Is Newtonian Physics Outdated? What Do Flat-Earthers Say?

In a previous blog, I discussed how two prominent flat-earthers misunderstand modern physics, thinking that light is either a wave or a particle. John Stunja (a.k.a. Quantum Eraser, a.k.a. QE) argues that light is a particle, while Austin Whitsitt thinks that light is a wave. Neither Austin nor John seems to be aware of the synthesis of these two ideas into the modern understanding of the nature of light that happened a century ago. Both Austin and John apparently have read many popular accounts of quantum mechanics, so they should have encountered this concept. As I said in the conclusion of my previous blog, Austin and John have selectively chosen quotes from authoritative sources in typical flat-earther fashion, ignoring the many quotes (often from the same sources) that contradict what they have chosen to believe. These two men have many fans within the flat-earth movement, so their misguided conclusions have influenced many other flat-earthers.

The Paradox of Gravity

The misunderstanding of these two prominent flat-earthers is not limited to the wave-particle duality introduced by quantum mechanics. They also have fundamental misunderstandings about the other pillar of modern physics, Einsteinian relativity theory. For instance, both Austin and John often say that Einsteinian relativity has replaced Newtonian physics, and many other flat-earthers parrot this line. It’s not as if flat-earthers believe either Newtonian physics or Einsteinian relativity. Rather, this appears to be a divide-and-conquer strategy to pit modern relativity against Newtonian physics with the goal of killing off both theories. Where did Austin and John get the notion that Einstein completely replaced Newtonian physics? Once again, they quote some prominent physicists saying something along those lines. However, if Austin and John had kept reading or searched a bit more, they would have found quotes that said something quite different. For instance, the textbook that I used the last year I taught general physics at the university had this to say in its conclusion to the chapter that discussed special relativity:

The sweeping changes required by the principle of relativity go to the very roots of Newtonian mechanics, including the concepts of length and time, the equations of motion, and the conservation principles. Thus it may appear that we have destroyed the foundations of which Newtonian mechanics is built. In one sense this is true, yet the Newtonian formulation is still accurate whenever speeds are small in comparison with the speed of light in vacuum. In such cases, time dilation, length contraction, and the modifications of the laws of motion are so small that they are unobservable. In fact, every one of the principles of Newtonian mechanics survives as a special case of the more general relativistic formulation.

The laws of Newtonian mechanics are not wrong; they are incomplete. They are a limiting case of relativistic mechanics. They are approximately correct when all speeds are small in comparison to c, and they become exactly correct in the limit when all speeds approach zero. Thus relativity does not completely destroy the laws of Newtonian mechanics but generalizes them.1

That puts a very different spin on things, doesn’t it? It’s not that special relativity and Newtonian physics are at odds with one another; they just describe how the world operates in different regimes. As the realm where special relativity merges into the realm of slower speeds and lower energies, we reach the Newtonian limit of Einsteinian relativity, and its description becomes indistinguishable from Newtonian physics. A similar thing is true in quantum mechanics. Quantum mechanics describes very small systems, such as on the atomic level, but as we consider larger systems, the description of quantum mechanics merges into the description of Newtonian, or classical, physics. In 1920, Niels Bohr coined the term “correspondence principle” to describe this. Both the correspondence principle (for quantum mechanics) and the Newtonian limit (for modern relativity theory) can be described as the classical limit.

The general physics textbook I used during my last year of teaching went on to briefly discuss general relativity, though it hardly went into as much detail as its discussion of special relativity. For a good understanding of general relativity, most physicists would recommend the classic book by Misner, Thorne, and Wheeler from a half-century ago. This is how this tome introduces the Newtonian limit:

Just as quantum mechanics reduces to classical mechanics in the “correspondence limit” of large actions, I>>ħ, so general relativity reduces to Newtonian theory in the “correspondence limit” of weak gravity and low velocities . . . This section elucidates, in some mathematical detail, the correspondence between general relativity and Newtonian theory.2

Over the next few pages, this book derived from general relativity Newton’s field equation for gravity in the Newtonian limit. Therefore, when properly understood, there is no real contradiction between the two theories.

