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We always hear how gravity bends space-time; why shouldn't velocity?

Consider a spaceship traveling through space at a reasonable fraction of the speed of light. If this spaceship, according to special relativity, gains mass as a factor of y as it approaches c, then its gravitational field should increase in strength as well. Hence, space-time should warp.

Note: Changes in space-time, gravity and mass should only be measureable by an outside observer with a different velocity. Those inside of the ship moving with it would not be able to measure the change in these properties.

Qmechanic
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6 Answers6

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Start with the gravitational field of the Sun. We are effectively stationary with respect to the Sun, because our relative speed is much less than $c$, and the Sun is rotating at well below relativistic speeds so we expect its gravitational field to be well described by the Schwarzschild metric. And indeed this is true: Newton's law of gravity is the non-relativistic limit of the Schwarzschild metric.

The metric tensor is invariant with respect to coordinate transformations, so if we take some observer moving at near light speed they would also find the gravity round the Sun to be described by the Schwarzschild metric. It will not look the same in the observer's coordinates, that is the individual components $g_{ij}$ will be different, but it will be the same tensor. Since in the observer's frame they are stationary and the Sun is moving, the conclusion is that velocity does not change the spacetime curvature.

Incidentally, this is why a fast moving object does not turn into a black hole.

Rexcirus
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John Rennie
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I assume you're asking whether a moving objects curves spacetime differently than a stationary one as its velocity increases.

Strictly speaking, no: it's the same spacetime geometry either way. The spaceship warps spacetime either way, and all we'd be is talking about it in a different frame. Because of this difference of frames, in a sense the gravitational field is different even though the geometry is the same.

If this spaceship, according to special relativity, gains mass as a factor of y as it approaches c, then its gravitational field should increase in strength as well.

This is not quite right. First, the spaceship does not gain mass. It's true that quantity $\gamma m$ is sometimes called relativistic mass, but this term is redundant with energy, bad at its intended purpose of preserving a superficial resemblance to Newtonian mechanics, and depreciated in physics. In special relativity, mass is invariant: $(mc^2)^2 = E^2 - (pc)^2$ is the same in all inertial frames.

Which is just as well, since the 'gravitational charge' isn't mass, but energy. But it is not a simple proportional increase when we view the spaceship's gravitational field in a frame in which it has a lot of energy.

This shouldn't be surprising if you know a bit of electromagnetism. A moving charge produces an electromagnetic field that has both electric and magnetic parts, since the motion of the charge, i.e. the current, matters. The electric field is enhanced in directions perpendicular to the direction of motion, which we can picture as the initially spherically symmetric field lines getting Lorentz-contracted, thus 'squishing' them close together in the perpendicular directions.

The gravitational analogue of electric current would be momentum, but because gravity is spin-2, stress in addition to energy density and momentum density is relevant to how spacetime is bent. You can see this described in the stress-energy tensor. So the gravitational field is more complicated, but it has an analogous behavior of being strengthened perpendicular to the direction of motion, although its quantitative behavior is different.

In the limit of lightspeed, the electromagnetic field of an electric charge turns into an impulsive plane wave, and the gravitational field of a point-mass behaves analogously, turning into a vacuum impulsive gravitational (pp-)wave.

Stan Liou
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Looking at the (spatial) velocity of an object alone is like looking at a plane and considering only its north-south velocity and neglecting its east-west motion.

Timelike objects are like cars with only one gear, no brakes, and a gas pedal that is stuck in place. They continue to go forward in time no matter what; all they can change are their direction through spacetime--how much velocity is going forward in time vs. how much is used to travel distances in space. (The steering wheel still works, but that's all, so to speak.)

When you consider both of these velocities together as one quantity, you find that this "four-velocity" of a given object has constant magnitude: the speed of light itself.

Similarly, the "four-momentum" has constant magnitude, proportional to the rest mass of the object.

These are all results from special relativity. General relativity is a bit more nuanced (not in the sense that four-velocity and four-momentum no longer have constant magnitudes--they do--but in how those magnitudes are calculated using the metric).

While different observers will disagree on obviously frame-dependent things like spatial velocity, they will all agree on the magnitudes of vectors in this fashion. You've been told that gravity depends on mass. I'm telling you that it depends on rest mass--which all observers agree on in this way.

