory does not explain, and does not try to explain, why the gravitational field exists. it just tells us the effects of the gravitational field being around.
I dont know what ST refers to.
In common English, "how" means the mechanics of a system, and "why" generally means for what purpose someone has undertaken an action. Think of "Why did you build a garage?" (for what purpose or to what end?) vs. "How did you build a garage?" (what exact method did you use).
To complicate things, for action by non-agents, "why" can also mean "how". Children might ask "Why is the sky blue?" but what they mean (or the answer they get) is "How is the sky blue".
So when it comes to physics, we generally say there is no "why" (teleology, or mental purpose of an agent), only "how" (observable physical mechanisms). As far as physics is concerned, there is no God who set up the universe in a particular way, because he liked things a certain way, or he wanted a certain outcome.
However, the ambiguity of "why" can still be re-purposed, if you will, in physics. It's not referring to the will of an agency, but instead to lower-order phenomena that give rise to higher-order phenomenal. Why does the Earth sky have to be blue instead of, say, red? We're asking about why the mechanics of Releigh scattering play out one way verses another.
1. Nature includes the force of gravity, described at low energies by Einstein's theory with a gravitational field.
2. Matter in Nature is quantum mechanical.
We believe that spin 2 particles will exist in any such theory (as they do in string theory), but even without this assumption it is still difficult to come up with such a theory.
The example you give of gluing the Standard Model to General Relativity does not achieve this goal and does not correctly describe nature. This is because the quantum matter of the Standard Model does not correctly back-react on the classical gravitational field.
When we couple the classical theory of General Relativity to classical matter, the gravitational field pushes matter around, and the matter in turn back-reacts on the gravitational field and changes it. When coupling quantum matter to classical General Relativity, the classical gravitational field can act on matter just fine, but it does not work in the other direction. The problem is that when matter is in a quantum superposition, it is not clear how to update the classical gravitational field (which cannot itself be in superposition). This missing part of the interaction means that SM+GR is not a complete theory.
-- Evan
The MOND folks would disagree here. They'd say that the phenomenon we call "dark matter" and "dark energy" are the result of quantized gravity.
Sure it's not a pointless mass?
Years ago someone on slashdot made an attempt. It's silly humor.
Your moma so fat even if I'd entangle
with her no information would be able
to leave her event horizon.
Nobody has managed to put gravitation
and QM together yet, and you want to
do it in a your-momma-so-fat-joke? Wow.
Sigh. I couldn't resist. It's one of my favorites. Let the downvotes begin.
I'm not an expert, but from what I understand GR and standard model don't exactly contradict each other, they are both considered "valid" and self-consistent on their own as they merely describe different processes on different scales?
On their own, they work just fine but if we try to stick the two of them together, to get a "unifying theory", we get different outcomes than we expected, that's why we are now trying to find the "glue" that fits the two of them together while still making sense in the end.
At least that's my layman's take on this whole situation.
the problem with string theory is that, in its landscape version, it has a hugely complicated and poorly understood high energy scale behavior, seemingly capable of producing a very wide range of possible observable effects, none of which have been seen
And providing testable hypotheses.
Because, even if a theory doesn't pan out, it' s still science: you've researched an avenue and came to certain conclusions.
And perhaps this is most of science work -- not the "found a new theory" but "checked whether this theory works" (and same for experimental science).
1) Science does not necessarily involve experimentation. 2) String theory has attempted many times at experimental verification. 3) Math are more experimental than most people think.
That is not to say, however, that all important work must be scientific. There is a certain value to exploratory work, which can guide the direction of experiments in such a way as "if we could get experiment X then we could turn hypothesis Y into a testable hypothesis." In my opinion that is important work. You can see this in fields like psychology and such where the systems are just too complex to be easily testable (even medicine to some degree). But if we don't keep pursuing we will never get there.
EG: We already consider "Dark Energy" a sort of made-up assumption to justify a model which, after recent observations was considered falsified. So, instead of focusing on others we decided to keep using it, we created a not-falsifiable nor testable assumption (dark energy indeed) and kept going with it, even if it violates everything Popper teach us.
Honestly it seems to me that, especially in physics, too many people are just too much mentally involved in their own theory that they have an hard time just dropping it when it doesn't seem to work anymore.
Most argue that the discovery of the neutrino saved the fundamental conservation laws, but we should never lose sight of the fact that we still don't detect them in a very large percentage of events we record (we say simply that they are there, but interact weakly).
One important measure of whether a hypothesis is likely going to be accepted as true in the future is that as we build better and more sensitive detectors we detect more outcomes favorable to the hypothesis. This is the way smart money bets when data are still hard to come by (see 'climate change').
> This was a non-scientific hypothesis at the time, but it told scientists the direction they needed to pursue in order to get to the point where the hypothesis could become scientific.
