Post by: talanum1 on 11/05/2021 10:48:46
Post by: Origin on 11/05/2021 11:54:21
When a train is moving with respect to a stationary observer, he will measure an object on the train as shorter than an observer on the train would measure it.Correct moving objects are length contracted.
However if he later climb onto the train and measure the object again he will find it longer than he measured it previouslyCorrect the object is no longer moving relative to him so it is not length contracted.
a contradiction for a global observer (Gods eye view)What is that supposed to mean? What is a global observer?
One can't say the object was really shorter.Sure you can.
Post by: talanum1 on 11/05/2021 12:03:03
What is a global observer?
An observer observing both observers from outside of time.
Post by: Origin on 11/05/2021 12:32:01
An observer observing both observers from outside of time.Since being outside of time is impossible, this observer is impossible.
In general mixing the supernatural with physics is not going to yield a meaningful result.
Post by: Eternal Student on 11/05/2021 13:21:50
When a train is moving with respect to a stationary observer, he will measure an object on the train as shorter than an observer on the train would measure it. However if he later climb onto the train and measure the object again he will find it longer than he measured it previously: a contradiction for a global observer (Gods eye view). One can't say the object was really shorter.
You're absolutley right, talanum. You can't say the object was really shorter because it assumes that length is a property that the object can have all on its own.
Length isn't a property of an object. Length is a property of the object AND the environment in which it is measured. Change the environment and the value of the length can change.
The colour of a wall in my house isn't a property of the wall. In daylight it's a pink wall but we have a multi-coloured LED lighting strip all the way around the wall which can throw light of various colours down the wall. If I change the light to red, then the wall does look quite red and not all pink. If I change it to blue light the wall actually looks quite grey. There's an interaction with the environment that determines what colour my wall will be.
There doesn't have to be any outside view of the universe, but if there is then that observer should be aware that length isn't a property the object can have on its own. It's a property that is determined by a mixture of things including the location of that object in the universe and the motion of that object.
Post by: Bored chemist on 11/05/2021 13:47:40
a contradiction for a global observer (Gods eye view).There is no absolute observer.
That's why the theory is called "relativity".
Why did you imagine that there should be some sort of "global observer"?
Post by: Halc on 11/05/2021 14:44:13
When a train is moving with respect to a stationary observer, he will measure an object on the train as shorter than an observer on the train would measure it. However if he later climb onto the train and measure the object again he will find it longer than he measured it previously: a contradiction for a global observer (Gods eye view). One can't say the object was really shorter.The 'God's eye view', or more commonly, the 'view from nowhere' is an accepted way of considering a system. In such a view, the train is a worldline and does not have a current state or a specific velocity. The length of the train (width of the train worldline) depends
1) on where along that worldline the measurement is taken since it can vary, and
2) on the coordinate system assigned to the events in the spacetime.
Since the view from nowhere does not have a location, time, or velocity, the coordinate system and event specification must be explicit.
Length isn't a property of an object.A rigid object has a proper length, which is considered the property of the length of the object, so I must disagree with your assertion. I mean, I'm a relational guy, so I see relations in almost everything that most people take as properties (including ontology), but proper length and proper mass are properties even in my book.
Length is a property of the object AND the environment in which it is measured. Change the environment and the value of the length can change.
Something where the endpoints move in relation to each other is another story. So I cannot ask "What is the proper separation between the centers of gravity of our galaxy and Andromeda, simultaneous with event X?". There's no obvious frame defined by that, so no correct answer.
Post by: Kryptid on 11/05/2021 14:47:48
An observer observing both observers from outside of time.
If you are trying to invoke an absolute reference frame, let it be known that there isn't any such thing in relativity.
Post by: Eternal Student on 11/05/2021 18:56:44
A rigid object has a proper length,
Hi Halc, hope you are well and of course you can disagree with me. It's great just to get any reply and discussion.
"Rigid bodies" are not a thing you can have in special relativity. A Rigid body is an idealisation used in Classical Mechanics but the concept of it is not consistent with special relativity. E.g. Simultaneity cannot be maintained in all frames of reference where there is some spatial seperation between the events. So, if a rod was "rigid" in one frame then all particles in it must have the same acceleration at the same time. Switch to another frame and that acceleration loses its simultaneity and the body isn't "rigid" anymore. You cannot maintain all the characteristics of an idealised rigid body from classical mechanics, the best you can do is choose a few characteristics to keep.
See for example, Bell's spaceship paradox ; Born-Rigidity or this youtube video for a demonstraton that a train cannot be modelled as a rigid body if you want to resolve one of the paradoxes that arises in special relativity.
(Jump to about 8:20 in that video and see that the train cars cannot be rigid bodies in both frames of reference if it is going to fall through the hole in the bridge).
Post by: Bored chemist on 11/05/2021 19:32:56
"Rigid bodies" are not a thing you can have in special relativity.Good catch.
The speed of sound in a rigid body would be infinite.
Post by: Halc on 11/05/2021 20:19:32
"Rigid bodies" are not a thing you can have in special relativity. A Rigid body is an idealisation used in Classical Mechanics but the concept of it is not consistent with special relativity.First of all, I'm just talking about something like a train where the front and rear of the thing are going at the same speed, assuming it isn't accelerating. Yes, trains typically vibrate, and thus are not 'Born rigid' objects, which is the idealization to which you refer.
Special relativity actually has no problem with Born rigidity. Such an object would be infinitely brittle, and thus any stress whatsoever will cause strain which will break it, but the object can be accelerated without applying any stress. However, it cannot be rotated, or more correctly stated: Its angular momentum cannot be changed.
