[old guy]

There's a nice quote from Einstein on the wiki about "real" and length contraction:
http://en.wikipedia.org/wiki/Length_contraction#Reality_of_Lorentz_contraction

The question as to whether the Lorentz contraction really exists or not is misleading. It doesn't "really" exist, in so far as it doesn't exist for a comoving observer; though it "really" exists, i.e. in such a way that it could be demonstrated in principle by physical means by a non-comoving observer.
—Albert Einstein, 1911

It also goes on to point out what most of us have been stating, that argument over what is "real" only depends on terminology.  If you define "real" to mean "in the rest frame" then your argument is right.  If you define "real" to mean "what is measured" then your argument is wrong.  In actual science, we don't use muddled terms like "real," and constrain ourselves to describing the predicted measurements, for which there is no ambiguity.

In essence, "real" matches your definition of "real" because you defined it to do so from the start.

Good link and very relevant Einstein quote. It confirms my previous quote of him (here or in my other thread) saying that the word "real" is "meaningless" because reality depends on what science chooses to observe. (Never mind that the universe is much larger than we can observe, but remains "real" even beyond our capability for observation, as new discoveries constantly inform us.)

My probe example intended to put some meat on the bones of the question, "What is real?" as a practical application, once we achieve relativistic speeds in space travel, of course. (See quote below.)
The retrieval team must know the "real length" of the probe in order to send out a shuttle with a large enough cargo bay. Obviously the shuttle must "come alongside" the probe, matching velocities, at rest with the probe to capture it. (The conjectures about colliding with the probe would be the result of a very poorly planned mission!)
Our cargo bay, as stated, is 10 meters. The "contracted length" of the probe, as seen from Earth's frame appears to be 10 meters. The point of the whole exercise was that 10 meters is not the probes *real length*, so sending out the above shuttle would be very stupid, because the probe is *really* longer than 10 meters. So there is your functional definition of "What is real?"

Your link, above the Einstein quote, directly addresses the topic of this thread:
Wiki:

In addition, even in a non-co-moving frame, direct experimental confirmations of Lorentz contraction are hard to achieve, because at the current state of technology, objects of considerable extension cannot be accelerated to relativistic speeds. And the only objects traveling with the speed required are atomic particles, yet whose spatial extensions are too small to allow a direct measurement of contraction.

The next section, however cites several “indirect confirmations.” I’ll quote the most famous/popular one and comment:

Another example is the observed lifetime of muons in motion and thus their range of action, which is much higher than that of muons at low velocities. In the proper frame of the atmosphere, this is explained by the time dilation of the moving muons. However, in the proper frame of the muons their lifetime is unchanged, but the atmosphere is contracted so that even their small range is sufficient to reach the surface of earth.

Essentially this argues that, because of “time dilation” (their rate of decay slows down at higher velocity), these mouns “live longer” than expected of lower velocity muons, and since length contraction is the math reciprocal of time dilation, for those muons “the atmosphere is contracted."
From “the frame” of the whole picture however (earth and incoming muons) , there is no contracting of the atmosphere around each muon. “For a muon” does not change  the atmospheric science of its thickness/depth.

(More later in reply to other posts as time permits, if I am so allowed.)

[old guy]

Regarding Imatfaal's comment on my thought experiment on length contraction "for a ship" going very fast to Alpha Centauri:
Imatfaal:

I posted a link and quotes to a very interesting blog which I believe correctly showed the principles and unsettling consequences of Special relativity.  Skulls in the Stars

I read the link and commented (post #7) in the New Theories version of this topic where I was told to go with my argument.

[David Cooper]

From the rest frame of the probe we now see the shuttles cargo bay as length contracted, and the probe is no longer contracted ...and the probe would still all fit in just before destroying itself and the shuttle!  The timings are very tough to reconcile, events seem to be switched, and with relativistic speeds you cannot be sure of any simultaneities without the maths; but in the end the same set of events happens - it must be the case because it is the exact same scenario.

That's a really good point, Imatfaal, so it's worth going through carefully how it's still works out that way. The force being applied inside the shuttle to accelerate the alien ship to the same speed as the shuttle is now going to be applied unevenly from one end to the other instead of to the whole ship at once, so that means it will start by compressing the front end of the ship just before it reaches the end wall of the cargo bay, and will continue to compress the rest in the same manner, compressing the tail end just after the tail has entered the cargo bay, at which point the door is closed on a compressed ship (although most of it is already expanding again). As the atoms push out and buckle the ship (or burst it through the ends of the cargo bay), the front end of the ship will have been expanding while the tail end was still being compressed behind it, but there's no difference to the end result of the buckling or bursting even if it's sequential along the length of the ship rather than simultaneous.

