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A benzene molecule has an intrinsic shape, a butane molecule does not.
From Lightarrow:QuoteImagine a massive neutron star, which is composed of two concentric parts spinning along a samr axis but in opposite directions, so that the outer bords of these parts are moving at relativistic speeds. 1. Which would be the measured radius of the star? 2. Which would be its "intrinsic" radius?We would have to imagine something a little different to that, because in SR length contraction operates only in the direction of motion, and rotation only describes motion perpendicular to the radius. So the intrinsic radius would be easily and unambiguously determined in an inertial frame that is stationary relative to the centre-of-mass of the neutron star.
Imagine a massive neutron star, which is composed of two concentric parts spinning along a samr axis but in opposite directions, so that the outer bords of these parts are moving at relativistic speeds. 1. Which would be the measured radius of the star? 2. Which would be its "intrinsic" radius?
Nevertheless it is possible to think of an "intrinsic" property that is ill-defined for a complex object -- in fact such properties are legion. What is the rest mass of the solar system? If we try to measure it in the frame of the centre-of-mass of the solar system, the orbital motion of each of the planets would ascribe to it a mass slightly greater than its own rest mass. So we are faced either with an object that has a mass somewhat greater than the sum of the parts or a mass that cannot be directly measured.
For a physicist, this creates a contradiction and a crisis, that can only be ultimately resolved with a denial that anything has "intrinsic" properties. For a chemist, though, there is no problem. We are forever ascribing an "intrinsic" weight to an atom which (for quite different reasons) is quite significantly different to the sum of masses of its protons, neutrons, and electrons.Even in a Newtonian universe, the solar system does not have a shape. The concept of "intrinsic" properties does not collapse as a result. A house or a raindrop still has an intrinsic shape. In the chemist's world an object may or may not have a particular intrinsic property. A benzene molecule has an intrinsic shape, a butane molecule does not.
Quote from: flr on 07/09/2012 21:11:34To be honest, my intuition tells me that objects deserve an absolute shape, and I find it hard to train my intuition to believe otherwise. Maybe from here arise my difficulty in accepting the relativity of lengths. Why should not some real object have a form of itself? Why an object cannot be something by itself?That's a good point, flr. The problem is that intuition only works when you're so familiar with something that it becomes intuitive! It can also be very misleading in science, since many effects are counterintuitive. A far better guide in science is logic. Once you know something is true (by observing or measuring it), you can logically figure out its consequences. That's the case in special relativity. Length contraction is very counterintuitive, but we know that the speed of light is constant for all observers from experiments. Once you know that's true, you can logically work through the consequences, and find that lengths have to contract. Our intuition fails because we hardly ever experience relativistic effects in daily life. You can definitely train your intuition by studying something thoroughly, but it takes a lot of effort to actually learn a subject so well that it becomes intuitive, especially when its as abstract as relativity theory.
To be honest, my intuition tells me that objects deserve an absolute shape, and I find it hard to train my intuition to believe otherwise. Maybe from here arise my difficulty in accepting the relativity of lengths. Why should not some real object have a form of itself? Why an object cannot be something by itself?
OK - Enough. This is a semi-official note that we either return to accepted ideas or the thread gets closed.
It is clear that many of you regard Special Relativity and/or its implications as a convenient fiction - This thread is rapidly descending into a melange of arguments from personal incredulity and ignorance of the subject and that is against the ethos of this Q&A forum.
Frame dependence, special and general relativity, and a privileging of observation, modelling, and mathematics mean that your false division into intrinsic and extrinsic is meaningless.
But how came the moun finds a shorter path in its way through atmosphere? By which mechanism its path is shorter than what I observe from Earth?Is our universe made of multiple "realities" that are superimposed? In each such "reality" length is shorter or longer and time "flow" faster or slower?
This warping of space and time is the most shocking part of special relativity when one encounters it for the first time. It is important to note that I use words like “perceive”, “observe” and “point-of-view” to describe the changes in time and space, but these are real changes — no physical experiment or measurement of any kind will disagree with the results.
I am not sure I found a satisfactory answer to the question: "what makes space and time be relative?" or its equivalent "what makes c be invariant?" Instead I found in many places the argument that the invariance of c is immediately equivalent with relativity of space and time.
I don't think there is a satisfactory answer to why the speed of light is constant. It's a postulate of special relativity, which means we take it as a fact. We have plenty of measurements that establish it as a fact, but we don't have some deeper theory that tells us why it is.
...but when it comes to state for example that a length contraction isn't 'real' it automatically drops into New Theories. Einstein defines it as 'real' from the frame measuring, and so did Lorentz too. To prove it wrong must then belong to New Theories, and doing it one need to present testable predictions,...
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.
The probe in my example is not actually 10 meters long, as (it was) observed from Earth's frame. Proof: It will not fit in the shuttle's 10 meter cargo bay. (Much too long, actually.)
I think it's fairly simple what the mods want. We can all soar, but that's for 'New theories', (although) it's quite hard not to put in your own interpretations in this Physics, Astronomy & Cosmology forum and we're all failable. This site was once constructed for giving people the chance to have in depth discussions, but when it comes to state for example that a length contraction isn't 'real' it automatically drops into New Theories. Einstein defines it as 'real' from the frame measuring, and so did Lorentz too. To prove it wrong must then belong to New Theories, and doing it one need to present testable predictions, and to be perfectly strict also the math behind the reasoning. And the last demand is definitely the hardest. But that is if one want people to take one seriously, and then one probably publish elsewhere
It will fit in the cargo bay for a moment, but it has to keep moving relative to the cargo bay in order to do so, with the result that it will only fit in it for a moment, so you're going to need a cargo bay with open doors at both ends. If you slow it to a halt or accelerate the shuttle to its speed so that you can capture the alien ship, it will then be too big to fit.
Actually, there may be a better way to illustrate things: you could do away with the door at the far end of the cargo bay and have some kind of device to decelerate the whole ship in an instant without crushing it, this being done by applying a levitation kind of force to it (as has been done with a frog), but obviously much stronger.