# The Naked Scientists Forum

### Author Topic: Is there a "force of gravity"?  (Read 13730 times)

#### Pmb

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##### Is there a "force of gravity"?
« Reply #25 on: 05/09/2011 19:15:53 »
It's quite all right to think of gravity as a force, so long as that force is not a 4-force. I explained all this at

http://home.comcast.net/~peter.m.brown/gr/grav_force.htm

Please forgive me for the misssing images. They are missing drawings and missing equations. I'll get it up to date sometime in the future.

The concept of force in GR is used in "Basics Relativity," by Richard M. Mould. Springer-Verlag Press

Well jolly good, but what exactly do you mean by 4-force? Einstein was pretty clear about gravity not being a force. Is this an alternative theory?
A 4-force is dwfined in the link above. I'll try to get it up today. In the mean time I'll just say that a 4-force is defined as F = DP/dt where D/dt is the absolute derivative defined here
http://home.comcast.net/~peter.m.brown/math_phy/absolute_derivative.htm

Also, Einstein never opposed the notion of gravity not being a force. In fact he used that notion in his text The Meaning of Relativity. Do you have a copy of this text? If so then turn to page 83 where Einstein answers your question. Einstein writes
Quote
The gravitational field transfers energy and momentum to the "matter" in that it exerts forces upon it and gives it energy:...
I wrote an article about how Einstein viewed gravity. It's online here
http://xxx.lanl.gov/abs/physics/0204044

#### Geezer

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##### Is there a "force of gravity"?
« Reply #26 on: 07/09/2011 07:24:07 »
Do you have a copy of this text? If so then turn to page 83 where Einstein answers your question. Einstein writes
The gravitational field transfers energy and momentum to the "matter" in that it exerts forces upon it and gives it energy:...

Found it, thanks! But now my brain hurts.

#### JP

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##### Is there a "force of gravity"?
« Reply #27 on: 08/09/2011 00:11:39 »
Do you have a copy of this text? If so then turn to page 83 where Einstein answers your question. Einstein writes
The gravitational field transfers energy and momentum to the "matter" in that it exerts forces upon it and gives it energy:...

Found it, thanks! But now my brain hurts.

Like everything else, so much misunderstanding comes from not realizing that fundamental concepts in Newtonian mechanics don't generalize simply to modern physics, be it relativity or quantum mechanics.  The word "force" groups together a lot of things in Newtonian mechanics that not only can change the energy/momentum of an object, but which also all satisfy F=ma.

In GR, gravity and all other forces can still change the energy/momentum of an object.  But gravity is modeled by Einstein's equations, which can be interpreted geometrically.  Non-gravitational forces all still satisfy something similar to F=ma.

So gravity is special for reasons that have been discussed at length in this thread already.  What you choose to label it as isn't nearly as important as understanding that it's different.

#### MikeS

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##### Is there a "force of gravity"?
« Reply #28 on: 11/09/2011 12:23:18 »
Quote Geezer
"It seems to me that there is no "force of gravity", although gravity clearly accelerates mass. What's wrong with this reasoning?
F = m.a"

Acceleration is a change in velocity over time.  So just by varying the passage of time you can have an acceleration.  Any gravitating body dilates the passage of time near its surface.

Gravity produces the effect of acceleration by altering the passage of time (by warping or bending space-time).
Normally one would expect a force to be required to produce an acceleration but in this instance it is brought about by the geometry of space-time.

#### Geezer

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##### Is there a "force of gravity"?
« Reply #29 on: 11/09/2011 16:34:49 »

Gravity produces the effect of acceleration by altering the passage of time (by warping or bending space-time).

So, if I'm in free fall and observe my velocity using my wrist watch, I will find that my velocity is constant? I don't think so

#### MikeS

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##### Is there a "force of gravity"?
« Reply #30 on: 12/09/2011 09:31:30 »
Geezer you are obviously quite right.  The question is why?  Each of the above statements (in my last post) I believe to be true but the time dilation factor is no where near enough to account for the acceleration.

Added 19-09-11
Using a gravity calculator to check gravitational acceleration over 2000km, equating acceleration to the strength of gravity at any altitude and equating that to the time dilation feature, it can be estimated, time dilation is enough to account for the acceleration.
« Last Edit: 19/09/2011 11:00:23 by MikeS »

#### Geezer

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##### Is there a "force of gravity"?
« Reply #31 on: 12/09/2011 21:13:41 »

The question is why?