-------------------------------------------------------------------

What I've also argued against  is the introduction of extra complexity to special relativity in order to preserve some preconception about the universe.  Science doesn't work that way.  We can always hold an infinite number of preconceptions about the universe and introduce extra hidden variables into a model to preserve them.  Obviously no one's arguing something this far out, but if I were more comfortable with unicorns causing length contraction, I could always introduce unicorns who zip about the universe shrinking our rulers with magical pony power.  As long as the unicorns are undetectable by measurement, and their pony power works identically to Lorentz contraction, I can claim that my results are completely in line with observations.  That doesn't this viewpoint valid science, nor does it make other viewpoints with far less absurd preconceptions valid science.  Science is generally about using the simplest possible model to describe some phenomenon, not choosing the more complex model because it fits your preconceptions.

That is completely right - no one wants unicorns, or anything else that adds completely unnecessary components to a theory.

Einstein's special relativity is the mainstream scientific explanation because it is the simplest model which introduces the fewest extraneous variables and preconceptions and explains observations.

I cannot see how that is the case. Einstein has a fabric of space called spacetime (which can be curved to cause gravity, but we don't even need to go there for evidence). Einstein himself spoke of the necessary role of an aether of some kind to enforce the separation between things in the universe (in terms of distance and geometry), which is something his fabric of space called spacetime does. Lorentz's theory also has a fabric of space which enforces distances and geometry, but it is three dimensional instead of four and has a Newtonian time instead of making time a special kind of space dimension. Both theories have the speed of light restricted to c through their fabric of space. Lorentz doesn't need to add anything else at all for his theory to fit the facts. Einstein doesn't either if he's only interested in the maths of measuring and predicting things. There is something extra required for Einstein's theory though, as he still has to explain the apparent pattern of cause-and-effect written throughout the universe - because he has no Newtonian time in his model, he is not able to generate effects from their apparent causes. There are long, complex chains of causation written through everything that's happened in the universe across billions of years of time, and for them to exist in that form without actually being generated in cause-and-effect order would be more than a little surprising. If you threw a sackful of grain out of a high window and the grain ended up on the ground with a complex pattern to it (perhaps forming a series of images telling a story with apparent cause-and-effect events unfolding from one picture to the next), you would be astonished [I suspect]. But that is exactly what Einstein's theory requires of the real universe, only the grain is never thrown from a window - the pattern is simply eternal and was not constructed in cause-and-effect order.

How can you be comfortable with that?

[old guy]

Imatfaal:

I am completely confused as to what you want from us.  If you want an admission that length contraction is illusory and in some sense not "real" - then you just won't get it (from me at least); mainly because SR deals in maths and not semantics, and secondly because I can fit the probe (very briefly) entirely within the shuttle's cargo bay.

My argument here simply questions how "real" length contraction is. Does a fast moving ship make stars move closer together? No, but its clock will "tick" more slowly the faster it flies, and its occupants will probably age more slowly.

Do objects have shapes and lengths independent of how they are measured? Yes, there is a "real cosmos with real objects" in it, but they may well appear contracted from very fast fly-by frames.

Can you really " fit the probe (very briefly) entirely within the shuttle's cargo bay?"
No.
Let's make the velocity of the probe relative to earth specific at 86.6 % of 'c.' If the "contracted length" as measured from earth is 10 meters, as established, it will then be 20 meters in its own frame and as measured from the shuttle once it enters that frame, at rest with the probe. A 20 meter probe will not fit into a 10 meter cargo bay, period, no matter how "briefly." Sorry.

[JP]

Lorentz doesn't need to add anything else at all for his theory to fit the facts.

Lorentz's theory postulates an unmeasurable reference frame, the aether.  The Lorentzian theory is essentially a historic holdover from the days when scientists thought matter of some form was required to support light wave propagation.  As more and more observations added up, it was found that this aether would have to be invisible and undetectable.  Say what you will about anything else, but Lorentz's theory has an extra, unobservable feature, while special relativity doesn't.  This is why it's been superceded by special relativity among scientists.

There is something extra required for Einstein's theory though, as he still has to explain the apparent pattern of cause-and-effect written throughout the universe - because he has no Newtonian time in his model, he is not able to generate effects from their apparent causes.

I'm not sure what you mean.  Can you give an example?  Special relativity does respect causality--that's one of it's important features.

