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David, "Einstein appears to offer a spacetime in which the contractions are not real - things appear to contract, but they're really just reorienting themselves in spacetime such that they appear shortened from other frames because part of their length is taking up part of the time dimension."To speak of it as wandering of in 'time' isn't that clarifying to me David
And to say that Einstein saw contractions as a illusion needs at least a citation from him. There are two views, some accept that time dilations exist, it's hard avoiding that, but adhere to that a LorentzFitzGerald contraction is a 'optical illusion', others as me expect it to be real, meaning true from the frame of reference finding/measuring it.
Let us assume that it really would be time ticking 'slow' in the muon-Spaceships 'inertial frame' (being in uniform motion), that will naturally include all decay and all 'change', including all 'force carriers' aka photons and 'virtual photons', add infinitum. What we now do is to introduce a 'variable speed of light' in where the constant 'c' has to adapt to the local frame of reference. That as it indeed will present a 'clock' for all natural processes assuming 'virtual photons', although we don't even need to do that. Just assume that the muon-ship has a device for measuring the speed of the light before leaving it, to then cross space to be received on Earth, aka a two way mirror sending that light of.So let us assume that you ('inertial frame' Earth) really can see that other, uniformly moving frames light , and that you too let that light 'bounce' between two mirrors, just to measure that lights speed. Can you expect the measurement to come out as 'c'? And if assuming that light to have changed its speed somewhere? To 'fit' our notion of 'c' on Earth, where would that be? And what would should I call that light changing its speed, a acceleration/deceleration maybe? And a variable?
The whole idea of contractions relative time dilations is that it is a symmetry as I see it, that's also why you can use a 'light clock' to illustrate it. If it was a illusion those light clock examples in where you have a contraction 'compensating' the time dilation by necessity would have to be wrong.
"Suppose we observe a body A to rest in space relative to our reference system. Let another body impinge on it, causing it to deform slightly as the force of impact is transmitted throughout, also setting A in motion relative to us. Such motion and deformations involve physical causes, yet they may well be described kinematically. Next consider a body B resting in free space relative to us,and now let us just begin to glide sideways away from it until we achieve a constant inertial speed. In Newton’s framework we say that B now seems to move away from us, and we call that a kinematical effect. In Einstein’s framework, we say that the body moves away relative to us and that its length is shortened relative to us, and we call those effects kinematical.
Einstein expected that the effect is identical to what would transpire if instead B were moving away from us at the same rate. For example, if a rocket accelerates near the Earth, and we are inside that rocket, then relative to us the Earth now should have a narrower length. Yet nobody will claim that thus something happened to all the molecules that constitute the Earth. The way we describe their cohesion relative to our rocket may change, but we would not say that there is any material change in the Earthly molecules partly because no such change happened relative to all other systems. By contrast, certain common changes in the configuration of molecules on a body are material changes, which might be observed from any system. Special relativity takes the effects of relative motion as fully reciprocal regardless of which system is regarded as oving. Thus observers on Earth would judge, instead, that the rocket is moving and contracted. Neither contraction is more real than the other, and neither is an optical illusion. Relativity of length means just that any two points on a given body are separated not by one universal length but by indefinitely many lengths." A.A. Martinez / Studies in History and Philosophy of Modern Physics 38 (2007) 209–215 213
As for Einstein I found this citation.In 1911 Vladimir Varićak asserted that length contraction is "real" according to Lorentz, while it is "apparent or subjective" according to Einstein. Einstein replied: The author unjustifiably stated a difference of Lorentz's view and that of mine concerning the physical facts. 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.[15] —Albert Einstein, 1911
The contraction is real in observational terms, but the object is not really contracted.
That would mean that there is an infinite range of equally valid measurements for the size of any object or the distance between any two points, and changing the frame from which you make the measurement simply gives you access to a different version of that length - you aren't changing the shape of it, but merely changing your viewpoint on it.... It's clear that there is a maximum measurement for the length of anything, and that measurement shows up when you measure the object from the same frame. I'd be surprised if Einstein didn't consider that to have greater validity.
David Cooper:QuoteThe contraction is real in observational terms, but the object is not really contracted.In other worlds, just to be clear, large scale length contraction is an illusion, if “the object is not really contracted.”That is what I’ve been saying (with no replies directly to my points) about Earth’s diameter, the distance to Alpha Centauri, and the length of the alien probe... all staying the same regardless of how they are variously observed/measured.
It is clear that measuring something from the same frame, at rest with the object, yields the valid and correct measure. I think that the dictum "there is no preferred frame of reference" is in blatant denial of what science already knows for sure about Earth's diameter, the distance to AC, the distance to the Sun, etc. They don't change with how one looks at them.
If objects like Earth have intrinsic shape independent of “your viewpoint” then there is a “valid” shape (both polar and equatorial diameters are well documented to a high degree of precision), and other measurements from all different varieties of viewpoint are not “equally valid."
I agree with your statement:QuoteWithout a preferred frame, the distances between things can be measured accurately within the frame in which they're stationary, and that would arguably be the truest measurement.
