Things get a bit "tricky" because the entire concept of multiple universes is a pipe dream. That's a nice opinion, but it has nothing to do with what I said. Things get tricky at singularities when it comes to describing time, because classical notions of "space" and "time" break down due to quantum effects.
No basis in reality or any way to prove. Whether it has any basis in reality remains to be seen; it may be a natural consequence of known laws of physics, whether or not we can verify that specific prediction. And there are some multiple-universe scenarios which do produce observational consequences.
if you look up the friggin' definition "the totality of known or supposed objects and phenomena throughout space; the cosmos; macrocosm." you begin to understand it's all from some geeks playing around with word definitions and nothing else It's not just word definitions; it has physical consequences. However, what physicists refer to when they speak of "multiple universes" doesn't agree with the dictionary definition.
I really hate it when 'scientists' start fabricating reality. Scientists don't fabricate reality. They fabricate theories, which make predictions, some of which are tested.
Am I stupid for thinking that, to a measurer moving forward at half the speed of light, the light coming from a torch he is holding will again only be moving at half of c? Not stupid, but not correct, either. The light will move at c according to any observer; that's the point of relativity.
You know that I mean that there has never been a prediction of a String Theory model, that was not also predicted by existing, simpler theories, that has been confirmed by experiment or observation. No, I don't. If you're talking about string theory having made any confirmed predictions, then of course that is true. But that's not an indictment against string theory; it's true of all QFT models beyond the Standard Model as well.
Recall, too, that you claimed that QFT has made "predictions" that string theory has not. This, too, is disingenuous. All of the QFTs so far studied can be embedded in string theory and are indistinguishable from it as far as experiments that have been performed are concerned. Thus, the data support QFT to no greater extent than they do string theory (and the string theories all have the advantage that they're consistent with gravity, to boot, unlike the QFTs). There are some QFTs which are not compatible with string theory's low energy limit, but those haven't made any successful predictions, either.
You speak of infinities as if they are all the same size. They are the same size: uncountably infinite. That being said, string theory certainly encompasses models beyond what any QFT can describe. That, too, is not an indictment against string theory; in fact, it's one of the main reasons for string theory's popularity.
But gravitons are mere perturbations of certain metrics in classical gravity. They are not analogous to the photons of E&M. That is not the same thing as having full consistency with gravity as so many string theorists boast. String theory goes far beyond mere gravitons. If you construct a theory of gravitons alone, all you get is a linearized theory of gravity (wrong) which is non-renormalizable (useless). Perturbative string theory introduces an infinite hierarchy of corrections that not only renders the theory consistent and finite, but recovers full nonlinear gravity (plus string-scale higher-curvature corrections). And if you don't like perturbation theory, AdS/CFT duality is a full non-perturbative and background independent description of some gravitational sectors of string theory, although it has not been extended to describe all sectors.
My understanding has been that String Theory (A 10 dimensional theory) can explain anything we observe in our universe if we invent a very specific theoretical parallel universe to satisfy the equation. This is troubling to me because it gives you free reign to invent new physics whenever they can't find the answers they are looking for. String theory can't explain any possible observation, but it can explain just about anything we can think of that behaves like quantum field theory at low energies. That's not too astonishing, however, since existing quantum field theories can also obviously explain just about anything we can think of that behaves like a quantum field theory at low energies. It's a matter of selecting an appropriate model; the process of particle physics consists of selecting models that can account for known observations, and then testing their predictions against new observations.
M-Theory [...] contains limitless parallel universes It doesn't have to, although you can construct M-theory models with "parallel universes". (You can also construct string theory models with "parallel universes". Depending on what you mean by that phrase, you might even construct "parallel universes" completely outside the context of string/M-theory.)
Therefore, String Theory is an expression of M-Theory and "alternative loop quantum gravity" (I think also referred to as the "Super Gravity" - originating in the 11th dimension) is also complimentary to M-theory. It is not known whether 11D supergravity has a loop quantum gravitational description.
My question - what's the problem? String theory and M-theory are not competitors, they are different parts of the same theory.
