This article is the first in a series of major problems in physics.

A successful scientific theory is one that makes accurate and precise predictions. Scientists are especially happy when two different hypotheses make predictions that agree. Thus, physicists suffer when they use their two best theories to predict the simplest possible quantum, and the result is such a staggering disagreement that it is often called “the worst guess in the history of science.”

The empty space is, well, empty. Being empty, it seems that calculating the energy in the empty space will be easy and the prediction will be zero. However, that expectation is incorrect.

The two theories that, when combined, underlie all of modern physics are called general relativity and the standard model of particle physics. General relativity describes the behavior of gravitational forces and applies to large structures in the Universe. In contrast, the standard model of particle physics is used to explain all other forces and governs the very small quantum world.

Both theories can be applied in a vacuum. So, what happens when these two theories are used to calculate the energy density in a real vacuum?

## A view from general relativity

Einstein’s theory of general relativity discusses the nature and motion of space itself. We have known for centuries that the Universe is expanding, and the theory that explains the evolution of the Universe is called the Big Bang. Basically, this theory says that the Universe was once small, and something caused it to start expanding.

Because gravity is an attractive force, this means that once it begins to expand, this expansion will slow down. Why? Because everything in the Universe attracted everything else.

Thus, it was very surprising when, in 1998, researchers investigating the evolution of the Universe discovered that the Universe was not only expanding, but also expanding rapidly. The only possible way is if the area had a small and different energy associated with it. If the force were in the right direction, it would result in a repulsive form of gravity. Researchers call this repulsive gravity “dark energy,” and they can calculate how much dark energy is needed to explain the evolution seen in the Universe. This energy is very small – it is equal to the energy of four hydrogen atoms in a cubic meter of space.

## A view from quantum mechanics

So, does the standard model predict spatial dynamics and, if so, how?

The standard model says that every space is filled with different fields. When those fields vibrate in certain ways, particles of the quantum world appear – electrons, quarks, etc. fields with a range of wavelengths. Because the world’s particles and waves are one and the same, this means that empty space contains a chaotic mixture of ephemeral particles that appear and disappear instantaneously. This lowest state of energy in various fields is called the zero point, and the energy at it is called “zero energy.”

To calculate the zero-point energy of the quantum world, add the result of all the quantum waves. In principle, there are no short waves, so you add shorter and shorter waves. Because shorter length means higher power, this means adding higher and higher power. Taken to the extreme, you can add zero wavelengths and infinite power – but we know that the standard model eventually fails at very high power, so you only add power to a certain frequency (and, therefore, only at a minimum of a small wavelength).

What exactly should be used in the calculation is a matter of theory, but most scientists agree that the highest energy at which the standard model can be used is called the Planck energy. If you use that energy as a cutoff in your calculations, you calculate the zero point energy too high. The mass of energy is equal to the mass of 100 quintillion twice the area of the entire visible Universe combined into a cubic meter.

## The worst guess in all of science

In fact, with this simple calculation, the power density predicted by the standard model is about 10^{120} times predicted by general relativity. That’s one followed by 120 zeros. This difference certainly earns the name “the worst guess in all of science.”

Issue 10^{120} it’s a very bad situation. Unproven theories have been proposed that improve the situation. For example, if a theory called supersymmetry turns out to be true, the discrepancy is “only” a factor of 10.^{60}.

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When there is such a large disagreement, there is something seriously wrong with one or both theories. It often happens that our current understanding of the theory is wrong, but general relativity describes the cosmos well and the standard model does a good job at the quantum level. It is only when the two are compared that the problem arises.

## Other possible solutions

What are some of the proposed solutions? Yes, there are many. For example, one explanation comes from the fact that the standard model assumes that there is no small unit of space. This means that the smallest volume you can imagine can be divided into smaller units in an infinite series. But what if there is an even smaller unit of space – ideally an “atom” of space? If that is true, then this changes the math, and in such a case, the disagreement between cosmic and quantum forces can disappear.

Another theory is that we are deceived by our emotions. As we see the world around us, we seem to be moving in three directions. If there were additional dimensions of space, then this would significantly change our theory of gravity, which would mean that quantum calculations (currently performed in three-dimensional space) are incorrect.

Although the final answer is unknown, it seems likely that the problem comes from our understanding of the world as being too small. After all, if the predictive model were correct, the Universe would have expanded so fast that stars, galaxies, and humans would never have existed.

But a secret is a secret. The simple truth is that researchers do not know why our concepts of the cosmic and quantum worlds make different predictions. Despite decades of effort, the answer has eluded some of the brightest scientific minds. We will have to wait until the next day when someone solves this cosmic puzzle and enters the pantheon of physics legends.

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