If it’s a spacetime warping, why are we looking for gravitons? And if it’s carried by a boson, what’s up with all those bowling-ball-on-a-rubber-sheet explanations so beloved of Intro Physics instructors? And if it’s somehow both, can the other 3 forces also be described as warps in spacetime?
The short answer is that relativity works on space-time and quantum mechanics rules particles. (That includes the other three forces so that no warping is required for them.) Neither one completely explains what we know about gravity. Therefore a larger and more complete theory melding the two must exist, or else pretty much everything we know is wrong.
Although gravitons are bosoms and stem from QM, it takes a larger theory to remove some of the complications. That’s why string theory gets theorists hot and bothered.
To be pedantic, nobody is “looking for gravitons”, in the sense that nobody is even dreaming of hoping to see evidence of the particle nature of gravitational waves (if in fact that is the correct description.) The big experiments that you might have heard of such as LIGO or LISA are designed only to detect the “wave” nature of gravitational waves; these would just be ripples travelling along the “rubber sheet” in that analogy.
I believe there were theories that put electromagnetism right in the scheme of spacetime warping by assuming a couple extra dimensions. They were thrown out early on (during the time of QM development) as being continuous and non-quantal, and hence “obviously wrong.”
Of course, before all that and before the photons there was the great conception of electromagnetism which painted it as waves that rolled through matter, vibrating it and being vibrated by it. It was a fantastically beautiful theory, from an intuitive standpoint. Same as general relativity. But it was continuous. And hence obviously wrong.
These ideas were replaced by photons and other shitty little particles, which work great at getting the model to work mathematically, but are crap when trying to understand how it all works.
Frankly, this whole quantum mechanics business is missing the forest for the trees. The old, non-quantal theories embody certain truth. A truth that must be appreciated and fully woven back into the theory. Fact is, those intuitive models aren’t really all that wrong. You can get a lot done forgetting all about stinking photons and imagining your microwave transmitter being permeated by rolling electric and magnetic fields. Do you think that the principles of quantum mechanics were set down after people accepted photons? Hell no. Quantum mechanics had its start by imagining little vibrating charges that created a magnetic field (which in turn induced an electric field, at infinitum) the way any accelerating charge does (and realizing that the frequencies of vibration had to be discrete).
Same with GM, I guess. The graviton thing may work mathematically, but noone can take the spacewarping away (or all the thought experiments that explain why and how the space should warp).
If anyone conclusively knows the answer to this question, there are some folks in Stockholm who would like to speak with you.
Usefulness and comprehensibility are of course significant factors, but they don’t make something truthful. For a lot of fields Newtonian physics are in practical terms almost always preferable to modern physics, but that’s only because the practical applications in question are of limited scope. Increase the scope, and the models break down. If modern physics is to be made more comprehensible, it will have to be on its own terms, not by integrating theories of lesser proportions.
I mean, keep in mind that the old theories were the default. If it was possible to make sense of new data in the context of the old models, that’s what would have happened. All the quantum and relativistic insanity only came to pass because they were the best ways anyone could think of to explain the data, not because physicists don’t sufficiently appreciate the value of comprehensible models.
If anything, physicists hate complicated ridiculous models. The Standard Model is the height of inelegance, and there is no end of attempts to refine it into something sensible. Everybody knows that relativity and quantum physics are mutually incompatible and will inevitably be superseded. But they stick because we haven’t thought up anything that better explains the data, and that’s the prime directive.
It is, however, incredibly unlikely that whatever replaces them will discard quantization. Non-quantal theories are ‘obviously wrong’; if they’re not (at minimum) going through serious gyrations to try to explain the copious evidence of quantum behavior, then they’re not up to the task of giving us a better understanding of the world than we already have, and so they’re irrelevant fictions.
But the bottom line remains that the present theories are only relevant fictions because we have nothing better. That, ultimately, is why scientists are constructing ever more intricate machines to look for this kind of stuff. Whatever is found (or not found) is necessary information to define the contours of a better theory.
You’re thinking of Kaluza-Klein theory, and, far from being thrown out, it’s actually pretty much the model for modern physical theories, which are formulated in terms of Yang-Mills theory, basically a generalization of Kaluza-Klein theory, which in itself couldn’t accommodate the nuclear forces that were discovered after its conception – though it’s of course still studied today, both from a physical and from a pure-mathematical standpoint. Also, the method of compactifying extra dimensions is used today in string theory.
Physics doesn’t really throw out something because it doesn’t fit some paradigm (if that was the case, there never would have been any quantum theory – or Einsteinian relativity, for that matter) – theories are tested on observation, and if that doesn’t fit, your theory has to go, no matter how pretty it looks on paper. Non-quantum theories quite simply don’t fit those observations.
Could you elaborate on what you mean by this? I always thought that the salient feature of Kaluza-Klein theories was that they were formulated by writing down Einstein’s equation on a spacetime with extra dimensions, whereas Yang-Mills theory doesn’t have such extra dimensions. I don’t think I’ve ever heard anyone draw a parallel between the two (at least not a direct one.)
Kaluza-Klein theory introduces an extra curled or compactified dimension. This is not the same as space-warping in relativity. Nor was it thrown out for being continuous and non-quantal.
The Wikipedia article shows this and also makes the Yang-Mills connection clear.
EDIT: points up Well, what he said.
Here’s a paper (pdf) that goes into the technicalities of when, exactly, the two can be said to be equivalent.
To be fair, in just the same way that an electromagnetic wave can be regarded as a stream of a great many photons, so too can a gravitational wave be regarded as a stream of a great many gravitons. You’re correct, though, that we can’t detect individual gravitons, nor, most likely, will we ever do so. Detecting an individual graviton is the sort of thing a Type III civilization might do as a technological tour de force.
On the topic of theories being thrown out for being non-quantum, it’s worth noting that quantum electrodynamics is the most precisely-tested theory in all of science, with experiment matching theoretical predictions to twelve decimal places. Any theory which can’t accommodate those results, therefore, has some serious issues.
Wait, what? Suddenly relativity and QM are much more interesting.