No string theory model has ever predicted anything that a QFT model couldn't. That's wrong. There are plenty of stringy phenomenological models that aren't QFTs, which make concrete predictions. We don't know if any of their predictions are right, but you can say the same about any specific QFT model beyond the Standard Model you may care to construct.
You would have to be smoking reefer to think that human scientists would invent the edifice that is string theory to handle the mere standard model. I said that the Standard Model could be embedded within the context of string theory instead of QFT. The point is, QFT is not "more predictive" than string theory, since you can write down models like the Standard Model in either framework, as you prefer, and they will make exactly the same low-energy predictions.
Plus they already had their chance with the strong nuclear force. What is that supposed to mean? QCD is not string theory, therefore string theory is useless for particle physics? There are a few gaps in your logic.
Incidentally, the original motivations for string theory in the strong nuclear interaction may yet pan out; the initial picture of flux lines as strings still has merit, and there is a small industry of string theorists working on that idea. QCD took over because of its successes at weak coupling when the theory is near asymptotically free. But it has bogged down at strong coupling, and numerical approaches on the lattice have been necessary. The AdS/QCD string approach has proven capable of addressing the strong coupling limit as a weakly coupled string theory for some supersymmetric QCD theories, and may be useful for actual non-supersymmetric QCD at strong coupling as well.
That's just the problem. It's far too predictive and broad. It's exponentially predictive and broad. It is not really less so than QFT, which was my original point. String theory can construct models that QFT can't, but even in QFT you can construct infinitely many models consistent with known physics (and infinitely many more that aren't).
And every time an experimentalist looks for something stringy and finds nothing, the string theorists simply readjust their parameters and make sure they have escaped the clutches of known reality. Again, this is no different from QFT. If you falsify one QFT, there are infinitely many more to take its place. That is, in fact, how particle physics has been done for the last 80 years ago: you construct models that make predictions beyond what is known, and when they get ruled out, you construct more until some work. That process can be within the frameworks of QFT or string theory.
You still persist in comparing string theory to a specific model like the Standard Model, instead of more properly to a framework for writing down models, like QFT. And all the string models at least have the benefit of being automatically consistent with gravity at all levels, unlike any of the QFTs, even in the string models which are indistinguishable from the QFTs at low energies.
There are string theory models that are falsifiable right now. Some of them have, in fact, been falsified. Some may be falsified in the very near future. We don't know, however, whether it will be falsifiable in the foreseeable future, because we don't know whether any of the "easy to falsify" models are true.
Smolin says that all the "universes" are connected to each other via black hole/Big Bang singularities. It's not really possible to compare times in different universes to say which ones exist "simultaneous" to ours, but it is possible to say that the one that gave rise to our univese came "before", and so on for its ancestor, defining a topological order. Things get a little tricky when classical spacetime presumably breaks down at the birth singularities, though.
In physics, the word "theory" is not used in the 8th-grade "it's a hypothesis or a theory" way; a "theory" instead refers to a set of field equations governing a phenomenon. That is, its empirical status is not relevant to whether it is called a "theory". In that sense, string theory is a "theory".
The point is rather that nothing can affect a universe's reproductive ability (either by "killing" it or altering how many descendents it can have), so no external selective pressures can be exerted on it. The number of times a universe can reproduce depends purely on its "genome", and not on any selection that the environment may impose. This isn't what is meant by "natural selection" in the biological context.
As I mentioned in the previous post, it's not clear that a Big Crunch is actually "death" for a universe, since both black holes and Big Crunches ultimately result in singularities. (Or, you could say, a Big Crunch may result in exactly one birth for every death.) I suppose an eternal universe could be thought of as "dead" in a reproductive context, if no new black holes form.
Due to some phyics which I don't understand, it is more likely to be antimatter which falls into it. It's not "more likely to be antimatter" which falls in. Matter and antimatter fall in with equal probability. It's the virtual particle with negative energy — be it matter or antimatter — which always falls in. That's the case 100% of the time because any virtual particle which enters the horizon must have negative energy "relative to" an external observer (crudely speaking).
