Tim88

The link sends you to a section without tensors, and the text is understandable. But yeah the symbols are difficult to figure out. The equation in Wikipedia for time dilation is correct: https://en.wikipedia.org/wiki/Gravitational_time_dilation#Outside_a_non-rotating_sphere That's the time dilation factor (see also just above that for the connection with the approximate equation). The length contraction factor in the radial direction (perpendicular to the surface) is the same. Note that the contraction is only in that direction; there is no gravitational length contraction in the tangential direction (parallel to the surface).

Yes indeed Memammal, we have seen how the understanding of people who adhere to "eternalism" have ideas about "presentism" that strongly differ from many of those who actually use presentism concepts; and also the other way round. Many arguments are wrong both ways, due to mutual misunderstandings of the concepts. For example, the picture that you posted contains the same fallacy about "presentism", as if past experiences were not real. And of course, I sense a similar reluctance from your side concerning a view of reality that doesn't fit with Memammal's reality, or that differ from a popular school of philosophy (who said that he didn't want a "popularity contest"?). There is no expectation from my side that ideas that have been developed in the course of ten or twenty years would tumble in a philosophical discussion, and neither should you have such an expectation. It's already very surprising - and a nice surprise indeed - that we are much on the same page about what "time" is. Luckily the topic of discussion here is only indirectly concerned with those issues. What matters is how to give helpful descriptions for understanding relativistic physics; the goal is to offer alternative metaphysical explanations to those who are not satisfied with the physics of "shut up and calculate". By now I have picked up enough of the clarifications to proceed with a first attempt to play "block universe" advocate for points 1-3 on behalf of those who want to promote that concept. In post #163 I made a more logical start of the kind of things that a metaphysical background should explain, and I used the 3D Absolute Space explanation. Now, 2 pages of discussion further, it may be possible to make a start with the competing explanation. 4D ABSOLUTE SPACETIME interpretation: 1. INERTIA. Acceleration corresponds to a curved trajectory through Spacetime. Acceleration can be easily distinguished, but eternal time is "frozen" so that more is needed for explaining dynamic forces. For that (as well as for explaining our experience of "now") it is proposed that there is a kind of dynamic time as well, corresponding to a "now" that is literally moving along the eternal time distance. That allows for the dynamics. Apart of that, Absolute Spacetime can of course just as easily as Absolute Space explain why rotating discs have absolute effects. I'll try to correctly formulate the block universe explanations: 2. SAGNAC. For simplicity of description, let's choose a Spacetime slice in which the disc axle only moves along the time direction. If a laser light is sent along the rim in both directions, the light rays will trace equal paths through Spacetime until they hit the detector, if the detector also only moves along the time direction. However the detector traces a curved path through Spacetime so that the point where the rays meet with the detector is not symmetric. The interference pattern is a function of the detector's trajectory relative to Absolute Spacetime. 3. EHRENFEST PARADOX. Lorentz contraction only occurs in the direction of motion. For simplicity we choose again a Spacetime slice in which the disc axle only moves along the time direction. But HERE MY MIND GOES BLANK; I don't know how to explain this with the block universe, as the trajectories through time complicate the description quite a lot. Anyone?

Of course Studiot, you don't seriously expect that anyone can prove philosophical ideas? That's already hardly so in physics, let alone metaphysics! The only thing that can be done, is disprove inconsistent claims as inconsistencies can be demonstrated.

Oh in fact you did forget gravitational length contraction, if you wanted to include GR.

