# md65536

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## Posts posted by md65536

### plot the revenge

1) the scenario cannot be an elaborate murder plot no physical harm can befall the victim.

2)I.D. forgery or credit fraud are out however forgery of letterheads etc. is acceptable.

3)vandalism and destruction of personal property is not permitted

4) there are no other rules just be original I think the rules make that an obvious requirement.

Step 1. Kidnap every person the victim knows, keep them in the well in my basement, and torture them daily.

Uh... I don't think your rules are very well specified, because I think this plan fits but that it isn't quite what you were looking for.

so you can clear your name

I think my plan would do that.

### Here's a great riddle I made up based on the movie "Wishmaster"

Why don't ask the Djin to just go up and down a 15 ft. pole until the natural end of planet.

Just to be on the safe side, make it a 30 ft. pole.

### Which has more gravitational pull, the moon or a nebula?

I recently saw an astronomy picture that had the moon in it and a nebula which looked as big as the moon.

I can't remember where I saw it... does anyone know if there are nebulas that actually look as big as the moon from Earth? And how far away would such a nebula be?

Here's my puzzle:

Suppose you had a nebula that looks as big as the moon.

Suppose that this imaginary nebula happens to be as dense as the moon.

Which would have a stronger gravitational pull on us?

You can assume the nebula is spherical and whatever distance away you want. Say, 100 light years.

### How to find 1 honest villager out of 99 with \$100?

At most 49 / 99 are troublemakers.

Ask any 50 a question and you'll get at least one honest person answering.

If you ask 49 people about 49 others, and you get all "no" answers, you know that each person asked is either an honest person talking about a troublemaker, or a troublemaker answering the question. Regardless, you know that all 49 possible troublemakers are involved and you can safely assume the 99th person that you haven't involved is honest.

Otherwise (if you get at least one 'yes'), take all the people whom you were told were honest, and ignore all the rest (the question answerers and the 'no's and the one you'd ignored).

You've now spent 49 dollars and have 51 to spend, and you have a group of at most 49 people who may be honest or troublemakers.

Also, for every one of this group that is a troublemaker, they got into this group because another troublemaker lied and said they were honest. At most, you could have 24 troublemakers getting 24 other troublemakers into a group of 49.

That means you can assume that this group is mostly honest.

You're in a similar situation to what you started with. So repeat the process until you get all "no" answers, and take the one who was ignored, or until you get a group of one (who will be honest).

(I guess I left out some details but if you have an even group, you'll get at least one "yes".)

### Which door should you take?

Go through both doors in superposition. Collapse the wavefunction of whichever reality is nicer.

### Relativity Question

Some great scientists have said that truth lies in simplicity. Physics without mathematics is not physics, it is philosophy. I am told that in some universities subject of relativity has been made optional. What does it indicate?

QM too is based on hazy concepts and so, on this count, there might be need for advanced mathematics. Once we come to know correct concepts governing a particle in motion, then everything, including mathematics, will become simple.

I think Feynman makes a good argument here:

This is from the first of a series of lectures that can be found here: http://www.vega.org.uk/video/subseries/8

If you take his quote out of context you might twist it to support your case.

But what he's saying is that the math isn't sacrosanct. It is just a set of techniques used to arrive at the "final count of beans" of a theory without counting every bean individually. It's not the math that's important but the predictions made by that math, and they're accepted when they best predict the outcome of whatever they model.

The story of the Mayans that he uses is that they were interested in predicting when Venus would show up in the morning vs. the evening, and they figured this out by observing it and counting days, and creating the math for that. This allows them to make accurate predictions. From this, they (we assume) had no idea why Venus followed those predictions, and as Feynman points out, trying to figure it out based only on the observations (the counts of days) is not likely to get you anywhere.

The explanation "it's because Venus and the Earth each orbit the sun with different periods" is nice to know and it's certainly important to us, but it gets you nowhere towards predicting when Venus will show up in the morning or the evening, without complicated math involved (more complicated than the Mayan method).

So beyond what Feynman says, my point is also:

- A new theory doesn't replace an old one just based on explanations, but based on new observations, and on predicting/modeling new phenomena or the same phenomena more accurately.

