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View Full Version : Straight forward question about measurement and true distance

tommac
2010-Jun-27, 05:16 PM
It seems from a bunch of posts that measurement of distances on a large scale is very difficult and somewhat pointless.

It seems that the more relevant questions is the time it would take light to travel to a distant point in space. I realize that we have the light year but does the light year take into account the expansion of the universe and co-moving coordinates?

Like if a galaxy is 10,000,000 light years from would we expect light to be able to get there in 10,000,000 light years OR would it be at a random point in co-moving space in 10,000,000 years?

loglo
2010-Jun-27, 05:43 PM
Its worse than that. Your galaxy isn't even in the same place where you think you see it. It certainly won't be in the same place when you get there, indeed accelerated expansion may not even allow light to reach there at all.

A light year is not a concept from GR, it is just a unit of measurement.

astromark
2010-Jun-27, 08:12 PM
I like the 'Loglo' view., and want to add. Relativity. We use the ly as a tool for understanding the relative distances across space.

Its a good tool for understanding distances. Nothing is stationary so every thing is not where it was when its light began its path to your eye...

Nereid
2010-Jun-27, 08:56 PM
It seems from a bunch of posts that measurement of distances on a large scale is very difficult and somewhat pointless.
May I ask how you came to that conclusion?

FWIW, it strikes me that perhaps you did not understand what you read; certainly, there are interesting nuances wrt 'distance' in an isotropic, homogeneous universe in which GR rules, but "somewhat pointless"?!?

It seems that the more relevant questions is the time it would take light to travel to a distant point in space.
Given that there is no absolute frame of reference, "a distant point in space" is somewhat ambiguous, isn't it.

On top of that, "the time it would take" is, as I'm sure you are very well aware by now, observer dependent (in general).

I realize that we have the light year but does the light year take into account the expansion of the universe and co-moving coordinates?

Is the answer satisfactory to you?

Like if a galaxy is 10,000,000 light years from would we expect light to be able to get there in 10,000,000 light years OR would it be at a random point in co-moving space in 10,000,000 years?
Ditto.

WayneFrancis
2010-Jun-28, 02:06 AM
It seems from a bunch of posts that measurement of distances on a large scale is very difficult and somewhat pointless.

pointless? To who?

It seems that the more relevant questions is the time it would take light to travel to a distant point in space.

What do you think the light year is?
Wouldn't the question being asked dictate the answer you get? In any case light year or parsec is probably going to be the unit of measurement used at that any scale beyond our solar system.

I realize that we have the light year but does the light year take into account the expansion of the universe and co-moving coordinates?

That would be the the qualifier you put on the distance.

Like if a galaxy is 10,000,000 light years from would we expect light to be able to get there in 10,000,000 light years OR would it be at a random point in co-moving space in 10,000,000 years?

How would you describe the distance between you, in a car travelling west at 50km/hr, and another car travelling east at 50km/hr? Say the fastest you can transmit the information is 120km/hr.

The answer you get should reflect the information you want. IE if you want the actual distance if you could freeze time then you'd ask what the co-moving distance. If you wanted the apparent distance then you'd ask for that. If you want the distance the object s where at when the measurement was taken ignoring any travel after that point in time then you'd ask for that.

Short answer is scientist will take into account the expansion as part of their equations to give them the answer they are looking for. Now when they say something like "object x is n light years away" they generally just mean the observed distance which is the distance the light has travelled to get to us not the co-moving distance or the distance the object was when it emitted the light. That isn't a fault of the system it is just the least confusing distance to tell the general public.

tommac
2010-Jun-28, 06:35 PM
May I ask how you came to that conclusion?

FWIW, it strikes me that perhaps you did not understand what you read; certainly, there are interesting nuances wrt 'distance' in an isotropic, homogeneous universe in which GR rules, but "somewhat pointless"?!?

Given that there is no absolute frame of reference, "a distant point in space" is somewhat ambiguous, isn't it..

Sorry ... I guess I am confused why you see offended by the words "somewhat pointless" but then seem to call the same situation "somewhat ambiguous".

The point I was trying to make is what good is saying that something is a light year away. What benifit can we glean from that? Would we be able to point a space ship at that galaxy and program it before hand to get there without it correcting itself as it goes along? Isnt the light year based on our perspective from earth? Wouldnt the spaceship have a diferent measurement of a light year ( assuming its clock is not 100% in sync with ours ) ...

