View Single Post
PeterDonis
#32
Feb19-12, 01:03 PM
Physics
Sci Advisor
PF Gold
P: 6,149
Quote Quote by nitsuj View Post
being overly subtle, Im saying EM away from you is length; EM towards you is time. The length measurement is the past tense of the time measurement.

Less subtle, and more "scientific" Length & time are defined by the path of a photon perpendicular to....the path of a photon. aka one light second per second.

This is also the presentation of a spacetime diagram, specifically when natural units are used. i.e. time axis perpendicular to length axis.

Are we done playing elusive debater?
I'm not "debating", I'm trying to understand what you think you're saying. None of the above helps much. The "time axis" is always perpendicular to the "length axis", regardless of what units you use, so that doesn't tell me anything useful. If you think "EM away from you is length, EM towards you is time", then does that mean if I shine a flashlight at you, I think it's length and you think it's time, while if you shine a flashlight at me, you think it's length and I think it's time? That makes no sense. Basically you are saying things that I can't make sense of in terms of any sort of standard physics. The only reason I keep bringing it up is that it seems clear to me that you do have something to say; you are just saying it in a way that I can't understand.

Quote Quote by nitsuj View Post
1.) i don't care for the "rules" of using a FoR for a photon. 'cause I posit that a photon cannot measure time. Perhaps better said as the distance it travels is purely spacelike. Your point here seems to get into the technicalities of applying theory. I have no formal education here so...but see the last comment of this post, I think it's relates to this.
So let me get this straight: you have no formal education, you don't understand any of the "technicalities of applying theory", and yet you think you can just choose to "not care for the rules" of the theory? No wonder you're not making sense.

I'm sorry if that comes off as snarky, but please understand that the theory has rules, and standard terms, and standard ways of talking about things, for a reason: so that the theory can make accurate predictions, and so that its concepts can be talked about with clarity and precision. You appear to be trying to express your thoughts in your own terms, using your own version of the theory, and it's not working well; but clearly you have *some* thought behind them. For example, you say "a photon cannot measure time...the distance it travels is purely spacelike". A photon's worldline is not spacelike; it's null; and it's the fact that it's null that accounts for why the "time measured by a photon" is zero (not the best way to put it, IMO, but a lot of people do use that phrase to refer to the null worldline). The "distance it travels" can be interpreted as a spacelike line, as I said in previous posts, but that spacelike line tells you nothing useful about the physics; in particular, it doesn't tell you that the photon's actual worldline is null, so it doesn't tell you anything about whether or not the photon can "measure time" in the sense you're using the term.

I would strongly recommend that you try to learn more about the standard theory and the standard terms. Even if you're going to end up deciding that you don't entirely accept the standard theory and the standard terms, you will find it a lot easier to communicate what you disagree with if you know the standard theory and the standard terms.

Quote Quote by nitsuj View Post
3.) I don't know of equation for measure of time and length from the perspective of a photon. perhaps this imagery would help, two photons; one is 299,xxx metres behind the other. what is the interval between the two? Is it purely spacelike no matter the relative velocity of whoever measures it? Why can there not be a time component in that interval?
Whether or not the interval has a time component does depend on the relative velocity of the measurer. By specifying that "one is 299,xxx meters behind the other", you are implicitly specifying that the measurer measures that length as the space component of the interval; you are also at least strongly implying that the time component of the interval is zero for that measurer--because specifying a distance between two moving objects is normally taken to mean "distance at the same time, according to the measurer".

A measurer that was moving relative to the first one would then see a nonzero time component to that specific interval (that is, the interval between the two specific events implied by your description--events on each photon's worldline that are "at the same time" according to the first measurer); but he would also see a *different* space component. However, a measurer moving relative to the first one would find it more natural to measure a *different* interval, one between events on the two photons' worldlines that were "at the same time" according to *him*, not the first measurer. By the relativity of simultaneity, these will be a *different* pair of events; to the second measurer, *this* interval, between that pair of events, will have a zero time component (and a space component different from 299,xxx meters); but *this* interval will have a nonzero time component (and a space component that is still different from 299,xxx meters) to the *first* measurer.

