Quantum Physics

the force of gravity diminishes with distance, but doesn't go away.  It is still there in orbit, but you don't notice it because you (and everything else with you) is falling together.

Before talking about orbits, let's talk about an elevator.  If you are in an elevator when the cable snaps, in the last few seconds of life, you have time for a quick experiment.  Pull out a pen, hold it in front of you, and let it go.  It will fall with you, so since you are falling together, it seems to float right in front of you.  It's as though there is no gravity.

If you shoot the elevator sideways out of huge cannon, you can do the same experiment: the pen is not only projected sideways at the same speed as you, but is falling with you.  It still floats right beside you.

Until you hit the ground.  But if the cannon fires you fast enough, as you go forward and fall, if you get the sideways coordinated with the falling just right, the earth will curve away at the same rate that you move over it and fall.  This is an orbit.  The cool thing about an orbit is you just keep doing it : you keep falling, the earth keeps curving away as you move over it and fall, and eventually you come back to where you started and do it all over again...and again...and again.

If you could stop the elevator's motion while in orbit, all your weight would come back...well, a little less because you are further from earth so the gravity's weaker, but not actually all that far.  Just above the atmosphere you'd be about 90% of your original weight.

Anyway, Einstein was so struck by the fact that simple motion seemed to cancel out gravity that he presumed they must be deeply related.  And since motion is just space and time....space-time itself must be part and parcel of gravity!  It only took him about 10 years to work it out from there....

Thats a great description Halfy

I'm just regurgitating classic imagery on the topic. Einstein actually used the image of a photon being shot through an accelerating elevator to illustrate how light would be affected by gravity. It's been elevators ever since.

 yeah that makes it easier to undersand.

so does that mean gravity is a product of space and time? or is space and time like that because of gravity, chicken or egg. is time just gravity moving things forward into space. (just chundering on to myself here )

Mainly we'd say space-time curves in response to the presence of mass, and the effect of that curvature is what we call gravity.

if the mass starts bobbing around it can emit gravity waves, in which case once you see the wave burbling past the connection to mass isn't so direct.

we'd say space-time curves in response to the presence of mass, and the effect of that curvature is what we call gravity.- Halfwise

But that brings me back to why do people think there might be gravitons then- gravity isnt a thing its just a word describing the effect of a thing.

So looking for a graviton is surely no different from looking for a dark particle to see how fast it goes (as Terry Pratchett observed its faster than light as the dark always gets there first   ).

Because everything else in the world is described to great precision by particles. Einstein spent decades of his life trying to describe the electromagnetic, strong and weak forces in terms of space time curvature, and failed. People who followed tried to do the reverse, and except for the frustrating flurry of unverifiable superstring possibilities, have also failed.

There is very recently a team that thinks they have resolved some things, but the claims don't really ring true to me, since we have very clear evidence of the universe expanding and this model seems to contradict that. http://phys.org/news/2015-02-big-quantum-equation-universe.html But these things come and go all the time.

is gravity connected to the expansion of the Universe then? is that what makes it expand.

What makes it expand is the initial push from the Big Bang.   We have two lines of evidence for the Big Bang:

1. Since everything is now moving apart, if you reverse time it must have been all together at some point
2. Everywhere we look there is leftover thermal radiation with a profile that fits with the idea of a hot compressed phase that has cooled with expansion

Gravity comes in by trying to answer a tantalizing question: given the rate of expansion, will gravity eventually pull it all back together again?  This is equivalent to escape velocity. If you throw up a stone, you and the stone are expanding, but it eventually slows to a stop and comes back down again.  If you throw it at or above escape velocity, it still slows down, but never slows down to zero so never comes back.  Gravity can only cause a slowing or reverse of expansion since it pulls things together, never an acceleration of expansion rate.

Our evidence right now seems to put the expansion at just barely above escape velocity.  But looking way back in time, we see a rapid acceleration rather than deceleration during a brief period immediately following the big bang.  This can't be gravity, what was it?  

Since this expansive period (called inflation) was during a far denser period than we presently have anywhere in the universe, subatomic particle interactions come into play, and I won't go into it.  There's a tentatively accepted theory for what was going on, but I wouldn't call it completely settled. Outside of this brief highly inflationary period, we now see evidence of a slower acceleration, too small to have been observed before the Hubble Telescope, which is where Dark Energy comes from.  This is still a very unsettled matter.

so there is no way to detect gravity and analyse what its made from? or dark matter/energy? I wonder if when they make one of those organic computer things it might be able to give answers. I am not sure what I mean by organic cumputers, erm those next phase things cant remember what they are called.

Yes, there is an organic computer that detects gravity. You utilize it every time you have a bit much and slide off your seat.

I think Petty's answer was best: gravity is just a state of our local space-time, it's not "made" of anything.

Unless gravitons exist, but nobody's completely sure they do, and nobody's detected them. Then you have to reconcile gravity as a 'state of existence' with gravity as 'due to things that exist, and I'm not talking about mass'.

The two responses are so different that they've caused considerable angst. I prefer to just stick with the state of existence explanation: mass causes curvature in space-time...and there you go. You simply can't go any deeper than curvature in metaphysical concepts. You're done. No more turtles.

halfwise wrote:Yes, there is an organic computer that detects gravity.  You utilize it every time you have a bit much and slide off your seat.

 You're done.  No more turtles.

