# Locally Lorentz

by JDoolin
Tags: locally lorentz
PF Gold
P: 706
 Quote by PeterDonis I should clarify this some more to make sure I'm clear: I'm not actually saying there have to be physical wave crests *all the way* along the line of simultaneity I've described. That's not necessary (although it makes it easier to interpret the relevant spacetime diagrams if you think about it that way). All that's necessary is that we can calculate a product (frequency / c) * (distance between emitter and receiver), where both are measured along the particular line of simultaneity I've defined. If "measuring frequency along a line of simultaneity" bothers you, just redefine (frequency / c) as (spatial wavenumber), which is perfectly well-defined along the given line of simultaneity--even if the wave crests don't reach all the way from emitter to receiver along that line (because the light pulse is too short), the concept "number of wave crests per unit distance along that line" is still well-defined.
The problem with this is that your "given" line of simultaneity, while it sounds like "THE ONE" is actually pretty arbitrarily chosen. You are saying there is something special about the reference frame where both emitter and receiver is at rest. And, I agree that there is something special about that frame. Namely, you've got built in observers (the emitter and receiver) that are in that frame.

You're making kind of an odd claim, though, regarding this reference frame. You say that the number of wavelengths is an invariant as long as we always use the same line of simultaneity, which is of course true, but it's rather like saying, "the radio will always be at the same volume, so long as we never turn the volume knob."

Another issue I see with your "given line of simultaneity" is that it can only apply when you have the emitter and receiver moving on parallel world-lines. If the emitter and receiver were moving toward or away from each other, there is no built-in line of simultaneity which asserts itself as "THE ONE"
Physics
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P: 6,174
 Quote by JDoolin You should distinguish between "distance between objects" and "distance between events."
I disagree; any use of the term "distance" implies a distance between particular events. What you are calling "distance between objects" really means "distance between two particular events, one on each object's worldline, that are somehow picked out as seeming natural to measure the distance between." However, that notion in itself only makes sense if the interval between the two events is spacelike; see next comment.

 Quote by JDoolin But distances can and should also be measured between nonsimultaneous events. For instance in this emission and absorption example, you shouldn't pretend that the distance traveled by the photon is equal to the distance between the emitter and receiver. Why? Because while the photon is under way, the receiver either moves toward or away from the emitter. This means that the distance between the receiver and emitter is irrelevant. It is the distance between the emission EVENT and absorption EVENT which is relevant.
I think your terminology is confused. What I think you are really talking about here is *not* the distance between the emission event and the absorption event; as I said before, those events are separated by a null interval so the term "distance" between them makes no sense. However, there is a distance you could be referring to. Suppose I draw a line of simultaneity in the emitter's frame that passes through the absorption event. This line will cross the emitter's worldline at some event, and the distance between *that* event and the absorption event is well-defined, since the interval between them is spacelike. Also, if the receiver is moving relative to the emitter, that distance will indeed be different than the corresponding "distance" I would measure if I drew a line of simultaneity in the emitter's frame passing through the *emission* event, and measured the distance from that event to the event on the receiver's worldline where the "emission" line of simultaneity intersects it.

Of course, the "distance" I've just defined is observer-dependent; if I go through the same process in the receiver's frame, I will come up with different answers if the receiver is moving relative to the emitter. See next comment.

 Quote by JDoolin First let me point out my snowflake example above. Who is correct? The guy in the car who thinks the snowflakes are coming right at him, or the person on the ground who thinks the snowflakes are coming straight down? You can pick out a frame of reference that meets some arbitrary criteria, but I think the most sensible criteria is "observer dependence." Use the distance as measured by a ruler in that reference frame, where your observer is. The driver and the person on the ground are equally correct, so long as they make it clear who and where they are.
Yes, they are. So what? I've never denied that distances, defined this way (more precisely, defined the way I did above) are observer-dependent. I'm talking about a different issue; the issue of what quantities can figure in the physical laws involved. Those quantities have to be invariants, which means you can't use observer-dependent quantities like the distance I defined above, *unless* there's a particular symmetry in the problem that picks out a state of motion. See my next post.

 Quote by JDoolin You may still feel like one of the drivers is incorrect, but remove the planet, and just have the car and the snowflake flying through space. The snowflake is coming at the car at 55 miles per hour and travels a whole second at that speed. That distance is well-defined, even though the events did not happen at the same time.
I assume you mean a second in the car's frame, since in the snowflake's frame the snowflake's speed is zero. Yes, if the observer in the car wants to know the "distance" the snowflake was, in his frame, away from the car one second before it hit the car, he can just multiply speed by time as you say. But that will *not* give him any sort of "length along the snowflake's worldline", because the snowflake's worldline is timelike. What it will give him is a distance defined as I defined it above, between the event on the snowflake's worldline one second (in the car's frame) before it hit the car, and the event on the car's worldline that intersects the car line of simultaneity through the snowflake event I just described.
Physics
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P: 6,174
 Quote by JDoolin The problem with this is that your "given" line of simultaneity, while it sounds like "THE ONE" is actually pretty arbitrarily chosen. You are saying there is something special about the reference frame where both emitter and receiver is at rest. And, I agree that there is something special about that frame. Namely, you've got built in observers (the emitter and receiver) that are in that frame.
More precisely, it's the very fact that there *are* two observers who are at rest in the same frame (i.e., at rest relative to each other) that picks out that particular frame as special *for this problem*. Another way of stating the specialness is that this problem has a symmetry that the general problem lacks: both observers have the same lines of simultaneity (or, equivalently, their worldlines are parallel, as you put it later in your post). Given that symmetry in the problem, the particular frame that respects that symmetry is not "arbitrarily" chosen; it's naturally picked out by the symmetry.