How the heck would I know

Gravity remains a great mystery. The bendy space/time model seems to work really well in terms of describing and predicting the effects. I wonder it we will ever get much further than that.

#### JP

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##### Is there a "force of gravity"?
« Reply #32 on: 12/09/2011 22:01:33 »
Gravity remains a great mystery. The bendy space/time model seems to work really well in terms of describing and predicting the effects. I wonder it we will ever get much further than that.

Is there a scientific theory that's can do more than being able to describe and predict effects?

#### Geezer

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##### Is there a "force of gravity"?
« Reply #33 on: 13/09/2011 00:01:29 »
Gravity remains a great mystery. The bendy space/time model seems to work really well in terms of describing and predicting the effects. I wonder it we will ever get much further than that.

Is there a scientific theory that's can do more than being able to describe and predict effects?

Picky, picky! Some theories provide a bit more insight into the underlying mechanisms than others. The underlying mechanism of gravity remains mysterious (unless somebody figured it out while I wasn't paying attention.)

As I can see no harm in teaching you how to suck eggs, I should point out that "science" literally means "knowledge" (which you would know if you had ever taken the trouble to study a bit of Latin.) The fact that we promulgate a bunch of theories to explain "what" stuff does does not necessarily contribute to the "knowledge" of how the heck it does it. Ergo (another Latin word), a "theory" does not necessarily contribute to science.

#### MikeS

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##### Is there a "force of gravity"?
« Reply #34 on: 19/09/2011 10:32:16 »
Quote Geezer
"It seems to me that there is no "force of gravity", although gravity clearly accelerates mass. What's wrong with this reasoning?
F = m.a"

Acceleration is a change in velocity over time.  So just by varying the passage of time you can have an acceleration.  Any gravitating body dilates the passage of time near its surface.

Gravity produces the effect of acceleration by altering the passage of time (by warping or bending space-time).
Normally one would expect a force to be required to produce an acceleration but in this instance it is brought about by the geometry of space-time.

Gravity produces the effect of acceleration by altering the passage of time (by warping or bending space-time).

So, if I'm in free fall and observe my velocity using my wrist watch, I will find that my velocity is constant? I don't think so

Geezer

An object free falling to Earth is seen to be accelerating.
An object free falling to Earth is experiencing time dilating more and more the closer it gets to the Earth.
Time dilating allows more distance to be covered per unit time.
This is acceleration due to time dilation.
This is an affect of gravity.

Your watch in free fall measures more distance being covered per unit time precisely because each unit of time is getting longer.  This effect is probably being diluted by other gravitating bodies like the solar system, our galaxy etc.

Time dilation due to the gravity of the Earth is a very small effect in the much larger picture.  To put this into context we must remember that gravity can make time stand still at the event horizon of a black hole or the passage of time could be infinite.  This gives a huge rage to the potential passage of time.
« Last Edit: 19/09/2011 10:37:21 by MikeS »

#### Geezer

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##### Is there a "force of gravity"?
« Reply #35 on: 20/09/2011 01:12:35 »

Your watch in free fall measures more distance being covered per unit time precisely because each unit of time is getting longer.

Erm, are you sure about that? If that were the case, you wouldn't observe any acceleration at all because the time to travel any distance would be proportional to the distance traveled, which would mean you had constant velocity.

#### MikeS

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##### Is there a "force of gravity"?
« Reply #36 on: 20/09/2011 07:59:07 »

Your watch in free fall measures more distance being covered per unit time precisely because each unit of time is getting longer.

Erm, are you sure about that? If that were the case, you wouldn't observe any acceleration at all because the time to travel any distance would be proportional to the distance traveled, which would mean you had constant velocity.

Geezer
Yes, when observed from a great distance this is true.  The geometry of space-time produces a constant velocity.  I 'believe' it is in accordance with relativity.

Normally one would expect a force to be required to produce an acceleration but in this instance it is brought about by the geometry of space-time.

#### imatfaal

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##### Is there a "force of gravity"?
« Reply #37 on: 20/09/2011 10:17:56 »
Mike - do the maths!  If I drop a ball out of my office window (well my old office) after a second the ball is doing 10 metres per second - after two seconds it is doing twice that speed; please don't tell me that this effect is due to time dilation.

