What exactly is gravity?

[juggernot]

Up until a few months ago I thought gravity was simply an unexplainable force which pulled matter together. I've recently discovered that gravity is actually a bending in Space/Time caused by matter itself!:) I always wondered why black holes can trap light, because I thought that gravity could only affect matter and not energy. Apparently light always travels straight and it is Space/Time that bends. Awesome. This new revelation has opened up some new questions for me as I researched the topic.

 

-What is a geodesic?

-what is the 4th dimension?

-how are a geodesic and the 4th dimension related?

-What are gravity waves or gravity radiation?

-What is Frame Dragging?

I-f spinning objects affect space time differently, do they have greater 'gravitational pull'?

 

I'm sure that these answers will also result in more questions. If people feel that these questions warrant seperate topics, I will edit this post and make it so. Thanks in advance for your help.

 

Note: To fully understand this topic, I will need to know more about space/time, but I will save that for another thread. Right now I only really know the principle through analogies.

[modest]

Up until a few months ago I thought gravity was simply an unexplainable force which pulled matter together. I've recently discovered that gravity is actually a bending in Space/Time caused by matter itself!

 

There's a recent thread discussing the differences between Einstein's interpretation of curved spacetime vs. a quantum mechanical explanation using gravitons:

 

15394

 

I always wondered why black holes can trap light, because I thought that gravity could only affect matter and not energy.

 

According to General Relativity... Regardless of how much energy a particular beam of light has, it will follow a null geodesic through spacetime. If spacetime is flat (representing no source of gravity) then the geodesic is a straight line. If there is a source of gravity then spacetime is curved and the geodesic for which light follows is curved.

 

In the case of a black hole spacetime is curved such that all future pointing null geodesics behind the event horizon lead to the singularity.

 

-What is a geodesic?

 

In GR a geodesic is the path a body or particle will take when no force is acting on it. As such, it feels no acceleration.

 

-what is the 4th dimension?

 

In relativity, it is temporal. There are 3 spatial coordinates and 1 time coordinate on a curved manifold (4-dimensional Lorentzian manifold) which is most often called "spacetime".

 

Einstein's field equations describe the curvature of this spacetime given some distribution of energy and momentum (mass).

 

-how are a geodesic and the 4th dimension related?

 

The geodesic exists on the 4 dimensional Lorentzian manifold (explained in the link above) as a straight line between two spacetime events. The most common way to think of this is with a globe. A geodesic is some line segment of a great circle (such as a line of longitude). The surface of the globe is 2 dimensional and the geodesic is straight in that 2 dimensional setting. It is, however, curved in 3 dimensions and when the globe is flattened into a map the geodesic and the path of the particle it represents is curved.

 

A real world example would be earth orbiting the sun. The geodesic of earth's path is straight on the manifold, but the path is curved from our perspective.

 

-What are gravity waves or gravity radiation?

 

I don't know what gravity radiation would refer to (?graviton?), but gravitational waves in relativity are most cool. Wikipedia does them more justice than I'd be able.

 

Gravitational wave - Wikipedia, the free encyclopedia

 

-What is Frame Dragging?

 

Rotating massive bodies affect space and time by 'dragging' them with the rotation.

 

Frame-dragging - Wikipedia, the free encyclopedia

 

I-f spinning objects affect space time differently, do they have greater 'gravitational pull'?

 

I'll have to look into this... I'm not sure what would be expected.

 

~modest

[juggernot]

Thank you for your answer. I'd like to ask a follow up question if I may. So when a force does act on an object, such as magnetism or kinetic energy it is bumped into another geodesic path, correct? So when someone is skydiving they are simply following their geodesic path. However if someone jumps on a trampoline they move to another geodesic path. This path however, also points to earth because mass warps space time creating the effect we call gravity. If a geodesic refers to the path an object will take if no forces are acting on it, how does this work with the law: an object in motion will tend to stay in motion, at object at rest will tend to stay at rest. If no forces are acting on an object and it is at rest, is it still on a geodesic path?

[modest]

So when a force does act on an object, such as magnetism or kinetic energy it is bumped into another geodesic path, correct?