Because spacetime boosts are analogous to rotations, boosting a particle and looking at its gravity just results in all spacetime variables (like the metric) being boosted as well. These transformation laws can be tricky for some tensors, but the underlying physical system can still be understood as equivalent to that of a stationary particle. It's just being looked at through another observer's eyes.

Muphrid
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When talking about gravity, mass, velocity, energy etc. it’s always good to give the context of the definition i.e. which theory we are discussing. Otherwise things get very confusing.

In General Relativity, gravity is usually said to be a consequence of the curvature (or bending) of space-time. Space-time curvature can be calculated from the total energy-momentum (the stress energy tensor) within a region of space.

In special relativity, the value of the mass/energy and velocity/momentum are relative quantities i.e. they depend on the observer and the measurement of another reference object. The total energy-momentum is invariant i.e. all observers can measure the same value without reference to other objects. The energy-momentum can be decomposed into values for the mass/energy and velocity/momentum but these can be different for different observers (as the observers can only measure them relatively).

For the spaceship in the question, one observer moving with the spaceship would say that the velocity/momentum of the spaceship is 0 and therefore not contributing to the total energy-momentum of the spaceship, another observer moving relative to the spaceship might say that the spaceship has a velocity/momentum which contributes to the total energy-momentum of the spaceship. Both observers will calculate the same value for the total energy-momentum for the spaceship however (as each will also calculate different values for the mass/energy of the spaceship). The same energy-momentum will result in the same space-time curvature. So usually, people don’t say that velocity causes space-time curvature because it is a relative quantity.

It is still an important point that moving objects do result in space-time curvature and therefore gravity – it is not possible for an observer to be at rest relative to all moving objects. This is true even without using General Relativity. A body of material (this could be anything, a lump of metal, a person, a star) is composed of huge numbers of atoms all oscillating in different directions – it is not possible for an observer to be at rest relative to every oscillation. The momentum of each moving component part contributes to the total energy-momentum of the object. In thermodynamics, the hotter the object the faster the component parts oscillate. So an observer at rest relative to the body of material as a whole will measure a higher mass for a hot body than a cold one. In Newton’s gravitational laws the greater the mass the greater the gravitational attraction it causes.

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The answers given so far make many good points. One problem with the question is that it's implicitly simplifying the context. So I'd like to give a more detailed answer, which is a qualified "yes". Curvature is "affected" not only by energy-mass, but also by momentum – which is related to velocity – and also by energy & momentum fluxes, like heat and pressure (see the pedagogical article by Ehlers & al. below).

First let's be more precise on a couple of points:

  • "Curvature", in its most general form important for general relativity, is not something which can be quantified by one number. It is expressed by a set of numbers, called a tensor, and their values depend on the coordinate system which we adopt in a region of spacetime. Simplifying things a little, your "curvature in the $x$-direction" could be a combination of my "curvatures in the $x'$ and $y'$ directions", if we're using two different coordinate systems $(t,x,y,z)$ and $(t',x',y',z')$.

    One particular set of curvature numbers, called the Einstein tensor, can be represented by a 4-by-4 symmetric matrix of numbers $G^{\mu\nu}$. This matrix can also be represented by an ellipsoid (or hyperboloid) in spacetime. This is the curvature tensor that enters directly in the Einstein equations.

  • If at a spacetime point where there's a non-zero amount of matter, by which we mean some substance, or baryons or leptons, then we can associate to that matter at that point a four-velocity vector $U^{\mu}$. This is also a collection of four numbers, which again depend on the coordinate system. If we are using one temporal and three spatial coordinates $(t,x,y,z)$, then the spatial components of the four-velocity $(U^x, U^y, U^z)$ are proportional to the coordinate velocities. Clearly we can choose a coordinate system in which the coordinate velocities are zero (at least in a small region).

    We represent the energy and momentum (their densities and fluxes) of matter by an energy-momentum tensor, which can be represented by a 4-by-4 matrix $T^{\mu\nu}$ of numbers, again dependent on the coordinates. In the simple case of matter whose internal pressure is negligible, called "dust", its energy-momentum tensor is directly given in terms of its four-velocity: $$T^{\mu\nu} = \rho\,U^{\mu}\,U^{\nu}$$ where $\rho$ is the rest-energy density.