The speed of sound in a rigid body would be infinite.Only in a Born rigid object, since yes, sound is by definition strain, and any strain at all will break the object, which doesn't violate SR. Yes, I realize that many websites (wiki) state this argument against it, but the argument is fallacious since sound cannot be transmitted at all through an object and still have it exhibit Born rigid motion.
Again my point is not about idealized Born rigid objects, but rather about ordinary real objects where the two endpoints are effectively stationary relative to each other. Such objects have a proper length, which is a property of the object, not a frame dependent relation.
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E.g. Simultaneity cannot be maintained in all frames of reference where there is some spatial seperation between the events. So, if a rod was "rigid" in one frame then all particles in it must have the same acceleration at the same time.This is incorrect. An accelerating rod has different proper acceleration along its length, else length contraction could not occur. That means that the front and back of the rod are not moving at the same speed in inertial frames other than the frame of the rod.
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Switch to another frame and that acceleration loses its simultaneity and the body isn't "rigid" anymore.We have a different definition of rigidity then. As long as the object is stationary in its own frame at any given time, the motion is considered to be rigid motion.
Post by: Origin on 12/05/2021 00:28:08
One can't say the object was really shorter.
You're absolutley right, talanum.No, he,s wrong and so are you.
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You can't say the object was really shorter because it assumes that length is a property that the object can have all on its own.There was no such assumption made. The length really is shorter in a moving frame relative to a stationary frame, just like time really is dilated in a moving frame relative to a frame at rest.
If you you don't think that is accurate then you don't think Special Relativity is accurate which, considering all of the rigorous testing SR has undergone, seems like you are on the wrong side of science.
Post by: Eternal Student on 12/05/2021 01:06:32
I think my earlier post was less than clear. Sorry.
You can choose exactly how you want to try and define a rigid body. That's what I was trying to say. You don't have to imagine it as a body where all parts accelerate together simultaneously - but you can. That is what a classical rigid body would do. If you try to keep this characteristic then you sacrifice the constancy of length in the reference frame where the rod is stationary (exactly as you said). Alternatively, you can consider it more in terms of a rigid motion, in this case the length of the rod is constant in the reference frame of the rod BUT you sacrifice the uniformity of acceleration of its parts (exactly as you said). (I'm not going to add all the details about in which frame and where, the sentences are already too long). You seem to favour a definition based on the idea of a rigid motion.
Whatever you choose to retain as your definition of a rigid body, you are telling me (I) something about the reference frame in which the length will finally be measured OR (ii) something about the motion (the acceleration) that the object can have in the frame where it was initially stationary.
Anyway let's have a look at your other points:
Again my point is not about idealized Born rigid objects, but rather about ordinary real objects where the two endpoints are effectively stationary relative to each other. Such objects have a proper length, which is a property of the object, not a frame dependent relation.I'm not following this, which is probably my fault. "The two endpoints are stationary relative to each other" - so there is a frame of reference involved, we can pick the front end to be the origin of the frame and the back end has 0 velocity in that frame. So it is the frame where the object is stationary (+ or - some translation). Yes, then there is a unique length you will observe for the object, although it seems that you have told me (up to translation) which reference frame we were using and I also know about the motion of the object in that frame (it is stationary).
Post by: Eternal Student on 12/05/2021 01:29:30
If you you don't think that is accurate then you don't think Special Relativity is accurate which, considering all of the rigorous testing SR has undergone, seems like you are on the wrong side of science.
I think we're on the same side of the fence, just trying to argue in different ways.
You think objects have a length that varies according to the frame of reference you measure it in. That's true and perfectly good.
I'm not Talanum but it seems that Talanum thinks objects should just have a length.
I was suggesting they don't have a unique length until we know something about the reference frames and motion.
Post by: Halc on 12/05/2021 03:37:31
You can choose exactly how you want to try and define a rigid body. That's what I was trying to say. You don't have to imagine it as a body where all parts accelerate together simultaneously - but you can. That is what a classical rigid body would do.By classic, you mean non-relativistic? But this whole discussion is about relativistic motion, so discussing Newtonian rigid motion is off topic, no?
My example was simply the proper length of a train, which is (sort of) the same regardless of frame of reference and length contraction. I say sort of, because I see real trains change in proper length by say 50 meters when changing from compression to tension. Real trains aren’t very rigid, but hypothetical ones can be.
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If you try to keep this characteristic then you sacrifice the constancy of length in the reference frame where the rod is stationary (exactly as you said).OK, you seem to be talking about relativistic motion, but with every part of the object experiencing identical proper acceleration, which will distort the object, which no longer has any frame in which the parts are at rest.
I think I need a citation somewhere that this is a valid form of rigid motion at relativistic differences in velocity. I will agree that the object cannot have a proper length for the same reason I gave above that there cannot be a proper separation of a pair of relatively moving objects like galaxies.
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Alternatively, you can consider it more in terms of a rigid motion, in this case the length of the rod is constant in the reference frame of the rod BUT you sacrifice the uniformity of acceleration of its parts (exactly as you said).You use the word ‘sacrifice’ like uniformity of proper acceleration (a violation of the equivalence principle) is somehow desirable to keep in this scenario.
