Jump to content


Photo
* * * * * 1 votes

Where does the energy come from when two objects gravitate towards each other?


  • Please log in to reply
37 replies to this topic

#21 Jonas Taelman

Jonas Taelman

    Lepton

  • Members
  • 5 posts

Posted 15 February 2017 - 02:08 PM

It's known that all objects with a certain amount of mass generate gravity. This is because of Einstein's relitivity theory. Look at it like this:

Get at towel or a blanket and stretch it on all four sides. Now put a poolball on the blanket. What you see is that the blanket is bent.

If you put a smaller ball on the blanket you will see that the smaller ball will roll over to the heavier poolball because the blanket is more bent at the poolball's side.

 

The same thing happens in space. All objects, with mass, bend space (and time). So the more mass the object has, the more gravity it will have.

On earth we consider to have 1G, if the earth had 2x the mass it has now it would be 2G of gravity.

 

When two bodies attract eachother, think about the blanket. Space is bent around both bodies and they just "roll" inside eachother's bent area.


  • 0

#22 Delta1212

Delta1212

    Primate

  • Senior Members
  • 2,469 posts

Posted 16 February 2017 - 12:08 PM

teaching_physics.png
  • 2

#23 mistermack

mistermack

    Atom

  • Senior Members
  • 201 posts

Posted 18 February 2017 - 05:23 PM

I've never liked the rubber sheet picture. I don't think it reflects the reality of 3D spacetime distortion.

 

To understand gravity you need to understand what mass is. And what energy is. Since we're not there yet, we have to make do with models that can represent what's happening accurately, so that we can calculate what will happen. 

But they are still just models.

 

My own mental picture is that a massive particle is just a huge amount of wave energy, rotating around itself in a tiny spherical ball. 

When it's in a gravitational field, spacetime is pulled out of it's natural symmetrical shape, and the ball becomes slightly unsymmetrical, in the direction of the object causing the field. A bit pear-shaped. So the wave isn't following the same curve everywhere any more. And it's the lack of symmetry that makes the whole thing want to accelerate in the direction of the mass causing the gravitational field.

 

That's my mental picture at the moment. It's sure to be bollocks, but it keeps me happy till I hear one I like better.


  • 0

#24 Eise

Eise

    Molecule

  • Senior Members
  • 438 posts
  • Locationthe old world

Posted 19 February 2017 - 11:33 AM

I nearly never post youtube links, but this one is worth it:

 

 


  • 0

#25 mistermack

mistermack

    Atom

  • Senior Members
  • 201 posts

Posted 19 February 2017 - 02:40 PM

I like that better than the rubber sheet. It's very good. But it's carefully phrased at the end. That's "HOW" gravity makes things fall. 

It describes it well, but it doesn't say "why" things fall. 

 

It sort of transfers the question. Instead of "why do things fall?" you now need to know, "why do things want to go "straight" when the orientation of local space is constantly changing? Why don't massive particles follow the orientation of the local space, as it changes with time?

 

So exactly the same question remains unanswered, it's just been transcribed.


  • 1

#26 Strange

Strange

    SuperNerd

  • Senior Members
  • 12,405 posts
  • Location珈琲店

Posted 19 February 2017 - 04:17 PM

So exactly the same question remains unanswered, it's just been transcribed.

 

So GR explains why Newtonian gravity works. But why does GR work?

 

That question will always be unanswered. If someone comes up with a new theory (let's call it "X1")  that explains "why" general relativity works then someone will just ask: "But why does X1 work?"

 

Eventually, there might be a new theory  (let's call it "X2")  that explains why X1 works. At which point, someone will ask: "But why does X2 work?"

 

Repeat for Xi (for i = 3 to infinity).


It describes it well, but it doesn't say "why" things fall. 

 

Actually, I have now watched the video. And you are wrong. It does explain perfectly why things fall in curved space-time. 


Edited by Strange, 19 February 2017 - 04:12 PM.

  • 0

#27 StringJunky

StringJunky

    Genius

  • Senior Members
  • 6,103 posts
  • LocationUK

Posted 19 February 2017 - 04:19 PM

 

It sort of transfers the question. Instead of "why do things fall?" you now need to know, "why do things want to go "straight" when the orientation of local space is constantly changing? Why don't massive particles follow the orientation of the local space, as it changes with time?

