How do we know that c is constant regardless of the motion of the observer

[Carlos Guevara]

From what i understand, relativity is based on the fact that, speed of light is always the same to any observer no matter how fast they are going.

 

for ex: regardless of an observer being steady or in motion (near the speed of light) will measure c to be exactly the same.

 

and time itself will slow down for the moving observer, to maintain that balance.

 

but my question is: how did we know that? was it experimentaly proven before relativity was thought of? or was it also theoratical?

 

if it was theoratical, then someone please explain to me how mr. albert e. came to this conclusion, that c is constant for all observers

 

thanks

 

p.s. im going nuts here

[lemur]

Maybe someone else knows about empirical evidence, but the logic of light having an an absolute limit makes sense to me, so I'll explain how I understand it and maybe that will be of some benefit to you: Basically, light is taken to have no (rest) mass in the sense that particles of matter have mass at rest (idk what experiment demostrated this or if it's just assumed). Anyway, if light in fact has no rest-mass, then its speed should become infinite with any amount of force since F=MA (i.e. as mass approaches zero for any force, acceleration would approach infinity). However, for something to reach infinite speed, its energy would also have to be infinite so there has to be some limit to the speed of light. Likewise, without rest-mass light cannot accelerate or decelerate due to external impulses so it always travels as fast as possible within the medium it is traveling through.

 

Now, supposedly when a light-source is moving toward its object, the velocity of the moving source does not change the speed of the waves but instead compresses the waves into a shorter wavelength. This is known as "blue shift." This is also somewhat logical since the momentum of the moving light-source has to somehow be imparted in the light-energy it is emitting, but since light has no means of accelerating or decelerating in relation to itself, it has to shift wavelength instead. I hope this all makes sense. I am also interested in the specific experimental tests, btw.

[steevey]

From what i understand, relativity is based on the fact that, speed of light is always the same to any observer no matter how fast they are going.

 

for ex: regardless of an observer being steady or in motion (near the speed of light) will measure c to be exactly the same.

 

and time itself will slow down for the moving observer, to maintain that balance.

 

but my question is: how did we know that? was it experimentaly proven before relativity was thought of? or was it also theoratical?

 

if it was theoratical, then someone please explain to me how mr. albert e. came to this conclusion, that c is constant for all observers

 

thanks

 

p.s. im going nuts here

 

C is just an observed property of photons. Because they have no mass, their momentum is infinite.

Edited January 18, 2011 by steevey

[Mr Skeptic]

It is based on an observation made hundreds (thousands?) of years ago, that the laws of physics are the same no matter how fast you are going, so that you can't tell how fast you are going if you are in a closed box, which was called the principle of relativity. The speed of light can be derived from Maxwell's equations. Also, we know that waves travel based on other factors than the speed of the emitter, so that you can't make sound go any faster by moving while you talk, same with waves in water. In those examples, the waves move within a medium at a speed dictated by the properties of the medium, and you can move faster relative to the waves if you are moving relative to the medium. People thought the same was true for light, calling the medium for light "luminiferous aether". However this idea was disproved by the Michaelson-Morley experiment.

[between3and26characterslon]

When it was believed their was an aether within which everything existed it was concluded that as our planet rotated about its axis, orbited the Sun which in turn orbited the galaxy which is also moving through space, that we should be able to measure our speed relative to this aether. By measuring the SOL in different directions we could work out the aether drift. It was discovered that there was no aether drift and the SOL was the same in all directions.

 

The SOL was measured using interference patterns so the degree of accuracy was very high.

 

If you are on a plane travelling at a constant velocity and you drop something, as far as you are concerned it falls straight to the ground (of the plane). Whatever the velocity of the plane, as long as it is constant, whatever you drop will fall straight to the ground.

 

This means for a body with constant straight line motion and a body at rest the laws of physics are the same i.e. you can not tell the difference between the two. This further implies that nothing is truely at rest, everything is in motion but if its motion is constant and in a straight line it feels like it's at rest. This is known as the 'Principle of Relativity' and is attributed to Galileo.

 

So how can the SOL be constant for every observer if every observer is in motion, All observers are travelling in straight lines in different directions all have different albeit constant speeds all measure the SOL to be the same.

 

Einstein concluded that for the Principle of Relativity and the constant speed of light to hold true something else must give, he realised that this must be time and space. That is to say the faster you go the slower you experience time and the shorter your length becomes in the direction you are moving.

[ajb]

C is just an observed property of photons.

