• [deleted]

  • Post #33

Giovanna

I would agree with your questioning of spacetime as a valid model of physical reality (and indeed with the basis of QM but that is a different story), but on more fundamental grounds. Neither space nor time physically exist!

Something exists, not space. Space is a way of conceptualising the physical presence of something (ie dimension, size, shape). In effect we are conceiving of a matrix of spatial points and then defining the relative occupancy of any given something being considered. Another way of putting this is that nothing cannot physically exist, only something can.

Time cannot be an attribute of physical reality, because it (or any sequence thereof) can only occur in one physically existent state at a time. In other words, time (or more precisely timing) is an extrinsic measuring system which differentiates the rate at which physical change occurs (ie from one reality to another).

Paul

  • [deleted]

  • Post #34

Dear Giovanni Amelino-Camelia

Your well-written paper, I think, supports the Leibniz view of space i.e. "Space is determined from the things it contains" and that space-time is a web of relations among things. I wander what will happen if this relation may not be uniquely determined and depends upon the things. If this is the case, the fabric of cosmos arena may consist of various emergent space-time patches of different sizes and properties, like black Holes and tunneling space. Maybe dark matter and dark energy reside in such spaces we do not have access so far (like the previous aforesaid two spaces). In some cases the emergent of a certain space-time patch entails the existence of another, e.g. (i) the elementary particles world entails the existence of the tunneling space, (ii)The grin of Cheshire cat and the feline itself abide obviously in different spaces, but the one space entails the other.

There are so to speak two intertwined conjugate spaces and maybe we can infer the laws and the properties of one from the other.

In a picture of the universe like this, what will be the role of the vacuum? And if "we do not have the luxury of referring to some objective space-time structure "as you mentioned in your paper, do we have it for the vacuum?

Best regards

Basileios Grispos

  • [deleted]

  • Post #35

I saw Giovanni in this documentary yesterday. He was riding his motorbike :) .

dear Saibal

I had not seen the documentary until this evening, since I am at a workshop in Budapest

I was just told I could see it on youtube, so now I know what you meant

I see,,,the BBC used a lot of the motorbike footage,,,I can imagine worse things they could have done,,,the director and the crue were really very nice and I am just glad they left out of the documentary some of the stupidest things I said,,,

and my motorbike allows me to bring us back to my essay, which after all is the topic of this thread: bulky motorbike, safely in the classical regime, space-time inferences very robustly reliable

cheers

Giovanni

dear Daniel

I have also posted on the page linked to your essay.

Thanks for suggesting I should read your essay.

I enjoyed it very much.

Of course we are pursuing different objectives, but there is a common drive toward seeking the building blocks of space-time notions in your essay and mine.

I am trying to take a certain leap in the (conceptually) unknown: doing physics without space, time, motion,,,,only particle detections and relationships among detectors,,,,this is after all what we really do operatively and I am intrigued by the possibility that if we stick to this minimalistic description, if we get read of the extra luggage of space-time inferences, perhaps we might travel more comfortably toward addressing some of the foundational issues we are facing

and by the way to me a clock is a box Alice gives to Bob: when the box is materially connected, in appropriate ways, to Bob's ``particle-detector box"

the combination of the two boxes produces readouts which assign a certain number, "time", to each particle detection,,,,,it seems to me this is what is actually done by the objects we call clocks,,,

if we found a steady source of particles in nature, let me call them particles of type A, it could be all in one box: detector distinguishes two types of particles and uses number of particles detected of type A as time whereas it handles number of particles detected of type B as its actual detections, so it times the detections by producing readouts of pairs of numbers, correlations n_A,n_B (had value of the counter B equal to n_B in correspondence of the value of the counter A equal to n_A)

best wishes for the competition

Giovanni

  • [deleted]

  • Post #38

Giovanni

Again want to go back to the issue of the divorce of space and time and again to ask you to read my essay.

Hi Giovanni

Good to note that space-time is questioned by forward thinking professionals who realise something has to give such that QM and gravity can find a common underlying structure.

You write: " v=sqrt[(E-U)/m]. 聽Since in quantum tunneling E − U < 0 this recipe for the speed (and therefore the corresponding derivation of the travel time) becomes meaningless."聽

Why meaningless, trust the mathematics; accept imaginary velocities. This you can do by adopting a hyper-space where each axis is complex.

Exactly such a complex hyper-space I propose in my essay, your comment to this idea would be interesting.

Regards

Anton @ ( 聽../topic/1458 聽)

I read your arXiv:1206.3805 paper a couple of time. I don't have time to write at length right now. However, this structure seems to be very similar to twisters. In fact I have today been thinking quite a bit about how one could construct T-theory with q-deformations. I will try to write more about the connection with T-theory after I have tried to bend metal on the idea.

I gave your paper a high score. It deserves it and I think it should be ranked higher on the list than it has been.

