Martin

http://podcast.thisamericanlife.org/special/405_Bonus_Bet_Against_the_American_Dream.mp3

All we gotta do, to make our dreams come true

is bet against

the American Dream.

============

a little journalistic background

http://www.propublica.org/feature/all-the-magnetar-trade-how-one-hedge-fund-helped-keep-the-housing-bubble

a video of the studio session, recording the song

http://www.thisamericanlife.org/radio-archives/episode/405/inside-job

op-ed piece in the NYT

http://www.nytimes.com/2010/04/25/opinion/25rich.html?hp

http://www.brainyquote.com/quotes/authors/p/paul_tillich.html

http://www.amazon.com/Courage-Be-Paul-Tillich/dp/0300084714

browse the ToC and look at headings for pages 155-186

http://en.wikipedia.org/wiki/Paul_Tillich

==quote==

Tillich's criticism against the traditional theistic God is that:

He deprives me of my subjectivity because he is all-powerful and all-knowing. I revolt and make him into an object, but the revolt fails and becomes desperate. God appears as the invincible tyrant, the being in contrast with whom all other beings are without freedom and subjectivity. He is equated with recent tyrants with the help of terror try to transform everything into a mere object, a thing among things, a cog in a machine they control. He becomes the model of everything against which Existentialism revolted. This is the God Nietzsche said had to be killed because nobody can tolerate being made into a mere object of absolute knowledge and absolute control. This is the deepest root of atheism. It is an atheism which is justified as the reaction against theological theism and its disturbing implications.[30]

==endquote==

... It stated that lightning was not produced solely by the cloud of it's origin but by the combination of the cloud's charged fields and a gamma ray burst coming from space in the form of a cosmic ray. Atleast I'm pretty sure that is what I heard...

This could be a great thread if you (or other contributors) keep asking questions.

Lightning is a really interesting topic.

How do clouds get their charge in the first place?

What is the breakdown volts/meter of dry air?

Either gammarays or cosmic rays passing thru some air can make it more conductive. So the air is more conductive and doesn't need such a high volts/meter for a spark to jump.

Radiation going thru can lower the threshhold. Lower the breakdown volts/meter ---triggering a bolt.

Why don't you google "breakdown voltage" and see what it says?

==========

I got curious and googled it, and I gather that in dry air it is about 3 million volts per meter. It also depends on the shape of the electrodes. Typically it is given for smooth round electrodes because any little bumps or pointy places can lower the threshold by providing concentrated gradient points from which sparks an jump.

It also depends if there is moisture or impurities in the air.

Spark breakdown is a CHAIN REACTION! Once a trickle of current starts, it further ionizes the air!

as soon as some electrons are pulled off their atoms, they begin to move and be accelerated by the voltage field. And as they move they bump into other air molecules and crash them and knock other electrons off. So then the action of the current further ionizes more air, and more air. Until the air becomes very conductive (very ionized).

Gammarays are photons (very short wave light)

Cosmic rays are mostly protons---hydrogen nuclei. A proton is a massive particle. Very different from light or from a gamma ray.

The point, I think, is that ANY energetic thing that passes thru the air can have collisions and knock electrons off the air molecules, and ionize. And so ANY of that stuff can make the air temporarily more conductive.

When it is more conductive then it doesn't take such a steep gradient like 3 million volts per meter to set off lightning.

Thanks Iggy! that's how I would have tried to respond to the question if you had not already replied.

This hyperphysics website link you gave is really good!

http://hyperphysics.phy-astr.gsu.edu/Hbase/vision/rodcone.html

Retina cells are two kinds: cones and rods. Cone cells do color vision and aren't as sensitive to low levels of light.

Rod cells do black-and-white (i.e. gray-level) vision and are more sensitive than cones are at most wavelengths. Except for red, the rod cells are not very sensitive to red for some reason. In any case they don't distinguish colors. When you see with rods your brain is only getting B&W images.

So that's why something that is intense blue in good illumination is going to look dull blue---and even gray---in reduced illumination.

Color vision is fascinating. That link Iggy gave has some stuff on it. There are a halfdozen short articles, all worth reading.

In the big bang model, our current universe started with a big explosion from a small object with extremely high density, the debris of this explosion re-grouped and evolved into stars and galaxies. The debris carried initial momentum of the explosion which makes the universe keeping expansion,...