 

OK

 

 

Because they have no mass, their momentum is infinite.

 

Not true. You have to be careful with the definition of momentum. For massless particle we have

 

[math]p = \frac{E}{c}[/math],

 

which comes from the mass-shell constraint on physical particles.

[Carlos Guevara]

Thanks to all those who replied. this helped a lot

 

So it seems, we have the equations and experimental proof of the concept. but no philosophically satisfying explanation to "WHY" nature behaves this way...

 

Am i right? pls do correct me if im not

[ajb]

So it seems, we have the equations and experimental proof of the concept. but no philosophically satisfying explanation to "WHY" nature behaves this way...

 

Am i right? pls do correct me if im not

 

But this is not restricted to the speed of light, all of physics is like that.

 

We can construct models and test them against nature. Theories that agree well with nature we say are "good". However, this does not tell us anything about why nature should be describable using mathematics. For example, even before Newton discovered his laws of motion and gravitation the planets went around the Sun obeying these laws.

 

Why nature somehow "chose" these laws is a not understood, let alone why they should be describable using mathematics at all.

[imatfaal]

There is no axiom-free basis to explanations of physical phenomenon - there is no fundamental "why" (is there in any field?) . We learn to explain, understand, model and predict observations and the results of experiments - as soon as the empirical side is completely removed then how can we test and prove our ideas. Philosophically your explanation is as good as any other; physically the explanation that match perceived facts and provides useful predictions is the one we follow.

[lemur]

Thanks to all those who replied. this helped a lot

 

So it seems, we have the equations and experimental proof of the concept. but no philosophically satisfying explanation to "WHY" nature behaves this way...

 

Am i right? pls do correct me if im not

It really depends on what you mean by "why," i.e. what kind of reason are you looking for? Maybe a better question is what you want to know and why you want to know it. It seems like a lot of people seek knowledge for the purpose of being knowledgable, which seems like a bad reason to me. The kinds of questions they ask are geared toward philosophical dominance, i.e. for the sake of claiming more general or fundamental knowledge of the widest array of phenomena merely for the sake of being broad or deep. Knowledge for its own sake is fine, imo, but knowledge for the sake of social status can get annoying. I'm not saying that this is what you're doing - it just came to mind with regards to various reasons for inquiry/knowledge.

[Carlos Guevara]

Well, its because "why" is a fundamental part of being aware of ones existense. I know before i seize to exist; i will come across more whys than i can possibly answer. But it is here we find the ultimate human experience. to fully understand what we understand less

[Mr Skeptic]

If you want to know why Maxwell's equations are what they are, you should probably ponder why they are so similar to the equations of fluid mechanics.

[swansont]

If you want to know why Maxwell's equations are what they are, you should probably ponder why they are so similar to the equations of fluid mechanics.

 

But that only leads to to another why, but science stops going down that path pretty quickly. That's because science exists to explain how nature behaves, and if one is dissatisfied with that, one can philosophize, or theorize in the lay sense, or probably a couple of other things that are nevertheless not science.

[lemur]

Well, its because "why" is a fundamental part of being aware of ones existense. I know before i seize to exist; i will come across more whys than i can possibly answer. But it is here we find the ultimate human experience. to fully understand what we understand less

Imo, the aesthetics of experience are an extreme luxury for those who have more or less total control over the conditions of their material (and spiritual) existence. To me the point of science and philosophy and other knowledge is to progress in terms of power and freedom. I don't see the point of "being aware of one's existence," as you say except to progress in that existence. I don't see humans or knowledge as static things but as constantly evolving dynamic processes in flux.

 

So now that we have shared personal philosophies of knowledge, you still haven't said what you mean by "why" when you ask "why nature behaves the way it does." Do you have a clear understanding of what kind of answer you are looking for, or are you just attempting to find an unanswered question to work on answering yourself?

 

 

 

But that only leads to to another why, but science stops going down that path pretty quickly. That's because science exists to explain how nature behaves, and if one is dissatisfied with that, one can philosophize, or theorize in the lay sense, or probably a couple of other things that are nevertheless not science.

I think it depends how you interpret the question, "why." You could say that plate techtonics explain why earthquakes happen. You could say lunar gravity explains why their are tides. Yes, science can and should (imo) dissect the mechanics of how such processes occur. But science often leaves me hungry for a better explanation of how that are not answered by current theories. For example, people often explain why light is produced by electrons, but they don't explain how an electron suddenly expels and electromagnetic field at the speed of light. If there was some medium, that would be a basis for understanding how, but most people seem adamantly opposed to re-opening the discussion of a medium for light. Personally, I think gravitational field force or some other massless connectivity in force-relations must act as a medium, but I don't want to hijack this thread (although that question is sort of related to why C is constant, I think).

