I put the light into a bag during the day, and opened it at night. Why is it gone? Where did it go?

This is a common sense question that has been discussed in the past, but many people still don’t understand it. In fact, many people know that no matter what container is used, light cannot be contained, but the essence of the question is: why can’t light be contained?

In theory, light can be stored like water.

If the container of water can be made leak-proof, then the water can be preserved forever, just like the wine someone hides in the cellar. After 50 years, the bottle of wine is still there, and it is getting more and more mellow.

However, wine is not entirely water, but a mixture of water and ethanol. Ethanol evaporates and sublimates much more rapidly than water, so the container must be more tightly sealed. But after being stored for a long time, even the most sealed jars will lose some of the alcohol.

Theoretically, if there is a container that does not leak any photons, light can be stored in such a container forever. However, light is not like water or alcohol. It is small, light, and escapes quickly. It is not as easy to find a container that can hold light as it is to hold water.

So, is there a container in our world that can hold light without leaking out a single drop? We will find out if we look at it patiently.

Basic properties of light

Light is the medium through which our eyes can see everything in our daily life. It is also a term in physics. Without light, our eyes would be useless and we would be blind. The essence of light is a photon stream in a specific frequency band. The reason why a light source emits light is that its electrons gain extra energy and release energy in the form of waves.

Light has the dual nature of wave and particle, that is, it is a particle, that is, a light quantum, and in statistical terms, it moves in a wave-like manner, that is, a large number of photons move in a wave-like state. Light has no rest mass, and its rest mass is zero, but it has energy, momentum, and dynamic mass. According to Einstein's mass-energy equation, the dynamic mass of a photon can be calculated.

The mass-energy equation can be used to deduce the photon's kinetic mass: E=mc^2=hv; hence the photon's kinetic mass m=(hv)/c^2. Here m is the upper limit of the photon's mass, h represents Planck's constant (h≈6.626*10^-34J·s or 4.136*10^-15 eV·s), v represents the frequency of any electromagnetic wave, and c is the speed of light. That is to say, the higher the photon frequency, the greater its mass.

Once a photon is born, it will not stop, but will move at a speed of about 300,000 kilometers per second (the speed of light in vacuum); there is no definite conclusion on the volume of a photon, but it can be considered to be the smallest in the known world, larger than the Planck scale (1.6*10^-35m); photons are the medium of the electromagnetic interaction force among the four fundamental forces, which means that the electromagnetic force relies on photons to propagate.

Photons can interact with any matter that participates in electromagnetic forces. Through absorption, reflection, refraction, and diffraction after interacting with various substances, we can discover various objects and the colors of these objects. The only thing that does not interact is dark matter, so people still do not know what dark matter looks like.

Light is actually electromagnetic waves, including radio waves (long wave, medium wave, short wave, microwave), infrared, visible light, ultraviolet, X-ray, gamma ray, etc. They all rely on photons as the medium of propagation, so they are all manifestations of light in different bands and frequencies. The light we are discussing today mainly refers to visible light, which is the light that we can see with our naked eyes, with a wavelength of about 380nm~780nm.

What "jar" can hold light?

Based on the properties of light and photons, we can infer that in order to contain light without "corrupting" the material, the following properties are required: no charge, no absorption of photons, and extremely airtight. To achieve these points, the container must be made of a material that we do not know, and the light reflectivity of this material must reach 100%, and the airtightness must not leak a single photon.

This kind of material does not exist in our world. Because all the substances we know are composed of atoms, and atoms are composed of positively charged protons and negatively charged electrons, photons can interact with these substances, so the phenomenon of "corruption" is inevitable.

Simply put, this container only needs to do two things: first, find or create a fully reflective material; second, use this material to make a container without any gaps. Unfortunately, these two points cannot be achieved.

Friends who use so-called bags to hold light are oversimplifying the problem. What kind of bag do you use? A cowhide bag or a flour bag? Or a reflective plastic bag? When these bags come into contact with light, the photons interact with the charged particles that make up the bag and are converted into energy. Therefore, when the bag is exposed to the sun, it will heat up.

Several high reflectivity materials

Theoretically, there is no material that absorbs or reflects 100% of light or radiation, which means that no object (except black holes) can "embezzle" all light. In space, except for black holes, celestial bodies will reflect light, and the light source is the stars. The reflectivity of our earth is about 35%, the reflectivity of the moon is about 58%, and Venus, Mars, and Jupiter all have strong reflectivity. Therefore, the bright moonlight can be seen from afar, and we can see those planets that originally do not emit light.

Reflection can be divided into diffuse reflection and specular reflection. Diffuse reflection means that the surface of an object is irregular, uneven and rough, so the direction of light reflected on it is scattered, so the light seen at one angle is not as bright as the specular reflection, but the total reflectivity of diffuse reflection and specular reflection of the same material is the same.