Incidentally, no astrophysical sized black hole is currently evaporating, because the cosmic microwave background radiation is currently hotter than any such black hole's Hawking temperature. Thus, it gains more mass by absorbing the CMBR than it loses by Hawking radiation. It won't shrink from Hawking radiation until the CMBR cools below its Hawking temperature, which won't happen for ~10^66 years for a solar mass black hole (see here. If very light holes were created in the Big Bang, they could be still around and shrinking at the moment, though.
Actually by definition superstrings could never be observed. No, that is not by "definition". Strings could be observed if we could reach Planck energies (unlikely but at least philosophically possible), and they could also be observed if certain large extra dimension scenarios are correct, lowering the effective Planck scale.
Also it's interesting to note that Edward Witten even declared that it's not even necessary to the theory that these strings exist. I am quite sure Witten never said that.
The example assumes that a laser fired from the front of a space freighter (travelling at velocity V) would - according to newton - have a resulting velocity of c + v. [...] maybe, because [light] has no mass too it is not effected by the velocity of the space freighter, maybe it just goes along on it's merry way at c regardless. Well, that's right. Light travels at c according to all observers because it is massless. That violates Newton's laws (in which nothing can travel at the same speed according to all observers), leading to relativity.
But frankly i find the idea that because light is travelling at a fixed speed and everything looks a bit wierd you make a giant leap to frames of reference and warping spacetime bizzarre. It's easy to see at least that a fixed speed of light must lead to time dilation: consider sending a light signal from the floor to the ceiling. According to you in the room, it travels a distance equal to the height of the room. But now suppose that the whole room is moving past someone on the ground. According to you, it's still traveling a distance equal to the room's height. But according to the ground observer, it moves that distance vertically, plus some extra horizontal distance equal to how far the room traveled during the (very brief!) time it took for the light to reach the ceiling. So the light travels farther according to the ground observer.
So far, nothing new. But if you postulate that light traveled at the same speed according to both observers, then to the ground observer the light took a greater amount of time to reach the ceiling (greater distance at the same speed means more travel time). In an extreme case, the light could take much longer to reach the ceiling, according to the ground observer, than you in the room measure.
Curved spacetime is a whole different ball of wax... it too can be justified but I don't have the time right now.
I think the classic objection to string theory is not so much that it isn't true, but that it cannot be falsified. That's incorrect. However, we do not know whether it can be falsified with experiments that we can perform in the foreseeable future.
Instead of making no predications about the universe it makes all possible predictions That's also incorrect; see the literature on the string "swampland". However, the range of possible predictions is very broad. That doesn't make it a failure, because it should be properly compared to a framework for building models, such as quantum field theory, rather than to a specific model, such as the Standard Model of particle physics.
String theory can produce plenty of models outside of what any quantum field theory can model. (Or at least, without said QFT being dual to a string theory.) The problem is that there aren't strong reasons to believe that the models which predict observable results are more likely than the ones that predict results that we'll never have the capability to measure.
Allow me to address those two quotes which supposedly render string theory "fundamentally flawed":
"Describing the self-interaction of gravitons consistently turned out to be a tough nut to crack. We now understand that the failure to solve this problem is a consequence of not taking Einstein's principle of background independence seriously. Once the gravitational waves interact with one another, they can no longer be seen as moving on a fixed background. They change the background as they travel."
String theory has a consistent description of the self-interaction of gravitons. Manifest background independence in the equations is not needed; physical background independence of the theory arises dynamically. (There are also manifestly background independent formulations of string theory, but it is not yet known how they are all related to one another.)
"This is because supersymmetry implies that there is a symmetry in time, the upshot being that a supersymmetric theory cannot be built on a spacetime that is evolving in time."
This is false. You can do superstring theory in time-dependent backgrounds. (You can find discussion of this in this thread, particularly in comments by Moshe and Aaron Bergman.) It also is not true that this would mean you can't study cosmology or black holes, either; you can study both even in time-independent backgrounds.
