How big is our Galaxy.

Galaxies in the universe can be visualized as frisbees moving randomly through space at an average distance apart of around 10 to 20 feet.--Percy

Since the furthest objects we can see are around 13 billion light years away, the diameter of the observable universe is around 26 billion light years. It gets at least 2 light years larger every year, more if we improve our observational capabilities.--Percy

Oh, I see what you're getting at. The size of the observable universe *is* about 26 billion light years. The 78 billion light year figure is for the current size of what can now see of the universe, which is much larger since it has continued to expand through the past 13.7 billion years (the estimated age of the universe). Of course, the current universe isn't observable.--PercyThis message has been edited by Percy, 01-14-2006 09:26 AM

I'm not so sure about this. I think our observable horizon is larger than 13.7 billion light years, and that the reason we won't be able to see anything further away, no matter how good our observational equipment becomes, is because that would predate the big bang. It would be a remarkable coincidence if right now just happened to be the point in the universe's lifetime where the expansion constant happened to place the limit of observability precisely at the big bang.I'd have to look up how far away the furthest observed object actually is, but I know we're approaching that period a couple hundred million years after the big bang when the universe first became transparent to electromagnetic radiation.--Percy

Hi Iblis,I think you may have misinterpreted what I was saying. I agree. My previous reply to Son Goku said pretty much the same thing when I said that the diameter of the observable universe increases annually by 2 light years. I don't agree with this if you're saying that objects at the limit of observability a billion years ago are no longer visible to us today. It's easy to see why this couldn't be so. A billion years ago we would have been able to see objects that were 12.5 billion years old. Now it's a billion years later, and we can still see these objects that are now 13.5 billion years old. Of course. I think when I said "limit of observability" it was confusing. I should have said "theoretical limit of observability". What I meant was that the expansion rate of the universe has not carried objects older than 13.7 billion years out of our theoretical observability horizon. The reason we can't see anything older than 13.7 billion years isn't because of the observability horizon, but simply because nothing existed before that time.--Percy

I think what you're calling the cosmological horizon is not the same thing as what I called the theoretical limit of observability, or the theoretical observability horizon. This is the point at which the cosmological constant causes expansion at a rate too large for light to overcome. What I was trying to say was that I don't think that point corresponds to the Big Bang.--Percy

I think we may be talking about two different things. I think you're saying that an object at the theoretical observability horizon a billion years ago would have passed over that horizon in the intervening time. I agree.Where we don't agree is that the big bang represents the theoretical observability horizon. I don't think it does. It would be a remarkable coincidence if we lived just at the point in time when the cosmological expansion rate had caused the theoretical observability horizon to correspond exactly to the the age of the universe.--Percy

You mentioned aging, so I just wanted to add a little about clocks in distant galaxies. These clocks move at the same rate as our own. That's because relativistic effects only apply to objects in motion relative to each other. These distant galaxies have only a small motion relative to ourselves, and so we observe only very tiny relativistic effects. This is because recession due to the expansion of space is not included when measuring relative motion.--Percy

Relativistic effects are only caused by relative motion within space. That's why the expansion of space has no relativistic effect. It's why when observing distant galaxies the period of objects like pulsars and Cepheid variables does not have to be adjusted for recession velocity, otherwise Hubble would not have been able to calculate his constant. Red shifting can occur because of relative motion, or it can occur because of the expansion of space. The reason why distant galaxies do not have observable time dilation, the reason why their clocks beat at roughly the same rate as our own, is because their recession is due to the expansion of space, and not due to actual relativistic recession velocities.Though there's no such thing as a fixed reference frame, this analogy still might help. Think of space like a rubber band that is being stretched longer and longer. Imagine two ants running away from each other at a speed relative to the rubber band of 1 inch/second, which means they are receding from each other at a rate of 2 inches/seconds. But as the rubber band is stretched longer and longer and they become further apart their recession velocity becomes greater and greater. However, their recession velocity measured against the rubber band is still 2 inches/second.It's the same way with distant galaxies. Though they might be receding from us at a great apparent velocity, their actual relative velocity to us as measured against some common reference frame is actually quite small, maybe around 50 km/sec, and is not necessarily away from us but in any direction.--PercyThis message has been edited by Percy, 01-15-2006 11:09 AM