[Carlos Guevara]

lol We can go on and on about this. but im trying to research and hoping to find a good answer, cause if not, i have to do it myself. What i meant by "why" is that; what happens in nature that causes it. That itme itself has to slow down to keep C constant for all observer.

 

Im sure that there is a better answer than "its just the way things are"

[ydoaPs]

lol We can go on and on about this. but im trying to research and hoping to find a good answer, cause if not, i have to do it myself. What i meant by "why" is that; what happens in nature that causes it. That itme itself has to slow down to keep C constant for all observer.

 

Im sure that there is a better answer than "its just the way things are"

When you derive the speed of an electromagnetic wave using the Maxwell Equations, there's no variable in it for the speed of the object or anything else. The only factors for the speed of light are the permativity and permeability of free space. So, if those changed, the speed of light would be different. However, it would still be the same for all observers.

 

The only way to make C dependent upon a frame would be to somehow discover that the permeability or permativity of free space is frame dependent.

[swansont]

I think it depends how you interpret the question, "why." You could say that plate techtonics explain why earthquakes happen. You could say lunar gravity explains why their are tides. Yes, science can and should (imo) dissect the mechanics of how such processes occur. But science often leaves me hungry for a better explanation of how that are not answered by current theories. For example, people often explain why light is produced by electrons, but they don't explain how an electron suddenly expels and electromagnetic field at the speed of light. If there was some medium, that would be a basis for understanding how, but most people seem adamantly opposed to re-opening the discussion of a medium for light. Personally, I think gravitational field force or some other massless connectivity in force-relations must act as a medium, but I don't want to hijack this thread (although that question is sort of related to why C is constant, I think).

 

Either you aren't familiar enough with the theory, or you are asking a why question. When you wiggle an electron, the change in the electric field induces a magnetic field, following Maxwell's equations — that results in an EM wave, which travels at the speed of light. So yes, the theory explains how this happens.

[D H]

I think it depends how you interpret the question, "why." You could say that plate techtonics explain why earthquakes happen. You could say lunar gravity explains why their are tides.

I wouldn't call those "why" so much as "what" questions: What causes earthquakes, what causes tides. Regardless of how one looks at those questions, there is some point at which the answer becomes axiomatic. In other words, "because we said so". Geology uses a lot of physics and chemistry in its models. At some point a geologist will answer some question with "I don't know the answer. You'll need to go ask a physicist." (If the answer is "You'll need to ask a chemist", poke deep enough at that chemist and she will eventually end up saying "go ask a physicist.")

 

We physicists have no deeper source to turn to. We could turn to metaphysics, but the problem with that is (a) the divorce decree between physics and philosophy was finalized hundreds of years ago, and (b) we want our axioms to be testable. Newton and others had a deep problem with the instantaneous action at distance that pervades Newtonian physics because it allows violations of causality. One could argue metaphysically that a causal universe must by necessity have a finite speed limit. A little more metaphysical handwaving and voila, only massless particles such as photons can (and must) travel at this universal speed limit. One minor problem here: These metaphysical arguments are not testable.

 

At some point it becomes better to stop the infinite regress of "why" questions with axiomatic statements that are testable.

[lemur]

Either you aren't familiar enough with the theory, or you are asking a why question. When you wiggle an electron, the change in the electric field induces a magnetic field, following Maxwell's equations — that results in an EM wave, which travels at the speed of light. So yes, the theory explains how this happens.

Ok, this is indeed the "how" at one level. But what about how the electric field induces the magnetic field and how the two fields launch into leap-frogging at C?

 

 

 

 

I wouldn't call those "why" so much as "what" questions: What causes earthquakes, what causes tides. Regardless of how one looks at those questions, there is some point at which the answer becomes axiomatic. In other words, "because we said so". Geology uses a lot of physics and chemistry in its models. At some point a geologist will answer some question with "I don't know the answer. You'll need to go ask a physicist." (If the answer is "You'll need to ask a chemist", poke deep enough at that chemist and she will eventually end up saying "go ask a physicist.")

