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Talk:Theory of relativity (view source)
Revision as of 18:00, November 11, 2009
, 18:00, November 11, 2009Trying to clear up some misunderstandings
::::::: You complain that relativity is different from Newtonian physics, and then you fail to see the relevance of what Newton said about the principle of relativity. I give up. The CP relativity article is filled with errors. [[User:RSchlafly|RSchlafly]] 00:27, 17 October 2009 (EDT)
::::::: You complain that relativity is different from Newtonian physics, and then you fail to see the relevance of what Newton said about the principle of relativity. I give up. The CP relativity article is filled with errors. [[User:RSchlafly|RSchlafly]] 00:27, 17 October 2009 (EDT)
== Some clarifications I hope will be helpful ==
I signed up for a Conservapedia account after reading this article and talk page, and particularly the back-and-forth between Aschlafly and Rschlafly. I saw what appeared to me to be some points of confusion and miscommunication, and I thought I might be able to help clear them up.
First, on the GPS question, I ''think'' I see what Aschlafly is saying about clocks. I think he means that Newtonian dynamics predicts no change in time from one reference frame to another, but rather that the ''forces'' on an orbiting satellite may affect a ''clock'' on board that satellite, just like bumping a record player can cause it to skip.
This is a very astute observation, particularly given the minuteness of the observed desynchronization. The trouble is, it can't possibly be correct under Newtonian dynamics. The equivalence principle — the most basic Galilean one — says that a freely falling reference frame is indistinguishable from a reference frame which is not accelerating. So there are no forces acting on a clock in an orbiting satellite. (This is an approximation, obviously; tidal forces are at work inside an orbiting satellite. But on that scale, tidal forces are far too minute to have any measurable effect, much less an effect on the scale that's been observed.)
Now, maybe there's some as-yet-unknown force acting on the particular type of clock in a GPS satellite, causing it to desynchronize from ground-based clocks. A magnetic force, maybe, since those clocks are solid-state. We can easily imagine such a force, and define it in a way that matches the observations, just like Newton imagined a "gravitational force" that caused objects to fall in the way he observed. But physicists are (I think understandably) reluctant to do that, since the predictions of general and special relativity combined match the observed effects to the limit of our ability to measure.
It's not accurate at all to say that the desynchronization of GPS clocks ''proves'' either theory of relativity. What is accurate is that the two theories of relativity ''predict'' that orbiting clocks will differ from ground-based clocks in certain ways (slightly faster by a factor X because they're at a higher altitude, slightly slower by a separate factor Y because they're in motion) and that the observed behavior exactly matches the sum of the two predictions, to the limit of our ability to measure. GPS clocks don't ''prove'' relativity, but they also don't contradict it.
It's also more than a bit misleading to say that "relativity has yielded nothing productive," since we wouldn't have PET scans without it for one example. But I'm a theoretician, not an engineer, so I can't contribute too much on that front. I'm also unsure whether you'd consider something like astronomical redshift or gravitational lensing (allowing us to better observe the universe) to be "productive" or not.
I'm generally confused by what you're referring to when you say "sweeping and implausible assumptions." Maybe you're confusing assumptions with predictions. In special and general relativity combined there are only three postulates — things that are assumed to be true by the theory: that the laws of physics are fundamentally the same no matter where you are or how you're moving, that the speed of light in a vacuum is the same to all observers, and that spacetime is everywhere (mathematically) continuous and differentiable.
The third postulate boils down, in layman's terms, to "We assume that mathematics works in the real world." It's admittedly unfounded, but it's also a fundamental assumption of every other theory in physics as well. The second postulate started out as a bit of an assumption, but all experiments conducted to date have produced observations that are consistent with it. And the first postulate is another one of those fundamental assumptions of physics, that the basic laws of physics don't just change arbitrarily from place to place or from observer to observer. Only three assumptions, and none of them are either sweeping or implausible.
Also, I think there's a bit of a misunderstanding (okay, a big, huge, giant misunderstanding) about what <i>E=mc<sup>2</sup></i> means. That's okay, because most people who know what the letters in that equation stand for misunderstand what the equation means. That equation is ''nothing more than a unit conversion factor.'' You can convert from feet to inches by multiplying by a constant, twelve. If we define <i>k</i> to be 12, to keep from having to write the number down, then we could say <i>I=kf,</i> or inches equals feet times a constant. The mass-energy equation is just like that, a conversion factor for talking about energy (joules) in mass units (kilograms), or vice versa, just as we can talk about length in either feet or inches by applying a conversion factor. ''That's all it is.'' It doesn't ''say'' anything about the universe.
If you want to take issue with the mass-energy relation, you should turn your attention to the stress-energy tensor <i>T<sub>μν</sub></i> in the Einstein field equations. That's where the prediction is made that mass and energy are related on a fundamental level. If you want to argue that point, argue it there, not in the unit conversion equation, which doesn't say what you seem to think it says.
Finally (and I'm only stopping here because I don't want to be ridiculously verbose), it's not ''exactly'' correct to say that relativity "[claims] that v can never, ever be faster than c." That's ''sort of'' true, if you squint, but it's not exactly right. What special relativity ''predicts'' — not ''claims'' — is that if you start with any object the speed of which is less than the speed of light — say, me, sitting here — and you ''accelerate'' that object, then its velocity as measured in any reference frame will never reach the speed of light, no matter how ''hard'' you accelerate it nor for how long. That's a more accurate way to describe the prediction. In fact, from a purely abstract mathematical standpoint, the equations of special relativity kind of say the ''opposite'' of what you said. Because it's an equally valid mathematical solution to say that objects can exist which move ''faster'' than the speed of light, but that when they are ''negatively'' accelerated (i.e., decelerated) their speed will never be so ''slow'' as the speed of light. These types of masses are called "tachyons."
Of course, there are some serious mathematical problems with tachyons as a solution to the equations of special relativity. It's true that the math works, but it's also true to say that if I have three apples and you take away five I have negative-two apples. A mathematically correct statement, but one that doesn't have meaning in the real world, since you can't ''physically'' take away more than I have; "negative-two apples" doesn't have a real-world meaning, and tachyons might also not have a real-world meaning. Special relativity doesn't say they ''do'' exist, or that they ''must'' exist, merely that ''may'' exist without contradicting the theory.
I do apologize for writing at such length, but reading the exchange between Aschlafly and Rschlafly made me deeply sad. You two argued with such vehemence over points of misunderstanding that are really very simple. The coincidence in your names, and the possibility that you might be kin, just made it all the more sad to me. Perhaps it makes me a busybody, but seeing two people in conflict over such a trivial set of misunderstandings made me want to help. I hope you take my comments in that spirit. --[[User:KSorenson|KSorenson]] 13:00, 11 November 2009 (EST)