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dmaivn
I face with this dilema when I think about the claim that the speed of light is absolute and nothing could travel faster. Let imagine the following experiment

Get a straight tube and drill a hole at the middle. Then put a small light source through the hole and turn it on. The light shoots out of the two open ends of the tube.

Now suppose that we consider the tube as the frame of reference and it's the relatively motionless point. The light that gets out at one end of the tube is moving away from the tube at the speed of light to one direction. And the light at the other end of the tube goes in opposite direction also at the speed of light. So the light particles from one end would be travelling away from the light particles from the other end at 2x the speed of light. But they say speed of light is absolute!

If you are on a ship travelling with either direction, the light particles in the other direction would be moving away from you at a speed higher than the speed of light. This is similar to the case of a light source pointing at the tail end of a plane, the light emitted from this light source would be drifting away from the airplane at speed higher than the speed of light.

Even though the speed of light is constant (once the light has been emitted)regardless if the light source is moving or not, but my common sense is telling me that relative to a moving light source, the light particles must be drifting away from the light source at a speed different to the speed of light.

This is what I find so frustrating with scientific reasoning. A lot of things just could not be comprehended by commen sense. All this quantum physics sounds just like pure theory. And the theory does not really fit with common sense. Are physicists just nuts who talk in a language no normal people could understand? I myself study philosophy and metaphysics ... but I still cannot understand what physicists say. Is my knowledge so inadequate even at University level?



Nessus
QUOTE
Now suppose that we consider the tube as the frame of reference

QUOTE (->
QUOTE
Now suppose that we consider the tube as the frame of reference

So the light particles from one end would be travelling away from the light particles from the other end at 2x the speed of light.

Relativistic velocity additions is given by
v = (v1+v2)/(1+v1*v2 / c^2)
which if v1=v2=c out pops v=c


QUOTE
If you are on a ship travelling with either direction, the light particles in the other direction would be moving away from you at a speed higher than the speed of light. This is similar to the case of a light source pointing at the tail end of a plane, the light emitted from this light source would be drifting away from the airplane at speed higher than the speed of light.

This is not how it works, time and space are adjusted so that everyone sees the light going at light speed. You cant assume that for all frames of refrence that time runs at the same rate, or even if distances are the same.

QUOTE (->
QUOTE
If you are on a ship travelling with either direction, the light particles in the other direction would be moving away from you at a speed higher than the speed of light. This is similar to the case of a light source pointing at the tail end of a plane, the light emitted from this light source would be drifting away from the airplane at speed higher than the speed of light.

This is not how it works, time and space are adjusted so that everyone sees the light going at light speed. You cant assume that for all frames of refrence that time runs at the same rate, or even if distances are the same.

This is what I find so frustrating with scientific reasoning. A lot of things just could not be comprehended by commen sense. All this quantum physics sounds just like pure theory. And the theory does not really fit with common sense. Are physicists just nuts who talk in a language no normal people could understand? I myself study philosophy and metaphysics ... but I still cannot understand what physicists say. Is my knowledge so inadequate even at University level?

This is akin to compaining about having to pay taxes, you dont have to like taxes but you still have to pay them.
You live in a world where the theories of quantum mechanics and general relativity predect things well, it does not require you to understand it smile.gif I guess if you cant accept light travels at the same speed in all frames of reference then i guess you have some things you need to learn...
rpenner
QUOTE (Nessus+May 24 2006, 09:33 AM)
QUOTE
This is what I find so frustrating with scientific reasoning. A lot of things just could not be comprehended by commen sense. All this quantum physics sounds just like pure theory. And the theory does not really fit with common sense. Are physicists just nuts who talk in a language no normal people could understand? I myself study philosophy and metaphysics ... but I still cannot understand what physicists say. Is my knowledge so inadequate even at University level?

This is akin to compaining about having to pay taxes, you dont have to like taxes but you still have to pay them.
You live in a world where the theories of quantum mechanics and general relativity predect things well, it does not require you to understand it smile.gif I guess if you cant accept light travels at the same speed in all frames of reference then i guess you have some things you need to learn...

One of the problems of Common Sense, is that it is derived from your lifetime of experience as a (relatively) slow-moving, medium-sized object interacting with other slow-moving, medium-sized objects. Making things somewhat worse, is that the range of speeds is typically very limited, and the human eye percieves less than 1 octave of EM radiation.

Quantum Mechanics is a fundamental theory about the behavior of individual elementary particles, all of which, by human standards are very small. In certain long-time or group averages, these particles act similar to our common sense, but at short time scales or for individual particles, these particles can exhibit weird behaviors, like quantum tunneling or the Heisenberg (sp?) uncertainty principle or the concept of a "ground state" in a bound system. Without quantum tunneling, some intergrated circuit electronic devices would fail to work. Without the uncertainty principle the electron would spiral into the nucleus and all atoms would be unstable (perhaps neutrons, also). These are the consequences of the non-zero value of Planck's constant.

