It would be more accurate to say that the models used to describe physics had to be modified.

As physics progressed, physicists revised their theories in light of new evidence and knowledge (and they continue to do so today). Here, Einstein describes how the special theory of relativity came about, in part, to solve the problem of classical mechanics' compatibility with electromagnetic theory.

Modern physicists consider classical mechanics an approximate theory that is useful for the study of non-quantum mechanical, low-energy particles in weak gravitational fields. It is usually the starting point for students learning physics.

Einstein's letter was originally published in the 28 November 1919 issue of The Times, a British daily national newspaper.

During the war, Einstein lived and worked in Berlin, Germany. Communication between England and Germany seems to have been limited during the war, even among scientists in those countries. Because science is a collaborative endeavor that spans countries and cultures, Einstein understandably appreciates the renewed communication between these two important countries.

Einstein published his equations that describe gravity with respect to space and time in 1915 —right in the middle of World War I (1914-1918).

Despite poor political relations during this period, two Englishmen published experimental evidence that supported Einstein's theory. Einstein commended the English scientists and institutions for prioritizing scientific pursuits over politics.

In 1959, the Pound–Rebka experiment confirmed that light moving out of a gravitational well is in fact red-shifted.

Read more in The New York Times:

https://www.nytimes.com/1959/12/13/archives/way-to-test-an-einstein-premise-found-by-2-harvard-scientists.html

In A New Determination of the Orbit of Mercury and its Perturbations (1843), Urbain Jean Joseph Le Verrier reported a peculiar precession of Mercury's orbit that was not accounted for by Newtonian mechanics.

Relativity theory, however, provides a robust account of Mercury's motion.

General relativity theory expands special relativity theory to include any reference frame (not just inertial ones).

With no constant frame of reference, general relativity theory cannot consider space and time as separable entities.

Instead, general relativity theory describes something called "space-time" which is warped by massive bodies. Objects traveling through space-time that has been warped by these bodies will follow a warped path (even though it may appear "straight" to the object). We call this effect "gravity."

Solids are 3D objects, such as cubes, spheres, or pyramid structures.

Euclid was a mathematician in Ancient Greece. His book Elements is arguably the first mathematics textbook. In the text, Euclid describes the geometry we seem to experience in our everyday lives in which parallel lines do not intersect.

This is also the same geometry we still learn in high schools across the country (2000 years after Euclid wrote it).

Inert mass (m) is conceptually different from heavy mass (M). Heavy mass is a value that determines the strength of gravitational attraction, whereas inert mass (or inertial mass) is a value that indicates how resistant a thing is to a change of motion.

Modern physicists use the term "gravitational mass" instead of "heavy mass."

The Eötvös experiment measured the correlation between inert and heavy mass (or inertia of a body and its weight) and showed that these two masses have the same value although are conceptually distinct.

General relativity theory allows us to describe space and time regardless of a choice of coordinate systems or reference frames.

Unlike special relativity, general relativity does not assume an inertial (i.e., static) frame of reference..

Physical laws are mathematical expressions that generalize physical phenomena. Physical laws explain and predict experimental results. Conservation of energy (ΔE=0) is one example of a physical law.

Einstein came up with the theory of special relativity before general relativity.

Special relativity assumes the laws of physics are constant in all inertial frames of reference (frames of reference that are assumed to be stationary).

General relativity expands on this, and describes the fabric of space-time on the scale of the universe (which Einstein suggests is warped by massive bodies).

Einstein expanded special relativity to general relativity over the course of about 10 years as he realized that special relativity was insufficient to explain some phenomena in the universe.

Special relativity theory concludes a relationship between mass and energy (E=mc²).

Therefore, the theory marries the conservation of mass and conservation of energy as one and the same law.

This kind of motion is impossible because it would require an infinite energy source, which violates the first law of thermodynamics (the Law of Conservation of Energy: The total energy of an isolated system is constant).

Einstein states a major consequence of special relativity: We can only say two events occur simultaneously when those events share a system of coordinates.

We can always put two events into the same coordinate system, but whether they are simultaneous depends on which coordinates we use.

Einstein uses a famous thought experiment to describe this. Imagine a train traveling on railway tracks and equidistant between two trees (one ahead, one behind). If lightning strikes both trees at the same time, an observer on the train will see the lightning strike ahead before the lightning strike behind. However, an observer standing still next to the tracks (also equidistant between the trees) will see both at the same time. Thus, whether the lightning strikes are simultaneous depends on the observer's frame of reference.

Kinematics is a branch of mechanics concerned with the geometry of motion.

(Einstein clarifies what he means by "kinematics" throughout the paper. When Einstein says kinematics, he specifically refers to the physics that treats space and time as separable entities.)

This is the second principle of special relativity:

Light can be thought of as many particles (called photons) that move through space. These particles always travel at the same constant speed regardless of the inertial system in which light is observed. This has been well established by the work of many physicists.

