# A beginners question

1. Mar 19, 2004

### Paul Fernihough

hi all,

i have a general interest in the big questions in physics but am no einstein. i am hoping that someone here may take the time to answer what i think maybe a basic flaw in my understanding of these things.

my understanding of einsteins notion of gravity is that a mass bends space-time & its this curvature of space-time that pulls objects in. therefore there is no force as such. objects are following the natural curvature of space-time. its the notion of no force that hit me yesterday. there has been no discovery (yet) of the mysterious graviton despite extensive searching & this would seem to back up the notion i have of no force of gravity. so, is gravity a force?

& if it is not a force then surely attempts to unify gravity & the 3 forces laid out in quantum mechanics would inevitably lead to peculiar results (infinities)? so why is this approach used to infer that general relativity must be wrong?

anyway, im fairly sure that i have only the vaguest handle on the concepts involved here, so im wanting someone to put me out of my misery.

many thanks,

PMF

2. Mar 19, 2004

Staff Emeritus
You are correct that in general relativity, gravity is a geometric effect. The force of gravity, which can of course be measured (weigh yourself on a spring scale!) then appears as what is called an "inertial force" - an apparent force that arises from a coordinate situation in spacetime (i.e. four dimensional geometry). Two other inertial forces are centrifugal force and coriolis force, both due to a coordinate situation in spacetime, in their case a rotating coordinate frame.

3. Mar 19, 2004

### Zhou Yu

I don't really have an answer to your first question, but I do have another question about gravity, so I figure this would be the best place for it.

"Are the effects of gravity instant"

For example, if earth is the only mass in the universe, then you add another large mass 2 light years away. Will the gravity from the new mass effect the earth straight away, or will it take 2 years...

My physics teacher seems to think it is instant, but I'm pretty sure I read somewhere that gravity isn't instant, probably somewhere in Einstein's theory of relativity.

Like "Paul Fernihough" I only have a limited knowledge on such things, then again being 15 doesn't help much in my case. Any information on the speed of gravity, or where I might find out about it would be most appreciated.

4. Mar 20, 2004

Staff Emeritus

General realtivity says that gravity - that is spacetime curvature - propagates at the speed of light in a vacuum, or c. But GR is only stated for smooth changes, so it just doesn't have these mass jumps in it.

5. Mar 20, 2004

### Zhou Yu

Ok, if you want to be absolutely perfect, then yes, a mass isn't just going to appear out of nowhere. I was just merely trying to simplify the question.

"There are no lack of candidates in Einstein's Relativistic Universe. Powerful gravitational waves are thought to be generated by non-spherical, large-scale vibrations of spacetime -- such as occur when a star explodes and its core collapses, two neutron stars collide, or two black holes spiral towards each other ,then coalesce."

http://archive.ncsa.uiuc.edu/Cyberia/NumRel/LIGO.html

So if any of the above events happened and caused a slight change in their gravity, would that change effect us instantly or not.

So it would seem that gravity is believed to propagate at the speed of light, does anyone else have a way to prove this? Either by experiment, observation, or in theory.

6. Mar 20, 2004

### Chen

It is the gravitational field that takes time to propagate, not "gravity" itself. Think of it this way: if you hold a sheet of paper between your hands and someone suddenly places a rock in the middle, will it instantly curve the paper or will the change happen gradually? And if there are two more rocks already on the paper, and one is closer to the point in which the new rock is placed, which one will start rolling towards the new rock (due to the curvature of the paper) first?

7. Mar 20, 2004

### modmans2ndcoming

On the strings board I think you mean you are no Ed Whitten ;-)

8. Mar 20, 2004

### modmans2ndcoming

something to mention....M-Theory predicts that Gravity is actually as strong as the EM force, but because it the gravitons can travel off our membrane into other membranes, the effect of gravity in our universe seems smaller.

check out "the elegant universe" on PBSs home page...it is a Nova program...really cool. ( I was kinda ticked that they protected their Quicktime movie from being able to be captured by QTpro though :-/ )

9. Mar 21, 2004

### Zhou Yu

http://members.optusnet.com.au/donchichio/string.html [Broken]

This was posted somewhere else in the forums, its a link where you can download the "Elegant Universe" movie. I'm not sure if its still working, but it worked for me a couple of weeks ago. Worthwhile to just check anyway.

