The extra dimension must be small, otherwise the universe will produce a lot of mini black holes, which will make the universe disappear in the blink of an eye.
This may be a way for the universe to go to destruction.
First, a pair of particles in the universe collide at an unimaginable speed. The huge energy they hit will produce a tiny, short black hole that is so small that it lasts for less than a second before evaporating.
Then, where the black hole evaporates, a “space bubble” with a completely different physical law from the universe we live in will begin to grow and expand outward at the speed of light. Wherever the bubble is wrapped, any atom will collapse, and everything that constitutes it will disappear in an instant.
This bubble is invisible. When it starts to swell, if you stand nearby, you can’t see it coming. If it hits you from below, your feet will disappear before you realize it.
However, to make the above horror scene really happen, there is a premise: the universe must have at least one extra dimension. But in reality, the universe has not gone to destruction, at least not yet. So what does this mean?
To understand this, let’s first talk about the impact of the extra dimensions.
The influence of extra dimensions on the universe
Everyone knows from experience that there are three dimensions in the surrounding space, for example, an object can move up, down, front, back, or left and right. In addition, we also know that time is one-dimensional, because there is only one direction in time. But many physicists believe that our space is likely to be higher than 3D, and these extra spatial dimensions can be called extra dimensions. Introducing additional dimensions into physics can help solve many of the current physics and cosmology problems, including the combination of general relativity and quantum mechanics, the nature of dark energy and dark matter, and so on.
We have never encountered additional dimensions in our lives, nor have we found any evidence of additional dimensions. Physicists believe that the extra dimension should be curled up in a small space, too small, and it is still difficult to detect.
Physicists speculate that even small dimensions can have an impact on the universe itself. For example, Newton’s law of universal gravitation states that the gravitational magnitude of two objects is inversely proportional to the distance between them. But if there are extra dimensions, on a very small scale, the inverse square law of gravity and distance may not hold. Because gravity may leak into these extra dimensions, this may explain why at the macroscopic scale, gravity appears to be much weaker than other fundamental forces. Conversely, because gravity will leak less to the extra dimension over a small distance, this will cause the gravitation to become stronger than expected over a small distance, no longer following the inverse square law (eg, gravity) May be inversely proportional to the cube of the distance or the fourth power).
Mini black holes will destroy the entire universe
So the problem comes: if the extra dimension really exists, the gravitation becomes stronger at the microscopic scale, which lowers the threshold for making mini black holes. If there is no extra dimension, gravity should be the same at the microscopic scale as the macro scale. If you create a mini black hole, you need more quality or energy.
This is why some people have worried that after the large Hadron Collider near Geneva in Europe started running in 2008, the collision of high-energy particles may create a mini black hole, and some people even worry that the black hole will swallow the earth. But so far, this has not happened.
However, the universe itself can produce particles of higher energy. The most energetic particles found so far are from ultra-high-energy cosmic rays, which are about 100 million times more energy than those produced by the Large Hadron Collider. If the collision between these particles produces a mini black hole, then the physicist can calculate how much extra dimension has an effect on the gravitational force, and the intensity of the gravitational force begins to increase as the distance becomes smaller.
However, the life of a mini black hole is very short. According to the theory put forward by British physicist Hawking in the 1970s, all black holes radiate outward, and they slowly evaporate and eventually disappear. Moreover, the smaller the quality of the black hole, the faster the evaporation. According to Hawking’s theory, any mini black hole formed by the collision of particles in cosmic rays will almost disappear in an instant.
This may be bad news for the entire universe. In 2015, several physicists at Durham University in the United Kingdom theoretically proved that when black holes evaporate, they are likely to push the universe into a state in which the laws of physics are so different that No atom can exist steadily. In this way, everything in the universe will disappear in the blink of an eye.
This kind of disaster is called vacuum decay. It depends on the view that our universe is in a false vacuum. A false vacuum is an unstable or metastable vacuum whose energy density is not theoretically the lowest. Because everything tends to evolve to the lowest energy state, the false vacuum can evolve into a vacuum with the lowest energy density, the true vacuum. However, the theory also shows that if the universe enters this true vacuum state, then the atoms and the things they constitute cannot be stable.
A false vacuum evolves into a true vacuum that may not occur for a long time, or suddenly inexplicably occurs at some point, or is triggered by certain events, such as the evaporation of black holes.
According to theoretical calculations, the physicist at Durham University found that after the black hole has completely evaporated, it is possible to create a true vacuum region at its location. Once the vacuum is present, the space around it will join the vacuum. This true vacuum area, like an inflated bubble, begins to expand outward at the speed of light, taking away everything it wraps around.
The extra dimension should be small
But this theory has led to a paradox. Particle collisions of ultra-high-energy cosmic rays in the universe should be a common phenomenon in the universe. If there are extra dimensions, and if they are large enough for the super-high-energy cosmic rays to produce mini-black holes, then there should be a lot of mini black holes in the universe. After they have completely evaporated, there are definitely some that will trigger vacuum decay. In fact, we are still alive, and vacuum decay has not happened. If the extra dimensions really exist, they must be smaller than previously thought, making it difficult for the ultra-high energy cosmic ray particle collision to create a mini black hole.
Two physicists from the United States calculated from the above ideas and found that any additional dimension must be less than about 16 nanometers in size, which is only a few hundredths of the original estimate. Of course, this research is not completely convincing. After all, whether the universe is in a false vacuum and whether the evaporation of black holes can trigger vacuum decay is something to be discussed. Despite this, the research still has some reference value.