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(I'm not going to add all the details about in which frame and where, the sentences are already too long). You seem to favour a definition based on the idea of a rigid motion.No, I favored a very loose definition, not the ideal Born rigidity definition. I way saying (in reply to your assertion that an object has no property of length) that (e.g.) a loaf of bread has a defined frame independent proper length at a given time despite the fact that its proper length changes if you accelerate it by application of a force at one end. That proper length is a property of the loaf.
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Not necessarily. For an inertial object, the front and rear are stationary relative to each other, which is a frame independent fact. For an accelerating object, this is only true relative to the inertial frame in which the object is momentarily stationary, which yes, involves that frame.Again my point is not about idealized Born rigid objects, but rather about ordinary real objects where the two endpoints are effectively stationary relative to each other. Such objects have a proper length, which is a property of the object, not a frame dependent relation.I'm not following this, which is probably my fault. "The two endpoints are stationary relative to each other" - so there is a frame of reference involved
it seems that Talanum thinks objects should just have a length.The unique proper length is a frame independent quantity and thus serves as the length property of the object. This is the length that Talanum probably means, but he wants it to be that length relative to any frame, which it isn't.
I was suggesting they don't have a unique length until we know something about the reference frames and motion.
Post by: Eternal Student on 12/05/2021 14:52:28
By classic, you mean non-relativistic?Yes. I'm old, so "classical mechanics" is just Newtonian mechanics. New definitions sometimes include SR as classical.
Some citations:
Pauli
wrote "the concept of a rigid body has no place in relativistic mechanics,” while Panofsky and Phillips
state that special relativity “precludes the existence of the ‘ideal rigid body’ .”[1] W. Pauli,Theory of Relativity(Pergamon Press, Oxford, UK, 1958) p. 132 .
[2] W. K. H. Panofsky and M. Phillips,Classical Electricity and Magnetism,2nd Ed.(Addison-Wesley, Reading, MA, 1962) p. 287.
The idea of a rigid body is easily defined in Newton Mechanics. However, the concept is not easily defined in SR and some of the properties that a rigid body has in Newton Mech. cannot be maintained.
Length contraction alone suggests that most definitions of a rigid body require explicit mention of the reference frame in which the lengths between particles could remain constant. There is little hope of maintaining constant lengths in all frames.
If you do define a rigid body in SR as something where length between particles = constant, in a frame where the object is at rest THEN acceleration of the object causes problems. As you mentioned previously, to allow for length contraction, there will be some inertial frames of reference where the back end has to accelerate more than the front end. In such a frame of reference the object has clearly lost something that a rigid body in Newtonian mechanics would have had (the object looks like it is being compressed as time evolves, which wouldn't be allowed in Newtonian mechanics).
If you want to ensure that there is never an inertial reference frame where the object seems to be deformed (so that's property of being "rigid" that we wanted to preserve) then you have to sacrifice the constancy of length in the rest frame of the object. Halc asked why we would want to do this or why we think it's a sacrifice - not seeing a body get deformed is a common sense idea of the body being "rigid". This is the sort of issue that troubled Pauli in the reference [1] given above. Another problem with such an object is that if you assume the body is rigid in this way then external forces applied to the front end must be instantly transmitted through the body to the back end in every reference frame. (If it wasn't then the front and back end won't retain the same acceleration and the object should deform as time evolves). I believe this is where the common reference to "the speed of sound" comes in. We aren't really interested in sound, it's just that in a real world object the forces between particles would be produced only by a small movement of one particle toward or away from another (there being a repulsion or attraction to restore the natural bond length). As such, real world transmission of force through a body propagates like a sound wave through the body. This notion of "rigid" is therefore an absurd way of approximating a real world object in SR since instant transmission of forces through the body assumes an infinite speed of sound. This lead people like Pauli to believe that there wasn't any place for anything like a rigid body in SR.
An alternative definition for "rigid" was developed. Here's one article I found online that discusses the development of "rigid motions". There are others but this post is already too long, I'm tired and you must be bored.
Rigid body motion in special relativity, Jerrold Franklin, Department of Physics, Temple University, Philadelphia, 2012 on Arxiv. (I've used Arxiv because it's free).
https://arxiv.org/pdf/1105.3899.pdf (https://arxiv.org/pdf/1105.3899.pdf)
Post by: Eternal Student on 12/05/2021 23:48:47
so discussing Newtonian rigid motion is off topic, no?I'm only guessing what was being considered as a rigid body and by whom.
My example was simply the proper length of a trainBut someone mentioned rigid objects, not sure who and it was days ago.
I think I need a citation somewhere that this is a valid form of rigid motionSome citations were given in an earlier post. It's not that it is "valid" in the sense of being the best definition, it isn't. It's just that this idea of rigidity can and has been considered by Physicists and still seems to be a view of "rigidity" that ordinary people may want to have ---> quickly leading to problems, including transmission of force at speeds ~ infinity etc. Nothing that would be a problem in Newtonian mechanics - but in SR nothing vibrates, moves or transmits that fast.
You use the word ‘sacrifice’Can't keep every characteristic that a rigid body may have in Newtonian mechanics when you make the transition to SR. Not all of them are important. Someone mentioned Violation of the Equiv. principle but that isn't a thing to worry about in SR, that's a GR thing.
Not necessarily... (is a reference frame involved)My original reply was intended to show that a suitable frame to measure that length (in the ordinary way with a ruler) can be constructed. However, this assumes Talanum would want to define and measure length that way - see later.
The unique proper length is a frame independent quantity and thus serves as the length property of the object. This is the length that Talanum probably meansThis is one of the more important items. Is Talanum willing to consider "proper length" as a property of the object? I'm only guessing but most people just want to consider length as something they can measure with a ruler. I guess we won't know unless Talanum replies.
