As I get closer to really understanding the theory (that is, knowing all the mathematical detail), I find myself becoming more and more of a string-theory fundamentalist, so I should attempt a response to this.
First, some basic quantum field theory. In quantum field theory, particles and fields are generally regarded as complementary. Particles are energy quanta from field modes, and field states are equivalent to certain superpositions of particle states. Every particle in the Standard Model corresponds to a field, and vice versa. But if you have to pick one description as more fundamental, it's probably the field, since the fundamental variables in the basic equation are field variables.
The classical theory of gravity is general relativity, and it's a field theory, so as a quantum field theory there should be gravitons. But as I just explained, a graviton state is a superposition of field states and vice versa.
The principal reason for the conceptual churn in quantum gravity, at least before string theory, was that quantum general relativity is nonrenormalizable, i.e. you cannot perform calculations with it. Renormalization (in its simplest, original forms) is a procedure for d...
There's been a proposal by Verlinde of treating gravity as an entropic force which is pretty neat.
There might even be some "predictions" of things that we already know from it - see It from Bit.
I'm surprised that nobody's mentioned this yet - it was a bit of a blogosphere kerfuffle quite recently.
This is a standard semiclassical motivation as to why gravitons most probably exist (I think from Steven Weinberg "gravitation and cosmology" but I have since long lost the book so I am not sure): In the limit of weak gravitation GR looks similar to the Maxwell equations. In particular there should exist gravitational waves.. (Have not yet been detected experimentally but if GR is (at least approximately) correct they should exist.) This means that you could in principle build a gravitational wave microscope. Say you want to measure the position of a test particle using this microscope. Now if gravitational waves were actually classical you could use arbitrarily feeble waves and thus arbitrarily small recoil on the test particle. And thus measuring position and momentum of the test particle with lower unaccuracy of position times momentum (along a given direction) than allowed by the Heisenberg uncertainty relation. But if gravitational waves are quantized in gravitons of energy = h times oscillation frequency Heisenberg uncertainty relation will be satisfied (Heisenberg's original semiclassical derivation goes through for any wave quantised like this).
Posts like this shouldn't appear on this blog. Adequately considering the question requires a truckload of special knowledge that most visitors don't have and that is not associated with the topic of the blog.
The rhetoric that is visible on the surface (comparison with something ridiculous, etc.) strongly privileges a hypothesis (of the teacher's password opaque kind, no less -- to most readers), which in absence of understanding of the subject may as well be pure Dark Art.
Being contrarian doesn't mean that we should be interested in entertaining contrarian hypotheses any more seriously just because they are contrarian. This way lies madness.
I am quite curious indeed as to how my post is any less topical than the quantum physics post by Eliezer that I linked.
I deliberately didn't write about anything that would have been foreign to any not-reasonably-scientifically-literate reader. If you disagree, please do point out where.
I do not think that the aether hypothesis was "ridiculous", and never said that I did. It was wrong, sure, but lots and lots of very smart people believed in it for quite a while. Similarly, I don't think gravitons are "ridiculous" and never said that they were.
The post being off-topic is not part of my argument. (That being said, if I were arguing that it is off-topic, then being no more offtopic than a given other post is hardly an argument against this post being off-topic in the absolute sense.)
The difference between this post and the quantum mechanics sequence is that this post doesn't get the readers up to speed, and so if they are not already up to speed, they can't understand what's going on (also, you won't be able to get the readers up to speed on this post via merely a blog sequence -- it's too much work). I said:
Adequately considering the question requires a truckload of special knowledge that most visitors don't have and that is not associated with the topic of the blog.
If you teach such knowledge on the blog, the argument no longer applies (though if it's too off-topic, the attempt might not be appreciated). If the knowledge is associated with topic of the blog, readers are expected to have it or seek it (e.g. standard biases). If most of the readers happen to already have the knowledge for whatever reason, then it can be assumed as well (e.g. calculus, because most readers happen to be well-educated).
...I deliberately di
The many-world interpretation of quantum mechanics was discussed here several times. People write about artificial intelligence here often. Evolutionary psychology is a standard topic. All these require a lot of specialised knowledge, which is not necessarily associated with rationality. Why not discuss the reality of graviton? Physics is probably more scary than evolution, since everyday intuition seems useless there, but that doesn't mean that everyday intuition isn't actually as useless in evo-psych.
