Discussion in 'Physics & Math' started by stateofmind, Feb 12, 2015.
Don't you think he'd be able to figure some of this stuff out by now?
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What would be stretched or compressed due to gravitational lensing? Do emitted photons only exist when we observe them or would they be expected to be where they are at any time whether we were there at the same time to observe them there or not?
The photon paths below exist in real time between the sources and the observer as long as the sources continue to rotate and emit during one complete rotation and the photons are not distorted or blocked.
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The distance between the rotating sources 1,0 and 3,0 and the observer (1,4 3,4) is 2 * Pi * R * c/v for all rotational velocities up to c. At c the distance/time of one complete rotation is 2 * Pi * R where the radius of rotation R is measured in light years. Path C, where the observer is 0 degrees to the plane of rotation, is the standard SR based relativistic rolling wheel solution which just uses x, y and time with z = 0 as the observer is within the plane of rotation. Path C has the most shift of all the lines while Path A, where the observer is 90 degrees to the plane of rotation, has very little shift.
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I don't think it would be falsifiable, if that's what you mean. Choosing a preferred frame allows us to make definitions but doesn't really affect anything. I don't normally have a problem with a harmless personal contribution to a theory (such as a preferred frame in SR), but in this case Farsight is doing it to preserve some sacred physical manifestation of "now" which seems a bit ad hoc to me.
Another issue here is that, while this would be permissible in a pure SR world, there is no such thing as a CMB rest frame in the real world. The remote CMB is accelerating away from us.
OK, my initial reaction was that the CMB local rest frame could not be used as a universal frame of simultaneity because of cosmic inflation (i.e. distant observers would disagree on the calculations)...but now I'm not completely convinced of that. Although there is "apparent coordinate acceleration", it's uniform in all directions. If we start at the Big Bang with all points colocated, and synchronize our clocks there, it seems to me that there might be a single "plane of simultaneity" for a given ticking of those clocks moving into the future.
Does anyone have input on this?
That was kind of my take on it, but you bring up an interesting subtlety that I hadn't considered. If the "universal now" clocks are defined such that they see an isotropic CMB everywhere, then they do not define a reference frame. Rather, they define (as you put it) a plane of simultaneity that cuts through many different reference frames that are accelerating away from one another. An odd side effect of this is that it would mean the laws of physics as we know them are, in fact, mostly wrong. Makes the whole thing seem even more ad hoc than it already did.
A good thing to read about is the cosmological metric. The cosmological clocks are comoving observers. Local proper comoving frames. So we measure the beginning to now one tick at a time. That's the time coordinate.
You seem to be slipping partially into the mistaken view of BB = 3D explosion from a point in a preexisting flat vacuum. Remember that, sans local inhomogeneities, in relativistic cosmology everywhere seems to be at the center of a uniformly expanding and redshifting cosmos. it follows there can be no surface of simultaneity except as local approximation. This is a manifestation of the notorious 'parallel transport' problem in GR. And btw any thoughts on my post #124?
The laws of physics work fine once redshift is allowed for. Sans a disputed tiny 'dipole anisotropy', fine structure constant for instance shows no sign of variation looking back as far as can be done. And btw, do you have any further thoughts on my post #125 re viability of TI?
Wow, I've just seen that ''People you ignore'' thingy. I have now two on ignore and already it seems like a different forum.
Thanks for the link! It's higher-level than my own experience with GR, but I think I was still able to get a lot from it. In particular, if I'm reading it right, section 4.9.1 says the comoving distance (i.e. the distance along a spatial hypersurface at constant t) between two spacetime events is a unique quantity. That must mean that the spatial hypersurface intersecting both events is also unique, which in turn means that defining the "real" time at any two points in space would fix the time at all points in space. Is that accurate?
This confuses me. I realize that the expansion of the universe looks uniform everywhere, but I don't see how that prevents one from defining a surface of simultaneity. Surely one can pick some set of coordinates that span the whole universe, then use the constant-t hypersurfaces in those coordinates as surfaces of simultaneity. Or did you mean that there are no unique surfaces of simultaneity, since many different coordinate choices can be used for this procedure? That I would agree with (and I'm pretty sure RJBeery would too), and it's what makes Farsight's idea seem so ad hoc.