As I already had debunked that misconception about presentism in the thread on 3D space and presentism, I now did it there once more, with more elaboration. Except for a few more comments to michel I won't comment further until after considering your references, and based on those adding what may be useful in answer to the points 1-3, as so far nobody else did. Yes, I agree; and it's even incompatible with presentism, as I elaborated here. It's more compatible with eternalism in a 4D Spacetime, as there "time" is a kind of distance that people progress over - which brings us back to the "double time" issue that you brought up. The good thing is that most of us now seem to roughly agree about what "time" is (as differentiated in duration, sequence and instant); the problem seems to be more a matter of understanding how that fits in with the different models of Space / Spacetime. Yes, exactly. What you have been saying was quite clear from the start. Moreover, we can walk to the left or to the right, but we cannot walk back in time - it's simple as that! The "moving arrow of time" on top of "eternal time" remains an issue for the block universe, and due to that issue as well as the mixing up of the two very different concepts "length" and "time", it does in my opinion more to make less sense of physics than to make more sense of physics. PS, this reference by memammal sketches very clearly the point that we deem very unconvincing (bold emphasis mine): In short, 4D Spacetime as reality can hardly escape a "double time" consisting of two very different time concepts that are equally true physical reality. But in fact, the "eternal" time distance is superfluous for physical reality. Removing that superfluous part we're left with 3D Space (+ the ordinary "time" concept) which doesn't have such issues. Moreover it has more explanatory power and is intuitively easier to understand. I already compared block universe to a cuckoo in a sparrow's nest; but increasingly I view it as people throwing the baby away with the bath water because nobody needs a baby, only to replace it by a Frankenstein's monster. OK, I'm already reading them, most of it goes rather quickly (I'll number them): 1. Once more, it's the math that the two models of reality share; apart of a single sound bite by Minkowski, there isn't anything particular about his block universe! Let me remind you of the fact that Langevin elaborated on that math formalism in order to promote the 3D Space model of reality. But maybe someone can explain how a literal interpretation of "curved" spacetime could be helpful. 2. I didn't know about that field; but IMHO it tries to deal with unproven interpretations of QM. For people who think that Feynman diagrams should be taken literally as action back in time (but apparently that's a fable), eternal "time" could be convenient. In contrast, for people who think that Bell's theorem implies "instant action at a distance", the Lorentz ether is most convenient, as Bell also said. In my eyes we should stay with our feet on the ground and focus on the explanations of the established facts of physics. 3. https://www.quantamagazine.org/20160719-time-and-cosmology/ At least that's about "block" vs. "presentism", citing the different opinions. apparently Smolin is also "fed up" with block, like I am now. And "blockers" are trying to explain the arrow of time. However, this is just trying to explain a problem that the model causes! Trying to fix a problem of the model isn't helping to make sense of the physics - just as repairing a broken saw doesn't cut wood. And the "growing spacetime model: "Space-time itself is growing as time passes". Sorry, but to me that's just hilarious! - causal set theory for predicting cosmological constant: that is suggested to be based on "block". However, at first sight, it's in fact based on GR math. 4. After shooting down the straw man, there's a discussion on block universe: - "It emerges that the feeling we have of the passing of time is nothing more than an illusion of human perception due to the asymmetry of the time axis: we can remember the past, but we cannot remember the future." That's again an attempt to solve a problem of the model, instead of showing how the model can help solving problems! - the link to "arrow of time" was broken - and forwarded me to a sex page! - "The conclusions presented here relating to the block universe model follow directly from Einstein’s theory of general relativity": that's just more bunkum. - "It is true that there is a time dimension defined within the universe. And for an observer within the universe, objects appear to change with respect to this time axis. However, this apparent flow of time is just an illusion of human perception due to the asymmetry of the time dimension." Is there anyone here who found that a helpful explanation of how "eternal time" can be experienced as a measure of change? - there is supposedly no beginning of the universe because "The entire spacetime block is laid out as one unchanging structure." But then, to be consistent, neither did anyone of us have a beginning; we were never born, but instead "laid out as one unchanging structure" as well. Problem solved?? The author regretfully shows the same lack of rigor in promoting eternalism as in criticizing presentism... - energy: nothing to do with "block" - Wheeler-DeWitt Equation : there's an issue that I don't know of, that "the rate of change of the state of the universe with respect to time is zero". No doubt that is open for different interpretations, but "block" explains it as follows: "The notion of evolution is not applicable to the universe as a whole since there is no external observer with respect to the universe, and there is no external clock that does not belong to the universe". Great. But totally unconvincing, as no "outside clock" is needed for establishing the existence of a rate. I now think more and more of empty perimedes-like philosophical debates... and that's quite the contrary of what I intend with this discussion. 5. Growing block in wikipedia. Interesting additional problem brought up there: we cannot know when "now" is with that model! And (of course), apart of the more intuitive aspect as it's closer to Absolute Space, there is no suggestion of how the concept can be useful for better understanding physics... Just one left: http://www.journals.uchicago.edu/doi/10.1086/392846 Regretfully my institution has no subscription to that journal. Memammal, please summarize useful aspects of "block" for physics according to that article, thanks!