 

Yes. It’s usually said that a world line (or particle path) that is acted on by a force is not a geodesic. But, I think you’re talking about a world line that is following a geodesic then gets redirected by some kinetic or electrostatic interaction and gets set on a new geodesic. And, that’s right. I agree with the way you characterize it being “bumped into another geodesic path”.

 

So when someone is skydiving they are simply following their geodesic path.

 

Yep. Air resistance makes it not quite a geodesic, but the idea is correct... the worldline of an object in freefall is a geodesic.

 

However if someone jumps on a trampoline they move to another geodesic path. This path however, also points to earth because mass warps space time creating the effect we call gravity.

 

Yes. Earth’s escape velocity is 11.2 km/s. Excluding air pressure as a variable, we could say that any worldline with less than 11.2 km/s would intersect the ground.

 

If a geodesic refers to the path an object will take if no forces are acting on it, how does this work with the law: an object in motion will tend to stay in motion, at object at rest will tend to stay at rest. If no forces are acting on an object and it is at rest, is it still on a geodesic path?

 

Sure. Even if we consider something "at rest" in the three spatial dimensions, it still moves through time. In a spacetime diagram, this would be a worldline that points straight up, and it surely can be a geodesic.

 

These, by the way, are really good questions. When being introduced to this stuff, I bet a lot of people have a bit too much trouble with the basic ideas to get to things like you're bringing up. Impressive.

 

~modest

[juggernot]

I understand how gravity affects things spatially, but I have a question about how if affects time. My basic question is why does gravity cause things to accelerate. After all, if matter is causing space itself to bend, I undersand why gravity 'pulls' on us. However I don't understand why we accelerate towards an object and not just move at a constant rate. I have a theory about why this is, but I'm not sure if it is right. When space/time is bent by the earth, it curves my geodesic so that I will end up on Earths surface, however it also bends time so that I reach my geodesic destination sooner than I should? This would mean we experience time differently though, which would probably have other consequences, making this theory very unlikely. I also don't understand why there is a maximum falling speed. Even though the Earth is curving my Geodesic, I am also curving the Earths right? So does that mean that the maximum falling speed is different for every two bodies of mass? For instance I would fall slower on another planet? Or a whale would fall faster then me on earth?

 

For example, you said an object at rest spatially, but moving through time would be a worldline that points straight up. If no forces were to ever act on that object it would travel through infinite time, but no space. Can Space/Time be simply thought of as a grid then? With Y being Time and X being space? If we say that the normal space/time ratio is a 60 degree angle, then mass could cause space/time to bend so that it is a 30 degree angle, meaning that we traverse the same amount of space in less time. What about light, and the speed of light? For the speed of light to be the fastest possible speed that would mean it would be a 0 degree angle(covers an infinite amount of space in no time). But wouldn't that make the speed of light something we couldn't measure?

 

If whoever answers could attempt a visual explanation that would be great.

[modest]

Sorry this took me so long to answer.

 

If whoever answers could attempt a visual explanation that would be great.

 

Yes, I think that will help.

 

I understand how gravity affects things spatially, but I have a question about how if affects time. My basic question is why does gravity cause things to accelerate. After all, if matter is causing space itself to bend, I undersand why gravity 'pulls' on us. However I don't understand why we accelerate towards an object and not just move at a constant rate.

 

Very good question. Considering time as curved into our spatial dimensions by gravity will explain this. It’s not easy to describe, but show’s pretty well graphically.

 

The first example has no gravity - I guess I should say, it has no spacetime curvature. It is a flat spacetime grid. Moving up on the grid is moving forward in time. Moving left or right is moving through the spatial dimension. The green line represents the surface of a planet and the red line is our “free falling” observer.

 

 

Notice the two do not intersect. This is because spacetime is not curved and there is no gravity.

 

The next pic:

 

 

shows time curved into the spatial dimension by the mass of the planet. The green line is the surface of the planet. It is not a geodesic. It feels acceleration (notice how you feel acceleration right now on the surface of a planet). The red line is the freefalling observer which is a geodesic. It moves toward the surface of the planet - first slowly, then more rapidly. It does this following a geodesic on the grid, feeling no acceleration. Yet from the planet’s surface it appears to accelerate toward the planet, and intersect with the surface.

