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Einstein’s Gravity is the New Heliocentrism

I’m sure you have heard of the millenia-long debate over whether Earth is at the center of the universe (geocentrism). Scientifically, in the 21st Century, this is mostly settled. As it turns out, even the premise of the debate was flawed! Today, Einstein’s gravity is the new heliocentrism.

Our ancestors can be forgiven for thinking that the Earth was the center of the universe. After all, when we look up at the night sky and even during the daytime, the Sun, the Moon, the stars, everything up there seems to be moving around us. However, when the sky is studied carefully, there are objects that give real hints that it is not as simple as it looks at first glance. Planets exhibit retrograde motion (they move one direction in the sky at a certain time of the year and then another later). Asteroids and comets show up occasionally, moving in strange directions and even displaying large tails.

Ignoring these smaller objects, the ancients felt the need to explain the motions of the Sun, the Moon, and the other known planets Mercury, Venus, Mars, Jupiter, and Saturn. Yes, in antiquity, even the Sun and Moon were considered planets. Historically, humans agreed on the geocentric model of the universe, which meant that anyone who came up with something different was seen as strange, weird, and maybe even crazy. In ancient Greece, if you could show something was true through reasoning, it had the chance to be believed. Aristarchus of Samos in the third century BCE reasoned that it was the Sun that was at the center of the universe and all the planets revolved around it. Later, incredibly persuasive thinkers such as Plato, Aristotle, and Ptolemy laid down the basis of geocentric thinking that would survive for over 1500 years. The Earth was simply too massive to be moved. It was unnatural to think that it could move. The objects in the heavens that moved were made out of aether, a lighter substance than what made the Earth.

Copernicus, Geocentric Model of Universe
Copernican Model of Universe

It was dangerous to even suggest something different. People who began to think differently based on astronomical evidence, like Copernicus and Galileo, faced persecution when their heliocentric hypotheses came to light. Copernicus published his major work describing his heliocentric model on his deathbed, avoiding criticism and persecution. Galileo, who not only shared his evidence that the planets (even Earth) went around the Sun, but that there were also other moons that went around the planets (Jupiter’s four largest moons), did not get away as easily, and suffered major religious persecution. The Catholic Church took more than 350 years to admit that they were wrong and apologize for their treatment of Galileo.

When Newton came on to the scene in the late 1600’s, he looked at all of the evidence, and explained the motions of the planets around the Sun and Kepler’s Laws with his idea that the force of gravity was causing all of it. With his new law, he found that even the Sun was in motion around the center of gravity of the solar system, which was located just outside of the Sun mostly due to Jupiter’s pull. He believed that it was this barycenter (center of gravity of the solar system) that was fixed in space, with everything else revolving around it.

Hundreds of years later, in the 1920’s and 30’s it was proven that the universe is much larger than we thought, is expanding, and contains many galaxies, of which the Milky Way is just one. We learned that our Sun is a star, stars come in many sizes and colors, and all of the other stars are very far away from us (many light years away). We narrowed down our cosmological theories of the beginning of the universe to the Big Bang model, in which we

Big Bang, Cosmology, expansion of universe

now accept that our place in the universe is not central. We are

simply on a small rocky planet, orbiting one star in the universe, of which there are a trillion billion others. We are not located in the center of our solar system, galaxy, or galactic cluster. There is no stationary center. The premise is false.

Today, Einstein’s gravity is the new heliocentrism. We have had hundreds of years to use and fully appreciate Newton’s version of gravity as a universal force and it has served us well. It helped us predict the orbits of planets and comets. It helped us discover the planet Neptune, and with it, we sent astronauts to the moon and back. Newton’s Law of Gravity is so close to Einstein’s General Relativity calculations that in many cases, we can use the simpler Law (Newton’s) when making quick estimates. However, when the differences are highlighted, you might as well be talking about apples and orangutans.

Newton’s Law is based on space and time being constant, fixed quantities. Einstein’s Relativity Theory dictates that not only space, but time itself changes under different

circumstances. Space warps; it bends near massive objects. Time slows near massive objects and moves faster farther away. Space contracts in the direction of motion when objects move fast. Time slows when objects move fast. Clocks really do move at different speeds in different locations/ elevations, and velocities, and with our advanced technologies, we have the black and white evidence to show this is true. Space really does curve around massive objects. With highly accurate astronomical observations, we have also proven that this is the case (most notably when solar eclipses occur, starlight is bent to the exact degree predicted by Einstein’s Relativity Theory). Because spacetime bends in this way, we can explain Einstein rings and Einstein crosses, images of galaxies that have been severely warped/lensed due to this effect.

