Gravity is the Universe’s Problem Child

“You dig deeper and it gets more and more complicated, and you get confused, and it’s tricky and it’s hard, but it is beautiful.” –Brian Cox

From Aristotelian physics to Newtonian mechanics, from Einstein’s Relativity to string theory, the more we learn about gravity the less it wants to follow our rules.

The first earnest attempt to explain gravity came from Aristotle, who believed all things had an innate desire to move to where they belong based on their gravitas. He explained in “Physics,” in the geocentric model with the Earth at the center of the universe, heavy bodies like earth and water are drawn to the center, whereas light bodies like fire and air are drawn upwards, because that is their origin. He said, “It is the nature of every kind of sensible body to be somewhere, and there is a place appropriate to each, the same for the part and for the whole, e.g. for the whole earth and for a single clod, and for fire and for a spark.”

Over the many centuries since then, our understanding of gravity has become far more refined, yet gravity has proven itself to be far stranger than Aristotle could have imagined.

Size Matters

What is applicable at one scale, breaks down at another. The best explanation for the behavior of supermassive stars, produces absurdities when applied at the quantum level.

Sitting in his garden at Woolsthorpe Manor in 1666, or so the legend goes, Newton was hit in the head by a falling apple, inspiring him to develop his well-known equation for gravity.  

Published years later in 1686, Newton’s law of universal gravitation explains the force of gravity between two objects is proportional to their masses and inversely proportional to the square of their distance.

Newton used this equation to calculate the orbits of planets with far more accuracy than ever before, and in fact, he used it to predict the existence of large planets in the outer solar system, which were later identified as Neptune and Uranus. At the time, Newton’s explanation of gravity was revolutionary, and for roughly 200 years, it held strong, despite a handful of short coming, such as its failure to explain the precession of the perihelion of Mercury’s orbit around the Sun.

As science marched on, it became evident Newtonian gravity was inadequate to explain objects of extreme sizes.

For extremely large objects, Einstein’s General Relativity is needed. This explains both space and time warp depending on the mass of the object. When it was published in 1916, although it was met with much skepticism, it explained the anomalies produced by Newton’s equations, like the orbit of Mercury. Since then, even though it’s counter-intuitive, it has been proven experimentally countless times, recently with the first detection of gravity waves in 2015 and the first picture of the Milky Way’s black hole in 2019. Despite Relativity’s success, it struggles to explain dark energy and spacetime singularities.

On the other hand, gravity is virtually meaningless with extremely small objects. At this scale, physicists rely on quantum mechanics, which explains particles exist in a cloud of probabilities until forced to choose a state. According Werner Heisenberg, one of the pioneers of quantum mechanics, “The atoms or elementary particles themselves are not real; they form a world of potentialities or possibilities rather than one of things or facts.”

At this scale, gravity is far too weak to measure, and Einstein’s Relativity is fundamentally incompatible. Trying to fit quantum mechanics with Relativity into a Theory of Everything is the most important open problems in physics. The leading contender for a such a theory is string theory, although this has yet to be fully accepted by the physics community.

The Elusive Graviton

Gravity is the only fundamental force whose carrier particle has not been found. It may not even exist.

According to quantum mechanics, the fundamental forces are carried by an elementary particle. This particle has been found for all of the fundamental forces, except gravity.

The strong force is responsible for holding protons and neutrons together, as well as their constituent quarks, and it’s carried by the gluon; W and Z bosons are responsible for carrying the weak force, which causes the radioactive decay of atoms; and the electromagnetic force is carried by the photon. Theoretically, the graviton carries the gravitational force. Finding it is one of the many goals for the next generation of particle accelerators, although some speculate it would take an absurd amount of equipment and resources to do so.

If the graviton were found, the problems don’t end there. First, the 3 known carrier particles fit nicely into the Standard Model of particle physics, which is essentially the periodic table of quantum particles. However, the graviton doesn’t have a place where it fits in. Second, gravitons are mathematically problematic. In quantum mechanics, it is common for infinities to appear while performing calculations, making arriving at a reasonable answer impossible. These infinities are dealt with through the process of renormalization, which essentially adds opposite terms to cancel them out. However, the mathematics of gravitons would involve an infinite amount of infinities, making renormalization itself impossible.

Do You Even Lift, Bro?

For reasons yet to be explained, gravity is orders of magnitude weaker than other fundamental forces.

As the name suggests, the strong force is the strongest, as it is over 100 times stronger than electromagnetism, more than a million times stronger than the weak force, and 10³⁸ times stronger than gravitational force.

While gravity may seem stronger, this is only because it has the longest range, but on small scales, gravity is pretty much irrelevant. More specifically, the strong force has a range of 10⁻¹⁵ meters; the weak force’s range is around 10⁻¹⁸ meters; both gravity and the electromagnetic force follow the inverse square law but the electromagnetic force tends to cancel itself out, leaving gravity to dominate on large scales.

Why gravity is so weak is a major open question for physicists. One of the leading explanations is the presence of other dimensions, which are hypothesized to be so small that only quantum particles can interact with them. Therefore, if gravity propagates though extra dimensions other than the 4 spacetime dimensions, then this might explain why some of it’s energy seems to get lost.

Is String Theory the Answer?

Our best hope of making sense of gravity rests with string theory. If it’s correct, the universe is composed of 1 dimensional strings that interact with 10 dimensions. How a string vibrates gives rise to different particles and their many properties. Using string theory, physicists have had some success understanding gravity at all scales, have derived the existence of the graviton, and explains why gravity is so much weaker than the other fundamental forces.

However, string theory has been heavily criticized for it’s complexity, it’s number of possible solutions, it’s lack of predictability, and it’s lack of experimental evidence. Lee Smolin, an outspoken critic of string theory, believes the physics community has gone awry due to hubris. He said “Some string theorists prefer to believe that string theory is too arcane to be understood by human beings, rather than consider the possibility that it might just be wrong.”

If he’s right and string theory is wrong, then it’s questionable if we will ever truly makes sense of gravity.

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