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Merged Puzzling results from CERN

Vorpal said:
If you're just having light in a vacuum, essentially all of the quantum nonconformity to common sense is the fact that the wave is that of probability describing particle. The all-paths summation for some weighting is already how classical waves behave. For light, this was noticed very early on in approximate way Huygens, as ben m said, and more precisely as a certain restatement of the Kirchhoff diffraction integral of classical EM.

This applies just as well to plain-vanilla, nonrelativistic QM. In the eikonal approximation of wave propagation used as an intermediary in deriving geometric (ray, short-wavelength) optics from wave optics, one only has to know about the de Broglie relations to get the standard Schrödinger equation, which also has an equivalent path-integral formulation. All the 'magic' is really in the idea that waves and particles have anything to do with each other (specifically here, de Broglie), not the path-summing. And though that de Broglie connection can be motivated to a large extent by the near-identical mathematical behavior of geometric optics and classical Hamiltonian mechanics, it's still pretty crazy from a common-sense point of view.
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Vorpal- did you ever see the Far side cartoon that shows what a dog hears when humans talk to it?
Something like this- " *&^%££$%$£^&()*&)*)(^*(&^%&$%&%££&£^%$%%R(&^(*&_)(_)(*_ Rover!"

While I really appreciate how hard people like yourself, Sol and others try to communicate complex concepts non mathematically, I find myself much in Rover's position, even with such verbal explanations. I know (to a certain tolerance) what each word means, but I still don't really understand what the sentences mean.:blush:
I persist in banging my head on something too hard for me. Occasionally I think I get a glimpse.
 
It is simple, Soapy Sam. It's like I was saying about Einstein, and that sand. Here's a modern paper on it: Inhomogeneous Vacuum: An Alternative Interpretation of Curved Spacetime. And here's a "fair use" excerpt from it:

"The refractive index of vacuum, as a special optical medium, may be changed under the influence of gravitational matter. In fact, there has been a long history of such an idea. In 1920, Eddington[26] suggested that the light deflection in the solar gravitational field can be conceived as a refraction effect of the space (actually the vacuum) in a flat spacetime. The idea was further studied by Wilson,[27] Dicke,[28] Felice,[29] and Nandi et al.[30 32] Recently, this thought of vacuum has been investigated further by Puthoff [13;14] and Vlokh.[33] In Puthoff's paper, the influence of gravitational field on the vacuum refractive index is analysed through the vacuum polarization. Vlokh discussed the value of this refractive index. In our recent paper [15] we analysed some simple cases of gravitational lensing by using an approximated expression of the refractive index for the vacuum outside the gravitational matter system. In this Letter, we emphasize the strong similarities between the light propagation in a curved spacetime and that in a medium with graded refractive index. These similarities suggest that an inhomogeneous vacuum may be the physical reality of the curved spacetime".

Read up on Huygens and refraction. They don't call it gravitational lensing for nothing.

Edit:

Soapy Sam said:
One of the (very many) things that baffles me is that in QM it seems to be considered reasonable to say a photon follows every possible path to get from point 1 to point 2...
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Just think of it in terms of a seismic wave rather than some billiard-ball particle.
 
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Nothing's ever simple.
:)

That paper costs 20 quid for example.
 
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Soapy Sam said:
One such was certainly "The Fabric of Reality" by David Deutsch. I've just been looking in my copy for the relevant passage, but no luck yet. I expressed myself poorly in the previous post- I don't mean anyone was saying the classical explanation is invalid, but that any classical explanation should also be explicable in quantum terms. I think most popularisations involving double slit experiments and explanations thereof stress that the quantum view of how photons move is real, despite it's apparent incompatibility with common sense. So I must suppose this applies as much to light crossing between galaxies as light moving through an experimental setup in a lab. It's also very likely I simply misunderstood what was meant.
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No, you didn't misunderstand - everything you said in that paragraph is correct. It's just that it's not really necessary to get at the thing you asked about.

What Vorpal was saying is that light, being a wave, behaves in much the same way as everything does in quantum mechanics, because QM is a theory of waves. For instance, if you shine a flashlight on two slits in a wall, you'll get an interference pattern on the other side, and you shouldn't be surprised by that.

