Register | Sign In


Understanding through Discussion


EvC Forum active members: 63 (9161 total)
0 online now:
Newest Member: popoi
Post Volume: Total: 915,585 Year: 2,842/9,624 Month: 687/1,588 Week: 93/229 Day: 4/61 Hour: 0/0


Thread  Details

Email This Thread
Newer Topic | Older Topic
  
Author Topic:   How do we really know how far away stars are?
Percy
Member
Posts: 22359
From: New Hampshire
Joined: 12-23-2000
Member Rating: 4.7


Message 16 of 25 (217952)
06-18-2005 8:09 PM
Reply to: Message 5 by GDR
06-18-2005 10:40 AM


The discussion progressed before I had a chance to reply, but about this:
GDR writes:
Thanks Percy. I understand that radio waves operate differently than light waves but I used radio waves to describe what I was getting at.
I was not drawing a distinction between radio waves and light waves. Radio waves and light waves are both electromagnetic waves. It makes no difference whether you use radio or waves or light waves to ask questions about the effects of gravity on electromagnetic waves.
The problem with your example concerning a transmission from the north pole to the south pole is that the bending of radio waves that makes such a transmission possible is by the atmosphere, not by gravity. The earth's gravity does bend both light waves and radio waves, but by negligible amounts.
In Message 15 you say:
I guess it goes back to my original example. I can stand at the south pole with my direction finder and have it sense the signal coming from one direction and the turn 180 degrees and find that same signal coming from a totally different direction. If I didn't know ahead of time I would likely think that I was receiving 2 different signals.
This isn't physically impossible, but think what would have to happen. The light travels to the earth from this distant object and you see it. Some of the light travels by some other path way beyond the other side of earth and along the way is bent so as to arrive at the earth from the opposite direction. I couldn't talk to the physics specific to this situation without spending some serious time thinking through this, but this just seems incredibly unlikely. Every instance of gravitational light bending we've seen is by a few degrees at most. Keeping the light sufficiently intact to form an image through a bends of 90 to 180 degrees just doesn't seem likely.
However, this question you asked in Message 10 seems more reasonable:
I'm not the sharpest knife in the drawer and so I'm having trouble figuring that out. The light that arrives here on Earth always appears to us to have travelled in a straight line but how do we really know just how straight that line is? Wouldn't we always get the same result no matter where we are in the orbit? The light just gets here and we can only surmise the route.
In Jar's replies he talks about triangulation, which can be used for relatively close stars. We know there are no nearby invisible objects with large gravitational fields because the effects on the paths of the local community of stars would be obvious.
But I'm guessing you were thinking of more distant objects where we can't really be sure what large objects might lie nearby the light's path. But keep in mind that gravity decreases by the square of the distance. Gravity at some distance from an object is too weak to bend light appreciably. This means that any light bent by an object will appear to be coming from fairly nearby that object. If an object is by itself in a region of the sky, then we can be sure its light came to us in a relatively straight line. Besides this, gravitational lensing produces distortions (they aren't like the perfect lenses you'll find in your camera), and my guess is that there are also tell-tale relativistic effects (perhaps Sylas or Eta can chime in). This would take care of the possibility of invisible highly massive objects like black holes that are no longer drawing in material.
Bottom line: The effects of gravity on the direction of light are detectable, and so we know that gravity does not turn the universe into a funhouse of mirrors where we don't know where anything really is.
--Percy

This message is a reply to:
 Message 5 by GDR, posted 06-18-2005 10:40 AM GDR has replied

Replies to this message:
 Message 17 by GDR, posted 06-18-2005 9:19 PM Percy has not replied

  
GDR
Member
Posts: 6199
From: Sidney, BC, Canada
Joined: 05-22-2005


Message 17 of 25 (217961)
06-18-2005 9:19 PM
Reply to: Message 16 by Percy
06-18-2005 8:09 PM


percy writes:
Bottom line: The effects of gravity on the direction of light are detectable, and so we know that gravity does not turn the universe into a funhouse of mirrors where we don't know where anything really is.
Your funhouse of mirrors quip is exactly what I was trying to get at. I guess that answers the question but because we can detect the effects of gravity on light can we always measure it?
Also, do dark energy or dark matter exert an gravitational field that will affect light or do we even know the answer to that. If it does and we have no way of detecting or measuring it are we back to the funhouse of mirrors?