Cheers LC

    • [deleted]

    • Post #41

    Giovanni,

    You would change Einstein's 1905 false light postulate only for the sake of your career, right?

    http://www.nature.com/nature/journal/v418/n6893/full/418034a.html

    Giovanni Amelino-Camelia: "Galileo-Newton theory was superseded by Einstein's theory of special relativity, but, after a century of success, that too is now being questioned. (...) Special relativity has only one absolute scale: the speed of light is the same for all observers, in all frames of reference; for particles with mass, the speed of light is the maximum attainable velocity. Last year, I proposed that the introduction of a second absolute scale, CHANGING THE POSTULATES OF SPECIAL RELATIVITY, would fit the needs of some quantum-gravity approaches and would affect the analysis of cosmic rays."

    Pentcho Valev pvalev@yahoo.com

    • [deleted]

    • Post #42

    The connection to twistor theory is I think not hard to see. The boost operator P_μ that acts on [x_i, x_0] = ilx_i such that

    P_μ > [x_i, x_0] = il P_μ > x_i

    The coordinates (x_j, x_0) we write in spinor form

    x_j = σ_j^{aa'}ω_{aa'}

    x_0 = σ_0^{aa'}ω_{aa'},

    where ω_{aa'} = ξ_a ω_{a'} ξ_{a'}ω_a. This commutator has the form

    [x_i, x_0] = σ_j^{aa'}σ_0^{bb'}[ω_{aa'}, ω_{bb'}]

    = iC^{cc'}_{aa'bb'} σ_j^{aa'} σ_0^{bb'} ω_{aa'}

    = i|C| σ_j^{aa'}ω_{aa'}

    where the magnitude of the structure matrix is |C| = l. In general this may be written for

    x_j = σ_j^{aa'}ω_{aa'}

    x_0 = σ_0^{aa'}ω_{aa'} iq_{aa'}π^{aa'},

    where the commutator [ω_{aa'}, π^{bb'}] = iδ_a^bδ_{a'}^{b'} and the general form of the commutator is then

    [x_i, x_0] = i|C| σ_j^{aa'}ω_{aa'} iσ_j^{aa'}q_{bb'}[ω_{aa'}, π^{bb''}

    [x_i, x_0] = ilσ_j^{aa'}ω_{aa'} - σ_j^{aa'}q_{aa'}.

    The boost operation B = 1 a^l_jP^j on the commutator [x_i, x_0] is then equivalent to the commutation between spinors [ω_a, ω'_b] for ω'_b = ω_b iq_{bb'}π^{b'},

    [ω_a, ω'_b] = [ω_a, ω_b] iq_{bb'}[ω_a , π^{b'}]

    = C^c_{ab} ω_c iq_{ab}.

    This could be explored more deeply. Ed Witten demonstrated the "twistor revolution" in string theory. If twistors are connected to κ-Minkowski spacetime there might then be a link between string theory and LQG and other "edgelink" type of quantum gravity theories. This would be potentially interesting, for this might serve to correct the difficulties with each of these.

    Cheers LC

    Greetings Giovanni,

    I would like to suggest the problem with Spacetime is that physical events are considered to be 'instantaneous' at time t. This fundamental assumption is in conflict with the Second Law of Thermodynamics which can be shown to state "every physical event needs some positive duration of time to occur" (see my Chapter, "The Thermodynamics in Planck's Law" ). Thus, the Second Law establishes 'physical time' to be 'duration' Δt and not 'instantiation' t . Each moment of a particle moving along a smooth worldline is a 'physical event' requiring a positive duration of time to occur. Spacetime violates this requirement of the Second Law. This, in my opinion, explains why Cosmology is in conflict with Thermodynamics.

    Constantinos

    "The Metaphysics of Physics"

    dear Anton

    I have nothing against adopting, if the formalism provides room for it, some formal notion of velocity that takes imaginary values, but you see from my essay that I am "detectors first",,,,readouts are real numbers,,,,surely you can combine TWO real numbers to get an imaginary one, but would that help? and what is the second number I should measure?

    In any case this is surely connected to the properties of the "Feynman-path time" (also imaginary) which I mention in parts of the essay

    best wishes for the competition

    Giovanni

    dear Lawrence

    thanks for your interest in the essay

    and your choice of expressing appreciation specifically for arXiv:1206.3805 tells me a lot....that is not the easiest paper to read among my papers...

    and most intriguing are your observations about twistors: I had been thinking about a connection between twistors and relative locality but only at a somewhat "intuitive" level, still looking for a formalization. It seems you have a case for one possible formalization of the connection.

    It would be nice if you managed to bring this to full fruition. I may be thinking about it (and we could be in touch for that) gradually as the end of this fall will approach (I have a couple of ongoing projects to finalize and two students presenting their PhD thesis over the next few weeks)

    cheers

    Giovanni

      Dear Giovanni,

      I read your essay with great interest. A few remarks/questions.

