What is gravity? Its not energy...its not force...?

Question

It can't be blocked or contained...it affects everything, but nothing can stop it... Created by mass... so do electrons have gravity? Does a spark have gravity? Do photons (light) have gravity? I've heard black holes suck in light. For gravity to affect it...a substance has to have mass, yes?...so light must have mass? Isn't light considered energy...so does energy exude a gravitational pull? If there was nothing in space...would gravity exist? I find gravity fascinating...maybe all of this is elementary and I'm wasting people's time.

Answer 1

Permit me to amplify what "PleaseStop." has said. And I'll repeat it even once more, at the risk of sounding like an echo chamber:



Gravity is NOT a force in GR.

It is a 'pseudoforce' -- it only seems to be a force because of a poor choice of one's frame of reference. In an accelerating frame, there is always a pseudoforce that is needed to make F=ma work. A familiar example is the so-called "centrifugal force" in a uniformly rotating frame.



In Newton's model of gravity, space and time are immutable, space is 3-dimensional and Euclidean (that is, its 'metric,' the rule that tells how to determine squared distance from coordinates, works according to Pythagoras' Theorem), time is an independent variable, both are unaffected by motion or mass; they are the 'background canvas' on which all physical phenomena are painted. Galilean relativity rules the mathematics of motion, and gravity is a force that acts mysteriously and instantaneously at a distance, between all massive objects in the universe. Philosophically, Newton was sorely troubled by this picture of gravity, but, as it explained observable phenomena so well, he just left it there. "Hypotheses non fingo." -- I make no hypotheses [about a causal mechanism for gravity]. (This is the conceptual problem you highlight in your last statement/question, under Additional Details.)



By the mid-to-late 19th century, this model had come into seemingly irreconcilable conflict with the lately well-established laws of electromagnetic (EM) phenomena. Some thought that the laws of mechanics would have to change; others thought it was the laws of EM that needed revision. Einstein found a third way -- change the description of the canvas itself!



So in Special Relativity (SR), space and time are a unified 4-dimensional entity, with a Minkowskian metric (same as Euclidean, only with signs on the 3 space-squared parts that are opposite the sign on the time-squared part). The "exchange rate" between space and time is c, the 'speed of light in vacuum,' which is constant.

[My own 2 cents -- c should properly be called, "the characteristic spacetime speed," because it is really a property, not of light, but of spacetime itself. Any massless field will travel at speed c.]



Once he had this all worked out (1905), he realized right away that gravity in the Newtonian formulation was untenable in SR. So after 10 more years, he came up with General Relativity (GR), in which, as "PleaseStop." says:

"Matter tells space how to curve; space tells matter how to move."

[in which the term, "space" is now understood to include time.]

-- a description coined by John Archibald Wheeler, who also coined the term, "black hole."



In Einstein's field equations for GR, the source term is not just mass, but also energy and "stress." So, yes, in GR, light, which is massless but has energy, does participate in producing 'gravity,' that is, it contributes in a prescribed way to the curvature of spacetime.



And Einstein's picture of gravity does away with action-at-a-distance, making physics local once again, because it has mass-energy-stress creating a local curvature of spacetime, which then propagates (at speed c) to neighboring regions of spacetime, and so on ... and Newtonian gravity is a pseudoforce in this picture; due to working in a noninertial frame. Inertial (free-falling) frames near Earth's surface are accelerating wrt that surface at 1g, downward. So standing 'still' on the floor, you are actually accelerating at 1g away from Earth's center, pushed up by the floor. The fact that neighboring inertial frames are accelerating slightly wrt to each other, at a rate that depends on their separation, and on their direction of separation, is simply one manifestation of the curvature of spacetime. GR describes the tidal (gradient of) force.



EDIT:

Thanks, PleaseStop. Bravo right back to ya.

I had to weigh in when I saw your emphatic denial that gravity is a force, which I've tried to impart many times in this forum in questions about gravity.



That said, I might not have been quite so hard on gintable; he makes a number of important points. While a single photon won't act exactly like a particle of mass hν/c^2 & momentum hν/c (for one thing, it travels a light-like, not a time-like geodesic of spacetime), yet if we had a massless, perfectly reflective box containing a photon gas, I believe it would behave just like a body of mass equivalent to the energy of its photons, for purposes of gravitation, as well as in response to any mechanical forces. In any case, it would take a phenomenal concentration of photons to produce any noticeable gravitational effect. I'm guessing it would take something like a supernova event to form such a concentration, and that to treat such an event in GR would require considering the effect of those photons on the gravitational field. I believe that would come close to an experimental verification that light participates in gravitation.

Answer 2

actually, gravity IS a force. Anything that has mass, has gravity. Yes, blackholes suck light. Light has no mass because nothing with mass can achieve light speed. Light does not work the same way as other things in the universe because light and time are warped by mass. If there was nothing in space, yes, gravity would still exist. Think of gravity like a blanket stretched tightly through the universe. When a planet is placed on the sheet, the sheet bends in a hole around the mass. If there was no mass, the "sheet" would still be there, just no hole.

Answer 3

If you accepted General Relativity as the best and current theory of gravity, then no; gravity is not a force by definition. I'll repeat that just everybody can hear that.



Gravity is NOT a force in GR.



By our definitions of what a force is, gravity would not fit as a force. There is good reason to believe this is the way it is as it seems nearly impossible the make a quantized field theory explanation of gravity. This does not necessarily mean that it won't ever found to be a force, but there is point when you have to accept what GR is telling you.



Matter is energy, energy tells spacetime how to curve and then spacetime tells matter how to move in spacetime. Everything travels in a straight path unless acted upon by some force. Objects in a gravitational field aren't acted upon by a force called gravity in GR; they take curved paths because a straight path in curved space is curved. The force is not generated in the exchange of a gauge boson like the other events you speak of because it is not a force that can be described by a quantized field theory.



Anything that has mass/momentum ie energy creates curvature...gravity is curvature.



What gintable is talking about is pretty much nonsense.



"You can argue that light DOES HAVE MASS, (if you want to). In Euclidean space-time, that is how light is understood. Light has mass of equivalent value per photon equal to h*f/c^2, and it receives gravitational forces just like anything else."



1. This comment makes no sense. As soon as you mention spacetime we are talking SR and GR. Gravity is a curved manifold that objects take straight line paths through. Yes, light could have a non-zero rest mass. This has not been experimentally ruled out; it still is possible photons have a very small rest mass. If there were some small rest mass, SR and GR would be unchanged. Giving it mass and saying it receives gravity like anything else is still a crappy explanation, because it is not an explanation.



"In Minkowski space-time, an alternative MODEL of light is put to use, whereby light doesn't have mass, and it is just following the inertial straight line through space-time that happens to be curved so that distant observers don't think it is straight."



2. Minkowski spacetime is a flat 4d spacetime chief. It is not an alternative MODEL, it is what the physical THEORY of Special Relativity is built upon. There is considered to be no gravity in Minkowski space since it is flat. So gravity in this case is weak enough to be approximately ignored.



A curved spacetime manifold would not be globally Minkowski; how could a curved surface be globally flat? darrrrrr......

Now a curved spacetime manifold is called a manifold because it is built up of locally small minkowksi spaces.



Bravo Fred.....Bravo. Standing ovation.

Answer 4

It is a force that just is. Just because we do not necessarily understand its innerworkings DOES NOT disqualify it as a force. It still is a method by which one object uses in attempt to alter the motion of another object.





Yes electrons do have gravity and yes electrons are affected by gravity



A "spark" is just a zone of air that changed to plasma phase. Yes it has gravity and yes air has gravity. Other kinds of sparks are just hot pieces of metal, hot enough to emit light, like the sparks from a grindstone. They have gravity just like the metal has gravity.





You can argue that light DOES HAVE MASS, (if you want to). In Euclidean space-time, that is how light is understood. Light has mass of equivalent value per photon equal to h*f/c^2, and it receives gravitational forces just like anything else.





In Minkowski space-time, an alternative MODEL of light is put to use, whereby light doesn't have mass, and it is just following the inertial straight line through space-time that happens to be curved so that distant observers don't think it is straight.





Either way, these are just MODELS. No experimental verification can yet confirm one or the other. They are just ways to REPRESENT what happens.





It hasn't yet been experimentally confirmed whether light actually makes or does not make gravity. I.e. could a pair of binary photons orbit each other? Not yet known.

Answer 5

Gravity is a space phenomenon.

Einstein explicated it in terms of the curvature of space.

If that is the case than the total Gravity Force enveloping the Earth would be equal to

MC^2 X Crv , Where Crv is the Space curvature at the surface of the Earth.. M is the mass of the earth and C is the speed of light.



Gravity was created synergistically with Time. That means one cannot exist without the other.



A new theory explaining the actual mechanism of gravity ,is the Prof Ricardo Carezani theory of Autodynamics.

Answer 6

Gravity is an effect which causes all mass in the universe to attract all other mass in the universe with a force inversely proportional to the distance between them and proportional to the product of their masses.

Here's where it gets complicated: gravity does not directly affect light as it is massless. What it does do is warp space-time, causing a curvature in the fabric of the universe which light follows. This is how black holes pull light in.

Answer 7

its a force.

yes

yes

yes (black hole)

light has mas because it is effected by a black hole.

energy can have mass (bonding energy)

no

Answer 8

It's a force

Answer 9

its a delicious topping that is commonly put on mash potatoes