- New theories do not replace math with logic, but with different math (typically the math doesn't get simpler, I should think).

I do agree that in the future, the explanations for why the math works will become simpler and more satisfying in many cases. As Feynman says several times in that lecture, nobody knows why it works. But that is not how it is judged. Feynman also points out that it's incredibly successful at predicting a large range of phenomena with high precision, and that is why the math is so valued.

### quantim leaping of electons

Their collective ideas involve alternative ideas that could be transformed into alternative theory, whereby the math would not change, only the explanations.

If the math doesn't change, the predictions don't change, and the theory doesn't change.

Different explanations for the same predictions are different interpretations of the same theory.

I agree, these ideas lead to new or modified interpretations of the existing theory. It happens all the time and it will keep happening, and progress will be made.

If one interpretation (ie. explanation) shows itself to be logically "right" vs another interpretation, it will probably only do so by improving or expanding the theory (and its math), or by suggesting a new way to test the different interpretations. Otherwise, even if an explanation is "completely logical", yet it makes the exact same predictions as another explanation, mathematically, with no test, then there is no way to show that the other explanation is wrong.

### quantim leaping of electons

Because it has never been detected, of course, does not mean that it never could be detected, re: physically traversing the distance between electron orbits. I consider it a matter of logic. If an electron is in fact physical, then it would have to disappear, transform into something else and then back into a particle. I understand almost the entirety of Quantum Theory has such assertions which seem illogical. I do not think leaping is a simpler explanation than traversing simply because detection in transit has never been observed. In this way my thinking is more in line with Shroedinger, Einstein, De Broglie and others who believe(d) that particles always have existence in one location or another, not disappearing and reappearing, or multiple locations at one time.

"Never been detected" is not the same as "not detectable".

If something is theoretically undetectable (and not just that detection is currently unfeasible), then it can never be detected according to that theory.

If it is ever to be detected, the theory must be modified or replaced.

It doesn't matter who you are (Shroedinger, Einstein etc); if an idea is based on belief and not theory and observation, then including that idea in a model is an interpretation of the theory.

Particles traversing distance may be a simpler to conceive interpretation than leaping.

However, "simpler" is not just about what seems to make the most sense without having additional questions to ponder. Its about specifying the model as efficiently as possible to minimize the number of additional assumptions that aren't a consequence of the observable evidence. So, the idea of particles leaping may be abhorrent, but if all that it means is that the particle is in one location at one time and in another location at another time and you don't specify or care about what goes on between those 2 spacetime coordinates, then it is simpler.

I'm not sure what QM predicts is undetectable or unobservable, but I know that it does predict that some things are. For example the uncertainty principle says that some measurements are physically impossible to make, even with any yet-to-be-imagined measuring technology. To be able to measure things that the uncertainty principle says are unmeasurable, you wouldn't need just better instruments, you would need a new theory.

I'm not sure what the various theories say about the detectability of particle traversal vs. leaping.

### Ontology of time

In this way, events happening in a different order from a vantage point of different stars is not hard.

Well yes, a finite speed of light makes it easy to consider events being seen at different times from different locations.

But consider this:

If the current moment is a universal present, then it seems safe to assume that any previous moment could also be considered a universal past moment.

Then if you consider a past momentary event, that event would have to have occurred in a universal moment.

Some other event at a different time would have to have occurred in another universal moment.

Since these moments are universal, they would have to occur in the same order according to anyone (otherwise, when the first event occurred according to one person in a universal present, it had already happened according to another person, in which case it was a past moment, not the present).

Therefore, presentism and this line of reasoning implies that lack of simultaneity (not just the appearance of it) is impossible. Yet SR implies lack of simultaneity.

Is there any flaw in my reasoning?

It might be possible to argue that once the "present" passes, it loses its universality, and that past moments are not universal, making it possible to change the order of past events. However I don't know if this argument could be tenable.

Lack of simultaneity is a reality; the simplest form of presentism must not be.

### quantim leaping of electons

Einstein and others believed that electrons like photons were particles in the classical sense in that they accordingly must "traverse the distance in between," whether this "traverse" can be detected or not..

