joigus

What you've got at the end is a rocket equation, in contradiction with your opening statement that it's reactionless. The force, \[\frac{d\boldsymbol{p}}{dt}=-n_{r}\boldsymbol{u}_{\textrm{rel}}\frac{dm}{dt}\] Is the reaction force (with mass transfer) of the fuel against a rocket. How did this reaction term come about from an allegedly reactionless mechanism? As pointed out above, also, you can't have self-propulsion from internal forces in outer space, because that would imply momentum is not conserved for a system of particles. This, in turn, would imply that empty space is inhomogeneous at small scales. There can be no exception. I could explain in further detail, but you must have either all three Newton's laws or none. They are not really independent. Otherwise you may have a system satisfying Newton's laws but it would be completely impossible to consider it made up of smaller parts that also satisfy Newton's laws. You can't just cross out the third law without giving up Newtonian mechanics altogether. Newton's third law, \[\boldsymbol{F}_{ij}=-\boldsymbol{F}_{ji}\] Is really a statement already implied by Newton's 1st and 2nd laws for binary partitions of a system into two subsystems exerting mutual forces (internal). These must cancel in pairs, \[\boldsymbol{F}_{ij}+\boldsymbol{F}_{ji}=0\] Otherwise the composite system in the absence of external forces (free) would be subject to accelerations. It would contradict Newton's laws for the COM (all of them, in particular F=ma for the COM and the corolary, the 1st law too).

I think your argument is quite correct. If you pressed the theory too much, you would need a working approximation of special relativity for the case when instantaneous forces act on bodies, either to stop them suddenly or to set them in motion instantly. In the case that a rigid ruler stops, by an instantaneous impulsive force applied when its tip reaches Earth, applied at that point, a wave would have to propagate carrying both the momentum of the impulsive "braking" and a Lorentz contraction factor, if I'm thinking correctly. In fact, you have pointed to a well-known logical consequence of special relativity: Namely: That no perfectly-rigid bodies can exist, for the simple reason that they would contradict relativistic causality. The reason being that one tip of the ruler would have to stop instantly and it would take some time for the local perturbation to reach the other end. This is in close analogy to the Doppler effect, in which a kinematic factor (length contraction/time dilation) travels along with a propagation effect by the receding speed that would be present even if Galilean relativity were exact. That's why in the last analysis you need waves (fields) to implement special relativity in an absolutely watertight manner. Rigid bodies can't play the part of physical objects. Point-like particles can do the job, but at the price of introducing densities, which are far removed from direct intuition and have nontrivial transformation properties. This is an extremely clever point. But I don't think @michel123456's arguments have to do with it.

I think it's in the eyes of the beholder. Or, in this case, in the ears of the listener. Rudeness is relative, because language is relative. The ultimate reason being that language's main purpose is to relate. And it is no coincidence that "to relate" means both "to interact" and "to tell."

I think this is a great question, and I would like to see it asked (as well as read proposals to answer it) in a more general context.

Have you completely missed @Ghideon's point, or have you completely missed Ghideon's post? Geometric patterns in Nature tell you nothing about conscience. Complexity is not the equivalent of conscience, nor have you shown that to be the case. If you think that to be the case, it is for you to come up with a clinching argument that proves you right. Then propose an experiment. You are conscious because you have sensory organs and a centralized processing organ for all the stimuli, not because your eyes are approximately spherical, or elliptical, or whatever shape. I would demand as much for a planet.

You're very, very likely right, @Bufofrog. For me it's a last-ditch attempt. After this I think I'm done. But I'll still keep an eye on the thread, only to read alternative complementary explanations, for my toolkit.

