Does velocity affect the mass's gravity?

[whydoyouwanttoknow]

I've read a few things in here that state that a mass's gravity influences its gravity, how much it gives off or whatever.

From what I've read though I thought only rest mass contributed towards gravity, otherwise the first time we built (or say, an alien civilisation) a spacecraft capable of travelling close to the speed of light we'd all die when the entire universe from craft's point of view collapses into black holes.

What I read stated that everyone sees the amount of gravity the same in all frames of reference.

Can someone help me out here, hopefully on a level I can understand.

[alpha_wolf]

IIRC, Newton showed that inertial and gravitational masses are one and the same. Since velocity increases inertial mass according to SR, it must therefore also increase the gravitational mass of the object.

[da_willem]

It follows from Einsteins theory of general relativity that all the components of the stress-energy tensor contribute to the gravitational 'field'. So besides energy (including mass) even momenta and pressures act as sources of the gravitational field.

This explains the existance of black holes. In a dense star the strong gravitational field requires a large pressure gradient. This adds to the gravitational field causing even larger pressure gradients. This process can in some cases turn the star into a black hole.

[Nenad]

pressures act as sources of the gravitational field.


I know velocity adds to gravity, but Im not shure about pressure. Where did u hear this?

[da_willem]

Newton had only mass as a source, but in the light of special relativity, where there is an equivalence between mass and energy, energy had also had to act as a source. Energy density is the (0,0) component of the stress energy tensor T. But to use only one component of a tensor would yield a noninvariant theory. So Einstein guessed that the source of the field should be the whole tensor T.

In general relativity einstein's field equations have T as the source of curvature. T is a tensor consisting 16 numbers including the energy density, momentum density and all sorts of forces like pressures.

You can find this in all the standard literature on Einsteins general theory of relativity.

[Nenad]

k, that explaines it.

[pervect]

I've read a few things in here that state that a mass's gravity influences its gravity, how much it gives off or whatever.

From what I've read though I thought only rest mass contributed towards gravity, otherwise the first time we built (or say, an alien civilisation) a spacecraft capable of travelling close to the speed of light we'd all die when the entire universe from craft's point of view collapses into black holes.

What I read stated that everyone sees the amount of gravity the same in all frames of reference.

Can someone help me out here, hopefully on a level I can understand.

http://math.ucr.edu/home/baez/gr/outline1.html
http://people.hofstra.edu/faculty/Stefan_Waner/diff_geom/Sec12.html

are among the simplest web references I could find for the stress energy tensor. Unfortunately they aren't that simple. It's true, though, that pressure is one of the components of this tensor.

Basically, the intent of the stress-energy tensor is to express the density of both energy and momentum in a small volume of space.

The way this is accomplished is that one passes the stress-energy tensor a 4 vector representing the time vector (also known as the four velocity) of the observer, and the stress energy tensor spits out the energy-momentum 4 vector of space-time at that point - i.e. it gives one both the energy density, and the momentum density and direction, at that point.

If you imagine that energy, rather than mass, is the source of the gravitational field in general relativity you won't go too far wrong.

One of the places you may go wrong with this simple view is inferring that if you travel fast enough, you'll collapse into a black hole. This happens if you ignore the role of momentum. It simply isn't true that if you go too fast that you'll form a blackhole (in spite of at least on pop-sci TV show that got it wrong). See the sci.physics.faq

going-too-fast (http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/black_fast.html)

There is a fairly simple explanation for why a fast moving object doesn't form a black hole in terms of gravitomagnetism, but unfortunately I've never seen anyone actually post it. Basically, just as a moving charge generates an magnetic field that affects other moving charges, a moving mass generates a gravitational field that affects other moving masses (but not stationary ones). This has been given the name "gravitomagnetism", and while it's more commonly associated with rotation, it's also present for linear motion.

The link below doesn't have anything to do with what I just talked about, it just talks about gravitomagnetism in general and gravity probe B specifically.

gravitomagnetism (http://science.nasa.gov/headlines/y2004/19apr_gravitomagnetism.htm)

[da_willem]

I've read a few things in here that state that a mass's gravity influences its gravity, how much it gives off or whatever.

From what I've read though I thought only rest mass contributed towards gravity, otherwise the first time we built (or say, an alien civilisation) a spacecraft capable of travelling close to the speed of light we'd all die when the entire universe from craft's point of view collapses into black holes.

What I read stated that everyone sees the amount of gravity the same in all frames of reference.

Can someone help me out here, hopefully on a level I can understand.

first of all, it is not at all true that everyone sees the same amount of gravity in all frames of reference. Einstein introduced the concept of equivalence, which states there is no way to distinguish gravity from acceleration. When you are accelerated in an closed elevator far away from all sources of gravity, there is no experiment you can do to decide wether you are at rest in a gravitational field or accelerated. This makes gravity another fictitious force, somewhat like the forces you feel on a rotating object. So the amount of gravity you 'see' depends on your motion. For example when you're in free fall it is like there is no gravity at all!!

