Does time exist?

roosh wrote: »

But in the specific instance of determining whether both observers will read the same measurement on the track ruler what we are interested in is not how much the light has travelled with repsect to Observer 1, but rather how much the light has travelled with respect to the track, to see how much of the track has been illuminated by the light.

with regard to the sphere(s), if it is the same sphere, with different centres, then it won't necessarily be a sphere - I picture it to be somewhat like how a wormhole is traditionally depicted.

Also, to say that the computer (or the rod) has two locations is somewhat of a misnomer, I think, because it blurs the lines between what is meant by "location"; just because we are standing different distances from it, it doesn't mean it has two locations. If you superimpose a theoretical, mathematical co-ordinate system on reality, and I do likewise, then we might argue that it has two locations, but that isn't actually the case because neither of us owns the "true" co-ordinate system.

I'm inclined to put more trust in your own mathematical ability - simply because I am familiar with my own but my initial impression, and this could quite easily be down to an error in my reasoning, is that saying that Observer 1 passes the 0, mark on the ruler at, t'=0 looks like a bit of a red herring - not in the sense that you are deliberately trying to mislead, but because when Observer 1 passes the 0 mark both the light and the track have already been travelling.

In Observer 1's reference frame, with the rod following behind the light, at 4/5 of its speed, the light will only ever have moved ahead of the rod by 1/5 of the overall distance travelled by the light, thus it will only have illuminated a length of track measuring 1/5 of that distance, and hence that graduation on the track - by the time it reaches Observer 1.

On the other hand Observer 2 will see a sphere of light centred on the rod, and so it will illuminate 5/9 of the distance by the time it reaches Oberver 1. Even allowing for contraction, it appears as though they will see different lengths of track illuminated.

All that being said, with respect to the derivation in post #54, the conclusion appears to be assumed in saying that t' = ɣ(t - vx/c^2). The derivation of ɣ as per the video you posted is something which the author also challenges.

In this reference frame none of those events are real any longer, but that would just be in this reference frame. My first birthday, with respect to this reference frame, is now an abstraction; but if that reference frame actually exists then it has it's own present i.e. if me as a one year old exists, the [he] is currently having [his] first birthday (according tho him). So if the entire universe is paused then the present is made up of all those present reference frames.

What would be the case thought if one of the presents was chosen as the moment to pause the universe; where that present is "the past" of another reference frame?

Oberver in motion
This is related to the point above about the "true present"; it might appear to be going back over old ground, but, through my own fault, there was an inherent assumption which was never addressed; the assumption relates to the notion of "past" and "future" reference frames.

The notion of the "universal pause button" was raised after this, while allowing the assumption of "past" and "future" reference frames; "past" and "future" reference frames are however challenged by the notion that Observer 1 would conclude that it is he that is moving relative to the rods and the "beam" of light.

To address the issue: "What must we be stationary with respect to in order to determine the true present?"

Our ability to determine the true present has no bearing on its existence, but in the context of Observers 1 & 2, if Observer 1 correctly [in my opinion] concludes that it is he that is in motion, with repsect to the rods and Observer 1, then he would come to the same conclusion - that the lightning strikes occured at the same time - as Observer 2.

Again, the issue of how Observer comes to the conclusion might need to be addressed, but you would imagine that if he arrives to the train station before his train departs and instead of boarding the train immediately he stands on the platfrom and watches the train start to pull out before boarding it, then he would conclude that it is, in fact, he that is in motion relative to the rods.

Alternatively, if he were to wait with Observer 2 until the train was approaching and then walked a distance to get picked up by the train (using it's sophisiticated device for picking up passengers while in motion), which allowed him to reach the inertial velocity of 0.8c before reaching the rods, he would surely conclude that it was he that was in motion and not the passing countryside.

roosh wrote: »

In a discussion on another forum someone has said that "because of technical limits" length contraction hasn't been tested directly, but has been indirectly tested through positive time dilation experiments. Is that accurate?

roosh wrote: »

I don't think it's quite as simplistic as that though is it? It would be more like "grid with centre of sphere located above rod" and "grid with moving rod with centre not above rod"; in terms of a ball, it would be similar to the ball being beside a post in one grid and not beside the post in another.

I don't think the author's experiment demonstrates any anisotropy in the speed of light either

that is what we are talking about though, isn't it? the locomotion of the train with respect to the surface of the earth?

But if you or I are travelling in a train and it slams on the brakes, we will be jolted forward initially - Is that something to do with us inhereting the momentum of the train?; if it were the earth that were moving the same effect wouldn't be felt, would it?

similarly though, if he were to stop suddenly wouldn't there be a discernable difference, which would help him to conclude that the log was spinning as well.

OK, I was getting a bit confused, I was thinking the blue dots (as per your initial diagram) corresponded to the black dots i.e. the blue dot "events" were the same as the black dot "events"; because that would have meant that events which were in the past of one reference frame were also in its present, while other events which were in its future were also in its present.

The "timelines", that are drawn there, only represent two reference frams of the infinite number of reference frames; the true present wouldn't consist solely of just those two reference frames, it would include all reference frames - how would that be graphed?

Using just the two reference frames above, the true present would consist of both lines. Sticking with the train example: at t=0 the lightning strikes would be in the present of Observer 2's reference frame and so those events would be real in Observer 2's reference frame; while the strike at B would have occurred already in Observer 1's referene frame so the lightning striking B is no longer a real event - in Observer 1's reference frame - it is in the memory of Observer 1.

