J.C.MacSwell

On my computer at work, just like right now...but in a different [b']state[/b]

Calmer and less paranoid?

Right where I'm sitting.

LOL !

 

BTW: I think You missed the point.

I think you're right. Care to elaborate?

Well I did drop the[/b']m and they are not crunching, maybe I dropped them in the wrong direction ?

I think you missed a few!

Now I am confused...

 

Why would a local movement of mass affect the global size or shape of the universe ?

Because if you dropped all of the balls in the universe you would have a big crunch.

Maybe I have misinterpret the question' date=' J.C.MacSwell will have to clarify...

 

But I feel the question is not if a Newtonian Universe will collapse or not, the question is how fast a GR Universe will collapse if there is enough matter compared to a small locally collapse of matter in space.

 

Since the main topic is Expansion Momentum, I assume he wants to know how the current speed of expansion will [b']counter[/b] such a collapse.

Yes, counter or resist (successfully or not).

If I could pose a question: If you drop a ball is the universe smaller than it would have been if you had not dropped it?

Ok that makes sense.

 

I have another question though. Why is the speed of light the barrier that we cant cross? Why is it that precise speed and not some other speed? Sure light goes that fast but what does that have to do with anything?

 

Thanks again.

We have nothing with which to propel it faster.

If light went faster, say 1.1 c, then that would be the barrier we could only approach and that would be the point where inertia would approach infinite.

Speed is not the variable to use' date=' J.C.Macswell.

When you throw something away from you, you exert a certain force on the object you throw. An equal and opposite-direction force is exerted on you.

 

 

A fan pushes a certain volume of air per second. If you attach a straw or similar device to a fan to constrict the volume of air, it must travel faster to keep the same amount of volume/second - it doesn't magically give the air more power, it just focuses it onto a smaller area of the sail.

 

That said,[b'] I can't imagine (and have yet to see) a design for a fan-equiped boat that worked best [/b]with the fan pointed the wrong way (at a sail, rather than straight behind you ... preferably an underwater fan, ie a propeller). If you actually do use a fan, you would want to reduce the drag on the front of the boat as much as possible, and make the whole thing very bouyant.

"Worked best" is not the issue. I'm not advocating it, I just claim it can be done.

From "A brief history of time" by Stephen W. Hawking 1988:

"If the numbers of stars was infinite and they where more or less evently distributed in an infinite space' date=' wouldn't the universe collaps since the stars shouldn't have any center to be pulled against, according to Newton. This way of thinking is an example of errors of the mind when dealing with the infinite. The correct way to deal with this cind of problem - which people realized later - is to start with a finite solution and interpret how the situation changes when adding on more and more evently distributed stars outside this area. We can add how many stars we want and they will still collapse to one single body."

 

From a more modern point of view you have to consider the shape of the universe, with a closed universe where gravity is stronger than dark energy, expansion will slowly halt and then reverse/contract.

 

From Wikipedia "Ultimate fate of the Universe":

"The fate of the universe is determined by the density of the universe. The preponderance of evidence to date, based on measurements of the rate of expansion and the mass density, favors a universe that will not collapse."

 

[b']But J.C.MacSwell assumes there is enough matter present to create a "big crunch"[/b].

Only to help pose the question. If you assume enough matter to slow it down the question is the same. What role does space play in the momentum of the expansion.

I am assuming that the expansion has momentum.

Which is accomplished by... magic? Seriously, though, that violates Newton's third law[/b']. You'd be having the air exert more force on the sail than it has to begin with. Nothing rebounds at a greater speed than it impacts.

I assure you it doesn't!

Try blowing through a straw if you want a simple example of the speed exceeding any "impacts". (and it's all impacts, though you can argue the molecule speeds are higher it is the same thing, the fluid can be accelerated, it must if it is funneled)

Does expanding space have inertia? That's a very curious thought.

 

AFAIK' date=' [b']the rate of expansion is measured by observing galaxie[/b]s that are approximately relatively stationary.

I think this is correct.

So based on that rate, what inertia is the space assumed to have that must be "subdued" in order to arrest the expansion?

As I think (?) you were pointing out the galaxies themselves are more or less at rest wrt the expansion, at least locally.

How is the momentum of the expansion thought to be measured?

Assuming there is enough matter present to create a "big crunch" is it assumed that space will "shrink" (or slow in expanding, stall, then shrink) to the same beat as would be consistent locally with a gravitational collapse "in" space?

I can`t see how That would work either tho Ed?

in theory it`s a perpetual motion machine' date=' you need only blow on the windmill once get moving forwards and that movement even without wind will drive the windmill.

y[b']ou can`t have any net GAIN by sailing into the wind using this surely[/b]!?

You need wind to sustain it, but it will go upwind. It's not a PMM.

No, it isn't. You would have to be pushing more air backwards than forwards in order to move forwards. Since the only air you're pushing backwards is in fact the same air you moved forwards, that can't possibly be the case.

Same amount at a faster speed. Net momentum transfer backwards of the air.

It actually mixes with other air so another way of looking at it is that it is not the "only" air displaced backwards.

Indeed' date=' a given, but I was stubbornly attempting to stick loosely to what I understood to be the original question.[/b']

 

However, a google for "rotary sail" shows what has already been achieved commercially with wind-powered fan type sails driving a water-propeller.