[whydoyouwanttoknow]

first of all, it is not at all true that everyone sees the same amount of gravity in all frames of reference. Einstein introduced the concept of equivalence, which states there is no way to distinguish gravity from acceleration. When you are accelerated in an closed elevator far away from all sources of gravity, there is no experiment you can do to decide wether you are at rest in a gravitational field or accelerated. This makes gravity another fictitious force, somewhat like the forces you feel on a rotating object. So the amount of gravity you 'see' depends on your motion. For example when you're in free fall it is like there is no gravity at all!!

I suppose if we one day actually prove gravitons that would have to be rethought?

[da_willem]

I suppose if we one day actually prove gravitons that would have to be rethought?

I don't know very much about gravitons (see my topic: http://www.physicsforums.com/showthread.php?t=16090), but it seems to me that the theory of gravitons is mathematically the same as general relativity. It yields the same observations, and so they do not exclude one another. So the question of wich theory is the 'true' one cannot be answered because they are two differenty interpretations of the same formulas!?!

[pmb_phy]

I know velocity adds to gravity, but Im not shure about pressure. Where did u hear this?

Imagine a gas in a box. As the velocity of the box increases then the box shrinks. Work is done on the gas as a result. This means we put in extra energy which, for small velocities, equals

\frac{1}{2}\frac{v^2}{c^2}pV

where V is the volume and p is the pressure. Add this to the kinetic energy of the gas then you'll get

\frac{1}{2}mv^2 + \frac{1}{2}\frac{v^2}{c^2}pV

where m is the mass of the gas. Note that this is related to the mass density as

m = \rho V

Substitute this into the above to obtain

\frac{1}{2}\rho Vv^2 + \frac{1}{2}\frac{v^2}{c^2}pV

This can be factored yielding an expression of the form E = (1/2) Mv2. (I can't seem to figure out how to use braces in Latex today). Thus the inertial mass increases with pressure and has a density of

\rho + \frac{p}{c^2}

Since inertial mass equals gravitational mass then pressure increases gravity.

Pete

ps - Note that the above addresses only the gas and not the container of the gas. Forgetting the container can lead to paradoxes

[pervect]

I don't know very much about gravitons (see my topic: http://www.physicsforums.com/showthread.php?t=16090), but it seems to me that the theory of gravitons is mathematically the same as general relativity. It yields the same observations, and so they do not exclude one another. So the question of wich theory is the 'true' one cannot be answered because they are two differenty interpretations of the same formulas!?!

"Gravition theory" isn't as advanced as you seem to think. Gravitions would be part of a unification of general relativity and quantum theory, which we don't have yet. So it's not very useful at this point to think about gravity in terms of gravitions.

As far as the original question goes, I imagine a lot of things will wind up being rethought when GR & quantum theory are united. It is of course difficult to say exatly what these will be. Meanwhile, thinking about gravity in terms of gravitons will take one far away from the mainstream. Among other things, this will mean that if one asks questions like "how do you explain this in terms of gravitions" the answer will come back "we don't explain it that way, that's not how our theories go at all".

[alpha_wolf]

I suppose if we one day actually prove gravitons that would have to be rethought?
It will not. Any threory of gravity has to agree with experimental results. As da_willem said, you do not feel gravity when you are in free fall. This is a fact with practical uses (this effect is used to help train astronauts), and it prooves that gravity is depenedant on the frame of reference. A graviton based theory that does not agree with this result is not a valid theory.

[sullyhet]

It will not. Any threory of gravity has to agree with experimental results. As da_willem said, you do not feel gravity when you are in free fall. This is a fact with practical uses (this effect is used to help train astronauts), and it prooves that gravity is depenedant on the frame of reference. A graviton based theory that does not agree with this result is not a valid theory.

i have a slight problem with statments like, YOU DONT FEEL GRAVITY,

you dont really feel gravity at rest either, you just feel the effects of gravity, such as, at rest you feel your being pressed against whatever you are at rest upon, and "in free fall", you feel yourself falling, wind rushing past you and what not, neither is actually feeling gravity, just the "effects of gravity"

and on top of that, why are you in free fall, because of the attractive force of gravity pulling you to the ground, if that is not, feeling the effects of gravity, i dont know what is

sorry if i seem rude, but things like this just bug me

[DaleSpam]

Hi sullyhet, you are necroposting (replying to dead threads) again. You should open a new thread rather than reply to one that has been dormant more than a year.

[tommac]

I found this thread while doing a search about the effects of momentum on gravity. Lots of interesting stuff. One question though .... does the direction of momentum effect the gravitational effects? In otherwords does an equal mass travelling away from an observer apply the same gravitaional force on an observer as one travelling towards the observer (assuming that they both have equal rest mass and are travelling at the same speed )