I guess at least we agree that the definition is somewhat in between, "somewhat ambiguous" and "somewhat pointless".

antoniseb
2010-Jun-28, 06:44 PM
Sorry ... I guess I am confused why you see offended by the words "somewhat pointless" but then seem to call the same situation "somewhat ambiguous".

... I guess at least we agree that the definition is somewhat in between, "somewhat ambiguous" and "somewhat pointless".

I don't agree. Rather than wave around vague terms, why not specify what the uncertainty seems to be, and the source of it. To go to your example, 10 million light years is fairly small in the cosmic sense, and so the corrections you'd have to make to account for expansion amount to less than one tenth of one percent. Likewise the distance that the target galaxy would have traveled in the 10 million years is small compared to 10 million light years... and all these things are calculable ahead of time, so you could target your photon pretty accurately.

Nereid
2010-Jun-28, 09:04 PM
Sorry ... I guess I am confused why you see offended by the words "somewhat pointless" but then seem to call the same situation "somewhat ambiguous".
Curious rather than offended.

In any case, why not ask the specific question you're seeking an answer to, in a straight-forward way, without editorial comment?

The point I was trying to make is what good is saying that something is a light year away. What benifit can we glean from that?
That rather depends on the benefit you're seeking, doesn't it?

If you're trying to model the future trajectories of stars, within a 100 pc radius say, having good estimates of their current positions and velocities would be rather important, don't you think?

Would we be able to point a space ship at that galaxy and program it before hand to get there without it correcting itself as it goes along?
Of course, that'd be a pretty simple thing to do.

However, if you have mis-estimated the proper motion of the 10 Mly distant galaxy, you'd be in trouble. Ditto the gravitational potential along the space ship's path, etc. However, that's no different than needing to know things like winds and tides if you're sailing round the world, to rendezvous with someone on another sailing ship (say).

Isnt the light year based on our perspective from earth? Wouldnt the spaceship have a diferent measurement of a light year ( assuming its clock is not 100% in sync with ours ) ...
Perhaps.

But how is that any different from needing to know (or have confidence in) how the GPS system works, in terms of using it to estimate travel times and trajectories?

I guess at least we agree that the definition is somewhat in between, "somewhat ambiguous" and "somewhat pointless".
Sorry, no, we can't agree on that.

A well-posed question involving distance will, very likely, have a clear, unambiguous answer; a vague one will likely have an ambiguous answer.

Just like anything else in life.

tommac
2010-Jun-29, 12:47 AM
Fairly accurately with respect to what?

I don't agree. Rather than wave around vague terms, why not specify what the uncertainty seems to be, and the source of it. To go to your example, 10 million light years is fairly small in the cosmic sense, and so the corrections you'd have to make to account for expansion amount to less than one tenth of one percent. Likewise the distance that the target galaxy would have traveled in the 10 million years is small compared to 10 million light years... and all these things are calculable ahead of time, so you could target your photon pretty accurately.

tommac
2010-Jun-29, 12:49 AM
That rather depends on the benefit you're seeking, doesn't it?

So what benefit could we get out of saying something is 10,000,000 light years away?

Nereid
2010-Jun-29, 01:12 AM
That rather depends on the benefit you're seeking, doesn't it?So what benefit could we get out of saying something is 10,000,000 light years away?
It helps in developing, and testing, hypotheses concerning galaxy formation? galaxy evolution? the nature of dark energy? the value of the Hubble constant? the universality of the Tully-Fisher relationship? the environmental dependence of SFRs?

But this thread is about a question you asked, tommac, not me.

If you see no benefit in putting effort into constraining estimates of dL (that's shorthand for one distance measure, the luminosity distance), for extragalactic objects, then why did you post the OP?

OTOH, if your question was really about the possible uses (benefits) of such estimates, why not start a new Q&A thread with that as the OP?

I must say I'm having a lot of difficulty trying to work out just what the question you're trying to get some generally accepted scientific mainstream answers to is; would you care to state just what that question (or questions) is?

Jens
2010-Jun-29, 02:15 AM
So what benefit could we get out of saying something is 10,000,000 light years away?

There is an enormous benefit, from an intellectual sense. There may not be any practical benefit. But surely you are interested in whether things in the universe are spaced evenly, or whether the space grows wider as they get further away, and things like that. It would have a bearing on the fundamental questions of cosmology.

Tensor
2010-Jun-29, 02:48 AM
So what benefit could we get out of saying something is 10,000,000 light years away?

Because is gives those who aren't professional astrophysicists or astronomers someway to grasp the distance, among other things, having to do with cosmological models? After all, those who actually work in the field use the z factor, not light years, when determining distance. This is primarily due to z being fairly dependent on the cosmological parameters one choses to model the universe with.

