Spiff,

Given that this statement does not assume or imply any kind of
absolute or invariant notion of distance, do you still disagree with it?
If so, what do you disagree with?

Different observers in different states of motion relative to me may
of course get different measures for the distance I walk, depending
in part on the details of their measuring techniques.

-- Jeff, in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

OK, this is my basic point. It seems to me this thread is an entry to discussing our current best understanding of how the concepts of "distance" and "time" interrelate, i.e., "is it distance then or distance now". Our current best understanding of distance and time is relativity, and so we can start with special relativity, but even that is not really our best understanding because gravity may play a crucial role in how space and time work (a la Mach). But it's useful to start with special relativity anyway, and I will share my own personal (non-expert) ideas about what the various flavors of relativity are trying to tell us:

In SR, we consider global arrays of mutually stationary inertial observers. Their rulers, and the Einstein simultaneity convention for time, generate our coordinate system. Then we need a concept of "proximity" between observer/events, which combines space and time and is called the "metric" of the spacetime. If the events are timelike separated, the invariant concept is proper time, which is the clock reading of an inertial observer (not in the stationary array) who is at both events. If the events are spacelike separated, the invariant concept is proper distance, which is the length of a rigid ruler that spans two events that are simultaneous in the ruler frame. Now, since we don't have an observer at both events, nor so long of a ruler, we actually end up needing indirect observations to infer these proper distances and proper times. And we just do the best we can, typically using light.

For example, we might do a "lunar ranging" experiment, where we bounce light off the Moon. Then we do a time measurement, not a distance measurement. However, in special relativity, the light will move at a known speed past that array of mutually stationary inertial observers, so a time measurement can be converted to a distance measurement. That conversion is consistent with the coordinate system, and can give distances between events that are not simultaneous in those coordinates, but the distance you get is actually the same as the proper "distance then", the proper distance between the source and the stationary receiving observer, at the moment that the light was emitted according to the stationary observer at the scene. So Jeff Root's statement in special relativity is claiming that the answer to the OP is "distance then", though he does not realize it.

Enter general relativity. Again, I do not think of GR as simply special relativity plus gravity, I think of it is as the "right" relativity, that corrects the misconceptions of special relativity (we get away with these misconceptions in many applications, but they are still not a correct representation of how reality behaves because they involve concepts that are not demonstrable, like the constancy of the coordinate speed of light when the light is somewhere you can't measure it). Now we find that there is nothing special about either inertial observers, or mutually stationary observers, as these both involve arbitrary conventions to even define. The conventions we would use to define these concepts amount to choosing a coordinate system, and the concept of distance amounts to specifying coordinates, like giving addresses to houses on your street. All the results can predict are the outcomes of experiments, but the "story" you tell, including the distances involved in the story, are completely arbitrary, because they depend on the arbitrary coordinatization.

Now there is certainly one standard coordinatization used in cosmological general relativity, which is comoving frame coordinates. There is also a list of types of distance measures, which can be used to try to infer from a dynamical model (not measure, because we have no direct access to the distance we are attempting to infer, as it involves simultaneous events that are widely separated) the "proper distance" in the comoving-frame coordinates. Other coordinates would of course require a different translation from the various distance measures to the proper distance. The value of proper distances is that they correspond to the metric, so they are a meaningful concept of "distance" that isn't obfuscated by mixing in some elements of time (because the events are simultaneous).

Notice that none of these ideas about distance support the contention that was made earlier that there is physical meaning to "the distance light travels" between its emission and its absorption. Also, even in the flawed special relativity version of reality, the distance "to the observer" between the emission and absorption of light is identical to the concept of "distance then", and is not distinguished from that concept by pretending the "distance between points in space and time" is somehow different from "distance then", as claimed by Jeff Root above, and supported as "exactly correct" by RussT.

__________________
Brett
Peters Creek, Alaska
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Nope. You're wrong. Whether you are doing it conciously or
unconciously, I don't know, but you are leaving out essential bits
of information which resolve the conflicts you raise. In this case,
you left out the information that Russ's statement applies to an
ordinary observer composed of atoms, while my statement applies
to the case where I am light, as you requested in post #192.
I quoted that request immediately before the statement of mine
that you quote. My statement was a direct reply to your request
that I consider the situation where I am light. So that is what my
reply assumes.

An "observer" who somehow consists of light would see no distance,
no passage of time, no anything. An observer who is a human being
sees light travel a definite distance in a given time. Different
situations, different results. Fifth-grader stuff. It isn't for lack of
knowledge or intelligence that you aren't getting this, Ken.

