About the Expansion of the Universe

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A bit of a question... Is

A bit of a question...

Is our sun massive enough to appreciably slow down time?

And will this be enough to extend it's lifetime as viewed from earth?

Similarly, will time pass more quickly in the (gravitationally lower) emptiness between galaxies?

Is our view of the CMB distorted at least partly due to the changing shapes and gravitational lensing over the eons as the galaxies have developed?

(And there's still an old discussion to pick up again sometime perhaps for another thread about whether time itself is actually quantised even though in our world we may only ever be able to resolve it down to discrete quantised intervals...)

Regards,
Martin

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Mike Hewson
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RE: There's something

Message 95133 in response to message 95131

Quote:
There's something rather tantalising there methinks...


Yup the 'divide' between the character of (rest)massless particles vs others is curious allright.

Quote:
That one is "Castrovalva" although I remember it as "Recursive Occlusions" and it was very cleverly done. A key plot feature was that the diarist for the inhabitants scrupulously recorded the day's events day by day in a large diary. There was also a second diary that had been filled with the history prior to the present diary. Both diaries, old history and new, both looked by the condition of their covers and the pages to be very new...


Ah! I missed that plot feature. Now, after all these years t'is clear ..... :-)

Quote:
Is our sun massive enough to appreciably slow down time?


Certainly is. Mercury's perihelion advance ( a slow movement of the closest point of approach to the Sun, not accounted for by classical effects ) is mainly due to GR time, rather than space, 'warping'. Indeed the GPS system here on Earth would drift in accuracy by some tens of kilometers per day if GR time corrections were not applied. A good rule of thumb is that light travels at a foot per nanosecond. ( Imperial foot ). So if I'm standing 30 feet from you, I'm also 30 nanoseconds behind the play on whatever is happening at your location.

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And will this be enough to extend it's lifetime as viewed from earth?


S'pose so. Stephen Hawking said you could live longer if you traveled on planes all your life. Though you'd only get a fraction of a second more than those who stayed on the ground. But, he says, the quality of in-flight food would likely shorten your life rather more than that gain! :-)

Quote:
Similarly, will time pass more quickly in the (gravitationally lower) emptiness between galaxies?


Yup. An important point to remember is that while Special Relativity can have symmetric viewpoints, General Relativity usually doesn't. By that I mean : when way out in space away from stuff, if you are in one spaceship shooting past me in another then we will see each others clocks running slower than our own. That's SR. But while constant velocities between inertial frames is purely relative, with accelerated frames that's not so. You can deduce that one's own frame is accelerated ( or affected by gravity if you like, as per the equivalence principle ) by finding non-inertial behaviour within. Like - 'I let go of the ball and it fell to the floor'. So if I'm down on Mercury you will see me going slower, but I'll see you going faster on Earth.

That's why the Twin's Paradox worries people. But it's not a paradox at all. The phrasing of the story can leave out or minimise the important point. Which is that you must use GR, not SR, to explain. So if my twin zips off at great speed and comes back later on, then yes he will have aged less. But there should be no expectation of symmetry ie. that I should have aged less than him. That's because he came back, and to do so he must have accelerated during some period of his whole journey to achieve that return. As the story is generally told the homebound twin wasn't accelerating. So you can't invoke SR and create a contradiction.

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Is our view of the CMB distorted at least partly due to the changing shapes and gravitational lensing over the eons as the galaxies have developed?


Absolutely! This does indeed concern those that measure and study the CMB. One important thing they do is to study angular correlations ( technically multipole moments ) which sort of assess how related are separated values. Hopefully, I guess one could assume, is that lensing and whatnot probably wont upset the boat too much - or at least could be accounted for. The Planck mission which is currently up and running is hopefully going to sort that, and much other stuff as well.

Not all SR viewpoints are symmetric mind you. Particularly with the ordering of events, that is : what event came before/after/same-as what other event. This is generically called simultaneity. I especially like the 'Barn Paradox' ( for want of a better title ). Again it's not really a paradox, just anti-intuitive. Imagine the following :

There is a barn. The usual big box type shape. It has big doors at either end. Suppose we place a long pole on the ground along the line between the doors and perpendicular to the faces of the door. You find that the pole is longer than the barn. So specifically you couldn't therefore fit the pole within the barn length ways with both doors closed. One door and/or the other needs to be open to have the pole, stationary on the ground in that orientation. Right?

Now let's get some super athlete and some hot shot door opening/closing machinery. Assume sliding doors not pivoting ones. After some practice we could conceivably get the following set of events :

- the guy runs very very fast into the open doorway of one end of the barn, with the pole held lengthwise. The other door is shut.