[JP]

Imatfaal:

I am completely confused as to what you want from us.  If you want an admission that length contraction is illusory and in some sense not "real" - then you just won't get it (from me at least); mainly because SR deals in maths and not semantics, and secondly because I can fit the probe (very briefly) entirely within the shuttle's cargo bay.

My argument here simply questions how "real" length contraction is. Does a fast moving ship make stars move closer together? No, but its clock will "tick" more slowly the faster it flies, and its occupants will probably age more slowly.

Do objects have shapes and lengths independent of how they are measured? Yes, there is a "real cosmos with real objects" in it, but they may well appear contracted from very fast fly-by frames.

Can you really " fit the probe (very briefly) entirely within the shuttle's cargo bay?"
No.
Let's make the velocity of the probe relative to earth specific at 86.6 % of 'c.' If the "contracted length" as measured from earth is 10 meters, as established, it will then be 20 meters in its own frame and as measured from the shuttle once it enters that frame, at rest with the probe. A 20 meter probe will not fit into a 10 meter cargo bay, period, no matter how "briefly." Sorry.

Do you really not see that your entire argument hinges on making "real" fit your definition of, and then telling us that's proof that you're right?  Of course "real" matches your definition if you tell us that you're redefining it to do so!

[old guy]

JP:

Do you really not see that your entire argument hinges on making "real" fit your definition of, and then telling us that's proof that you're right?  Of course "real" matches your definition if you tell us that you're redefining it to do so!

You misunderstand me. It is not about my personal definition of "real."  I speak as a "realist," not some crank trying to debunk all of SR. That is why I used the example to demonstrate a "real world application" of length contraction as a sub-theory of SR... in a future world, of course, with near 'c' space travel.

If you were on the retrieval team, you would know that the observed length of 10 meters, from earth's frame, was not its true length, in its own frame. You would not send out a shuttle with a 10 meter bay to retrieve it.

Please do not make this personal, as if I invented realism. Einstein endorsed a form of idealism when he said that there is no reality independent of observation/measurement.
If one is not allowed to disagree with that, then SR has become a realism- intolerant dogma in favor of SR's version of classical idealism.

How about you? Are you an idealist or a realist. (Rhetorical question.) Does the falling tree make a sound without an observer to hear it?
Does Earth change shapes with every possible different perspective viewing it. I can't believe that you believe that!

[JP]

Now we're at the heart of the matter.  You're discussing metaphysics behind special relativity.  That's a perfectly valid and interesting area, but it's not a matter for this science forum.  If we were a philosophy forum, I'd say go at it!

We had some interesting discussions here, but unless you'd like to discuss science, I'll have to ask you again to keep it to "new theories." 

Thanks!

[David Cooper]

Lorentz doesn't need to add anything else at all for his theory to fit the facts.

Lorentz's theory postulates an unmeasurable reference frame, the aether.  The Lorentzian theory is essentially a historic holdover from the days when scientists thought matter of some form was required to support light wave propagation.  As more and more observations added up, it was found that this aether would have to be invisible and undetectable.  Say what you will about anything else, but Lorentz's theory has an extra, unobservable feature, while special relativity doesn't.  This is why it's been superceded by special relativity among scientists.

The unmeasurable reference frame is simply the fabric of space which restricts the speed of light within itself to c. The fact that the aether was originally regarded as something more like matter when these theories were originally being worked out does not disqualify the final form in which the fabric of space does the entire job, and having that fabric of space is not anything extra over having a fabric of space called spacetime. The difference between the two theories is really limited to the nature of time - one has Newtonian time while the other attempts to get rid of that entirely and to replace it with something which is almost identical to a space dimension.

There is something extra required for Einstein's theory though, as he still has to explain the apparent pattern of cause-and-effect written throughout the universe - because he has no Newtonian time in his model, he is not able to generate effects from their apparent causes.

I'm not sure what you mean.  Can you give an example?  Special relativity does respect causality--that's one of it's important features.

Special relativity respects it in the sense that the patterns of causality are there and appear to "work", but without Newtonian time it appears to be impossible to have the causes happen before their effects, which is something they would have to do in order to cause their effects. Perhaps you can point me to a way round that.

Let me illustrate the issue clearly by taking a version of a thought experiment with a lot in common with the (incorrectly named) twins paradox. This pushes relativity to the extreme to show up the difficulty, but I'll ramp it up in stages. Let's begin with the idea of a rocket leaving a planet, reaching a high speed, then stopping after a while, turning round, returning home at a high speed and eventually arriving back at the planet. If the rocket was to travel at 0.866c both on the way out and the way back, and with a rapid turn-around which can be taken as being close enough to instantaneous that we can ignore it, a year may have ticked by for the rocket during its trip while two years have ticked by on the planet.