-- Jeff, in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

Sorry Jeff, you are completely and demonstrably incorrect here. RussT's statement is perfectly clear, he is talking about the "path of photons" and he clearly claims "the proper distance for that could never be zero". You claim that "everyone" here knows that the proper distance between the emission and absorption of light is zero. Well sorry to tell you, but the "path of the photons" connects the emission and absorption of light. Ergo, those events do have a proper distance of zero, in obvious contradiction to the statement of RussT that I quoted. So no, not "everyone" knows that proper distance is zero, and at the moment it's not even clear that you do, despite your protestations to the contrary. You are completely mistaken that simply identifying us as the observers, or what we are made of, somehow makes the proper distance between those events not zero, you should look up the meaning of an "invariant", as this seems to be a continuing source of error for you.

Whew.... I just read most of this and am perplexed as to just why it has gone on so far. A few years ago, when tempers were often less bridled, such a conflict would have gotten this thread closed by now. My compliments to both encampments, at least as far as courtesy goes.

I suspect that Spaceman Spiff introduced something that may have much to do with the more Newtonian camp view -- the Hubble flow. Though non-special, there seems to be a corelation between our local frame and spacetime itself, at least within our region in and around the Local Group, and from the perspective of almost any other being that might be on any other planet.

As Ken has noted, a very long ruler would stretch to about 2.5 million light years, assuming we extend it from our local frame here on Earth. Note that there is no imaginable way that we could accelerate, stand on our head, or doing anything else that would cause us to discover a ruler reading greater than 2.5 million light years. [Or am I wrong?] Thus there is a sense that the 2.5 million light year distance is more than just one from our frame of reference; all other observers on other planets within the Local Group would agree with our distance, more or less.

But there is nothing wrong with presenting the bigger picture.

But it is a round hole, and a very big one at that, much larger than the little square peg. Just because the case of our place in space presents a very large insert that has a little square hole within it does not mean there isn't more that is out there.

An analogy might be that we are on the first floor of a very tall skyscraper, with only a stairway, no elevator. The first floor is grand but those that have climbed to the higher floors can see things that those on the first can not. This isn't because of a lack of anyone's intelligence, of course, but because what is seen from the higher floors is simply different. Nevertheless, the "real world" happenings are commonly found on the first floor and it is the only floor that allows one to move out into the world (ie. Earth).

Regardless, what Ken and Grant, and others, have been presenting are very pertinent to the essence of the O.P. question as the view from higher floors are pertinent to science in the 21st century.

My point is that both camps have their merit, but to deny relativity its place and impact upon galactic distances and times measured seems unjustified.

__________________
Lighten up! This is a stellar board!

The distance will not be greater only if we consider ourselves stationary to the other galaxy and all other observers stationary to ourselves as well, more or less, and remain that way, or if measured immediately after we quickly accelerate. Otherwise, however, if there is any continued relative motion between ourselves and the other galaxy, either before or after acceleration takes place, then the distance will eventually become greater or less, of course, depending upon the direction of travel and when the measurement is taken according to a particular frame, so the question might then become something like "Is it 2.5 million light years then, or 2.5 million light years now?" Oh, wait...

__________________
Let's put together the pieces of The Grand Puzzle . (website - now revised)

"Let's define another operator, Sz, which we won't pay any attention to."
"This transformation will automatically make zero equal zero."
"It may be true that zero equals zero -- and that is certainly an equality -- but I don't want to go into the details at this time."

What Russ says is often difficult to parse.

His claim is that 2.5 million years does not equal zero time, and
2.5 million light-years does not equal zero distance.

Based on what he said in post #78 and post #114, it is clear that he
is objecting to your use of light as the observer who is measuring the
distance. He is not saying that an observer traveling at c would
measure anything other than zero. Instead, he is saying that there
is no such observer, never has been such an observer, never will be
such an observer, and that your continually resorting to this fantasy
observer is utterly irrelevant to answering the original question, as
well as irrelevant to understanding what he and I are saying.

Russ,

Do you agree that an observer traveling at the speed of light would
measure all distances as zero?

Have you ever meant to claim in this thread that an observer traveling
at the speed of light would ever measure a distance other than zero?

No. I claim that everyone here knows that in the frame of the light,
the proper distance between emission and absorption is zero.

However, it is not possible for anyone to be in that frame.

In particular, distances of galaxies are never measured or reported
for such a frame.

That's fine. I have no disagreement with that statement.

They may have a proper distance of zero in the frame of the light,
but they do not have a proper distance of zero in any other frame.
In particular, they do not have a proper distance of zero in any
frame in which anything is actually measured.