My point is that it is possible to do this. Direct more momentum backwards in spite of the fan facing forward.

As long as the sail/fan system can direct more air momentum backward than forward it can propel the boat forward.

Also a fan can be set up to catch the wind and turn a water propellor which propels the boat straight upwind. No energy source other than the wind required.

I may be totally off but since none has answered' date=' I remember this thread by Martin:

 

[b']Astronomy puzzle (angular-size/redshift)[/b]

 

http://www.scienceforums.net/forums/showthread.php?t=22200

Thanks Spyman that's what I was after.

This is all very interesting. Really. But just so I'm clear' date=' [b']all of the forces 'unify' (or whatever) at higher energies[/b], in the same way that at higher energies the EM force and W force are the same. In other words, there's no particularly special connection between EM and W; we've just seen them 'unify' in the lab?

 

Sorry if I'm way off.

This is conjecture at this point. We have no idea of how or why. I thought the EM and W were the same at higher energies, not just connected, but I may have been mislead as Severian suggested. I thought Weinberg, Glashow and Salam won the Nobel prize for that?

I think physicists, for the most part, are pretty good at staying objective. But I do question the degree of "faith" they seem to have in "theories" such as the Big Bang Model. Built up on strong circumstantial evidence, and having no serious alternatives "remaining standing", has seemed to have produced a paradigm of certainty beyond what seems warranted. I think "science" puts the Big Bang Model as the clear front runner, but faith, mob mentality ( ) or just by default it gets pushed into the all but certain range.

Time took a serious beating back in 1905... and still hasn't recovered.

Come on - the guy is a respected physicist. You may disagree with him, but to completely discount his opinion, and urge others to ignore him, is a bit much.

In your opinion would he give a good review to a well written anti-string book?

I know people that, where I respect their work, I would take with a bucket of salt their opinion of a competitor or colleague.

Not that I know anything at all of Dr. Motl, but why shouldn't Martin urge others to ignore him, if he feels the guy is not being objective and is on a crusade?

Well' date=' I just got the two books I was waiting for from Amazon...

 

"The Elegant Universe" by Brian Greene ( I read his other one)

"Warped Pasages" by Lisa Randall.

 

I'm reading WP first. If I like the book, [b']I will read it very slowly and only for an hour before I sleep.

[/b]

Bee

 

Martin.... Look at the review one person gave to smolins book... http://www.amazon.com/gp/product/customer-reviews/0618551050/ref=cm_cr_dp_pt/104-7422758-3593505?ie=UTF8&n=14560&s=books

You could also try a warm glass of milk.

I think you are asking by what effect can DM be seen

 

so far we just have evidence from its gravitation effect

 

there is a bunch of interactions called "weak force" that a DM particle might be involved in and so be revealed

 

I think if you look back in this thread you may find a post of Severian about this. He's probably the one to ask about weak interactions revealing a DM particle.

 

and then there's Wikipedia and the arxiv search tool

 

BTW MacSwell' date=' just now thinking of Dark Matter ("things invisible to see") reminded me of this lovely old Donne song

 

http://www.pjbsware.demon.co.uk/DonneS1.htm

 

maybe you know it.[/quote']

 

Thanks Martin. I was not clear again. I was thinking of an effect we could "see" from Earth to detect it where theory "needs it". I guess if we detect something on Earth, via collider or something, that could strengthen the case for it or argument for it. I will have to check the Wiki and other links. My main interest was in the possibility of it not being "needed", though that sounds unlikely with current measurements and thinking.

I seem to be having trouble with English. the neutrons don't radiate their quarks in the sense of "spitting them out" (ordinarily)

 

but they can interact with light because they have charged particles inside them

so they can absorb/radiate or anyway scatter light VIA the charged components inside them

 

even tho their total overall charge is zero

 

I am just acting as MacSwell's interpreter here

 

===========

must say that I personally do not have a very clear idea of what kind of stuff DM could be

 

it can't scatter light' date=' or interact with light at all, of any wavelength (or correct me if I'm wrong) so it can't have electric charge, and none of its component parts can have electric charge----otherwise it would couple to the EM field.

 

it apparently is able to pass thru friggin walls. it interacts so little with usual normal baryons and leptons of regular matter that it cant even feel them

 

[b']so a cloud of DM can pass thru a cloud of hydrogen without noticing[/b], while two clouds of hydrogen doing otherwise the same collision will heat each other to millions of degrees temperature

As in a gravitation only effect, the weakest known force, or are there other possible effects they are looking for?

I think MacSwell didnt mean a component of a quark' date=' he meant that the nucleons can radiate via their quarks, which are charged.

 

even if overall the thing is neutral, if it has some part which is charged then it can radiate.

 

think that's what he meant, so no serious confusion hopefully[/quote']

 

That's correct. I meant that the quarks were components of the neutron. Is that not acceptable terminology?

Just one more point. It turns out that the speed at the center (for uniform density) equals

 

[math]\sqrt{\frac{GM}{R}}[/math]

 

Where M is the mass of the Earth and R its radius.

 

This is also equal to the orbital velocity for an object at the surface of the Earth (ignoring air resistance)

Interesting how that works out.