So, where did Austin and John get the notion that Newtonian mechanics (and gravity) is dead? They stopped reading when they read quotes from famous physicists saying that general relativity has replaced Newtonian gravity. If Austin and John had kept reading, they might have found that many of the same physicists eventually said that general relativity reduces to Newtonian gravity in the Newtonian limit. But using quotes like that would undermine their divide-and-conquer strategy.

In another aspect of their war against physics, Austin and John often quote some of the same physicists as before, saying that gravity is not a force. We observe in the world that masses tend to attract one another, resulting in acceleration. In Newtonian mechanics, accelerations require forces, so in Newtonian mechanics, gravity is a force. But general relativity treats what we see as the acceleration of gravity differently. In general relativity, objects move along geodesics in curved spacetime. The curvature of spacetime gives rise to what we perceive as the acceleration of gravity, but since objects in free fall do not deviate from their geodesics, there is no acceleration, and hence there is no force. Indeed, this does appear to be a contradiction. But perhaps a better term for this is that it is a paradox. At the very least, Austin and John commit the all-or-nothing fallacy, demanding that either general relativity or Newtonian mechanics must be correct, rendering the other false. This is a bit disingenuous because, as I’ve already commented, Austin and John reject both Newtonian gravity and general relativity.

Resolution of the Paradox

The resolution of this paradox is to realize that physicists have more than one way of looking at the world. For instance, in classical mechanics, one can solve problems by treating accelerations that result from forces, or one may solve the same problems by considering energy constraints. The advantage of the second method is that energy is a scalar while accelerations are vectors, resulting in much simpler mathematics. Similarly, in solving problems in thermodynamics, physicists sometimes treat heat as a fluid, while in solving other problems, they may employ the kinetic theory of heat. As I already pointed out, general relativity is a more generalized understanding of gravity than Newtonian theory. That is, Newtonian gravity is a special case of the more broadly applicable theory of general relativity. In solving most problems involving gravity, such as here on the earth, there is no appreciable difference between the results of using either theory. However, the use of Newtonian gravity is far simpler than general relativity. Therefore, pitting one theory against the other reveals a fundamental misunderstanding of the relationship between these two theories.

Do physicists treat gravity as a force? You bet. As evidence for this, consider the fact that all general physics textbooks define and treat gravity as a force. For instance, the textbook I used the last year I taught university physics discussed four fundamental forces, one of which is gravity.3 When discussing modern physics much later in the textbook, it describes the current thinking of how the four fundamental forces are mediated by exchange particles.4 The exchange particles of three of the four fundamental forces have been detected. The lone holdout is the hypothetical graviton as the exchange particle mediating the force of gravity. Since we don’t have a theory of quantum gravity yet, we don’t even know the properties of the graviton. This sort of discussion is typical of general physics textbooks. I suppose that there may be a few general physics textbooks that, in later chapters when discussing modern physics, say that general relativity does not treat gravity as a force, but I have yet to see one that did.

Conclusion

So, what about the quotes from prominent physicists that Austin and John like to use, indicating that gravity is not a force and that general relativity and Newtonian gravity contradict one another? Those quotes are meant to illustrate how the results of Newtonian gravity and general relativity are very different when applied to situations where it matters, such as around black holes. Similarly, Newtonian physics doesn’t work very well in the atomic and subatomic worlds, which necessitated the development of quantum mechanics a century ago. Both general relativity and quantum mechanics are more complete theories that have classical physics as special cases. The point is that even general relativity and quantum mechanics are incomplete, but Austin and John treat them as complete.

As a creationist, I find it humbling that our physical theories, as impressive as they describe the world, are incomplete. It tells me that God has created a far more complex and fascinating world than most people realize. Like many biblically grounded scientists before me, I view my attempts to understand God’s creation better as a high calling and a form of worship.