Well, if I've arrived at a misconception it won't be the first time. My source is Sylas, but I could have misinterpreted him or he could be wrong. My reading of my exchange with Sylas about this (see What's the Fabric of space made out of? starting at Message 211) is that the time dilation effect you're observing for distant galaxies is actually due to simple recession due to expansion of space, and not to relativistic effects. See the first paragraph of Sylas's post in Message 226.--PercyThis message has been edited by Percy, 01-15-2006 11:37 AM

Yes, I referred you to paragraph one of that post in my previous reply. Is that the part you're looking at? If so, I think we have a different interpretation of it.--Percy

Might it be a matter of perspective and approach whether one prefers to draw distinctions or not? Sylas was clearly drawing a distinction between relativistic versus "just getting further away" effects. In other words, he was identifying and distinguishing between the different contributions. I can tell that you prefer including them all in a single category, i.e., just measure the recession velocity and be done with it. This has obvious advantages, but understanding the nature of the fabric of space and what the effects of its expansion are is not one of them.One of the interesting questions that came up in that earlier thread, and that Sylas was unfortunately no longer around to address, was what would we would observe for a distant galaxy that was stationary relative to us. Such a galaxy would be moving at tremendous speeds relative to its neighbors, but relative to us it would be stationary. However, relative to us in space (and this was where Sylas's comment about long distances rendering these things questionable has me still scratching my head) it is moving toward us at a tremendous velocity. We would see no red shift because the expansion of intervening space cancels out the compression of wavelengths, but the relativistic effects from our tremendous relative velocities should cause its clocks to appear to be moving very slowly.I have a feeling I'm about to receive another yuk-yuk smilie, but I'll click the submit button anyway.--Percy

Agreed. You didn't quote what you're replying to, but if it's my example of the distant galaxy that appears stationary to us, I was only talking about observations from our own perspective. I can see you and Cavediver agree about relativistic and Doppler effects being the same, but there still seems to me an important distinction is being missed. For example, what do we observe with that distant stationary-relative-to-us galaxy I described, and why?--Percy

Okay, but now let's apply a gradualistic argument similar to yours for the atomic clock. Someone is approaching me at high speed from the other side of the room, so we know we're in the same inertial frame. What I observe of him has both a relativistic and a Doppler contribution, and I can clearly make measurements. Now what if he's approaching me from the moon. Same thing, right? Now from Pluto. Same thing, right? Now from Alpha Centauri? Same thing, right? Now from Andromeda? Same thing, right? But as the distance increases, somewhere along the line you're going to claim we're no longer in the same inertial frame. I know we can't fix an exact distance, but can you explain how you know a distance is too far to make inertial frame determinations? And even if we can't do this for great distances for practical reasons, why can't we still do it theoretically for thought experiments?I can tell that this is what Silas was alluding to, and I'm guessing he chose not to address the specifics of my surmise because of the uncertainty.--Percy

Though I'm replying to your message, Iblis, this is actually a reply to both you and Cavediver.I guess my goal is to understand the general principals, and to be able to have a small library in my mind of "when this, then that" situations. Those with a more informed understanding can look at the layperson approximations and say that they're wrong or inaccurate or misleading or whatever, but that gives the layperson nothing to understand.So while SR is an approximation, it is as far as most laypeople are going to get, but maybe I can go part of the way. Can someone explain why we need to involve GR in a thought experiment not involving mass or acceleration?About my galaxy example, some of my readings have taken care to consider separately the relativistic and Doppler effects. Can someone explain why this isn't correct?--Percy