Can someone explain Einstein's relativity theory?

[JesseM]

So why should their be any argument? Whose arguing the other side? In other words who's arguing that science does explain anything. Why don't physicists just respond to the so-called crackpots by saying, "Oh we don't bother trying to explain anything anymore, we've all become nothing more than engineers. There are no physicists left. Physics has died and engineering is all that survived from it."

Engineering is about applying existing laws to technologies, physics is about finding new laws, so they're pretty different regardless of whether you believe in "mechanisms" or not.

There also seems to be a double standard here. I mean, everyone claims that Newton's mathematical description of gravity wasn't actually an explanation but Einstein's General Relativity explains gravity in terms of warped spacetime. Physicists seemed to like Einstein's explanation better than Newton's mere description, not merely because it is more correct, but also because it offers an intuitive mechanism.

Aside from the fact that GR gives more accurate predictions, I think physicists like GR better because it follows uniquely from a few basic assumptions like the equivalence principle, while Newtonian gravity doesn't. Similarly, SR is elegant because it follows from Einstein's two postulates. While modern physicists don't look for "mechanisms", they may want to find a few set of basic assumptions which can be used to derive a more complex-looking theory.

Particle physics proposed the theory of quarks to explain particle physics. Now String theory is proposing the existence of strings to explain particle physics. But anytime someone wants to explain relativity all of a sudden explanations aren't important?

Well, again, you're talking about a different sort of explanation than a search for a "mechanism". Physicists may try to explain some set of laws as a limit of some more fundamental laws, like with thermodynamics or with the search for more fundamental particles. If you want to derive SR from some more fundamental theory, though, that's not the same as looking for a "mechanism" which accords with our ordinary physical intuitions.

 

[NeutronStar]

My $.02 on "LR". It's possible to do "LR" correctly and get the same answers as standard relativity theory, in which case it's just an interpretation of special relativity and not a different theory, as it makes the same predictions.

I agree.

I never meant to imply that SR is incorrect. All I've ever said is that there are other interpretations that are just as correct and that in the case of LR this interpretation can include a concept of an aether.

However, because LR focuses on what changes (space and time) rather than what does not change (the Lorentz interval), it is more difficult to get the correct answers with the "LR" approach than the one that is usually taught. This is a good enough reason for not teaching the "LR" approach, in my opinion.

I also agree that the precise mathematics of LR should not be taught in colleges. That would just be a waste of everyone's time. But I believe that more emphasis should be given to the theoretical possibility. At least let the students know that it's a viable description. Too many students come out of Modern Physics courses thinking that any concept of an aether is completely forbidden. That's actually incorrect, and should be frowned upon.

Furthermore, many of the "LR" proponents you see in the newsgroups or on the webpages are seriously confused (probably, in part, due to the fact due to the aforementioned difficulties). They get results that conflict with standard relativity theory. To make matters worse, they don't always seem to realize when their results are different than standard results (even when this is pointed out to them very directly) - probably because they don't understand the standard approach.

With all due respect,… there are a lot of people out there who are seriously confused about SR and get results that conflict with relativity.

I mean, just because some people can't do math is no reason to toss out a whole theory is it?

The fact that many LR proponents don't realize when they are wrong has to a certain extent "poisoned" the name. There is at least a 50% chance that when someone talks about relativity in terms of "LR" that they are making predictions that are not compatible with standard relativity without realizing it -(or at least without admitting it) - a typical false claim of an LR enthusiast is that the ether can be detected.

Well, I agree that the whole concept of an aether has been "poisoned". This is why I feel that colleges should address the issues a little better in their Modern Physics courses. Try to clear things up a little bit one class at a time.

While, we are talking about educational formalism I'd like to mention that I actually took Modern Physics twice in my lifetime (almost 20 years between those classes). Nothing has changed! I'm mean from a pedagogical point of view. They still don't make it crystal clear at the beginning of the course that relativity is just a description and not an explanation of what's going on. And they still don't touch the word "aether" with a ten-foot pole other that to say that it died with Michelson and Morley.