Just a further point on that: I'm not sure where Karl Popper's philosophy, with regard to falsification, ranks among the scientific community, but if applied in this instance then the author's observation would be sufficient, if not to falsify the theory, then to at least challenge it.

Is there a possible explanation for the alleged phenomenon, as outlined by the author?

roosh wrote: »

The issue I have, i suppose, is with stepping out of the mathematical hypershpere and into "real space", and vice versa; because it's not quite as simple as the ball being in front of one post and behind another, it is a case of remaining beside the post in one reference frame, while the post is moving away from it in another, where both balls and posts are the same.

It also seems as though the problem of the constancy of the speed of light would be addressed if the observer on the train were to conclude that it is the train that is moving relative to the surface of the earth, and not the earth that was moving.

The analogy doesn't accurately represet the [alleged] observed phenomenon; because the dot doesn't shift from centre in the direction of motion, but in the opposite direction; so it doesn't actually inheret speed.

It would actually have to inheret speed to hit the centre, because the location of the sensor - in "real space" - would have changed from when the the pulse was sent, with the orbit of the earth in "real space", or reality.

It is more like "space invaders" where both the ship and the "alien" are moving in the same direction, say to the right of the screen; if the ship fires a shot when it is located directly at the centre point of the alien, then it won't hit the alien in the centre point, but in a point to the left of the aliens centre, becuase the alien will have moved by the time the pulse reaches it.

I don't doubt that the two can be reconciled, because the nuance of relativity is such that it allows for the "re-graphing" of all observed phenomena from an infinite number of reference frames; to the extent that, as far as I can see, the geocentric worldview cannot necessarily be determined to be incorrect - and isn't necessarily incorrect.

It also appears to rely on an apparently ridiculous assumption; namely, that for the relevant observer, the rotation of the earth just happens to increase, such that the relative change in distance is exactly the same as they would expect were it the train that were moving relative to the surface of the earth, at the given velocity; not only that, but the rotation of the earth just happens to increase at the exact moment that the accelerator is pressed; not only that, but the rotation of the earth also slows down at exactly the same rate as they would expect the train to, when the brakes are applied; again, all this occuring to co-incide with the exact time they apply the brake.

The consequence of that reasoning is that the engine of a train doesn't actually propel a train forward.

does the application of the brake cause the geometry of spacetime to slow the earth?

also, are there not effects directly (or even indirectly) related to the speed of the earths rotation that could be tested to see if the rotation of the earth has actually changed?

Not necessarily. If you are typing at t=0 (the pushing the pause button) then I amn't reading it, I am doing something else.

again, it's not that the event is real or isn't, it might be clearer to simply state that it isn't in that reference frame; it may be similar to the manner in which a sphere of light can remain centred over a moving rod and remain stationary elsewhere, simultaneously.

cheers, I must have a look at those, but my immediate reaction is how does a changing electric field verify anything other than a changing electric field.

roosh wrote: »

Would we have to use Jupiter; for our solar system could we not use the sun?

It might not necessarily "fall behind" the mirror, but it would not necessarily hit the mirror where expected, meaning that it would be somewhat off; this effect would then be amplified as the light is bounced from one mirror to another, meaning that it would also miss the detector. As the effect is dependent on the direction of motion it could affect just one of the beams, which could potentially give rise to the null result.

As is abundantly clear, I am no expert on such matters, but I haven't really seen anything that has tested this possible systematic error of the MMX.

"a twin makes a journey into space in a high-speed rocket and returns home to find he has aged less than his identical twin who stayed on Earth"

"Einstein, Born and Møller invoked gravitational time dilation to explain the aging based upon the effects of acceleration"
Twin Paradox

"Gravitational time dilation is the effect of time passing at different rates in regions of different gravitational potential; the lower the gravitational potential, the more slowly time passes"
Gravitational Time Dilation

The twin paradox, as it is usually formulated, involves one twin going into outer space and the other remaining on earth - I presume this means that both occupy regions with different gravitational potential.

However, if we formulate the thought experiment using the two twins as our observers, then it doesn't involve either twin going into outer space; theoretically the experiment could be conducted using the high speed train traveling around the surface of the earth, with the track designed such that both remain equidistant to the earths core - presumably this would mean that both occupy regions with the same, or very similar, gravitational potential.

In this scenario, where there is no change in gravitational potential, both "should paradoxically find the other to have aged more slowly".

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The illusion of time scene, I'm pretty sure, is the "twin paradox", or at least a variation on it, demonstrating the same phenomena that the "twin paradox" is supposed to demonstrate; they simply added a region of space that would have [am I right in saying] a much, much higher gravity potential.

My misapplication of the term may lie in it's application to the "twin paradox" as opposed to an incorrect use of it, per se; that is, the "twin paradox" may not involve a gravity well, but it does involve one twin moving between regions with different gravitation potential - I took this to mean, moving through a gravity well.

It is still the difference of "mangaroosh" being at rest relative to the tree and the tree moving relative to "mangaroosh" - which is a paradoxical situation.

The "light cone" structure only represents two spatial dimensions, because it would no longer form a cone if it represented three.