I agree that the topic of the OP is a difficult and specialised question, but there are some aspects of it which can be accessible to non-experts, e.g. the question of testability of string theory or loop gravity, use of anthropic reasoning in physics, or how much popularity of theories influences research. I do not want to have here a strict policy of not discussing specialised topics.
My estimate is that most (>90%) of the opinions about these topics expressed on LW are wrong. After all, we are not all specialists, and even specialists are often wrong. But as long as we are trying to evaluate the questions rationally, albeit with limited knowledge, and as long as we are aware about our limits, I don't see why we ought to stop.
There can only ever be one reality, and so there can only ever be one correct theory of reality.
I disagree with this. A theory is basically a model (or pertains to one). Models by necessity leave out details of the thing they're modelling (if you disagree, then the best model of reality is simply reality, and we already have that). So depending on which features of reality you think are relevant, you can have multiple models of reality bringing out each of those features. The theories based on those models will sometimes make different predictions, ...
Loop quantum gravity actually does have at least one testable conclusion: non-constancy the speed of light at high energies. There's even some support for this as of a couple of weeks ago, with measurements from the Fermi telescope.
Tom, I upvoted this because I'm a fan of the LW "physics for rationalists" series that Eliezer started two summers ago. Interesting stuff.
My uneducated intuition is that you're right, and the graviton is the same naive-hypothesis-extension failure mode as aether.
But it's interesting to note that these two meta instances of reasoning-by-similarity are apparently acceptable:
"Making a hypothesis about a different physical phenomenon by naively extending common properties of a group of well-understood physical phenomena is bad." (paraphrase)
"Theories of physics have a known tendency towards elegance and simplicity."
they were created by the influence of mediator particles called (respectively) gluons, W and Z bosons, and photons
The particles (and the specific number of particles for each force) are a consequence of how the different forces are modelled. The local gauge groups for the electromagnetic, weak and strong forces are U(1), SU(2) and SU(3) (this is a simplification, especially for the weak force).
The one dimensional U(1) generates a single force carrier, the photon, the three dimensional SU(2) generates three (W+, W- and Z), while the eight dimensional SU(...
You correctly decry popularity as a non-rational measure of veracity, but to the extent that it expresses a sort of straw poll, it may be a good indicator anyway. The idea of expert futures markets comes to mind.
My point is related: is it not also a fallacy to assert it's GOT to be simple? That's awful close to demanding (even believing?) something's true because it ought to be, because we want it so bad. Occam's razor has worked like a champ all these years but inference is risky and maybe now, we find ourselves confronted with some hard digging. I too hope some crystalline simplification will make everything make sense, but I don't think we've a right to expect that, or should. What you and I want doesn't matter.
The analogy between æther and graviton has some appeal. Photons, W and Z are more or less directly detectable. There is no hope of doing that with graviton. Hence, speaking about graviton brings probably no good: such language has no direct correspondence to observed reality and there is a danger that it can enforce some incorrect intuitions. As far as it goes, I agree. But note also that neither gluons are directly observed.
What I disagree is that the new theories are "graviton-based". They are rather "gravitational-field-based". The g...
Actually, E&M has been described as a warping of a 4+1-dimensional space-time (in combination with gravity). http://en.wikipedia.org/wiki/Kaluza–Klein_theory
It had some problems.
Interesting. I think there are fundamental things we don't know about gravity yet.
A basic physics question about light waves: I understand how water waves are a property of the medium. My analogy for how it works mechanically is potential energy and kinetic energy swapping in a (mostly) self-sustaining rhythm. I don't understand light waves though. What causes them to oscillate?
Um. I'm having one of those I-can't-believe-I've-been-this-stupid-over-the-last-ten-years moments.
I went back and reread what you wrote and the part I missed before was this:
The wave described is light.
So it isn't that light "happens to follow" this wave equation. That wave equation IS light -- that is, that specific interaction between the electric and magnetic fields is light.
Honestly, I'd never thought of it that way before. I can go back to that chapter in electromagnetism and see if I understand things differently now.
I look at the light bulb on my desk and I wouldn't even call it 'light' anymore. It is electromagnetic interaction.
