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Everything posted by md65536

  1. Yessssss... but What you described can be true, but there are complications you should understand. Lets say the lightning bolts hit the 2 ends of the train simultaneously in the train frame, and M1 is in the middle of the train, and sees the lightning bolts simultaneously at the same moment M passes on the embankment (at a negligible distance from M1). Then M also sees them simultaneously. And yes, each says "The ends of the train where the char marks are, are both the same distance from me at the moment that I see the lightning." First, because of length contraction, there isn't a f
  2. A lot of the cautious best practices depend on whether you're talking about a battery pack or a single cell. I'm not sure if charging a single cell to 4.2 v will have a noticeable effect, but if you charge a 2-cell series pack to 8.4 v, and they're not perfectly balanced, you might eg. charge one to 4.15 v and the other to 4.25 v, more likely shortening its life. Avoiding charging to full might make a pack last longer, but do nothing for charging a single cell? Letting a cell drop below 3 v once might shorten its life a little, but driving it negative in a very unbalanced battery pack wil
  3. I just quoted the wiki page. I can't explain other than with another quote from the page: "But because the principle is so vague, many distinct statements can be (and have been) made that would qualify as a Mach principle, and some of these are false." Definitely Mach's principle doesn't answer the questions here. I see it more as philosophical in that it provides questions that can't be answered, or at least aren't settled by science. I disagree that there's evidence of what would be observed if all matter was taken away, as far as I know no one has performed such an experiment!
  4. This is a variation of a variation of Mach's Principle https://en.wikipedia.org/wiki/Mach's_principle "Mach7: If you take away all matter, there is no more space." I think that if time and distance are emergent properties of the universe then nothing requires spacetime, because everything could be described in terms of whatever they're emergent from. Eg. maybe causal connections could be described using topology without geometry. In that case, c is a property of the relationship between distance and time, something describing how things are connected in the underlying universe
  5. But the frequency of the light changes depending on the motion of the emitter. Why is the frequency different if it's not actually in motion?
  6. Based on others' answers, I think I'm misunderstanding what you mean by this. What you described here is fine, if you set up the experiment to get those results. Having the lightbulbs next to the observer is a good idea, because if they light up at the same time and at the same place, that can be considered a single event, and then everyone in every reference frame will agree on whether the bulbs lit at the same time or not. Similarly you can set it up so that M and M1 are at the same place at the moments the light bulbs light (ie. their world lines intersect at that event) and then every
  7. "The interval between the two events is space-like" describes everything you're talking about here, and it has a precise definition. "Event" has a precise definition as well. There no problem here. A lightning bolt could also be described not as a single event, but as something with spatial and temporal extent. Then it could be described with some other precise definitions that effectively describe sets of events, such as "world line". If "coexist" is given a precise definition, it would be fine, but there are already words that suffice, and it's a problem if you want it to mean some
  8. If your brain was in a simulation, where every stimulus you received was somehow artificial, that could mean that everything you know (which you acquired through stimuli) might be made up. You probably wouldn't need to distinguish external stimuli (eg. the sense of touching things) vs. internal (eg. having a memory of having lived your life up to this point). That would mean the senses of remembering things could be made up... so maybe it is simulating a few minutes or just one moment in time, including any made-up memories of time before that. It could also mean that things like atoms could b
  9. That's not a square, only a rhombus.
  10. Interesting shape! Am I understanding correctly that 2 opposite corners of the cut face are on the middles of edges (diagonal length 10√2) and the other two are on opposite corners of the cube (diagonal length 10√3)?
  11. Maybe I missed the point, but the extract is basically explaining that in relativity, you deal with spacetime, not just space or just time. When you do, the things described in the text become simple. It's simple and mathematical, and the nature of the subject doesn't complicate it. If anything it seems to make issues irrelevant.