In summary: an "interval" is a Lorentz interval between a specific pair of events; for any timelike or spacelike interval there will be one particular FoR (one particular measurer) in which only one component (time for timelike intervals or space for spacelike intervals) is nonzero. (For timelike intervals this is called the "rest frame"; for spacelike intervals there is no simple term in common use, but "simultaneous frame" would seem to me to be a good term for it). For null intervals, the time and space components in any FoR must be equal, but their actual magnitude will vary from frame to frame.

Quote Quote by nitsuj View Post
(this is just by my possibly wrong reasoning, I don't know the simple math of calculating intervals, but assume that since nothing goes faster then c, the separation between the two is of purely length, no cause from the trailing photon can effect the leading photon no matter how much time is given)
This is false; the leading photon could certainly interact with something that could then propagate back in the other direction and meet the trailing photon.

Quote Quote by nitsuj View Post
4.) lol, this one is funny, Peter the fundamentally different things are null & spatial, the "same thing" is the path of a photon, hence "fundamentally different perspectives of the same thing." (I know a photon doesn't have an FoR, let's play pretend like they did when defining the metre & second)
You are still missing my point. The path of the photon, meaning its worldline, is null; it is *not* spatial, and no amount of change in perspective will make it spatial. When you talk about a "spatial path of the photon", you are talking about a *different thing*--a spacelike line, the projection of the photon's worldline into a particular spacelike surface, that has nothing to do with the photon's physics.

Quote Quote by nitsuj View Post
next.) I can't help you understand it any further then; if you can't catch up to a beam of light the interval must be purely spatial. for some reason, in the next part you reason exactly what I've been saying. Yay we agree!
So in your lexicon, "the interval must be purely spatial" is equivalent to "the path of the photon is a null line". Hmm. At least you appear to agree with what I said.

Quote Quote by nitsuj View Post
Answer.) it's yes & it would be perpendicular to each other, where the distance of the path of a photon are equal parts. i.e. one meter up for a time measure, 1/299,xxxx of a second for a length measure, and there is a meter and a second defined. they are of the same distance (interval) but of different units, referred to as natural units. As defined, separated merely by orientation relative to what an observer calculates as a null path, where time & length are zero. yay invariance!
A "null path" does *not* mean time and length are zero; it means "length in time" and "length in space" are equal (speaking somewhat loosely). The rest of what you wrote just seems like a long-winded way of saying what I just said in the previous sentence.

Quote Quote by nitsuj View Post
This from wiki seems to address that technical stuff from point one that I am unable to address; "Massless particles like the photon follow null geodesics. Spacelike geodesics exist. They do not correspond to the path of any physical particle, but in a space that has space-sections orthogonal to a timelike Killing vector a spacelike geodesic (with its affine parameter) within such a space section represents the graph of a tightly stretched, massless filament"

Yay! I learned a new word, I should have be saying "orthogonal" instead of "perpendicular".
Yes.

Quote Quote by nitsuj View Post
Maybe you can help me understand something better, this back and forth regarding the FoR of a photon, worldlines ect, does addressing those "issues" come from the "killing field" stuff? I can't understand the wiki entry well enough to know.
A Killing vector field is a very general concept in differential geometry; it has nothing specifically to do with the case of a photon. The only reason the concept was brought up in the Wiki entry in reference to spacelike geodesics is that it's a lot easier to give a physical meaning to spacelike geodesics if the spacetime as a whole has a timelike Killing vector field. The best simple way I know of to picture what that means is that the spacetime is stationary: i.e., the metric "looks the same" at all times. Since the metric determines which curves are timelike, spacelike, and null, that also means that curves of each type "look the same" at all times; for example, if null lines (photon worldlines) are 45-degree lines on a spacetime diagram at one time, they are 45-degree lines at all times. That makes it easier to picture what's going on in the spacetime.

Quote Quote by nitsuj View Post
In this context I amount "killing field" to the myth buster line "I reject your reality [photons measure of time/length] and substitute my own [observers measure of time / length]". Is that what the "killing field" does?
No; see above. But there is no such thing as a "photons measure of time or length", because photons don't have a standard FoR in which they are at rest.