      1. I agree wholeheartedly that modeling classical spacetime as a manifold over the continuum is at the very best redundant. Some would argue that it "does no harm" and that it "makes no difference" what the structure is at arbitrarily small scales, but something that bothers me about this view, in addition to the issues you raise (tunneling, etc.), is the fundamental role the representation theory of the Poincare symmetry group of Minkowski space plays in constraining the properties of particles in quantum field theory. I do not think it is at all obvious that altering this symmetry at small scales would have no practical effects.

      2. What is your favorite interpretation of quantum theory? I ask this because it seems to me that the conceptual role of spacetime depends on this to a large degree. For instance, if you prefer a sum-over-histories view, you can separate the properties of classical spacetime from quantum effects arising through superposition of universes. If you prefer to think about a single universe and view classical physics as a limit via the correspondence principle, then it seems clear that such a universe is very unmanifold-like.

      3. On a related note, one possible way of thinking about nonlocality is to provisionally banish the spacetime metric and simply regard direct influence between events as defining locality. The idea is that spacetime is a way of talking about relations among events, that it closely resembles a Lorentzian 4-manifold at large scales, but that "nonlocal interactions" with respect to the metric of this "nonphysical manifold idealization" are simply indications that the manifold is really fictional. Personally, I tend to think that the most prominent nonlocal phenomena such as entanglement have more to do with superposition than nonmanifold structure, but I do think that the large-scale properties of spacetime arise from more primitive microstructure, or, more conservatively, that models based on primitive microstructure will eventually outperform manifold models experimentally. See my essay here for more on this: On the Foundational Assumptions of Modern Physics.

      3. I will have to have a look at your papers about observer-dependent association of processes to points in noncommutative spacetime. That is an idea I have not heard of before.

      Thanks for the enjoyable read. Take care,

      Ben Dribus

      • [deleted]

      • Post #47

      I will think about this more as well. There may be a more general isomorphism of some type.

      I find myself wondering if twistores might be some type of connection to between loopy and stringy approaches to quantum gravity.

      Cheers LC

      • [deleted]

      • Post #48

      Giovanni,

      If the absurd "energy-dependent but observer-independent" speed of light is a failure, why don't you try, for a change, "energy-independent but observer-dependent" speed of light? I am not sure about the energy independence but "observer-dependent speed of light" sounds quite reasonable. I have tried to show this in my essay:

      http://fqxi.org/community/forum/topic/1398

      Pentcho Valev

      dear Pentcho

      consistently with the rest of my posts, let me just confirm that for me any proposal which is consistent with available experimental data and can be fit within a logically-consistent mathematical framework is good-enough for testing. So I am fine with speculations about "energy-independent but observer-dependent" speed of light. I do not "BELIEVE" something is right or wrong. What can happen occasionally is that I (or someone else) rigorously SHOW that some proposal is either in conflict with available experimental data or lacks internal logical consistency. But as long as no such proof against the proposal is given I will not build any prejudice/belief against a proposal. One might have to go to a next level of assessment ("plausibility", "intuition", "conservativeness") when tough decisions must be made about which proposals deserve top priority, considering the limitations of our resources, but that is whole other challenge.

      best wishes for the competition

      Giovanni

      dear Ben

      thanks for your comments

      I will read your manuscript with interest

      In the meantime let me start by giving a concise answer concerning my "interpretation" of quantum mechanics. For me quantum mechanics is just like any other theory in physics: it is a formalism predicting certain correlations among the readouts of detectors (and clocks) built following certain craftmanship procedures. Its predictions are successful so I am "happy" with quantum mechanics. The part which is successful of quantum mechanics, the part which I use, the part which produces succesful predictions, is the same in all of its "alternative" reformulations. So its alterantive reformulations are not of interest to me within the confines of quantum mechanics itelf.

      There is however an area of speculation about the applicability of quantum mchanics (or some modification of it) to a class of measurement procedures we have never managed to perform, the class we colloquially label "quantum gravity observables". In that realm we have at present no experimental basis. It is then legitimate to speculate that quantum mechanics might have to adjust at least a bit. And then the alternative formulations of quantum mechanics can become of interest also to someone like me (equivalent reformulations of the starting point may well not be equally efficacious in getting us to the finish line)

      best wishes for the competition

      Giovanni

        5 days later

        Dear Giovanni,

        I appreciate the response! I think we agree on how quantum theory should be viewed in general. As you suggest, however, different interpretations might generalize in different ways. For example, in Feynman's original sum over histories formulation (the 1948 paper), he sums over "particle trajectories" on a fixed background. If you believe GR, though, you know that the "spacetime" itself should respond in different ways to different trajectories, so each trajectory has its own spacetime, and suddenly you are summing over universes, but the underlying concept of a sum over histories comes through unscathed. The alternative formulation of a probability density function on spacetime, which works fine for a fixed background, doesn't generalize as easily to the background-independent scenario. Take care,

        Ben