Because they have Inertia, which is resistance to change in velocity or position when stationary which is a function of their mass and/or momentum.


Edited by StringJunky, 19 February 2017 - 04:22 PM.

  • 0

 Education, like life, is a journey not a destination


#28 mistermack

mistermack

    Atom

  • Senior Members
  • 201 posts

Posted 19 February 2017 - 06:09 PM

Because they have Inertia, which is resistance to change in velocity or position when stationary which is a function of their mass and/or momentum.

Yes, I thought of that. But change in position relative to what? Obviously not the space around the object. 

Is there some fixed set of coordinates that an object is stationary in?

If space in a gravitational field is curving with time, what frame of reference is there that is independent of that?

If there was a master frame of reference, that everything moved relative to, that you could get a fix on, it would all be a lot easier to picture. 


  • 0

#29 StringJunky

StringJunky

    Genius

  • Senior Members
  • 6,103 posts
  • LocationUK

Posted 19 February 2017 - 06:46 PM

Yes, I thought of that. But change in position relative to what? Obviously not the space around the object. 

Is there some fixed set of coordinates that an object is stationary in?

If space in a gravitational field is curving with time, what frame of reference is there that is independent of that?

If there was a master frame of reference, that everything moved relative to, that you could get a fix on, it would all be a lot easier to picture. 

The idea of stationary is dependent on  the observer co-moving with an object i.e moving in the same direction with the same speed. Along with motion, it's always relative to something. If you picture yourself on a rock in space and there''s nothing around you except another rock coming towards you... or are you moving towards it?  There's no way to know because an absolute reference doesn't exist.


Edited by StringJunky, 19 February 2017 - 06:47 PM.

  • 0

 Education, like life, is a journey not a destination


#30 DimaMazin

DimaMazin

    Protist

  • Senior Members
  • 843 posts
  • LocationVolgograd

Posted 26 February 2017 - 04:38 PM

Scientists should define is vacuum energy less closer to big mass. If so then object , increasing space/time and  increasing its energy , reduces vacuum energy  ( falling in gravity).


  • 0

#31 Pugdaddy

Pugdaddy

    Meson

  • Senior Members
  • 80 posts

Posted 26 February 2017 - 05:48 PM

I never have quite bought into the rubber sheet analogy. It suggests to me that spacetime and a massive object are separate entities that interact with each other. It just seems to me more plausible that the massive object is a fluctuation of spacetime that has caused spacetime to contract around that fluctuation drawing in the surrounding spacetime making the area around the massive object or fluctuation, for lack of a better analogy, less dense causing any other massive object to alter it's course through spacetime. 


Edited by Pugdaddy, 26 February 2017 - 05:58 PM.

  • 0

#32 Strange

Strange

    SuperNerd

  • Senior Members
  • 12,405 posts
  • Location珈琲店

Posted 26 February 2017 - 06:23 PM

I never have quite bought into the rubber sheet analogy. It suggests to me that spacetime and a massive object are separate entities that interact with each other.

 

 

That is what the theory says.

 

If you have an alternative model, please feel free to post the mathematics and the evidence in the Speculations forum.


Edited by Strange, 26 February 2017 - 06:24 PM.

  • 0

#33 JamSmith

JamSmith

    Quark

  • Members
  • 21 posts

Posted 28 February 2017 - 09:54 AM

I have read somewhre regarding this topic and I come across this information. 

 

Gravity is a weak force. The two objects will have an attraction to each other. But it will be so weak in comparison to other factors and to the attraction to the ground that the attraction to each other will not be noticeable. There is a concerted effort to obscure our understanding of gravity. So we do not see much research on the attraction between small objects.


  • -2

#34 imatfaal

imatfaal

    lazy do-nothing mudslinger

  • Moderators
  • 7,472 posts
  • LocationSt James's Park

Posted 28 February 2017 - 10:41 AM

I have read somewhre regarding this topic and I come across this information. 

 

Gravity is a weak force. The two objects will have an attraction to each other. But it will be so weak in comparison to other factors and to the attraction to the ground that the attraction to each other will not be noticeable. There is a concerted effort to obscure our understanding of gravity. So we do not see much research on the attraction between small objects.