Again, I'm not sure I follow. One thing I know about GR is that in the coordinates defined by any given reference frame, the same laws of physics hold. I'm also pretty sure that because our universe is expanding, any two inertial clocks will disagree about which is right; an observer at rest w.r.t. clock A will see clock B as slow, and vice-versa. (Correct me if I'm wrong here.) To give the universe a "real time", in which both clocks A and B are correct, we need to pick a set of coordinates that do not correspond to any reference frame. I have to assume that in such coordinates, the laws of physics as we know them stop being correct.
As for post 125, I'm glad you brought the point up, and I'm sorry I let it slip my mind. I agree that it would be too contrived to assume future absorbers change their "brightness" to compensate for decreasing density. Instead, I assume that one of two things would have to be true for transactionalism to remain viable:
1. The currently derived form of time-symmetric quantum field theory, in which every emitter has an absorber, is a useful but incomplete piece of the whole theory. A more general form will include cases where one or both of the interacting particles is replaced by the universe's boundary conditions, e.g. photons that come from the big bang or are lost to endless expansion. Or,
2. Near the boundary of the matter-filled universe and/or near the critical point in time where the universe's expansion speed exceeds the speed of light, electrons' ability to emit light decreases for lack of absorbers. This would mean that the transactional interpretation actually predicts changes to the laws of electromagnetism, but we are currently far enough from the "edges" of the universe that we wouldn't notice them.
Sorry to be tardy, I'm away from home at the moment.
Yes, everything is deterministic. There is no magic, no mystery, no mysticism. The light wave is one wavefunction, the electron is another, the result of their interaction is something pointlike.
A light cone is an abstract thing. It doesn't exist. Light moves, we make electrons out of light, electrons spin.
I dispute that. I do not hold with instantaneous spooky action at a distance. Can you send information faster than light? No.
No it isn't. It's explaining what actually happens in the dual slit experiment without resorting to some unscientific multiverse. Would you dismiss the logical explanation in favour of many worlds that can never be proven?
I have no issue with that.
Or that. I was trying to give the simplest situation rather than make light of complexity.
Have a read of the Wikipedia article on quantum decoherence. It says things like decoherence applies when wavefunction becomes entangled with the wavefunction of the measuring device. But IMHO entanglement is "quantum mysticism".
It's like Jeff Lundeen said. You don't disturb it enough to destroy it. I think the seismic wave analogy is pretty good. We are dealing with waves here, not point particles and probability.
I reject the insinuation that understanding the world is mere philosophy. We do physics to understand the world, not to predict experimental outcomes.
I say it's definite and deterministic. A gamma-gamma photon interaction will result in pair production.
If Einstein was still around, I am absolutely confident that he would have no issue with the CMBR reference frame. I imagine he'd say something like "relativity is presented from the local viewpoint, but the God's eye global viewpoint is equally valid".
It's tosh, Fednis. We make electrons out of light. We can diffract them. Then we can do Compton scattering such that an electron absorbs part of a photon, and we can do something similar with a bound electron. Then we can do the inverse. To suggest that these emission and absorption events must be linked across 13.8 billion light years of expanding space is an unscientific fantasy for which there is no evidence whatsover.
The relativistic definition of a surface of simultaneity relies on a clock synchronization scheme that can only hold in an inertial reference frame:
On that score my distinction made in #167 was somewhat misleading as it implied a surface of simultaneity might be possible for 'tagged' gas molecules in an expanding gas cloud in Minkowski spacetime. Not so - what I meant to convey there was that in that scenario one could 'triangulate' to a unique center and use such as an 'absolute' reference - not possible in the cosmological case.