Of course the distance between pulses slows, but that is because the visual "recording" of the event is being stretched out over a longer "beam" of light. No, it's not just that. As I said before, there is also real time slowing, plus the finite speed of light. Relativity is not just Newtonian physics with a finite speed of light tacked on. It makes fundamentally different assumptions about time.
Everything I've read just assumes that time travels at a different rate, and so the math must account for it. Why is this? Einstein originally assumed it for reasons that may sound esoteric (he wanted a mechanics that was compatible with Maxwell's theory of electromagnetism). There are many different sets of assumptions from which it can be derived.
You can get to relativity if you assume that light has a finite speed that is the same according to all observers. That's the usual way in which it is derived.
Has it been proven? There are plenty of experiments which demonstrate time dilation (see here).
No, "QFT" has not predicted anything. Specific models of particle physics have predicted things. You can embed those models in either QFT or string theory, as you prefer. If string theory had been around then, physicists could have proposed the Standard Model within the context of string theory instead of QFT if they had wanted. String theory is not less predictive than QFT; it is a framework for model building (and in a broader and more consistent one than QFT).
Now, I can't claim to be up to date on the pop-physics scene, but do most people writing for it actually subscribe to the Copenhagen Interpretation? This poster summed up most physicists' true attitudes towards the Copenhagen interpretation pretty well.
Whatever you think of its actual merits as a theory, surely it's still firmly in the outer reaches of "scientifically-minded philosophy", rather than accepted scientific dogma? It's not a theory, it's an interpretation of a theory. That's why it's called an "interpretation" and not a "theory". Most would say that it's not a coherent interpretation, either. But you can't experimentally distinguish any of the interpretations of quantum mechanics from each other; if you could, you'd have an alternative to QM, not an interpretation of it.
That's what really bothers me about the theoretical physcis groupthink: it seems they've completely dispensed with the concept of falsifiability Interpretations are not falsifiable, because they're not scientific theories. They don't make predictions, they just provide a philosophical framework for interpreting the predictions of a theory.
String "theory" is of course the poster child for this phenomenon. That's nonsense. String theory is both falsifiable and motivated by physical considerations, most notably the quantization of gravity. See here for a deeper discussion. Physicists didn't just sit around and think up string theory because it sounded cool, they invented it to solve real problems with existing theories
The time dilation is not just an apparent effect due to the finite speed of light, it is also a real effect of time slowing down. Light not only shows up late, it comes slower. i.e., pulses of light sent at time 1, 2, 3,... don't arrive (according to B) at times 2, 3, 4, but more like 2, 4, 6,...
You'll probably have to just sit down and learn enough special relativity to calculate what happens. To get the flavor of things, though, you can read this FAQ. I find the spacetime diagram explanation to be the clearest.
Unless he is also proposing that the daughter universes shared some characteristics of their parents, natural selection is a nonsense term. Actually, he does propose that. But natural selection is still an inappropriate term for his theory, because there is no selection. Or at least, not universally. There is reproduction with mutation, but universes don't have to die. Many universes are eternal, and even the ones that succumb to Big Crunches might lead to new universes in the same way that black holes do. So the "metaverse" just becomes populated with the universes that reproduce the most, rather than the ones that "survive" some "fitness test".
I'm not so sure about the GR part, since the spacetime background is assumed to be static. The background spacetime in string theory is unobservable; it receives dynamical corrections from string interactions which render it observationally equivalent to GR (+ high energy corrections).
One of Smolin's big points in favor of loop quantum gravity is formulated in way that's manifestly consistent with the basic principles of GR (background independence), whereas string theory isn't. That's a nice philosophical goal, but it's not a physical objection. Furthermore, string theory does have background-independent formulations (via matrix compactification, AdS/CFT and various dualities, etc.)
Also, it's now been shown that an asymptotically flat spacetime is a solution to loop quantum gravity. I don't regard that to be as formally well established as Smolin does, especially since in the presence of the Hamiltonian constraint problems, it's not clear what the LQG equations actually are. But even so, LQG still hasn't been shown to have GR as its classical limit.