There's a beginner's mistake in phrasing: the speed v is not absolute, you may just as well pretend that the rod ends up in rest, and so the Earth is moving at speed v. Therefore you can't say that it's the "real" length! Apart of that it looks OK to me. Assuming that you meant it as swansont specified, then that's the length according to Earth.

PS in particular, as I showed, the following claim is wrong (bold emphasis mine): I'll try once more. In presentist models, time is not a 4th dimension like distance; the things that can be moving according to such an interpretation of reality, are physical objects. Motion v over distance d implies a conceptual missing parameter, which concept we call "time" t (more precisely, here it refers to duration). Although our internal navigation system may confound these things, time can obviously not move in any presentist view including model, incl. Absolute Space: it's the wrong parameter! "Now" is just a "snapshot" that we make in our brains of a situation that progresses, and immediately after is relabeled "past". The Spaceball movie here above correctly depicts that fact (at 1:00). It's different of course with block universe time, as in that 4D Spacetime view it's a 4th dimension like distance.

Bold face added: On a second look, my phrasing in bold was imprecise (and it somewhat mismatches with the rest). SR only took care of effects due to speed; eventual effects from gravitation were not yet included in optics, and new corrections to mechanics were also foreseeable. The research program of that time was not yet completed. So, Einstein used the equivalence principle to predict the effects of gravity on optical phenomena and related physical measurement tools. As I could not quickly find the derivation, I now re-derived it myself as follows. (Funny enough, I next found Einstein's derivation which was more complex, as he started from SR in combination with light-based simultaneity; but next he simplified so much that he effectively didn't use SR!). We can think of a rocket in "deep space", accelerating from v=0 to v= 9.8 m/s in 1s. Inside, the phenomena should look just as on Earth. Also, special relativistic effects are negligible for this case. At t0=0 a light pulse is sent from the floor upwards to a receiver at the ceiling, at a height h. We'll assume that the ceiling will not move much before the light arrives. Then the light distance = h. The light pulse arrives at time t. The receiver is then moving with speed vr: vr = at (1) The light pulse travels the distance ct: h = ct => t = h/c (2) (1) + (2) -> vr = a h/c (3) For this, classical Doppler is good enough: fr/f0 = (c+vr)/c (4) (3) + (4) => fr/f0 = (c + ah/c)/c fr/f0 = 1+ah/c2 (5) The same observation should be made on earth with gravitational acceleration g: fr/f0 = 1+gh/c2 (6) After he had found the gravitational frequency equation, he reasoned how that equation should be understood. With only clocks in rest, there cannot be a Doppler effect. And wave crests cannot get lost in transit. There was only one logical solution possible: a clock should run at a faster rate when it has been placed higher up in a gravitational field. And as he had visited a polytechnic college, he immediately thought about how one should deal with that in practice. It's no good to do measurements with clocks that run at different rates! So, he already considered that very precise clocks that are put at a high elevation, should be corrected for their height so that they will run at the same rate as clocks at sea level. In this way the clocks can be used as part of a single reference system. If some part is not understandable, please tell; it's all simple math.

In view of some confusion in the thread from which this one was a spin-off, here some more elaboration about the role "time" plays in a Lorentzian or relativistic "Absolute Space". Absolute Space uses different "times", but it has a single time concept. As I see it, different clock times occur as direct physical effects of location near heavy mass and motion through Space; time as a comparison of the progress of physical processes implies that there's only one kind of "real time". Any sensation of a "flow" of time (as in dx/dt) is only in our head, as physical processes occur in our heads and we are conscious of our own physiological changes as well as the changes that occur in our surroundings. Of course, there is also a "local time" as set up by humans for measuring time, due to the impossibility to establish absolute speed and absolute simultaneity. Although "true time" is inaccessible to us, the relationship between "true" and "local" time is straightforward. And it's even possible to add a theoretical "Newtonian absolute time" in that view for conceptual convenience, standing for an imaginary "zero speed, zero gravitational field clock"; but in that view of reality, such an "absolute time" is merely a mathematical construct of our minds, a thinking aid. A friend who is writing a book about "time" sent me the following short video link which, I think, sketches in a funny way "time" - the first section is a bit like how "eternalism" operates, very different from "presentism", which is more like the second part:

Instead I said that I will do that, in order to try to find things that I may be able to answer for those o so many adherents of block universe. Yes indeed this thread got side tracked, and it risks to get sidetracked further. As you here are replying to my comment in the thread about presentism, I'll reply there. I don't know any other thread that even attempts what we attempt do here. Please show what other models that I never heard about offer concerning points 1-3, thanks!

