histoman

has the probability density function for a particle ever been directly observed? it's impossible to do because the function collapses into a definite result as soon as you try. the evidence for it is indirect - ie, by looking at the distributions of several observations.

einstein objected to this probabilistic interpretation of quantum phenomena. he thought there was a deeper, detrministic theory.


if we can confirm the existence of other dimensions, then other dimensions will be fact. until then, they are conjecture, albeit maybe very useful conjecture.


there's nothing wrong with inventing new ideas. i'm just saying that when we do we should recognize that we've also created new questions, such as what exactly is a brane and why should we believe it exists?

"a happens because of b" only makes sense when we know what b is. otherwise, all you've done is "passed the buck", so to speak.


god and the spaghetti monster would also be discounted because they don't postulate natural causes and therefore can't be tested.

the uniqueness theorem argument applies to other scientific theories, in particular to theories that may be created. it simply says that two (or more) theories may exist that have the same explanatory power yet make very different assumptions about underlying phenomena - ie, very different and even conflicting assumptions can coexist if there is no empirical evidence to favor one.

if there is no evidence of branes or other dimensions, then, in principle, other theories may exist using different assumptions that have equal ontological status - ie, a questionable, or contingent status.

histoman

i haven't studied this, but isn't planck time part of one of the proposed quantum gravity theories? if so, then doesn't it also have the same contingent status as the theory itself? how do we know that time is discrete and not continuous?

UberCryxic

Einstein did object to quantum mechanics, but, again, Einstein was also wrong. Bell's inequalities and the associated experimental data showed that quantum theory was right. There are no "local hidden variables" that guide the behavior of fundamental particles. Bell's inequalities are just one out of a myriad pieces of evidence that confirm the predictions of quantum theory. What does the probability density function have to do with empirical observations? That is a mathematical formulation. As I said, QM is one of the most empirically satisfying theories ever.

The existence of other dimensions is more than just "conjencture," but something less than "fact," at least at this point. Language problems aside, we do have very good reasons for believing that four-dimensional spacetime is not all there is. In fact, we're almost positive of that supposition, but just not enough to give it the definitive label of "factual."

The questions that you ask surrounding M-theory are pertinent, but obviously at this time, lacking any reliable measurements, we can't commit ourselves either way. I can describe the essence of these latest TOE attempts in the following way: one-dimensional strings "vibrate" in the 11th dimension and create the fundamental particles responsible for this universe, among others. The strings are responsible for everything. Why should you believe these things exist? Because the theoretical mathematics in which we're dabbling right now reveals a remarkable degree of accuracy to the reality of our universe. I do not mean to affirm these entities absolutely; I just want to give you a good reason for why they might exist so you don't dismiss the propositions of the theory right away. String theorists also speak about the "elegance" of their theory, implying that it's too "beautiful" or efficient to be incorrect.

I am understanding your position somewhat better, but you should know that I am a scientific realist, and I do not really take kindly to the skeptical attitudes of people like Fraassen and Quine. The latter's undetermination of theory argument is really unappealing, giving scientific theories way too much credit in their ability to survive scrutiny. In reality, they are much too brittle. A scientific theory does not exist as an independent, extra-worldly unit that makes certain statements about nature. Certainly that's an important characteristic, but not a defining one. We can gage the utility and accuracy of scientific theories based on both the nomological and empiricial results they yield. For example, one could like GR better than Newton's conception of gravitation because the predictions of GR are more empirically compatible with the observed aphelion of Mercury. One could also like GR better because it is used to keep accurate clocks on a satellite. One could like QM because it is used to build cesium clocks, better televisions, and so on. Scientific theories have the wonderful property that the entities thinking about those theories can apply them in the real world and see if they hold. In that sense, science is getting more and more accurate than in the past. It is reaching a "final destination," if you will. That destination will be the TOE. Despite repeated misrepresenations of his views, Thomas Kuhn was also a firm believer in scientific progress, although maybe not in the same ways as I.