I may have got this wrong but when people are questioning if length contraction can happen, then it seems unlikely they are considering length as "proper length". If they were aware of "proper length", then "proper length contraction" isn't something they would have worried about, since it doesn't happen.
Some kind of conclusion?
We need to find out what Talanum is considering as "length".
None-the-less , some of the twists and turns in this discussion have been enjoyable. Thank you to everyone who has put in some time.
Post by: Halc on 13/05/2021 03:46:20
Pauli wrote "the concept of a rigid body has no place in relativistic mechanics,”I’d agree with this, but I’m not claiming any ‘mechanics’ in the normal sense of the word. I said that if you exerted any stress on a Born rigid object (like touch it with your finger), it must break, so including such an object into the usual mechanics isn’t going to work.
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while Panofsky and Phillips state that special relativity “precludes the existence of the ‘ideal rigid body’ .”Agree again, but quantum mechanics pretty much does that job nicely. I’m simply claiming that mathematically, Born rigid linear motion and acceleration does not violate SR. A rotating object can exhibit Born rigid motion, but cannot undergo angular acceleration, so it has to have always been rotating.
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The idea of a rigid body is easily defined in Newton Mechanics.Defining it in mechanics that are wrong in the context under discussion seems to be starting on the wrong foot.
For instance, Born rigid linear motion can be defined as every particle undergoing the exact same proper acceleration, which isn’t going to work under SR.
It can be defined as all particles having the same velocity at a moment in time, which works again only in the Newton universe but not one with relativity of simultaneity (RoS).
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However, the concept is not easily defined in SR and some of the properties that a rigid body has in Newton Mech. cannot be maintained.Under SR, Born rigid linear motion is one where at all times the proper distances are maintained between all components of the object. That works for both theories, and thus seems to be the better definition, but it needs modification if the rotating case is to be considered. Seems like an easy definition if you ask me. I made it up, so I’m not quoting any official definition. Yes, some of the Newtonian properties (the ones in the more ambiguous definintions above) are lost. Particles in an accelerating object do not experience identical proper acceleration (as is well illustrated with the Bell’s spaceship scenario), and due to RoS, relative to some frame in which one particle is in motion, other particles of the accelerating object are going to be moving at different speeds, again well illustrated by something like Rindler coordinates.
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If you do define a rigid body in SR as something where length between particles = constantProper separation, but yes, I chose to define it that way.
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in a frame where the object is at restProper distance isn’t a frame dependent quantity.
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THEN acceleration of the object causes problems. As you mentioned previously, to allow for length contraction, there will be some inertial frames of reference where the back end has to accelerate more than the front end.Acceleration is absolute, and thus also not frame dependent. Yes, acceleration of the various parts of a rigid body is higher at the rear and lower at the front (front being in the direction of acceleration).
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In such a frame of reference the object has clearly lost something that a rigid body in Newtonian mechanics would have had (the object looks like it is being compressed as time evolves, which wouldn't be allowed in Newtonian mechanics)It is a problem with Newton’s theory then, not a problem with reality.
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If you want to ensure that there is never an inertial reference frame where the object seems to be deformedHow things appear is a relation between the thing and an observer, not a property of the object. Linear length contraction is not deformation. It is merely a coordinate effect.
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Another problem with such an object is that if you assume the body is rigid in this way then external forces applied to the front end must be instantly transmitted through the body to the back end in every reference frame.As I said, any external imbalanced force (not applied everywhere in proportion) will constitute finite stress and will break the rigidity of the object, but it won’t violate SR.
And no, the far end will need to move immediately in its own proper frame but not in others, where one or the other end of the object begins to move first. This is clear if one understands Rindler coordinates.
I started a thread some time back about the minimum time it would take to move a stationary rigid object of length 100 light years a distance of one light hour. I think the answer was about 55 days. I learned plenty running through that exercise.
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(If it wasn't then the front and back end won't retain the same acceleration and the object should deform as time evolves). I believe this is where the common reference to "the speed of sound" comes in.Sound cannot pass through a Born rigid object. That would constitute stress.
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We aren't really interested in sound, it's just that in a real world object the forces between particles would be produced only by a small movement of one particle toward or away from anotherThere are no forces between particles of a Born rigid object. That would be stress. This is a big reason why QM forbids such objects since there’s no way to do away with such forces. Hence we must drop to the abstract mathematical case where the object is a continuous solid with zero forces anywhere within. Such a homogeneous object would have no temperature, as opposed to the one made of particles at temp absolute zero.
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This lead people like Pauli to believe that there wasn't any place for anything like a rigid body in SR.He should conclude there is no place for a rigid body in reality, but SR is a mathematical model which doesn’t forbid Born Rigid motion.
It’s because length was being discussed, and a non-rigid object doesn’t have a proper length and thus has no one length property. A hypothetical vehicle (car, train) is essentially rigid in that it has a meaningful length despite the fact that real trains change length considerably between compression and tension states.Quote from: HalcMy example was simply the proper length of a trainBut someone mentioned rigid objects, not sure who and it was days ago.
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None that defined rigid motion that way, in the context of relativity.Quote from: HalcI think I need a citation somewhere that this is a valid form of rigid motionSome citations were given in an earlier post.
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Is Talanum willing to consider "proper length" as a property of the object? I'm only guessing but most people just want to consider length as something they can measure with a ruler.Proper length can be directly measured with a ruler. Coordinate length isn’t so easily measured with a ruler, but it can be done.