That's good Sriman - but be aware that your original question was about GR! Einstein found the approximate gravitational time dilation equation of posts # 13 and #31 with the equivalence principle. The equivalence principle is already known from Newton's mechanics: if you are inside a free falling elevator (let's hope it never happens!), the effects of acceleration compensate the effects of gravity - it will look as if you are floating. Inversely, if you are far away from Earth in a fast accelerating rocket, the effect that you feel due to acceleration at 1g is just as if you are at rest on Earth. Einstein reasoned that this also should hold for measurements of clock rates: the effect from gravity should be the same as the effect from acceleration. When a rocket accelerates, there is a Doppler effect due to the changing speed. In that way it was possible to predict the effect of gravity on observed frequencies between two clocks at a distance. Now I cannot find a link to the derivation, but it's not so difficult to work out - maybe someone else will fill that in, or else I can look it up later. After he had found the gravitational frequency equation, he reasoned how that equation should be understood. With only clocks in rest, there cannot be a Doppler effect. And wave crests cannot get lost in transit. There was only one logical solution possible: a clock should run at a faster rate when it has been placed higher up in a gravitational field. And as he had visited a polytechnic high school, he immediately thought about how one should deal with that in practice. It's no good to do measurements with clocks that run at different rates! So, he already considered that very precise clocks that are put at a high elevation, should be corrected for their height so that they will run at the same rate as clocks at sea level. In this way the clocks can be used as part of a single reference system.

I'm also going to sleep now! The article that Memammal referred to can be useful to explain some kind of block universe view, and that's why he linked to it. You got bugged by a straw man that the writer invented concerning "time" according to the Absolute Space view - as I explained earlier in the "presentism" thread.

Koti, the "moving now" issue is the "double time" issue of block universe - I already discussed that in a thread on that topic [edit: see here]. That reference is thus utter nonsense. See instead the bottom part of my last post #180, just before your post.

In my opinion, Wikipedia is hardly of more value for philosophical citations than this thread itself. Moreover, in this thread we are comparing disagreeing explanatory views of relativistic physics with each other - and not classical physics with relativistic physics. Thus, here above I filtered through the parts that seem to relate to explanatory power of the block universe on top of the mathematical explanatory power of the Poincare-Minkowski space-time formalism which is shared by the two models. I'll next read your links and try to see if I can distill more out of those that can be used to phrase the causal block universe explanations of the phenomena of points 1-3 of post #163. But it is true that apart of the "odometer" explanation, I find the block universe concept rather incomprehensible. For example, does "curved spacetime" (which is a mathematical term) correspond to a curved Spacetime in block universe (thus with a 1:1 metaphysical correspondence), so that the clocks have different trajectories due to "curvature" of the Spacetime background? (and if so, what explains that curvature? How should it be interpreted? And the same for geodesics, is the "block" view helpful here?). Therefore, I'll be happy if someone else with a better understanding of that view explains inertia, and effects from rotation with the help of "block" instead of me! Apart of that: You mean that Absolute Space needs "something like double time"? I don't think so. [edit:] Although it uses different "times", it has a single time concept. As I see it, relativistic Absolute Space implies that different clock times occur as direct physical effects of location near heavy mass and motion through Space; time as a comparison of the progress of physical processes implies that there's only one kind of "real time". [edit:] Of course, there is also a "local time" as set up by humans for measuring time, due to the impossibility to establish absolute speed and absolute simultaneity. Although "true time" is inaccessible to us, the relationship between "true" and "local" time is straightforward. And it's even possible to add a theoretical "Newtonian absolute time" in that view for conceptual convenience, standing for an imaginary "zero speed, zero gravitational field clock"; but in that view of reality, such an "absolute time" is merely a mathematical construct of our minds, a thinking aid. So far, no comparable logical explanation has emerged concerning the "double" time (two conceptually different "true times"?) of the Spacetime view; it remained more of a mystery than an explanation. If you would like to start a topic on that: yes please!