Without a preferred frame, the distances between things can be measured accurately within the frame in which they're stationary, and that would arguably be the truest measurement.
But lightarrow said:QuoteIf you are still with respect to Earth, every Earth's dimension has a value, in your frame of reference; if you are moving with resperct to Earth, those dimensions are different, as measured in that new frame of reference (because of #).In relativity you *cannot* say: "the lenght is 2 metres", you have to say: "the lenght is 2 metres as measured in this specific frame of reference".Does this claim that Earth changes shape as it is measured from different frames of reference?
If you are still with respect to Earth, every Earth's dimension has a value, in your frame of reference; if you are moving with resperct to Earth, those dimensions are different, as measured in that new frame of reference (because of #).In relativity you *cannot* say: "the lenght is 2 metres", you have to say: "the lenght is 2 metres as measured in this specific frame of reference".
This, of course, is impossible and has never been empirically observed.
So the claim seems to be that there are no actual objects with intrinsic properties (or distances between them) independent of how they are observed/measured?
Here is another “reality check” against large scale length contraction:Say an alien probe is discovered heading toward Earth at a significant fraction of light speed. From Earth’s frame it is measured to be ten meters long, length contracted because of its velocity relative to earth. It is decided to go out and intercept/capture it in one of our very high speed space shuttles (of the future.) Our shuttle has a ten meter cargo bay. Will the probe fit into the bay?
... A very practical test of “actual length” vs “contracted length.”The answer is “no” because the probe’s “actual length” must be longer than its “contracted length” for it to appear as ten meters long from earth’s frame in this case.Lightarrow, please address this challenge and the “earth changing shapes (diameters)" challenge. Thanks.
Yes - we're all pretty much agreed now (I think) that the objects don't change, but that the measurements do,
heh No we haven't David, agreed that is. I will stand by it is frame related (observer dependent) and as 'real' as can be, from the frame measuring.
Physics concerns measures; if an object's measure changes, then the object changes.(Also):You always have to specify a frame of reference, it *doesn't exist* a "lenght" independent of it.
Certainly. In a frame of reference which is still with respect of our planet, the Earth is spherical...; in another, moving, frame, it's not (and of course every human being is flattened too). Where is the problem?
You're wrong for both assertion. Maybe you still haven't totally grasped what "measure of lenght" means, read again my first post. A measure of lenght *is* frame-dependent, and so is, as consequence, an object's shape.
Do you think there are no natural objects with intrinsic properties or distances between established by gravity as they were formed in space? All the cosmos depends on how it is observed? Is this not relativity's version of classical subjective idealism, with 'frame of reference' as the abstract, virtual "subject."
In philosophy, Realism, or Realist or Realistic, are terms that describe manifestations of philosophical realism, the belief that reality exists independently of observers.
Quote from: David Cooper on 04/09/2012 20:06:11Yes - we're all pretty much agreed now (I think) that the objects don't change, but that the measurements do, Two contradictory statements, put in this way. Physics concerns measures; if an object's measure changes, then the object changes.Unless you intended something else...
Say 1000 ships pass by Earth going 1000 different (but near 'c') velocities, all going in 1000 different directions. Does Earth change into 1000 different shapes with its diameter contracting variously in all those directions? Of course not!
Earth is actually nearly spherical. AC is actually 4.37 light years from Earth. The distance to the Sun is actually about 93 million miles, which would not change if it were measured by a ship flying by very fast. The probe in my example is not actually 10 meters long, as observed from Earth's frame. Proof: It will not fit in the shuttle's 10 meter cargo bay. (Much too long, actually.)
Yep, it's frame dependent.
QuoteYep, it's frame dependent.And that means the observed object changes its shape as the observer changes its (say inertial) frame of reference?
As far as I know, there is no direct experimental proof of length contraction. The time dilation was experimentally proved, but length contraction was only assumed to obey the invariance of "c". I understood the time dilation in seconds from that photon clock imaginary experiment. However I still cannot picture the length contraction in a nice intuitive way. This is the closest I could get to Lorentz contraction: I force myself to accepting that the lengths and therefore the shape of objects, as observed from various frames, are not intrinsic properties of objects but rather relationships between observer and the measured object. Seeing lengths as relationships between frames rather than an absolute property of the measured object, I can then digest that they could change with changing the frame of reference. But then we can always ask: "are these relationships the real lengths or apparent" - and ... here we go again ..... I cannot really understand why and how the space of a muon is different from mine (staying on Earth) in such a way that the muon somehow finds a path of only 1 meter to travel through what for me is 50 km layer of atmosphere.
So the sort of thing I was saying in my previous post is that the "measured length" of an object (like the Earth's atmosphere) -- is not an intrinsic property of that object -- but its "rest length" is.
As far as the muon travelling through the atmosphere is concerned, there will be no inappropriate change in the physics of its interaction with the atmosphere because the relativistic decrease in the length of path will be matched by a relativistic increase in the number density of atmospheric molecules, and the relativistic increase in the observed mass of each molecule (which operates quite separately to the increase in number density) will produce an exact match with the physics we observe from an Earth-stationary frame.