Of course it's not the case that quantum light - namely photons - behave in all respects like classical light. For instance if you turn down your "flashlight" enough, the detectors behind the wall will only register (say) one "click" per second. Those are photons, and the fact that they come in discrete (i.e "quantized") bunches is weird and inconsistent with classical physics. If you make a histogram of the positions of all those clicks, you'll still get an interference pattern. That's even weirder.

But that second weirdness is only important because of the first weirdness. If you forget about the first, you're back to classical physics, and that's just fine for understanding how fast things propagate.
 
Soapy Sam said:
Nothing's ever simple.
:)

That paper costs 20 quid for example.
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You can probably get it for free at arxiv.org.

But while I would never recommend not reading something, that looks to have little to do with what you asked and (based on the abstract) it's probably either wrong or trivially equivalent to general relativity. Moreover, "alternatives" aren't the place to start when you haven't yet understood the thing they're the alternative to.

EDIT - I looked at it (just google the title, you can get it free). It's trivial, it literally does nothing but equate one formula to another. Not only that, at least after two minutes of consideration it looks simply wrong. Equation (7) means that 4 pi r^2 is not the area of the sphere at distance r from the center of the lens. That means that equation (5) is very dubious and most likely wrong, but eq. (5) is what their analysis is based on.

Of course in the weak-field limit this problem may go away, but lensing is observed in the strong field regime, and they don't say that their result only works in weak field.
 
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sol invictus said:
EDIT - I looked at it (just google the title, you can get it free). It's trivial, it literally does nothing but equate one formula to another. Not only that, at least after two minutes of consideration it looks simply wrong. Equation (7) means that 4 pi r^2 is not the area of the sphere at distance r from the center of the lens. That means that equation (5) is very dubious and most likely wrong, but eq. (5) is what their analysis is based on.
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This relates to Einstein's disk. See The Rigid Rotating Disk in Relativity by Michael Weiss:

"Einstein's 1916 paper on GR [5] makes no mention of elevators; instead, the Equivalence Principle is introduced via the rotating disk. Einstein reproduces Ehrenfest's argument, but with a different conclusion: since we are no longer assuming flat Minkowski space, Einstein asserts that geometry for the rigid rotating disk is noneuclidean. The Equivalence Principle now implies that geometry in a gravitational field will also be noneuclidean...

Surely you know about this?
 
Farsight said:
This relates to Einstein's disk. See The Rigid Rotating Disk in Relativity by Michael Weiss:

"Einstein's 1916 paper on GR [5] makes no mention of elevators; instead, the Equivalence Principle is introduced via the rotating disk. Einstein reproduces Ehrenfest's argument, but with a different conclusion: since we are no longer assuming flat Minkowski space, Einstein asserts that geometry for the rigid rotating disk is noneuclidean. The Equivalence Principle now implies that geometry in a gravitational field will also be noneuclidean...

Surely you know about this?
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Of course.

And indeed, that is related my point - you cannot simply treat gravitational lensing as if space were a material with a variable index of refraction. What they've done in that paper is invalid. The result might be valid in some sense (and it's probably fine in the weak field limit), but the method clearly isn't.
 
If we're looking at light deflection and don't want a weak field, we should be able to make it work for an asymptotically flat static spacetime, at least if the refractive index is also anisotropic (which is more than they apparently consider in the paper and removes the utility of doing so), but no more than that. But what's the relevance here? The title suggests something much more general:
Inhomogeneous Vacuum: An Alternative Interpretation of Curved Spacetime​
If that's really what they're after, their approach is dead from the get-go, because no amount of messing about with refractive indices is going to reproduce gravitational redshift.

Spacetime is curved not because light bends, but because of gravitational redshift. If you have two observers A,B at different elevations in a static, spherically symmetric gravitational field, say A sends a radio signal of fixed frequency to B, immediately followed by an identical signal. Because of redshift, the proper time of A to send the signal (AA') is different from the proper time of B to receive it (BB'). Hence spacetime is curved.
Code: 
      B        B'    A start of 1st signal, A' start of 2nd.
      *-------*      No matter how the signal bends about,
    _/      _/       if spacetime is flat and the situation
  _/      _/ ^space  is static, then AB is congruent to A'B',
 /       /   |       so AA' must be congruent to BB'.  
*-------*    +-->time     But experimentally, it isn't.
A       A'
Making an flat-spacetime alternative to curvature that predicts the right redshifts would be much more impressive. Or at least conformally flat, for that matter. There's really no way to do it without also making the flatness unobservable.
 
