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Gravity/Space-time Question

Started by carterfox, 08/08/2018 03:20PM
Posted 08/08/2018 03:20PM Opening Post
Hi Friends,

I posted a question here about the properties of light several months ago and got some really helpful answers.  I now have a question about gravity and would love to here what you all have to say!

So, I have been trying to understand gravity in general relativity and here is where I am getting confused.  If I understand correctly, relativity says gravity is just the curvature of spacetime, induced by the presence of mass.  Masses are just following straight lines in space, but their 'straight' lines are warped by the curved spacetime.  When a moving mass reaches a curved region, it's 'straight' line is following the curve, leading it to orbit the larger mass or appear to be attracted to it.  I totally understand that, but where I am lost is how two stationary masses would attract eachother.  How does this warped spacetime view of gravity explain the acceleration of stationary masses directly towards a larger mass?  How could the stationary mass go from not moving, so it has no 'straight' line, to being accelerated across a new 'straight' line right towards the larger mass?  

I have been reading books and watching videos but just cannot find a good explanation for the acceleration of stationary objects.  I see all these visualizations of spacetime as a sheet and mass makes a dent in it, making other masses 'fall in', but that explanation requires the old attractive force gravity to make the mass 'fall down the hill'... If masses are just following straight lines, then why should they be 'attracted' to sources of curvature in space?

Thank you
Posted 08/13/2018 02:56AM #1
Think of space time as having a 'pressure' like water. Space time looks at matter as an infestation and it pushes on it. Two objects in space time would thin out the area between them more so than the area around outside them, thus the objects would be pushed together.

Try this thought experiment. Think of a single bit of mass in and entire universe by itself.  The space time around the mass is thinner closer to the mass and denser further away.

I don't like using the following idea because gravity is a property of space not matter. It is how space reacts to matter. None the less, think of matter as sucking in space time, like a vacuum cleaner sucking in air in a room. The air will be thinner closer to the vacuum cleaner. If you have two vacuum cleaners, the air between them will be thinner than the outside air around both objects. 

 It's actually the same thing in reality only reversed so to speak. It is not gravity from Earth that pulls you down, it is the pressure of space time on the Earths presence that crams you into the Earths surface.

If you want a real twist, try thinking about absolute spin.  If something is spinning, then it is spinning relative to some other object(s). What if there was only one object in a universe? Lets put two rockets on it opposite from each other and fire them off to spin up the object. The object will feel the force and react accordingly with rotational acceleration. Then turn the rockets off. Is the object spinning? Spinning relative to what? There is nothing else for it to be spinning in relation to. This is where the fabric of space time comes in. The object will be spinning relative to the 'stationary' space time that was there prior to the rockets firing. It's kind of like an object in water. Spin the object in water and the water will begin to rotate with it.
Posted 08/13/2018 10:14PM #2
The spacetime pressure explanation makes a lot of sense to me but what I do not understand is why spacetime would be 'thinner' closer to mass.  If the presence of mass makes spacetime push on the mass, wouldn't there be more spacetime closer to mass because it is being pushed towards the mass?
Posted 08/13/2018 11:35PM | Edited 08/13/2018 11:39PM #3
Good thinking! This stuff is something that can really be a trick to get your head wrapped around it. Space time tends to act opposite in ways that matter energy do. Unfortunately, all we are and all we do, how we study, the equipment we use, the pencil and paper we take notes on, all is based in the existence of matter and energy. This fact makes it tuff to explain as well as grasp.  To show how some of this stuff works by using this world of matter and energy we live in ,well, ...hmmm, very careful analogies maybe.  I'm winging this so....OK....here is an example of something working rather opposite to how we experience things:

Consider the total energy in a spring. OK wait....think of a spring that is compressed. Never mind what is compressing it, just consider the existence of a spring in a compressed state. Now consider all the energy it encompasses. It has the energy contained in it's mass. Good ol' E=mc2. This is a lot of energy.  It also has some potential energy in being compressed. The potential energy will pale in comparison to the energy in it's mass. This is how it will be in just about any such situation as we experience it. Now, how about something that can contain massive potential energy with very little 'substance' ?  Welcome to the world of space time, except that there isn't any matter energy here to think about, to make it tangible. Come to think of it, it does go a long way towards making sense of dark energy being much more prominent compared to dark matter as calculated by the physics. (physics gives math purpose)  Lets move to your question now, I have a sad but hopefully functional analogy.

Consider a room of people, crowded, standing around, casual, comfy, just crowded, shoulder to shoulder, OK with the situation, stable but rubbing right against each other....and a door opens.  The door is not sucking people out, it's the people that naturally go out on their own. They approach the door, it's just what they do, they aren't under pressure like air in a balloon, they just approach the door, it's what they want to do. The crowd is thinner towards the door, thicker away from the door.  Lets try something better because space time would rather be proactive on matter so.....how about the white corpuscles. Those guys are proactive after infestations. They are wandering around in a nice uniform distribution, then that disgusting matter shows up. The corpuscles go for it, push onto it. The corpuscles distribution gets thinner close to the matter by their own nature as they attack and remain denser further away. The disgusting matter isn't sucking them in, it's what the corpuscles do.

Now you could go into saying things about how the corpuscles would be dense on the surface of the matter or what about the people outside the room. Analogies aren't perfect. You'll start to get into digesting concepts of singularities, and infinite densities, and never ending pondering. Ok, once I get going things kind of settle in so here's one for you. Think of an infinitely long magnetic spring. The spring is relaxed just hanging around. Then a piece of iron shows up. The end of the spring latches out to the iron. The spring is less dense close to the iron and the effect travels out along the spring forever to infinity. ooooohhh, ahhhhhh,  ok if you want to actively find failures in an analogy you can. I don't have a lot of time for this and it is a bit arduous to try to convey.  Hope this has helped.
Posted 08/14/2018 12:01AM #4
That analogy of the crowded room of people is really interesting, seeing that there is no force or anything moving them into the open door.  It kind of makes me think of entropy and that if there is a void where spacetime could be, but is being taken up by mass/energy, it will naturally try to fill the void to move towards maximum entropy.  

That really helps me make sense of things.  Thanks so much for taking the time to write out such helpful answers.  I am a physics student trying to make sense of the world and hearing from people like you is where I find the best answers.  Really appreciate it