- we shut that entry door when the pole is just inside.

- wait for some brief moment

- and then open the other door

- the guy runs out of the barn, having neither sped up, nor slowed down, nor changed the orientation of the pole.

How can this be? You would guess right if you said his high speed caused length contraction of the pole along the line of motion. So he could be inside the barn when both doors were closed and the pole held lengthwise.

But I'm describing this from the point of view of a nearby observer, who is not moving with respect to the barn. What does the runner himself see? After all he's not moving with respect to the pole, as he is carrying it. So for him the pole has not changed length. Has the barn changed length for him? Yes it has! But he would see shortening of the length of run through the barn, not a greater span between doors.

Well, what happens is for him the doors are never simultaneously closed. What he sees is:

- he enters the open doorway of one end of the barn, with the pole held lengthwise. The other door is shut.

- the other door is opened

- he is in mid-barn with the pole hanging out through both doors

- the pole clears the entry door. That end is now completely within the barn. The other end is sticking out of the barn's other door.

- we shut that entry door.

- he continues out of the barn, having neither sped up, nor slowed down, nor changed the orientation of the pole.

What I'm illustrating is that in SR ( which is all that is needed for the barn story ), there is this thing called the spacetime interval between two events. In this case it's the door opening/closing events. You can always swap a time interval for a spatial one by changing viewpoint. The total spacetime interval is the same for any ( unaccelerated ) viewpoints, but I can partition that with varying amounts of space and/or time by choice of observer.

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[aside]
for the mathematically inclined :

ds^2 = c^2 * dt^2 - (dx^2 + dy^2 + dz^2)

ds = interval in spacetime
c = speed of light
dt = interval in time
dx, dy, dz = components of the interval in length

hence while keeping ds constant : with dt = 0 ( doors closed together ) then (dx^2 + dy^2 + dz^2) must reduce ( shorter pole ) : compared to (dx^2 + dy^2 + dz^2) unchanged, whence dt must be non-zero.
[aside]


Cheers, Mike.

( edit ) Re : barn. It is significant that the event ( exit door opening ) which occurs earlier for the runner ( compared to the barn based watcher ) is ahead of him in his direction of motion. Whereas the event which occurs later ( entry door closing ) is behind him. He's meeting the light from the exit door earlier than otherwise because the distance to it is reducing. The light from the entry door has to traverse a longer distance as he's running away from it. The speed c doesn't change, but the runner's position certainly does while light travels from either door to him.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

hockeyguy
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I was recently thinking about

I was recently thinking about this topic again, and I had somewhat of a revelation. If time is created as space expands, and inflation is true (which I doubt), then the universe is far older then we believe it to be. Any thoughts on this?

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RE: ... If time is created

Message 95135 in response to message 95134

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... If time is created as space expands...


Well, what is 'time'? And what does time exist 'in'?

Also note that things can exist at different rates/scales of time. On a geological timescale, our Earth changes in just a blink of an eye. Yet, for our small human timescales, the changes for the Earth appear to be so slow as to be unchanging.

As for the expanding universe and being able to see into the past when viewing ever greater distances... Does that not assume that:

The universe has in the past expanded faster than the propagation of light and that expansion has now slowed enough to allow the light to catch up to our present viewpoint?

And/or:

The universe 'folds back upon itself' such that as you look in any direction, you get to see an uninterrupted line of space and time even though that length of a line of sight could not have existed as that length in earlier times?

Just as you can have "reverse time domain" reconstruction of a sound source if you have a suitably large array of receivers, can we do a similar trick with the CMB to reconstruct an image of the 'Big Bang'?

Keep searchin',
Martin

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Mike Hewson
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RE: RE: ... If time is

Message 95136 in response to message 95135

Quote:
Quote:
... If time is created as space expands...

Well, what is 'time'? And what does time exist 'in'?

Also note that things can exist at different rates/scales of time. On a geological timescale, our Earth changes in just a blink of an eye. Yet, for our small human timescales, the changes for the Earth appear to be so slow as to be unchanging.

As for the expanding universe and being able to see into the past when viewing ever greater distances... Does that not assume that:

The universe has in the past expanded faster than the propagation of light and that expansion has now slowed enough to allow the light to catch up to our present viewpoint?