The human eye mainly relies on diffuse reflection to distinguish objects. Different diffuse reflections make different objects appear in their original form. If it is completely mirror reflection, it is difficult to distinguish the appearance of the object. The higher the reflectivity, the less it is absorbed, and the longer the light can be retained.

Among common metals, according to different wavelengths of light, silver has the highest reflectivity, which can reach 98-99%, followed by copper, which can reach 60-98%, gold, which can reach 47-98%, and aluminum, which can reach 86-92%. Among them, silver and aluminum have the most balanced reflection of the entire visible light band, so silver or aluminum is generally used to make mirror coatings.

Mirrors commonly used in households are generally made of aluminum as the reflective material. Since the glass processing technology and the purity of aluminum cannot be perfect, and the glass transmittance is about 90%, the reflective surface of everyday mirrors is behind the glass. In this way, 20% of the light is lost when passing through the glass. Even if all of it is reflected back, only 80% will remain.

Therefore, the reflectivity of commonly used mirrors can only be between 75% and 85%; while mirrors used in telescopes and high-tech industries will use silver as the reflective material, and do not transmit through glass, and the processing technology has higher precision, so that the reflective ability can reach more than 95%, and the reflective ability of top scientific research instruments can reach more than 98%.

The world's highest reflectivity mirror

It is reported that a team of scientists from the University of California, Berkeley, has developed a nano-mirror that no longer uses silver as a reflective surface. Instead, it uses a high-tech nano-material with a thickness of only 0.23μm and a reflectivity of up to 99.9%.

A team of scientists from the Massachusetts Institute of Technology also published a paper in the journal Nature, claiming to have created a "perfect mirror" with a reflectivity of nearly 100%. In the past, the reflectivity of mainstream solar photovoltaic power generation mirrors was only around 94%. If this "perfect mirror" is used, the power generation capacity can be greatly increased.

But how close is this mirror to 100% reflectivity? The Berkeley team reported a 99.9% reflectivity, which is probably not even close to that. The question is, even if there is a mirror with such a high reflectivity, how can people make a seamless container and how can they put light in it?

That is to say, even if you make a perfect container, how can the light you want to put in get in? And how can you observe it? Because once you can observe the photons inside, it means that it has run into your eyes, there is a loss, and the container is "leaky".

Assume that a maximum reflectivity gapless container is filled with light

We assume that we use a material with 100% reflectivity to make a perfectly sealed container without gaps (note, not a pocket), and then inject a beam of light into it without loss. No matter how the light inside collides, it cannot be absorbed or escape. In this way, the light in the container can be stored there.

But this is just a beautiful wish. There is no such perfect thing in the world. If there is any master who can conceive of this device, even if it is a thought experiment, I think he will win the Nobel Prize.

In order to satisfy the imagination of some friends, let us now assume that there is a seamless container that can hold a beam of light without loss, and the reflectivity of the inner wall of the container can reach 99.99%. Let us simply calculate how long this beam of light can be contained in the container?

Assuming the inner diameter of this container is 1 meter, we will now hold sunlight for 1 second. It is known that the radiation energy of the sun shining vertically on the earth is σ=1.4×10^3 J/(s·m^2), the energy occupied by visible light is η=45%, and the wavelength of visible light is about 380nm~780nm. According to the energy of a single photon = hc/λ (λ is the wavelength), it can be concluded that the energy of a visible photon is between 2.55*10^-19 and 5.23*10^-19 J.

Let's make a compromise and calculate based on the average energy of 3.89*10^-19J. For every square meter exposed to vertical sunlight for 1 second, there are about 1.6*10^21 visible light photons. Even if the six sides of the container receive the same amount of direct photons, the total number would be about 9.7*10^21.

How long can these 97 trillion photons be preserved?

The speed of light is 300,000 kilometers per second. If we ignore possible refraction and reflect vertically, it will reflect 300 million times per second on the inner wall of 1 meter away. If the loss is 0.01% each time, after 5,000 reflections, there will be less than 1.5 photons left in the container. After 5,100 reflections, theoretically there will be only half a photon left. However, it is impossible for a photon to be half, so in reality there is no photon.

How long does it take for 5,000 reflections? That is 1/60,000 seconds. That is to say, even if a sphere is perfectly sealed and has a reflectivity of 99.99%, and 97 trillion photons are put into it, after 1/60,000 of a second, none of these photons will be left. What is the concept of 1/60,000 of a second? The human eye cannot distinguish it.

What's more, if you use a bag that is leaky on all sides to hold light during the day, and then after several hours, you open the bag and still want to see the light rippling inside, this situation is only possible in a dream.

However, today's solar cells store solar energy during the day, convert it into electrical energy, and emit light at night. This "light-catching" method is much easier than using containers to hold light. Thank you for reading, and welcome to discuss.

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