The Planck time and the Planck length are derived from thought experiments using some fundamental assumptions made in quantum theory and GR, yes, but this "contingent" talk is a bit deceptive. It is sort of like saying (note: "sort of" like saying, I'm not accusing you of saying anything either way) that because light "bending" was merely a proposition in Einstein's head, its existence is predicated on being in Einstein's head. Planck time and Planck length, like the "bending" of light," are true. They are "contingent" on well-established theories and assumptions.

histoman

the natural state of quantum particles is the indeterminate state described by their wavefunctions. yet it's impossible to observe particles in their natural state. all observations are of determinate states after the wavefunctions have collapsed. how, then, do we know that the quantum description of particles is even correct? true, the results of the calculations are right. but, in principle, the same results could have been generated by another theory using different assumptions.

the indeterminacy of particle states is a basic assumption of qm, yet it appears that it can't be directly tested. (or did i fall asleep in class and miss something?!)



ok. so it's too soon to say that space and time are quantized, right?

the bending of light is true because it's been seen. but no-one has yet observed that space and time are discrete. i don't doubt that someone did a thought experiment and calculated these numbers. it's the interpretation of planck time and length that i question.

btw, if planck length and time are indeed indivisible, then they also set limits on the resolution of any theory. if the planck time is the moment when gravity separated from the other forces, this means a toe won't tell us anything about what the unified force was like because this existed in the first planck time (and space) increment about which nothing can be known. am i missing something because this seems a little paradoxical. don't you need resolutions finer than the planck time to understand the very earliest moment of the universe? how are we going to know this if space and time are discrete?

histoman

you mean the predictions of the theory have already been confirmed by experiment?

histoman

is this because it's needed in string theory to derive the desired results? or is it because there is independent evidence that extra dims exist?

UberCryxic

The predictions of M-theory have not been confirmed experimentally yet. What I meant was that if you make the mathematical and nomological assumptions that those theoretical physicists are making, you get numbers that are true for this universe. The added benefit is, of course, that the theory also explains several problems in physics, chief among them the weakness of gravitation. Again, I do not want to suggest that M-theory is correct, it's way to soon to make that judgment, but we should realize that it has benefits along with flaws.

The curvature of light is true because it's been seen? That's an odd comment. So...you're saying that light started curving when Eddington made that observation in 1919? Obviously I know that's not what you mean, but the statement is poorly worded. Regardless, the curvature of light is true because of the physical principles that guide its behavior, not because humans have observed it. It would have been true with or without us in the Universe, everything else being equal. Likewise, we have Planck time and Planck length because that's how the Universe fundamentally works. We don't need to observe them, although in principle it should be possible to experimentally verify Planck length (and then Planck time follows from that). You can say that spacetime is quantized, and you can say it fairly confidently. There's nothing amazing with interpretation: Planck time is around the order of 10^-44 s and Planck length is something like 10^-35 meters. Basic facts of nature. Moving on....

Your characterization about what the TOE will and won't allow in light of the Planck units is both correct and incorrect. The TOE should be able to tell us what happens at the singularity, and that's all that matters. If we know that, then the Planck times won't mean much. All the properties and laws of the universe will be shown to be a consequence of something that happened at t=0. But you are right that we'll never know anything about the "intervals," not that those are that relevant. What happens at the singularity is the most important issue. We're G-O-D if we can discover that.

Well, we do test for indeterminism in QM. Maybe not "directly," thinking that I vaguely understand what you mean here, but this is an unreasonably harsh standards to hold. It's sort of like questioning friction even though we don't fully understand how friction works at a microscopic level (and it may be even worse, given the level of empirical data for QM). I recommend the following article: http://physicsweb.org/articles/world/12/12/19. It does a better job at answering your questions on this specific topic than I could (particularly your worries about the EPR paradox).

UberCryxic

Unfortunately, the latest TOE candidates propose dimensions so small that humans would never be able to perceive them. We know about the four-dimensional world and that's it. The extra dimensions are used to gain the desired result; strings apparently need to "vibrate" in 10 dimensions (or 11 in M-theory).

histoman

i'm going to guess that those numbers are "true for this universe" precisely because of those assumptions - ie, it isn't really independent verification.



the "we know" that was omitted was obviously implied and understood.


yes, but we would not have known it was true w/o seeing it.



you can certainly calculate something called planck length and time, but whether it's really a fundamental unit of space and time has to be proven - ie, the theory that postulates this has to be verified.


but wait a minute. at the singularity, space and time were both smaller than the planck length and time. if nothing can exist that is smaller than this, how can a theory that takes these quantities as fundamental units describe anything at all at the singularity?!



it's not hard to understand. in fact, it's one of the oldest criticisms of qm, one that einstein himself made.


"even though"? i think you mean "because", right?

friction is not a topic that engenders controversy like the interpretation of quantum physics does. i may be wrong, but i don't think einstein ever objected to it - friction, i mean.

histoman

are the dimensions smaller than the planck length? if so, how is that possible if space is quantized?