Now, a lot of confusion comes into things if you try to explain things in terms of clocks ticking at different rates on the planet and in the rocket, because if all frames are genuinely equal there can be no difference between the fundamental rates at which clocks tick and you can't have some ticking faster than others. This can be got round by allowing the rocket to take a shortcut into the future by travelling at a high speed and thereby at a different angle through time, so while its clock is ticking at the same rate, it's also on a different course through spacetime which allows it to get back to the planet far in the future while the planet is still only half way there - the two can still meet up because the future of the planet is already there for the rocket to interact with. The whole business of clocks ticking is actually Newtonian time, so you must actually get rid of it altogether and just have an eternal block universe where the future is linked to the past by many pathways of different time-dimension lengths. We can still imagine time ticking at constant rates along each time-dimension paths if we like and picture things taking shortcuts into a ready-built future, but you always have to be aware that in doing so you would be adding a Newtonian time to the model, and that is not part of SR.

Now let's ramp it up. Let's make the rocket faster such that it records only one second by its clock (not practically possible, but this is a thought experiment in which the details of the effects on time, distance, etc. are correct). We can now put in place a series of complex events on the planet which all feed into the next so that there's a chain of causation which couldn't be predicted in any less time, and this might involve running a computer program which could end up producing any one of billions of different results. When the rocket returns a year later it is immediately able to access the answer of the computation, even though it only took one second to get there according to its clock. Thirty million seconds' worth of processing have taken place, but from the rocket's point of view this has all happened in one second.

Ramp it up again and you could have the rocket trip timed within the rocket as a billionth of a second while a billion years have gone by on the planet. There is a ratio of events here of thirty thousand quintillion to one. It appears that you have time running thirty thousand quintillion times faster on some pathways than others, but the block universe model has a billion year's worth of future (and more) ready-built and waiting for things like the rocket to take shortcuts into. By eliminating Newtonian time altogether, the rocket doesn't actually move though - it simply is, and all of its atoms are stretched through the model like spaghetti such that they exist in all times. This is what SR does to the nature of the universe.

There is nothing in SR to account for how such a universe can be generated, so it just exists eternally without the future ever having been generated from the past and without causes having any opportunity to generate effects. As soon as you attempt to have a cause generate an effect you introduce a Newtonian kind of time back into it to provide a before and after, and then you suddenly have ratios of events like thirty thousand quintillion to one coming into play for different paths in the construction - a single clock cycle occurs in the computer on the rocket during its trip while a computer on the earth has thirty thousand quintillion of them, and because we're now trying to construct a block universe, we're necessarily working with one which doesn't have a ready-built future to allow shortcuts - the rocket has to wait for the thirty thousand quintillion sequential events on the planet to run through before its computer (the rocket's) can have its second tick.

This is where SR hits its major snag - it appears to be impossible to generate a universe without introducing a Newtonian time back into things, so what you appear to be forced to do instead is just accept that it can't be generated while at the same time accepting that the patterns of cause and effect which are written through it are just an extraordinary coincidence. Now, I'm not asking anyone to reject SR on that basis, but I do think it's absolutely fair for me to insist that people accept that SR is in no way superior to Lorentz's theory to the point that the latter can be rejected and the former treated as if it is as good as a fact. It looks to me as if Lorentz's theory is more likely to be correct than Einstein's, but I don't need to go anywhere near that far in making the point that things are in no way clear cut in favour of Einstein's SR.

If any of the above needs clarification or further explanation, please ask and I'll provide it. What I'm hoping is that someone can see some way to overturn what I've said and reveal to me that SR can eliminate this difficulty, because if mainstream science is actually right, I want to be part of it.

[JP]

Lorentz doesn't need to add anything else at all for his theory to fit the facts.

Lorentz's theory postulates an unmeasurable reference frame, the aether.  The Lorentzian theory is essentially a historic holdover from the days when scientists thought matter of some form was required to support light wave propagation.  As more and more observations added up, it was found that this aether would have to be invisible and undetectable.  Say what you will about anything else, but Lorentz's theory has an extra, unobservable feature, while special relativity doesn't.  This is why it's been superceded by special relativity among scientists.

The unmeasurable reference frame is simply the fabric of space which restricts the speed of light within itself to c. The fact that the aether was originally regarded as something more like matter when these theories were originally being worked out does not disqualify the final form in which the fabric of space does the entire job, and having that fabric of space is not anything extra over having a fabric of space called spacetime. The difference between the two theories is really limited to the nature of time - one has Newtonian time while the other attempts to get rid of that entirely and to replace it with something which is almost identical to a space dimension.

