Well, because experiment tells us this is so. It’s the same reason that elementary particles have certain properties and not others, and certain energy levels, etc.
I’m assuming the hypothesis of extra dimensions solves some problem or simplifies some theoretical framework. But why choose spacial dimensions to fiddle with–i.e., how is it any more valid or helpful than postulating there’s a “Krusty the Klown” field that underlies everything and has certain properties which magically explain the link between gravity and the other forces, etc…/
Experiment tells us that there are 3 large dimensions in our particular universe, and in our part of that particular universe. Experiment also tells us that general relativity is on the right track. General relativity says that space can bend and curl into different geometries. Experiment also tells us that quantum mechanics is on the right track. Quantum mechanics tells us that at short distances the gravitational field probably curls and even foams into all sorts of geometries. So while yes, we see that there are 3 large dimensions around us, there is really no compelling theoretical reason why there are no many more dimensions that just happen to be “curled up”, or smaller than the distance scale we live in.
Basically, don’t be a “dimension snob”!
Just because you can’t see some doesn’t mean they are silly or un-motivated. See my other post about why 3 dimensions is completely arbitrary. We have no reason to feel entitled to 3 only!
Yes, adding extra dimensions sort of “magically” simplifies and unifies parts of our physical theories. It works tantalizingly well, enough that it has gotten a lot of physicists excited over the years. String theory is still pretty exciting, and is something that requires extra dimensions in order for the theory to work. And believe me, it is a theory that is very beautiful and philosophically pleasing in many ways, but it is also one of the only promising ways of unifying gravity with quantum mechanics.
I didn’t realize the idea of curled up extra dimensions was something that falls out of the science–I thought it was just an excuse to explain why we can’t see them…
P.s., I guess what bothers me is trying to figure out at what point a theory that’s not really testable just becomes equivalent to postulating that the earth sits on a giant tortoise (before they had the ability to make any observations to test it.)
I’ll tell you, even a lot of my fellow physicists feel the same way! Regarding string theory, there tends to be a division into two camps:
those who think it’s veering dangerously into philosophy, and
those who think believers in 1) are ignorant of the compelling nature of the theory
Both sides have a point; indeed string theory is not currently falsifiable in practice. But also it is very difficult to convey to those in the 1) camp the reasons why it is nonetheless probably correct. At the end of the day the people in the 2) camp want to scream: “look, we aren’t stupid!” Personally, I wouldn’t be enormously perturbed if we decided to re-label string theory as philosophy (of physics) rather than physics. But I would still feel it deserves a large amount of funding and smart people working on it. Your Krusty example just doesn’t give them enough credit. It “smells right” in a lot of ways, and it may, unfortunately, just happen that we live in a universe where the correct theory is unfalsifiable (in practice, not in theory). Stepping back, you have to realize that there is no strong reason why we should live in a universe where the final theory is falsifiable in practice! In fact, when you step back and look at the possible theories and the energy scales involved, it starts to become obvious that at a certain point in order to move forward we will have to push further and further into philosophical territory.
As a not-physicist, isn’t the space of things that aren’t testable in practice bigger than the set of things that are testable in practice? Meaning, there’s a whole lot of energy scales where things could be tested, but a whole lot more bigger numbers that we couldn’t hope to achieve? It seems to me to be combinatorically likely that there’s a great deal of stuff in the universe that’s simply not practically testable just because of this property.
Wow, you seriously should write a book. I get the same sense of clarity reading your posts as listening to Feynman.
I realized the Krusty example was extreme and could be taken as insulting–sorry.
But I agree totally with your last point. It makes a certain sense, in fact, that we live in a universe where a “final” theory is not falsifiable.
But why believe it’s final and, if it has no predictive value, you always come up against “why bother?” (other than the pleasure people get in developing the theory.)
That it’s probably not a “final” theory to me is supported by the sense one gets that time and space are artifacts of some deeper physical reality (to paraphrase Brian Green.) My understanding is that String Theory doesn’t really get at something underneath time and space (?)
You are correct. However when we ask that a theory be falsifiable (in practice), we are asking that there be some prediction of the theory that can be tested in practice. We are not asking that all predictions be necessarily tested. Just that there be at least one prediction that can be tested, and that there is a possible result of the test that would disprove the theory.
String theory predicts many things that have already been tested, something that is often overlooked. Those are called postdictions. It also predicts, for example, supersymmetry. This is something that is incredibly compelling, even aside from being required by String theory. Many theorists just assume supersymmetry to be true. Unfortunately, evidence for supersymmetry may very well be at a high energy scale that is practically inaccessible to us. So technically it is practically unfalsifiable. There are lots of things like this. Magnetic monopoles are predicted by String theory, but they are also assumed to be true by many physicists anyways. And they are probably so massive that we will never be able to create one in the lab. Oh well. They are still working on it, and maybe one day, even if they can’t make practically testable predictions, they will at least have the only viable theory of quantum gravity. And that will be something.
That’s kind of you. I’m glad my attempts at clarifying a difficult subject is at times successful!