"What is Philosophy of Physics?"
Not by definition, but by one specific definition -- from the popular epistemology by Popper (who also coined the relevant term). Others (Feyerabend for one) have pointed out that actual science (in practice) doesn't work that way.
I went through Stanford CS in the mid-1980s, just as it was becoming clear that expert systems didn't do very much. But there were many faculty heavily invested in the technology. Feigenbaum was running around telling Congress that the US would "become an agrarian nation" unless they funded a big national knowledge system lab headed by him. Strong AI was supposed to work Real Soon Now. What actually happened was the "AI Winter" - for about two decades, not much happened. The AI startups from the 1980s went bust.
Yes, my university still has a class were you build some using CLIPS...
At about the same time somebody was running around telling people we were going to be in a whole lot of trouble if we didn't train a bunch more physicists. A few years later we entered the era of the 40 yr old post-doc.
Maybe it's a marketing problem. If string theory would move outside of the realm of physics and into that of mathematics I don't think it would have these issues.
String theorists seem to think their theory is so beautiful that there is no need for it to be able to pass this sort of test.
I don't see any evidence that string theorists don't believe that having practically-testable predictions is desirable.
IIRC, actually, there was quite a bit of noise a few years back when string theory was the basis for in-principle-testable predictions regarding quantum entanglement, but I don't know if any practical tests have been constructed.
https://www.nature.com/news/scientific-method-defend-the-int...
So yes, they would like it if there were practical tests, but no, they don't think they are necessary.
The problems string theory are attempting to answer are many magnitudes more complicated than general relativity and electromagnetism and hence require tests many magnitudes more precise and complicated. It's possible such tests may never be developed and that there may be many competing theories of everything with no way to ever ascertain which one is correct.
New theories take not just decades, but centuries to develop. There was at least a three-hundred year gap between the development of Newtonian gravity and general relativity. Much of string theory is only a couple decades old. If physicists and mathematicians decide that string theory is untenable, they will gradually abandon it, similar to how in capitalism products become obsoleted in favor of newer, better ones. There is no need for someone to determine which theories are worth pursuing or or not; the ‘marketplace’ of ideas does that automatically, gradually nudging the natural progression of research towards better ideas as old ones are discarded or modified. For example, 120 years ago aether theories were conjectured the propagation of light and gravitation, but these theories were later were supplanted by special relativity.
http://greyenlightenment.com/falsifiability-string-theory-an... String Theory succeeds because it offers a means that is logically consistent to bridge gravity and quantum mechanics, and the strings themselves are building blocks for all particles. If someone can come up with a logically consistent theory that is as encompassing as string theory and can be tested and verified with existing technology, the physics community is all ears.
The tyranny of verificationism impedes scientific progress. Let string theorists do their thing/string.
> as you study the idea more carefully, do you get closer to testable progress or farther away
but didn't really answer it. the author points out that negative results in the LHC don't disprove string theory (because it would be impossible, given the lack of testable expectations it creates), but show that it's even harder to test than we thought.
In his words, that makes it seem farther away than ever.
At what point is something so far away that you move on to ideas that are less tested but more grounded?
I have noticed this is common with string theory defenders. They often don't really address the objections, but instead move off in some other direction they find more congenial.
Most new theories usually take days to months to develop, and are small increments over older theories that will then take years to test and, if validated, become the basis of newer theories. There are very few exceptions of theories that took years, but then those were developed by a lone individual, not a mainstream thing.
The tyranny of verificationism is the only thing guiding theory development. Without it people are blind. Yeah, it's useful to walk blindly for a while, but it has never been useful to wander very far.
This is a very dangerous path. Science without empiricism is religion.
this is idealistic and I think ignores a very important effect that is present in both consumer capitalism and in scientific research: the institutional interests of those on the supply-side tend to dominate those on the demand-side. In scientific research the supply-side would be research funding institutions such as universities, and the academic-industrial complex of publications.
in consumer capitalism we see many examples of products getting worse over time (from the consumer point of view) because it serves the interest of the producer. the best example I can think of is home appliances. In the 1950s and 1960s they were engineered for durability and maintainability. In 2017 they are engineered to be cheap to produce and to be disposable. Washing machine repair service is basically a thing of the past, for example. Now you are expected to just buy a new one. Similar effects can be observed in a lot of products.
what might this effect look like in scientific research? a lot of the "replication crisis" is related to this effect. there is pressure to write up a positive and interesting result, rather than a negative result. there is also pressure to write up theoretical research that attracts attention even if it doesn't yield experimental procedures to test the theory. is this going on with string theory? I don't know, but maybe.
my point is just to call into question the notion that there is some kind of positive selection pressure that improves research over time. maybe there is, but I wouldn't count on it being strong enough to overcome institutional pressures.
(Could we get a mod to fix this? Thank you!)