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Bill S. Offline OP
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How old is the Universe? That seems like an easy question to answer. It’s 13.7 billion years, or close to.

Where do we measure that time from? That’s another easy one: “now”.

Relativity tells us there is no universal now, all we can identify is the spacetime event that is “here and now”. So, what do we mean when we say that we measure back from now? Any reference frame that is in motion relative to ours will identify a different here and now. Every part of the Universe is in motion relative to every other part, and the faster that relative motion, the greater will be the discrepancy between the measurement of here and now with respect to each part.

We are assured that the more distant (from us) parts of the Universe are moving away from us at speeds faster than c. What can we say about the measurement of here and now in those areas? All these areas are parts of the Universe. If we can’t identify a “present” for them, how can we say how their “present” relates to the start of the Universe from their perspective? OK, the Big Bang may not be the start of everything, but that’s not the issue here.

We could assume that any measurement taken from any part of the Universe would show that the Big Bang occurred at 13.7 billion years before that local “present”, but what does that actually mean? Is our measurement of the age of the Universe just a feature of our particular location in spacetime?

Co-moving coordinates and cosmological time may be considered as “answers” to this, but they are only arbitrary coordinates, set to “work” in the 4+1 dimensional universe which we observe.


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Ok, that one (or those?) may be a little bit tricky. Orac may have a better take on the answers. But let me make a start at it.

Let me start with the question of the parts of the universe that are moving away from us at greater than the speed of light. This is kind of difficult, but they aren't really moving away from us. They are separating from us. That looks like just saying the same thing in different ways, but it isn't really. What is happening is that space is stretching. The distant galaxies aren't really moving at greater than the speed of light. I have thought about that and thinking in a science fictional way I have come up with a sort of a thought experiment. Assume we have some kind of space travel that will allow us to travel any place in the universe instantly. So let us measure the average speed of all the near by galaxies with respect to our galaxy. I'm not sure what the value would be, but the average would be relatively low. It wouldn't be high enough to cause any huge variation due to time dilation. Now let us turn on our transport and travel 14 billion light years. Remember that this is that science fictional transport system that is "instantaneous". One thing we take with us is our proper motion with respect to all the galaxies in this neighborhood. That is we are still moving at the same speed we were moving, with respect to our neighboring galaxies here. Now let's measure our speed with respect to near by galaxies in the location we have moved to. If I understand it correctly then we will find that our proper motion with respect to all our new neighbors will be at least in the same order of magnitude as with our old neighbors. That's because they aren't really moving away from us. They are just getting farther away because there is more space between here and there.

I hope you can understand what I am saying there. The bottom line is that at no time have the different parts of the universe actually moved at close to the speed of light with respect to each other. So that clocks all over the universe are keeping approximately the same time.

I recognize that this is one of those weird concepts that come with modern physics. Some of it I am afraid we just have to accept on faith if we don't want to go back to college and enter an advanced physics program.

Bill Gill


C is not the speed of light in a vacuum.
C is the universal speed limit.
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Bill S. Offline OP
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In the OP, the FTL bit was little more than window dressing. I wondered if it was a mistake including it, but I’m glad I did as your response has raised some interesting issues.

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…..they aren't really moving away from us. They are separating from us. That looks like just saying the same thing in different ways, but it isn't really.


I have to disagree with that, Bill; the galaxies (or rather the galaxy groups) are not moving relative to space, but they are moving relative to one another. Consider the balloon analogy: the spots are not moving relative to the surface, but each spot is moving relative to every other spot. Surely if there is more distance between any two galaxy groups today than there was yesterday they are moving apart, whatever the mechanism.


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Originally Posted By: Bill S.
Surely if there is more distance between any two galaxy groups today than there was yesterday they are moving apart, whatever the mechanism.

