md65536

swansont, I'd be much more comfortable having the expert label that's on every post show the specific field of expertise, rather than removing it completely. Can that be done? Thanks!

Yes thank you for renouncing the label. What field of physics do you have expertise in? It would be a lot less confusing if the "expert" label showed what specific expertise you have because a lot of replies you've made do seem to be appropriately labelled "expert", just not I believe the ones I've mentioned. It would be vastly different to myself and others to be discussing relativity with an expert in some other field of physics vs. discussing it with an expert in relativity, especially when we disagree on something. It causes great self-doubt and confusion to disagree with an expert, even when all external evidence shows they're wrong. The problem isn't how you understand relativity, it is how you mislead others on the parts you misunderstand, resist corrections, and use the "expert" label as an indicator of authority. I didn't ignore your references on rapidity, I replied to everything I thought was relevant, and I figured in the end we came to an agreement about the topic.

Yes, to show that your non-expert lack of knowledge on the topic spans multiple threads. My reply to your mathematics is, 'Did you write out the post yourself or copy it from somewhere without attribution? Do you understand that what you wrote does not explain, clarify, or relate to the statement "The worldine is the transition between Alice and Bob's reference frames."?' The quotes link to the threads. There should be an arrow icon in the top right corner of the box.

"The worldine is the transition between Alice and Bob's reference frames." is not re-inventing the wheel, it's reinventing the stick and calling it a wheel. I asked for clarification and didn't get it (or, I don't understand it). Does "The worldine is the transition between Alice and Bob's reference frames." make sense to you as a description of the established meaning of "worldline"? This is not just a one-off error, but a repeated source of confusion for others as well. For example in a thread talking about different engines on a single train: reply: It is either merely confusing to the point of nonsense, or it is evidence of a non-expert misunderstanding of the term and the concepts being discussed. Can you explain what you mean by this? If you're correct then the Alice and Bob statements are consistent. For example, say "lightning striking the front of a moving train" is a typical event in SR examples. What is the reference frame of that event? I asked google and the first result is from https://www.physics.udel.edu/~jim/PHYS309_16F/Class Notes/Class_2.pdf "Events are not tied to any individual reference frames. Events are measured by observers who do belong to particular inertial reference frames." I think that's correct. Did you write out the post yourself or copy it from somewhere without attribution? Do you understand that what you wrote does not explain, clarify, or relate to the statement "The worldine is the transition between Alice and Bob's reference frames."?

Then I just want to say, I don't do this gleefully. Early on getting interested in science, I'd search on the internet, and put huge stock in expert information, and I see others now doing it and being told they don't understand, and are then told nonsense that is not possible to understand. I think it can affect a person's approach to science for years if they're new and can't distinguish pseudoscience when they're told it's expert knowledge. Alright, here's the first example: A confused or incomplete use of the word "worldline". Are they referring to an object that they fail to mention? Inconsistent statements about Alice and Bob. Claiming an event has its own reference frame indicates a lack of understanding of the basics of events. Claiming the spacetime interval is a distance and a path, not even getting the dimensions right. In one single reply, this demonstrates a pattern of using established terms to mean whatever else they want to, which is pseudoscience. I ask for a reference for this information but didn't get one. I asked how the use of "worldline" could possibly be consistent with the normal use, and got this: Is this word salad? It certainly has nothing to do with a transition between reference frames originally described. If it was designed to be confusing use of jargon to avoid supporting or correcting the original statement, it worked. Here's the previous misuse of terms (rapidity and boost) that I called out: https://www.scienceforums.net/topic/133833-why-lorentz-relativity-is-true-and-einstein-relativity-is-false/?do=findComment&comment=1264941 Discussed in thread: https://www.scienceforums.net/topic/133837-is-rapidity-a-measure-of-acceleration/ In the above threads, there are often pages of equations posted without citation, that use the same terms correctly and identically to what can be found elsewhere but are unrelated to the question being asked. This is another form of pseudoscience, mixing in real science to give an air of legitimacy to previous false and only tangentially related statements. The misunderstanding of so many established terms in relativity in just 2 posts demonstrates a level of understanding that is less than "expert." The pattern of attempting to argue and confuse instead of admitting or correcting mistakes demonstrates being intentionally misleading. The constant posting of content identical to that which can be found elsewhere, but does not answer the question being asked, shows a lack of understanding of the topic. Please tell me that you think all the examples I posted are reasonable expert statements and I'll accept I'm mistaken and give up on this site.