  12. ... continued from previous post. But why stop there?! From here, I think you could add any number of wormholes from anywhere to anywhere, as long as they each linked two events using the same t parameter as all the other wormholes, ie. they each connected two events that were the same instant according to a single common frame of reference, and you wouldn't create any causality paradoxes. But then!, I think you could also add any number of wormholes, each with a different time parameter (so, some wormhole entrances link an event in your frame's present to an event in your frame's past
  13. If you could travel at all faster than light, multiple times in different directions, you could create paradoxes. I don't think the instantaneousness of it matters at all, unless you can apply it arbitrarily in different frames of reference. For a counter example without paradoxes (I think), if spacetime is multiply connected between A and B, where one connection is flat spacetime with a proper distance of 10 LY between A and B, and another connection is a stable wormhole between A and B, allowing instantaneous travel in either direction at any time t in one given inertial frame of refere
  14. Yes, it's not a problem as long as everyone understands a statement consistently. Even "traveling to someone's future" could be discussed without confusion if someone defined what that meant and everyone agreed on it. Combining this and OP's example: Say there are 2 events, A-2020 ("now" at A) and B-2020 ("now" at B). A ship (or message) travels FTL from A-2020 to B-2020. It travels from "the present" at A to "the present" at B (that is the present relative to the two events, in A & B's shared frame of reference). Then say B immediately accelerates away from A. In the new fram
  15. But that's not a paradox. I think that a one-way teleporting of information would not break causality (not sure though). To make the situation into a paradox you'd have to add something to it. You could assume generalization and 2-way travel, but I think you could also add restrictions to make a paradox impossible (use one-way wormholes or black holes). Yes, I think without more specifics, we can't conclude that there's for sure a paradox, or no possibility of a paradox. What does it mean for B to be five years in A's future? Can you explain in terms of events or coordin
  16. After thinking about this, I think this is not a paradox. Instead of "space ship" you could have said "tachyon". You haven't described the ship doing anything that would break causality or do anything paradoxical. If by "you travel" you mean literally a person, then you can come up with something paradoxical, but it's not a paradox yet. (Edit: As for the rest of what you wrote after the above, I don't think it's right. Especially anything that adds a paradox, the paradox seems to come from an incorrect description.) Relativity doesn't specifically disallow nor predict the possibility
  17. This doesn't sound like a scientific argument. Besides, you haven't described anything that you're seeing that is at all different than if you'd never stepped through the wormhole. You're seeing light from about a hundred years of Earth's worldline arrive at planet P. Why describe it as a story? Why not speak of events and light cones, etc? Why not use defined scientific terms? Your story doesn't help at all explain the meaning of "[you] are in effect travelling into the 'future' [...] of the person standing 2 m away from you". I doubt you could explain the meaning in that, because it see
  18. I disagree. J.C.MacSwell has merely described the situation that others set up, in real terms like frames of reference, without any assumption about how that situation was arrived at. No false claims were made. Your idea of "traveling into the future of a person" doesn't make sense to me in any of the frames mentioned (which events are you comparing, and in which frame? If the events are in the same place (star B), it seems to describe only events in a mutual present). I don't see any paradoxes mentioned yet, but one could be built from the situation. In the frame of reference of an objec
  19. Here's a bit of a meander through how I understand this topic. There are other ways to look at it that you might prefer. First, if you move a light clock across a timelike interval in a particular frame, you can derive the time dilation factor for the moving clock using Pythagoras theorem. It looks quite similar to the scene you described in the initial post. I'd take a look at this if you've never seen it, I could post a video. If you repeat this for a bunch of different frames, you'll find you're looking at a bunch of different triangles that all have one of their sides in common: the s
  20. Thanks for that. Is the repeated use of the word "true" in the video not a standard scientific term (even misleading)? If someone said that if a measure of something being length contracted isn't a true measure of length, I'd argue that's wrong, whereas saying it's not its proper length is using a scientific term. About exact specification of intervals: If you have 2 events and you don't care about their locations, only their relative separation, then you're talking about their spacetime interval. Further if you don't care about how they're measured in one particular frame of refer
  21. I think you should! If you think it's a bad job and don't know how to make it better, then we haven't done a good job in explaining it. There are aspects of this topic that I'm going to keep getting wrong until I see it in the right way. I know for myself it'll take repetition, to keep looking at it. Besides, I don't think that you did a bad job. Originally you didn't specify the two events of the interval precisely, but 1) it was good enough to understand what you implied, and 2) the imprecision only changes the 200 seconds value by +/- 3.3 microseconds, so imprecision is not a problem t
  22. No, I think you're right. When OP wrote, you're basically saying that the 200 LS is imprecise or an assumption about where the reflection point is. I was treating it as though it was supplying the previously missing information, but that's not explicit.
  23. No, I didn't see that as a problem. The original spec is, "So we have 2 events ; the emission and the recapture of the signal," and "200 light.seconds taken by the signal to make the round trip." It's not specified where the reflection point is, but I don't think that matters because it's only used to establish the time between the two events, and that's given. For me the light reflection path is irrelevant. It's only used here as a clock, and any stationary clock would do. Yes, there are light-like intervals between the reflection point and each of the 2 events, but I wasn't thinking of
  24. You're in over my head! Hopefully someone else can help? That's the derivative of a hyperbola. I don't see it saying anything about switching places. When y (or r) is small, it changes quickly. As y gets bigger, it approaches x (or ct), and the rate of change approaches constant; a unit hyperbola asymptotically approaches the line y=x. If you take the spacetime interval and make r a function of t, I think what that means physically is... It describes how the spatial distance of the interval changes as a function of the time component of the interval, as you go through differen
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