The electrons are believed to be in discrete orbits, so the photo must be a discrete packet. Bollocks. You're right, that is a bogus justification of the quantization of light. It only demonstrates the quantization of electronic orbits. You are, however, wrong that quantization of light is a mistake. The quantization of light can be justified, but you have to be much more subtle in your arguments — the theory of stochastic electromagnetism got pretty far using classical light + quantum matter before it ran into insurmountable obstacles. I think Milonni has a text which details the evidence you need to justify true quantum electrodynamics.
Slashdot Mirror
Recall, too, that you claimed that QFT has made "predictions" that string theory has not. This, too, is disingenuous. All of the QFTs so far studied can be embedded in string theory and are indistinguishable from it as far as experiments that have been performed are concerned. Thus, the data support QFT to no greater extent than they do string theory (and the string theories all have the advantage that they're consistent with gravity, to boot, unlike the QFTs). There are some QFTs which are not compatible with string theory's low energy limit, but those haven't made any successful predictions, either. You speak of infinities as if they are all the same size. They are the same size: uncountably infinite. That being said, string theory certainly encompasses models beyond what any QFT can describe. That, too, is not an indictment against string theory; in fact, it's one of the main reasons for string theory's popularity. But gravitons are mere perturbations of certain metrics in classical gravity. They are not analogous to the photons of E&M. That is not the same thing as having full consistency with gravity as so many string theorists boast. String theory goes far beyond mere gravitons. If you construct a theory of gravitons alone, all you get is a linearized theory of gravity (wrong) which is non-renormalizable (useless). Perturbative string theory introduces an infinite hierarchy of corrections that not only renders the theory consistent and finite, but recovers full nonlinear gravity (plus string-scale higher-curvature corrections). And if you don't like perturbation theory, AdS/CFT duality is a full non-perturbative and background independent description of some gravitational sectors of string theory, although it has not been extended to describe all sectors.
Incidentally, the original motivations for string theory in the strong nuclear interaction may yet pan out; the initial picture of flux lines as strings still has merit, and there is a small industry of string theorists working on that idea. QCD took over because of its successes at weak coupling when the theory is near asymptotically free. But it has bogged down at strong coupling, and numerical approaches on the lattice have been necessary. The AdS/QCD string approach has proven capable of addressing the strong coupling limit as a weakly coupled string theory for some supersymmetric QCD theories, and may be useful for actual non-supersymmetric QCD at strong coupling as well. That's just the problem. It's far too predictive and broad. It's exponentially predictive and broad. It is not really less so than QFT, which was my original point. String theory can construct models that QFT can't, but even in QFT you can construct infinitely many models consistent with known physics (and infinitely many more that aren't). And every time an experimentalist looks for something stringy and finds nothing, the string theorists simply readjust their parameters and make sure they have escaped the clutches of known reality. Again, this is no different from QFT. If you falsify one QFT, there are infinitely many more to take its place. That is, in fact, how particle physics has been done for the last 80 years ago: you construct models that make predictions beyond what is known, and when they get ruled out, you construct more until some work. That process can be within the frameworks of QFT or string theory.
You still persist in comparing string theory to a specific model like the Standard Model, instead of more properly to a framework for writing down models, like QFT. And all the string models at least have the benefit of being automatically consistent with gravity at all levels, unlike any of the QFTs, even in the string models which are indistinguishable from the QFTs at low energies.
There are string theory models that are falsifiable right now. Some of them have, in fact, been falsified. Some may be falsified in the very near future. We don't know, however, whether it will be falsifiable in the foreseeable future, because we don't know whether any of the "easy to falsify" models are true.
Smolin says that all the "universes" are connected to each other via black hole/Big Bang singularities. It's not really possible to compare times in different universes to say which ones exist "simultaneous" to ours, but it is possible to say that the one that gave rise to our univese came "before", and so on for its ancestor, defining a topological order. Things get a little tricky when classical spacetime presumably breaks down at the birth singularities, though.