 

I also don't understand why there is a maximum falling speed. Even though the Earth is curving my Geodesic, I am also curving the Earths right? So does that mean that the maximum falling speed is different for every two bodies of mass? For instance I would fall slower on another planet? Or a whale would fall faster then me on earth?

 

The speed that you fall is determined by how curved spacetime is. A more massive planet will curve spacetime more and you will fall faster toward it. Ignoring Earth’s atmosphere (pretending it didn’t have one). We could consider an earthling falling some 50 meters height and hitting the surface of the earth. A mooninite falling 50 meters on the moon would fall with less speed toward the ground and would hit the ground of the moon slower than the earthling. This is because earth has more mass and curves spacetime more.

 

However, two objects of different mass on the moon will fall at the same speed. If both are dropped from the same height, they will hit the ground at the same time. I believe an astronaut did this with a feather and a hammer. Because there’s no atmosphere they both fell at the same speed. Both follow the same geodesic toward the ground. Both accelerate equally toward it.

 

Can Space/Time be simply thought of as a grid then? With Y being Time and X being space?

 

Yes, it’s not quite as simple as it sounds, but what you describe is a Minkowski diagram of spacetime.

 

If we say that the normal space/time ratio is a 60 degree angle, then mass could cause space/time to bend so that it is a 30 degree angle, meaning that we traverse the same amount of space in less time. What about light, and the speed of light? For the speed of light to be the fastest possible speed that would mean it would be a 0 degree angle(covers an infinite amount of space in no time). But wouldn't that make the speed of light something we couldn't measure?

 

Here you've steered us into special relativity. On a spacetime diagram, nothing can move faster than light. If light moves one unit of space per one unit of time, then it looks like this:

 

 

Light is massless, and it is the fastest moving thing. This means all mass will have a steeper worldline on the diagram than light:

 

 

This is a ridiculously sparse explanation I'm giving you, so I encourage you to check out wikipedia's articles on special relativity and Minkowski diagram

 

And this is also a wonderful little applet to play with,

 

http://www.adamtoons.de/physics/relativity.swf

 

I will be here for any questions as best as I can answer them and I apologize again for letting your post fall through the cracks and taking so long to answer.

 

~modest

[Pluto]

G'day from the land of ozzzzzzz

 

Hello Modest, I'm learning just by reading your response.

 

Thank You.

 

======================================

[ChunTzu]

what is the 4th dimension?

 

I really hope I'm not bastardizing this but I'm going to attempt a brief explanation of another interpretation of the 4th dimension, which is not altogether related to you question of gravity, I just want to put it out there to chew on. Maybe somebody more qualified can back me up, here. I'm not a scientist, just an enthusiast...

 

According to Hyperspace Theory (as explained by Michio Kaku in Hyperspace: A Scientific Odyssey Through Parallel Universes, Time Warps, and the 10th Dimension, really easy to find on Amazon, etc), the 4th dimension is not made up of time or spacetime, but is actually the next spatial dimension above the 3rd spatial dimension. In a more macro view, the universe is actually made up of 10 dimensions that were split into 6 and 4 as a result of the Big Bang. The 6 dimension universe shrank to such an infinitesimal size that it can no longer be detected while we reside in the alternate 4 dimensional universe.

 

The 4th spatial dimension should be thought of in the context of the human grasp of 1D (x dimension, a straight line) , 2D (i.e. x & y dimension, folding a piece of paper), and 3D (i.e. x, y, & z dimension, folding a piece of paper into a cube). Control of the 4th dimension can more easily be understood in a simile: imagine a person (existing in 3D space) peels stickers off a piece of paper (existing in 2D space); now imagine a being of some kind in 4D space peeling stickers off a 3D piece of paper. As I understand it, there's no proof of anything in 4D space being detected though there has been projections of 4 dimensional shadows from unseen objects.