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I may have got this wrong but when people are questioning if length contraction can happen, then it seems unlikely they are considering length as "proper length".Agree, but that length is a relation with a frame, not a property of the thing being measured.
Post by: yor_on on 13/05/2021 08:22:06
Post by: yor_on on 13/05/2021 08:54:13
The only thing I might want to add to the end of that sentence is 'relative the observer', but it is implied by your reference frame using 'the eye of a God' . We do have rest frames, as ideally defined by being in a 'same frame of reference'. What I tend to look as the local definition, the one giving us repeatable experiments in where we agree on a cm being a cm, and a second a second.
Post by: yor_on on 13/05/2021 08:59:48
That should be interesting.
=
This was not what you meant, I know that, but for those thinking of it as a creation, or using that euphuism 'God' for a creator it becomes a problem. Not only is 'God' expected to see and know everything, now we also have to include those other observers reference frames into it.
And the point I would like to make is that all differentiating reference frames are equally valid. That we can transform one frame of reference to another, or agree on there existing a abstract logic connecting them, doesn't make any of those observers locally wrong. So we don't even need a outside for it (the universe) to become a mosaic.
what it means is that it is perfectly correct to state that we can transform away gravity by being in a local 'free fall', as in space in a relative motion. Locally defined gravity disappears. And I don't think we have to assume it being a 'test particle' in that free fall. Not the way I look at it.
And it is there I find quantum mechanics lacking something, not in the failure of describing gravity, but in the way it ignore frames of reference.
Post by: puppypower on 13/05/2021 14:08:02
Each relative reference may see the other reference appear contracted in space and time. However, if you include mass, as a stand alone variable, one has a way to determine absolute reference priority. The mass and relativistic mass allow one to do an energy balance. Relative references tend to create more that one energy balance, thereby violating energy conservation.
My classic example of these affects was something I called the relative reference workout. This is a way to exercise and burn extra calories, all without exercising. One only has to use relative reference assumptions. We have one person running along a track at velocity=V. At their brisk pace, they burn X calories per hour.
The rest of the people, who pay big buck to work out, without working out, are placed on comfortable easy chairs around the track. They pretend to be the moving reference. We are told that there is no absolute reference, so pretending to run is as good as actually doing it. The time and distance contractions affects will appear the same from either reference, so they feel good about this workout assumptions. They are in relative motion, so they can burn the same calories without doing anything but assume and pretend. It is win win for all.
The problem is the energy, connected to mass and relativistic mass, will not be the same for all. Only one person; runner, is burning the calories indicative of the brisk pace. This is the correct or absolute energy balance reference. However, we could game the system, by only allowing so many clients in occupy the easy chairs, so the sum of calories burned by all the sitters, is the same as the calories burned by the one runner. Then the illusion will appear better, since the energy balance will also appear relative.
If we had two beakers, each wth the same amount of a radioactive isotope, and we gave one beaker sufficient velocity to contract space-time to a measurable amount, its half life will appear to get longer due to time dilation. The beaker that is not moving, if it had consciousness and an imagination, can pretend to be the moving reference, via relative reference. However, the concentrations will never be the same when we stop the experiment and compare. There is an absolute reference priority for half life, based on mass/energy plus space and time.
From a practical POV, when we look out at space, we do not have a good way to know the energy balance ahead of time, so we can assign absolute references. We are sort of stuck using two out of three variables by default. The various photons we use to determine the properties of the universe are all composed of wavelength and frequency or distance and time. Two out of three variables is the default. This does not allow one to determine absolute reference using SR. Relative reference is connected to the practical limits of applied reality, but it is not a statement of pure reality. It is useful to engineers but not to purest.
The work around the practical and applied science limitations of relative reference is the speed of light reference. The speed of light is the same for all references. There is an absolute reference that is the same in all relative references. This provides a common ground state reference, from which each and every relative reference can set it own absolute reference priority. This is easier said than done.
Post by: Bored chemist on 13/05/2021 14:54:51
Relative reference is an illusion that appearsYou seem to be the only one who thinks it exists (outside of computer addressing).
What do you think it means?
We are told that there is no absolute reference, so pretending to run is as good as actually doing itNobody tells anyone that.
It's another bit of "made up by Puppypower" nonsense.
The beaker that is not moving, if it had consciousness and an imagination, can pretend to be the moving reference,No, it can't because it didn't experience the accelerations which the other beaker did.
Why do you post this sort of trash?
Post by: Eternal Student on 13/05/2021 15:07:38
Thanks for adding more to think about. I've got some real world nonsense to deal with for a while but I'll probably reply later.
Bored_Chemist, this seems like it may just be "fighting talk" and yo mama jokes but be careful not to drive people away from taking an interest in science, please. I don't agree with Puppypower's exercise plan but I love the idea and I might steal it for use one day if I'm ever teaching SR to a group of students. It's like another paradox to be resolved.
Post by: Origin on 13/05/2021 15:26:49
Bored_Chemist, this seems like it may just be "fighting talk" and yo mama jokes but be careful not to drive people away from taking an interest in science, please.You have not been here long, so you will soon find out that puppypower has absolutely no interest in science.
Post by: Origin on 13/05/2021 15:35:42
Each relative reference may see the other reference appear contracted in space and time. However, if you include mass, as a stand alone variable, one has a way to determine absolute reference priority.For once, instead of just making inane statements, please demonstrate the math that shows that including mass will give an absolute reference frame.