Please stop right there. It sounds like you are picturing it as an explosion of matter outwards into empty space----as flying debris. That's a common misconception and it's not what the big bang model says.

"Big bang" is a bad name for the model which was given to it by someone with a rival theory who wished to discredit the model.

Please read the article called Misconceptions about the big bang.

http://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf

The first page is blank, so scroll down. The very first confusion they deal with is the "explosion" misconception.

So your saying the universe is flat then?

No. Please read my post.

so all i was taught about the big bang, and how it universally expanded in a spherical shape is technically wrong

I don't know what you were taught, or what you understood from reading old-ass textbooks.

I said we do not know. Each year the data gets a little better and we measure the large scale curvature better and narrow down the estimate.

There is a 95% errorbar for the curvature that roughly speaking looks like

[0.99, 1.01]

where if it were to turn out to be exactly =1, the curvature would be zero and space would be, on average, at large scale, flat.

But if it should turn out to be 1.01 that would mean positive curvature and space would have the overall largescale shape of a 3D hypersphere.

That is, something analogous to the 2D surface of a balloon.

The standard model, the cosmo model that essentially everyone uses, includes both these cases, just by varying one or two key numbers that you plug in. The U can be infinite volume or it can be finite volume, but in either case NEARLY flat.

And according to the standard cosmo model there is no OUTSIDE space that the world is "expanding into". As in the balloon example, there is no inside of the balloon or room outside, only the 2D surface. All existence is concentrated on the 2D surface. But that is just an analogy for the 3D hypersphere where all existence is concentrated---there is no inside or outside of it. You couldn't "get outside" and look.

Nor could you in the infinite volume, perfectly flat, case.

In standard cosmo there is no outside edge. Matter is uniformly distributed thoughout space and has always been so. There is no boundary. There is no explosion of matter outwards into surrounding empty space.

You could read the mso.anu.edu article in my sig if you want more clarification. The file starts with one blank page so scroll down.

http://wmap.gsfc.nasa.gov/

 

i stumbled across a new (to me at least) idea, that being the universe is flat.

wtf?

when did this happen.

No big deal. "flat" is technical parlance for not being curved.

Not being curved means for instance that whatever triangle you construct with lightrays will have angles add to 180 degrees.

Or for instance it means that the volume of a ball increases as the cube of the radius. (assuming we are talking about a 3D space.)

For an ordinary 3D space like we live in to be nearly "flat" just means it has the geometry that you are taught to expect in 9th grade or highschool---with 180 degree triangles and all that.

In fact space is NOT exactly flat. It has local bumps and hummocks.

what NASA is talking about is LARGE SCALE AVERAGE geometry. and even that we do not know to be exactly flat. All the WMAP data showed is that it was NEARLY so. Even if you only look large scale and ignore the local humps like around the sun, we still don't know exactly flat.

You ask "when did this happen?" "when did we find this out?" :-D

Well we've always known that over small distances, in and around our solar system locale, the geometry was nearly flat.

But measuring the large scale curvature, over billions of lightyears, is quite challenging. So we didn't know for a while. Evidence has been building up for either zero curvature, or maybe a very slight positive curvature (so that very large triangles would add to very slightly more than 180 degrees.) The WMAP spacecraft has been taking data for something like 8 years. Other methods involve counting galaxies within larger and larger volumes. The work goes on, the estimates get more accurate. It is still not finally settled.

... greater than the atmospheric pressure on Earth. ...the pressure is very strong, thus making it hard for large lifeforms to form.

...

How does increasing pressure (other things being equal) obstruct the development of large lifeforms?

If the earth had sealevel air pressure of 10 atm, instead of 1 atm, but other things equal (e.g. temperature) would that have made it hard to have animals comparable to whales, elephants, giant redwoods, sharks, rhinoceros etc. etc.

Especially large plant lifeforms. Why would a tenfold increase in pressure have obstructed their development? Curious about your reasoning.

... He briefly mentioned, however, that certain exemptions to this natural tendency do exist (gravity, biology, and geological sedimentary 'sorting' mechanisms being a few examples). ...