This message is a reply to:
 Message 16 by Percy, posted 06-18-2005 8:09 PM Percy has not replied

Replies to this message:
 Message 18 by Sylas, posted 06-18-2005 10:17 PM GDR has replied

  
Sylas
Member (Idle past 5250 days)
Posts: 766
From: Newcastle, Australia
Joined: 11-17-2002


Message 18 of 25 (217968)
06-18-2005 10:17 PM
Reply to: Message 17 by GDR
06-18-2005 9:19 PM


Your funhouse of mirrors quip is exactly what I was trying to get at. I guess that answers the question but because we can detect the effects of gravity on light can we always measure it?
There is a serious possibility of a kind of funhouse of mirrors in cosmology. I'll get to this below; it has nothing to do with gravity or curving of light.
We certainly can detect the effects of gravity on light, and we have a mathematical model which allows us to infer the curvature given a gravitational mass, or infer a mass when we are able to estimate the amount of curvature.
Also, do dark energy or dark matter exert an gravitational field that will affect light or do we even know the answer to that. If it does and we have no way of detecting or measuring it are we back to the funhouse of mirrors?
Dark matter certainly exerts a gravitational field; but dark energy is a bit different; it is a kind of cosmological anti-gravity and cannot be detected in the same way.
The whole reason for postulating dark matter is to account for gravitational fields. Put another way, dark matter refers to matter that is not luminous, but can be detected by its gravitational effects.
This includes the effects of gravity on light. In fact, there have recently been several attempts to map out dark matter in the cosmos by the amount of curvature in light. This requires a lot of data and computationally heavy analysis. Here, for example, is the APOD for 14 Aug 2003, showing a dark matter map for the giant galaxy cluster CL0025+1654; a Yale press release on"Substructure Maps Show that Dark Matter Clumps in Galaxies"; and another report from a team of French scientists who reported amap of dark matter over a comparatively large section of the sky in March 2000. This is hot research and progress in mapping dark matter is likely to continue.
But now for the weird funhouse of mirrors. It's not exactly mirrors, but it's a funhouse for sure. There is another way in which you might see the same galaxies in different directions. The universe may have an unusual topology, in which space is interconnected in unexpected ways. We use the analogy sometimes of the universe being like a giant hypersphere; but in cosmology there is serious consideration of the possibility that 3d space might be more complex, in which case there could be alternative paths through space a bit like alternative paths around the Earth, but even more complex. This is not a consequence of gravity.
There was a report in Nature magazine just two years ago on a "soccer ball universe", in which some scientists proposed a "12-sided" topology for the universe. This was indeed based on looking at different directions in the sky and looking for similarities, as would be expected if we could see the same regions from different directions. The proposal was falsified almost as soon as it was published; it seems that the topology of the universe is simple at least to the bounds of visibility. But the report does indicate that scientists are considering this seriously. For discussion and more links, go to Ned Wright's News of the Universe archive, and search down for "A twelve-sided Universe? - Probably not."
Cheers -- Sylas

This message is a reply to:
 Message 17 by GDR, posted 06-18-2005 9:19 PM GDR has replied

Replies to this message:
 Message 19 by GDR, posted 06-18-2005 10:53 PM Sylas has replied

  
GDR
Member
Posts: 6199
From: Sidney, BC, Canada
Joined: 05-22-2005


Message 19 of 25 (217970)
06-18-2005 10:53 PM
Reply to: Message 18 by Sylas
06-18-2005 10:17 PM


Thanks Sylas
I don't understand how we can tell that light is being bent when we can't see the mass that is causing the gravitational pull but I won't ask for an explanation as I'm sure I wouldn't understand that either. At any rate it seems pretty certain then that light waves can't be bent without us detecting the bend.
Sylas writes:
We use the analogy sometimes of the universe being like a giant hypersphere; but in cosmology there is serious consideration of the possibility that 3d space might be more complex, in which case there could be alternative paths through space a bit like alternative paths around the Earth, but even more complex. This is not a consequence of gravity.
Would light travel along those alternative paths? You say it is 3d space so presumably time isn't involved in this theory.
I have trouble with things like the soccer ball universe as I'm trying to get my head around the idea that the universe has no outside edge and no center. When I start looking at soccer balls I see both an outside edge and a center.
Thanks for the reply