I'm not 100 % sure this is at the root of @michel123456's problem, but I have a feeling that it may be. Please, pay utmost attention to this: (Emphasis mine.) I think this observation by Janus is absolutely essential. Here's something you said a while ago, that I think is very significant and, to me at least, betrays that you haven't understood what Janus was trying to say: The length and time that undergo dilation and contraction in relativity are not the naive "as seen" lengths and times that one intuitively gets from common experience, the projections on your retina, so to speak, and on your inner clock. Your use of "see" and "measure" as interchangeable has set off my alarms. In relativity you must be careful about what you mean by position and time (and thereby by length and duration) from the get go. So much so that I think you're not there yet. Coordinatizing events is the groundwork for everything else you do in special relativity. When objects are distant from you, you must establish some kind of measurement standards to guarantee that you know what you're talking about when you say "it is at x at time t." This is at the core of what Janus meant by "see" and "say" in my previous quotation of him. Distinction, this one, that becomes absolutely crucial when discussing the relativistic Doppler effect, because there is a mixture there of both aspects. But it is very important in general. The standard observer in SR is not one that simply believes what she sees. She sets up measuring devices so as to be able to say where and when something distant happened, knowing full well that the more distant something is, the later it gets to her. Einstein pictured a frame of reference as an infinite set of observers relatively at rest with respect to each other, and with clocks that have been sync'd at t=0. That's not necessary. You can set, as @md65536 said before, one reliable clock and get any timing from light signals back and forth from it. The standard observer in SR already knows objects are expected to be "seen" different from what they "are." Please, take a good look at Fig. 2.8 here (from https://www.academia.edu/39463786/R_dInverno_Introducing_Einsteins_relativity) by Ray D'Inverno: It is plain to see that, for any event, t2 is the time when you see it happen, but that's not the time when you infer (measure, in your wording) that it happened. The time when you infer (measure) it happened is, \[t=\frac{1}{2}\left(t_{1}+t_{2}\right)\] The difference between one and the other is source of much confusion for relativity students. When Janus says "see" he refers to t2 in Fig. 2.8 (or increments of it). When he says "say" he refers to coordinate time (or increments of it), which is different, and corresponds to t. The timing of events in relativity, the coordinate timing, is the time that you say (guess, if you know SOL), not the time that you see (paraphrasing Janus.) The times and lengths that you see are only too obviously deceiving you already, because the image of the head of the train, e.g., that's coming to you, obviously predates the image of the tail, as seen by you at the same "psychological" time. I really hope this helps. If it only adds confusion, feel free to ignore it. But in that case, I don't know what it is you don't know.

That seems right. But that's a sufficient condition. Suppose the 20 aminoacids were the same, and all the molecules had the same handedness. How would we know which forms of life came first? Maybe we would have to go to genetic trees, as @Endy0816 suggests...

I think that's a very sharp observation. I'm in two minds about that, to be honest, and ultimately I can't give you an answer. But that shift seems to be going on. For one thing I rather like geometrical theories. They are mathematically complicated, but very beautiful and simple to formulate in principle. But for the other, principles of negative content, the "don't go there" kind or principles, like thermodynamics' laws; IOW, the "impossibility to tell this from that", kind of principles, tend to be very stable, very robust. They may be signaling that new variables, or that a new creative splitting of the significant variables must be proposed. Maybe the distinction between source and interaction field cannot be assumed to be always valid. Or maybe, as Nima Arkani-Hamed says, "space-time is doomed." I wish I knew the answer to that. Edit: I'm not even sure I'm addressing your question properly.

He means low-energy experiments have been used to study the properties of elementary particles from the very beginning of the subject. Meanwhile, in Saint Petersburg... Attempts to study deviations from the standard model with simple spectrometers https://www.natureindex.com/institution-outputs/russia/peter-the-great-st-petersburg-polytechnic-university-spbpu/5139070b34d6b65e6a001c3a?utm_source=facebook&utm_medium=social&utm_campaign=nindx-Sep20RH&utm_content=SPU&fbclid=IwAR0vPuvAw1jImwSoe-Hk07f5e6xyCe9qDK_xQLm-pmizjpq6nOKk14uMXLU#highlight Just in case it is significant. High-energy experiments are important, and they probably will always be. But they're not the only game in town. Edit: The reason why high-energy experiments are so important in studying small things lies in Heisenberg's uncertainty principle. Probing small distances generally requires using very high energies.

I address what may be the origin of your confusion in this thread:

Conscience in other planets is an idea about aliens. Conscious planets is an alien idea altogether.

Poor old Zeus. All the other gods stole his thunder.

Point taken. I see other knowledgeable users are addressing the more traditional/geometric point of view. Just to try to be complete, and because I have a feeling that your idea that gravity may not "be real" might come from a certain trend in theoretical physics that started in the late 90's, let me tackle the "information" perspective: In the last decades some messages have been filtering out from the community of theoretical physics in the way of "gravity is an illusion", "gravity is not a real force", "gravity is entropic", etc. https://www.scientificamerican.com/article/the-illusion-of-gravity-2007-04/ Sometimes physicists try to develop an intuitive picture of what really is much more subtle and complicated, and involves a high level of mathematical sophistication to express precisely. Strong suggestions have cropped up that the best language to describe what gravity consists of is the language of information. That gravity is made up of information. That may turn out not to be the case, but it's the direction the research seems to be going. Now, it is for you to decide whether information is real or not. I'll take the view that an action is real if it can destroy something, crash it to smithereens, modify it, change its course of motion. Do you think information can modify, change the course of events, even destroy things? If you do, and it is true that gravity is made up of information, then gravity is real. Gravity can bend your spine, and it will, if you live long enough. So can information. Information can bend your spine, make you turn your head, nod in agreement, etc.