If there is no possible detectable difference between particles "leaping" from one place to another, vs. traversing the distance in a classical sense, then either case would be equally valid in a model of reality. Unless some test can show that one or the other is invalid, there is nothing to say that one is right and the other is wrong. If there's no difference, the two should be interchangeable. But, I think it would be pointless to say that something exists in a location if there is no way to detect that it was ever there, so if there is no evidence that particles traverse distances, then leaping those distances would be a simpler explanation (ie. a model would have the particle existing in one detectable place, and then in another detectable place, and not specify any other locations for which there is no evidence of the particle existing).

I think it's important to consider that any "leaping" would have to obey the law of causality. If information is transfered over a distance (the leap), then it must do so at a speed <= c (but probably =c). So if energy disappears from one location and appears in a distant location, it would also need to appear at a later time than it disappeared.

### Ontology of time

In contrast, one version of presentism (with which I agree) asserts that now is ongoing, everywhere, i.e., that the present is present everywhere, that there are not an infinite number of "time environments" as in the "light cone" model.

Philosophically this perspective transcends relativity's focus on (and obsession with, I would say) what frame of reference sees what images (or gets what information) from where and when. In other words this version of presentism does not take the speed limit of light and resulting "signal delay" to be the ultimate limit and ontological absolute for what is real and existing in the present in the whole universe. Relativity can not be denied for for what it covers (leaving length contraction and time dilation out of it for now.)

Once we get over the reification of time as an entity (see my argument for time as only event duration for physical processes, EDPP) we can begin to see that the present is omni-present.

We must begin by realizing that "is" means now... the present, and there are no local "time environments" for each supposed "point in spacetime" at the apex of each imaginary "light cone."

How do you reconcile this presentism with lack of simultaneity?

No matter how you explain it, you will be able to find examples where events--non-causally related ones--are seen and determined to occur in different orders for different people. Do you have an explanation for that that is compatible with presentism?

I have my own interpretations of SR that I thought were compatible with presentism.

I tried to make it work but I just can't provide an explanation of how different events could occur in different orders, and yet have a single "present" that is shared by the different observers.

The only way I've been able to make it work is to "flatten" time at all locations to a single instant (meanwhile treating the universe as a point singularity).

This treats "duration" as completely subjective or perceptual.

If you remove duration to make presentism work, then time becomes nothing but causal relationships with no concept of anything "taking time".

But this removes so much from the concept of time, that the meaning of present is completely changed, if not lost entirely.

Are you willing to consider such bizarre ideas, or does your version of presentism work with evidence from "plain everyday experiences", while denying things like lack of simultaneity? If the latter, then I'm not interested.

### General Relativity

Equivalence principle: is it exact? If yes, then why A. Einstein put more than one mathematical model for the GR in the beginning and waited for the proof? And, what about the negative sign that seems to have reversed the meaning of the Equivalence principle, as asked by many scientists?

I dunno but if no one else has an answer I'll stab at it...

Yes?

There are multiple mathematical models because there are multiple possible "shapes" of gravitational fields.

The typical example of an accelerating rocket is not exactly equivalent to typical gravitational fields, ie. those around spherical bodies.

I believe the rocket would be exactly equivalent to a homogeneous gravitational field maybe?

An observer in a rocket sitting on a (tiny) planet's surface might be able to detect subtle differences in the direction of gravitational force at different locations within the rocket, or the gravitational gradient between the top and bottom of the rocket.

I suppose the equivalence principle works exactly for different types of acceleration and corresponding gravitational fields. Gravity on Earth would be equivalent to some imaginable rocket whose parts are not all accelerating exactly the same.

### Gravity and Light

The question is: Why do photons move with the maximum possible velocity permitted in nature? Because it is their nature. Then, there may be a relation between Relativity and Electromagnetism. This is what Einstein were trying to do, if I am not wrong.

I believe that anything can be considered to be moving at c. It is more of a standard speed than a maximum speed.

c is essentially a constant linking distance and time.

I'm pretty sure this view requires some misinterpretations of accepted science! Hopefully someone can correct me.