If we take two observers who are at rest relative to each other, then they will both label each other as "being at rest" and there will be no observable increse in distance between them. For an increase in distance to occur between them there must be relative motion; this establishes a fundamental difference between being in motion and being at rest, such that it is not possible to be both at the same time; it also establishes that being in motion and being at rest is not simply a matter of labelling - the one or the other must occur before it can be labelled.

For there to be relative motion between the two observers either one, or both, of them must be in motion; both can't label themselves as being at rest [and both be correct] because at least one of them isn't.

But let's say they do, as per the thought experiment; but they decide to communicate with each other; one contacts the other and says "why did you start moving?"; the other will reply "I didn't start moving, you did!"; the other will reply, "I didn't start moving, you did!"; now, this could go on ad infinitum, or they could come to the conclusion that one of them, or perhaps both of them, but at least one of them, is mistaken.

Presumably this applies to all parts of the body, including the head?

roosh wrote: »

Very true. How does an observer determine then that they are "at rest"?

As with with Etienne Brauns, I am just questioning whether these potential systematic errors have been accounted for, not necessarily that they be used in conjunction with each other.

If the mirrors in the MMX were set up with a specific light-path in mind, then a variation on the path of the light would result in the beam being reflected to somewhere other than the intended location. If this meant that the beam did not enter the detector, then it would surely account for the null result. How do modern, more sophisticated versions demonstrate that this [potential] systematic error doesn't occur?

My apologies, while I knew that I didn't fully understand all the principles of GR, there were certain basics that I thought I had understood; which I am entirely less sure about now. I may also have been applying the everyday meaning of terms to terms which are used differently.This unfortunately (or fortunately depending on how you look at it) leads to more questions.

My understanding of gravity was that we experience "more gravty" while standing here on earth than we would if we were to go into space on a shuttle. That doesn't appear to be the case though; is this where the equivalence principle comes in?

I also find it somewhat confusing that they refer to "the traveling twin" as undergoing acceleration and deceleration, and switching between reference frames; which I would generally understand as being in motion, or "traveling".

They say that "the travelling twin" would analyse "the stay-at-home twin" as "freely falling in a gravitational field"; why would the "stay-at-home" twin not analyse it similarly; and why, from the perspective of "the travelling twin" would the "stay-at-home" twin not be observed to undergo the same acceleration and deceleration?

https://www.youtube.com/watch?v=SN77b9DqEbc

https://www.youtube.com/watch?v=xrjosOjvlOc

I had a look at that section but couldn't really understand the maths of it. Am I right in thinking that the reason that the paradox doesn't arise is due to the asymmetry (asymmetry of what I can't recall); that one of the observers undergoes acceleration and deceleration?

I would say that the paradox arises in the fact that both observers are labelled as being simultaneously "in motion" and "at rest". I don't think your formulation of the issue is representative of the problem; an observer being labelled as both "mangaroosh" and "roosh" at the same time is not the same as being labelled as "in motion" and "at rest", because the latter are mutually exclusive, referring to two different states.

Which, assuming it's veracity, would represent the present to the omniscient being, were he to press the "universal pause button".

We are, arguably, free to label either observer as "in motion"; however, both observers cannot label themselves as "at rest" if there is relative motion between them. Again, if they were to communicate with each other they may disagree with each other over who is "in motion", but they would have to conclude that they both cannot be "at rest", so one of them must be wrong.

On the question of consistency: if both observers are at rest relative to each other and certain objects in their reference frame, then their definition of "at rest" is, "at rest" relative to those objects. If observer A then experiences relative motion between himself and all of those objects, but observer B only experiences relative motion between himself and Observer A, then Observer A would not be consistent in his application of labelling, if he labels himself as "at rest"; or at least, he would be less consistent than Observer B.

If an observer is capable of moving every part of his body, and he exercises that ability, why wouldn't he conclude that he was "in motion" and not "at rest"?

[*]How did we move on from the Ptolmeic model of the solar system, if an observer would always label themselves at rest?

[*]How did anyone determine that the earth is rotating?

[*]How does the "laser gun" scenario mentioned earlier, reconcile with the "expanding sphere" phenomenon?

[*]If two observers are at rest relative to each other, but then undergo relative motion, there must be a cause of that motion; if one observer is "running", the other observer will presumably attribute the relative motion to the "running", but what will the "runner" attribute the relative motion to?

roosh wrote: »

Are the observers free to agree on the labeling i.e. can an observer label himself as being in motion?

The part that is causing confusion for me is, I think, to do with the use of the term "acceleration" because I associate that with motion.

If both observers start off on the zero-gravity station, will they consider themselves as freely falling in a gravitational filed? Why is it that the twin, who detects the gravitational field, doesn't conclude that he has accelerated and is therefore in motion?

Does that mean that his clock will tick slower? Again, my interpretation of the term "acceleration" would lead me to say that this is so bcos the force of acceleration/gravity would cause the other's clock to tick faster.

This is another source of confusion for me, as I was always under the impression that gravity was a force; a "fundamental force" at that.

I don't fully understand what is meant by "an expression of the geometry and chronometry of events" either; does that mean it is just a matter of labeling also?

Which of SR or GR applies to the following:
- Where the stay at home twin is on earth, and the other goes into space?
- Where both stay on earth?
- Where both start at the zero-gravity station?

I'm thinking SR for the latter 2 (but not presuming), and I'm not sure at all for the first one.

