Why Gravity Isn't "Travelling" Faster than the Speed of Light (c - Causality)

...from Google AI:

Retardation cancellation in gravity refers to the relativistic phenomenon where the delay in gravitational interaction (due to gravity traveling at the speed of light) is nearly perfectly cancelled out by velocity-dependent terms in general relativity. This cancellation causes gravitational forces to point toward a body's current, "instantaneous" position rather than its delayed (retarded) position, thus preventing orbital instability that would otherwise occur. 

Core Concepts of Retardation Cancellation

The Problem of Retardation: If gravity travels at the speed of light (c), Earth should technically feel the Sun's gravity from 8 minutes ago (its "retarded" position). If this were the only effect, the resulting torque would cause Earth to spiral into the Sun in months, which does not happen.

The Cancellation Mechanism: According to general relativity, the retardation effect is canceled by velocity-dependent terms that act as a form of correction. This is a fundamental difference between gravity and electromagnetism.

Why It Matters (Stability): This cancellation ensures that orbits remain stable because the gravitational field appears to act instantly, even though it actually moves at light speed.

Role of Velocity and Acceleration: For non-accelerating (or slowly accelerating) objects, the retardation effect on gravity is practically non-existent. However, for systems that radiate energy via gravitational waves, this cancellation is not complete.

Alternative Explanations (MOND): Some theories suggest that in very low acceleration environments (like the outskirts of galaxies), these retardation corrections are related to Modified Newtonian Dynamics (MOND) and can account for effects usually attributed to dark matter. 

Essentially, retardation cancellation ensures that General Relativity works with observation: gravity travels at c, but behaves as if it travels instantaneously, thus maintaining stable orbital motions.

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The Gravitational Static Field Around Massive Objects Fills the Space Between the Objects in it like a Rigid Structure...

from Google AI

Gravitational fields, created by massive objects, exist as 3D structures extending through space, influencing other masses via attraction. These fields, described by general relativity as spacetime curvature, act as a continuous, energetic medium bridging objects rather than merely empty space, forming a structured, invisible "web" that directs motion. 

Key aspects of this gravitational "structure" include:

Three-Dimensional Structure: Gravitational fields are not just lines on paper, but continuous three-dimensional influences filling the space surrounding and between massive objects.

Structure of Space-Time: General relativity describes these fields as the actual curvature or warping of space-time, which can be perceived as a, sometimes, rigid or fixed structure.

Energy Density: These fields, especially in the context of GR, are not "empty" space but contain non-zero, measurable energy that determines the motion of objects within it.

Field Lines as Visual Aid: While not physical wires, field lines are used to visualize the direction and intensity of this structure, pointing towards the center of massive bodies. 

Force Propagation: The field acts as an invisible force field (or "cosmic field"), allowing objects to influence each other without touching. 

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from Google AI:

A gravitational field acts as an invisible, static structure that surrounds any massive object, with its intensity and direction determining how other objects move near it. While classical mechanics often treats this as a force field pointing towards the center of mass, modern physics describes this "structure" as the curvature of spacetime itself.

Key aspects of this gravitational structure include:

Static Nature: The field is "static" because it does not move relative to the object creating it (e.g., Earth's gravitational field stays with the Earth).

"Rigid" Characteristics: The field is often depicted as a "potential energy landscape" or "well" that stays fixed around the massive object.

Structure of Space: Einstein's general relativity explains that this field is not a rigid substance holding objects, but rather a warping of space-time geometry, where objects follow straight-line paths (geodesics) that appear curved because the space itself is curved.

Attractive Force: Within this field, any mass (or energy) experiences a force pulling it toward the source of the field, with strength inversely proportional to the square of the distance. 

Analogy as a "Rigid Structure"

This idea is commonly visualized as a "frictionless ski hill" or a "funnel" in space around a massive object, which directs the motion of nearby objects. For spherical objects like planets, this structure is generally uniform and acts like a rigid "nesting set of shells" extending through space. 