It is certainly possible to come up with an ad hoc definition of surface of simultaneity - for instance such that all clocks on a receding shell centered about you have 'the same time'. However such clocks will not agree amongst themselves as to 'the current time'. I think what you are getting at is that one can devise a formula that corrects for all such relative motions - relative to a given observer. Probably so - but such corrections imply the chosen hypersurface does not comply with the standard notion of simultaneity.
If local enough to qualify as inertial then sure Newton's laws hold as originally framed. Otherwise 'fictitious forces' enter but I do not consider that a breakdown of the guts of Newtonian physics which reduces to a locally defined F = ma.
Agreed about relative motion screwing up simultaneity - as per above discussion. As you know if the physics is cast in covariant tensor form then such physics locally holds across all reference frames. One likely majority but not universally agreed property of FLRW cosmology is the failure of global energy-momentum conservation. So in that limited sense there is failure of an otherwise considered sacrosanct principle.
That option imo is an ad hoc position that destroys the fundamental physics claim implied in TI. So as stated in an earlier post, either it always works, or it's dead. Not merely an interpretation of QM but a falsifiable variant theory.
Notwithstanding inflationary epoch and current dark energy accelerated expansion, in GR cosmology there always was and continues to be a causal edge i.e. horizon beyond which no communication is possible. Implying 'lost photons' applies at every epoch. So imo option 2 is also falsified based on the simple laser pointer test given in that other forum thread. No detectable change in output intensity, whereas by any reasonable estimate of future cosmic transparency, a dramatic variation should have shown up - pointer aimed skyward vs e.g. darkish ground. Also, if true the expectation would surely be dramatic differences in ionization energies hence spectral lines as a time evolving function of particularly opaque-to-transparent transition in early universe. Again - constancy of fine structure constant seems to rule that out.
This was written as a response to a comment on optical fourier transforms, so I'll treat it as such. There's no argument that optical fourier transforms can be explained without "mysticism", or indeed, without quantum mechanics of any kind. Some other experiments are more troublesome, but we'll get to them in a bit.
Sure, the light cone is abstract. When I say "the entire output distribution must be in the future light cone of the lens that produced it," that's shorthand for "if the output distribution is measured at a time t after the light interacts with the lens, then the entirety of said distribution must be within a distance ct of the lens." This restriction becomes important in quantum entanglement, where entangled pairs can be much further apart.
Explaining the dual slit experiment is definitely NOT the big challenge for an interpretation of quantum mechanics. If it were, people would still be considering EPR-style local hidden variable theories. Have you heard of Bell's inequality? If you have, then you know any interpretation of quantum mechanics must be either non-deterministic or have non-local hidden variables. You've explicitly said you think QM is deterministic, so what is your non-local hidden variable? Saying it's "something like an optical Fourier transform" is not an answer.
You don't believe in entanglement. Alright, then. Consider a pair of photons, produced by parametric down-conversion, such that conservation of angular momentum guarantees they have orthogonal polarizations. If we send them off to two observers, too far away to communicate at sub-luminal speeds, the observers will always measure orthogonal polarizations in any basis (linear, circular, or anywhere in between). The statistical anti-correlations between the measurements are too strong to be explained by any hidden variable that was set before the measurement basis was chosen. What is this if not entanglement, and how do you interpret it?
I'll agree with that. I just wanted to make sure you weren't talking about measuring just part of the wavefunction without disturbing the rest of it, or otherwise gaining more total information about the wavefunction than a strong measurement would allow.
That's a whole other can of worms - in the interest of keeping this thread on track, I'm not touching it.
That does make sense. I've now gotten myself a bit confused about the CMBR reference frame, but those questions can wait for a subsequent post.
No interpretation has any more or less evidence for it than any other - that's what makes it an interpretation rather than an alternate theory. I just find transactionalism compelling because it's the only interpretation I know of that allows for an observer-independent universe without contrived restrictions on FTL signalling. As for your own model (not interpretation, because you've yet to demonstrate that it makes the same predictions as quantum mechanics), I'm happy to pick up discussion on the Alternate Theories thread whenever you like.
