In the universe we live in, everything needs to operate within the established physical framework, and these frameworks are what we call science, and they are also the basic laws of the universe.
One of the laws is deeply rooted in our hearts, but we still sometimes question it, and we are also willing to challenge this so-called basic law.
It was proposed by Einstein more than a hundred years ago, that is: there is an ultimate speed limit in the universe, that is, the speed of light propagating in a vacuum (299,792,458 m/s).
In physics, we believe that the speed of any massless particle in a vacuum must be the speed of light. At present, we know that there are three light-speed particles, namely photons, gluons and gravity son.
Any particle with mass must run at a speed lower than the speed of light, even the lightest known neutrino has a mass of only 1.1eV, which is lighter than the second lightest electron (5.11×10^ 5eV) is nearly 1/500,000, and its speed cannot reach the speed of light in a vacuum.
Then the question is, why can the space-time structure of the universe expand at a speed exceeding the speed of light? Even causing some huge galaxies to move away from us faster than the speed of light?
This seems to violate the basic law that the speed of light cannot be surpassed? Let's talk about this problem below.
Scale effect
In 1905, Einstein proposed a revolutionary special theory of relativity, which changed our previous view of inherent space-time, such as: scale effect and time dilation effect.
The magic degree of these two effects is no less than that of magic. The cornerstone of the theory of relativity.
1. No matter who you are, no matter what state you are in, the speed of light is constant for you. 2. No matter where you are, no matter how you move, what you seeThe laws of physics are all the same. 3. The propagation of light does not require any medium, only time and space are sufficient.
These three The most troublesome thing about the basic proposition is the principle of constant speed of light, because this principle violates our daily life experience, and speed superposition can be seen everywhere in our life.
For example: two cars traveling in the same direction, the speed between them satisfies the principle of superposition; you ride a bicycle and throw a ball forward, in the eyes of people on the ground , the speed of the ball must be added to the speed of the bicycle.
But the speed of light is not like this, it is the same constant speed relative to everything.
Basic schematic diagram of Michelson interferometer< /p>
This matter is completely incomprehensible to us, but as early as 1881, scientists Albert Michelson and Edward Morley confirmed that the speed of light is constant through experiments.
The basic principle of the experiment is like this, make an interferometer, emit a beam of light at the light source, when the light passes through the beam splitter, it will divide the light into two beams perpendicular to each other Coherent light, the two beams of light have the same wavelength.
Then the two beams of light travel the same distance and are reflected back again, forming a new interference pattern. There is a key point here. If the speed of light on one component changes, it will lag behind the light on the other component.
Interference fringes change when two beams of light meet. But the result of the experiment is that even if the earth orbits the sun at a speed of about 30 km/s in the universe, the interference fringes do not change.
This shows that no matter which component of light, the speed will not change because of the rapid movement of the earth. The speed of light will still be maintained. Does not satisfy the principle of velocity superposition that we think.
So the constant speed of light is not Einstein's imagination, but the result of experiments. Although the speed of light is not satisfied with simple speed superposition, but the movement relative to the speed of light will let us see the red shift and blue shift of lightPhenomenon.
In 1916, Einstein's greatest success was to incorporate gravity into the theory of relativity, which reshaped the space-time structure of the universe for us. Before that, we used Newton's theory of gravity to explain everything.
However, some problems have arisen, such as Newton’s gravity is not accurate enough under large-mass celestial bodies or energy, and the problem of explaining the bending of light is the same, and the emergence of general relativity solves the above problems. Newtonian gravity has also become an approximation of relativistic gravity.
The general theory of relativity interprets gravity as the curvature of mass/energy on the space-time of the universe, and the curved space-time affects the movement of everything in the universe. The anomalous precession of Mercury's orbit and Eddington's observation of a solar eclipse in 1919 proved the correctness of general relativity.
The new gravitational theory and space-time concepts let us realize that the universe we live in cannot be exquisite. It either contracts or expands under the action of gravity. In short, Einstein’s universe is a dynamic universe .
In fact, as early as 1924, astronomers observed that there is a close relationship between the distance between galaxies in the universe and us and the redshift of starlight, although galaxies are relatively In motion, there is no absolutely static object, but this relative speed is only a few thousand kilometers per second.
And we observe that the redshift of starlight is far greater than the redshift effect brought about by the relative motion of galaxies. And we found that the farther these galaxies are from us, the greater the redshift.
According to Einstein's theory, scientists immediately realized that we are in an accelerating expansion of the universe, the retrograde speed of galaxies increases with the distance because the space-time of the entire universe is accelerating swell.
It's like steaming bread. The raisins in the dough are like galaxies in the universe. When the dough expands, the raisins inside will move away from each other, and the farthest raisin seems to be far away. faster.
But it's not because the raisins or the galaxies are moving, they think they have been in a static state, and it's the whole bread or space-time that gets bigger.
For example, you live in Shanghai now, and another person lives in Beijing. You haven’t left your city for a year. You both think that you are static. If the earth is expanding, Then you will also think that the other party is away from you.
The same is true for galaxies. The first galaxies in the universe appeared hundreds of millions of years after the birth of the universe. When they first appeared, the light they emitted began to spread outward.
It took 13 billion years or more for the light from galaxies to reach Earth. When this light was first emitted by the galaxy, it may have been in the ultraviolet range, but because of the expansion of space, the light has been stretched into the infrared range.
We receive these light rays, and after measuring the redshift value, we can conclude that the galaxy is accelerating away from us, and the speed has exceeded the speed of light.
But in fact, these galaxies are not moving at all, let alone moving at the speed of light relative to space. Their speed relative to space is estimated to be only 2% of the speed of light, or even lower.
We say that the speed of light is the limit speed in the universe, which means that the speed of everything relative to space cannot exceed the speed of light. And this law does not limit the space itself, there is no upper limit to the expansion speed of space, and it will not violate the theory of causality, nor will it transmit any information.
The most important thing is that the expansion of space is not the speed of objects we usually think of, but the rate of change of speed per unit distance, the unit is km/s/mpc, more like a frequency .
The accelerated expansion of space has caused the universe to be only 13.8 billion years old, but the galaxies we can see now, or the scope of the universe, has reached a distance of 461 light-years.
Moreover, 18 billion light-years away, the speed of galaxies has exceeded the speed of light. Even if we chase these galaxies at the speed of light, we will never be able to reach them.