I used to read his serious physics blog posts, and sometimes look at his papers. I found them increasingly speculative and misleading.

He's bright, a good talker, and an attractive media personality. His popular book is selling very well. But I stopped paying attention a year or two after he failed to get faculty tenure at Chicago, moved to Caltech, and became increasingly air-castles and media-oriented.

If you have the impression that biology somehow goes against thermodynamics that could be because of a slightly imprecise slippery way he talked in a TED situation. Or maybe he didn't say that, but qualified it with some conditions and reservations which didn't come through.

I don't want to take the time to listen to carroll's TED so I can't give you a fair assessment, or find out what he really said, versus the impression that he gave.

=================

entropy in the universe (forgetting Carroll popularizations and looking at recent research by some top people) is a great topic!

Berkeley Nobel laureate George Smoot has just co-authored an important paper about it.

there are exciting new developments going on as we speak.

If you want a link to Smoot's February 2010 paper, just ask.

Erik Verlinde's January 2010 paper (entropy and gravity--which verges on the topic of entropy and the universe) also got a huge shock reaction.

The new link is http://fr.calameo.com/books/000145333186209bbc0f5

The author is Tom Heymann. You? or a family member?

In any case, congratulations on a nice piece of work.

I don't know how to catagorize this, however. The author is, I would guess, a student at the EPF-Lausanne. (Ecole poly. fed.) But is it usual for Swiss bachelor students to produce papers like this? Clearly a lot of work went into this paper.

It is impressive. But not yet, I think, at a level of scientific journal publication.

Good point. :-D

And pendulum was what you suggested already in post #2.

Whether or not that is the final project, it is valuable experience with a rather accurate simple timing device.

Basically copying what Galileo did in the early 1600s---about 400 years ago. A savvy dude, that we owe a lot to.

This source says G experimented with pendulums in 1602

http://cnx.org/content/m11929/

and described the constant swingtime feature in a letter to a friend

also that around then another friend of G's began using a pendulum to time the pulse of medical patients.

But G did not publish his pendulum findings until much later, in the 1630s.

For more, google "galileo pendulum" or check the Wikipedia.

If life ... continues to grow ... forever, would it be safe to assume that over a long enough period of time that it would basically consume...

A good way to consume any kind of matter is to compress it into small black holes.

Small black holes are hot, the smaller the hotter. They evaporate, producing Hawking radiation.

It is more efficient to get heat that way than by fusion. If you start with hydrogen and do fusion you can only get up to iron. Only the fusion reactions up to iron nuclei release energy. There is always the iron left over.

so only a relatively small fraction of the mass is converted to energy.

With small black holes a large fraction is converted.

After all the stars die out, life forms could still get energy by gathering together any kind of matter and compressing it into small black holes.

Eventually all the extra matter that was lying around would be gathered and consumed.

A dilute soup of waste heat radiation. All the dead stars used up, and other junk.

There are several different ways to harvest gravitational energy, I left out a lot. But one way or another all the matter that they could get hold of that wasn't needed for life itself could, in principle, be used.

Your scenario seems reasonable to me. maybe I'm wrong and someone will correct me, but I think you are right.

From any given bunch of life, most of the universe escapes being eaten simply because it is expanding. Most of the galaxies that we can now see with telescopes are receding from us so fast that it is physically impossible to catch them. (Most galaxies recede faster than the speed of light.) There is something called the "cosmic event horizon" beyond which our local civilization can not, even in principle, exercise control.

So there is a limit to the amount of matter which any one bunch of life can gather and consume. But given whatever is available, your scenario of gradual complete consumption is plausible. (if it didn't go extinct earlier for some other reason)

... people have a predetermined amount of time in the form of two beeps from a computer or stopwatch...

edit: oh and human interaction IS allowed throughout.

Thanks for the detail. So human interaction is allowed throughout.

That is important. It means you could be sitting there counting pendulum-swings, instead of having to contrive a device to do the counting mechanically, like the notched wheel you see in old clocks.

Why don't you right now get a piece of string and a metal weight and see how close you can come to timing seconds?

Adjust the length of the string so each round-trip swing takes one second, and a round-trip takes two seconds.

See how good you can do with just that simple method.