This means that it does not matter how fast you travel towards or away from a source of light—it will always travel at a constant velocity, the speed of light.

An inertia system of coordinates is a coordinate system that we can approximate to be stationary.

For instance, we can describe the displacement of a baseball (the object of interest) relative to the baseball field (an inertia system of the baseball). Since both the baseball and the field are subject to the same rotational motion of Earth, the movement of the baseball can be described relative to the seemingly stationary field.*

Einstein gives us a way to determine if a system of coordinates is an inertia system:

A system of coordinates moving in the same direction and at the same rate as a system of inertia is itself a system of inertia.

*For the purposes of this example, we have ignored the (negligible) effect of Earth's rotation. An observer on the surface of Earth must be accelerating to follow a circular path, and the entire system is subject to the non-inertial effects of gravity.

Systems of coordinates indicate the location of a point in space in reference to some other point. In doing so, systems of coordinates form a mathematical map of physical space.

In Einstein's example, the system of coordinates for the railway train is the ground.

Einstein says the analytic method is used to form theories of principle, whereas the synthetic method is used to form constructive theories. We can understand the methods as contrasts.

The synthetic method supposes premises that help explain natural phenomena, while the analytic method reduces all natural phenomena to their common denominator. The synthetic method is a bottom-up approach, whereas the analytic method is a top-down approach.

Another way to think about the difference is that the analytical approach starts with observations, whereas the synthetic method starts with hypotheses.

(For those who are philosophy-inclined, Immanuel Kant discusses this distinction between the analytic and the synthetic.)

The kinetic theory of gases describes the motion of atoms in the gas phase and makes a number of assumptions such as:

1. The gas molecules have negligible volume compared to their container.
2. The molecules are constantly moving.
3. All collisions are perfectly elastic.

It is only by embracing Newton's ideas that physicists such as Einstein were able to consider exceptions and, in doing so, build on them.

Although we don't often think about it, scientists improve on each others' ideas all the time and revise theories in light of new evidence generated by new technologies and methodologies.

Euclidian geometry presumes a uniform and constant arrangement of space. However, Einstein proposed (and others demonstrated) that space is "warped" around objects depending on their mass. The larger the mass, the greater the degree of warping.

Because time is connected to space, it is also warped around massive objects.

Einstein introduces the need for a concept of general relativity, which describes motion with respect to any two coordinate systems, not just inertial coordinate systems.

Einstein's question points out that coordinate systems are not a product of nature, but a way of understanding nature. It is because of this that physicists are able to revise the tools and methodologies they use in light of new evidence.

Aristotle contemplates absolute and relative motion in his book On the Heavens. He describes how heavy bodies move down and lighter bodies (like air or fire) move up relative to the center of the universe.

Read more: https://plato.stanford.edu/entries/spacetime-theories/#2

Einstein's theory of relativity is a two-part theory. Together, the two parts explain the motion of subatomic particles and gargantuan masses and address some of the shortcomings of earlier theories of motion.

The theory is well known for combining the concepts of space and time, revealing that they are inseparable.

Again, this was found by Arthur Eddington and Frank Watson Dyson in their paper.

Maxwell's equations describe how electric and magnetic fields manifest from charged particles. Together, Maxwell's equations suggest the speed of electromagnetic waves (i.e. light) is constant.

The Lorentz Force Law says that the force felt by some charged particle is related to the surrounding electric and magnetic fields.

Together their work describes special relativity, but only for electromagnetism.

The analytical method begins with empirical observations, from which principles and formula are inferred (which can be applied to other cases).

Empirical methods involve observation and experience, rather than logical deduction alone.

Einstein's "English colleagues" are Arthur Eddington and Frank Watson Dyson, astronomers who obtained experimental evidence of Einstein's theory of relativity.

Modern physicists have debated whether the error bars in the Eddington experiment were larger than the effect they measured. Nevertheless, the results have been confirmed.

Read Eddington and Dyson's work here.

In the image below, the solar gravitational field influences the sun's rays framing the moon. Notice that the light bends around the moon due to the gravitational field, rather than forming a spherical halo.

A value that indicates how resistant a thing is to a change of motion.

Modern physicists use the term "inertial mass" instead of "inert mass."

Modern physicists call "inert" and "heavy" masses "inertial" and "gravitational," respectively.

James Clerk Maxwell and Hendrik Antoon Lorentz were physicists of the late 1800s and early 1900s. Their work is the foundation of the branch of physics called electromagnetism.

This is the first principle of Special Relativity:

The laws of physics are the same for all observers in uniform motion relative to one another.

This is the second kind of theory that Einstein delineates. He tells us that a theory of principle is one that is formed from the most consistent and basic observations seen across all natural phenomena.

A constructive theory is one that is built up (i.e., constructed) from assumptions to explain a natural phenomenon.

A proposition states a claim that can be either true or false.

The science that studies the relationship between heat (thermo) and work or movement (dynamic).

The region of space around the sun that is attractive to other massive bodies (i.e., the planets).