I photographically recall the poster over an exhibit at a science museum, "Light Is Electromagnetic Radiation'. I thought that meant that light was radiation (obviously, it radiates) that was associated in some way with electromagnetic theory and I remember thinking it was a decidedly unpleasant verbal construction.
I'm thankful, and sorry...
And finally, the Michelson-Morely experiment showed that the "aether" was always stationary relative to Earth, even though the Earth changed direction every six months as it moved about in its orbit! Shortly thereafter, the inconsistencies were resolved with Albert Einstein's Theory of Special Relativity, and everyone realized that aether was imaginary.
I actually think this is a misleading characterization. Those inconsistencies with resolved by positing that physical bodies contract as they move through the aether. Lorentz even gave us the ma...
Well, first: Does any collapse theory have any experimental support? No.
With that out of the way...
If collapse actually worked the way its adherents say it does, it would be:
- The only non-linear evolution in all of quantum mechanics.
- The only non-unitary evolution in all of quantum mechanics.
- The only non-differentiable (in fact, discontinuous) phenomenon in all of quantum mechanics.
- The only phenomenon in all of quantum mechanics that is non-local in the configuration space.
- The only phenomenon in all of physics that violates CPT symmetry.
- The only phenomenon in all of physics that violates Liouville's Theorem (has a many-to-one mapping from initial conditions to outcomes).
- The only phenomenon in all of physics that is acausal / non-deterministic / inherently random.
- The only phenomenon in all of physics that is non-local in spacetime and propagates an influence faster than light.
WHAT DOES THE GOD-DAMNED COLLAPSE POSTULATE HAVE TO DO FOR PHYSICISTS TO REJECT IT? KILL A GOD-DAMNED PUPPY?
- Eliezer Yudkowsky, Collapse Postulates
In the olden days of physics, circa 1900, many prominent physicists believed in a substance known as aether. The principle was simple: Maxwell's equations of electromagnetism had shown that light was a wave, and light followed many of the same equations as sound waves and water waves. However, every other kind of wave- sound waves, water waves, waves in springs- needs some sort of medium for its transmission. A "wave" is not really a physical object; it is just a disturbance of some other substance. For instance, if you throw a rock into a pond, you cannot pluck the waves out of the pond and take them home with you in your backpack, because the "waves" are just peaks and troughs in the puddle of water (the medium). Hence, there should be some sort of medium for light waves, and the physicists named this medium "aether".
However, difficulties soon developed. If you have a jar, you can pump the air out of the jar, and then the jar will no longer transmit sound, demonstrating that the wave medium (the air) has been removed. But, there was no way to remove the aether from a jar; no matter what the experimentalists did, you could still shine light through it. There was, in fact, no way of detecting, altering, or experimenting with aether at all. Physicists knew that aether must be unlike all other matter, because it could apparently pass through closed containers made of any substance. And finally, the Michelson-Morely experiment showed that the "aether" was always stationary relative to Earth, even though the Earth changed direction every six months as it moved about in its orbit! Shortly thereafter, the inconsistencies were resolved with Albert Einstein's Theory of Special Relativity, and everyone realized that aether was imaginary.
Shortly thereafter, during the 20th century, physicists discovered two new forces of nature: the strong nuclear force and the weak nuclear force. These two forces, as well as electromagnetism, could be described very well on the quantum level: they were created by the influence of mediator particles called (respectively) gluons, W and Z bosons, and photons, and these particles obeyed the laws of quantum mechanics just like electrons and mesons did. The description of these three forces, as well as the particles they act upon, has been neatly unified in a theory of physics known as the Standard Model, which has been our best known description of the universe for thirty years now.
However, gravity is not a part of this model. Making an analogy to the other forces, physicists have proposed a mediator particle known as the "graviton". The graviton is thought to be similar to the photon, the gluon, and the W and Z bosons, except that it is massless and has spin 2. I posit that the "graviton" is essentially the same theory as the "aether": a misguided attempt to explain something by reference to similar-seeming things that were explained in the same way. Consider the following facts:
And, with reference to the graviton itself:
So, what's really going on here? I don't know. I'm not Albert Einstein. But I suspect it will take someone like him- someone brilliant, very good at physics, yet largely outside the academic system- to resolve this mess, and tell us what's really happening.