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from Google AI:

The de Sitter effect (or geodetic precession) is a general relativity prediction where an orbiting gyroscope's axis precesses due to the curvature of spacetime caused by a central mass, such as Earth. Confirmed by Gravity Probe B at 6.6 arcseconds per year, it highlights how spacetime geometry dictates motion. 

Key Aspects of the De Sitter Effect:

Origin: Predicted by Willem de Sitter in 1916 to correct Earth-Moon orbital motion, it describes how a spinning object in a curved space does not return to its initial orientation after a full orbit.

Gravity Probe B (2004): This mission verified the effect by tracking gyroscopes in Earth's orbit, confirming the precise misalignment caused by the Earth bending spacetime.

Mechanism: It is purely a consequence of spacetime curvature (geodetic effect) and is distinct from frame-dragging (Lense-Thirring effect), which is caused by the rotation of the mass itself.

Modern Context: Often discussed alongside "de Sitter space"—an expanding, empty universe with a constant positive cosmological constant used to model early universe inflation. 

Difference from "De Sitter Space" and "Gravity"

While the de Sitter effect refers to this specific gyroscopic precession, de Sitter gravity often refers to a theoretical framework (or a form of "toy model" in 1 + 1 dimensions) where gravity acts within a de Sitter universe, often related to cosmological studies of dark energy.

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The Gravitational Field is Dragged with the Object travelling (Spinning) through Space-Tme?

from Google AI:

The statement above is ✅ True, but it requires a specific physical understanding of how gravity "moves." 

In the framework of General Relativity, a gravitational field is not a separate entity that exists on top of space; rather, it is the curvature of spacetime itself [14, 31, 32]. 

Explanation

• Spacetime Curvature: Mass and energy tell spacetime how to curve [8, 32]. As an object moves through space, the source of that curvature moves with it, effectively "dragging" the distortion of spacetime along its path [21, 24].
• Speed of Gravity: Changes in a gravitational field (such as those caused by a moving mass) do not happen instantaneously throughout the universe. They propagate at the speed of light (
  

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  ) [37].
• Retarded Potential: Because gravity travels at a finite speed, an observer at a distance experiences the gravitational field of a moving object as it was at a slightly earlier time (the "light-travel time" between the object and the observer) [37].
• Gravomagnetism (Frame-Dragging): For massive objects that are rotating or moving at high speeds, they don't just "drag" their static field; they actually "twist" the surrounding spacetime, a phenomenon known as frame-dragging or the Lense-Thirring effect [21]. 

Key Distinction

While the field "travels" with the object, it is more accurate to say that the source of the curvature is moving, and the surrounding spacetime continuously re-adjusts its shape at the speed of light to reflect the object's new position [37]. 

Gravity is the Shape (Geometry) of Space-Time Itself

from Google AI:

According to Einstein’s theory of General Relativity, gravity is not a traditional force acting at a distance, but the geometric curvature (shape) of 4D space-time caused by mass and energy. Objects, including light, follow the straightest possible paths (geodesics) along this curved space-time, which we perceive as gravitational attraction. 

Key insights on this concept include:

Mass-Energy Curvature: Massive objects like the Sun warp space-time, creating a "dimple" that causes objects to orbit rather than move in straight lines, as explained on this Reddit thread.

Not a Force: Gravity is not a force pulling objects through space, but rather the result of objects moving through curved space-time, notes a user on this Physics Stack Exchange page.

Time Distortion: On Earth, most of what we experience as gravity arises from the warping of time (making clocks run slower) rather than the warping of space, notes the Stanford University website.

Evidence: This theory explains phenomena like the deflection of starlight near the sun, the precise orbit of Mercury, and the existence of black holes, says a user on this Reddit thread. 

In essence, gravity is the geometry of the universe, and the universe tells matter how to move along that geometry, argue users on this Reddit thread.