This message is a reply to:
 Message 18 by Sylas, posted 06-18-2005 10:17 PM Sylas has replied

Replies to this message:
 Message 20 by Tranquility Base, posted 06-18-2005 11:08 PM GDR has replied
 Message 21 by Sylas, posted 06-18-2005 11:18 PM GDR has replied

  
Tranquility Base
Inactive Member


Message 20 of 25 (217973)
06-18-2005 11:08 PM
Reply to: Message 19 by GDR
06-18-2005 10:53 PM


GDR
Remember that the soccer ball (or balloon) analogy of a universe with no boundary or centre is making an analogy of the soccer ball's 2D surface with our 3D space.
The 2D surface of the soccer ball indeed has no buondary or centre. Sure it has a centre in an 'unobservable' dimension (ie the third dimension which the 2D world of the ball's surface can't directly see) but not in its 2D world.
The real truth of the no centre, no boundary 3D world is that the centre and curvature is occurring in a forth dimension (not time) and we simply can't picture it because we are 3D beings. Some GR people prefer not to talk of a centre in a forth (spatial) dimension, but others agree that the curvature is as real as that.

This message is a reply to:
 Message 19 by GDR, posted 06-18-2005 10:53 PM GDR has replied

Replies to this message:
 Message 22 by GDR, posted 06-19-2005 12:10 AM Tranquility Base has not replied

  
Sylas
Member (Idle past 5250 days)
Posts: 766
From: Newcastle, Australia
Joined: 11-17-2002


Message 21 of 25 (217974)
06-18-2005 11:18 PM
Reply to: Message 19 by GDR
06-18-2005 10:53 PM


I don't understand how we can tell that light is being bent when we can't see the mass that is causing the gravitational pull but I won't ask for an explanation as I'm sure I wouldn't understand that either. At any rate it seems pretty certain then that light waves can't be bent without us detecting the bend.
I'm not sure how it is detected either; and I'm not confident that we can always be sure of detecting when light has been curved. It's enough to take advantage of cases where it can be detected; probably by distortions of some kind in images.
Would light travel along those alternative paths? You say it is 3d space so presumably time isn't involved in this theory.
I have trouble with things like the soccer ball universe as I'm trying to get my head around the idea that the universe has no outside edge and no center. When I start looking at soccer balls I see both an outside edge and a center.
Actually time is involved as well. Some kinds of topological defect might allow a kind of time travel; weird though it is this is seriously considered as well, with an attempt to figure out what kinds of topology are consistent with what we know of the universe so far. Here is a recent report on such topics in the popular press: Wormhole 'no use' for time travel, From the BBC on May 23, 2005. Note the conflicting speculations in the report.
The problem with outside edges and centers for a soccer ball is a defect of the analogy. There is no perfect analogy here, except to say that there is no real reason to insist that space is necessarily simply connected.
To grasp the soccer ball analogy, I think you need to consider the universe as being the patches on the dodecahedron; not the whole dodecahedron. Furthermore, the universe is every patch. If you move in a straight line on a soccer ball, you will cross into another patch; but in the universe you are just moving through one patch sized universe and can get to different part of the patch by moving in different directions. This kind of connectedness extends from the 2 dimensions of the skin of a soccer ball up to the 3 spatial dimensions we are used to; but you should not think of this as embedded in a higher dimensional space like the skin of the soccer ball being embedded in 3d-space.
The simplest connected topology to imagine is a checkerboard topology, in which moving off one edge moves you back though another edge. Some computer games have this effect, in which on the 2d screen moving left off one side sees you entering again from the right. You can easily get the same effect in 3d with a cube; moving past any edge just has you smoothly coming in again from the opposite edge.
You can map a 2d checkerboard universe onto a doughnut shape, to get a finite 2d universe in which you can move indefinitely in any direction, but you just keep returning to where you started. It could be the same in our universe; though so far there's no positive evidence of it; it is only speculative.
Cheers -- Sylas

