Is Length Contraction in SR physical??
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From: J. Clarke on 2 Feb 2010 18:52 Ste wrote: > On 2 Feb, 17:38, PD <thedraperfam...(a)gmail.com> wrote: >> On Feb 2, 10:30 am, Ste <ste_ro...(a)hotmail.com> wrote: >> >>> On 2 Feb, 13:55, kenseto <kens...(a)erinet.com> wrote: >> >>>> On Feb 1, 6:38 pm, artful <artful...(a)hotmail.com> wrote: >> >>>>> On Feb 2, 1:55 am, "kens...(a)erinet.com" <kens...(a)erinet.com> >>>>> wrote: >> >>>>>> Is Length Contraction in SR physical?? >> >>>>> Define 'physical' >> >>>> Look up the dictionary. >> >>> I have a better suggestion. "Physical" is what the world would look >>> like if observed by God, where information is conveyed >>> instantaneously and is not subject to noise imparted by any other >>> physical process. >> >> That's an interesting concept. >> What we've gleaned from the laws of physics so far is that no >> information is conveyed instantaneously, ever. There does not seem to >> be a visible exception to this regularity of nature. >> So it appears that what you think should be regarded as physical is >> how nature is, if the laws of physics that nature appears to respect >> were violated. >> Or another way of saying it is, physicists should be trying to >> understand the regularities and rules that nature operates by, but by >> discarding the regularities and rules so far observed. >> Do I have this right? > > No, it's about stepping outside of mere observation and having a > hypothesis about what is *really* happening, which is (to a certain > degree) not supported by the observations alone. > > Physicists seem to be stuck in this problem of recognising that what > is observed depends on the observer's circumstances (much in the same > way that the Sun rises and sets at different times depending on where > you are on the Earth), but they don't seem to be able to (or even > concerned to) take a leap and actually start talking about reality, > instead of just conceiving of infinitely complex mathematical rules > that will determine when the Sun rises and sets based on geographic > position, without actually saying *anything at all* about the > fundamental physical basis of these observational discrepancies. Physicists deal with what is observed. If you want to "take a leap and start talking about reality" as if "reality" is something other than what is observable, then you're in the wrong shop.
From: Ste on 2 Feb 2010 19:49 On 2 Feb, 19:33, PD <thedraperfam...(a)gmail.com> wrote: > On Feb 2, 10:39 am, Ste <ste_ro...(a)hotmail.com> wrote: > > > > Then I would ask the following questions of you. > > > Is velocity a physical property? It is *clearly* observer-dependent. > > > Velocity is a physical property that does not vary with observer- > > perspective, yes. The measurement of velocity is observer-dependent, > > yes. > > Ah. Then I'll ask you what your physical velocity is right now. Along > the way of answering that question, please also explain how you would > go about determining what that velocity value is. I actually have no idea what my velocity is right now. I also recognise the problem of measurement, but that does not detract from my belief that there *is* a material reality independent from observation. > > The discrepancy between those statements is accounted for by the fact > > that I believe in *material reality*, where the measurement of > > variables does not necessarily reflect the value of the variable > > itself. > > OK, so here's an interesting statement by you. There is the claim > about the existence of properties that will not necessarily be > revealed by measurement. I'd like to clarify that variables are often manifest without necessarily being able to accurately quantify them (which I presume quantification is what you mean by "measurement"). > One then immediately might ask what values > those properties have, and how would that be determined outside of > measurement? The truth is that some variables may be unquantifiable. That is different from saying they don't exist. > And if all we have is a set of observations > (measurements), then how do we glean the reality of those nether- > properties at all? Inference, of course. > Finally, a basic notion: When we say we know how the universe works, > what we mean is that when we *see* circumstances A, B, and C prevail, > then we can expect to *see* an outcome X in quantity Q. It is our > ability to make this prediction reliably that gives us confidence that > we know what is going on -- otherwise we can't be sure. But since the > prediction has entirely to do with *observed* circumstances and > *observed* outcomes, what other connection do we have with the > operation of nature than by what is observed? As I've said repeatedly Paul, the question is not about what is observed, but about how to interpret it. > > > Having answered that, is kinetic energy a physical property, given > > > that (at least at low speeds) the kinetic energy is proportional to > > > the square of the velocity, and so is also clearly observer-dependent. > > > Having answered that, is the law of conservation of energy, which > > > includes kinetic energy as one of the contributions to the conserved > > > sum, a law about physical properties? If not, then what is it a law > > > about? > > > I'm afraid you'll have to explain what you mean. > > The law of conservation of energy says that the sum of measured energy > contributions in a closed system (and one of those contributions is > measured kinetic energy) will remain constant regardless of > interactions inside the system (including exchanges among different > types of energy). This statement is true regardless of choice of > reference frame, though the value of the system sum will be a > different constant depending on the reference frame. It is a > remarkable physical statement about a remarkable physical property, > and it is considered one of the core lynchpins of our understanding of > nature. Indeed, I broadly understand the law of conservation. > Now, you just made a statement that the *measured* velocity is not > necessarily the *physical* velocity. Indeed. > Since the kinetic energy is > proportional to the *measured* velocity squared, then it too must not > be related to a *physical* quantity. Therefore the law that makes a > statement about this *measured* and other *measured* contributions to > a system sum must not be dealing with physical properties, right? I dare say the law of conservation can apply to both "physical" velocity and relative velocity. > Therefore, what is this law about that makes a remarkable statement > about *measured* properties but not physical properties? I still don't understand what you're really asking. The law of conservation can apply in all cases. > > > > and if physical > > > > properties do indeed seem to change depending on how they are > > > > observed, then there should be some coherent explanation for it rather > > > > than just a statement that things are "observer dependent", which in > > > > most people's minds rather leaves the glaring question "why". > > > > I think there may just be a clash regarding an unfounded expectation > > > that physical properties "should" be observer-independent. Why would > > > this be an expectation? > > > Oh I know that is the source of the clash. The reason I expect > > observer-independence is because I believe in material reality, where > > variables exist independent of observation or measurement. > > Oh the variables do exist. My question is why you think there is one, > "true" value associated with a physical variable that is independent > of reference frame? I think there is a physical variable that is independent of its measurement. > And on what basis do you have the expectation that material reality > has variable values that exist independent of observation or > measurement? It ties in with what I understand to be the definition of "material reality" - namely, that the properties of the physical world exist independent of observation and measurement. This stands in contrast to "idealism". > Lest you think that where physics has gone wrong is in the last 100 > years or so, I'll remind you that it was Galileo in the 1600's who > made the observation that properties of material reality depend on the > reference frame. He noted, for example, that the speed of an object > has no inherent value other than by choice of reference frame. This is > not modern physics. This is CLASSICAL physics, pre-Newton. But I don't devalue the concept of the "reference frame" in understanding why the measurement depends on the circumstances of the measuring equipment. The point is that, as a realist, I hold that the variables being measured have values that are independent of the circumstances of measurement.