There is no contradiction with what Russ said.

I'm not sure I've deciphered everything Russ has said in this thread,
but what he has said that I have been able to parse, I agree with.

Everyone here except perhaps you knows that the distance between
the Andromeda galaxy and us is not zero. Everyone here knows that
an observer moving between the Andromeda galaxy and us at the
speed of light will measure a distance of zero between emission and
absorption. Everyone here except perhaps you knows that there is
no such observer, and never has been.

-- Jeff, in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

No, his claim is what it is, not what you might want to pretend it is. What kind of argument is that? Your position now is "everyone on this thread understands what you are saying, and anyone who makes statements completely in contradiction to that should simply have their statements 'parsed' to say something other than what they said." This is a new one: "argument by indiscriminate parsing".
Let me say this one more time: it doesn't make the slightest difference who is measuring the distance, if you specify two events at the ends of a "path taken by light", and you say the "proper distance could never be zero", you are making a false remark and demonstrating that you do not understand my statements about that proper distance in fact being zero. Period. This i
s the beauty of facts, "parse" them anyway you want.
You are obsessed with the frame of the observer, but as I've told you, if you look up the concept of an "invariant", you find that the nice thing about them is that they are observer independent. What's more, proper distance is an invariant-- that is the whole point of it. Finally, being as science is based on whatever fraction of the shared experience of intelligent beings that is objective, science is focused entirely on the purpose of explaining the invariants. All else is sheer pretense, scientifically speaking.
You see, you are incorrect that everyone in this thread understands that proper distance is zero. You do not understand it yourself! Since you won't take my word for it, you really need to google "invariant" and "proper distance", then get back to me. Until then, you are just wasting both our time.

Wrong. Look up "null geodesic" while you are at it, and then reflect on the role of a metric in relativity.
Actually, I never said that distance was zero. I said the proper distance connects two simultaneous events, one in our galaxy and one in Andromeda, which contradicts both your statement and RussT's about proper distances crossed by light paths (which are null geodesics). I also said that we have no direct measurable access to that distance, so we must infer it the best we can using the data we do have access to, and any such inference comes with it an uncertainty based on what is not known about the relevant dynamics, i.e., the gravitational environment that actually determines the meaning of concepts like distance.
That is not what I want everyone to know-- for that, read my actual posts.

You see, I personally feel it is a common misconception, or at least a poor way to think about relativity, that moving at high speed somehow "contracts" the distance one travels, and light "contracts" the distance to zero. But there's no need to contract anything-- the point is, the distance travelled is really less for a fast moving observer, and it is really zero for light. So it's not that light is some kind of "weird observer", it is that light actually finds a shortcut. The distance between two galaxies depends on the path taken between them, and the path taken by light is a zero-distance path. It has nothing to do with who is doing the observing, that is an invariant of relativity. Now you understand why I say there is "no such thing" as "the distance" that any given observer "perceives" between two events-- the distance depends on the path that connects two events, but a path that connects them in such a way that they are simultaneous is the "proper distance" between them, if such a path exists (that's the ruler that spans the events).

I am curious how any oberserver, relativistic or not, that is near the Earth could claim a much more distant Andromeda. Is not 2.5 million lyrs. the maximum distance any observer would claim? [I am ignoring peculiar motion, of course.]

I wonder whether or not we are approaching Andromeda or Andromeda is approaching us? It would be hard to determine just how much each is contributing to the gain in distance. Assuming we knew their masses, that would not be enough. It depends on whose reference frame is chosen, which is the point that many are making.

__________________
Lighten up! This is a stellar board!

Sorry, but if as your example seems to indicate that you really believe that an observer on the train and an observer anywhere else in the universe (even, for example, at rest relative to the tracks) actually measure the distance between the events you describe (or any two events), then you do not understand relativity -- plain and simple. To the contrary, these observers you mention can each measure a distance between the events in space and in time. The only invariant among intervals of space, time, and space-time is the last of these. But then, Ken G has already pointed out these things numerous times. You reply by claiming that you understand and accept relativity, but then claim that Ken G is missing the boat somehow, and then go on to give examples to illustrate your point -- many of which indicate that you don't understand the concepts.

There is no the distance, whether meant or anything else.

Spiff,

Your objections are to what you want me to have said. You haven't
disagreed with what I actually said.

-- Jeff, in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

Oh okay, I see what you are saying. So ignoring gravitational effects and acceleration, and just considering it inertially, if all observers coincide in the same place when making their measurements to Andromeda, although travelling at different relative speeds, I believe it would be the observer that is stationary to Andromeda that would measure the greatest distance, the one that observes no overall redshift or blueshift due to motion.