In regards to gravitons being closed loop strings, and therefore being able to travel off our membrane. Well I've got a couple of questions...

1) When they travel off our membrane, do they immediately pass into another membrane, or is there a higher dimension of space between the membranes that it can also exist in?

2) If we know gravity is N times weaker than EM, could we then calculate how big the higher dimension of space is ( in which the membranes exist ) ?

Also just for the sake of the movie, my final question is "what does the M in M-Thoery stand for?"

Last edited by a moderator: May 1, 2017
10. Mar 21, 2004

Staff Emeritus
In the theory where gravity propagates off our brane, the brane is thought of as the boundary of a higher dimensional brane, as a two dimensional surface can be the boundary of a three dimensional solid. The physics on the lower dimensional brane entirely determines the physics on the higher dimensional one; this is called the holographic conjecture, and is one of the active parts of string physics. The gravity idea is just a feature of it. And yes, they can work out from their theory the approximate order of magnitude of gravity to the other forces and it matches. But I suspect they tuned the theory to get this result, so it wouldn't count as a prediction.

11. Mar 21, 2004

### modmans2ndcoming

12. Mar 21, 2004

### modmans2ndcoming

now if I can just find a way around the DRM so I can transcode it.

13. Mar 26, 2004

### neotrekkie

dark energy from the higher branes?

1) When they travel off our membrane, do they immediately pass into another membrane, or is there a higher dimension of space between the membranes that it can also exist in?

2) If we know gravity is N times weaker than EM, could we then calculate how big the higher dimension of space is ( in which the membranes exist ) ?

Also just for the sake of the movie, my final question is "what does the M in M-Thoery stand for?"[/QUOTE]

1) sorry don't no this, but can "particles" or "strings" in the higher brane infuence our brane? could it have someting to do with the missing dark matter or dark enery? If gravity is the only force to efffect outhet branes then perhaps things in the higer branes effects us? Of course how can you prove the a things in the higher brane you can't detect is the reason we have dark mater end energy :-)

M stands for magic, mystery or matrix. Whatever of thiese you like. At least this is what Ed Witten said in "the Elegant Universe"

14. Apr 3, 2004

### Javier

Hello,
some comments: the particular model where our 4D world is a brane at a boundary of a 5D bulk spacetime is known as the "braneworld scenario". This is distinct from the more general AdS/CFT conjecture relating (with technicalities of course) a 5D spacetime with a particular string theory (and its low energy supergravity theory) to a 4D spacetime with supersymmetry but no gravitation. As mentioned, one can do computations with the 4D supersymmetric field theory and get information about the particular 5D string theory (or its low energy supergravity limit); this is a holographic principle.
In the braneworld scenario, the correspondence is realized literally: a 4D brane without gravitation is embedded in the bulk 5D spacetime with gravitation. Even though gravitation does not "live on the brane", the other fields on the brane couple to the gravitational field in the bulk 5D space. One can tune things so that the appearance of gravitation from the perspective of 4D observers is very weak compared to the interaction strengths of the other interactions.
However, this is not M-theory, rather a particular feature in a particular string theory (which is an effective theory from the point of view of M-theory) that may point out fundamental features of whatever M-theory ends up being.
String theory is not needed to get a relatively large number of candidates for the source of "dark energy". For example, supersymmetry alone introduces new scalar fields, some of which are candidates.
cheers.

15. Apr 4, 2004

### Dlanorrenrag

Membrane Time

TIME
Under string theory, in what time do the hypothetical metaphorical strings vibrate? Does the vibrating go on in our time, or is there a 12th dimension, being a metaphysical time in which the strings vibrate? Does the whole of the universal membrane to which all the particular strings cohere keep its own time? Relative to its own time, is the universal membrane able to impulse all of its metaphorical strings in unison? If not, how is any one universal membrane separately identified in relation to all other universal membranes?

VIBRATION
Is our entire universe merely what manifests in sync with a grand vibration of the universal membrane, as well as in sync with all the strings which cohere to it? If, somehow, we were to get out of sync, would we be in a different universe? Do our strings and universal membrane “really” vibrate, or are we merely repelled, to occupy a higher, vibrating, metaphysical space time that is not occupied by any other universe?

16. Apr 4, 2004