This is not about "a number is equal to itself"; indeed that would be meaningless! Here both Einstein and I already explained why the idea that frequency might change "in mid air" is absurd, and why that fact is useful for correctly interpreting gravitational time dilation - as a matter of fact, it was an important step on the road to GR. But never mind we'll leave it at that, just as you said. Yes, that's how to tune everything into a single system - and as cited, this procedure was already foretold by Einstein in 1911. That equation includes the time dilation effect, which makes it invariant between reference systems. See here for the classical Doppler effect equation: https://en.wikipedia.org/wiki/Doppler_effect#General

The rest was just exemplification and elaboration. Indeed I forgot to ask you what you meant with that, as we already know that both concepts avoid the "twin difficulties". Such difficulties arise when one assumes that there is no background at all. And as was elaborated on the last pages, it seems as if there isn't much more that the block universe concept helps with. Interesting idea! Even if we assume that the "Space" part of Spacetime is finite, how about the eternal "time" part that is supposedly present and existing? That does sound huge, and "infinite" sounds as quite too much... I don't know how you want to use reduced time consciousness for this discussion; but since that person's movements of arms and legs probably uses a kind of PID control (and that implies again activation of speed cells and "time" dynamics), I don't think that that would be a totally different "ball game". And I really wonder if we did a poll, what percentage of people would vote for "block".

They use somewhat different definitions. An elaboration can be found in the well known "Physics FAQ". (disclaimer: I influenced some of the corrections in the update).

OK. Apparently such person will still have a sense of time; however it will be less developed, as there isn't much more than that person's internal "clock" to go on. Interestingly, yesterday I heard in a talk that even red blood cells have a biological clock, so that their conductivity changes with the hour of the day. However, it's not clear how that is useful for explaining laws of physics... Ehm no, not really, and different scientists perceive time differently; even this thread is witness thereof. A lot of things are pushed into Einstein's shoes (as no doubt also with Newton), but that one was mostly not Einstein's doing. His philosophy was formed and influenced by the people he frequented such as Lorentz and Minkowski. But I read that according to his autobiography he was first of all influenced by Hume and Mach. Not that it matters much for us; we must make up for ourselves what metaphysics we deem most plausible. Wow, thanks! Of course, better would have been if you had made a synthesis yourself, but I understand that you also are busy. I'll look at it and see what of that material can be used for showing how "block universe" metaphysics can be helpful for making sense of physics - if only to be fair. I reasoned that since there is no sub forum for metaphysics, it's more appropriately put under "philosophy" than under "physics" - especially as it may be unavoidable to occasionally make short excursions into "pure philosophy" and discuss ideas from philosophers. However we're here really at the boundary of physics and philosophy, far from the arguably "empty" and "useless" philosophy that Feynman made jokes about. Well, I hoped (and still hope) to find out what those concepts mean for practical understanding, by more "hands on" explanations than the usual philosophical debates that tend to neglect practical usefulness. What's the use of either concept for developing intuitive understanding of the physics? That's the focus here. Maybe you lost me here! It appears to be a discussion (by means of 3D Space + dynamic time concepts!) of someone drawing (and next cutting) mathematical curves... I think that it's essential - as done by both Langevin in 1911 and Einstein in 1916 - to distinguish the mathematical formalisms (as well as their pictorial representations) from the metaphysical concepts that they were based on, or that we may infer from them. Was that perhaps not sufficiently elaborated in the "mother thread"? By the way, also that thread was put here in "philosophy", probably with the same reasoning.