The problem, as I see it is, again, that both observers cannot label themselves as being at rest, because at least one of them isn't. This issue isn't resolved by the use of two separate and arbitrary co-ordinate systems where both observers label themselves as being at rest.

Again, if they contact each other and ask why the other moved, they would conclude that one of them must be mistaken - how would they determine which one was? Again, this isn't resolved by each determining that the other is mistaken, bcos they can subsequently contact each other and have the same dispute.

I don't think that comes down to Gallilean or Lorentz transformations.

--

But going back the to the issue stated above. If they are at rest relative to each other and then there is relative motion between them, both cannot label themselves as being at rest. If they communicate with each other, they will come to the conclusion that one of them must be wrong.

That's true, but from the perspective of the universe you would have the exploded sun, with the light traveling towards the earth.

From the perspective of the universe, would there past, present and future?

Is a frame of reference not defined by the objects in it?

Would this not mean that his model does not accurately reflect his empirical experience?

If someone labels the earth as rotating, and as orbiting the sun, does this mean that they cannot consistently label themselves as being "at rest" when on the earth?

I was just thinking in terms of how each observer would observe themselves as being at the centre of an expanding sphere of light; just for visualisation purposes, if we imagine that one of "sliver" of that sphere was the laser pulse, it would mean that in one frame of reference it would hit the observer on the platform, but in the other it would miss him. But that isn't the case, so I'm wondering how it is resolved; or how I've misrepresented the phenomenon.

But how? Would he not have to conclude that both he and his friend were actually in motion before, and that he had erroneously labelled himself as "at rest"? Wouldn't he have to come up with an explanation for why his running caused him to remain at rest, and why his friends decision not to run caused him to fall behind?

roosh wrote: »

That xkcd comic strip might actually be helpful in demonstrating where I am having difficulty. I understand that we are free label things as we please, and that we can perform calculations on that basis. I understand that we can label things as being in motion and ourselves as at rest; but, in the case of the cartoon, when Bond is at rest relative the scientist i.e. not spinning in the scientists device he won't "die" but something will happen which will cause him to "die" - or at least have a different experience to that which he had when at rest e.g. he will experience increased "G-force".

Again, I know he can calculate it from the perspective of him being at rest, but I don't understand what he will deem to be the cause of that change in experience; for example if he says he is experiencing a gravitational force, what will he say occurred to cause that change; will it be that everything started spinning in relation to him?

--

Again, I have similar trouble here. If both twins start off in the zero-gravity station, they will consider themselves as being in free-fall won't they? Will they not reason that something will have to happen for either of them to experience a gravitational force i.e. if they remain at rest they will continue to experience zero-gravity, but they will experience an effect similar, or equivalent, to a gravitational force if they start moving?

If the "traveling twin" experiences this gravitational force, while he maintains that his twin remains in free-fall, why doesn't he conclude that he is the one who is in motion?

Again, I know that he can calculate it from the perspective of him being at rest, but his experience of being at rest in the station with his twin is different to his being at rest when moving relative to his twin.

Has that scenario been tested; where both clocks start off at zero-gravity and one undergoes acceleration?

When you say that gravity is an expression of that geometry, what do you mean?

OK, sorry, so the paradox is resolved under SR because of asymmetry, but asymmetry doesn't apply in GR - have I that much right? Why is there no asymmetry in GR, does one twin not undergo acceleration and therefore change reference frames under GR, leading to asymmetry as well?

OK, I understand the part about the labeling conventions, but the issue I'm having might be more to do with what the labels "in motion" and "at rest" refer to. As said before, if both observers are at rest relative to each other and then there is relative motion, both of them cannot be "at rest", because at least one of them has to be moving. How do they determine which one moved? They can't both maintain that they remained exactly where they were and that the other moved, because if that was the case, then the distance between them wouldn't have increased.

That is what I was trying to get at with the idea of a universal present i.e. "what is 'real' in all reference frames"; no doubt you have a different interpretation of this.

Do all these hypercones combine to form the block universe?

In the physical world we can't remain at rest relative to an imaginary reference frame, can we, does it not have to be in relation to other objects?

This again is something I have trouble with. I understand the idea of being able to switch labels arbitrarily and being able to calculate taking either position, but as has been established, the act of being in motion is different from being at rest. If we arbitrarily switch labels we are saying that one thing is moving at one time and stationary at another.

Sorry, I probably confused matters introducing the sphere, instead of clarifying them. I didn't mean that the sliver would be at all angles, because that isn't necessarily how the laser would work is it. As you mentioned, the laser would miss the "stationary" observer in both reference frames, but if the laser were fired at the same instance as another light source (which forms a sphere), and from the same position, then the laser would form the radius for one sphere but not the other.

This is true, but if you start of by labeling the ground as being at rest, and you are standing, at rest, on the ground, then something must occur for there to be relative motion between you and the ground - or something must cause there to be relative motion, otherwise you would remain at rest relative to the ground.

If you then start running, but maintain that you are remaining at rest then there must be a reason why running can cause you to stay at rest; if the reason you give is that the ground has started moving, then there must be a reason why the ground has gone from being at rest, to being in motion, which leads you to label it as being in motion.

roosh wrote: »

OK, I'm not sure if I'm articulating very well, what it is that I'm asking. Sticking with the example of the zero-gravity station:

Both twins start off in the zero-gravity station - both are free falling in a gravitational filed (is that correct?).

