#gravity #relativitytheory #quantumgravity #physics #quantumphysics

Ch:- 3

Pg:- 47,48 slightly went out of textbook

Gravity in Classical Physics:

• Isaac Newton's theory of gravity (1687) is a force-based model, in which gravity is a force acting at a distance between two objects.

• The force is described by the equation:

where G is the gravitational constant, m1, and m2​ are the masses, and r is the distance between them.

• Instantaneous Action: Gravity acts instantaneously over a distance (spooky action at a distance), which was later challenged.

• Inverse-Square Law: The force decreases as the square of the distance between objects.

• Works well for macroscopic objects and for systems like the solar system, where speeds and masses are relatively moderate.

Limitations:

• Fails at High Speeds or Strong Gravitational Fields: Newtonian gravity can't describe extreme cases, such as black holes or the bending of light by gravity (gravitational lensing).

• Incompatibility with Special Relativity: Instantaneous action is not allowed in Einstein’s relativity, as nothing can travel faster than light.

Gravity in General Relativity:

• Albert Einstein’s General Relativity (1915) reinterprets gravity as not a force, but the curvature of spacetime caused by mass and energy.

• Matter tells space how to curve, and curved space tells matter how to move.

• Described by the Einstein field equations:

no need to focus on the equation for now, i will make a post explaining this equation

• Non-instantaneous Effect: Gravitational interactions propagate at the speed of light via gravitational waves.

• Curved Spacetime: Gravity is not a force but a geometric property of spacetime itself. Large masses (like planets and stars) create “dents” in spacetime, and objects move along paths (geodesics) in this curved geometry.

• Accurate Predictions: General relativity has been confirmed in many tests, including gravitational time dilation (time runs slower near massive objects), light bending, and the precise orbit of Mercury.

Limitations:

• Breaks Down at the Quantum Scale: General relativity fails to describe gravity at very small (quantum) scales or very high energies (inside black holes or at the Big Bang).

• No Quantum Description: It is a classical theory and incompatible with quantum mechanics.

Gravity in Quantum Physics:

• Quantum physics treats the other forces (electromagnetism, weak and strong nuclear forces) through quantum field theories, but gravity has resisted such treatment.

• Quantum Gravity is the hypothetical theory that would describe gravity according to the principles of quantum mechanics.

Approaches:

• Gravitons: One potential approach involves gravitons, hypothetical particles that mediate the gravitational force, similar to how photons mediate the electromagnetic force. However, gravitons have never been observed.

• Loop Quantum Gravity: A theory attempting to quantize spacetime itself by proposing that spacetime is made up of discrete loops rather than continuous curves.

• String Theory: Proposes that fundamental particles (including the graviton) are one-dimensional strings vibrating at different frequencies, potentially unifying gravity with the other forces.

Challenges:

• Incompatibility with General Relativity: General relativity and quantum mechanics use different mathematical frameworks, and merging them has proven difficult. For instance, singularities like black holes or the beginning of the universe lead to breakdowns in both theories.

• Experimental Limitations: We lack experiments that directly probe quantum effects of gravity. Current particle accelerators cannot reach the energy scales needed.

Equations we can use to hypothetically prove quantum gravity:

Video on classical physics gravity:

Video on relativity theory:

Video on quantum gravity: