Science Questions

Why does mass cause space-time to bend?

Fri, 22nd Apr 2011

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@grahamshort asked:

Why does the presence of mass cause space-time to bend?




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@grahamshort asked the Naked Scientists: Why does the presence of mass cause space-time to bend? What do you think? @grahamshort, Wed, 20th Apr 2011

Ah! Good, an easy one. 

Physics is good at explaining some things; mechanisms, relationships, and models - other things it just shrugs its shoulders at.  This tends to be one of them - we are still at the point of investigating how curved space effects things and how to reconcile the large scale classical theories of GR and SR with the small scale ideas of Quantum Mechanics. 

Why the world behaves as it does on a fundamental level tends to be an avoided question.  This particular 'why' question might get close to being answered once we have properly linked quantum theory and general relativity and once we have proof that the symmetry breaking of the Higgs particle is responsible for endowing other particles with mass.  imatfaal, Wed, 20th Apr 2011

In ordinary (Euclidean) space, gravity bends the path of light. In Minkowski space-time, the path of light is the definition of a straight line. Straightening the path of light by definition changes the rules of geometry. The internal angles of a triangle no longer add up to 180, and two straight lines may cross more than once. If you divide space-time into 4D hypercubes, you will find that they don't fit together perfectly in any 3D slice of space-time, holding one dimension constant. To make the hypercubes fit perfectly, you must view the whole 4D continuum. Computers do this easily; human minds need to be reprogrammed at a fundamental level to do so. This is what we call the warp of space-time.

The warp of space-time is often called the cause of gravity. That's like saying the curvy lines on a topographic map are the cause of mountains. To explain the cause of warped space-time, you must explain what causes light to bend in Euclidean space. The concept of force is foreign to general relativity, but in Euclidean space, force is the time rate of change of momentum; f = dp/dt. (F = ma does not work at relativistic speeds.) A photon's speed is constant, but its changing direction is changing momentum; so gravity does exert a force on a photon in Euclidean space. Where does that force come from?

As yet, there is no widely accepted theory which explains the cause of gravity. Look in the New Theories section for ideas. Phractality, Wed, 20th Apr 2011

Another excuse to post the Feynman video!

Basically, I'm just echoing what Matthew said, but with videos!  And the take-home point of the video is that it's easy to explain a less fundamental theory in terms of a more fundamental one.  If you keep asking why each successive theory works, you eventually end up running out of theories!  That's the problem here.  The best accepted theory we have on gravity is general relativity, which doesn't in itself explain why mass bends space-time, just that it happens.  There are tentative theories like string theory or quantum loop gravity that try to explain why space-time bends as a result of mass.  But these theories haven't been tested, and therefore they aren't accepted science yet. jpetruccelli, Wed, 20th Apr 2011

The answer is frame-dragging effect
source: EEK, Wed, 8th Apr 2015

That is quite wrong. Frame dragging does not explain why mass curves spacetime.

Curved spacetime is just another term for tidal forces. The cause of tidal forces is due to the varying strength in the gravitational force with position in space. PmbPhy, Wed, 8th Apr 2015

It is clear that currently, no one knows the answer to this question with  accuracy. The following observations, however, may be relevant.  Quantum mechanical theory indicates that zero is not a possible value of electromagnetic energy, and this is supported by the discovery of the Casimir Effect, which is interpreted as detecting wavelike energy in empty space. Quantum mechanics tells us that matter is composed of wavelike energy. It is not a great leap of logic to conclude that empty space and matter are the same thing, differing only in their energy states. (We could generalize by saying that space-time and matter-energy are the same thing.) If that is so, the phenomena that are known to exist in matter when it changes its energy could also apply to the transition between empty space and matter. One such phenomenon is expansion. The transition between "empty space" and matter could involve some sort of expansion or contraction of the underlying phenomenon of matter-energy-space-time by reason of the laws governing its existence, which, of course, remain imperfectly understood. We might be able to say with some confidence that gravity is the evidence that those laws, whatever they are, are not strictly linear, and that this nonlinearity is the basis for the expansion. But if so, that raises other questions (as usual), such as, is G constant? G, the universal gravitational constant that links mass and the curvature of space-time, if it is an expression in nonlinearity in quantum effects (probably at the string level), then field theory, as concieved as the basis for the existence of electromagnetism, may have to be rethought because the photons, being integer-spin particles, are capable of unlimited superpositions; but the formulation in which their superimnposability is ordinarily expressed are linear, which is not consistent with the notion that the difference between the presence and absence of a photon would have gravitational effects. (It is here assumed that, just as the difference between the presence or absence of an electron is known to have gravitational effects based on the particle's mass, so must it be so also for the photon, because the photon, although having no proper mass, nonetheless has momentum and should therefore some type of gravitating effect just as any other particle). But if the photon exerts a gravitational effect, and does so because of nonlinearity acting on the difference between the field energy state corresponding to no photon and the state corresponding to one photon, then , because photons are superimposable, adding more photons should result in further nonlinear effects, which of course would translate into increased gravity, exactly as expected so long as the degree of nonlinearity between states remained constant. Whether that can be assumed is unclear, which brings into question the constancy of G. Viewing the quantized electromagnetic field as something compatible with photons exerting gravity therefore results in a description of the quantized electromagnetic field which, being nonlinear, is somewhat different than what one normally thinks of. 

Arguably, however, the gravitational effect of a few photons, or even quite a goodly number of them, is so small that it could never be observed (under ordinary circumstances), so that there is hardly any point in trying to modify the understanding of the quantized field to accommodate nonlinearity. You need something much heavier to make that significant, e.g., ordinary matter. Or even supercondensed matter. The problem with approaching the problem on the basis of such matter, however, is that ordinary and even supercondensed, matter is built on particles of half-integer spin, to which the Pauli exclusion principle applies. That means that once a state is occupied by one particle, it is not available to be occupied by any others, so that the piling of particles upon particles such as was envisioned in the case of photons is not possible, so that the whole question of space-time-matter-energy nonlinearity becomes inapplicable. 

And because G as we know it is observed only in connection with objects massive enough to permit such observations, which are composed of half-integer particles, any questions as to the ultimate non-linearity of the quantum field become practically irrelevant. Of course, that might change if a very large number of photons could be crammed into a very small space, such as if two very large pieces of matter and antimatter were to suddenly collide; however, carrying out such experiments is a bit difficult. Atomic-S, Thu, 9th Apr 2015

Please read the last paragraph in the following link.

EEK, Sat, 11th Apr 2015

On reading the said reference, it appears that it is speaking only of the effect caused by rotating bodies. A stationary body does not "drag" a frame. Frame-dragging, of course, would be an outgrowth of the more general law as to how energy and momentum in a given location distort space-time there, and by extension, in its neighborhood also.  All of which is probably the macroscopic description of the way everything is constructed at the quantum level, which of course is a major question in physics today, and which may be answerable, in rough terms, by saying that empty space-time and space-time occupied by one or another type of recognizable matter or energy are simply different quantum states, but that this difference is not simply one of energy, but also geometry, which could be the manifestation of a weak nonlinearity in the states resulting in a slight incompatibility of fit between portions of space-time in different states.  Or perhaps some other effect we don't yet understand. That is the case argued in general terms. At some point, science has to get more specific, dealing with many issues such as the hypothetical existence of the graviton, which allegedly would be the carrier particle accounting for the gravitational attraction in the way that the photon accounts for electromagnetic force. But of course, that means that the picture of gravity being propagated via a particle would have to be reconciled with the picture of it being the consequence of a distorted medium (space-time), and at first, those pictures seem to be highly incompatible, inasmuch as there is no particular reason why particles being exchanged between objects and creating a force between them implies anything at all about the space and time around them. But we have been here before: the picture of sound being a form of quantized energy existing as a result of various energy levels of phonons (which has been verified in the context of thermodynamics etc.) and the picture of sound as a change in the density of chaotically moving other particles also seems fundamentally irreconcilable, but it turns out that because these other particles are subject to quantum laws, their exists with respect to their motions descrete energy states that are equivalent to phonons. Science may someday likewise be able to reconcile the two descriptions of gravity, and doing so appears as a major objective of string theory. Atomic-S, Sun, 12th Apr 2015

I was curious, we always look at space as vast emptiness (not accounting for light rays and all the other energy waves, just talking about pure space), where once we thought it was an aether. Since we have so much secondary evidence (as in, we have not perceived space per se, but we see the affect it has on matter), do we have any theories that support space being itself a medium of some sort? It can be warped, it provides enough drag as to keep all but photons from going light speed. Light can be bent through it just as light can be bent through water or glass. It seems to me that space could be the "aether" past scientists thought it was, only it is a substance so inert that only matter attempting to travel at relativistic speeds are even affected by it. Again, I am no scientist, but I was curious as to what you guys and the community think and to maybe gain some better understanding of the universe.
If the above were true, I think matter is like the oil to space's water. If you drop oil onto water, it automatically gets pushed into a circular shape. Could gravity not be a force of matter attracting to matter, but space pushing matter into "oil" globules? mike.hatcher, Wed, 9th Dec 2015

Here is the best answer available I think:
arthur.manousakis, Thu, 10th Dec 2015

Too funny that you link to your own theory.  Kinda a bit arrogant, no? IAMREALITY, Mon, 13th Jun 2016

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