The details are:

- All energy, unobstructed, travels at c.

- Energy and mass are equivalent. One might consider mass to be "made of" energy, however I remember reading comments that suggest this is misleading?

- Therefore all particles or energy that can be considered to be moving < c according to some frame of reference (including their rest frame), must oscillate or change directions. If the energy moved in a single direction, it would move at c.

- If energy is moving at c, but oscillates or changes direction, then after a time t its total displacement will be less than the total distance traveled by the energy. So if you consider a moving particle, the energy that makes up that particle is moving at c, but the particle itself can move at speeds < c. As an analogy, if a sailing ship moves from point A to B in a time of t, its (average) velocity is ||B-A||/t, but if it is tacking in the wind its speed through the water can be much greater.

So, if you are considering a mass moving in a straight line, its velocity is its displacement over a given time, while the speed of its constituent energy is the total distance that energy oscillates over the same time (which will always be c).

With this view, the maximum speed occurs when displacement and distance are the same, ie. movement in a straight line.

In which case, light etc. travels at c in a vacuum because it travels in a strictly straight line in a vacuum.

### Maintaining the appearance of fixed distance in spite of length contraction?

The thought experiment you propose sounds like "Bell's Spaceship Paradox":

Is this the situation you're thinking about?

Yes, I think that the link provided gives an answer to the question.

In the "Analysis" section, they set up a good simplification (treating Frank and Bert in our examples as point masses).

The applicable bit is: "This implies that xA(t) − xB(t) = a0 − b0, which is a constant, independent of time. In other words, the distance L remains the same. This argument applies to all types of synchronous motion."

They go on to show that "when switching the description to the comoving frame, the distance between the spaceships appears to increase by the relativistic factor [gamma]. Consequently, the string is stretched."

In other words: If the front and back of the ship can be accelerated using synchronous motion, then yes, the length of the ship will remain the same, and will appear to remain the same for observers which see the front and back synchronized.

It should be easy to show the following: Iff all points on the ship can be accelerated using synchronous motion, observers which see it synchronized would see no length-contraction deformations of the ship. Otherwise, sections in between the synchronized points would stretch in accordance with the wikipedia article.

### Frame of Reference as Subject in Subjective Idealism

Yes. More like the impossibility of "visualizing" a fourth spatial dimension, since three axes fully describe "space" as a 3-D matrix. (What direction is signified by a fourth axis?... not "time" as a fourth spatial dimension.))

This will make little sense until you understand the ontological debate about space and time.

I have provided links (many times) to the ontology of the spacetime debate. Little response. The International Society for the Advanced Study of Spacetime has presented many scientific/philosophical papers on the subject over the last decade of conferences on the subject. Mostly ignored as I have presented links in this forum.

I suggested that you read my favorite author on this Euclidean/ non-Euclidean debate, (as pertains to relativity) but you have not commented on the paper linked above. Too busy to read new information? I don't know. Just guessing. You must be busy as a science website administrator.

Please put this thread in the philosophy section where it can be formally ignored by physicists who don't care about the ontology of time.. or space.. or "spacetime."

Thanks.

I couldn't find any actual links that you're referring to. Can you repost the links, or a link to the post containing the links? Links to specific papers and even references to sections within them would be appreciated... no one wants to wade through the ISASS site trying to find writings that back up your views.

Is the author you're referring to Dennis Dieks?

Have you read any of Hans Reichenbach's work? I myself haven't, but I see references to him in stuff that makes sense to me.

I don't get Dieks, personally. For example, in the first section of http://www.phys.uu.nl/~wwwgrnsl/dieks/becoming.pdf, he references Reichenbach and an idea (Conventionalism) that makes sense, but then concludes from it an idea that I can't make sense of (a "global shifting Now").

Conventionalism (http://en.wikipedia.org/wiki/Philosophy_of_time#Conventionalism) seems like a useful idea for your "ontology of time", because it seems to provide a means to sidestep GR, perhaps treating it as an arbitrary interpretation of time that is agreed on by convention. However, defining an authoritative distance (a fixed diameter of the Earth, etc) seems to be aiming in the exact opposite direction (Asbolutism or something).