 

The existence of multiple spatial dimensions numbering greater than the tried and true 3 folds into one of the proposed greater calculations for the Theory of Everything. The idea, here, being that each higher dimension would effectively give us more "room" to figure out the laws and formulas of the dimensions "below" it, which are most importantly assumed to be simpler and more compact. Therefore, if humans ever harnessed the power to achieve existence in something like the 10th dimension, we could possibly have one calculation which could explain every single thing in our known 3D universe, thus the Theory of Everything. This would undoubtedly touch upon your question, "what is gravity?" while possibly giving us an avenue to control it.

 

This is, interestingly, rumored to be what Einstein was trying to figure out for the last 30 years of his life, after his presentation of the Theory of Relativity. Papers full of notes of this wispy "all-encompassing, universal view", which he himself did not fully understand at the time, were left on his desk after his death.

 

That's as accurate and brief as I can make it. Corrections are welcome because this stuff is fascinating to me and I want to be right.

 

 

 

P.S. I haven't read this entire site and can't speak for its credibility but there are some more resources related to multiple spatial dimensions available here.

[ChunTzu]

Ah ha! I knew something had to be around here: Hyperspace on Hypography

[truth_united]

juggernot

what is the 4th dimension?

 

As explained by modest, 4th dimension is time. Further, gravity is a direct consequence of interaction between the 3 dimensions of space, the 4th dimension of time, and the matter existing within this space-time.

 

Theoretically, we can have even more dimensions but our human capabilities would perhaps be limited in perceiving them. One can only wonder what other phenomena would be a consequence of interactions between these other unknown dimensions and the known 4 dimensions!

 

truth_united

[Pluto]

G'day from the land of ozzzzzzz

 

Matter has three dimensions.

 

Time is a measure of motion. Giving the 4 dimension.

 

===================================================

 

What is the difference between a graviton an a photon?

[freeztar]

What is the difference between a graviton an a photon?

 

Observation?

 

-or-

 

One is real, the other is not? ;)

 

-or-

 

A bipolar/tri-polar spectrometer? :)

 

It reminds me of the joke I just made up.

 

A graviton and a photon walk into a bar. The bartender asks "What'ya havin'?". The photon says, "Whatever he's havin'". :)

[Pluto]

G'day from the land of ozzzzz

 

In reference to gravity I came across this link.

 

It maybe of interest to some, or maybe a discussion.

 

Do I understand it,,,,,,,not yet

 

[astro-ph/0608567] Gravitational Wave Sources from New Physics

 

Gravitational Wave Sources from New Physics

 

Authors: Craig J. Hogan

(Submitted on 25 Aug 2006)

 

Abstract: Forthcoming advances in direct gravitational wave detection from kilohertz to nanohertz frequencies have unique capabilities to detect signatures from or set meaningful constraints on a wide range of new cosmological phenomena and new fundamental physics. A brief survey is presented of the post-inflationary gravitational radiation backgrounds predicted in cosmologies that include intense new classical sources such as first-order phase transitions, late-ending inflation, and dynamically active mesoscopic extra dimensions. LISA will provide the most sensitive direct probes of such phenomena near TeV energies or Terascale. LISA will also deeply probe the broadband background, and possibly bursts, from loops of cosmic superstrings predicted to form in current models of brane inflation.

[modest]

In reference to gravity I came across this link.

 

It maybe of interest to some, or maybe a discussion.

 

Do I understand it,,,,,,,not yet

 

[astro-ph/0608567] Gravitational Wave Sources from New Physics

 

Gravitational Wave Sources from New Physics

 

Authors: Craig J. Hogan

(Submitted on 25 Aug 2006)

 

The mission is explained well at this link:

 

LISA and the search for Einstein's waves

 

Scheduled for launch in 2016, LISA will be the largest scientific instrument ever constructed, consisting of three spacecraft, each separated by 5 million kilometres (3 million miles). Its task will be to detect the elusive gravitational waves which were predicted by Einstein’s Theory of General Relativity, published in 1916. To date, although astronomers have indirect evidence of their existence, none have yet been detected directly.

 

LISA will be one of the most challenging space science missions ever flown. In order to detect the passage of a gravitational wave, the distance between the spacecraft must be measured by laser beams to an accuracy of ten picometres, about one millionth of the diameter of a human hair!