The examples are misinformation about basic concepts, where at least twice I've supported my position with the first line of a wikipedia entry. Is it appropriate to link to examples in this thread, or should such issues generally be handled with private messages?

Yes, and there's evidence that they're not just common errors because when they're pointed out, not only are they not acknowledged or corrected, but they're repeated or supported with tangential arguments.

It's come up for me repeatedly. Is there anything I can do about it? Are posts made by experts noticed by other experts? I assume occasional errors are noticed and ignored as inconsequential? I've gotten into several arguments with an expert but don't remember another expert ever weighing in on either side. Are such arguments not noticed? It feels like gaslighting because it suggests the possibility that other experts have seen the posts and ignore them because they agree with them, and that I'm the one who doesn't understand the basics. I don't suppose it would be useful or appropriate to post examples here?

re. How does a person get expert status? Does the same process apply to losing status, or is it permanently applied with no possible further consideration? Say for example an expert is consistently posting pseudoscience, specifically of the type "Use of misleading language -- Using established terms in idiosyncratic ways, thereby demonstrating unfamiliarity with mainstream work in the discipline," in a way that misinforms readers. Is it appropriate to "report" such posts as pseudoscience? Or is there a better way to call staff attention to the behaviour? Or is it allowed that resident experts can consistently reply to questions in the mainstream science forums with pseudoscience?

Where are you getting your information? How is this consistent with anyone else's use of the term "world line"? Can you please provide a reference? https://en.wikipedia.org/wiki/World_line "The world line (or worldline) of an object is the path that an object traces in 4-dimensional spacetime." How can that possibly be a transition between Alice and Bob's reference frames? How many dozens of people have to come to this site and be told by an "expert" that they "have a fundamental misunderstanding", and then be given such utterly embarrassing nonsense from you? How many go away believing it, knowing only that an expert told them something beyond the possibility of understanding, and never get the truth? You really should request that the site removes your "expert" status, for the sake of everyone. I would like to request that they do that. Is there an official way to do that?

If the equivalence principle is used, then there should be an equivalent relative velocity between source and receiver. Consider 2 sources falling into a black hole, and only one of them realizes it and accelerates outward so that it can remain stationary relative to the EH. Near the EH, it would need to have a velocity approaching c relative to the free-falling source, so there would be a Doppler shift between the two. The sources would have to appear differently, and external sources would have to appear different to them, depending if they're stationary or falling. Consider a rocket accelerating upward, so that the bottom of the rocket is equivalent to being deeper in a gravitational well relative to the top. Light from the top is blue-shifted when seen at the bottom, but the top and bottom remain at relative rest. However, consider two sources at the top of the rocket, both emitting a single pulse of light. One of the sources is fixed to the rocket and accelerating, and the other is inertial but set up so that it is momentarily at rest with the other source at the moment the pulse is emitted. Both pulses should be blue-shifted the same amount when seen by the bottom of the rocket, even though the rocket will have a relative velocity with one of the sources when it is seen. Or to put it another way, since the rocket is accelerating and light takes time to cross the distance of the rocket, the velocity of the receiver at the moment of reception will be different than the velocity of the source at emission. The blueshift can be entirely attributed to this difference in velocity, based on reasoning when source and emitter are replaced with equivalent but inertial particles.

No. The centripetal force is Fc = mv^2/r, so greater r means greater v, which means a larger Lorentz factor relative to the centre of the wheel. Would that give the same answer as a gravitational redshift analysis assuming negligible spaceship mass?