### Maintaining the appearance of fixed distance in spite of length contraction?

No. Bert would require Frank to finish adjacent to the bow of the Old Model Spaceship, which would have required him to move in the opposite direction at much higher than light speed, yet remain at rest with respect to him at the same time.

I defy you to find any rate of acceleration that wouldn't produce a similar contradiction (ie. Frank moving backward or seen moving backward by Bert, at any speed) by your reasoning.

Meanwhile I'll try to explain your hypothetical scenario differently...

In the scenario, the New ship is synchronized to Bert at the back's clocks.

Bert sees Frank at the front begin to move at the same time that Bert does.

Bert also sees the ship reach full speed just as he's pulling alongside the back of Old ship.

Bert writes: "Cabooseman's log, stardate... whatever. We have begun our journey at the same time that Old ship came by, and we are side-by-side."

Old ship was contracted to 0.5 LY length, but now it is back to 1 LY length. The front of Old ship should be beside the front of New ship, just as full speed has been reached.

However, Bert will not immediately see this, because light from the front of the ship is still delayed by 1 LY.

What Bert will see might be described as a wave traveling up the length of the length of the Old ship, changing the ship from length-contracted to rest length, but not all at once.

I believe this would take a year to complete. Only after a year, does Bert see the front of Old ship coincide with the front of New ship. He sees Frank wave and give a thumbs up, 1 year into the journey.

In other words, Bert sees the front of the ship continuing to move forward, catching up to Frank after a year of Bert's time.

This is consistent with what Frank sees.

Frank's clocks are synchronized to Bert's. According to Frank, he starts moving long before Bert does.

Frank might write in his log: "Captain's log... I have begun our journey long before the back of Old ship is scheduled to catch up to Bert."

Frank matches the Old ship's velocity, but this happens a very long way away, so Frank doesn't observe this happening immediately.

Frank will observe the Old ship continuing to appear to be moving toward Frank, even though Frank knows he's moving forward too.

I don't know how to explain this properly. One might say: If Frank were now at rest relative to Old ship, the light from the observations of Old ship would still be catching up to Frank at a velocity of c. No matter what velocity Frank takes on, the observations will still catch up to Frank, so Frank can never see himself outrunning (or at rest relative to) those observations... In other words, Frank will still see the Old ship continue to catch up.

I think it's valid to say that Frank does not actually reach full speed AND enter the rest frame of the distant Old ship simultaneously. This is not a contradiction, because the 2 are separated by a great distance when Frank reaches full speed.

I'll stop with the details here because I know I'm going to screw them up.

Frank sees himself start the journey early, and after a long time (1 year I guess it must be) he sees Old ship's front catch up and come to rest next to him.

He waves to Bert and gives a thumbs up.

Different observers see different events happening simultaneously.

There is no contradiction or impossible situation here.

Addendum: Bert does not join the rest frame of the back of Old ship at the same time he joins the rest frame of the front of Old ship, even if it's the same frame.

Frank does not join the rest frame of the distant front of Old ship at the same time that Frank reaches full speed.

Things from one frame can switch to a different shared frame at different times, if those things are separated by distance.

### Maintaining the appearance of fixed distance in spite of length contraction?

If at noon, you begin moving away from me at v=.866c, and then immediately (negligible delay) you send a signal to tell me to start, I will receive that signal after 0.5s by your clock, which is after 0.5 *gamma = 1 second according to my clock, as observed by you. So again it works out.

This description and calculation is so wrong its embarrassing.

I got the relativity of motion and of light wrong.

I applied time dilation backwards.

### Maintaining the appearance of fixed distance in spite of length contraction?

I would say it implies that Frank and Bert can not remained synchronized.

But Bert can remain synchronized to Frank, according to Bert.

Then they are not synchronized, according to Frank.

### Maintaining the appearance of fixed distance in spite of length contraction?

Can anything accelerate while all it's parts stay in exactly the same shared rest frame at all times?

Yes, iff it can be treated as a point particle.