I know you won't, but it would be so refreshing to see you at least attempt it.
I know it's much easier to make absurd statements and just move on than to actually back them up, but come on and at least try?
Post by: Bored chemist on 13/05/2021 18:22:09
It's like another paradox to be resolved.It's a washed up version of the Twins paradox.
Pointing out that Nonsense is nonsense won't scare of those with a real interest in science, but it might persuade some of the other trolls to steer clear.
Post by: Eternal Student on 14/05/2021 10:36:44
About rigid bodies and all the paradoxes that have recently been mentioned:
I was going to mention the bug - rivet paradox which was discussed back in my day. I think the textbook "Spacetime Physics" byTaylor and Wheeler have a similar problem using a detonator switch and call it by another name (resting bomb problem?). Anyway, it's a fairly well known paradox in Special Relativity where "rigidity" is something you must consider carefully to resolve the poblem and reach the same conclusion in both frames.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fhyperphysics.phy-astr.gsu.edu%2Fhbase%2FRelativ%2Fimgrel%2Fbugrivet.gif&hash=c6130a0cc332babb3239f8ef7d5b3d7d)
Above Image taken from "Hyperphysics" website. I hope it displays but there's also a JPG file of it attached at the bottom, I hope. I've never tried using images in a post before.
Anyway, the discussion of rigidity was interesting and maybe there was some small justification for considering ideas from Newtonian mechanics in the context of Special Relativity. Think of the bug, we've got to try and save the bug.
Best wishes to everyone.
Post by: Halc on 14/05/2021 13:31:01
I was going to mention the bug - rivet paradox which was discussed back in my day.I wish they'd stop calling things paradoxical that are not. Nothing in SR is paradoxical, else it would be demonstrated to be self-inconsistent. This one is a mild variant on the barn-pole or ladder 'paradox' which isn't paradoxical for the same reason.
There are only two physical events: the squashing of the bug, and the meeting of the head of the rivet with the wall. In the high speed case, those two events are outside each other's causal light cones and thus cannot have an effect on each other. Relativity of Simultaneity says that the events can be abstractly ordered with either before the other, as your picture has shown. But considering the situation in various abstract coordinate systems cannot change the outcome of these physical (frame invariant) events. Thus the bug dies and the rivet head hits the wall at full speed, and these physical events happen in all frames, so no paradox.
Post by: Eternal Student on 14/05/2021 15:48:04
those two events are outside each other's causal light conesYes, I agree and I've always liked the explanation using light cones and causal effects. You've presented the explanation very well.
It's a great attempt to bypass any consideration of rigidity rather than just saying "the rivet can't be rigid in the way you may want it to be". You have skillfully side-stepped the issue rather than making it go away. Here's another light cone analysis:
The event A = "the hitting of the rivet head on the wall" cannot be considered the cause of event B = "the rivet end stopping" unless A is in the past light cone of B. An analysis of what happens in the bug's frame will fail if it endows the rivet with ideal rigid characteristics that it cannot have (which is what most people, including myself will do when they first see this problem). It is not possible to assume one end of the rivet stopping should instantly cause the other to stop. It's not even enough to replace idealised rigidity with some rapid but smooth deformation of the rivet in an easy way. For example, it's not enough to assume the rivet end decelerates rapidly but smoothly to restore the rest length of the rivet in the bug's frame. The minimum time required for event A to fall into the past light cone of event B is such that in the bug's frame the rivet must extend beyond the rest length of 0.8mm.
Although I do like the explanation based on light cones that you have mentioned, it doesn't avoid causing consequences for how rigid bodies can be modelled in special relativity. This paradox is not exactly like the barn pole or ladder paradox. Those paradoxes are resolved easily just by using the Lorentz transformation and turning the handle to do the calculations. None of them really need the student to worry about rigidity or trouble themselves with light cones and causal inference.
Post by: puppypower on 19/05/2021 13:43:59
For example, in the twin paradox, one twin ages at a different rate. However, that same twin does not show any permanently change in his size in the direction of motion. Distances may appear to contract with motion, but once the system is returned to it's starting point, this apparent change in distance does not persist. However, the time change will remain as demonstrated with clocks. There is an asymmetry.
What that tells me is if universal red shift, is based on relativity, it would need to be frequency driven, with wavelength following the time/frequency shift, due to space and time being unified into space-time. Maybe we need to think in terms of time-space.
Distance, apart from time, as shown in the clock experiments is fully reversible. But time, even by itself as a a clock, is not reversible. The universal red shift appears to be a time modulation, since only time displays a permanent change under various conditions of relativity.
It is strange that most thought experiments, model relativity in terms of distance changes, even though distance appears to be a reversible passive/reactive variable. Time is the dynamic/active variable that can undergo permeant changes. The clock experiments demonstrated this. This reverse of the order; space-time, may have led to some unintended practical problems.
If we had light moving through a billion light years of space, encountering massive objects and then wide open gaps of space, distance/wavelength will red shift and blue shift in a reversible way, but time will show a permanent accumulative affect, with the final wavelength reaching us dependent on the accumulative permanent frequency shift. The dynamic variable leads.
We could demonstrate this by doing the clock experiment, while traveling to all the planets of the solar system, as well as the gaps between. The clock will show a variables rate of time propagation, depending in where it was, with this accumulative time change adding up, to a final clock reading. The size of the clock may appear to change; slightly larger or smaller, from the earth reference, but this is not a running total, but ends the same as we began. In terms of energy, when permanently altered time/frequency ends up in single place in space-time, reversible passive distance aligns with dynamic accumulative time change, due to space-time.