 Quote by JDoolin You're making kind of an odd claim, though, regarding this reference frame. You say that the number of wavelengths is an invariant as long as we always use the same line of simultaneity, which is of course true, but it's rather like saying, "the radio will always be at the same volume, so long as we never turn the volume knob."
No, it's like saying that if there's a particular radio station I want to listen to, there's only one place I can put the tuning knob. Given the particular symmetry in the problem, the line of simultaneity is not arbitrarily chosen; there is a particular property that naturally picks it out.

 Quote by JDoolin Another issue I see with your "given line of simultaneity" is that it can only apply when you have the emitter and receiver moving on parallel world-lines. If the emitter and receiver were moving toward or away from each other, there is no built-in line of simultaneity which asserts itself as "THE ONE"
You're correct; the case where emitter and receiver are in relative motion is more complicated. I think the idea of "number of wave crests counted along some particular line" can be generalized to cover that case, but I haven't completely worked it through yet (though I suspect that the fact that I've only stated it so far in terms of space instead of spacetime is an issue--instead of counting wave crests along a line, the answer may be to count them in a spacetime area, which would translate into a 4-volume if we put back in the other 2 spatial dimensions we've been suppressing). I am pretty sure, though, that no notion of "distance" by itself, such as the one discussed in my last post, will work, because of the observer-dependence I mentioned there.
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P: 706
 Quote by JDoolin Emitter \ \ Reciever \ \ \ \ \ \. (Reciever chasing emitter.) \/\ (Nearing a zero distance in space and in time) \ \ (the light and receiver are going in opposite directions) \ \ . Reciever \\\ Emitter \\\ \\\ \\\ (Emitter chasing receiver.) \\\ (Nearing an infinite distance in space and time.) \\\ (The light has to go a long way to catch up.) \\\ \\\ 
Peter,

From your description of what you think I mean, I can tell that this ascii picture was woefully insufficient to get my idea across, so I took the time to attempt to remedy the situation with the attached diagram.

As you can see, in the diagram on the left, the emitter is chasing the receiver, and vice-versa on the right. I'm not saying something about their motion relative to each other but about their motion relative to a given observer.

I don't want to go through and respond to your last response point-by-point, because if you've missed this idea, you won't make sense out of anything else I've said.

However, I do want to clarify a couple of ideas.

One, the difference between a distance between events, and a distance between objects.

If you draw a line of simultaneity in some observer's reference frame, intersecting two object's worldlines, this represents the distance between objects. This is an observer dependent quantity. If those objects happen to have parallel worldlines, this distance is subject to the lorentz contraction.

If you have the space-time coordinates of two events, (regardless of whether their separation is null, timelike, or space-like) you can calculate the distance between events by using the distance formula, (sqrt(x^2+y^2+z^2). This is also an observer dependent quantity.The distance between two events, is to be distinguished, also, from the space-time INTERVAL between the two events, which of course is an invariant sqrt(t^2-x^2-y^2-z^2).

There is one other aspect of your argument where I think you should take a bit more care, because at one point, I think you have suggested that the "distance between events" is meaningless unless those events have a space-like separation. Yet, you were able to fairly well describe the distance between time-like separated events from the frame of the car, and from the frame of the ground, and the frame of the snowflake, so I know you cannot really believe these ideas to be meaningless.

 Quote by PeterDonis So what? I've never denied that distances, defined this way (more precisely, defined the way I did above) are observer-dependent. I'm talking about a different issue; the issue of what quantities can figure in the physical laws involved. Those quantities have to be invariants, which means you can't use observer-dependent quantities like the distance I defined above, *unless* there's a particular symmetry in the problem that picks out a state of motion. See my next post.
Here is where we may have a central disagreement, which is the question, "Do observer dependent quantities have any place in physics?" I would say, emphatically YES. In any actual description of phenomena, one invokes a hypothetical observer,from whose viewpoint one measures all of the phenomena. Often called "the lab frame." It's not a symmetry of the problem that picks out the state of motion. It's the velocity of the rulers and clocks and apparatus that you're using that picks out the state of motion.
Attached Thumbnails

Physics
PF Gold
P: 6,174
 Quote by JDoolin I took the time to attempt to remedy the situation with the attached diagram.
Thanks, the diagram and your clarifications make it clearer what you're using your terms to refer to. I don't think we have any disagreement about the physics of this particular scenario, only about what terminology is "appropriate" to describe it.