In general, no. I think we all agree that the sharing of frames is relative, and that if all parts of the velocity-changing ship share the same frame according to one observer (point) location on the ship, then other observers on the same ship will not see all the parts of the ship remain in the same frame.

The instant acceleration is just to simplify the thought experiments.

If you could build a ship where all of its parts can accelerate the same way (ie every point location on the ship is part of a propulsion system that accelerates the same way as every other point), and you synchronize the time at all locations on the ship to one point location's clock, and you have all parts of the ship coordinated to accelerate and decelerate at the same specific times according to their own local clocks, then the whole ship will remain in the rest frame of that one point location, according to that one point location. But the same observation won't be seen by any other part of the ship.

Is there an absolute limit on acceleration that depends on length?

I don't think so.

If it's possible to build a ship that stays in the rest frame of a single observer's location on the ship, then according to that location, the ship need not experience any stresses while accelerating, at any rate.

Therefore, the ship need not be torn apart due to the impossibility of synchronizing its parts.

If it doesn't get torn apart according to one observer, it won't be torn apart according to another observer. Other observers will necessarily see the ship affected by length contraction, and its various parts moving at different times, however it must be that other observers also observe the ship experiencing no stress. Any deformations (of space or material) observed will balance each other to allow no stress.

There are likely engineering reasons that make such a ship impossible or infeasible to build, but I don't think SR itself limits acceleration.

What happened? Is Frank in the same rest frame as Old Model Spaceship? Is he in the same rest frame as Bert? Is Brand New Spaceship still intact?

Yes, Frank is in the same rest frame as Old Model Spaceship, and the same rest frame as Bert. The ship can remain intact.

When you say the ship was synchronized, that must be done according to a specific clock.

In this example it seems like it's synchronized to Bert's clock.

Your example is certainly puzzling, but I think you're failing to consider changes to relative simultaneity that occur during frame switches.

I'm sure that if this was considered, then you'd find that if Bert was next to the stern of Old Model Spaceship immediately after finishing accelerating, then Frank would be next to the bow immediately after accelerating, even though the timing of various events would be different for the different viewpoints.

I think your outcome implies that Frank and Bert remained synchronized both to each other and to the Old ship???, which is impossible.

### Chronular fantasia theory

Quite a few people on the internets seem to simply not "believe" in SR, and are trying to figure out time while holding onto that belief.

I don't get why anyone would try to figure it out while denying the reality of SR. I can see questioning the evidence of SR, but given the evidence, trying to figure out time according to ideas incompatible with SR seems to involve figuring out something incompatible with reality. How can you figure out time while reasoning about ideas that are not based in reality? It is essentially reasoning based on fantasy. You can claim anything you want to, if you're not concerned with reality.

For fun I thought I'd try it (while still considering the forum rules). Perhaps consider the topic to be "any fantastic ideas about time" and feel free to suggest wildly different directions. This is for fun but the value of it might be in inspiring new ideas?

Chronular Fantasia Theory (CFT)

Assumption 1a: SR is wrong wrong wrong!

Assumption 1b: (Corollary) Reality is also wrong.

Reasoning:

1. Events are clearly made up of a series of individual instants. Instants can be considered infinitely thin (in the time dimension) slices of reality.

2. Since instants have no depth in the time dimension, it is impossible for any 2 instants to overlap each other. Therefore no 2 instants can occur at the same time. One must always precede or succeed another. That is, no part of an event may happen at the same time as another part of any event. Therefore, no 2 events can happen at the same time.

3. Since all instants are separate, they can be put in a definite order, like a stack of library index cards. The passing of time involves going through these index cards in order, and implementing the "instant" described on the index card.

4. Since any 2 particles moving would have to be considered separate events, it's clear that an instant can only involve at most one particle. So an instant must be a single change that occurs in only one place in the universe.

Conclusion:

The passing of time is the sequential processing of tiny individual changes to single particles.

Events that you witness, such as a blade of grass blowing in the wind, involve many particles each waiting their turn to move, in order. These changes are interspersed with all the other minuscule changes in the universe.

It only seems to happen all at once because of the super high rate at which the instants occur. I'm talking about millions of instants per minute!