This makes sense since we measure distance with a meter stick. This is a passive tool. Time is measured with clocks which use some form of energy; wires, battery or spring. This is a dynamic tool. Distance is a passive variable and time is a dynamic variable. Relativity can impact both the passive and dynamic tools, but only the dynamic tool is not fully reversible, since slowing or speeding its energy flow, via time change, will cause permanent changes in its output parameters. The meter stick has no energy flow, so when the experiment is over it still measures one meter. A stopped clock, not using energy, will not gain or lose time since it becomes passive.
Post by: Bored chemist on 19/05/2021 14:04:40
I wish they'd stop calling things paradoxical that are not.Yes.
You are right; it's not a paradox.
The twins have different experiences, so it is no shock that they have different outcomes.
Post by: Origin on 20/05/2021 00:01:16
An interesting observation about relativity is connected to clock experiments where clocks will permanently gain or lose time. Paradoxically, although time will change in a permanent way clock distance/size changes due to relativity are fully reversible.This is incorrect. The traveling twins clock ticked slower and his ruler was shorter than the at home twins clocks and rulers. When the traveling twin returns and stops his clocks now again ticks at the same rate as the at home twins clock and his ruler is the same length as the at home twin. Since the traveling twins clock ran slower during the trip, less time passed for him than the at home twin. Since the traveling twin had a shorter ruler he traveled fewer miles than his at home twin measures. The time dilation and length contraction are analogous to each other. The age difference is permanent and the distance traveled difference is permanent.
Post by: CrazyScientist on 20/05/2021 08:34:20
https://en.wikipedia.org/wiki/Twin_paradox
(https://upload.wikimedia.org/wikipedia/commons/thumb/c/ce/Twin_Paradox_Minkowski_Diagram.svg/333px-Twin_Paradox_Minkowski_Diagram.svg.png)
But this is not the case in one-directional motion with a constant velocity. Lenght contraction is fully symmetrical - distance in th moving frames will be contracted by the same amount from the perspective of both twins, no matter which one is treated as the moving one. However time dilation is not symmetrical.
Let's say that initially both twins are separated from each other, while having the same age and one of them is moving at a constant velocity towards the second one. According to the laws of relative motion, it shouldn't be possible to tell which one of them is moving and which one is stationary - in their own rest frames they both will be stationary and it will be the other twin, that is moving. Lenght contraction will be in such case the same for both moving frames. But when at some point those twins will meet in space, one of them will be older than the other - how can it be, if in the rest frames of each twin it was the other twin, that was moving?
And when both twins will then compare their ages, they will be able to tell which one of them was in fact moving and which one was stationary - and the motion of one twin will become an absolute property of his frame, violating the primary law of relative motion (relative velocity is NOT a definitive property of a frame).
Post by: Origin on 21/05/2021 03:05:09
Let's say that initially both twins are separated from each other, while having the same ageThat can only happen if they both accelerated and decelerated at exactly the same rate.
and one of them is moving at a constant velocity towards the second one.That means the moving twin must have accelerated to the constant velocity.
But when at some point those twins will meet in space, one of them will be older than the other - how can it be, if in the rest frames of each twin it was the other twin, that was moving?The twin that accelerated will be the one that is younger when he decelerates to the other twins frame, just as SR predicts.
Problem is, that in SRT the twin paradox can be explained only in the case of two-directional motion, where one twin is traveling to a distant destination and comes back to the second twin.A one way trip will still result in the traveling twin being younger. Say you fly to a planet at relativistic speeds that is 10 ly away. When he lands on the planet (let's assume the 2 planets are relatively stationary with each other) he will be younger than the stay at home twin.
Post by: Halc on 21/05/2021 03:55:02
No, that can only happen relative to a few very specific frames and not relative to most others. The comment by CS is making the very same mistake as the OP: Assuming an absolute ordering of events, such that the comment has meaning without a frame reference.Let's say that initially both twins are separated from each other, while having the same ageThat can only happen if they both accelerated and decelerated at exactly the same rate.
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Given the usual definition of twins (born at the same event), yes, but any pair of people can be in this situation, moving towards each other with the same age in some frame, and having never significantly accelerated. This technicality isn't what invalidates CS's point.Quote from: CrazyScientistand one of them is moving at a constant velocity towards the second one.That means the moving twin must have accelerated to the constant velocity.
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But The twin that accelerated will be the one that is younger when he decelerates to the other twins frame, just as SR predicts.No. The one moving at the higher speed in the frame in which they are initially they are the same age will be the younger. Acceleration doesn't directly come into play and need not occur at all. Nobody needs to 'decelerate' (as you put it) at all. Acceleration would only be necessary to keep them in each other's presence after they meet, and then it matters completely not at all which ones do the accelerating.
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A one way trip will still result in the traveling twin being younger. Say you fly to a planet at relativistic speeds that is 10 ly away. When he lands on the planet (let's assume the 2 planets are relatively stationary with each other) he will be younger than the stay at home twin.Only relative to a subset of reference frames, and not relative to others. Relativity of simultaneity means that there is no objective age difference between twins that are not co-located, but your comment seems to imply otherwise.
OK, you probably mean relative to the frame of the planets, but then the comment should include that reference since it is meaningless without it.