 

Gravity is weak at human scales - the usual example is that a fridge has enough attraction to the fridge door to overcome all the mass of the earth; however gravity does not come in two polarities that are opposite and cancel out and as such is always attractive and does not get shielded this means it works on cosmological scales where the other forces tend to be unimportant. 

 

The force between two objects of a human scale is not noticeable in everyday life - but it is measureable Henry Cavendish was measuring the gravitational attraction of two objects in his lab over 200 years ago

 

There is no concerted effort to obscure our knowledge of gravity.  There is ignorance and there is culpable ignorance - if you had the will you would be able to download for free (and legally) texts/lessons from the very basics on places like Khan, through tougher stuff on edx.org etc, and finally to graduate level with Feymann's courses and Sean Carrol's lecture notes on GR.

 

There are huge numbers of programs dealing with measuring gravity at the human level - G Newtons Gravitational Constant is not known to  high precision so there is lots of work to try to lower the error margins on this very important constant


  • 2

A little learning is a dangerous thing; drink deep, or taste not the Pierian spring:
there shallow draughts intoxicate the brain, and drinking largely sobers us again.

- Alexander Pope

 

feel free to click the green arrow  ---->

 


#35 Strange

Strange

    SuperNerd

  • Senior Members
  • 12,405 posts
  • Location珈琲店

Posted 28 February 2017 - 12:53 PM

So we do not see much research on the attraction between small objects.

 

 

What about all the direct measurements of G performed by Cavendish and many others?

 

What about the CERN Alpha project that aims to test the gravitational effect of anti-matter using a small number of (anti) hydrogen atoms. You don't get much smaller than that.


  • 0

#36 imatfaal

imatfaal

    lazy do-nothing mudslinger

  • Moderators
  • 7,472 posts
  • LocationSt James's Park

Posted 28 February 2017 - 03:40 PM

What about the CERN Alpha project that aims to test the gravitational effect of anti-matter using a small number of (anti) hydrogen atoms. You don't get much smaller than that.

 

Perfect example - I was trying to think of a high profile one but couldn't get my mind off the Watt Balance at NIST and wasn't sure that was as relevant as I thought it was


  • 0

A little learning is a dangerous thing; drink deep, or taste not the Pierian spring:
there shallow draughts intoxicate the brain, and drinking largely sobers us again.

- Alexander Pope

 

feel free to click the green arrow  ---->

 


#37 JamSmith

JamSmith

    Quark

  • Members
  • 21 posts

Posted 1 March 2017 - 06:22 AM

Good answer not sure I could have said it better myself.

Lets look at another basic definition.
 

Thanks for the new insight.


 

Gravity is weak at human scales - the usual example is that a fridge has enough attraction to the fridge door to overcome all the mass of the earth; however gravity does not come in two polarities that are opposite and cancel out and as such is always attractive and does not get shielded this means it works on cosmological scales where the other forces tend to be unimportant. 

 

So can follow this information right?

 

The force of gravity is proportional to the size of the object (mass) and your position relative to it (distance from the center of gravity).

 

Here is the formula:

 

g=-GM/(r*r),

 

where, g is the acceleration of gravity,

G is the gravitational constant,

M is the mass of the object and

r is the radius or the distance of the observer from the center of gravity.


  • 0

#38 imatfaal

imatfaal

    lazy do-nothing mudslinger

  • Moderators
  • 7,472 posts
  • LocationSt James's Park

Posted 1 March 2017 - 11:19 AM

Thanks for the new insight.


So can follow this information right?

 

The force of gravity is proportional to the size of the object (mass) and your position relative to it (distance from the center of gravity).

 

Here is the formula:

 

g=-GM/(r*r),

 

where, g is the acceleration of gravity,

G is the gravitational constant,

M is the mass of the object and

r is the radius or the distance of the observer from the center of gravity.

 

That is the acceleration due to gravity (as you say) - the force is proportional to your mass, the objects mass, and the distance between

 

F = \frac{GMm}{r^2}


  • 1

A little learning is a dangerous thing; drink deep, or taste not the Pierian spring:
there shallow draughts intoxicate the brain, and drinking largely sobers us again.

- Alexander Pope

 

feel free to click the green arrow  ---->

 





0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users