The point is that the round-trip swing time is nearly independent of how much swing angle. A wide swing going fast or a narrow modest-size swing going slow---the roundtrip time is always the same. It is what makes it an ideal simple timer.

Try timing 15 seconds. Within reason, it should not depend on whether you give it a large nudge to start or a small nudge. What matters is the length of the string. See if you don't find that to be the case. This should take you only a short time to do, and then you will know something by your own direct experiment--instead of just reading or being told.

Or make the string longer so that each one-way swing takes one second.

I'm not suggesting you use pendulum (weight on a string) for your final project design but I suggest you quickly, in the next half hour, get some experience directly measuring time with a simple device. Pendulums can be very accurate. Just don't let it swing wildly, a moderate angle of swing is good.

Compare it with the second hand of a regular clock and, like, time 15 seconds with 15 swings. Adjust the string length until it is perfect. or nearly perfect. Easy, and good experience.

Hi, I am taking part in a science competition in which I must design a device that can measure time. I must measure 5 different lengths of time that range from anywhere from 10 to 300 seconds. .

The guy still has not told us what the timepieces are being judged on. Is it accuracy or, say, how original and funny they are.

He has not told us how the 5 intervals to be measured are to be chosen.

they range from 10 seconds to 5 minutes.

Very likely he will have to set the timepiece to run for some interval, like

4 minutes and 25 seconds, say. And then it will have to run by itself without assistance for that whole time. And then stop, or ring a bell---generate some recognizable event signifying the end of the period.

That's how I would set up the contest, I think. And screw originality and humor. I would judge on accuracy.

Other people might imagine the contest differently.

Let's agree on some rules, criteria, format or something. Unless the original poster gets cooperative and obliges us by telling us what they are.

Hi, I am taking part in a science competition in which I must design a device that can measure time. I must measure 5 different lengths of time that range from anywhere from 10 to 300 seconds.

Skeptic already mentioned pendulum, the obvious thing that comes immediately to mind.

The thing is though, I don't understand how the competition is set up.

Do YOU get to pick the 5 intervals which must all be 10-300 seconds?

Or do you walk in with your timepiece device and the judges hand you a piece of paper with 5 lengths of time that THEY chose that you weren't told ahead of time and which you now have to measure?

Or do they tell you 2 weeks in advance what the 5 intervals of time are going to be which you have to measure?

Which is it?

Amazon lets you browse this new HEP book by a CERN theorist

Gian Francesco Giudice . The book went on sale three weeks ago, 25 January, 2010, and is currently sold out and back-ordered. It went to the top of the physics bestseller list.

http://www.amazon.com/Zeptospace-Odyssey-Journey-into-Physics/dp/0199581916/ref=sr_1_1?ie=UTF8&s=books&qid=1266250510&sr=1-1

See what you think.

The guy is good with words, images, and stories. Has a keen sense of humor. Plus he is a smart theorist who has been there at CERN since before 2000, sociable, insider perspective.

If space is not curved, then you will travel towards infinity forever in one direction, right?

The simple straightforward answer is yes, definitely! Very few cosmologists bother with weird exceptions to that in the form of flat "pacman" models.

Almost everybody, when they say flat (zero curvature at least on average, after smoothing out local bumps) thinks infinite. Very much the usual idea of infinite space.

But there are models that are flat (in the sense of triangles adding to 180 degrees) but have some odd topology. Example take a square and identify opposite edges, so pacman running off to the right reappears coming in from the left.

Or take a cube and identify opposite sides, so you get a 3D analog. I think you know those examples already---we've discussed that kind of thing.

But basically, ignoring these examples with peculiar topology, the answer is yes.

In your estimation, if you traveled in a straight line at the speed of light in one direction, how long would it take before you arrived at your starting point, IF space is slightly curved? Since the universe is expanding, it must be a lot longer than 900 Billion years, or not?

You are focusing on the case where space has a slight curvature and is a hypersphere (3D analog of the balloon surface). In that case the circumference COULD be a lot larger than 900. The 900 figure is just a current estimated lower bound.

To make the question more definite, let's assume this finite (slightly curved) hypersphere case and moreover assume that the circumference is 900, right at the estimated lower bound.

It's a good question. As I understand it you are asking could you ever get around? (And if you could, how long would it take?) Traveling at the speed of light.