Now we can say this unambiguously: four-velocity "affects" curvature. In fact this comes directly from the Einstein equations: $$G^{\mu\nu} = \kappa T^{\mu\nu}$$ where $\kappa$ is a proportionality constant. In the case of "dust" matter we have $$ G^{\mu\nu} = \kappa \rho \,U^{\mu}\,U^{\nu} $$ which clearly says, for instance, that the curvature $G^{xy}$ is proportional to the $x$ and $y$ coordinate velocities. So yes, if the matter at a given point has a higher $x$-velocity, then it's also generating higher $G^{x\nu}$ curvatures there.

This will also be reflected in the metric tensor, which will contain off-diagonal terms proportional to $\rho U^x$, and called "gravitational vector potential". In a first approximation they are somewhat analogous to the magnetic field, but for gravitation. Indeed the curvature effects coming from velocity are called "gravitomagnetic effects". These effects are concretely considered by the International Astronomical Union in their modelling of the solar system; see references below.

[For the sake of correctness, the fact that the four-velocity (and hence the coordinate velocities) affect Einstein curvature is not a pure consequence of the Einstein equations, but a consequence of them together with the fact that the energy-momentum tensor of matter depends on the four-velocity. One can also create models of matter whose energy-momentum tensor does not depend on the four-velocity, for instance $T^{\mu\nu}=\lambda g^{\mu\nu}$, where $g^{\mu\nu}$ is the inverse metric tensor. Such matter would correspond to a cosmological constant; see the pedagogical article by Ehlers & al.]


But it's important to emphasize the coordinate-dependence of velocity components, curvature components, and their interpretation. A picture may help. Consider a point in spacetime where we have matter with four-velocity $U^{\mu}$ and Einstein curvature $G^{\mu\nu}$, related by the equation above. We can visualize them as follows:
Illustration of curvature and 4-velocity
Note that this is not a spacetime-diagram. It's just a (quite faithful) illustration of the four-velocity and of the Einstein tensor in two dimensions. Also, we are considering matter that does have internal pressure. There is a relation between four-velocity and Einstein curvature because if the four-velocity vector were longer, then the ellipse would have a longer major axis as well.

Now we can introduce a coordinate system $(t,x)$ as follows (the axes looks orthogonal, but that's unimportant):coordinates t,x
In these coordinates, the matter at this spacetime point has non-zero coordinate $x$-velocity, and the Einstein curvature has a particular $G^{xx}$ component, represented by the intersection between the ellipse and the $x$-axis. This value is proportional to the square of the $x$-velocity and to the pressure. Note how the ellipse is skew with respect to this coordinate system. We interpret this skewness as being an effect of the non-zero velocity with respect to this coordinate system.

But we can also introduce a different coordinate system $(t',x')$ as follows:coordinates t',x'
In these coordinates, the matter is at rest, with zero $x'$-velocity. The Einstein curvature has a particular $G^{x'x'}$ component, proportional to the pressure. Note how the ellipse is now not skewed with respect to this coordinate system. We interpret this absence of skewness as being an effect of the zero velocity with respect to this coordinate system.

Clearly the physical situation is the same.

References

For the representation of symmetric matrices as ellipses:

On gravitomagnetism there is introductory material in many places; for instance:

Gravitomagnetism in the International Astronomical Union specifications for coordinate systems for solar system and Earth:

On the cosmological constant, interpreted as a kind of matter:

pglpm
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Equivalence requires acceleration to curve space the same way gravitating mass does. Hermann Weyl, Zur Gravitationstheorie, Annalen der Physik, 54, 117, (1917) argued that kinetic energy should curve space just as gravity and electromagnetic fields do by entering into the stress energy tensor.

The concept of inertial mass increasing at high speed has been used a lot in science, but is often replaced by non linear terms in the energy momentum equation which has better verification. An argument is made that particle accelerators running at high kinetic energy would not be able to levitate a beam of particles if they had relativistic gravitating mass unless the electric charge was also increasing relativisticly, which is not observed. To conserve the equivalence principle many scientists no longer use the concept of relativistic inertial mass.

In summary an object that is changing speed or direction is expected to generate a gravity wave, usually very small. It is proposed but not proven that an object of high kinetic energy could contribute to space curvature.