In physics, the word "theory" is not used in the 8th-grade "it's a hypothesis or a theory" way; a "theory" instead refers to a set of field equations governing a phenomenon. That is, its empirical status is not relevant to whether it is called a "theory". In that sense, string theory is a "theory".
The point is rather that nothing can affect a universe's reproductive ability (either by "killing" it or altering how many descendents it can have), so no external selective pressures can be exerted on it. The number of times a universe can reproduce depends purely on its "genome", and not on any selection that the environment may impose. This isn't what is meant by "natural selection" in the biological context.
As I mentioned in the previous post, it's not clear that a Big Crunch is actually "death" for a universe, since both black holes and Big Crunches ultimately result in singularities. (Or, you could say, a Big Crunch may result in exactly one birth for every death.) I suppose an eternal universe could be thought of as "dead" in a reproductive context, if no new black holes form.
You can read more about this point here.
Incidentally, no astrophysical sized black hole is currently evaporating, because the cosmic microwave background radiation is currently hotter than any such black hole's Hawking temperature. Thus, it gains more mass by absorbing the CMBR than it loses by Hawking radiation. It won't shrink from Hawking radiation until the CMBR cools below its Hawking temperature, which won't happen for ~10^66 years for a solar mass black hole (see here. If very light holes were created in the Big Bang, they could be still around and shrinking at the moment, though.
So far, nothing new. But if you postulate that light traveled at the same speed according to both observers, then to the ground observer the light took a greater amount of time to reach the ceiling (greater distance at the same speed means more travel time). In an extreme case, the light could take much longer to reach the ceiling, according to the ground observer, than you in the room measure.
Curved spacetime is a whole different ball of wax
String theory can produce plenty of models outside of what any quantum field theory can model. (Or at least, without said QFT being dual to a string theory.) The problem is that there aren't strong reasons to believe that the models which predict observable results are more likely than the ones that predict results that we'll never have the capability to measure.
Allow me to address those two quotes which supposedly render string theory "fundamentally flawed":
"Describing the self-interaction of gravitons consistently turned out to be a tough nut to crack. We now understand that the failure to solve this problem is a consequence of not taking Einstein's principle of background independence seriously. Once the gravitational waves interact with one another, they can no longer be seen as moving on a fixed background. They change the background as they travel."
String theory has a consistent description of the self-interaction of gravitons. Manifest background independence in the equations is not needed; physical background independence of the theory arises dynamically. (There are also manifestly background independent formulations of string theory, but it is not yet known how they are all related to one another.)
"This is because supersymmetry implies that there is a symmetry in time, the upshot being that a supersymmetric theory cannot be built on a spacetime that is evolving in time."
This is false. You can do superstring theory in time-dependent backgrounds. (You can find discussion of this in this thread, particularly in comments by Moshe and Aaron Bergman.) It also is not true that this would mean you can't study cosmology or black holes, either; you can study both even in time-independent backgrounds.
You can get to relativity if you assume that light has a finite speed that is the same according to all observers. That's the usual way in which it is derived. Has it been proven? There are plenty of experiments which demonstrate time dilation (see here).
No, he means a Higgs boson. The Higgs is what is responsible for a particle's mass, not the graviton.
MOND has not been "replaced" by TeVeS; TeVeS is merely one proposal for constructing a MOND theory that is consistent with relativity.
No, "QFT" has not predicted anything. Specific models of particle physics have predicted things. You can embed those models in either QFT or string theory, as you prefer. If string theory had been around then, physicists could have proposed the Standard Model within the context of string theory instead of QFT if they had wanted. String theory is not less predictive than QFT; it is a framework for model building (and in a broader and more consistent one than QFT).
The time dilation is not just an apparent effect due to the finite speed of light, it is also a real effect of time slowing down. Light not only shows up late, it comes slower. i.e., pulses of light sent at time 1, 2, 3, ... don't arrive (according to B) at times 2, 3, 4, but more like 2, 4, 6, ...
You'll probably have to just sit down and learn enough special relativity to calculate what happens. To get the flavor of things, though, you can read this FAQ. I find the spacetime diagram explanation to be the clearest.