How to detect additional dimensions

Looking for an additional dimension sounds like a science fiction, fortunately, the gravitational wave was found. The key to this breakthrough is the gravitational wave incident observed in August 2017 GW170817: A pair of neutron stars circled together, stirring the structure of time and space in their death spiral, and the gravitational wave observation decking the ripples.

Unlike the invisible black holes, the merging neutron star will explode spectacularly, and then it is detected as a gamma ray violence. During the GW170817 time, Gama radiation flash arrived 1.7 seconds after the gravitational wave, and then the light in the entire electromagnetic spectrum, eventually discovered the distant galaxy that exploded. This optical recognition gives measuring surveys that is completely independent of gravitational waves, which allows us to make some very important conclusions on how gravity spreads in space, which enables us to measure how much dimensions actually have.

We believe that space is three -dimensional, and one -dimensional time is added to us four -dimensional time and space. In addition to the usual three -dimensional space, adding additional space dimensions can actually explain many problems: the difference between gravity and other forces can also be explained, and the essence of dark energy can be explained. But before we understand all over -dimensions, let's think about the behavior of air gravitational, light and material in 3+1 dimension.

Brightness attenuation

A light pulse spreading from a distant light source, we can imagine that light is evenly distributed on the expansion ball shell. If we see this pulse, this means that our eyes or telescope intercept some of them. The brightness of the pulse depends on how much light we intercepted, so the light becomes more scattered with the expansion of the ball shell. The surface area of ​​the brightness is inversely proportional to the surface area of ​​the shell, and the surface area of ​​the shell is proportional to the square of its radius, that is, the square inverse proportions of the square of the distance from the source of the source, which is the famous square inverse law law.

However, if we change to living in a 2 -dimensional space, then the same pulse will spread on an enlarged circle rather than the sphere. It's no longer the law of the square.

We can see that the brightness of the pulse depends on the number of dimensions, and it is usually inversely proportional to the power of the dimension of the distance. Therefore, the brightness in the 4th dimension should decrease faster than in the 3D space. This relationship is also applicable to gravity. In our universe, gravity declines according to the law of the square compliance, which is reflected in Newtonian gravitational formula. We did see a slight deviation in a very strong gravitational field (such as near the sun), but even there, Einstein's general theory of relativity also perfectly described the gravity with three space dimensions.

But there are some things that look strange for gravity. For example, its poor strength, gravity is much weaker than the other three basic forces. The only reason we see that such a large gravity is that it has a large scope. Many people want to find a theory of all things and merge natural forces into the same super force, which means that gravity must also be as strong as other forces in small scale, and then weaker in low -energy and large -scale areas. This intensity may not match because the gravity and other forces are indeed different. An interesting method is to add an extra spatial dimension. As mentioned above, the more dimensions you have, and the intensity drops faster.

Four-dimensional space

Let's understand these theoretical objects called membranes. We can regard them as a geometric structure. It is the dimension that may exist in the quantum field and its corresponding particles. They are usually used for strings, and they usually have 11 dimensions. But in the theory, except for the three space dimensions of the membrane, all other spatial dimensions are unavailable. They are limited, and they are compactly around them, allowing us to stuff them into three space dimensions.

But we can also flip the idea: a three -dimensional membrane is embedded in time and space with four space dimensions, and the extra dimension of the space is extended rather than compact. Most things in the universe, including all basic forces other than gravity, will be limited to three -dimensional membranes. If the theory is adjusted just right, we will get the law of the three -dimensional membrane material and force, but in other space dimensions, if the gravity is in four dimensions rather than three dimensions, the performance of gravity will be very different. It should become weaker.

This can also be used to explain the mysterious phenomenon dark energy. The expansion of the universe seems to be accelerating, which is usually considered to be the role of vacuum energy. But in the universe we assume four space dimensions, there is another method to get this acceleration. Gravity is weak in the scale of the solar system and the galaxy, but it may become weaker at a larger scale. It complies with the law of cubic inverse ratio at a larger scale. In fact, the three -dimensional membrane of the three -dimensional structure that we can observe the universe can actually be expanded to the additional fourth space dimension that looks like accelerated.

Gravitational wave test

So how will we test such a crazy idea? Here we still return to the gravitational wave. If the gravitational field can be extended to the extra space dimension of this assumption, the gravitational wave should lose energy to that extra dimension when passing through space. In the conventional three -dimensional space, gravity seems to follow the law of the square inverse ratio, and the decline in the intensity of gravitational waves is proportional to the square of the distance. However, if there are four or more dimensions in space, the speed of gravitational waves decreases faster than expected in three dimensions. So this principle gives us a simple test, just observe a gravitational wave and calculate how much its strength drops within its marching distance. If the intensity drops too much, there is evidence that the extra space dimension may exist. Now we only need gravitational wave detectors and a way to disseminate distance from independent measurement waves. Fortunately, we have the GW170817 incident mentioned at the beginning.

The electromagnetic signals from these merging neutron stars enable us to be able to measure its distance independently of the gravitational wave signal. This is impossible to achieve the combination of black holes. There is another important factor. In order to determine the strength of the gravitational waves, we need to know how much its initial intensity is. A super convenient feature of gravitational waves is that this can be figured out by viewing the other characteristics of the merger event, that is, the quality and frequency of the merged object and the frequency of waves and our independent distance measurement.

So, what is the final conclusion? How many additional space dimensions have we found? The answer is zero additional dimension. The gravitational wave lost appropriate strength in 3+1 dimension, and no observed gravity leaked to the extra spatial dimension. By the way, the comparison of electromagnetic waves and gravitational waves also allows us to verify that gravity does spread at the speed of light, which eliminates or restricts various alternative theories of general relativity.

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Source: Vientiane experience

Edit: Muyu

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