I think we'd both have been helped a lot if it was more common on this site for people to discuss and accept corrections, but it always feels like a fight to try. Because of that I think this site is harmful for learning at least about relativity. All of that makes sense now. The constant acceleration implies constant proper acceleration here, I just point that out because I've confused it with constant coordinate acceleration in a single reference frame before. Am I wrong to assume that someone who understands what you wrote here would see that previous statements from you (going back years) don't make sense?

Rapidity is not equal to proper velocity. The chart at the top of https://en.wikipedia.org/wiki/Proper_velocity shows how they (and velocity in natural units) relate. Also, Rapidity is not a type of boost.

You're referring to "celerity", which is different. https://en.wikipedia.org/wiki/Proper_velocity https://en.wikipedia.org/wiki/Rapidity "For one-dimensional motion, rapidities are additive." I'm sorry to hear that, but yes he's not using the terms correctly.

So that's the same Lorentz boost, just represented using a different expression for the constant velocity. Where's the acceleration? Everything you wrote seems to be for a constant velocity ie. a constant rapidity. If one changes, the other changes. I don't see that expressed anywhere. Are you still using "boost" to refer to a Lorentz transformation or are switching between meanings of the word "boost" here? If it's "a type of boost called rapidity" you're saying you can boost the boost? What does that mean? It looks to me like you're just showing the rapidity form of the (constant velocity) Lorentz boost, which still describes constant rapidity. However since it's easier to use changes in rapidity to describe relativistic acceleration because rapidity is additive, you're assuming that rapidity describes the change in velocity itself (ie. change in rapidity) rather than just representing the velocity alone. Your problem example was, "Lets have a constant acceleration for however many years." What's the result?

Well I look forward to the details because nothing you wrote here makes sense to me. You are an expert on this? I was hoping someone else would chime in because the only confusing parts of it are things you wrote. A boost is a Lorentz transformation, do you agree? Rapidity is a measure for relativistic velocity, do you agree?

I've seen this posted on this site many times over the years and I think it's wrong but never saw a correction or explanation. It's repeated often in posts labelled "expert" but I don't understand what it means. As a Lorentz transformation doesn't a boost imply constant velocity? How can a measure of velocity be called an acceleration? How is a measure of velocity a type of Lorentz transformation? Is there some sensible meaning to what I quoted that I'm just not comprehending?

It's the constant speed of light that allows times to be expressed in terms of a distance that light travels, and distances to be expressed (and literally defined) by how far light travels in a given time. Consider a beam of light from one event to another event. Those two events will be generally different distances apart for different moving observers, meaning different frames will have the same beam of light travel different distances, meaning that the time between the events must be different in different frames. Draw a beam of light as a line. If you draw it in the ct dimension, that represents the time between two events at the same spatial location, like an abstract beam whose distance is measured only by time, or you could make it a real beam in the y direction. Now consider the same "beam" in a frame that is relatively moving in the x direction. It is orthogonal to ct, so if you draw this out, you end up with a right triangle. The hypotenuse can represent the same time as measured in another frame where the the start and end events are at different locations, ie. the distance between them is longer ie. the time is dilated. The relationship between the lengths of the edges of the triangle is given by the Pythagorean theorem. The meaning of it comes from the maths. Maybe you could say it's a geometrical representation of the invariance of the speed of light. For any timelike spacetime interval, there's always a rest frame where the spatial distance between the two events is zero, so you can always represent the interval with simple right triangles. I suspect if you want more meaning than that, it would be found in a more mathematical description.

To add to this, these are choices that are made in defining the interval, to make it useful and simple, not to make it somehow supremely meaningful. It's not even a distance, but a square (so that you can deal with negative squares, instead of imaginary times or distances). However since it's made up of distances and times, you can use maths to convert it into something with the meaning you want. The proper time along an arbitrary world line should be the same as an integral of infinitesimal times measured in momentary rest frames at each event on the world line, so you could integrate the square root of infinitesimal spacetime intervals.