emphasis mine: Once more, we use only a single reference system for a "global" analysis; otherwise it's meaningless and one is almost sure to mess up, as I explained. As a matter of fact GPS uses a single reference system with detectors at different altitudes and speeds. If there was a change of frequency in transit due to altitude (or if photons would turn into chocolate mousse), GPS systems would be messed up. The conservation of cycles is not really different from for example conservation of total current in a river or a conductor. One doesn't need to "count in different frames" for those laws of physics; in fact one should not do that. Yes, exactly. A difference is not a change. Maybe I explained this less well than Einstein; and as he explained it in a very clear manner, I'll cite his explanation next as I now found it back: "[..] it is difficult to discover whether the inferred influence of the gravitational potential really exists. On a superficial consideration equation (2) [f1 = f2 (1 + gh/c2] [..] seems to assert an absurdity. If there is constant transmission of light from S2 to S1, how can any other number of periods per second arrive in S1 than is emitted in S2? But the answer is simple. [..] we must certainly define the time in K in such a way that the number of wave crests and troughs between S2 and S1 is independent of the absolute value of time ; for the process under observation is by nature a stationary one. [..] If we measure time in S1 with the clock U, then we must measure time in S2 with a clock which goes 1 + gh/c2 times more slowly than the clock U when compared with U at one and the same place." - Einstein 1911, "On the influence of gravitation on the propagation of light" (in the last sentence I substituted gh back for "phi").

That's great Sriman. And I may have narrowed down the issue of Strange, it's probably a matter of definition. - frequency = number of cycles per second. - to change = to become altered or modified. For example, if I measure a length a stretch of water in nautical miles, I will find a different distance than if I measure it in sea miles. Nevertheless, the length of that stretch of water cannot be altered or modified by my measuring of it with different length standards; it certainly did not change. By definition, if a frequency changes in transit, this means that the number of cycles that is received in one second is different from the number of cycles that is emitted during the same time, according to the same time standard. But that is impossible in a stationary situation - just as impossible as photons turning into chocolate mousse. And as E=hf, also the energy cannot change. I think that that's just what Sriman already understood in the first post.

Thanks - sorry for that, indeed it may be ambiguous out of context. The context: "Essential requirement [for consistency]: we must do the analysis with a single reference system." I did not mix reference systems without specification in a single sentence (in fact, I try to never do that). In more elaborated phrasing, including the things that I had stated separately: If for 24h, as measured with a system on the ground (which is assumed to be in rest), a certain number of cycles is sent upward to a detector (which is also in rest), then - according to the ground system - that number of cycles must also be received by the detector in the same time interval. Else cycles would go missing "in mid air"! That excludes a change of light frequency "in transit". I hope that now it's clearer.

First of all, it's not exactly so. If he thinks that he is not traveling at 0.8c, but in rest, then he should find c. How that is calculated is shown in the post just above by DimaZin. But if as you say, he thinks that he is traveling at 0.8c, and a light ray is according to him "traveling parallel to him", that means that he assumes that he is not in rest - he actually thinks that he is traveling at 0.8c (relative to the ground?). Right? Then he should make relativistic corrections for his speed. Next he should find that the ray is traveling at the velocity c-v "relative" to him (but note that not everyone uses the same definition of "relative velocity"). Effectively he puts himself in the shoes of a "third party observer". The third party observer method is used in the GPS system: the radio signals that GPS satellites exchange travel at c-v relative to the satellites, which themselves travel at v.

I'm sorry if you found it confusing, even in combination with the elaborations by Okun. But I remember that many years ago I was puzzled by it too, due to the conflicting explanations. The OP was satisfied because I agreed with his logical deduction and he had nothing to "unlearn".

Oops I was thinking about the one variant while I wrote about the other, and so it got mixed up... I intended to write that, as I now understand it, "growing" block universe has a specific problem that michel spotted and strikingly named "double time"; if I correctly recall it was in order to account for the advancing past. However, both the "block" and "advancing" interpretations should anyway explain our consciousness of a progressing "now", and I got the impression -thanks to ajb and Mordred- that both block universe variants also suffer from a need for "double time" if they want to explain dynamic effects. Yes, I agree. Those negative issues make block universe concepts look unappealing to me. Still, the focus here is more on useful features of the block universe. So far I discern two clearly useful features for making sense of relativity (but only for specific aspects): a 1-1 relationship with the standard mathematical presentation, and the rather neat "odometer". If nobody comes here to clarify how otherwise a block universe model of reality is useful for explaining relativistic physics, then it will start to look rather poor and weak compared to Absolute Space... I did expect it to be a little more attractive than that!

The formula's for gravitational time dilation are given in Wikipedia, in the first section of https://en.wikipedia.org/wiki/Gravitational_time_dilation For experiments on Earth, the last, simple equation in that section is usually good enough: time dilation factor = 1 + gh /c2