 Quote by JDoolin As you can see, in the diagram on the left, the emitter is chasing the receiver, and vice-versa on the right. I'm not saying something about their motion relative to each other but about their motion relative to a given observer.
Got it. No disagreement here.

 Quote by JDoolin If you draw a line of simultaneity in some observer's reference frame, intersecting two object's worldlines, this represents the distance between objects. This is an observer dependent quantity. If those objects happen to have parallel worldlines, this distance is subject to the lorentz contraction.
No disagreement here.

 Quote by JDoolin If you have the space-time coordinates of two events, (regardless of whether their separation is null, timelike, or space-like) you can calculate the distance between events by using the distance formula, (sqrt(x^2+y^2+z^2). This is also an observer dependent quantity.The distance between two events, is to be distinguished, also, from the space-time INTERVAL between the two events, which of course is an invariant sqrt(t^2-x^2-y^2-z^2).
I understand now what you meant by "distance between events" (and how it's distinct from the interval, but I was never in any doubt that you meant something different than the interval by your "distance" terms). I would point out, however, that there is one key difference between this "distance between events" and the "distance between observers" as you defined the latter term above: the "distance between events" does not Lorentz transform like you expect a distance to transform. It can't possibly, because it's longer if the emitter and receiver are moving in one direction, and shorter if they're moving in the other direction. Lorentz contraction doesn't work like that, as I pointed out in an earlier post.

 Quote by JDoolin There is one other aspect of your argument where I think you should take a bit more care, because at one point, I think you have suggested that the "distance between events" is meaningless unless those events have a space-like separation. Yet, you were able to fairly well describe the distance between time-like separated events from the frame of the car, and from the frame of the ground, and the frame of the snowflake, so I know you cannot really believe these ideas to be meaningless.
But you'll note that I was careful to define "distance" in such a way that it was always measured along a spacelike line; I just obscured one aspect of that by defining the events such that one of them was on the worldline of the observer whose lines of simultaneity were being used (so the "spacelike distance" from the observer to that event was zero). The scenario in your diagrams above doesn't have that property, so one has to be more explicit in defining exactly what "distances" are being used. In your scenario, I would say that the "distance between events" that you've defined is actually the difference between two distances that are each measured along spacelike lines: the distance between the observer and the emission event (measured along the observer's line of simultaneity that passes through that event) and the distance between the observer and the absorption event (measured along the observer's line of simultaneity that passes through that event). Your formulation in terms of the differences in x, y, and z coordinates implicitly uses this definition, because the coordinate values at each event are defined as the components along each spatial coordinate axis of the distance from the observer's worldline to the event along the observer's line of simultaneity through that event.

 Quote by JDoolin Here is where we may have a central disagreement, which is the question, "Do observer dependent quantities have any place in physics?" I would say, emphatically YES. In any actual description of phenomena, one invokes a hypothetical observer,from whose viewpoint one measures all of the phenomena. Often called "the lab frame." It's not a symmetry of the problem that picks out the state of motion. It's the velocity of the rulers and clocks and apparatus that you're using that picks out the state of motion.
You may be misunderstanding the point I was making. I wasn't saying that observer-dependent quantities don't have any place in physics; I was saying only that observer-dependent quantities can't appear in the *laws* of physics. The law that determines what light intensity a particular observer will measure from a particular source is one of the laws in which observer-dependent quantities can't appear. So the correct statement of that law can't use "distance" in general (but see below for the special case where the emitter and receiver are at mutual rest), since that is observer-dependent (regardless of which of the definitions discussed above we use). However, it also can't use the spacetime interval between the emission and absorption events, because that's always zero even though different (source, receiver) pairs produce different measured intensities. So there must be some *other* invariant, observer-independent quantity that appears in the law.

I think you're also somewhat mis-stating the role of symmetries in a physical situation. For the special case we've been discussing, where the emitter and receiver are at mutual rest, there is a special state of motion in which the problem looks simplest: the state of motion which corresponds to the mutual rest frame of emitter and receiver. This is what I meant by saying that this particular problem (*not* the general problem, which allows emitter and receiver to be moving relative to one another) has a "symmetry"--it looks simpler if you pick a particular reference frame in which to describe it. In that frame, all the different kinds of distances we've been discussing are equal: the "distance between observers" is the *same* as the "distance between emission and absorption events". This distance is also the "proper distance" between the emitter and the receiver, and "proper distance" has the same kind of special status in relativity as "proper time" does--it qualifies as an "invariant", because it can be defined in a coordinate-independent way, just as proper time can. (It's true that it can't be the general invariant that appears in the light intensity law I referred to above, because it only works for this special case, as I noted in a previous post. The general invariant, I think, is a generalization of this one, but I haven't completely worked it through yet.)

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