The thing that I really don't understand is why colleges do this to people? I looked around me at all the younger students trying to beat their heads against the wall to comprehend the explanation. I felt like screaming at the top of my lungs, "There is no explanation! Just do the math and accept the results!". I mean really. That's the only way they are really going to survive the course. Why don’t the textbooks and professors make that clearer to make it easier on everyone?

Personally I think that the professors get a kick out of watching the students bang their heads against the wall trying to understand the explanations of relativity. Also, if they made these things crystal clear at the onset everyone might pass with an "A" and that would make the course look too easy.

So they like to keep the students in the dark about these things.

 

[Hurkyl]

I can assure you that my reasons for believing in an aether have absolutely nothing to do with not wanting to believe in SR.

I didn't say they're you're reasons (but I probably meant it) -- just that it's a common reason people have.

I mean, it really doesn't offer a mechanism for time dilation.

Sure it does -- as soon as you stop viewing measurements as a black box. It follows from the constancy of c, which was proven by the fact Maxwell's equations are not frame dependent. (One of) the first example(s) people learn about SR is that of the light clock formed as light bounces between two mirrors. If that doesn't satisfy your definition of "[offering] a mechanism", then nothing will.

The only part that's interesting is why all clocks should agree with light clocks. But, given that macroscopic matter is shaped by electromagnetic forces, it's no stretch for me to believe one could actually prove it for mechanical clocks as well.

To me, that is much more attractive than just doing engineering calculations using SR.

Again, I'll counter with my own experiences. I agree that sometimes, understanding from where an equation comes can help with solving a problem, but I find the exact opposite is more frequently helpful -- specifically, by eliminating any biases I might have formed from knowing the origin of the equation.

But, both tools are in my bag of tricks. Sometimes I'll add biases that might help me work through a problem (which usually only works when I'm not very familiar with a subject), and sometimes I'll abstract away to try and eliminate biases that are preventing me from seeing the solution.



Anyways, my question to you is this: what is the gain from assuming an aether? Unless I'm missing something, having a "preferred" frame nets you nothing more than a label for a special frame, but raises questions that have no relevance to solving the problem, like "why is that one preferred?"

Whenever I knowingly add biases to a problem, I have a specific reason. For example, when working with a particular (finite-dimensional) vector space V, I'll write its vectors as matrices with a single column (coordinates with respect to an unspecified basis, of course), and I'll write vectors in V*, the dual space, as matrices with a single row. (aka column vectors and row vectors, respectively) This approach assists me with organizational issues, and allows me the to use the familiar matrix multiplication syntax. As such, adding this bias helps me solve problems, and I know why, but I know enough to avoid this approach when it will confuse an issue.

 

[NeutronStar]

Modern physicists don't really believe that "mechanisms" are important any more. Any mathematical rule we can describe could be a rule that nature uses in her "program". Check out chapter 2 of Feynman's book The Character of Physical Law, "The Relation of Mathematics to Physics", for a good articulation of the way most physicists think about this.

By the way, thanks for pointing to this information. I'm definitely interested in reading Feynman's thoughts.

That doesn't mean that I'll agree with the direction that physics has taken. I'll probably just conclude that physics has been infiltrated by the pure mathematicians and that all the real physicists have been secretly murdered and buried in empty sets.

 

[jcsd]

If you really must have 'mechanisms', you could view time dialtion in special relattvity as simply the result as farirly intutive result of a 'rotation' in spacetime.

 

[yogi]

A couple of comments by Posters which I believe are incorrect. Specifically it is implied that LR is the same as classical Lorentz Ether Theory. It is not the same - LR is based upon a local geogravitational effect. I simply says that in the Earth's gravitational field, light is one way isotroptic and therefore the Lorentz transforms hold for measurements of time dilation relative to Earth clocks, and it comports with those measurments. It is not based upon physical contractions of the type originally introduced by Fitzgerald and Lorentz to explain MMx. Secondly, since the transforms are only applied one way, it is simplier than SR - none of the so called SR paradoxes arise (I know, they are not paradoxes to relativists - just to everyone else).

 

[Garth]

I felt like screaming at the top of my lungs, "There is no explanation! Just do the math and accept the results!".

What! Who taught you relativity? I must admit that at undergraduate level, with so much thrown at me all at once, as a student I felt the same way. But that was because at the time I failed to appreciate the deep truths of SR space-time.