Intuitively, I would say the answer is no, you could never circumnavigate, because of expansion. The point to emphasize is that we don't know the future rate of expansion, the Hubble rate H(t) is 71 and according to the standard model it is slated to decline to around 61 (km/s per megaparsec) and what 71 means is that a distance increases one c for every 13.7 billion ly. The distance 13.7 is increasing at rate c. The distance 27.4 is increasing at rate 2c. The distance 41.1 is increasing at rate 3c.

So the distance 900 is increasing at rate 900/13.7 c. That is 65.7 c. In that case you would have to travel faster, say 66 times the speed of light, in order to make any progress at all, and the net progress towards the goal would be fairly slow.

The Hubble rate is slated to decrease to an asymptotic value around 61, and what that 61 means is that instead of using the figure of 13.7 we get to use a figure of around 16, so the break-even speed is 900/16 or around 56 c.

Laluskiie,

Think about the balloon surface example. It is a toy 2D version. The real 3D picture is analogous but harder to imagine.

Think about all existence concentrated on that 2D sphere surface. Only 2D creatures can live there. Flat amoebas with no thickness sliding around in the 2D surface.

No rubber. The balloon does not exist, only it's surface. No inside space, no air inside. No outside space, no room for the balloon to sit in. Only the pure 2D surface, and all existence is concentrated in that surface. flat galaxies, flat stars, flat planets flat amoeba people. Nobody can point in any direction that is not in the 2D surface.

Google "wright balloon model" and get the animation and watch it a few times.

Concentrate on thinking that no surrounding space exists, the only space is the pure 2D surface. You will see galaxies getting farther apart and the ittle wiggly things are photons of light going from one galaxy to another.

We can study geometry internally. We can learn the shape of space without there being an outside. To discover the curvature of space all you have to do is measure the angles of various size triangles. If they add up to more than 180 degrees then it's curved. The 2D people on the 2D sphere can do that too.

We know our 3D space is curved but we don't know that there is any space outside it. We know distances between clusters of galaxies are increasing but we don't know that there is any "outside" space to expand into. doesn't need to be. To keep things as honest and as simple as possible we have to not make up unnecessary baggage. We assume no outside. Most cosmologists don't anyway. A few fantasize about higher dimensional surroundings and stuff like that, but it's not useful. doesn't help model the U or fit the data.

Are the forces of expansion sufficient to separate our Local Group..I thought it was between clusters or superclusters?

No, according to the projections I've seen. Our local group will stay together but gradually merge (on a very long timescale).

So "our galaxy" will consist of both the present Milkyway and Andromeda and other stuff merged into a large elliptical (not so pretty as a spiral).

Iggy is right, all we will see is "our galaxy".

But the local group will not be pulled apart.

Since I'm not an expert in this , I will just point you to Larry Krauss' article on the longrange future. A lot of it is written in clear non-math english. He's a recognized expert in cosmology:

http://arxiv.org/abs/0704.0221

Just go there and click on PDF to download the article.

=========================

mickmeister and swaha,

the headline was misleading "Universe measured: we're 156....wide"

the result was a LOWER BOUND.

Like saying "it is at least 300 miles from San Francisco to Paris, France."

If someone determines that the distance is at least X they haven't measured it, in the usual sense.

The 2004 article did not give a handle on the actual size. I think you both realize this, but in 2008 we got a NASA report from the WMAP mission that presented evidence that the U was either INFINITE in spatial extent, or, if space was slightly positive curved, then it was spatially finite but the circumference was AT LEAST 600 BILLION lightyears.

I understate the figure the 2008 WMAP report gave for lower bound was somewhat over 600.

Now there is a new 2010 WMAP report from which one can derive a larger lower bound. With 95% confidence the circumf is in excess of 800 billion lightyears (assuming it is positive curved so space closes on itself analogously to a sphere.)

And also the evidence continues to be consistent with infinite extent---in other words still much much larger than that.

From the 2010 WMAP report one can also derive a 68% confidence lower bound on the circumference, in that spatially closed case. 2pi*13.7/sqrt (.0079) billion lightyears. That comes to 968 billion lightyears. The circumference has to be at least that.

Picture there is no center, no radius, it is the 3D analog of the surface of a 2D sphere with all existence on that 2D surface (so no center exists). That is why I give the circumference.