Both label themselves as being at rest. If they remain at rest then they will continue to be in free-fall in the gravitational field.

One twins experience of the gravitational field changes. What does he attribute this change to i.e. what is the reason for the change in his experience of the gravitational field; or what happens to give rise to this different experience of the gravitational field?

---

If I am at rest relative to the ground, and then start running, and the ground starts moving because of a gravitational field, what caused this change in the gravitational field i.e. why wasn't it in effect before I started running, and how does it come into effect at the precise moment I started running?

Is the example of GPS clocks not different; does it not involve a clock in space and a clock on earth, as opposed to two clocks both either in space or on earth?

Is an accelerating object necessarily in motion?

Sorry about this now, this could be my association of the term acceleration with motion, if one twin is singled out as accelerating, how does he label himself as being at rest; and why can't either twin be labelled as accelerating under SR?

Also, why doesn't the paradox arise under GR?

Actually, you may be able to explain this also, because it is something I can't immediately imagine. In the portrayal of the block universe, with the reference frames, or the "slices" of "nows", the past is said to exist and also the future; or the reference frames do - is that roughly correct? Did you characterise it as there being a reference frame where I am celebrating my 8th birthday existing in the past?

I'm not sure what terminology to use, but to put it crudely, if we say that this reference frame we are in is tending towards the future, are all the other reference frames not tending towards the future also?

I'm just wondering how "my 8th birthday" didn't "keep up" with my reference frame i.e. it should be sharing the "now" with me.

I've grouped the following couple of quotes, because I think the point is related:

What I have difficulty with, is what the labels refer to, that they refer to mutually exclusive actions; which, for me, takes away the arbitrariness with which they can be applied.

In the physical world we can't be at rest relative to an imaginary reference frame, because it isn't physical; we will always be at rest, or in motion, relative to physical objects.

Given the fundamental difference between moving and not moving, it seems like some justification is required for the labeling.

Sorry, I was trying to explain what it was I was visualising, but didn't do it very well.

If we imagine an LED on the barrel of the laser gun, which flashes the moment the shot is fired; in the reference frame of the observer on the trian he will remain at the centre of the sphere and the laser pulse will form the radius of the sphere directly in front of him; but, for the observer on the platform the laser pulse will not form the radius directly in front of him, but to the side of him.

But, if we trace the imaginary line of the photon* which forms the radius of the sphere that is directly in front of each observer, in the reference frame of the observer on the train, the line the photon* takes will trace the line of the laser pulse; but in the other reference frame it won't.

I'm just wondering why the light behaves differently?

*I'm not sure if the photon is real or imaginary, bcos I'm not sure what the physical make up of the sphere is

roosh wrote: »

If we stick with the zero-gravity station, it might be easier to explain what I'm trying to get at, and might make it easier to highlight the error in reasoning.

If both observers are on the zero-gravity station they will experience certain effects: weighltessness; objects floating around the place bcos of the aforementioned; and whatever other effects one experiences in a zero-gravity station. These effects must have a cause; I'm wondering what that cause is.

My understanding is, that one twin will then experience certain effects which will differ from the "stay-at-home" twin. The cause you mentioned is a gravitational field, but I'm wondering what causes the expression of geometry to change such that it results in the "traveling" twin experiencing the different effects?

I understand that, but the twin paradox appears to represent a different scenario than GPS sattellites; that they are the same equations doesn't necessarily mean that predicted effect will be observed.

If one twin is singled out as undergoing acceleration under SR, then how can he label himself as being at rest, if he is, by necessity "in motion"?

OK, so the parts I don't understand are:
- Under SR, how can one twin label himself as "at rest" if he is, by necessity, "in motion".

- Why doesn't the paradox arise under GR, if neither is singled out as being in motion? My reasoning is that the paradox should arise, unless there is a mitigating factor; unless there is some asymmetry between the twins - by asymmetry, I simply mean a difference, other than merely visual, between their experiences.

What I am trying to do, is develop an understanding of what it means for the past to exist. I would agree that there is no unique "set of 'nows'", I would say that there is just the present moment - not present moment in time. I would agree that my 8th birthday happened before this moment, but I wouldn't say that my 8th birthday still exists; I have a memory of it, but I would say that it exists in my memory only.

By definition an imaginary reference frame needs an observer to imagine it, if "I" was an observer at my 8th birthday, then that observer "moved through time" to this present; if there is a reference frame where "my 8th birthday" represents the present, then do "I" exist in both frames of reference i.e. in my past and in my present?

And as you pointed out [in not so many words], it wasn't a correct model of reality, because the centre doesn't actually exist. What we can do however is speak about the relation of the sphere to the physical matter, with respect to which it moves; there will always be matter located at the co-ordinate we designate as the centre of the sphere.

In the physical world, you cannot move or be at rest relative to an imaginary reference frame, because it is not physical - this doesn't stop us using it as a means of communication, but it does affect our application of definitions, given that we cannot move relative to it.

This is probably the difference between absolute motion and relative motion. Absolute motion is simply the question of whether or not there is movement; if there is relative motion then, by definition there is movement, and so there is absolute motion. Determining what is moving, however, is a different matter.

Does length contraction cause the sphere to rotate?

roosh wrote: »

It appears as though what is being said, is that, according to relativity, the different effects that each twin experiences is just an artefact of co-ordinate labeling; that the effects each twin experiences is as a result of how they label the scenario. This may be an over-simplification, but it almost seems as though, according to relativity, the distance between the two observers increases because each labels the other as being "in motion".