And if you read the thread on falling into black holes you will find that time doesn't actually come to a stop a the Event horizon of a black hole - that is an artefact of using the wrong form of coordinates for the job; whilst schwarzchild coordinates are great for outside the black hole near the eh they create a mathematical singularity (ie you get undefined/infinite answers) - better choices of coordinates remove this anomaly.

#### MikeS

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##### Is there a "force of gravity"?
« Reply #38 on: 21/09/2011 16:06:48 »
Mike - do the maths!  If I drop a ball out of my office window (well my old office) after a second the ball is doing 10 metres per second - after two seconds it is doing twice that speed; please don't tell me that this effect is due to time dilation.

And if you read the thread on falling into black holes you will find that time doesn't actually come to a stop a the Event horizon of a black hole - that is an artefact of using the wrong form of coordinates for the job; whilst schwarzchild coordinates are great for outside the black hole near the eh they create a mathematical singularity (ie you get undefined/infinite answers) - better choices of coordinates remove this anomaly.

imatfaal
Yes, I think it is due to time dilation and I agree it sounds impossible but, I suspect, that may just be a matter of scale and our arbitrary way of measuring the passage of time.
"gravity can make time stand still at the event horizon of a black hole" "from the perspective of a distant observer" I should have added.

#### Pmb

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##### Is there a "force of gravity"?
« Reply #39 on: 22/09/2011 02:52:52 »
I forgot to mention that Newton defined force as F = dp/dt, not F = ma. The former is in the beginnning of the Principia and the later is Euler's expression for the force/mass/acceleration relastionship. The expression F = dp/dt is relativistically correct, where generally F = ma is not.

#### Geezer

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##### Is there a "force of gravity"?
« Reply #40 on: 22/09/2011 08:22:18 »
I forgot to mention that Newton defined force as F = dp/dt, not F = ma. The former is in the beginnning of the Principia and the later is Euler's expression for the force/mass/acceleration relastionship. The expression F = dp/dt is relativistically correct, where generally F = ma is not.

What's the difference?

#### Pmb

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##### Is there a "force of gravity"?
« Reply #41 on: 27/09/2011 19:08:37 »
I forgot to mention that Newton defined force as F = dp/dt, not F = ma. The former is in the beginnning of the Principia and the later is Euler's expression for the force/mass/acceleration relastionship. The expression F = dp/dt is relativistically correct, where generally F = ma is not.

What's the difference?
They don't always have the same value, e.g. for v << c the mass of a rocket which is accclerating from its engines being turned on, and when the velocity is relativistic. Its best to define F = dp/dt and when v << c and m = constant.

#### Geezer

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##### Is there a "force of gravity"?
« Reply #42 on: 27/09/2011 22:33:46 »
I forgot to mention that Newton defined force as F = dp/dt, not F = ma. The former is in the beginnning of the Principia and the later is Euler's expression for the force/mass/acceleration relastionship. The expression F = dp/dt is relativistically correct, where generally F = ma is not.

What's the difference?
They don't always have the same value, e.g. for v << c the mass of a rocket which is accclerating from its engines being turned on, and when the velocity is relativistic. Its best to define F = dp/dt and when v << c and m = constant.

Can rockets achieve relativistic velocities?

#### Pmb

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• Physicist
##### Is there a "force of gravity"?
« Reply #43 on: 27/09/2011 22:51:35 »
Quote from: Geezer
Can rockets achieve relativistic velocities?
Theoreticaly, yes. But that would be to impractical to build.

#### imatfaal

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##### Is there a "force of gravity"?
« Reply #44 on: 28/09/2011 10:57:57 »
Quote from: Geezer
Can rockets achieve relativistic velocities?
Theoreticaly, yes. But that would be to impractical to build.

Theoretically - Not really.

It is clear from the above (MR is mass ratio of Massfull over Massempty) that to accelerate a 1000kg rocket with a exhaust velocity of 10000 m/s (ie double what is currently achievable to make maths simpler) to a tenth of the speed of light you would need the mass of the full rocket to be 1000e^3000kg  - That is the mass of many many universes. Even taking the velocity of exhausted gases up to 10^6 m/s would require a rocket that outweighs an asteroid (10^17kg) - and that's at maximal efficiency and with make-beleive technology

Realistically - it is doubtful that even .05c is on the cards, even with the exotic tech envisaged for the future such as ion drive and fusion drive

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##### Is there a "force of gravity"?
« Reply #44 on: 28/09/2011 10:57:57 »

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