Since it's so approximate, just a 68% likely lower bound, 968 is too precise looking. Let's say circumf at least 900 billion lightyears.

If you want the professional article it is Komatsu et al. Ask if you want a link. Currently the trend on size estimates is UP as they accumulate more and more data.

I regularly listen to the Naked Scientists Podcast and this topic came up a few weeks ago. They estimated roughly 160 billion light years wide. I can't recall where I read it, but I read that the universe is expanding a billion miles every hour in all directions.

yah but how did theymeasure it? what's the method?

Read this article on Space.com: Universe Measured: We're 156 Billion Light-years Wide!

thanks brother mickmeister.

That Space.com article was dated 2004. I remember the article when it came out---Cornish, Spergel, and somebody else.

That is not current. I wouldn't go by it. The number is not correct if you are talking about the OBSERVABLE portion of the universe. And if you are talking about the size of the whole universe according to the standard model then there is now a different size estimate as of 2008-2009.

And a different method is being used to measure, from the one in the 2004 paper that got the 156 number.

I have to go, but I think Iggy knows this stuff. Urge reading. I'll get back to this later.

I guess everybody should know how to calculate the falling time at approximately uniform acceleration.

Here's a problem. Say Milky masses a trillion (10^12) solar and there is nothing else around and you release a brick at a distance of 3 million lightyears. How long does it take to fall 0.5 million light years?

It's elementary. Type this into google:

sqrt(9 million (light year)^3/(G*mass of sun))

I've canceled 10^12 in the numerator and in the denominator. Otherwise it is just (approx.) uniform acceleration GM/r^2

and distance fallen = (1/2)a t^2.

the point is that google calculator is a remarkable tool because it handles all the units for you. It knows the mass of the sun. It knows what a light year is. It knows Newton's constant G.

All you need is simple algebra, solve for the time t.

It will even tell you the answer in years. Nice calculator.

... we can

probably be sure that at some point in time Andromeda was nudged

into our path (or vice-versa) or that the velocity was altered at some point ?

...

That is reasonable. The velocity in any case would have been changing if only because of the gravitational attraction of the two galaxies!

But also the velocity could have been affected by the fact that the Hubble rate has changed greatly over the past 8-10 billion years! It used to be much larger than the 71 it is today. It is still declining (but partly because of the dark energy effect the decline is slowing---as I recall H is supposed to level out around 65 or so.)

In order to go further with this you need to crunch some numbers.

Why don't you do this problem, pur?

Forget about expansion. Assume Milky is one trillion solar masses. Assume you release a brick at a distance of 3 million light years and it starts falling toward Milky. How long does it take to get to within 2.5 million light years?

Very very roughly. Get it within a factor of two of the correct answer and you are OK.

That's about the simplest problem in this general area that I can think of. If you can't tackle it then there is no way you can usefully pursue this stuff, and we can end our discussion.

Seems like there might be a chink

in the armor of something.

There is a chink! But the chink is a flaw in your argument (not in mainstream cosmology). You assume that Andromeda has a constant velocity towards us for all that time. No reason to assume that.

It might have been moving away from us part of that time and then later started closing in. You really can't estimate what the distance was 12 billion years ago, merely by assuming constant velocity.

The structure-formation people do this kind of thing with elaborate computer models where they simulate the formation of clusters of galaxies. To do it properly takes a lot of numbercrunching.

What I gave you was a hypothetical example, because you asked how far A would have to be. If you magically transported Andromeda today to 6 million lightyears away and kept everything else the same (ie same velocity relative to background, still pointed at us)...

I didn't suggest you try to extrapolate back into the past, that's a computer modeling job.

Good stuff guys!!

I hadn't ever heard about the "Great Attractor". Can someone provide a

well-respected link to learn about it.

well, I started a Great Attractor thread. It has some links. We heard a lot about the Great Attractor back in the 1980s or 1990s. Later some people seem to have found that it was not as massive as initially supposed and there is another large concentration of galaxies out beyond it that may be more important. I'm not sure about this. Anyway one does not hear so much about G.A. nowadays.

In any case it was never imagined that we are getting nearer to G.A. Even back in the 1980s it was realized by the people who detected it that the distance from us to the G.A. is increasing.