Well, the history and everything else doesn't matter (I provided it because it's interesting to know where the Lorentzian model came from, IMO).  What matters to scientists is which one has an extra undetectable feature and which one doesn't.  At least on the count of an undetectable reference frame, the Lorentzian model has it, while special relativity doesn't. :)

The rest of your post is quite long, so I'll get to that separately...

[David Cooper]

What matters to scientists is which one has an extra undetectable feature and which one doesn't.  At least on the count of an undetectable reference frame, the Lorentzian model has it, while special relativity doesn't. :)

The Lorentzian model has a fabric of space which can't be detected, and Einstein's model has a fabric of space called spacetime which can't be detected either. The fabric of space in the Lorentzian model is automatically the preffered frame, so there's no extra undetectable thing tied up in it.

[yor_on]

David, SR is a consequence of 'c'. And so becomes time dilations and LorentzFitzGerald contractions. Einstein first made his assumption that the contradictions he found in his thought experiments would become logically explained, if you defined light as having the same speed in all uniformly moving frames, no matter what relative motion they had. So I don't get where from you get the idea that relativity doesn't know how it is constructed? When it comes to Lorentz there's been a lot of discussions about whom first may have thought out the consequences, but it was Einstein that published and developed it, on his own. Him and Lorentz corresponded at times, exchanging thoughts, but so they all did. But relativity was a mind concept of Einstein, not Lorentz, although it may well be that Lorentz was the more gifted mathematician. Einstein struggled with finding the right mathematics for his concept over the years, testing different views and finding them lacking. He never actually stopped doing that either, as I see it as he still was trying to fit relativity to a 'fifth dimension' at his death, trying to find the right math for describing it, which apparently seems quite close to the mathematics string theorists use today. Lorentz never had anything bad to say about Einstein getting the recognition for his mind child, but both was deserving a Nobel prize for their contributions to relativity (that neither got).

Here is The Einstein Theory of Relativity, by H.A. Lorentz. as well as 'mathpages' comments on the same. Who invented relativity?   
 

[JP]

What matters to scientists is which one has an extra undetectable feature and which one doesn't.  At least on the count of an undetectable reference frame, the Lorentzian model has it, while special relativity doesn't. :)

The Lorentzian model has a fabric of space which can't be detected, and Einstein's model has a fabric of space called spacetime which can't be detected either. The fabric of space in the Lorentzian model is automatically the preffered frame, so there's no extra undetectable thing tied up in it.

Except that the preferred frame is undetectable...

[yor_on]

If it is 'time' and its 'arrow' you're questioning then 'time' is a mystery still :) Just as why 'c' must be 'c'. The arrow though is a direct consequence of its equivalence to 'c' as I think of it. Locally invariant, and using light as a clock, splitting it down to Plank scale you get a situation that describes a arrow as close as we can get, one 'tick' at a 'step'.

[JP]

Now, a lot of confusion comes into things if you try to explain things in terms of clocks ticking at different rates on the planet and in the rocket, because if all frames are genuinely equal there can be no difference between the fundamental rates at which clocks tick and you can't have some ticking faster than others.

Clocks don't tick at the same rate for all observers.  That's time dilation.

This can be got round by allowing the rocket to take a shortcut into the future by travelling at a high speed and thereby at a different angle through time, so while its clock is ticking at the same rate, it's also on a different course through spacetime which allows it to get back to the planet far in the future while the planet is still only half way there - the two can still meet up because the future of the planet is already there for the rocket to interact with.

You can chart the path through space-time of the planet vs. the rocket and show that the rocket's proper time (time measured by the clock on the rocket in the rocket's frame)  is less than the earth's proper time.  If that's what you're saying, I agree.  Talking about "the future of the planet is already there" is somewhat confusing, but I think we agree on what the clocks see.

The whole business of clocks ticking is actually Newtonian time, so you must actually get rid of it altogether and just have an eternal block universe where the future is linked to the past by many pathways of different time-dimension lengths.

Actually, I am partial to the block universe (and the even more bizarre many-worlds interpretation of QM), but this is introducing philosophy into the mix...  I certainly wouldn't go telling people that the many-worlds interpretation has to be true just because I philosophically prefer it!

By eliminating Newtonian time altogether, the rocket doesn't actually move though - it simply is, and all of its atoms are stretched through the model like spaghetti such that they exist in all times. This is what SR does to the nature of the universe.

OK.  Each object can have a world line through space-time of all the points in space and time it has occupied.  I guess a line is like spaghetti.  So what?