From: Ste on 2 Feb 2010 20:02 On 2 Feb, 20:10, PD <thedraperfam...(a)gmail.com> wrote: > On Feb 2, 9:24 am, Ste <ste_ro...(a)hotmail.com> wrote: > > > But the question remains, what is the *physical* cause of the > > discrepancy when the object to be measured is moving differently from > > the measuring equipment. It should be a question taken seriously in > > physics. > > And it is. > I will give you just a sketch, meant to inspire further investigation, > not to teach. > The physical cause is traced to the *meaning* of the physical property > length, which is the difference in the locations of the endpoints of > something, where those locations are measured *simultaneously*. The > importance of the simultaneity becomes apparent if you try to measure > the locations of the endpoints of, say, a moving car and attribute the > difference to the length. Then in turn, it becomes clear that length > is frame-dependent if simultaneity is frame-dependent -- the frame- > dependence of simultaneity is the physical cause of the frame- > dependence of length. Ah, now I know this is the difference between us. I hold that "simultaneity" is *not* frame dependent. > So now the question becomes, what is the > physical reason for the frame-dependence of simultaneity? This in turn > is traced to the *meaning* of simultaneity, and how we would know that > two spatially separated events are simultaneous; this in turn hinges > on the synchronous arrival of equal-speed signals from the events at > the midpoint between the events. From here it is relatively simple to > show that the frame-independence of the speed of light *demands* that > for a given pair of events, that definition of simultaneity will be > satisfied in one reference frame and not in another reference frame. > That is, the frame-independence of the speed of light is the physical > cause of the frame-dependence of simultaneity, which in turn is the > physical cause of the frame-dependence of length. So then the question > is, what is the physical cause of the frame-independence of the speed > of light? The answer to this question has to do with the hyperbolic > structure of space-time and the fact that causal relationships are > limited to a region of spacetime called the past- and future-light > cones. This hyperbolic structure of space and time is thus the > physical cause of the frame-independence of the speed of light, which > is in turn the physical cause of the frame-dependence of simultaneity, > which in turn is the physical cause of the frame-dependence of length. > > To expand this in a way that would be compellingly easy to follow > would take more pages than I'm willing to invest. Fortunately, this is > done in numerous books designed to explain these things in a > compellingly easy-to-follow fashion. Would you like some > recommendations? You have in fact answered the whole of my question here Paul. The difference lies in our concepts of "simultaneity". As I said, my concept of "simultaneous" is "those events which occur at the same instant, and would be observed to do so universally, *if* information about the occurence of those events could be conveyed instantaneously".
From: eric gisse on 2 Feb 2010 20:11 Ste wrote: [...] > But I don't devalue the concept of the "reference frame" in > understanding why the measurement depends on the circumstances of the > measuring equipment. The point is that, as a realist, I hold that the > variables being measured have values that are independent of the > circumstances of measurement. Is a magnetic field the same in every reference frame?
From: Ste on 2 Feb 2010 20:13
On 2 Feb, 20:14, PD <thedraperfam...(a)gmail.com> wrote: > On Feb 2, 10:46 am, Ste <ste_ro...(a)hotmail.com> wrote: > > > > Physics is decriptive. > > > Not according to Paul Draper. Apparently it is necessary that a > > "scientific theory" be predictive (and therefore prescriptive), not > > just "explanatory" (i.e. descriptive). > > > In any event, the question of "why" has perhaps been misinterpreted. I > > really meant "how". That is, what is happening in the physical system > > to cause a change in measurements which are "observer dependent". > > Then let's start with the basics. > Let's suppose there is, as you claim, an observer-independent value of > the speed of your coffee cup right now. > For a different observer, that coffee cup will be measured to have a > value of velocity that is different than the observer-independent > value. > What is happening in the physical system of the coffee cup and that > observer to yield a different measured value of velocity? Nothing (i.e. nothing of relevance here) is physically happening to the coffee cup. The difference is in the circumstances of the observer. Incidentally, if indeed there is any way to measure an observer- independent value, then obviously that value should not change with the circumstances of the observer. > Since this is so basic, we should be able to answer it, if it has any > answer at all. Indeed. |