sol invictus said:
Light in vacuum (and anything else that's freely moving) follows the shortest path through a curved spacetime. The shortest path is what you'd usually call a straight line, with no acceleration. Since the spacetime is curved, the path looks curved, but it really is the shortest path.
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I do not understand the concept of shortest path because it sounds as if light could choose other paths to reach the same destination. Is it not so that light does not know its destination, but merely goes "straight out", even though "straight out" may actually be curved, as in curved space time, and the light hits whatever blocks it along that path?
 
steenkh said:
I do not understand the concept of shortest path because it sounds as if light could choose other paths to reach the same destination. Is it not so that light does not know its destination, but merely goes "straight out", even though "straight out" may actually be curved, as in curved space time, and the light hits whatever blocks it along that path?
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You're right that they are conceptually different things, but physically they are equivalent.

What does it mean for a curve to go "straight out"? A natural interpretation is that it keeps on going in the same direction. But that means we need to be able to compare directions (tangent vectors) at different point of the curve. The mathematical contraption that does this for a curve is called the 'connection', and except for some consistency criteria, it's arbitrary. You have lots of freedom to define which curves "go straight out". There's no mathematical reason for those curves to be the same as those that extremize length, but it turns out that there is a unique choice of connection which precisely matches them, and we can literally define a completely local "straightness" that exactly reproduces length-extremizing curves.

It also makes many, many things very simple. And physically, we can look to see if spacetime mismatches those two concepts of straightness... and don't detect any difference. (Btw, the jargon is 'affine geodesics' vs 'metric geodesics'.)
 
Another way to think of it is to remember your definitions from geometry. A line is the shortest distance between two points, right? Therefore, if light always travels in a straight line, then the route it took must, by definition, be the shortest distance between where it originated and where it ended up. No one is saying that photons start out, pull out the map, and plot a course. It's just that wherever you happen to detect a photon, if you can back-track it's path, it took the shortest route to your detector from it's source.

Hopefully that makes it a bit clearer for us laymen :)
 
Thanks Vorpal and Hellbound.

Math was always my weak point, and Vorpal's explanation was the hardest to understand because of my lack of knowledge, but it was the logically most satisfying, because I think it can be used to predict where a photon might end, whereas the back-tracking method can only be used to determine what path the photon took.
 
Mikemcc said:
Some initial results from the re-runs have been released:

http://news.sciencemag.org/scienceinsider/2011/11/faster-than-light-neutrinos-opera.html

There's still some issues, but it is increasingly looking like a 'oh s***!' moment in experimental physics! :D
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Not really yet.

"A major concern among the dissenters is the fact that the "time window" within which neutrinos were detected by OPERA in the most recent run had a width of 50 nanoseconds, something that the leader of the superluminal analysis, Dario Autiero, only revealed once the tests had been carried out."
 
Dancing David said:
"A major concern among the dissenters is the fact that the "time window" within which neutrinos were detected by OPERA in the most recent run had a width of 50 nanoseconds, something that the leader of the superluminal analysis, Dario Autiero, only revealed once the tests had been carried out."
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What does a time window of 50 ns mean? Did the 2 ns pulse of neutrinos arrive spread out to 50 ns? If so, I believe that there is a problem because it looks as if each neutrino had its own individual path or were subjected to forces that the other neutrinos might not experience. Was that your reservation too?
 
I did a little research about c .. and I think we generally have 3 types of c.

1) defined c. Today meter is derived by light, and so is time. So c is defined to be 299,792,458 m/s EXACTLY.
2) measured c. Here I include every direct method of measuring speed of light in vacuum. Here we have several values with different errors.
3) derived c. It means we measure different quantities, and derive c from it. Again there are several values with different errors.

What I'd like to see is comparison of all these values with OPERA data. I failed to find all of the values in same units (especially true for errors), and I'm not sure I can convert them correctly.

What if light really does not move at full c in vacuum ? Ie what if c(2) is not equal to c(3) ? Maybe the errors in measurement allow for that.
 
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