Exactly. There is a period of time, just after the 'singularity' but before the 'Big Bang', called the 'inflationary epoch'. It's a really, really small time like 10^(-30) of a second or somesuch. The idea is to wind the clock back from the present day to said singularity, which is nominated as the choice of time = zero. So for our thinking about the timing of events in the universe, and for clarity of discussion, we have chosen that. The singularity is a polite way of saying that all the various numbers - temperature, pressure, density - go to infinity as we get closer to Time Zero. In other words we haven't a clue what actually happens. Neither our thinking or our maths can properly grasp the infinite - except that it is bigger than anything finite! :-)

Now the CMB, and other evidence, indicates that regions way our there in space have properties remarkably like each other but despite that the deduced geometry show them to have never been in 'causal contact'. This means that if one assumes the speed of light is the fastest rate of travel of any influence ( ie. not just light but any force at all ), then how can these widely separated parts of the universe have so similiar features? We believe the universe isn't old enough for these areas to have influenced each other yet - they have been too far apart.

[ It could be a fluke that the areas resemble each other. But like total strangers in a chance meeting at an airport lounge who note considerable physical likenesses, we might hypothecate a closer common ancestry even though the detail is obscure. ]

So the concept is that maybe, at some time, they were close enough to have influenced each other - hence have properties like each other - but then were flung away from each other faster than light speed. That could explain what is seen now and is called inflation. This is expansion of space at a speed exceeding c, not signals within space traveling faster than c. And so as time proceeds parts of the universe that were earlier in causal contact, and then punted off at superluminal speeds, have now had the time to send light or other influences.

What is also worthy of note is the concept of 'observer' in physics discussions. This can be confusing as it can then involve all sorts of philosophy, consciousness and other aspects ( which is a valid topic ), but perhaps unnecessarily overstates what is often included in the meaning of the term 'observer' in a given conversation.

The minimal idea of an observer, is simply 'something that can be influenced'. So it might be an electron sitting around, and then something comes along to change it's state, like a photon say. In our minds eye we are imagining this interaction taking place according to some model being discussed - waves, particles, strings etc - so we are 'observing' but not actually there. But it's really a thought experiment. Now when we talk of lightspeed influences as above we are merely saying that nothing can travel any faster than c, so whatever particle/effect began in area A that then travels to area B to interact, needs at least a certain time minimum to travel the span between A and B.

Another concept of observer has more to do with how we receive signals at a human scale and deduce the meaning of them. Say when we talk of a single electron going from A to B we don't personally, as human beings, directly measure that. We have instruments to do that for us, but in doing so there is a layer of assumptions about how said devices work - hence how they report to us about goings on at a scale we can't perceive. So a reading on current meter, which we can immediately sense without assistance, that is linked to a photomultiplier tube tells us how many photons are hitting some surface. We must understand photomultipliers ( and a lot else besides ) to reach consistent deductions. What we don't often see revealed in experimental summaries is a vast amount of cross-checking that seeks to ensure this. But the reality is the use of proxies for our blunt human senses.

[ The Schrodinger's Cat business can get awfully tangled when observer concepts are not well defined. It actually challenges one to be firm in such an idea of 'observer'. Different conversations about that bloody cat usually suffer from disparate assumptions. I take a simple 'relativistic' view : the cat knows it's health of course, but I don't because I'm not in the box with it. When I open the box I'll find out. Replacing 'cat' with 'wave function for a cat' doesn't change the logic. Nor does linking the moment of it's possible death with a single radioactive decay. I could have a mouse gnawing ( or not as it pleases ) on a rope that holds a crushing weight over the cat, as per Tom & Jerry. So a 'classical' cat is really no different than a 'quantum mechanical' one. Big deal. No paradox. ]

The two observer ideas can be combined. Einstein defined time as 'that which is measured by clocks'. A bit of a cheeky definition that sidesteps stating what time 'really is' in any philosophical sense, but was very useful for precisely that reason too. So if I have two identical clocks, however you might agree to define their structure and operation, then I may send one down to nearby a black hole horizon. I hence compare what I see of it with respect to it's twin that remained with me well away from the black hole. So I relate the readings from the two without necessarily firmly defining what is meant by 'time' with either clock. I assert the clocks are of the 'same type' and different behaviours are thus ascribed to their respective environments.

Or I could imagine myself traveling down with the first clock to the hole ( as I wouldn't want to actually do that even if technically able to do so ) and looking back up at the twin clock - which will spin around like crazy from that viewpoint if I go deep enough into the black hole's gravity well. If I use a clock in either circumstance as a measuring instrument for a nearby ( 'local' ) process then my understanding of such process will reflect the use of the clock. So frequencies of photons, movement speeds of particles etc ...