This is an authoritarian, human-centered view of science. Another perspective would say that nature (empiricism) is the ultimate authority that has to be consulted by anyone, expert or otherwise to reach valid scientific explanations. Obviously anyone can reach a point where they don't know the answer. However, you can use your existing knowledge and creativity to contemplate how the question could be answered and/or what information or knowledge would be needed to move forward with answering it.

 

We physicists have no deeper source to turn to. We could turn to metaphysics, but the problem with that is (a) the divorce decree between physics and philosophy was finalized hundreds of years ago, and (b) we want our axioms to be testable. Newton and others had a deep problem with the instantaneous action at distance that pervades Newtonian physics because it allows violations of causality. One could argue metaphysically that a causal universe must by necessity have a finite speed limit. A little more metaphysical handwaving and voila, only massless particles such as photons can (and must) travel at this universal speed limit. One minor problem here: These metaphysical arguments are not testable.

Interesting. I wouldn't call an axiom that is testable an axiom. I think of axioms as assumptions taken-for-granted in order to build on them. This gets into the arbitrariness of paradigms a la Kuhn. I also wouldn't say that physics or any other science has ceased to be a form of philosophy. You make a good point that not all philosophy insists on empirical rigor, but philosophy that does is called science, imo. You can't escape the necessity of thinking and reasoning in physics or any other science. Computers can't do science by themselves, can they?

 

At some point it becomes better to stop the infinite regress of "why" questions with axiomatic statements that are testable.

It depends on the kind of "'why' question" in question. "Why" questions that are refinable into empirically viable "how" questions are scientifically interesting, no? I don't even think all valid scientific questions have to be testable. It is more important that they play a critical role in existing knowledge/models. E.g. it will never be possible to observe the electron as it passes through the slits but that doesn't make it unscientific to wonder what it is doing as it moves from one side of the slit to the other, does it?

 

 

 

[D H]

This is an authoritarian, human-centered view of science.

You apparently completely misunderstood what I wrote.

 

 

Interesting. I wouldn't call an axiom that is testable an axiom. I think of axioms as assumptions taken-for-granted in order to build on them.

That is what axioms are in mathematics, but not science. Postulate is perhaps a better word, but postulate and axiom are essentially synonyms. F=ma, the invariance of c, the equivalence principle: All are axiomatic and all are very testable. In physics at least, we really like the base assumptions to be testable.

 

 

 

[lemur]

You apparently completely misunderstood what I wrote.

You wrote that when one expert fails to know the answer to a question, they defer to an expert in a different field. You did not mention the authority of nature/empiricism/reason and neither did you mention that any person is capable of refining their inquiry through systematic research and reasoning from observation and logic. Sorry if "authoritarian" is a harsh-sounding word. All I meant is that the authority of living scientists is not the ULTIMATE authority on scientific matters. There is a "higher power" that they must defer to, whether you call it nature, empiricism, reason, or . . . oh never mind.

 

That is what axioms are in mathematics, but not science. Postulate is perhaps a better word, but postulate and axiom are essentially synonyms. F=ma, the invariance of c, the equivalence principle: All are axiomatic and all are very testable. In physics at least, we really like the base assumptions to be testable.

In practice, axiom and postulate are indeed similar. "Postulate" taken literally sounds more like something put forth as a basis for something else, so I can't decide if it automatically implies or rejects empirical/critical validity testing. Suffice it to say that some approaches to knowledge assume knowledge to be axiomatic and relative to some foundational paradigm, thus implying that all knowledge and truths are relative to arbitrary underlying assumptions. Other approaches to knowledge do not take knowledge and truth to be relative and apply critical rigor to the task of pushing knowledge and truth ever further. Personally, I think that when paradigms collapse, it is because obfuscation has either given way to reveal a clearer picture of reality or obfuscation has established a more solid basis for further obfuscating. To me, true science involves a willingness to question paradigmatic assumptions and coherence where contradictions emerge. I don't see any reason why one or more paradigms would "hold fast" waiting for a paradigm-shift unless the "scientists" involved were subjects of an authoritarian regime instead of being independently critical.

 

 

[D H]

You wrote that when one expert fails to know the answer to a question, they defer to an expert in a different field. You did not mention the authority of nature/empiricism/reason and neither did you mention that any person is capable of refining their inquiry through systematic research and reasoning from observation and logic. Sorry if "authoritarian" is a harsh-sounding word.

"Authoritarian" is, to be blunt, a stupid word here.