Yes, a timelike spacetime interval is the square of the proper time measured by an inertial clock moving between the 2 events in flat spacetime. If the sword remained at rest the whole time, and gravity was neglected, the interval would be the square of how much the sword aged between the two events.

I thought there might be a paradox but I can't create one after all. Suppose the universe "wraps around" 1 light year in distance, and assume it behaves the same as it it was flat. Then you could see what appears to be an infinite row of Earths, each subsequent one looking one year older than the last. One that looks n years older is "old light" from Earth that has made n loops around the universe before reaching you. Lets say you can travel near enough the speed of light that it takes about a year Earth time to loop around the universe. If you leave Earth at the start of 2024, you'll return to Earth at the start of 2025 Earth time, even though the journey is almost instantaneous according to proper time of the traveler. Before you start, the clock on Earth as seen 1 LY away shows 2023, the one beyond it shows 2022. Thinking only of how things appear, there's no need to worry about relativity of simultaneity. As you travel one loop, you see 2 years pass on the "destination Earth" clock, so you see it showing 2023 when you start, and 2025 when you arrive. The next clock beyond it shows 2022 when you start, and also must have 2 years appear to pass during your journey, so it shows 2024 when you arrive. If you keep going, it shows 2026 when you get to it. There's no paradox there. From the perspective of Earth, the ship and the image of Earth in 2024 travel around the donut in opposite directions and meet at the far end after half a year, and the ship returns at the start of 2025. Now if you add another loop that's 2 light years long, it's the same thing, just double everything. You could have one ship travel the first loop twice, and meet a ship that travels the longer loop once, after 2 years Earth time. Negligible time would pass for both travelers. Or, you could have one traveler do the long loop in 2 years Earth time, and the other do the shorter loop in 2 years, ie. at a speed of c/2. One would see 4 years pass on their "destination Earth" and the other would see 3, where they would meet. Ie. they both start in 2024, and one sees Earth around the long loop looking like 2022, and arrive in 2026; the other sees Earth looking like 2023, and arrives in 2026. One would have aged a negligible time and the other would age 2x .866 years (according to Lorentz factor). In terms of distances everything should be similar. Traveling at near c, the lengths would be negligible. At half c, traveling a proper light year would be measured as .866 light years traveled distance. I can't see any paradoxes here. I think it would be equivalent to if you had a flat universe with a set of copies of Earth spaced a light year apart, all at relative rest and with synchronized clocks. Also add copies of the traveler so they could see "their distant selves". Creating that with copies wouldn't introduce any paradoxes.

Two objects falling directly towards a BH can diverge (as with spaghettification). Two objects falling indirectly and parallel can diverge, eg. if only one of them has escape velocity due to different distance from the BH. I think the analogy needs more details. On the other hand, if you have two side-by-side geodesics both directed toward a single point (like a CoM or barycenter), they shouldn't be parallel at any finite distance, in general?

Or use wheels that are the same size and curve the road intrinsically... like with a trampoline. How is your analogy "correct"? What do the wheels represent and are you saying that spacetime (the road) is not really curved??? This also shows that gravity is not needed to show curvature in the trampoline analogy. Pin a rubber sheet flat against a wall in zero-g. Stick a large ball representing a gravitational mass under the sheet, stretching it (or even a long pipe sticking out from the wall, to imagine it more extremely). Roll an axle with 2 wheels of the same size along it, and the path will curve, analogous to null geodesics.

Then the radiation field of an accelerating charged particle drops off as 1/r because it propagates perpendicular to the acceleration of the charge, the field lines distributed over a circle for a given r rather than a sphere? An oscillating charge radiates EMR with a frequency equal to that of the oscillation. Apparently, Maxwell's equations imply that even a charge with a constant acceleration must also radiate. However, the frequency and energy would be zero, or at least approach zero as time approaches +/- infinity. So one could say that a charge at rest on the surface of Earth does not radiate energy, or that it radiates light with infinite wavelength, which is not physically detectable nor has an absolute meaning, but is consistent with all physical laws. I hope this is right instead of me just getting more confused.