So now clock can generically mean 'any time dependent process'. Again we haven't defined the meaning of time per se, but merely stated what things in the universe ought depend on it. This is called an 'a priori' assumption, meaning that we just accept what we choose to, and proceed on. Perhaps then I can talk of 'gravitational redshift' based on the belief that processes nearby the hole ought to be producing, say, gamma rays but all I see emerging are radio frequency photons. I might track some lump of material descending inwards and follow what I see arriving ( well way from the action ), and note that progressively lower frequencies of radiation are received. From that I say that gravity 'slows' or 'warps' time.

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The universe 'folds back upon itself' such that as you look in any direction, you get to see an uninterrupted line of space and time even though that length of a line of sight could not have existed as that length in earlier times?


This is the area of 'manifolds' which are extended or global geometric properties that then define what would be observed if you were within one.

A good example is a neat spaceship arcade game from about 25 years ago ( now that dates me! ). The screen is rectangular, but if you go off the left side you'll come back on the right, if you go off the top you come back up from the bottom. And vice versa. It has the same properties as the surface of a donut/torus in 3D space.

The Earth's surface is another good one. I remember a sci-fi set in the vast future when the globe was entirely covered with man made structures, and much history was lost or forgotten. Probably the IQ's had gone down too. A guy took a train traveling East, but he was restless for adventure and just kept going. Eventually he returned to his home town and quite puzzled. He made enquiries with the transport company, but they also could not understand why there was some 'apparent circularity' built into the system!! :-) :-)

But many other variants exist. Indeed the Calabi-Yau spaces of string theory, used for describing extra unseen dimensions, are like this but on steroids! :-)

Quote:
Just as you can have "reverse time domain" reconstruction of a sound source if you have a suitably large array of receivers, can we do a similar trick with the CMB to reconstruct an image of the 'Big Bang'?


Yes, indeedy do!! The Planck mission is currently tasked to try and measure some very subtle variations in the CMB that could actually reflect the mass distribution of the universe well before the current 'image' limit of about 300,000 years after the Bang. If successful it could map patterns due to gravitational waves that span a good fraction of the size of the universe!

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Gundolf Jahn
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RE: ...A good example is a

Message 95137 in response to message 95136

Quote:
...A good example is a neat spaceship arcade game from about 25 years ago ( now that dates me! )...


Does that happen to be "Asteroidsâ„¢"? (Okay, I've revealed my age in my profile :-)

Gruß,
Gundolf

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Mike Hewson
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RE: RE: ...A good example

Message 95138 in response to message 95137

Quote:
Quote:
...A good example is a neat spaceship arcade game from about 25 years ago ( now that dates me! )...

Does that happen to be "Asteroidsâ„¢"? (Okay, I've revealed my age in my profile :-)

Gruß,
Gundolf


:-) :-)

There were a few of that ilk, Asteroids being one. I also liked the other various tweaks on it like a central black hole, repulsive gravity and randomly timed and targeted jumps. Not to forget all the silly scooting about while trying to get my inertial thinking right! You know .... rotate the ship then apply the thrust ... more often than not it was mayhem. The gun never recoiled on the ship though. Newton would have rolled in his grave! :-)

Cheers, Mike.

( edit ) Here's another piece of stunningly dated trivia. The Thunderbirds - 60's UK puppet series by Gerry Anderson - has five Tracy sons. That's Scott, Gordon, John, Virgil and Alan. Each was named after a famous real life astronaut. Given that the first episodes were made in 1964 that narrows it to the Mercury & Gemini programs. Any guesses who were the actual astronauts?

( edit ) The names Yuri & Alexei didn't get a run evidently. Nor Leica ... ;-)

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

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Arrow of Time

Arrow of Time

There are some who can live without wild things and some who cannot. - Aldo Leopold

tullio
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The astronauts' names might

The astronauts' names might be (from memory only): Alan Bean, Virgil Grissom, John Glenn, Gordon Cooper. So far so good from my memory. Scott is not McNealy. Could he be Scott Carpenter?
Tullio
Laika is not a Leica, they are different. Homage to both.
Yuri is Gagarin. Alexei? Titov?
No, Eliseev, but I had to consult a book. Not fair.

Rod
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RE: The astronauts' names

Message 95141 in response to message 95140

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The astronauts' names might be (from memory only): Alan Bean, Virgil Grissom, John Glenn, Gordon Cooper. So far so good from my memory. Scott is not McNealy. Could he be Scott Carpenter?
Tullio
Laika is not a Leica, they are different. Homage to both.
Yuri is Gagarin. Alexei? Titov?
No, Eliseev, but I had to consult a book. Not fair.

I believe its Alan Shepard. The first American in space. And it is Scott Capentar

There are some who can live without wild things and some who cannot. - Aldo Leopold

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