The mechanism is the metric with signature (-1,+1,+1,+1) or with the opposite convention if you prefer.
1. Is time a dimension like space, and should be treated with it in a space-time continuum?
This can be considered the basic postulate of SR.

If so then:

2. Is the metric, which connects events with their neighbours and gives a structure to the space-time manifold, given by an extension of Pythagoras' theorem:
dtau² = dt² - [dx² + dy² + dz²]/c²?
[In which case time is a dimension like the space dimensions, but it is not exactly the same. Mathematically time has the same relationship to length as the imaginary numbers have to the real numbers. The suggestion that if time is a dimension then it is subtly different from the others is, in my opinion, intuitively obvious.]
This can be considered the second postulate of SR.

If so, then:

SR is all about answering the question, "What is the nature of time, and how does it relate to space?" All flows from this metric and this concept of time. If an object is stationary in the coordinate system of the observer then
dtau² = dt²
and tau, proper time, is seen to be that time as measured in the rest frame of the object. Four-momentum follows immediately as a 4D equivalent of ordinary Three-momentum, with Four-velocity being the rate of displacement across space-time wrt the proper time tau.

Consideration of the Four-momentum of a stationary object in the rest frame of the observe leads immediately to
E = mc².

Consideration of the fact that t is not the same as tau for objects moving relative to the observer leads to the realisation that simultaneity is realtive to the observer and hence to the resolution of the Twin Paradox and the other paradoxes of SR.

The question is, "Does the theory work experimentally?" I think it does. Others may properly argue on the basis that the disputed results of various experiments may, or may not, agree with its predictions, but to argue on the basis that the theory cannot be understood and you have to, “Just do the math and accept the results“ only exposes your ignorance.

Garth

 

[yogi]

Garth: ..."only exposes your ignorance" The reciprocal nature of the of the transforms in SR have created many dissidents who have spend years trying to visualize its implications. NeutronStar is one such person - that does not justify a personal attack.
That SR is confusing, can be seen by the many conflicting descriptions of its consequences: I recently came across the following:

The reciprocal effect of length contraction and time dilation, which appears by logical necessity to emerge from the kinematic part of the special theory of relativity, has been variously explained as
1. true but not really true (guess who)
2. real
3. not real
4. apparent
5. the result of the relativity of simultaneity
6. determined by measurement
7. a perspective effect
8. mathematical.
Here is a small selection from the literature; Unless placed in quotation marks, authors' assessments are summarized.
1. Effects are true but not really true:
Pride of place goes to Eddington [1928, 33-34]:
"The shortening of the moving rod is true , but it is not really true."
2. Effects are real:
Arzelies [1966, 120-121]:
The Lorentz Contraction is a Real Phenomenon. ...
Several authors have stated that the Lorentz contraction only seems to occur, and is not real. This idea is false. So far as relativistic theory is concerned, this contraction is just as real as any other phenomenon. Admittedly ... it is not absolute, but depends upon the system employed for the measurement; it seems that we might call it an apparent contraction which varies with the system. This is merely playing with the words, however. We must not confuse the reality of a phenomenon with the independence of this phenomenon of a change of system. ... The difficulty arises because we have become accustomed to the geometrical concept of a rigid body with a definite shape, whatever the measuring system. This idea must be abandoned. ... We must use the term "real" for every phenomenon which can be measured ... The Lorentz Contraction is an Objective Phenomenon. ...
We often encounter the following remark: The length of a ruler depends upon its motion with respect to the observer. ... From this, it is concluded once again that the contraction is only apparent, a subjective phenomenon. ... such remarks ought to be forbidden.
Krane [1983, 23-25]:
It must be pointed out that time dilation is a real effect that applies not only to clocks based on light beams but to time itself. All clocks will run more slowly as observed from the moving frame of reference. ...
The length measured by the moving observer is shorter. It must be emphasized that this is a real effect.
Matveyev [1966, 305]:
The dimensions of bodies suffer contraction in the direction of motion ... A body is, therefore, "flattened" in the direction of motion. This effect is a real effect ...
Møller [1972, 44]:
Contraction is a real effect observable in principle by experiment. It expresses, however, not so much a quality of the moving stick itself as rather a reciprocal relation between measuring-sticks in motion relative to each other. ... According to relativistic conception, the notion of the length of a stick has an unambiguous meaning only in relation to a given inertial frame. ... This means that the concept of length has lost its absolute meaning.
Pauli [1981, 12-13]:
We have seen that this contraction is connected with the relativity of simultaneity, and for this reason the argument has been put forward that it is only an "apparent" contraction, in other words, that it is only simulated by our space-time measurements. If a state is called real only if it can be determined in the same way in all Galilean reference systems, then the Lorentz contraction is indeed only apparent, since an observer at rest in K' will see the rod without contraction. But we do not consider such a point of view as appropriate, and in any case the Lorentz contraction is in principle observable. ... It therefore follows that the Lorentz contraction is not a property of a single rod taken by itself, but a reciprocal relation between two such rods moving relatively to each other, and this relation is in principle observable.
Schwinger [1986, 52]:
Each will observe the other clock to be running more slowly. This is an objective fact. It is not a property of clocks but of time itself.
Tolman [1987, 23-24]:
Entirely real but symmetrical.
3. Relativistic effects are not physically real:
Taylor & Wheeler [1992, 76]:
Does something about a clock really change when it moves, resulting in the observed change in the tick rate? Absolutely not! Here is why: Whether a clock is at rest or in motion ... is controlled by the observer. You want the clock to be at rest? Move along with it. ... How can your change of motion affect the inner mechanism of a distant clock? It cannot and it does not.
4. Relativistic effects are apparent:
Aharoni [1985, 21]:
The moving rod appears shorter. The moving clock appears to go slow.
Cullwick [1959, 65, 68]:
[A] rod which is at rest in S' ... appears to the observer O to be contracted ... Similarly, a rod at rest in S will appear in S' to be contracted...
Jackson [1975, 520]:
The time as seen in the rest system is dilated.
Joos [1958, 243-244]:
The interval appears to the moving observer to be lengthened. A body which appears to be spherical to an observer at rest will appear to a moving observer to be an oblate spheroid.
McCrea [1954, 15-16]:
The apparent length is reduced. Time intervals appear to be lengthened; clocks appear to go slow.
Nunn [1923, 43-44]:
A moving rod would appear to be shortened. An interval is always less than measured by the other observer.
Whitrow [1980, 255]:
Instead of assuming that there are real, i.e. structural, changes in length and duration owing to motion, Einstein's theory involves only apparent changes, and these are independent of the microscopic constitution and hidden mechanisms controlling the structure of matter. [Unlike]... real changes, these apparent phenomena are reciprocal.
5. Relativistic effects are the result of the relativity of simultaneity:
Bohm [1965, 59]:
When measuring lengths and intervals, observers are not referring to the same events.
French [1968, 97],
Rosser [1967, 37],
Stephenson & Kilmister [1987, 38-39]:
Measurements of lengths involve simultaneity and yield different numerical values.
6. Relativistic effects are determined by measurements:
Schwartz [1972, 113]:
Each observer determines distances to be foreshortened.
7. Relativistic effects are comparable to perspective effects: Rindler [1991, 25-29]:
Moving lengths are reduced, a kind of perspective effect. But of course nothing has happened to the rod itself. Nevertheless, contraction is no illusion, it is real. Moving clocks go slow, a 'velocity-perspective' effect. Nothing at all happens to the clock itself. Like contraction, this

 

[jcsd]

That's silly, as I said, evryone knows that time dialtion and length contraction are the result of changing basis in spacetime

Until 'real', 'not real', 'apparent', etc, are well-defiinde physical concepts then it's just a matter for that age old question of ontology as the actual predictions of relativity are unambiguous. Personally I have to say within the rleativstic paradigm length contraction etc must be described as 'real' as cleraly if you take a spatial slice from a givcen cordinate system then there is no objevctive way to tell whether the lengths of objects are due to length contraction or not (though you could argue that by only examing a single spatila slice we're not looking at the bigger picture).

 

[Janus]

This thread has drifted off the initial question to the point that I'm just going to lock it.