5. It must be that all instants occur at fixed intervals in the time dimension. This is what makes it appear that all clocks tick at the same fixed rate.

### cycles of reality

If I am correct, one might possibly time travel if you could get some very specialized equipment to record the pattern shifts of a particular field of atoms in space, and you'd also have to have a some very specialized equipment to somehow run this cycle again, but, this would be a new instance of a past moment. Kinda like that movie, Ground Hog Day.

I've always thought that if you could take a system and restore its state to a previous state, you'd effectively have it time travel to that past state.

If it could be done so that there is no way to tell the original state from the restored state, there would be no difference between this, vs. the system time traveling to the past.

Unfortunately, it is not possible to do this with systems where distances are involved, due to lack of simultaneity.

Why? Basically, the idea requires saving and restoring the state of a system as it exists in a single instant. But a single instant is different according to different locations within the system.

Or as you suggest, recording the patterns of atoms in space... this could only be done according to the timing of one observer, I think.

I think that what this practically means, is that the system can return to a previous state only according to a single observer.

Since doing this would not restore the same past state according to other participants in the system, it would involve restoring the system to a different state than the past one, and thus it would be impossible to replay the system the same as it was the first time around. Even without the uncertainty principle (which I kinda suspect is really just a consequence of or similar to lack of simultaneity), you could not return to the past (ie. a past state) and relive it.

Or perhaps I'm wrong or there's another way around it. But I think time travel to the past is impossible for any system with more than 1 dimension.

### Light velocity independent of the source velocity.

What is not given is that when the star moves across the line of sight, still it will not acquire velocity of the source. In short photons emitted by the star in the direction perpendicular to the motion of the star will not move along with the star. If this was not the case then we would miss the photons and we will not be able to see the star when it is moving perpendicular to the line of our sight.

Am I correct?

Yes. The propagation of light is isotropic, meaning the same in all directions.

You can't directly observe signals sent perpendicularly to you, but you can receive information sent from different events (the simplest being if the light signals are also reflected to you at both the source and destination), and will always calculate the speed of light between any 2 points in a vacuum, to be c.

### Maintaining the appearance of fixed distance in spite of length contraction?

I agree with swansont. I can't find a way to force a single observer to see the ship's length change, without there being a simple way to compensate.

Most other observers would see the length between the observer and the object change. It's easy for me to mix up frames and get stuck thinking that their rest length must change.

Is a rest length between two relatively moving points even defined?

Sharing a frame with something that changes velocity, will depend on timing. Due to lack of simultaneity, there is no absolute sharing of a frame by multiple objects. Whether 2 objects are in the same frame or not depends on how they are observed.

In clock synchronization you account for this delay. If we are 1 light-second apart, and I send you a signal at noon, you know you receive it at 12:00:01

If at noon, you begin moving away from me at v=.866c, and then immediately (negligible delay) you send a signal to tell me to start, I will receive that signal after 0.5s by your clock, which is after 0.5 *gamma = 1 second according to my clock, as observed by you. So again it works out.

But from my perspective, if my clock is synchronized to yours, and I know that at noon you'll begin moving...

Suppose for now I'm just observing signals from you, and not trying to stay in your rest frame.

If gamma = 1 (no movement, just a signal), I would receive it at 12:00:01.

But with gamma = 2, your frame switch would change the synchronization of our clocks. When it's 12:00:00.05 for me, I think it's noon for your clocks, according to me??? (This is only for an instant where you've instantly accelerated to 0.866c but before you've covered a considerable distance.) I would receive your signal after 0.5s, again at 12:00:01.

In previous posts I must have been incorrectly dealing with the synchronization change.

So it seems the details of a complicated viewpoint balance to match the outcome of a simple viewpoint, which should be expected.

But there are always further complications to consider!

According to the above,

When it's noon my time, it's noon your time and you are 1 LS away.

When it's 12:00:00.05 my time, it's noon your time (in your new frame), and you are 0.5 LS away.

However, due to the travel time of light, I won't actually observe any disruption in your passage of time, and I will see you instantly jump from 1 LS away to 0.5 LS away, at 12:00:01.