 

Suppose you ask a scientist a question to which they do not know the answer. The first thing that person should do is acknowledge that they don't know. If they do know someone who might know the answer, giving you the name of that knowledgable person is a courtesy, not a deference to authority. It simply is not possible any more for any one person to know all of science. It isn't even possible for one person to know all of a single body of science. You are at best being naive to think that someone should know all of science.

 

 

In practice, axiom and postulate are indeed similar. "Postulate" taken literally sounds more like something put forth as a basis for something else, so I can't decide if it automatically implies or rejects empirical/critical validity testing. Suffice it to say that some approaches to knowledge assume knowledge to be axiomatic and relative to some foundational paradigm, thus implying that all knowledge and truths are relative to arbitrary underlying assumptions. Other approaches to knowledge do not take knowledge and truth to be relative and apply critical rigor to the task of pushing knowledge and truth ever further. Personally, I think that when paradigms collapse, it is because obfuscation has either given way to reveal a clearer picture of reality or obfuscation has established a more solid basis for further obfuscating. To me, true science involves a willingness to question paradigmatic assumptions and coherence where contradictions emerge. I don't see any reason why one or more paradigms would "hold fast" waiting for a paradigm-shift unless the "scientists" involved were subjects of an authoritarian regime instead of being independently critical.

What does this have to do with scientists wanting their postulates to be testable?

 

[lemur]

"Authoritarian" is, to be blunt, a stupid word here.

I already explained this word to refer to the belief or assumption that a human expert is the ultimate authority in some form of knowledge. It is not a stupid word. It very clearly describes a deviation from authority by nature, empiricism, reason, etc. If you provide factual proof or logical reasons why or how knowledge is inadequate or can be improved and some "expert" tells you that you don't have the authority to question expertise, that is authoritarian, is it not?

 

Suppose you ask a scientist a question to which they do not know the answer. The first thing that person should do is acknowledge that they don't know. If they do know someone who might know the answer, giving you the name of that knowledgable person is a courtesy, not a deference to authority. It simply is not possible any more for any one person to know all of science. It isn't even possible for one person to know all of a single body of science. You are at best being naive to think that someone should know all of science.

I agree that people should say so when they don't know something, but they may (and probably should) apply their knowledge to elaborating why they don't know what they don't and how their existing knowledge could be extended to find the answer. I also agree that referring someone to a knowledgable person could be helpful. I just took what you said to mean that all roads of inquiry lead to experts (human authority) and not to your own research process that may include but should not be limited to consulting the expertise of others.

 

What does "knowing all of science" have anything to do with this? Questions are answered one at a time. The answers are not passively waiting to be uncovered in books. Everything you know is just a set of resources you can draw on to answer questions and pursue further research. The end goal isn't to be knowledgable but to answer the question in question or postulate further questions to elaborate/develop knowledge. I don't know why else you would set up a dichotomy between knowing everything yourself or consulting others for what they know except to depict an authoritarian view of science where scientists are the keepers of knowledge instead of researches in active pursuit of such knowledge, whose authority ultimately lies beyond any single individual or text.

 

What does this have to do with scientists wanting their postulates to be testable?

I guess that means you didn't really read or understand what I wrote there.

 

 

[Mr Skeptic]

lemur, why is saying "I don't know but such and such might know" authoritarian?

 

---

 

That is what axioms are in mathematics, but not science. Postulate is perhaps a better word, but postulate and axiom are essentially synonyms. F=ma, the invariance of c, the equivalence principle: All are axiomatic and all are very testable. In physics at least, we really like the base assumptions to be testable.

 

I don't think things can be axioms and testable at the same time. And some of the stuff is definitions, which are true by virtue of that we said they are such. F=ma I take as the Newtonian definition of force, rather than as specifying some relation between m*a and whatever it is that force is, and as such isn't really testable.

[swansont]

I don't think things can be axioms and testable at the same time. And some of the stuff is definitions, which are true by virtue of that we said they are such. F=ma I take as the Newtonian definition of force, rather than as specifying some relation between m*a and whatever it is that force is, and as such isn't really testable.

 

Which is an example of why the use is different in math vs physics. In math there are things that are true but cannot be proven. In physics, you construct a model of some behavior. A good model will fail if it doesn't describe nature, which is the requirement of falsifiability and the desirable feature of precision. That can be a test of an axiom that is included. It doesn't have to test the axiom, because a model can fail in more than one way. F=ma is tested in that the relationship is linear, and you can construct experiments to show this. The constancy of the speed of light is tested in that there are ramifications of it, such as time dilation, and you would not get those results if c were frame-dependent.