Thanks for clearing that up. I was talking about some time-fixing hypersurface that is apparently not a "surface of simultaneity" by the conventional definition, and I understand why the conventional kind would be impossible to define for the whole universe.
That's interesting - I didn't know that energy-momentum conservation could fall apart in FLRW cosmology. As a rough intuition, would it be correct to think of this in terms of the expanding universe redshifting energy out of photons as they travel about? Either way, it makes sense that local invariance of the laws of physics should be the important thing in GR, and a CMBR preferred reference frame wouldn't violate that.
I do have one question about the CMBR frame, though. Since observers who see an isotropic CMBR will be moving apart in an expanding universe, it seems like they must be in different reference frames. So why do people talk about the "CMBR rest frame" as though it was a single reference frame? Am I missing something, or is the GR concept of a reference frame more flexible than in SR to deal with the simultaneity problems you mentioned?
I was contemplating possible responses to this argument when I realized that the transactional interpretation is probably already of the form I described in option 1. According to the link I posted above, the transactional interpretation has been generalized to the point that it covers all of quantum field theory. In quantum field theory, every field has a corresponding particle, and vice-versa. Interactions between things are described in terms of fields coupling to each other in various ways. When looking at time-symmetric electrodynamics, it makes sense to treat electrons as the emitters/absorbers and photons as the signal carriers, but in broader field theory such a distinction is unwarranted. Instead, any one particle should be able to act as emitter, absorber, or signal for any other particle it couples to. Saying that photons (or any other particles) can eventually fly off into the void without being absorbed is then equivalent to saying that the universe has boundary conditions, which seems like a pretty reasonable assertion.
Really good point. I guess option 2 doesn't work.
Why would he since it's just another choice of coordinates. We choose the coordinates which are most useful for doing the physics. That's how some coordinates are preferred, because of usefulness.
Yes that's the likely majority viewpoint. Throw in dark energy though and overall for current and future epoch there is an implied net gain with time. But it more broadly ties in with the non-uniqueness of defining a total energy in a non-stationary spacetime. Not everyone agrees and there are two camps - total energy either not conserved or inherently ill-defined, and the 'zero energy universe' crowd who maintain one can sensibly have an exact cancellation between positive energy matter + radiation (plus dark energy), and negative energy gravitation. Sean Carroll an example of the former, Philip Gibbs the latter camp. You might think there would be unanimity on such a topic but no! I'm still undecided.
Given what we have already covered, it amounts to no more than an often unstated ad hoc 'earth centric' viewpoint. A dipole anisotropy in CMBR enables us to determine our 'peculiar velocity' wrt local Hubble flow, but otherwise the notion of a universal reference frame is foreign to modern cosmology.
Thanks for that interesting angle on QFT which adds something to my extremely meager understanding of that field. You may be right about universe boundary condition being a natural component of TSQFT. I would need to do a lot more study of the subtleties and implications.
Now, what was the OP topic again? Ahh yes.....Please Register or Log in to view the hidden image!
Thanks for clearing up my questions on the CMBR frame, Q-reeus! Your answers have been very informative. And I'm glad to help with your understanding of quantum field theory, although I'm not exactly a leading expert myself. As for the OP, I don't think we've gone completely off the rails; one important factor in determining whether many-worlds is a reasonable interpretation is deciding whether there are any more reasonable alternatives. And the discussion of GR is on topic because... uh... concept integration! Yeah! Everything is connected, which is what makes science so great. Please Register or Log in to view the hidden image!
I haven't proposed my own model. The optical Fourier transform is a merely a demonstration of a simple mundane dual-slit explanation that doesn't require a multiverse or mysticism. If you think that deserves to be in alternative theories, whilst time-travelling communication across 13.8 billion years is "compelling", you just aren't being scientific. Sorry, but that's the size of it. And apologies, I can't stop. Hopefully I'll be home tomorrow, and will respond properly then.
NB: the zero-energy universe is garbage. Gravitational field energy is positive. A gravitational field will convert potential energy into kinetic energy, but conservation of energy applies. You do not end up with less energy than you started off with.
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