I think...

### Maintaining the appearance of fixed distance in spite of length contraction?

If we are neglecting the acceleration time as per the OP, why?

Because to maintain clock synchronization, the object would have to get a head start on the observer.

A signal to begin a synchronized start would have to come at the same time as the observation of the object beginning that start, and these would be delayed by the speed of light.

So you're changing the rest distance between object and observer, and are thus forced to change the lead time that the object acts with.

But I think this reasoning is wrong!???

I'll try to work through an example with "signals" instead of "observations", which usually gives some insights.

Suppose observer O and remote point P are 1 LY apart, and P is sending 1 signal every day.

At some arbitrary time, P will move in the direction away from O, at a speed of 0.866c, for a given number of days, and then stop.

O knows this much information and will attempt to remain stationary relative to O.

At rest, O is receiving signals sent "a year ago" from P. There are currently 365 signals "en route" from P to O.

When it happens, P begins moving away from O at 0.866c.

This length contracts the initial distance between them to 0.5 LY.

Due to an update in simultaneity, only a half year has passed at P relative to O. There are now only about 365/2 ~= 182 signals currently "en route" from P to O.

According to causality, O will not be able to observe any change in the signals, including their rate, or their source distance, for at least half a year. It must receive those 182 signals exactly as if no length contraction has taken place.

O then receives the signal to begin moving, 0.5 years after P begins moving.

O instantly accelerates and is now at rest relative to P.

Let's suppose this can happen in much less than a day, and timed so that O never observes any of the daily signals being length-contracted.

I would assume that the rest distance between them will now be greater than the original rest distance, because P has been moving away during its head start.

But is this a mistake??? The rest distance now applies to a different frame, so I don't think I can easily claim this.

Okay, I can't figure my way past this part.

In a "stationary observer's frame", P has had a 0.5 year head start at 0.866c, so the rest distance between O and P is now 1.433 LY, however this stationary observer would not currently observe O or P at rest, so would probably observe that distance under length contraction.

What is the rest distance according to O and P, who are currently at rest relative to each other?

It would have to be the same distance for both of them, so how is this possible, when P observes O starting much later???

I'm guessing it would have to still be 1 LY, but I can't figure out how this is so according to P's perspective!

Okay so according to P's perspective...

P starts moving. Distance to O contracts to 0.5 LY, and O moves away at 0.866c.

O continues moving away until it is 1 LY away, and then it remains at rest relative to P for as long as P remains in this inertial frame.

I don't have a firm grasp on why this is.

But yes, I now agree:

1. If an observer can sync its movements perfectly with a distant object, the observer will always appear to be at rest relative to the object.

2. In such cases, the rest distance between object and observer seems to remain fixed.

A distance can be maintained, from the viewpoint of the observer or the object, but not both.

Agreed, but I was wondering if it was even possible from just one of those viewpoints, because it didn't seem intuitive in my example.

I suppose the solution is typical: If one looks at it in the simplest way, the right answer may come easily. If one looks at it in the most complicated way, the wrong answer seems to come out, until all the effects of SR are properly accounted for, at which point the right answer again works out.

### Maintaining the appearance of fixed distance in spite of length contraction?

It will always be 1 light-second away if it stays in the same reference frame, i.e. it is at rest with respect to the observer.

Is this true even if the frame isn't an inertial frame?

If the observer and object change velocity (relative to some other frame) at "the same time", even if separated by say a light year, does their relative simultaneity remain fixed as long as they don't move relative to each other? And thus we can talk about "simultaneously" for the entire frame, no matter how big it is?

If the object and observer moved independently, but in sync... Suppose the object was allowed to choose whether to move along with the observer's reference frame, or stay stationary relative to the other frame.

If the observer moved at 0.866 c for 1 second and then stopped, but the object was 1 LY away, the observer would not be able to tell whether or not the object moved, until about a year later. Does this mean that any possible visible length-contraction effects would appear one year later?

I assume the answer is "no", and that length contraction would be apparent immediately. In which case, for the object to be synchronized with the observer, it would actually have to start its movement early... (by half a year in this case, with gamma=2, I think).

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