I would see it the other way around, that each [rightly or wrongly] labels the other as being "in motion" because the distance between them increases. I'm just wondering what causes the distance between them to increase?

If one twin attributes it to "acceleration" how can he then, consistently, label himself as being "at rest" if, by necessity, he is in motion, and attributes the effect to necessarily being in motion?

I think it's somewhat different; I think it's more like saying, that, although the equations calculate that when I drop my pen off a building on earth it will behave the exact same way as if I dropped it off a building in a zero-gravity compound, that doesn't necessarily verify that it will.

Which experiments are you referring to specifically, because, again it could be my lack of understanding, but there are a number there which involve the measurement of a clock at different locations on the earths surface with different degrees of gravitational potential - or something to that effect.

It might be helpful to repost the video from earlier in the thread; it might be able to help clarify where I am going wrong.

https://www.youtube.com/watch?v=DRDN7ceu6UU

If we take the thought experiment as it is in the video; with both observers labeling themselves as being at rest and the other as being in motion, with each carrying a clock. As the thought experiment is set-up there, the paradox does arise; because each should observe the same effect in the others reference frame i.e. both will observe the others clock ticking more slowly.

In the physical world, however, such a scenario will not arise naturally, because at least one observer would have to undergo acceleration, and asymmetry would occur. This appears to be how the "paradox" is resolved under SR.

Under GR, however [to my understanding], where asymmetry doesn't resolve the issue, the paradox should arise - unless there is another mitigating factor. I'm just wondering what that mitigating factor is.

Another query I have: If we go back to SR however, where the paradox is resolved due to asymmetry, arising from the fact that one twin undergoes acceleration, the question arises, how can one twin label himself as being "at rest", if by necessity he is "in motion". This also raises the question of whether length contraction actually occurs.

The derivation of the Lorentz factor, as per the video posted, is carried out on the basis that the photon, in the clock on the train, travels a path perpendicular to the mirror. It certainly appears that way to the observer on the train. However, if he is the twin singled out as undergoing acceleration, then his clock is also undergoing acceleration. If his clock is undergoing acceleration then, according to what we have discussed, the photon must inheret the velocity of the mirror, such that it doesn't travel a path perpendicular to the mirror, but at the angle as observed by the observer on the platform. This would mean that there is no actual length contraction; the clock ticks slower because the photon has to travel a longer distance due to the motion of the clock.

If "my 8th birthday" is happening now for someone, am I there?

I ask, because that 8yr old observer has aged since then and is here in this present. How can I still be 8yrs old and at my birthday celebrations in "someone elses present", while simultaneously being in "my present"?

You may be able to define movement with respect to a frame of reference, but that definition would remain entirely theoretical, and wouldn't accurately represent the physical world. In the physical world you cannot move relative to an imaginary frame of reference, for the simple fact that it is not physical. To say that you are moving, or at rest, relative to an imaginary reference frame is to imagine that you are.

Absolute motion cannot be relative, because that would make it relative motion; relative motion, however, proves absolute motion. Again, absolute motion is simply a question of whether or not there is motion; if there is relative motion then there is absolute motion, simply for the fact that if there was no motion, there would be no relative motion.

Again, this seems to suggest that two observers, a good million miles apart, will observe themselves as being at the centre of the same expanding sphere because of how they apply an imaginary set of co-ordinates.

To my mind, their application of the imaginary set of co-ordinates won't determine whether or not they are at the centre of a physical sphere; their location with respect to the physical sphere will determine that, and their co-ordinates should reflect their physical location.

How I imagine it is thus, two observers side by side in an expanding sphere, at the centre; their co-ordinate systems reflect this. One starts moving from that location, such that he is no longer at the centre; his co-ordinates should reflect this - otherwise the sphere isn't a sphere. If length contraction, due to a co-ordinate transfer puts him at the centre, then the co-ordinate transfer must be erroneous - unless he's not in a sphere.

Granted, that is based on a fairly basic understanding of spheres.

roosh wrote: »

It does seem as though cause and effect get lost in transformation, or "transformed away" as you put it, such that the suggestion appears to be that simply re-labeling something can result in a physical change.

To put it in more blatant terms, it would be like suggesting that two observers label a piece of fruit as "an apple", but then one observer changes the label to "an orange" and this results in the flesh of the fruit, and the skin changing. That would be an example of where an effect is attributable to a change in labeling convention. Which seems ludicrous, but appears to be what is being suggested - in less crude terms, of course.

If the increasing distance between observers is just an artefact of co-ordinate labels, then this seems to suggest that the physical effect of the increasing distance arises due to labeling.

Did you mean that one twin is singled out, by the other, as undergoing acceleration, as opposed to one twin being singled out as undergoing acceleration?

Because if each twin attributes it to the other's acceleration, then that just leads us back to the paradox.

There is a critical difference between the two, in that one takes place under the force of gravity, while the other doesn't.

By "take one on a journey" do you mean that it undergoes accleration?

and hence the paradox.

The equivalence principle is partly the reason why the paradox should occur; if neither is singled out as undergoing accleration then both should observe the other's clock as ticking slower hence the paradox.

[According to SR] Does he acknowledge that he is "in motion", or has undergone acceleration?

Again, apologies, I might have confused matters more than clarifying them.