I think (I hope) that many people are like me, and really just want to uncovers some philosophically satisfying explanations for the nature of reality. For example, we just found the Higgs boson. It cost a LOT of money! We not doing it because it will help cure cancer (although that would be nice!). We are not doing it because it is likely to have any technological side-benefit (it probably won’t, although don’t tell that to the people who give us our grant money! shh!). We do it because we want to learn more about the universe. Because we are curious. Science is the most fool-proof way of satisfying that curiosity, but sometimes your theories have to push the envelope a bit in order to open up the possibility of a truly revolutionary and philosophically satisfying idea.
And a minor nitpick: it’s important to distinguish between a theory with genuinely no predictive value, and one whose predictions are not practical to test. For example, we currently don’t have a complete theory of quantum gravity that has any predictive value! String theory at least promises to have some predictive value, which, although perhaps not practical to test, would at least be intellectually stimulating!
Not completely. There are even more audacious theories that attempt to do that (ones that make Krusty look like Niels Bohr!), but they are much further from being able to have any predictive value.
The Higgs field is everywhere, throughout all space. This is not the same as the Higgs boson, however, which is more like an energetic ripple in the Higgs field (technical terms are “an excitation of the field” or a “quantum of the field”). In quantum field theory, all particles are understood to be excitations of various fields (e.g. an electron is a ripple in the electron field, etc.)
In the LHC’s case, the Higgs boson is something that exists in the particle accelerator for a few seconds when you smash the protons together, and then rapidly decays into other stuff.
Note that even though the Higgs is, so far as we know, fundamental – i.e. not made of other stuff – it can still decay into other stuff. This is typical of particle physics; one kind of particle can change into other particles as long as the total amount of certain special quantities – “conserved quantities” – doesn’t change.
Just as the Higgs decays into other particles even though it’s not made of that stuff, likewise parts of the proton turn into a Higgs (some of the time, when you smash them together with enough energy), even though they weren’t made of Higgses to begin with. The Higgs wasn’t in the protons, it was produced by the act of smashing the protons.
Not a dumb question at all. I hope my explanation helps.
Indeed, it could turn out to be a supersymmetric Higgs (there’s actually multiple Higgses in supersymmetry), and as you note supersymmetry provides a natural dark matter candidate.
But more generally, whatever dark matter is, if it couples to the Higgs field it can have interactions with the Higgs boson. So looking for discrepancies between how the Higgs decays and how we’d expect a Standard Model Higgs to decay could give us a clue as to what sort of Beyond the Standard Model particles we should be looking for.
I’m not currently a professional physicist, but I tend to doubt most physicists really think string theory is in essence untestable. I’d guess most physicists tend to fall into these categories:
String theorists, who say “Yes, there’s nothing testable yet, but let us keep working on it and we hope to have some testable predictions.”’; and
Skeptics, who say "You haven’t made any real predictions in how many decades of work? String theorists may be a little too in love with the cool math rather than realizing it’s pretty unlikely to come up with good predictions from this kind of model with the observations we have now. Maybe it’s time to put our energy on different kinds of theories. "
What you say is absolutely true; all of my comments so far have not said so explicitly, but I will say so here: string theory may indeed yield testable predictions in the future. Just as it currently stands, it does not.
To both questions: nobody knows. What causes the electron field, or the quark field? Nobody knows. What causes the gravitational coupling to be smaller than the electromagnetic coupling? Nobody knows.
It’s not the existence of mass, per se, that’s the quandry. Rather, we’ve got a mathematical formalism that works beautifully for describing particles and their interactions, but this formalism breaks if you try to write down the terms in the equations that correspond to mass. You literally cannot calculate anything meaningful. The introduction of the Higgs field fixes this rather elegantly. It induces terms in the math that look exactly like what you wanted to write down originally, only now the whole formalism holds together and the theory suddenly works (and makes spectacularly precisely verified predictions).
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As iamnotbatman says, nobody knows. The Standard Model has a knob in it for the strength of each fundamental particle’s interaction with the Higgs field, and we have no idea why the knobs are set the way they are. A conundrum is that these knobs seem to be set to wildly different values (which is another way of saying that the particles have wildly different masses), and such a spread in values warrants explanation. No one knows the explanation.
It’s precisely this that makes me feel - intuitively - that the multiple-universe theories are correct.
Why not? All possible combinations of settings can be out there.
Like to see some proof of it, though. Not likely in my lifetime, I’m afraid.
First, I just want to chime in my agreement that iamnotbatman does a great job of explaining things.
My question: Am I correct that the Higgs boson is not something that we would ever expect to exist “in the wild” on its own? Its “existence” (in quotes because of what I just said) is something that is predicted by the equations that define the Higgs field. And so the fact that we have (presumably) been able to create this very short-lived particle serves pretty much only as confirmation that the Higgs field exists. In other words, we have no way (currently) to directly test the existence of the Higgs field, and so discovering/creating the Higgs boson is currently our only way to confirm the existence of the Higgs field.
So then is there any way, theoretically, that the Higgs boson could exist or be created naturally and thereby do something or cause something to happen or cause something to change?
For example, assuming I understand correctly, the photon is really the result of an excitation of the electromagnetic field. So detecting photons provides support for the existence of the electromagnetic field. But a photon also transmits electromagnetic energy from one part of the universe to another, right? Even without intelligent observes to view the light, the energy still gets moved around. Is there anything remotely analogous to that that might be said about the Higgs boson?