Also, I was wondering how far away would we have to be from

Andromeda before gravity would be overcome and the expansion of

space would take over. (Approx.- I'm assuming that no other gravitational force would interfere)

That's a really good question! Are you familiar with the idea of motion relative to the microwave background? That would be a convenient way to address this. Here's a way to get a handle. The expansion of distances (between points that are not moving relative to background) is given by the Hubble rate

71 km/s for a distance of one Megaparsec.

That means that if two observers are not moving relative to background and they are separated by a distance of one Megaparsec, then the distance that separates them is growing by 71 km/s.

Now Andromeda is approaching at some speed that is on the order of that, for simplicity say it is exactly 71 km/s.

Maybe it is 50 and maybe 100 but this doesn't matter, call it 71.

A parsec is 3.26 lightyears. A Megaparsec is 3.26 million lightyears.

To oversimplify let's pretend Milky is stationary and Andromeda is moving towards us at 71.

Then if you kept everything the same and put Andromeda 3.26 million lightyears away, the expansion of the distance would just cancel Andromeda's random motion.

I'm not saying that gravity would be overcome, only that Andromeda's motion toward us would be overcome. (If it were magically repositioned that far away from us, everything else the same.) I haven't completely answered your question, but maybe this gives a rough idea. Some quick-and-dirty handle on it.

To be sure, double the distance. Imagine putting Andromeda 6 million lightyears away. That would surely break it loose. It would continue drifting away forever. Goodbye Andromeda.

Someone else may be willing to make a more detailed analysis, using estimates of the masses of the two galaxies including the surrounding clouds of dark matter and all that. But my impression is that we do not have reliable estimates of the relevant masses. The mass figures have changed over the years. Look up mass of Milky on google and see what you get. I've seen 400 billion solar and 1000 billion solar. I've seen Milky estimated more massive than Andromeda and less massive. Mass estimates of galaxies involve guesswork. But the actual observed speed, from Doppler, is more directly measurable.

http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/990924a2.html

"...is now thought that the Great Attractor is probably a supercluster, with Abell 3627 near its center..."

The date on that NASA public outreach page is 1999. Anybody have an update? Additional detail?

Wikipedia refers to the concentration around Abell 3627 as "ACO 3627" and says there could be an even more massive concentration ("Shapley") out beyond the G.A., discovered in 2005.

http://en.wikipedia.org/wiki/Great_Attractor

So the G.A. may have been "demoted"

"The survey also confirmed earlier theories that the Milky Way galaxy was in fact being pulled towards a much more massive cluster of galaxies near the Shapley Supercluster which lies beyond the Great Attractor.[3]"

Wiki (not always reliable) does cite a source!

http://www.ifa.hawaii.edu/info/press-releases/kocevski-1-06/

And the source in turn cites two 2005 scientific papers:

http://arxiv.org/abs/astro-ph/0510106

http://arxiv.org/abs/astro-ph/0512321

So here is some unresolved controversy and you have to weigh and decide for yourself.

The topic of the G.A. came up recently. I recall hearing about G.A. in the 1980s, and an estimate that the Milkyway galaxy, and collectively the Virgo supercluster which we and our little local group of galaxies belong to, are all moving towards the G.A. Not that we are getting closer to it, just that we are moving in that direction.

By the early Naughties (bad pun for the 2000s) the microwave background data had come in and it was estimated the speed was roughly 500-600 km/s relative to the Background. (But this did not cause a blueshift, it only meant there was less redshift than otherwise.)

That picture may have been revised some, but assume all that is at least approximately right. Then you might intuitively suppose that we are getting CLOSER to the G.A. That given sufficient time we might meet and merge with it. This was conjectured in another thread.

But cosmology buffs will realize that this doesn't follow. The G.A. is receding from us, or if you prefer, we are receding from it. The distance is increasing. What astronomers actually measure is a REDSHIFT in the light from the Abell 3627 supercluster. The redshift is just not as large as you'd ordinarily expect for something at that distance.

In other words we are not expected ever to meet and merge with the G.A. supercluster, or supersupercluster, however you like to classify it.

It would be great to get some recent links about local motions like this. How are they measured, or at least estimated, what kind of inference is used?