Is not the Tripitaka a holy book? And the Vedas? The Upanishads? The Sutras? And the ZhuangZi? And the Popol Vuh? And the Edda? And the Book of Mormon? Why those three?

I'm not sure that would work. You can't have two people doing the same job. Dramatization: Afflicted person: "Why, oh God, why!!" God: "Well, that's because..." Science: "If you allow me, God, I'll do the talking"

Aaaah. That rings a bell. I think we should have a thread dedicated to impersonate other users (nicely, of course). I would be very happy to have an impersonation of myself. We did that years ago in some language-learning forums. We had some fun. Edit. I think it's a great idea. If more people think it's fun and the mods don't mind, we could do that.

Thanks for pointing it out. I got lost in the nesting of replies.

It's not that bad. Your apologies are accepted, and they didn't have to be supreme. You just missed some aspects of the interface: I didn't say that. You did.

<joke> Last time I checked, it did. It was painful. With SARS-Cov-2, who knows. Maybe they abolish it tomorrow. </joke> I suppose you've heard a bad account of the idea "gravitation is of entropic origin."

Good point. Also, a die, or a slab, or a person, all have some proper length, width and height, Lx, Ly, Lz, but they don't have a "proper time extension", T. In elementary relativistic discussions objects can be considered to have an instantaneous presence, so to speak. They are time-pointlike. That's why the treatment is not completely symmetric in space and time. From the POV of integrated, or elapsed, or covered time and/or space: The twins differ in the proper time elapsed for each of them. But if you try to find an analogue of that in space (the proper length covered, integrated to the whole respective paths), you find it's always zero for both of them, because with respect to themselves, they haven't moved. You can be always at x=0, y=0, z=0. But you cannot keep at t=0. This question has arisen before. You always are moving in the time direction, unless you are a photon.

Exactly. Whether foreshortening is real or not is quite immaterial a discussion to me. My example of the garage was meant to illustrate that foreshortening is "real" enough to anyone interested in getting a slab of wood inside a garage.

Always when I'm sleepy I make mistakes like this. I meant, of course, "if you introduce it with its length parallel to the garage's door baseline." And @The victorious truther, you misquoted me.

I know. It's out of the question. Let me give offer you an analogy of why foreshortening is not as inconsequential as you seem to think. You try to get a very long slab of wood into a garage. If you introduce it with its length (analogue of proper time or proper length) perpendicular to the garage's door baseline, so to speak, it doesn't fit. But if you foreshorten it by rotating it, it does fit. Picture yourself inside the garage helping others to get it inside. You may even close one eye so you don't have parallax errors and you can more precisely estimate the foreshortening. The slab has "shortened", and you know it's safe to push it in. So foreshortening has real consequences. If you "rotate" muons in space time with hyperbolic angle v/c, they last long enough that they can reach you from the ionosphere. If they were at rest, they wouldn't "fit." (In this case they would be too "short", they wouldn't last long enough.) I hope that helps. Analogies sometimes help.

IMO, fields are the tipping point. When you review a handful of examples with Alice and Bob, the twins, the car getting in the garage, etc., it's all very useful, it's very illuminating. But somebody will always come around and tell you, "what about a guy with a flashlight traveling on a spaceship..." and so on, presenting you with a new "paradox." Studying relativistic fields really makes you give up on trying to find either mechanical explanations, or counterexamples. Thought experiments, or gedankens, are not the reason Einstein came up with relativity. It was field theory. And then he devised some of these impressively clever ways to reason it out. The tipping point for him were Maxwell's equations for the electromagnetic field. Everything else must comply with it. You think about the paradoxes very carefully and some argument always saves the day for Einstein. In the twins' paradox example, it would be really complicated to calculate how much time the non-inertial twin rejuvenates in doing the trip. The spirit of the argument is not in the calculation really. It's in the concept. You immediately say "doh!" So it's the other way around, really. It's like Darwin's theory. Some people never get it. This theory must be true. The arguments are so compelling. Different organisms produce almost equal offspring organisms. The external conditions change much more slowly than the cycle of reproduction, so to speak. Some differences will be maintained; others will disappear. Now let's explain why different protists have different layerings of membranes. And Darwin's theory gives you the right line of reasoning. Aren't you convinced? Do an experiment. And the results always confirm the theory. Same in relativity. Only in relativity it's so much easier to do an experiment. Eise told you about muons lasting longer when they move fast, for example. How else do you explain that? I think the time has come to call you to task and challenge you to come up with a better explanation for that than time dilation, which has been checked to extraordinary precision. And that really is my two cents. I meant: Geodesics are obtained by making the action stationary (doesn't matter if they're maxima or minima; they happen to be maxima).