I will get my coat

halfwise wrote:You're done.  No more turtles.

Are you sure?

William James, father of American psychology, tells of meeting an old lady who told him the Earth rested on the back of a huge turtle. "But, my dear lady", Professor James asked, as politely as possible, "what holds up the turtle?" "Ah", she said, "that's easy. He is standing on the back of another turtle." "Oh, I see", said Professor James, still being polite. "But would you be so good as to tell me what holds up the second turtle?" "It's no use, Professor", said the old lady, realizing he was trying to lead her into a logical trap. "It's turtles-turtles-turtles, all the way!"

A touch of perspective-

A nice rule of thumb is that Jupiter is 10x the diameter of the earth, for a volume a 1000x larger, and the sun is 10x the diameter of Jupiter, another 1000x larger. So the sun can fit a million earths inside it.

But then those big stars are over a thousand times the radius of the sun, which gets way past the capability of my imagination. I think the final picture in the video with the earth as a speck hanging above one these stars captures it: the star is so big pretty much looks flat when the earth is a speck.

mind. blown. but if we cant even imagine sizes, what about the really weird stuff? our little monkey brains just aren't equipped enough.

Its ok we have developed a coping mechanism thats enshrined in only four words- "Fucking hell, thats big!"

its so big it takes 1000 years to get round is really big.

Fucking hell, thats big!

See works every time.

Finally getting back to quantum physics, a team claims to have made an image that shows light acting like both a particle and a wave. Though the article sort of explains it, they show the picture with no explanation or labels whatsoever, so you can't figure out what exactly they are talking about.

http://phys.org/news/2015-03-particle.html

I have a question, of sorts, for the boffins of Forumshire.
I am sure its all very simple and I am simply ignorant of the thing that makes it all very simple.

Consider a spinning disk. If you are old enough consider LP playing.
Now take a marker pen and put a mark on the outside edge, then draw a line to the middle and put another mark next to the hole at the centre.
Now every time the record makes 1 revolution the two marks go round once. But the mark on the outside has to travel a greater distance in that time than the mark at the hole near the centre.
If they are both turning at the same speed, but one travels less distance round than the other how does that work.
And more importantly if you could locate the exact centre of the disk in the hole- whats going on there as it turns regards Time?

And if you had a massive disk- I mean ginormous, and you got the centre to spin at just under the speed of light- what happens at the edge?

I don't think there's any problem understanding why a mark further out moves faster than a mark further in: they are going around at the same frequency, NOT the same speed.  Damn sloppy use of language to say they are going at the same speed, that's all it is.

As for approaching the speed of light, a professor of mine used this example to explain how length contraction leads to curvature of space.  but first we have to deal with length contraction.

Actually it's easier if we first talk about time dilation.  I'm going back to the tried and true light clock that Einstein first imagined and has been used ever since to explain relativity.  If you make two perfect mirrors and place them a fixed distance apart, it will take a fixed amount of time for light to bounce between them.  If you have an endlessly bouncing pulse of light, this forms a clock.  Place it so the light is bouncing vertically between the mirrors.  Now make a second mirror clock of the same dimensions, give it to your friend, along with a train ticket.

Your friend comes riding past you with an identical clock to yours, light bouncing up and down.  But wait a minute: in order to bounce up and down as the train is moving, you'll see her beam of light making a zig-zag pattern as the clock moves along.  Which means during each bounce it's moving further than yours!  You can figure out the extra distance using pythagoras and the speed of light.

But if it's moving further each bounce, and the times are the same, then either the speed of light on the train must be faster (a simple vector addition of vertical and horizontal velocities, you might think), or something fishy is going on to keep light at a constant speed.

Einstein realized that the electromagnetism of light couldn't work if you allow the speed to change, so something else had to be sacrificed.  Since we are talking about time, you guessed it: he decided time in the moving train had to slow down.  But only from our perspective outside the train: to the person inside the train the light is still bouncing straight up and down.

But if time slows down and the speed of light stays constant, that can only happen if lengths get shorter in the direction of travel!

So what happens with your record, is as the edge goes faster and faster (you increase the rotation rate), the distance around the edge gets shorter and shorter, so as to keep a point on the edge from exceeding the speed of light.

But how can you make the circumference of a circle get shorter while keeping the radius the same?  why, you curl the edges up like a bowl!  But this isn't evident to an ant crawling around on the record - the curvature is too gradual to notice without careful measurements.  Even more, the curvature happens in a third dimension, not the flat 2 dimensions of the record.  In our 3 dimensional space, the curvature happens in the 4th dimension, and is only noticeable when you do measurements and notice they don't add up as expected.

And if all this doesn't make sense to you, just comfort yourself in the thought that Einstein was a damn wacko.

Damn sloppy use of language to say they are going at the same speed, that's all it is.- Halfwise

Well at least I got something right- it was down to ignorance!

'In our 3 dimensional space, the curvature happens in the 4th dimension, and is only noticeable when you do measurements and notice they don't add up as expected.'

Hold on, does that mean the curvature of space only happens in Time, not in Space? But if thats right why is it called the curvature of SPACE! And aren't the two things the same thing looked at in different ways?

No doesn't happen in time. In a 4th dimension of space we can't access, like the ant can't crawl off the record that's bent up into a bowl.

That sound a bit, handy. A bit too handy, it happens somewhere in space but we cant see it or know about it. Is it the same place God lives?