I heartily endorse The Mechanical Universe and Beyond. A good complement for Physics for Poets + any Modern Physics course.
Regarding whether the Second Postulate of Special Relativity an assumption or not -- it is an assumption based on a record of experiment. We looked for evidence it was not true at a time when we assumed it might be not true, but we couldn't find that evidence. Now we assume it is true and occasionally look for evidence it is not true. Some current theoretical models predict very-very-high energy gamma rays might travel very slightly faster than visible light.
Today, Pentcho Valev posted a link to a paper I partially agree with. (Naturally, Pentcho Valev fails to extract anything useful from it, but that's another matter.)
http://www.uio.no/studier/emner/matnat/fys...lativAJP193.pdfIt includes a quote from
Banesh Hoffmann,
Relativity and Its Roots Freeman, New York, 1983, p. 92
which I think clarifies the original connection to Common Sense.
QUOTE (Banesh Hoffmann+ 1983)
The second of the two principles in Einstein’s paper said that the motion of light is not affected by the motion of the source of light. Nothing, it would seem, could be more orthodox and obvious [in the Maxwell-era thinking in which light was just a wave-like disturbance of the lumiferous ether]. For if a source of light generates light waves in the ether, once the waves are launched they are no longer linked to their source; they are on their own, moving at the rate set by the elastic properties of the ether....
If it was so obvious, though, why did he need to state it as a principle? Because, having taken from the idea of light waves in the ether the one aspect that he needed, he declared early in his paper, to quote his own words, that “the introduction of a ‘luminiferous ether’ will prove to be superfluous.”
We see in all this the working of an extraordinary intuition. The beautiful thing about Einstein’s cunningly chosen pair of principles is that each by itself seems harmless, yet the two together form an explosive mixture destined to rock the very foundations of science.
Ralph Baierlein's point about the LOW-SPEED BEHAVIOR OF THE LORENTZ
TRANSFORMATION seems misguided, as the Lorentz tranformation equals the Galilean Tranformation in the classical case where c = infinity. (Here c is the "speed limit of the universe" not the speed of light, where the two are the same in Einstein's Relativity.) Mathematically he is correct that when the ratio v/c is small, there are non-Galilean effects in Special Relativity, but that is not the usual classical limit. A special issue of Scientific American indicates that Time Dilation of a relative walking speed is predicted to affect the future generations of atomic clocks. (That's a 10^-17 effect, but would open up a new realm of classroom experiment.)
I'd post more URLs but I have a taxi waiting.....
MDT
27th May 2006 - 11:50 PM
If I had a superconducting wire with electricity traveling at C and I gave the wire motion in the direction of the current, wouldn't the distance displacment of the wire in space allow the electric signal to move farther than calculated by C? Or would electricity slow down to maintain the distance defined by C?
rpenner
28th May 2006 - 07:09 AM
QUOTE (MDT+May 27 2006, 11:50 PM)
If I had a superconducting wire with electricity traveling at C and I gave the wire motion in the direction of the current, wouldn't the distance displacment of the wire in space allow the electric signal to move farther than calculated by C? Or would electricity slow down to maintain the distance defined by C?
Nessus has quoted the Einstein velocity addition equation:
QUOTE
Relativistic velocity additions is given by
v = (v1+v2)/(1+v1*v2 / c^2)
which if v1=v2=c out pops v=c
So if the speed of electrical signal propagation (which is different than drift velocity of electrons) is 1.0 c, and you move the conductor at speed v = 0.01 c relative the laboratory, then the speed of electrical signal propagation is measured from the laboratory to be (1.01 c) / (1 + 0.01) = c.
Likewise, in a different material, if the propagation speed was 0.5 c and you also moved it at 0.5 c, then speed of the signal as measured in the laboratory would be:
(1.0c)/(1.25) = 0.8c
You should prove to yourself that if v1 = c and v2 = 0..c, then the answer is always c. You should also prove to yourself that if 0 <= v1 < c and 0 <= v2 < c, then the answer is always < c. Most physicists work in Minkowski space -- a mathematical model of space time where some of the preconceptions of Special Relativity are built-in -- it has topological properties different than those of Euclidian space-time.
ralph walton
28th May 2006 - 11:27 AM
I believe astronomical space observations a few years back noted velocities of solid material that were in the region of 10x lightspeed. Sorry I have no detail on this
dmaivn
29th May 2006 - 03:03 AM
The foundation of our knowledge (and desires) is built on common sense experience combined with the logic built into our brains. It's hard to deal with this kind of abstraction. The usefulness of the relativity theory is that it explains things otherwise could not be explained. But it also raises things that look quite incompatible to our normal perception. I will be happy to accept that it's a theory and stop at that.
Speed is based on spatio-temporal change. If the way we observe speed is based on time and space, and when this shift is near or at the speed of light, time and space are changed then how on Earth could we apply a mechanistic Newtonian way of thinking? The causes are changed by the effect. This is beyond my comprehension. What I fear is that some people might abuse the reputation of Einstein and use his formula indiscriminately to support arguments. Formulas don't prove anything. They are only conjured up to support a theory or conjecture. Formulas don't prove that anything is true or false. Let me just put up a very simple observation
1 + 1 = 2
No one owould argue that? This simple mathematical formula is really beyond argument. I don't think any scientists would argue that it might be false. And apart from being self-evident, similar equations can be used to "prove" things!
However, speaking metaphysically we might see problem here. The formula is not bullet proof as we may want to think. It depends on the way our mind works. Our minds seem to conjure up whole numbers and perfectly rounded entities. In nature no one can find identical entities. No two entities are same in size, shape or weight (or any attributes). Therefore equality is either a built-in function of the mind or just fiction. We accept equality as it is and not try to question it. But we cannot prove that this equality exists outside of our minds. Similarly there are vexed problems such as the value of squareroot of 2. It's an irrational number that is never terminates (therefore indeterministic). That begs the question why we would want to break up a prime number which is by definition could not be divided by anything but 1 or itself. And still we do try to break it up, and come up with only an approximate value of what its squareroot would be.
We would never get a chance to be on a spaceship that travels near the speed of light to witness how time and space change and try to measure the speed of other objects. But in anather sense, we might actually be moving near the speed of light relative to other havenly objects as nothing is considered stationary in this Universe. What I am saying is that it's very dangerous to use formulas to attempt to prove something, especially on things that we can hardly use experiments to study.
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