__________________
Let's put together the pieces of The Grand Puzzle . (website - now revised)

"Let's define another operator, Sz, which we won't pay any attention to."
"This transformation will automatically make zero equal zero."
"It may be true that zero equals zero -- and that is certainly an equality -- but I don't want to go into the details at this time."

I quote your own quote, tell you what you have written is a misconception (specifically, in bold), and your response is that I have disagreed with a statement that I have imagined?

You say you "understand" relativity, and then go on to say that of course "everyone" understands your argument. But then when prompted to explain that argument, you say things which indicate that you do not understand relativity. You can't have it both ways, but that appears to be 'the game' you've been playing with Ken G, whether intended or not.

I'm afraid so, yes.

I just looked back through the 44 most recent posts in the thread,
and could not find any place where I said that I understand relativity.
I have said that I understand various things, I've said that Ken's
understanding of relativity is good, and much deeper than my own,
and I've (rashly) said that everyone here understands certain things,
some of those being aspects of relativity. I don't recall ever saying
that I understand relativity. Can you show me where I said that?

Oh, no. I certainly never said that everyone understands my argument.
I did say, for example, that "everyone here knows that in the frame of
the light, the proper distance between emission and absorption is zero."
From Ken's response to that assertion, I can see that I may have got
the physics wrong, in which case the statement as a whole is wrong,
too. Even if I got the physics right, the statement as a whole could
be wrong. But I have not yet analyzed Ken's response fully enough to
determine whether what he says about the physics actually conflicts
with what I said. I thought that what I said was equivalent to what
he has said numerous times in this thread, but if he disagrees-- which
he does-- then I need to figure out why he disagrees. I'll work on it.

In any case, I never said that everyone understands my argument.

My argument does not depend on relativity. Ken's does; mine doesn't.
So whether I understand relativity or not has no bearing on whether
everyone or anyone here understands my argument.

-- Jeff, in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

Relative to what?

Again, relative to what?

The key thing about relativity is that it's all relative. There is no standardized "ether" against which things moving in the cosmos are measured. Things move relative to other things, period. If there is a galaxy (Galaxy A) which is moving at .5 c relative to a cluster of galaxies (Cluster B) whose relative velocities to one another are less than .0001 c, one might assume that the .5 c galaxy is the speedy demon, but one would be wrong. It is just as relevant to say that Galaxy A is experiencing a half lightspeed flyby from Cluster B.

Exactly! I knew you were going somewhere with this, George. My bad if I didn't see it coming.

__________________
If I set the budget, we'd have Ares and more. Unfortunately, I don't set the budget, and Ares is just too expensive and too far out for us to accomplish our goals within the budget we were given.

If we halt the ISS, all versions of Ares, and transport Orion and Altair aboard DIRECTv3's Jupiter family of Shuttle-Derived Launch Vehicles, we just might make it back to the Moon by 2020.

Yep. As Grant has shown much earlier, the peculiar motion (blueshift) is quite small and not of particular interest at this point.

What I am hoping to demonstrate is that the 2.5 million lightyears distance could be considered "the" distance because of the fact there can be no greater straight line distance. This may explain the hang-up for Jeff and Russ. Am I close, Jeff? [This Andromeda case is easy since the other ways to see it, as Grant demonstrated from his page one link, are not that important, but much more distant galaxies will require one to make a choice.]

Of course, the distance to Andromeda can be considered to be much less due to relativity, and that is an important point that Ken initiated.

__________________
Lighten up! This is a stellar board!

__________________
Lighten up! This is a stellar board!

BTW, my compliments on your latest avatar, Publius. It took me a second to recognize who you were lifting up.

__________________
Lighten up! This is a stellar board!

George,

The idea you are pursuing, in which distance measurements made with
zero velocity relative to the place you are measuring to are considered
particularly meaningful, is something I've thought about, a few years ago,
but isn't what I'm arguing now. What I'm on about at present is simpler--
as I've already said too many times, its something most fifth-graders would
have no trouble with. It doesn't require any knowledge of relativity, though
an explanation of it could suggest the concept of at least Galilean relativity.

I don't understand this comment:
I can guess, but I really don't know what you are referring to. My best
guess is that you're just saying that observers in certain states of motion
relative to the Andromeda galaxy will see it as being closer than will other
observers who are at the same location, but in other states of motion.
If that's all you meant, then, okay, yes, of course.

-- Jeff, in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

I'll start here...

That is simply because there are two 'truths' being expoused in this thread.

The first was stipulated to very early on...