 

Gravitational waves are emitted when very massive objects such as black holes spiral violently together or when neutron stars collide at high speed. These invisible waves squeeze and stretch spacetime as they travel to us from distant parts of the universe,

 

The indirect evidence that the link talks about was done by Hulse and Taylor and got them the '93 Nobel prize for physics. It is summarized somewhat sparsely here.

 

Gravitational waves are indeed very cool and observing them directly would be an amazing achievement. I hope LISA fares better than gravity probe B.

 

~modest

[Pluto]

G'day Modest

 

 

"Thank you very much" as Elvis would say.

 

Now I have to do more reading.

 

When will it ever stop?

 

Would we ever know what gravity is and its properties?

[Pluto]

G'day modest

 

I have read the links.

 

But! it has open another door.

Reference to recyling pulsars and waves from compact matter sounds very interesting, I'll be back in a month of Sundays.

 

They do mention that they did not observe gravity waves from LIZA.

 

I wonder if the senses are too close.

[maddog]

Up until a few months ago I thought gravity was simply an unexplainable force which pulled matter together. I've recently discovered that gravity is actually a bending in Space/Time caused by matter itself!:) I always wondered why black holes can trap light, because I thought that gravity could only affect matter and not energy. Apparently light always travels straight and it is Space/Time that bends. Awesome. This new revelation has opened up some new questions for me as I researched the topic.

I am currently reading a book I bought awhile ago: Quintessance: Missing

Mass by Lawrence Kraus. I am currently reading the discussion about

Newtons' own concern over his theory over that particular issue - "Action

at a distance". At the time the mathematics of fields of force was not yet

formulated.

 

What is a geodesic?

Imagine moving around on a two dimensional surface (two coordinates).

Now imagine that surface wrapped onto a sphere. This new warped

surface will be a geodesic with still two coordinates relative to the sphere

and three coordinates relative to before the transformation (cartesian or

polar).

 

-what is the 4th dimension?

This is covered a lot in a book by Abbot, Flatland. Read it. A four

dimensional manifold (assumed to differentiable) is similar to the three

dimensional variety you familiar with just by adding a fourth coordinate.

The huge difference here this extension with all coordinates equivalent

would become a 4-dimensional vector space. We are more familiar with

one of these coordinates being a scaler (no direction). So adding a

dimension can be either way, it becomes important to know how that

added parameter is to be utilized.

 

-how are a geodesic and the 4th dimension related?

The physics of this was answered otherwise. Mathematically, a geodesic

is an example of vector space (coordinate system) being transformed in

one of one or more dimensions higher (two into three above, or three into

four).

 

-What are gravity waves or gravity radiation?

a) Gravity Waves are described much in more detail in a book called

Gravitation, Misner, Kipp, Thorne. Without the detailed mathematics,

you can imagine the curvature of space rippling like the waves on water.

This "rippling" is expanding in all directions in most simple case (no

colamination). So at each point in space the radius of curvature of the

space is vibrating one way then another. By using three satilites

positioning and position them in an L shape, a laser can be shot from one

to the other two against a mirror and return. Knowing all positions to high

accuracy and measuring the distortion in the interferometry pattern you

can determine and detect a Gravitational Wave to (hopefully) high enough

accuracy.

:) Gravitational Radiation is the expression of Gravity as either a Wave

(GR) in a) or as a Graviton.

 

-What is Frame Dragging?

I think of Frame Dragging as a force causing your reference frame

(rotating) to slow down. This could be many things so would need more

specificity. With respect to Gravity, think of the Procession of the

Perihelion of Mercury that Einstein used to vindicate his General Theory of

Relativity.

 

If spinning objects affect space time differently, do they have greater 'gravitational pull'?

Yes. Both in Special and General Relativistic case.

 

Note: To fully understand this topic, I will need to know more about space/time, but I will save that for another thread. Right now I only really know the principle through analogies.

I suggest a good book "Spacetime Physics" by Taylor (Freeman Press).

 

What is "exactly" is Gravity is currently not a fully answered question.

This is because no current theory can both accommodate Quantum Field

Theory in the Standard Model and General Relativity. NO Current Quantum

Gravity Theory. All else is speculation.

 

maddog