Post by: Origin on 21/05/2021 12:04:34
No, that can only happen relative to a few very specific frames and not relative to most others. The comment by CS is making the very same mistake as the OP: Assuming an absolute ordering of events, such that the comment has meaning without a frame reference.The question was about twins separated by space but both having the same age. My thought was this could be accomplished by one twin flying at say .6c to a point 5 ly away from earth. At the same time the other twin flys at .6c to a point 5 ly away in the opposite direction. Each twin will say that 4 years have passed and the earth will say 5 years have passed. Both twins will agree that they are 10 ly apart and are the same age, which is one year less than there age as calculated on earth. Is that incorrect?
Post by: Halc on 21/05/2021 16:01:09
The question was about twins separated by space but both having the same age.That part is ambiguous, as I have explained. CrazyS also stipulated that they're moving towards each other, not reflected in your example which has them moving apart.
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My thought was this could be accomplished by...It can be accomplished in any number of ways, including one of them being inertial the whole time. Each 'twin' represents a worldline, and them being age X (unspecified) defines a pair of events, one each on their respective worldlines. Events are frame independent facts. There are two possibilities regarding the relationship between the two events: Either they are outside each other's light cones or they are not. In the latter case, one twin is objectively older than the other, so this is inapplicable. In the former case, the ordering of the two events is frame dependent, so saying they're the same age is the same as saying the two events are simultaneous, which they are only relative so some very specific frames. Relative to most frames, one of the events occurs before the other. This is straight up relativity of simultaneity. Saying they're the same age without a frame reference is the same as saying two spatially separated events are simultaneous without a frame reference. It's not even wrong.
In your scenario, due to symmetry, they're the same age relative to the Earth frame, but if they're moving apart at over .88c relative to each other as you have them, then in their respective frames, each will say that the other ages ~1.88 years after those 4 years. So different answers depending on the frame used.
Similarly, one could stay home and the other zooms to a star 10 LY away, turns around and starts the journey home. This actually corresponds to the scenario CS mentioned where they're moving towards each other. In that scenario, there are frames in which either is older than the other, and some where they are the same age. It all depends on your arbitrary choice of frame.
Post by: CrazyScientist on 21/05/2021 20:30:19
Let's say that initially both twins are separated from each other, while having the same ageThat can only happen if they both accelerated and decelerated at exactly the same rate.and one of them is moving at a constant velocity towards the second one.That means the moving twin must have accelerated to the constant velocity.But when at some point those twins will meet in space, one of them will be older than the other - how can it be, if in the rest frames of each twin it was the other twin, that was moving?The twin that accelerated will be the one that is younger when he decelerates to the other twins frame, just as SR predicts.Problem is, that in SRT the twin paradox can be explained only in the case of two-directional motion, where one twin is traveling to a distant destination and comes back to the second twin.A one way trip will still result in the traveling twin being younger. Say you fly to a planet at relativistic speeds that is 10 ly away. When he lands on the planet (let's assume the 2 planets are relatively stationary with each other) he will be younger than the stay at home twin.
To solve the problem we have to get to the most basic form of this scenario - that means to focus on the relative motion itself and exclude the mass of twins or any influence of gravity (that means no planets). Let's say for example that the twins are "suspended" somewhere in empty space.
Without the mass and it's inertia, acceleration becomes relative as well - that means it is not possible to tell which frame accelerates towards the other frame - acceleration becomes fully symmetrical just like lenght contraction. Yet time dilation isn't symmetrical and one twin is aging faster than the other, so it's motion (and acceleration) becomes absolute.
Post by: CrazyScientist on 21/05/2021 20:38:10
The question was about twins separated by space but both having the same age.That part is ambiguous, as I have explained. CrazyS also stipulated that they're moving towards each other, not reflected in your example which has them moving apart.QuoteMy thought was this could be accomplished by...It can be accomplished in any number of ways, including one of them being inertial the whole time. Each 'twin' represents a worldline, and them being age X (unspecified) defines a pair of events, one each on their respective worldlines. Events are frame independent facts. There are two possibilities regarding the relationship between the two events: Either they are outside each other's light cones or they are not. In the latter case, one twin is objectively older than the other, so this is inapplicable. In the former case, the ordering of the two events is frame dependent, so saying they're the same age is the same as saying the two events are simultaneous, which they are only relative so some very specific frames. Relative to most frames, one of the events occurs before the other. This is straight up relativity of simultaneity. Saying they're the same age without a frame reference is the same as saying two spatially separated events are simultaneous without a frame reference. It's not even wrong.
In your scenario, due to symmetry, they're the same age relative to the Earth frame, but if they're moving apart at over .88c relative to each other as you have them, then in their respective frames, each will say that the other ages ~1.88 years after those 4 years. So different answers depending on the frame used.
Similarly, one could stay home and the other zooms to a star 10 LY away, turns around and starts the journey home. This actually corresponds to the scenario CS mentioned where they're moving towards each other. In that scenario, there are frames in which either is older than the other, and some where they are the same age. It all depends on your arbitrary choice of frame.
It's exaxctly as you say: "It all depends on your arbitrary choice of frame". Because of time dilation, one frame becomes arbitrary choosen as stationary and other becomes arbitrary choosen as the moving one - but isn't it against the idea of relative motion, where it's not possible to tell, which frame is moving and which one is stationary?
Post by: Origin on 22/05/2021 04:20:14
In your scenario, due to symmetry, they're the same age relative to the Earth frame, but if they're moving apart at over .88c relative to each other as you have them, then in their respective frames, each will say that the other ages ~1.88 years after those 4 years. So different answers depending on the frame used.Excellent point that I neglected to include.