We have all heard of "shadow following" and "inseparable", which means that the relationship between two people is as intimate as a shadow. So, does the shadow that follows us like a "tail" also have the weight of an animal's tail?
To answer this question, first understand what a shadow is. Shadow is an optical phenomenon. Light travels in a straight line in a homogeneous medium, and when light encounters an opaque object, it produces a projection, which is what we call a shadow. Since the shadow is a phenomenon, the shadow naturally has no weight.
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However, scientists have discovered an interesting fact that although a shadow has no weight, when the shadow is projected on an object, it will reduce the weight of the object. According to the theory of quantum mechanics, light is capable of carrying a certain amount of momentum and energy. In a vacuum, the magnitude of the momentum of light is proportional to the magnitude of energy. When light is illuminated on an object, the object absorbs or reflects light, a process that changes the momentum of light, which in turn changes the energy of light. Therefore, when an object is illuminated by light, the object is subjected to the force of light. We call this force light pressure. It can be said that while we are soaking up the sun, we are also bearing the pressure of the sun, but the amount of this pressure is negligible.
Therefore, with regard to the reduction of the weight of the object projected by the shadow, we can understand it as follows: when the shadow of the b object is projected on the a object, it means that the light that should be illuminated on the a object is blocked by the b object, and the b object bears a part of the weight from the light for the a object, so the weight of the a object becomes lighter. However, just as it is difficult for us to feel the light pressure under normal conditions, the weight of an object is also minimal.
However, when the light pressure is concentrated in a very small space, this force cannot be ignored. The laser surgical forceps used in surgical procedures apply this principle.
The light pressure we receive on Earth is insignificant, so will the light pressure we receive in the solar system closer to the sun be different? Can we turn that pressure into a driving force for navigating in space?
As early as the 19th century, the writer Verne described the solar sail as a device in his novels. He believes that light will exert force on the object, hit and bounce out of the object, the process of impact will produce energy, when a large number of photons act on the object and produce enough energy, it can push the solar sail forward.
In the 20th century, scientists began to study related matters, and solar sails officially became the target of scientific practice. Through the unremitting efforts of scientists, the solar sail can use the reflected light pressure of the sun on the special film to propel the spacecraft forward without consuming additional chemical fuel and working medium. Small quality, large exhibition ratio, low cost, low power consumption, long voyage... These are the advantages of solar sails. The "Tianfan-1" developed by the Shenyang Institute of Automation of the Chinese Academy of Sciences has verified the advanced technology of a number of solar sails in orbit.
Scientists believe that solar sails are promising to become spacecraft that travel outside the solar system, and have broad prospects for asteroid detection, solar polar exploration and space debris removal. Perhaps in the future, we will see that in the vast universe, a probe traveling by light is moving firmly towards the depth of the boundless.
This article was scientifically checked by Liu Huan, a second-level teacher at Fenglin School in Weihai City, Shandong Province.
Xinhua Network Science Popularization Division
Popular Science China - Scientific principles are one-point
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