What is Gravity? - Part 2
- kieronconway
- 6 hours ago
- 6 min read
© 2026 Kieron Conway - All rights reserved.
This article will look at Einstein's alternative view of gravity and follows on from “Part 1 of What is Gravity?” in which we looked at Newton's theory.
Phase 1 - Development of Special Relativity
Two hundred years after Newton, Albert Einstein (1879 to 1955) embarked on the development of special relativity, which shows how particles behave traveling at velocities close to the speed of light.
To do this he used a mathematical construct that fused time and space together in what is known as space-time, developed by one of his professors, Hermann Minkowski (1864 to 1909).
Minkowski space-time provides a mathematical framework in which all observers, each in their own inertial frame of reference, can have their observations mapped onto a single space-time reference frame.
Any one of the observers is designated as being stationary and is only moving through time. All common events to all the participant observers are then mapped onto Minkowski space-time according to how the stationary observer views them.
Each of the other observers is mapped onto the Minkowski space-time context according to their relative velocity to the stationary observer.
This provides the context in which particles travelling at speeds close to that of light can be investigated.
Indeed, computers at accelerators, like the Large Hadron Collider (LHC), use a version of Minkowski space-time to reconstruct the particle tracks that result from high-speed collisions.
Minkowski space-time demands all participants are either stationary or moving at constant velocity meaning that that there are no forces in action.
In this inertial frame-work, no account is made of gravity or acceleration.
Phase 2 - Development of General Relativity (GR)
Einstein wanted a version of space-time that allowed him to model gravity and its effects on motion, including that of light.
He produced a mathematical model of space-time that included gravity and acceleration having already established the equivalence of gravity and acceleration.
In other words, gravity can be interpreted as a form of acceleration.
He produced a set of equations that showed how the presence of matter and energy curved space-time and indicated how matter and light must move in the presence of gravity.
From this mathematical arena, he was able to model the universe and make predictions about how things should work in the real world.
A new version of Gravity
Remember, in Newton's theory of gravity, the attraction between two masses is given by;
F = G(m1 x m2)/r²
According to Newton, the force acts between two masses. Photons, having no mass do not experience this force in the same way as material bodies.
In Einstein's space-time, matter must follow the curvature created by gravity and this is also true for light.
Think of it like this;
Imagine a boat following the earth's curvature, it moves along a curved line known as a geodesic, which is the shortest distance between two points on the earth's oceans.
In Einstein's theory, the “straightest” possible path through curved space-time is also a geodesic.
A Geometric Interpretation of Gravity
Einstein's theory of general relativity, provided a geometric interpretation of gravity, in which mass and energy bend space-time rather than exerting a force as in Newton's theory.
Gravity, in general relativity, is a result of the geometry of space-time and this explained and predicted things that Newton's theory could not.
Important Point:
It must be stressed, as Einstein himself was at pains to point out, a geometric interpretation of gravity is NOT a replacement for Newton's gravity.
It's an alternative description that becomes essential when dealing with strong gravitational fields or the need for extreme precision.
The Bottom Line:
In Einstein's space-time, gravity is no longer considered as a force, but it creates a geometry that influences particles and light travelling through it.
The curvature of Einstein's space-time leads to the shortest distance between two points, in the vicinity of a gravitational field, being a geodesic.
This can be carried through to the 3D space with which we are familiar.
In fact, the space constraints of space-time have little effect in every-day gravitational fields, like that of the earth or our sun, where the effects are very small.
It's the time component of space-time curvature that has the maximum impact on how matter and light behave in gravitational fields.
Gravitational Time Dilation
This is another revolutionary outcome of general relativity (GR).
Time runs slower for someone on the surface of the earth, compared to someone high above the earth.
The effect is very small in earth's gravitational field, but becomes massive as you approach close to a black-hole.
So, Gravity is not a force?
If you believe that gravity is not a force, try the following experiment:
a) Drop a brick on your stationary foot.
b) Try to shout "Gravity is not a force" with conviction.
This humorous example emphasises the point that Einstein's theory is only an alternative to Newton's and not a replacement as your throbbing foot will testify.
Space-time is a mathematical abstraction that deals with gravity without considering the force created between two bodies.
The Predictions of General Relativity
Einstein predicted from GR the following;
1) Gravity bends light.
2) The orbit of Mercury deviates slightly from Newton's prediction.
3) Light coming from a strong gravitational field will have its wavelength altered when measured by someone on earth receiving the light.
These are the three predictions that Einstein himself made, which could be used to verify his theory.
All three predictions were found to be correct.
The sun bent the path of light from behind it, during a total eclipse in 1919, making the star visible from earth. In a world with no internet, this sent Einstein's reputation viral a short time after he completed his theory.
Einstein calculated the perihelion precession of Mercury with total accuracy: not as dramatic as being able to see a star hidden by the sun, but equally important.
Just before his death in 1955, the technology became available to determine the shift in wavelength measured on earth of light emitted from a white dwarf, with a strong gravitational field. This measured gravitational-shift was just as Einstein had predicted.
None of these predictions can be made from Newton's theory of gravity.
Other Predictions of GR
There are many predictions from GR. Here are two of the most important.
Ripples in Space
GR predicts that accelerating massive objects can generate ripples in space known as gravitational waves.
The propagation of these waves, created in high-mass mergers, for example, was predicted by GR to generate gravitational waves at the speed of light.
Gravitational waves are now detected on a regular basis by instruments such as LIGO.
This solved Newton's dilemma of a change in gravity being propagated instantaneously.
Black Holes
GR predicts how gravitational collapse can create super dense objects.
The shadows created by such objects have now been photographed.
Other Predictions
GR also predicts a host of other fascinating possibilities, which you can read about in Part 2 of A Journey into Modern Physics.
A Key Benefit of Einstein's Relativity
Without special and general relativity, we would not have a viable GPS system.
The two theories provide us with the corrections needed to be made constantly to keep GPS navigation working to the accuracy that allows us to have confidence in "you have reached your destination".
Without these frequent corrections, GPS would accumulate errors of about ten kilometres every day.
So, What is Gravity?
Still no real answer from either Newton or Einstein.
Both theories described the effects, but not the actual physical mechanism that causes those effects.
Most likely, we have to wait for a verified theory of quantum gravity to get to the bottom of this.
Quantum gravity may explain what gravity is, similar to how the other three fundamental forces of strong nuclear, weak nuclear and electromagnetic, work.
To sum up
We still can't answer the question, what is gravity? But we have good descriptions of how it works.
If you ask engineers to design a rocket to travel to the moon, they will use Newtonian physics to get it there.
If you ask astronomers to indicate when the planet Mercury will be at the closest point to the sun in the year 2050, they will use Einstein's physics to give you the answer.
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