That is what I was trying to differentiate between; moving and separating. It is one of those weird physics things. I don't think there are really any good analogies. Going back to the red shift thing with distant galaxies. Typically a shift in wave length is associated with the Doppler effect. But the red shift for distant galaxies isn't really a Doppler effect. It is because the space between has been stretched and the light waves which are 'anchored' to space have been stretched with it. As I understand it if you can do the math you can figure it all out using GR. I can't and am not ready to go back to school to work on an advanced degree just so I can. So I think as far as I am concerned I will just accept the word of the people who do understand the math that it works that way.

Sorry I can't be more help.

Bill Gill


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Quote:
So let us measure the average speed of all the near by galaxies with respect to our galaxy. I'm not sure what the value would be, but the average would be relatively low. It wouldn't be high enough to cause any huge variation due to time dilation.


Quote:
Now let's measure our speed with respect to near by galaxies in the location we have moved to. If I understand it correctly then we will find that our proper motion with respect to all our new neighbors will be at least in the same order of magnitude as with our old neighbors.


As I see it, the reason we would be measuring our speed relative to our neighbours as the same in both places is that the location we left and the destination location would both be travelling at approximately the same speed relative to their neighbours. In the case of an expanding universe, this would apply whether the mechanism involved galaxy groups being transported by expanding space, or travelling through space.

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Sorry I can't be more help.


This is exactly the help I need - something to make me think.


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Originally Posted By: Bill S.
As I see it, the reason we would be measuring our speed relative to our neighbours as the same in both places is that the location we left and the destination location would both be travelling at approximately the same speed relative to their neighbours. In the case of an expanding universe, this would apply whether the mechanism involved galaxy groups being transported by expanding space, or travelling through space.

The thing about this is that just moving from here to there wouldn't change our proper motion. We would still have the same proper motion with respect to our neighborhood. And our proper motion with respect to the new neighborhood would be roughly the same. That is both places have roughly the same proper motion.

Bill Gill


C is not the speed of light in a vacuum.
C is the universal speed limit.
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I’m not sure how helpful the introduction of proper motion is. Proper motion, if I understand it correctly, relates to the change in position of celestial bodies due to their motion through space, as that would be observed from the centre of the solar system. Obviously that is quite different from changes in relative positions due to the expansion of the Universe.

When you say: “just moving from here to there wouldn't change our proper motion”, what are you actually referring to?


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What I am trying to say is that if we measure the average movement of all the nearby galaxies we will find that we are moving with respect to that average. One more addition to our thought experiment. Let's define a mythical North for the universe. Now assume that we are moving NNE at 100 Km/hr with respect to that average. That is what I am referring to as our proper motion. Now we make the big jump. When we arrive at our new location we will still be moving NNE at 100 Km/hr. We will in fact be moving parallel to the direction we were moving before the jump. That is why I say our proper motion with respect to our original location will be the same.

What I am saying is that if we then measure the average movement of all the galaxies in our new location we will find that our proper motion with respect to that average will be in the ball park of our original average motion.

The thing I am trying to get across is that just because space is stretching between here and there doesn't mean that things are moving faster there than they are here.

Bill Gill


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I think you may be both correct and perhaps there is a bit of confusion because of not well defined term us. I am a little confused by both arguments as we seem to have several concepts jumbled up together so try parting them .

The first concept is physics homogeneity. That basically says

1.) No point is space is special the laws of physics are the same everywhere
2.) No point in time is special, so the same basic laws of physics should govern all of time.
3.) No direction in space or time is special moving in any direction the physics laws remain the same.

So what do you both think of the concept of physics homogenity?

The second concept is universe expansion and it being uniform and homogenious. So if the universe is expanding like a ballon not uniform would be the idea on part of the ballon has a weak spot and that section may expand faster than another so you get a ballon with bubble spots


I think you need to work out we are not mixing those two ideas in before discussing motion?

If you are both happy to have homogeneity in both of those two concepts you can move on to discussing motion. If we don't have that agreement it gets more complex to discuss motion.

Last edited by Orac; 04/17/15 03:03 AM.

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If you both are happy that we have homogenity of both we then face the issue of perception when dealing with motion.

The problem of perception is easiest to show even using classic physics with an example we have done before.

Assume the world was suddenly frictionless and perfectly round and I bowl a ball. I describe the ball as moving away to me with some speed (and with a speed gun I could even determine it's speed) away from me. Sometime later it smacks me in the backside smile

All motion direction is only perception to what you know!

Can I suggest a perhaps more robust way to discuss it which is via energy or work done from a reference point. That is fairly robust and holds up usually.

When you sit on your chair you have a lot of motions going on but you say you are not moving because you are exerting no energy. When you move the reference to a point in space you suddenly realize your not moving state is incorrect and you equate a proper energy to yourself relative to some motion to some point in space.

I think if you try the discussion in that manner it may make more sense.

Last edited by Orac; 04/17/15 03:36 AM.

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Let’s try getting rid of the instantaneous jump, lest some scientist accuse us of using a thought experiment that is invalid because it breaks the laws of physics. Could we achieve the same if we specified that we were leaving Earth, using our warp drive and arriving at another planet, 1,000 ly away an hour later.

Are you saying that in our new location our proper motion (as per your definition) would be “in the ball park of our original average motion”. If you are, I think I can accept that. All we need to know now is whether Orac would agree with that, and we could have taken a step away from confusion. smile


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Quote:
I think you may be both correct


I like that idea.

Quote:
So what do you both think of the concept of physics homogenity?


The concept makes sense to me. My only provisos would be:
1. We can apply the concept with reasonable certainty only to that part of the cosmos (or Universe) of which we have direct knowledge. Beyond that would be speculation.
2. I would have some reservations about applying “any direction” to time.

I don’t think either of those would seriously impinge on the present discussion.

(Not finished, but out of time)


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As far as the homogeneity of expansion goes, I’m not aware of any evidence that there is inhomogeneity that might be observably; so I’m happy to go with that, “locally” at any rate. I add that because I am aware that it has been seriously suggested, as a possible cause of the accelerated expansion, that our observable Universe could be a relatively low density “bubble” in a higher density universe. Again, I think that’s probably tangential to the present discussion.

What does seem particularly relevant is the relativity of simultaneity, as this seems to insist that we cannot identify an absolute “now” throughout the Universe. Is that something on which we could all agree?


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Originally Posted By: Bill S.
Let’s try getting rid of the instantaneous jump, lest some scientist accuse us of using a thought experiment that is invalid because it breaks the laws of physics. Could we achieve the same if we specified that we were leaving Earth, using our warp drive and arriving at another planet, 1,000 ly away an hour later.

But that is the thing about thought experiments. We can ignore the laws of physics. And 1,000 ly is not enough to make a serious demonstration of what I am talking about. That is barely moving in terms of the things you were talking about in your OP.

Originally Posted By: Bill S.
Are you saying that in our new location our proper motion (as per your definition) would be “in the ball park of our original average motion”. If you are, I think I can accept that. All we need to know now is whether Orac would agree with that, and we could have taken a step away from confusion. smile

In my example I am talking about 2 different situations. One is our proper motion with respect to the average motion of our neighboring galaxies. Call that PM1 The other is our proper motion with respect to the average motion of the galaxies in our new neighborhood. Call that PM2 In my experiment PM1 would not change. We would be offset from the our old location, but we would still be moving in the same direction at the same speed. In my last example I used the mythical universal North and assumed we were moving NE at some given speed. In our new location we would still have proper motion PM1 moving NE at the same speed. PM2 would be different, but it should be in the same ballpark. That is it won't be a large fraction of the speed of light. The thing is that from here we would see a large red shift in the light from those galaxies, indicating that they are moving away from us at high speed, but when we got there we would see that they aren't moving at that speed. That is because they aren't moving, space is expanding.