Special Relativity is a fundamental theory about how two particles can relate to each other. The first assumption is that there is no preferred rest frame of the universe that you have to work in to make your physics equations work out right. The second assumption is that there is an upper speed limit to the universe, and that it is equal to c, the speed of light in vacuum in the Maxwell formulation of Electromagnetism. For slow moving objects, the most obvious effect of Special Relativity is the concept of kinetic energy which is approximately: KE = 0.5 m v^2 + 0.375 m v^4/c^2 + .... -- Other effects mean that observers in relative motion have different, but justifiyable, concepts of what "simutaneous" means for events that don't happen at the same spot. The Time-dilation and other conflicts with common sense are all consequences of the non-infinite speed limit to the universe. If you set c = infinity, KE = 1/2 m v^2.

But is a world where we set h = 0 and c = inifinity really a world of common sense? If you run off a cliff does common sense predict that you will move in a straight line until you run out of inertia and then fall vertically? This Wile E. Coyote model of physics was the standard for over one thousand years after Aristole.

Galileo demonstrated by experiment that Aristolean physics is wrong.

If you are going twice as fast in a car, does common sense say you will need twice or four times the breaking distance. I think this is in the California Driver's Manual because common sense says twice the distance while Newton and experiment show that it is much closer to four times the distance.

If humans could exert themselves to the point where running speeds ranged from 2 to 200 miles (or km, doesn't matter for this argument) an hour, perhaps common sense would more intuatively understand KE = 1/2 m v^2 and braking distance. If humans grew up in a vacuum with a much better perception of the 3-D nature of the trajectories of ballistic objects, then perhaps Aristole would have discovered Kepler's observation that gravity cause objects to travel in conic sections. If humans could see more of the EM spectrum, perhaps common sense would not allow for universally transparent substances like water, glass and diamond so the inutionist concept of a material ether would not arise.

Depending on your school of philosophy, you might not easily accept that human common sense is shaped by perception, experience and perhaps evolution, but the scientific evidence is that we are shaped by our environment. Even our language and culture shapes the answer to what seems like such a basic question: How many basic colors are there?

dmaivn
Thanks for the enlightenment. I remember I did read a bit about this theory of relativity from the online Wikiepedia. I saw the equation for adding velocity that ultimately give c as the absolute.

So the theory makes the "assumption" that the speed of light is the same and absolute. There is also a relationship between time and the speed of light. Our common sense perception of speed is based on our perception of time. Without perception of time we cannot perceive speed. Our perception of time is so limited to what our senses could afford us.

So it comes back the fact that we beleive in a theory just because it helps to predict not because it's correct. We derive the value of a theory or its correctness from its ability to help us to predict. I am aware that it's dangerous to rely too much on theory and reasoning. A philosopher by the name David Hume put it so concisely that reasoning was a very poor tool to arrive at knowledge. He gave the example of the billiards. Imagine you have never seen the billiards. The first time you hit a ball, it moves and about to hit another ball. Right at that moment if some one covers your eyes and ask you what your reasoning would predict the outcome. You would never be able to predict what would happen. It's just like no one would be able to reason that water could drown a man just by looking at it. A large bulk of our knowlede is from experience. Theories we create only attempt to bridge the gaps of what experience cannot tell us. We give them a level of trust only on their ability to help us to predict outcomes.

This reminds me of the axiom of subsets which is also known as the Axiom of comprehension. This axiom is the corner stone of human knowledge as it describes how our brains classify our perceptions into sets. And from this step we go on to catalogue all information from the external world. When I first learnt of it in pure mathematics at 2nd year University, I thought it was ridiculous to trust human knowledge on this axiom. After all an axiom is only an intuition that cannot be proved. It's something your minds come up with, and you just use it on faith. So far no one has tried to work out how our minds come up with this axiom or try to disprove it just because it's so useful for constructing knowledege. So I suppose I just have to accept Einstein's theory of relativity until one day another genius comes along with something better.









Dave Grossman
QUOTE (dmaivn+May 24 2006, 05:27 AM)
This is what I find so frustrating with scientific reasoning. A lot of things just could not be comprehended by commen sense. All this quantum physics sounds just like pure theory. And the theory does not really fit with common sense. Are physicists just nuts who talk in a language no normal people could understand? I myself study philosophy and metaphysics ... but I still cannot understand what physicists say. Is my knowledge so inadequate even at University level?

People used to think that it was common sense that the Earth was flat or that the Sun travelled around the Earth. Common sense is no substitue for the Scientific Method.

Quantum Physics and General Relativity are models that rely on complex and abstract mathematics that few people, if any, truly understand. However, they are very good models in that they make predictions that can be verified through experiment. From what I understand, no experiment has ever contradicted a prediction made by QM or GR.

There are problems with these theories in that they are actually incompatible with each other. I'm confident that this will be resolved at some point and a new theory will supercede them both. There are many theories that claim to do this, such as String Theory.

For the time being, both QM and GR are used successfully in the design of particle accellerators, electronics and GPS satellites.

- Dave
Dave Grossman
QUOTE (dmaivn+May 25 2006, 02:06 AM)
So the theory makes the "assumption" that the speed of light is the same and absolute.

It's not really an assumption. Experiments designed to detect the motion of the Earth attempted to use a varying speed of light. Those experiments failed and the constancy of the speed of light was accepted as a fact.

See http://en.wikipedia.org/wiki/Michaelson-Morley_experiment

Also, you should check out the series, The Mechanical Universe and Beyond at:

http://www.learner.org/resources/series42.html

You can watch it online.

- Dave
rpenner
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.pdf

It 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
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
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
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
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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