There is nothing in SR to account for how such a universe can be generated, so it just exists eternally without the future ever having been generated from the past and without causes having any opportunity to generate effects.

We discussed this in your other thread and you insisted you weren't talking about generating a universe.  Now you are again.  Talking about generating a universe.
As I told you there, arguing that "there is nothing in SR to account for how such a universe can be generated" is pointless unless you're arguing against a theory that deals with the generation of the universe.  Special relativity doesn't.  It works within the universe to make predictions.  All you've offered is a theory with an additional, undetectable element that does the same job and tries to explain how the universe is generated.  But then again, so does believing exactly in special relativity and then adding "God made the universe."  That's simply not a scientific way of doing things.

Your point about causes being unable to generate effects is completely wrong as well.  Causes obviously generate effects in SR.  If you can connect a cause to an effect by a valid world line, it can generate that effect.

[old guy]

Now we're at the heart of the matter.  You're discussing metaphysics behind special relativity.  That's a perfectly valid and interesting area, but it's not a matter for this science forum.  If we were a philosophy forum, I'd say go at it!

We had some interesting discussions here, but unless you'd like to discuss science, I'll have to ask you again to keep it to "new theories." 

Thanks!

It is difficult to see how a thought experimental practical test of the physics of length contraction is metaphysical, not "science." Granted, Einstein's version of idealism (only observations are "real") in contrast with realism (things exist and have properties independent of observation) is a philosophical issue...  but one I am sure is of central relevance to this topic. Denying that objects have intrinsic properties is not an argument against realism.

Anyway, I have complied with your demand that I move it to my length contraction thread in New Theories.
Please reply to my last post there.
Also, since imatfaal will not be allowed to reply to my last challenge to him in this thread, I have moved that exchange as well and hope that he will reply there.
See you there, I hope.
Edit, Ps: It is weird that David Cooper is allowed to continue to hijack this thread with his pet theory/opinions about different versions of SR, but I am once again gagged here.

[David Cooper]

Actually, I've found a problem with my version of the shuttle experiment. It works fine in a Lorentzian universe if the shuttle is stationary, but in SR there's the problem that you can't make claims about simultaneity at a distance, so it's wrong to say the whole of the alien ship fits inside the shuttle for a moment, as revealed by the other case where the shuttle is moving and the ship is not - in this case the ship gets compressed from its front end as it's accelerated up to the speed of the shuttle and it starts to expand and buckle before the tail end has been compressed to fit inside the shuttle. In one frame it appears to fit for a moment, but in another frame it fails to.

The thought experiment provided by Damocles may eliminate this problem...? If the star and telescope are moving instead of the disc, then they will be flattened a little in their direction of travel and the disc will then be larger than the telescope aperture. The flattening of the star doesn't matter as it's acting as a point source effectively at infinite distance, but how can it be that light will enter the telescope throughout? The light that appears to be coming from the star is actually coming from where the star was earlier, so the light reaching the telescope  (just before and after the disc gets in the way) is passing the disc at an angle and making it act as if it's narrower than it is... except that the disc could be replaced with a ball, so that isn't going to help. I'll have to think about this for a bit longer, but I'm stuck for now.

Edit: we do know though that the disc/ball will still appear as if it is contracted even if it's stationary and the telescope and star are moving instead. It must be something to do with the different length of time it takes for the light to reach opposite sides of the telescope and to work their way through to the focal plane - I'm beginning to visualise it. Yes: the light from the trailing edge of the telescope aperture takes a lot longer to reach the focal plane, so it's still travelling through the telescope while the eclipse of the disc/ball has begun, whereas the light which gets into the opposite side of the telescope when the eclipse ends has a shorter trip to make to get to the focal plane and will arrive there before the last of the light from the trailing edge gets there.

[David Cooper]

Now, a lot of confusion comes into things if you try to explain things in terms of clocks ticking at different rates on the planet and in the rocket, because if all frames are genuinely equal there can be no difference between the fundamental rates at which clocks tick and you can't have some ticking faster than others.

Clocks don't tick at the same rate for all observers.  That's time dilation.

In the thought experiment, it can be observed that the rocket's clock has recorded less time than the planet during the trip, so you might claim that the rocket's clock has been ticking slower than the planet's clock throughout the trip, but if all frames are equally valid it cannot be the case that any frame can make its clock run slow as compared to another frame unless there is a preferred frame being used as a standard to control their relative rates. Without a preferred frame, the rocket cannot know whether to slow down or speed up its clock when it leaves the planet and sets off on its rapid journey, so it cannot be doing either. If the planet is stationary, the rocket would slow its clock when it moves away from it, whereas if the planet is moving fast, the rocket may be slowing down by leaving the planet in which case its clock would have to speed up. These things would work exactly like that in a Lorentzian universe, but cannot do so in SR unless they are to tick both more slowly and more quickly at the same time.