I was referring to the clock on the train. In the derivation of the Lorentz factor, the path of the photon is depicted as traveling in a line perpendicular to each mirror, such that it forms a right angle to the mirror. When coupled with the path of the photon as observed by the observer on the platform, the application of pythagoras's theorem gives rise to the length contraction formula, or time dilation formula.

Now, if neither observer is singled out as undergoing acceleration, then the paradox would arise, because each would observe the others clock as ticking more slowly.

If, however, one observer is singled out as undergoing accleration, and that observer is the observer on the train, then if he was to model the path of the photon as forming a right angle to each mirror, he would be mistaken, because not only is he under acceleration, but so too his is his clock.

If, as was previously discussed, light inherits the velocity of the device from which it was emitted, then in order for the photon to give the appearance [to the observer on the train] of traveling in a line perpendicular to the mirrors, it would have to travel the path as observed by the observer on the platform.

This would mean that his clock ticks slower, not because of length contraction, or time dilation, but becuase the photon has a longer distance to travel between the mirrors.

If we step into the physical world for a moment, and pretend that you and I were at my 8th birthday; now both of us have grown up since then (in many ways); that is, those two 8yr olds (assumption for the basis of explanation) didn't remain 8yrs old, we both moved "through time" to the ages we are now, and no co-ordinate system can change that. How then can our 8yr old selves still exist? They can't exist in our past (and someone elses present) because we remained in our present all the time and are in our present now; we didn't get stuck as 8yrs olds.

I wouldn't say there is an absolute set of co-ordinates, because co-ordinates are imaginary; in the physical world we can only move with respect to physical objects.

We may not be able to identify which body is in absolute motion, but we can deduce that some bodies must be in absolute motion because if there weren't bodies in absolute motion, there would be no relative motion.


If we take our two observers for example, and put them in "empty space" apart from themselves. If they are at rest relative to each other and then the distance between them increases, we can know that at least one of them has to be in absolute motion. We may have no way of distinguishing which one it is, but we know that at least one of them must be - if not both of them.

We measure their motion relative to each other, beacuse that is what measurement does, it seeks to quantify one thing in terms of another.

Not necessarily. Both observers can still measure the speed of light to be c in all directions, even if one observer is moving in the same direction as part of the sphere, and the opposite direction of another part of it.

Is a Lorentz transformation necessary though? As above, if an observer is singled out as being in motion, then a more accurate representation of the EDIT: phenomenon might negate the necessity for a Lorentz transformation.

roosh wrote: »

OK, this is mainly where we are at odds I think, the idea that nothing physical changes.

Apologies for restating some of this, but I'm doing it more for my train of thought than anything; here is my thinking on it:

If both observers are at rest relative to each other, and there is no physical change then they should remain at rest relative to each other; they cannot label the distance as increasing unless something happens to increase the distance; well, actually, they could, but that wouldn't mean that the distance actually increases between them. This is what I meant by the difference between labeling an apple as an orange; where in one scenario the labeling changes the flesh and the skin of the fruit.

Cable Guys
If we imagine our two observers both holding the end of a length of cable such that, when they are at rest relative to each other, side-by-side, the cable is slack.

Here both label themselves as being at rest relative to the other, as well as the space station around them. They acknowledge that if they remain at rest relative to those objects, then the cable will remain remain slack. They acknowledge, that in order for the cable to extend to its full length, and become taught between them, one or both of them will have to undergo acceleration.

So, it happens; the cable becomes fully extended and taught between them. Now, labeling can't cause the cable to extend between then, and become taught; something must happen to cause this physical change in the cable.

The observable change in the cable is attributed to the relative motion between the two observers; now, labeling doesn't cause relative motion, things are labeled as being in motion relative to each other because at least one undergoes acceleration. To express it in the terms you used earlier, I would say that the change in distance between the two observers is not an artefact of co-ordinate labeling, rather, the co-ordinate labeling is an artefact of the relative motion.

Defining the co-ordinate system
Here is where I think there is another breakdown in communication:
if the cable was slack when both observers were at rest relative to each other, and side by side, then they would have defined a co-ordinate system based on their relative position to the objects around them. This would have included the other twin, and their relative position to the same objects. They would have labeled this as being "at rest", such that their definition of being "at rest" was based on their relative position to the aforementioned objects. They would also have labeled the other twin as being "at rest", which equally would have been based on their [the other twin's] relative position with respect to those same objects.

When defining this initial co-ordinate system, where they are at rest relative the other observer, they would have defined certain things about the geometry, and therefore gravity and acceleration. They would have said that both were in a uniform gravitational field and neither was undergoing acceleration.

Then they observe relative motion between them and the objects, while observing that there is no relative motion between the other twin and the objects. This means that what they previously defined as being "at rest" they are now labeling as "in motion". Which seems somewhat inconsistent.

Here is where I think there is another breakdown in communication:
They would see the slack cable extend to its full length, and become taught. Something, other than labeling, must cause the cable to extend. You mentioned that I thought that the twin who attributed it to acceleration would have to label himself as being "in motion" but you mentioned that So here, both attribute it to the acceleration of the other, while they themselves remain stationary while experiencing a gravitational force; is that correct?

Will this actually be the case though? Will the twin who remains in the zero-gravity station experience a gravitational force? My thinking would have been that the twin in the zero-gravity station would not experience a gravitational force, so the twin that experiences a gravitational force should, due to the equivalence principle, conclude that he is undergoing accelerationl, and hence, "in motion".