This message is a reply to:
 Message 19 by GDR, posted 06-18-2005 10:53 PM GDR has replied

Replies to this message:
 Message 23 by GDR, posted 06-19-2005 12:25 AM Sylas has replied

  
GDR
Member
Posts: 6199
From: Sidney, BC, Canada
Joined: 05-22-2005


Message 22 of 25 (217981)
06-19-2005 12:10 AM
Reply to: Message 20 by Tranquility Base
06-18-2005 11:08 PM


Thanks Tranquility Base
These concepts are also mind bending.

This message is a reply to:
 Message 20 by Tranquility Base, posted 06-18-2005 11:08 PM Tranquility Base has not replied

  
GDR
Member
Posts: 6199
From: Sidney, BC, Canada
Joined: 05-22-2005


Message 23 of 25 (217982)
06-19-2005 12:25 AM
Reply to: Message 21 by Sylas
06-18-2005 11:18 PM


That all helps. It seems that in the end its like a lot of science. They have a pretty good idea about what's going on, but in the back of their mind there is a huge lump of doubt.
Science has come so far so quickly and yet it seems that everytime we get through one door of knowledge there's more doors in front of us that we didn't even know were there.
I remember reading that back in the late 1800's students were told not to go into physics as they had already knew pretty much all there was to know.
It seems strange reading books like Brian Greene's "The Fabric of the Universe" and reading about things that are so small you can't begin to imagine them and then you read about things that are so large you can't imagine and at distances that are so large as to be meaningless. Then you put it all together and that even those huge things at unbelievable distances are still made up of those same tiny little particles that make up even my body.
It is going to be interesting to see just which way that next forward step is going to go.
Thanks again
Greg

This message is a reply to:
 Message 21 by Sylas, posted 06-18-2005 11:18 PM Sylas has replied

Replies to this message:
 Message 24 by Sylas, posted 06-19-2005 12:41 AM GDR has replied

  
Sylas
Member (Idle past 5250 days)
Posts: 766
From: Newcastle, Australia
Joined: 11-17-2002


Message 24 of 25 (217983)
06-19-2005 12:41 AM
Reply to: Message 23 by GDR
06-19-2005 12:25 AM


I agree wholeheartedly. Cosmology is exploding with discoveries at present, in part because there are so many unanswered questions combined with a flood of new data from space based observatories.
The weird speculations are an indication of how far out of the box people are willing to go; but even so, my guess is that the major breakthroughs are not going to come from wild speculations, but from some oddity that doesn't fit anything anybody has thought of so far.
The most exciting phrase to hear in science, the one that heralds new discoveries, is not 'Eureka!' but 'That's funny...'
-- Isaac Asimov
A case in point was the discovery of accelerating expansion about five years ago: this was almost totally unexpected as far as I can tell.
Two websites to keep an eye on:
  • Ned Wright's News of the Universe, with new items every couple of months or so; big news will show up here.
  • Universe Today; with daily updates on what is going on. Scanning this page will give a hint of the enormous rate of discovery at present in astronomy.
These are very exciting times for anyone interested in astronomy and cosmology. Cheers -- Sylas

This message is a reply to:
 Message 23 by GDR, posted 06-19-2005 12:25 AM GDR has replied

Replies to this message:
 Message 25 by GDR, posted 06-19-2005 12:53 AM Sylas has not replied

  
GDR
Member
Posts: 6199
From: Sidney, BC, Canada
Joined: 05-22-2005


Message 25 of 25 (217984)
06-19-2005 12:53 AM
Reply to: Message 24 by Sylas
06-19-2005 12:41 AM


Thanks Sylas. They're bookmarked.

This message is a reply to:
 Message 24 by Sylas, posted 06-19-2005 12:41 AM Sylas has not replied

  
Newer Topic | Older Topic
Jump to:


Copyright 2001-2023 by EvC Forum, All Rights Reserved

™ Version 4.2
Innovative software from Qwixotic © 2024