In SR, if a rocket makes a trip timed at a billionth of a second for its clock, you are not allowed to say that its journey actually took a billion years and that its clock was running slow. The rocket really did make its trip in a billionth of a second, and it did so by taking a shortcut into the future. The planet took a billion years to complete its journey from the same starting point in spacetime to the same end point, but the rocket didn't have to wait for a billion years of time to run through before it could land again.

By eliminating Newtonian time altogether, the rocket doesn't actually move though - it simply is, and all of its atoms are stretched through the model like spaghetti such that they exist in all times. This is what SR does to the nature of the universe.

OK.  Each object can have a world line through space-time of all the points in space and time it has occupied.  I guess a line is like spaghetti.  So what?

The point is that if you have no Newtonian time in the model you can have things which you call past and future, but you have lost any real kind of before and after - the future is already in place for you to take shortcuts into without having to run slow so that paths which need to pack a lot more action into them can keep pace.

There is nothing in SR to account for how such a universe can be generated, so it just exists eternally without the future ever having been generated from the past and without causes having any opportunity to generate effects.

We discussed this in your other thread and you insisted you weren't talking about generating a universe.  Now you are again.

You're misremembering - the issue was always about how in SR you can generate the future out of the past without Newtonian time.

As I told you there, arguing that "there is nothing in SR to account for how such a universe can be generated" is pointless unless you're arguing against a theory that deals with the generation of the universe.  Special relativity doesn't.  It works within the universe to make predictions.

If you did say that there (I've just read the thread again and still didn't notice), then I appologise for not picking up on that. Thank you for stating it clearly here. I take it from this then that it is a matter of philosophy to think about how an eternal block universe can be generated without adding Newtonian time to it and therefore it doesn't matter whether a block universe can't be generated or not as it is not important to the laws of physics in terms of making predictions.

All you've offered is a theory with an additional, undetectable element that does the same job and tries to explain how the universe is generated.  But then again, so does believing exactly in special relativity and then adding "God made the universe."  That's simply not a scientific way of doing things.

I'm simply looking at two theories and trying to see why one of them is "wrong" and the other "right" when the one that's "wrong" has as its fault that its preferred frame (which is its fabric of space) can't be detected while the one that is "right" has a fabric of spacetime that can't be detected either and when only one of the two theories is able to allow a universe to be generated (meaning that the future is generated out of the past through cause and effect). Now it is clear that this is unimportant to physics as it is mere philosophy, and physics doesn't do philosophy. That's fine then, so long as you aren't putting things across as facts when they are philosophically unsound - to do so on the basis of simplicity is actually bringing bad philosophy into play (inadvertently).

Your point about causes being unable to generate effects is completely wrong as well.  Causes obviously generate effects in SR.  If you can connect a cause to an effect by a valid world line, it can generate that effect.

Without a Newtonian time being introduced, that is simply wrong. Effects are clearly not being generated by their apparent causes in SR because it is possible to take shortcuts into the future to access the effects at the end of long chains of causation without waiting for those chains to run through - the result of a calculation taking a billion years can be accessed in a billionth of a second by taking a shortcut into the future, but that's only possible with a block universe where the future is ready-built. If you attempt to repeat the thought experiment in a block universe in which the future is not ready-built such that causes actually generate their effects, you have then changed the situation into a construction phase running under a Newtonian time in which the rocket's clock must run in slow motion for a billion years to allow the result of the billion-year-long chain of causation to generate a result which the rocket is to interact with on its arrival home, and that would be running under the rules of a Lorentzian universe.

For SR to be correct, you would need to demonstrate that a block universe can be generated in some way without going through a Lorentzian phase first. Can you do that? You don't have to, I suppose - you can always just write the problem off as philosophy and pretend it doesn't matter.

[JP]

I take it from this then that it is a matter of philosophy to think about how an eternal block universe can be generated without adding Newtonian time to it and therefore it doesn't matter whether a block universe can't be generated or not as it is not important to the laws of physics in terms of making predictions.

Yes.  That is exactly correct.

Everything you're arguing hinges on you assuming that "the eternal block universe" exists, and the block universe is a philosophical interpretation of the model. 

I think there are only two scientific points left to discuss that could possibly justify the Lorentzian model as scientifically preferable (in terms of being the simplest predictor and explanation of observations):

1) Does the Lorentzian model offer any predictions of measurements that differ from SR?