If we look at it from the perspective of each individual twin:

Twin A maintains that it is Twin B that undergoes acceleration, so he should observe Twin B's clock ticking slower, and displaying fewer ticks when they return to common ground.

Twin B maintains that it is Twin A who undergoes acceleration, so he should observer Twin A's clock ticking slower, and displaying fewer ticks when they return to common ground.

Both twins agree that one twin underwent acceleration, but they disagree on which one it is and so the paradox remains.

Again, we must have somewhat different reasoning on this, but hopefully it can be resolved.

As I see it, if each twin labels the other as undergoing acceleration, the paradox arises, because each should observe the others clock as ticking slower, and as registering fewer ticks when they return to common ground.

To my mind there doesn't seem to be much difference between the two; in both cases each twin maintains that it is the other that has undergone acceleration and themselves that has remained at rest, which is what gives rise to the paradox.

That much is probably clear

I'm just wondering if I drop a pen off a building on earth, will it behave identically were I to drop it off a building in a zero-gravity compound; or it might be better to ask if, if I knock a pen off my desk on earth, will it behave the same way as if I knocked it off a desk in a zero-gravity station?

Apologies, the term "uniform gravitational field" is throwing me somewhat; I take that to mean that the gravitational field is the same for both observers; is that accurate?

OK, this is another area I don't fully understand; how, if he acknowledges that he has undergone acceleration can he consistently label himself as being "at rest" if by necessity he is in motion?

I would agree, to a certain extent, that we must conclude that time is "labeled differently"; but we may differ on what that means. I would say that they "label time" differently because the clock of the observer on the train is ticking more slowly, than that of the observer on the platform.

If the speed of light is constant, and the observer on the train is in motion, then the photon pinging between the mirrors of his clock will have to travel a longer distance between the mirrors, than the photon of the stationary observer, and hence it will actually tick more slowly.

Does SR actually allow us to physically distinguish inertial and non-inertial reference frames? I know it's stated that one reference frame, or observer, has undergone acceleration, but if both observers label themselves as at rest and the other as in motion, then is that not just the equivalence principle in effect, if not in name?

I have trouble with the idea of us existing as a worldline, because I struggle to see how there is evidence for this without assuming a priori that both the past and future exist.

I don't question our ability to develop a theoretical model where we can trace the worldlines of particles, and even ourselves; I don't doubt the ability of such a model to predict things about "the future"; what I do question is how we can test the assumption that past and future actually exist; particularly when any predictions of such a model will always manifest in the present. I know we can remember (imagine) the past, and project (imagine) the future, and we can construct a predictive model on that basis, but any such model appears to be based on an a priori assumption that both exist.

The world-line issue is one I have difficulty with, because, to my mind, it assumes the existence of past and future.

OK, we mean pretty much the same thing by the term absolute (I think).

To say that there can exist a physically distinguished set of co-ordinates, where things can be labeled as they "absolutely" are, may be somewhat contentious, because a co-ordinate system requires a person to imagine them, or to think them. However, the inability of the human race to determine the absolute motion of an object does not mean that there is no absolute motion. It is sufficient that we can reason that there is absolute motion, and so that a physically distinguished "set of co-ordinates" must exist.

How I would characterise it is thus: if we imagine the universe full of objects all at absolute rest; in this case there will be no relative motion. For relative motion to occur, something has to, absolutely, move. The measurement of it's movement will be relative, because the nature of measurement is comparison i.e. it is to describe one thing in terms of another. This may preclude us from saying which one is absolutely in motion, but we can reason that at least one of the objects has to be in motion, and so must be absolutely in motion.

Would the effects of "the Big Bang" necessitate that everything be in motion?

That is because the speed of light, by definition, is relative, not absolue; insofar as speed is a form of comparison. That doesn't mean that a photon is not absolutely in motion.

Wouldn't that depend on how he measures the speed of light? If he measures his own speed relative to the "stationary" background, then he can also measure the speed of light relative to the "stationary" background. He would still calculate the speed of light to be c and his own speed as 0.8c.

If he is measuring the speed of light relative to himself, is there not the potential for error; because the speed of light is defined in terms of a stationary clock, and not a moving one. While the speed of light relative to him might actually be slower, his slower ticking clock would make it seem as though it is traveling at the same speed as when he is stationary.

This would be my reasoning, just going on the derivation in the video you posted; the modeling of the photon [in the photon clock], for the observer on the train, would be erroneous, if the train is considered to be "in motion".

The video shows the path of the photon from the perspective of both observers; from the perspective of the observer on the train, the photon is depicted as traveling at right angles to the mirrors; while the observer on the platform sees it travel the path as depicted by the hypotenuse of a right angled triangle.

However, if the train is in motion, and the mirrors along with it, then assuming that the photon inherits the direction of the emitter - which was the phenomenon that spared the observer on the platform from death, by the laser pulse from the train - then, for the observer on the train, the photon also travels the path as depicted by the hypotenuse of the right angled triangle.

"Time" dilation, or rather clock dilation, will occur due to the increased distance the photon has to travel in its journey between mirrors, without the need for invoking length contraction. The notion of length contraction is only necessitated if the observer on the train assumes that he is "at rest"; which gives rise to the [arguably] unresolved paradox as outlined above.