Newton conceived of a gravitational force that was active everywhere all at once, and admitted to not knowing how it could possibly work this way. He believed that if the Sun winked out of existence, the Earth would immediately fly off in a straight line away from that missing central mass. Einstein’s Relativity Theory predicts that Gravity is conducted by gravitons and gravitational waves, which move at the speed of light. He believed that it would take 8 minutes and 20 seconds for the information that the Sun was gone to reach Earth and then the Earth would fly off in a straight line. Einstein also believed that, because gravity waves are so incredibly hard to detect, human beings would probably never detect them. In this case, Einstein was wrong. In 2015, we detected the first gravity waves and since then, we have detected almost a hundred more!

Last week, I read a fascinating article from Big Think in which scientists found a 1.7 second delay between the light waves and the gravitational waves sent to us during a kilonova explosion in deep space (130 million lightyears away) when two neutron stars merged. You can read about it here, but the basic idea is that, just as Einstein predicted, the gravitational waves move at exactly the speed of light, and the light from that event was either slowed down by objects in its path or the light was produced soon after the neutron stars merged, and therefore arrived late. So boom, yet another major win for Einstein’s Relativity theory.

Here’s a brief list of other things that can only work or be explained using Einstein’s Relativity Theory: the planet Mercury’s orbit, GPS, electromagnets, the color of gold, the element mercury being liquid at room temperature, the existence of black holes, Sun-delayed radio signals, frame-dragging (how spacetime spirals when a mass spins). If Newton’s conception of gravity were correct, these things could not be explained. Only with Einstein’s Relativity theory do these, and the others mentioned before, make sense.

Scientifically, when a new theory comes along that explains not only all the phenomena previously explained by the old one, but predicts new ones, as mentioned above, for which there is now a mountain of evidence, that new theory then replaces the old one and becomes the new accepted theory. That’s how it works, in “theory”. In reality, as we have seen, it sometimes takes hundreds, or even thousands of years to change the public’s mind on this, much less the minds of the scientists.

Since mathematically the two theories produce close results, and Newton’s Law is simpler and easier to comprehend, for over a hundred years now, educators, all the way up through undergraduate physics, have chosen to stick with what they know: Newtonian gravity. This also follows the idea of Occam’s Razor: when presented with multiple competing hypotheses or explanations, the simplest one is usually the best or most likely. The key word here is usually. Asked which theory is more technically correct, according to the data, scientists will agree that it is Einstein’s Relativity Theory. Shown that there is no way to explain the above phenomena according to Newton’s Law of Gravity, they will agree. Only Einstein’s Relativity Theory survives scientific scrutiny, predicting so many things for which our modern science is now finding evidence. Even though it is more complex, in this case, for now, it is the best description we have of how gravity works.

But, even Relativity Theory is suspect in the extreme environment of a black hole and in the smallest reaches of quantum theory. Does that make it an invalid theory? Not at all. All theories are models of how we think the universe works. As models, they have their strengths and weaknesses. General Relativity Theory’s strengths are explaining to a high degree of fidelity the gravitational interactions of bodies in our universe. It predicts so many things that have only recently been shown to be true with evidence gathered by modern technologies (again, see the list above). When we probe the weaknesses of a theory, it typically leads to new discoveries and new science. Never in the history of science have we developed a theory of everything that explains all that we see and predicts everything else we cannot see. There is always a frontier of new discoveries and our current models help show us the way.

Einstein’s General Theory of Relativity is a 180 degree turn on how gravity works, akin to viewing the Sun as the center of our solar system instead of the Earth. Accepting this model requires our minds to change about the world and how it works. The speed of light is fixed and space and time are variable. Gravity is not a force, but a warping of spacetime around massive, energetic, and/or spinning objects. When you drop a ball, the Earth accelerates upward to meet it, since there is no force on the ball. Wrapping our minds around these fundamental changes in worldview is not easy, but climbing a mountain begins one step at a time (and gravity helps us along the way). Let us take these first steps together, and usher in a scientific revolution that is already over a hundred years in the making.


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