2) We already know that the Lorentzian model assumes the existance of the Lorentzian aether (an undetectable, preferred reference frame), while SR doesn't.  So that's one complicating factor for the Lorentzian model.  While only using the measurable predictions of both theories, does SR have an additional, unmeasurable feature that the Lorentzian model lacks? 

Of course, it may be philosophically preferable.  But that's not a matter for discussion here, so let's take that to New Theories.  :)

[David Cooper]

1) Does the Lorentzian model offer any predictions of measurements that differ from SR?

No, so that just leaves (2).

2) We already know that the Lorentzian model assumes the existance of the Lorentzian aether (an undetectable, preferred reference frame), while SR doesn't.  So that's one complicating factor for the Lorentzian model.  While only using the measurable predictions of both theories, does SR have an additional, unmeasurable feature that the Lorentzian model lacks?

The Lorentzian model assumes the existence of an undetectable three-dimensional fabric of space plus Newtonian time while Einstein assumes the existence of an undetectable four-dimensional fabric of space in which time turns into a special space dimension. Neither brings anything else in, so they are equally simple in terms of their requirements. To say that Lorentz then needs to bring in an undetectable preferred frame is not correct - it automatically comes along with having a three-dimensional fabric of space with Newtonian time and is not an addition because the fabric of space is the preferred frame. It would be equally incorrect for me to claim that SR has to bring extra things in when they automatically come along with having a four-dimensional fabric of space in which the time dimension behaves oddly. Neither theory can claim with any justification to be simpler than the other - the differences between them are down to a single thing, and that is the nature of time chosen for each model.

Of course, it may be philosophically preferable.  But that's not a matter for discussion here, so let's take that to New Theories.  :)

That would be fair, provided that everyone is careful not to make statements here about one theory being right and another wrong when such claims cannot be justified on logical grounds. It appears that in physics there is a much lower standard of logic being applied in determining which theories are accepted and which are rejected than there would need to be to justify more global assertions about their validity, but science fails to make that clear on the tin - people tend to look up to it as an authority and to take a considerable amount on trust. In this case we have a theory which is treated as correct in science but which can be shown to have serious problems logically, but logic is being branded as philosophy and ignored on that basis. At the same time, we have a rival theory which is being rejected through incorrect reasoning relating to claims that the other theory is simpler.

Anyway, thanks for taking on the discussion so far as you have - it has been very helpful in clarifying to me why SR holds sway at the moment. Everything is falling neatly into place.

[old guy]

I was just reviewing this thread to see if anyone ever offered any direct evidence for large scale length contraction. I didn't see any. In lieu of that, what is the best indirect "evidence" and how (exactly, please) does that translate to large scale?

[David Cooper]

The Michelson Morley experiment. You won't see an arm of it contracting in any obvious way as you can't get it up to a sufficiently high speed, but when you look at the way light behaves when it goes through it, you can see that the length must be contracting in the direction of travel in order to enable the light to reach the leading mirror and get back in the same time as the light following the other path. Another way to demonstrate this is to use two light clocks at 90 degrees to each other - they will keep perfect time with each other, while your theory (the one disproved by the MM experiment) predicts wrongly that one will run more slowly than the other when it is aligned with its direction of travel.

[wolfekeeper]

Also, the apparent position of the stars move about as the Earth orbits the Sun. That's length contraction, among other effects.

[old guy]

David,
We all agree that the speed of light does not change with the speed of its source or the speed of observers relative to the source. Nor can light can be "pushed" faster like a bullet from a speeding gun. Also there is no "aether" from which to figure an absolute velocity, so velocity is always a matter of "relative to what?" in all cases. So far so good.

Now, by what force are physical objects and distances contracted, as objects in-and-of themselves, independent of how they may be observed differently from different frames?
If Earth is said to have various diameters when variously observed, what force makes the real physical Earth change shape? Ducking the question does not answer it.
wolfekeeper,
The apparent movement of the stars has nothing at all to do with the claims of length contraction. Read some basic astronomy.

[yor_on]

David, your examples of light contraction are 'on the spot' as far as I'm concerned. If I remember right Lorentz 'invented' length contraction just to explain the MM experiment. And when it comes to light clocks I have a very sweet link doing the math in a understandable way, using geometry. All Moving Clocks Are Slowed by Motion   

And this site is also a pleasure to read, shows that relativity is explainable if you try, not just reserved for those whose brains have outgrown their skulls..

Why does relativistic length contraction (Lorentz contraction) happen?

Why does Lorentz contraction only act in the direction of motion?