And Orac is quite right about the homogeneity of physics. That is one of the basic assumptions in almost all of physics. People are still testing to see if they can find any variations, but so far nobody has come up with any. I believe that there have been a few experiments that seemed to indicate some slight possibility of variations, but nothing that has achieved any serious level of probability. I don't even think about it when I start talking physics. It is just one of those things that I take for granted.

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Originally Posted By: Bill S.
Are you saying that in our new location our proper motion (as per your definition) would be “in the ball park of our original average motion”. If you are, I think I can accept that. All we need to know now is whether Orac would agree with that, and we could have taken a step away from confusion. smile

I have no issue so long as we note we have made assumptions.


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Originally Posted By: Bill S.
As far as the homogeneity of expansion goes, I’m not aware of any evidence that there is inhomogeneity that might be observably; so I’m happy to go with that, “locally” at any rate. I add that because I am aware that it has been seriously suggested, as a possible cause of the accelerated expansion, that our observable Universe could be a relatively low density “bubble” in a higher density universe. Again, I think that’s probably tangential to the present discussion.

Absolutely correct Bill S but it does muddy the waters if anyone believes that.

Originally Posted By: Bill S.
What does seem particularly relevant is the relativity of simultaneity, as this seems to insist that we cannot identify an absolute “now” throughout the Universe. Is that something on which we could all agree?

I am going to sit on fence on that. You may be able to create one or not it depends on unknown physics answers. QM entanglement is measured to propogate at least 10,000 times faster than light and in theory it is instantaneous over any distance. The idea has been used to set time records here on Earth and there is a project to deploy a space based version. If theory holds and it is instantaneous and over any distance then you can have a global "now" clock.


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Originally Posted By: Bill
But that is the thing about thought experiments. We can ignore the laws of physics. And 1,000 ly is not enough to make a serious demonstration of what I am talking about. That is barely moving in terms of the things you were talking about in your OP.

Totally Agree and as you would need to travel at a giddy 99.9999999999999999998% of c to catch the event horizon of the universe expansion.

The background of UDFy-38135539 (http://en.wikipedia.org/wiki/UDFy-38135539) shows you the pure size and scale you are really working with.

It also introduces the new term I would suggest you both start using Comoving distance (http://en.wikipedia.org/wiki/Comoving_distance)

Originally Posted By: Bill
The thing is that from here we would see a large red shift in the light from those galaxies, indicating that they are moving away from us at high speed, but when we got there we would see that they aren't moving at that speed. That is because they aren't moving, space is expanding.

Close they may be actually moving but the expansion movement would be greater (no-one has seen a local ref movement at near the speed of light). The composite two movements make up the comoving distance and speed you first noted before you did your warp jump.

Originally Posted By: Bill
I don't even think about it when I start talking physics. It is just one of those things that I take for granted.

We all sort of did that until the universe started throwing up lots of strange things. I like you believe it is uniform but I am just a little short on data to be able prove it smile


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Originally Posted By: Orac
I am going to sit on fence on that. You may be able to create one or not it depends on unknown physics answers. QM entanglement is measured to propogate at least 10,000 times faster than light and in theory it is instantaneous over any distance. The idea has been used to set time records here on Earth and there is a project to deploy a space based version. If theory holds and it is instantaneous and over any distance then you can have a global "now" clock.


That's a very good point, but in the OP I was looking at the situation from the viewpoint of relativity rather than QM. You may recall that I have some unconventional ideas where entanglement is concerned. smile

Would you still be on the fence if we were working only with relativity?


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If we can't use QM and only GR it would be near impossible because of the time differentials.


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As for your entanglement beliefs Bill S try recent experiments smile

Science article:
"Experimental Proof of Nonlocal Wav...asurement"

Layman version:
http://www.engadget.com/2015/03/30/spooky-experiment-proves-quantum-entanglement-is-real/

Last edited by Orac; 04/20/15 08:49 AM.

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