What usually happens is that some astrophysicist or article mentions that some galaxy or quasar is 10,000,000 light years away. What they don't usually tell you, is that the particular galaxy or quasar has a red shift of some z value, and given some assumptions, that z value equals the stated distance. What they additionally don't tell you is that this particular z value is only good if you use the cosmological parameters that are given in the actual peer reviewed paper. This usually involves assuming some form of the lambda cold dark matter universe, a density factor, a hubble parameter, etc. If you change any of those parameters, then the particular z value, along with the distance mentioned in the paper, changes.

This is one of those situations where most people (and I don't mean you here, as you are asking a question about it, you obviously know that something isn't right) don't know enough (in this case, determining distance with light-years) to know that what is being said isn't quite right (10,000,000 light year distance). They certainly don't realize they may have to know a whole lot more to actually understand the technical distinctions that are used to get that distance. In this case, how the z factor is related to the distance and how the z factor is both determined by and dependent on the cosmological parameters of the universe you are modeling.

WayneFrancis
2010-Jun-29, 04:12 AM
Seems you are fishing for an answer to a vague question that you can use in some other post to state that people agree with you tommac. This is why so many of us tread lightly and get very detailed with our answers, trying to remove all ambiguity, around your questions.

As suggested perhaps you can tighten up your question but as it stands the answer is "No, it is not pointless." Do you really think that scientist don't consider all these factors if their fundamental question would be effected by the questions? Its a bit like asking a maths professor if when then solved the problem 11 + 9 if they remembered to carry the 1. This is what peer review is all about. If there was a paper that made conclusions that would be effected by cosmological expansion and they didn't factor that in then I'd be very surprised if it got past the editor let alone the multiple reviewers without the paper being kicked back because of said oversight.

I've got a friend that is constantly convinced that scientists don't factor in everything they need when coming up with answers. He literally thinks that most theories are hanging by a shoe string that most 9th graders could cut because something simple is being ignored that would cause the whole house of cards to come tumbling down.

Every once an awhile something big comes along that changes our view but for the most part it doesn't completely invalidate the last century of observations and theories.

antoniseb
2010-Jun-29, 08:18 AM
Fairly accurately with respect to what?

In your OP you talked about ending up in some Random point in space. I said that even not taking the (easily calculated) expansion of space into account, or proper motions, you would end up within a tenth of a percent of your intended place. And as Nereid emphasized, if you have the technology for a near-light-speed ship you probably have the ability to measure proper motions, and do MUCH better than that. Also, please note that your original request stipulated no course-correction. What a strange-piece-of-rhetoric reason to declare that measuring distances is pointless!

StupendousMan
2010-Jun-29, 01:20 PM
After all, those who actually work in the field use the z factor, not light years, when determining distance.

Correct.

This is primarily due to z being fairly dependent on the cosmological parameters one choses to model the universe with.

Ooops. You must have left the "in" off the front of "dependent" there. The reason we use "z"
is because it is based on direct measurements. A spectrum of the galaxy shows the
hydrogen alpha line at 7000 Angstroms, so its redshift is

z = (7000 A) / (6563 A) - 1 = 0.067

This does NOT depend on one's choice of cosmological parameters.

Converting "z" to a distance DOES depend on the cosmological parameters.

Tensor
2010-Jun-29, 01:58 PM
Ooops. You must have left the "in" off the front of "dependent" there. The reason we use "z"
is because it is based on direct measurements. A spectrum of the galaxy shows the
hydrogen alpha line at 7000 Angstroms, so its redshift is

z = (7000 A) / (6563 A) - 1 = 0.067

This does NOT depend on one's choice of cosmological parameters.

You are, of course, right. I've got to remember to stop posting after 10pm, after taking my meds.

Converting "z" to a distance DOES depend on the cosmological parameters.

This was what I was trying to get across and, of course, with the meds, I screwed it up a bit.

WayneFrancis
2010-Jun-29, 02:09 PM
I got what you meant Tensor but thanks for the clarification StupendousMan...more accurate descriptions are always welcome.

antoniseb
2010-Jun-30, 11:37 AM
One other note, as far as astrometrics go, if you have a baseline for parallax studies of even just 1 parsec, you can with equipment available today, measure the apparent distances to objects in galaxies a mere 10 million light years away to great precision with direct trigonometry.

tommac
2010-Jul-01, 03:48 AM
There is an enormous benefit, from an intellectual sense. There may not be any practical benefit. But surely you are interested in whether things in the universe are spaced evenly, or whether the space grows wider as they get further away, and things like that. It would have a bearing on the fundamental questions of cosmology.