Here KenG and Grant and Tim Thompson all agreed that there IS a "light travel time", that is "Known" and that all of Humanity "Knows" it to be "True"!!!

Now, everyone that has replied to this thread, especially Spaceman Spiff, answer this question...in writing, as a simple response to this question...

The distance to M31 is known to be ~2.5 light Years from Earth, so IF a Super Nova exploded/imploded in M31 2.5 million years ago, how long would it have taken those photons to travel to the Earth, and when would we be detecting them, in Earth's frame???

Good...now compare your answer to this statement...

And Ken's correct...that statement is what Relativity dictates...

So, you wind up with two supposed 'Truths'...1. light has a specific travel time based on a well known ~ distance, and 2. light doesn't have a travel time or a distance.

Both Jeff and I have repeatedly shown, that those distances, to Local objects ie; the Moon, Sun, all our planets, alpha Centauri, and even M31 were NOT determined using SR/GR or SR>>GR...

So, based on your answers to the SN in M31, it doesn't take a rocket engineer, But it might take a fifth grader, to understand that light/photons do actually travel the distances shown...oh yeah...in Earth's Frame...can't forget that little tid bit, even though just mentioning light travel time should have been obvious to anyone who knows a little 'bout it.

Now, IF there are Aliens in spaceships out there, traveling at Relavistic speeds, and even all the way to "c", great, when they get here they will undoubtedly be able to teach us a thing or three...BUT all of these that no one has responded to do apply!!!

Let's see if we can determine how that 'Arguement' will play out...

"If" we become that 'observer' (IN fact jump in the seat right next to him/her)you/KenG/Relativity/mainstream and Grant says 'we' should...

Say that 'alien in a spaceship' traveling toward us, at "c" right next to our sun,
and let's say all the planets are lined up right behind us, like happens about every 175 years...

and let's say that ship you and the alien are on is going to Mars, Jupitor, Neptune, when you get there "Instantantly", did the 'space' between there and the sun contract to Zero? What happened to Mercury, Venus, the Earth?

How are "You and The Alien" going to convince anyone, that you actually just came from the Sun in the First Place, when you could have just as easily come from a galaxy that was 1000 billion^1000 billion light years away?

And If you landed at NASA, you may be able to convince someone that you actually did travel 'instantaneously', but, now that you are in Earth's Frame, how are you ever going to convince anyone that the "Proper Distance" the Sun is or should be, is Zero distance???]

__________________
RussT
________________________________
Everything is, as it should be, otherwise, it wouldn't be!

I am going to respond to yours George, because it has some very key points and is well thought out.

And yes, this has been a wild ride, and if I were not very careful, this could have been closed...

I suspect that Spaceman Spiff introduced something that may have much to do with the more Newtonian camp view -- the Hubble flow.

I don't think the Hubble flow has anything what-so-ever to do with this discussion, as we are talking Local neighborhood of M31 inward to Earth.

But, yes, I covered...and from the perspective of almost any other being that might be on any other planet
Well, from their Planet rest frame, yes, and we them.

Yes, there is no doubt about this!!! And I have never said differently.

KenG keeps on saying things like...."If Jeff had said..."...so I will do the same here...

If kenG had said..."well that's all well and good for the Local stuff, BUT if you really want to know the difference between 'distance then VS 'distance now' as the expansion of the Universe comes into play, here is some important info"

Then we wouldn't be having this conversation, and now that he is stuck trying to defend...There's no such thing as the distance the light travels. There isn't even such a thing as the time the light travels

He is pulling every trick in the book and even making up new Relativity!!!

__________________
RussT
________________________________
Everything is, as it should be, otherwise, it wouldn't be!

That is exactly what I did claim, because Relativity did not and cannot determine distances in the Local neighborhood!
But why don't you respond to my posts instead of trying to work jeff and I against each other?

Those are both very good Posts and speak for themselves.

I even suggested that KenG could use an observer 'right next to the sun' travel at "c" toward us for his emission/absorption at Zero, and then show us how Relativity derives the 93 million miles...he passed on that.

I have agreed that that is what Relativity says, but that it is irrelevent in the Local neighborhood.
Now, that is a whole different kettle of worms... you will have to look at some way earlier posts, where I got warned to get a glimmer of that.

Knows that Relativity "Says"!!! It's a "Theory"...hence KenG's...the distance depends on the path that connects two events, but a path that connects them in such a way that they are simultaneous is the "proper distance" between them, if such a path exists in post # 219

Bingo and Bingo


Again...Bingo.

__________________
RussT
________________________________
Everything is, as it should be, otherwise, it wouldn't be!