Gravity and spacetime

Tail

If I understand everything correctly, gravity affects particles with mass/energy. How can it possibly affect spacetime? Doesn't it have to "grab" something?

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Tyger

Bodies with mass/energy produce gravity which is the effect of bending space-time. Anything around that body has to follow a path determined by the curvature of the space-time.

Tail

Hadn't occured to me to think about it like that... gravity warps ONLY spacetime, it doesn't "pull" things.

That right?

pmb

Originally posted by Tail
Hadn't occured to me to think about it like that... gravity warps ONLY spacetime, it doesn't "pull" things.

That right?
Gravity *can* curve spacetime. I doesn't mean that in all cases that it does. And it can curve space as well as spacetime.

But matter reacts with matter - an matter defines spacetime (loosely speaking)

Pmb

Twister

Originally posted by Tail
If I understand everything correctly, gravity affects particles with mass/energy. How can it possibly affect spacetime? Doesn't it have to "grab" something?
What about neutrinoes? Or other non realistic particles that matter!

Tail

Originally posted by pmb
Gravity *can* curve spacetime. I doesn't mean that in all cases that it does.
When doesn't it? Can you mention at least one case?
And it can curve space as well as spacetime.
Spacetime includes space.

matter defines spacetime (loosely speaking)

Doctor Luz

Re: Re: Gravity and spacetime

Originally posted by Twister
They have mass/energy.

Originally posted by Twister
Or other non realistic particles that matter!
What non realistic particles are you talking about?

Originally posted by Tail
If I understand everything correctly, gravity affects particles with mass/energy. How can it possibly affect spacetime? Doesn't it have to "grab" something?
There's a story about gravity and relativity that is not told as often as it should. It goes back to Newton and is quite revealing of how physicists think.

You see, we are so used to think of gravity as a "force" that it doesn't occur to us there's something strange about it. What's strange about it is that it's somewhat "spooky", as it acts without physical contact between bodies. To us the concept seems ordinary, but it's well documented that Newton himself disliked the idea, and adopted it because he had no other option. In Newton's time electromagnetism was not known, and the idea of two bodies interacting across empty space was quite out of the ordinary.

Another problem revealed itself as time went on. The force of gravity has the peculiar feature of being proportional to mass. For physicists, that's a sure sign that it's not a force at all, but rather what they call a pseudo-force, something that seems to be a force simply because we are looking from the wrong perspective.

Imagine a closed truck with a box and a camera filming the box and the interior of the truck. The box is not attached to the floor while the camera is. If you are looking at the movie of the box, you will notice something interesting. When the truck is moving in a straight line, the box stays in place, but when the truck is turning the box appears to undergo acceleration in the opposite direction. From the perspective of the movie, it's as if the walls of the truck attract the box. From the perspective of someone watching the truck from the outside, it's clear that the box is just trying to follow its inertial path while the truck is being accelerated. The impression that the box is experiencing a force is an illusion.

How do you know if something is really experiencing a force or if it's just the appearance of a force? Well, one sure sign that you're not dealing with a real force is when the force is proportional to the mass of the object being accelerated. That is just too convenient to be correct. Gravity happens to be such a force (proportional to mass). Together with the fact that it's an ad-hoc solution conceived by Newton to explain planetary motion, physicists have always suspected gravity to be a pseudo-force. With that in mind, they set out to find the correct perspective which would reveal that objects appearing to be accelerated by gravitational forces are not accelerating at all.

Einstein's General Relativity achieved exactly that feat. In GR gravity is not a force but simply a feature of the geometry of the universe. Two objects moving in a "straight" line ("straight" from a GR perspective) are bound to collide, not because they are being accelerated, but because spacetime is not really "straight" the way we perceive it. To borrow from the truck analogy, it's as if we were the box while spacetime is the truck; we are constantly trying to move in a straight line but the truck is going around in circles and as such we experience acceleration.

So to answer your question, gravity does not affect particles with mass/energy, it's spacetime that does by the fact of being "curved". Just like a crooked road "affects" the load of a truck.

Hope this helps.

pmb

Tale wrote
When doesn't it? Can you mention at least one case?Spacetime includes space.[/B]
I can give you three examples

(1) A uniform gravitational field
http://www.geocities.com/physics_world/uniform_field.htm

(2) A straight cosmic string cosmic string
http://www.geocities.com/physics_world/cosmic_string.htm

(3) A vacuum domain wall.
http://www.geocities.com/physics_world/domain_wall.htm

Spacetime includes space.
Obviously. However I think you missed my point. It's possible for space to be flat and spacetime be curved. In fact that's exactly the case for a flat universe - I.e. in a flat universe space is flat = however spacetime is curved.