Yes but can all of that be answered using light-year measurements???

tommac
2010-Jul-01, 03:52 AM
Thanks ... this clarifies things quite a bit!

Because is gives those who aren't professional astrophysicists or astronomers someway to grasp the distance, among other things, having to do with cosmological models? After all, those who actually work in the field use the z factor, not light years, when determining distance. This is primarily due to z being fairly dependent on the cosmological parameters one choses to model the universe with.

What usually happens is that some astrophysicist or article mentions that some galaxy or quasar is 10,000,000 light years away. What they don't usually tell you, is that the particular galaxy or quasar has a red shift of some z value, and given some assumptions, that z value equals the stated distance. What they additionally don't tell you is that this particular z value is only good if you use the cosmological parameters that are given in the actual peer reviewed paper. This usually involves assuming some form of the lambda cold dark matter universe, a density factor, a hubble parameter, etc. If you change any of those parameters, then the particular z value, along with the distance mentioned in the paper, changes.

This is one of those situations where most people (and I don't mean you here, as you are asking a question about it, you obviously know that something isn't right) don't know enough (in this case, determining distance with light-years) to know that what is being said isn't quite right (10,000,000 light year distance). They certainly don't realize they may have to know a whole lot more to actually understand the technical distinctions that are used to get that distance. In this case, how the z factor is related to the distance and how the z factor is both determined by and dependent on the cosmological parameters of the universe you are modeling.

Nereid
2010-Jul-01, 07:29 AM
There is an enormous benefit, from an intellectual sense. There may not be any practical benefit. But surely you are interested in whether things in the universe are spaced evenly, or whether the space grows wider as they get further away, and things like that. It would have a bearing on the fundamental questions of cosmology.Yes but can all of that be answered using light-year measurements???
Assuming "that" refers to fundamental questions of cosmology, then no, you need a great deal more than merely estimated distances (whether expressed in light-years, feet, mega-parsecs, or furlongs).

For example, observations of the cosmic microwave background's dipole, and its angular power spectrum.

For example, estimates of the two-point correlation in the distribution of galaxies.

For example, the estimated primordial abundance of the elements, and their isotopes.

For example, estimates of the direction of rotation of spiral galaxies.

For example, estimates of the temperature of the CMB as a function of redshift.

And much, much more.

Tensor
2010-Jul-01, 12:51 PM
Thanks ... this clarifies things quite a bit!

Please note the corrections that StupendousMan made. That the z value is independent of the model used, as it is a direct measure. However, converting the z value to a particular distance is dependent on the model, and the assumptions used.

Nereid
2010-Jul-01, 01:24 PM
Thanks ... this clarifies things quite a bit!
One more thing: you can do an awful of cosmology using only the observed redshift.

A very great deal of effort has gone in to trying to estimate the value of the Hubble constant, i.e. a key term in all cosmological models, relating the observed redshifts to distances. Much of that effort has been devoted to nailing down the distance ladder; however, there is at least one method of estimating H0 that is direct (no intermediate distance steps). Also, while most methods seek to estimate the luminosity distance, there is at least one which sets out to estimate the angular diameter distance.

StupendousMan
2010-Jul-01, 02:05 PM
Much of that effort has been devoted to nailing down the distance ladder; however, there is at least one method of estimating H0 that is direct (no intermediate distance steps).

A direct measurement of H0 with no intermediate steps? Sounds interesting -- can you provide any details?

Nereid
2010-Jul-01, 02:36 PM
A direct measurement of H0 with no intermediate steps? Sounds interesting -- can you provide any details?
Sure. But first to emphasis that the "no intermediate steps" refers to distance ladder steps, not that there aren't other, non-distance ladder, steps (there are)!

Kochanek and Schechter's (http://nedwww.ipac.caltech.edu/level5/March04/Kochanek/frames.html) 2004 symposium presentation "THE HUBBLE CONSTANT FROM GRAVITATIONAL LENS TIME DELAYS" is a good place to start.

The physics of time delays is well understood, and the only important variable for interpreting the time delays to determine H0 is the mean surface mass density <*> (in units of the critical density for gravitational lensing) of the lens galaxy at the radius of the lensed images.

The 49 references to it, in ADS (http://cdsads.u-strasbg.fr/cgi-bin/nph-ref_query?bibcode=2004mmu..symp..117K&amp;refs=CITATIO NS&amp;db_key=AST), are a good place to go next.

* the kappa symbol should go here