Pete

Twister

Re: Re: Re: Gravity and spacetime

Originally posted by Doctor Luz
They have mass/energy.

What
non realistic particles are you talking about?
Hello DOC, I am thinking virtual particles.Yes neutrinoes have a little mass as they say.Condensed high frequency powerfull points of energy are always full of something.

FZ+

They still have mass/energy.(Albeit temporarily) Hence Hawking Radiation.

Twister

mass.

FZ, That tells me that matter is either made by the spontainious episode or that matter is a result of,or virtual particles bring mass with them.From a mass origen.The question is ---What piece of anything does not have mass.The ether must be the origen of this substance.

FZ+

Er... no... The existence of virtual particles is due to quantum uncertainty, where probability creates so called "quantum foam". The nature of this "foam" is very different from classical ether.

Twister

lost it

I replied to you,my post was lost due to pop ups sorry.Bottom line where do virtual particles come from!

Tail

Field fluctuations or something like that.

Twister

field fluctuations

The field must condense in certain points to produce a particle,And the low energy level or stability must make it decay as fast as it poped into existance. For on to stay in our dimension it would seem it needs more ummph.More energy to stabalize it and to be taken care of by other photons of the same frequency nearby.

wakeball32

Rubber Sheet Analogy

I know you have all heard the rubber sheet analogy when it comes to general relativity. If not this is it in layman's terms: If you were to stretch a sheet of rubber and put a bowling ball in the center it would depress the rubber. Now any object (round) placed on the outer edge of the depression will roll towards the heavier object, i.e. gravitational pull. But there is one flaw in this....while the bowling ball makes a depression in the rubber because "gravity of earth" pulls it down...the depression in space-time IS gravity, not an effect of gravity. So you can't say that a planet's "gravity" causes a bend in space-time, because that bend in space-time IS gravity. Just making a point, goodnight and Godspeed.

Twister

Two sides

There is always two sides to every depression!The mass indenting spacetime causing more mass to fall or get pulled or close its orbit to collide with the bowling ball also has a indent into the spacetime it is denting--Does this make any sense!Almost a oposite reaction on the --other side.The push of spacetime reflects or is opposed to anti matter .Just looking on the other side! TWISTER

LURCH

Re: Two sides

Originally posted by Twister
There is always two sides to every depression!The mass indenting spacetime causing more mass to fall or get pulled or close its orbit to collide with the bowling ball also has a indent into the spacetime it is denting--Does this make any sense!Almost a oposite reaction on the --other side.The push of spacetime reflects or is opposed to anti matter .Just looking on the other side! TWISTER
I believe the situation you are describing reffers to negative energy, not antimatter. Nagative energy (and whatever negative matter might be created from it) is gravitationally repulsive. Though I've not seen experimental data, it has been side several times here in the Forums that antimatter is attracted by gravity.

Twister

Re: Re: Two sides

Originally posted by LURCH
I believe the situation you are describing reffers to negative energy, not antimatter. Nagative energy (and whatever negative matter might be created from it) is gravitationally repulsive. Though I've not seen experimental data, it has been side several times here in the Forums that antimatter is attracted by gravity.
Sorry,New here! Should the anti-matter be attracted to ant-gravity?

LURCH

Re: Re: Re: Two sides

Originally posted by Twister
Sorry,New here! Should the anti-matter be attracted to ant-gravity?
That is not the generally held view, no. Again, I have not seen laboratory varification of this. Indeed, I can't imagine how one would go about varifying it experimentally with the tiny amounts and short duration of the antimatter we have so far been able to create. But the general consensus among particle physicists seems to be that both matter and antimatter are attracted by gravity. Antigravity, if it exists, would be generated by negative energy, and should repel both matter and antimatter.

However, I would assume that negative energy is attracted by antigravity. But this is just a supposition on my part.

Twister

last stand.

Has anyone chacked out MCcoin unikef theory?

wakeball32

Theory

Speaking of theories, the so called "vortex theory" is almost one of the dumbest things I have ever read. I went to that web site and the author makes no clear hypothesis and states no real theory at all. Of course every one is entitled to their own opinion and I might be wrong. Also, I will check out that mccoin unikef theory. Thanks

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