Light will bend by gravity, but the source of gravity must be very huge. The gravity of the earth is too small, which is basically negligible for light about 300,000 kilometers per second.

Category：hotcomm

2025-05-18

This article is based on answering similar questions from netizens, see screenshot:

Simple answer: Light will emit in an infinitely close to a straight line.

Light will be pulled and bent by gravity, but the source of gravity must be very huge. The gravity of the earth is too small, which is basically negligible for light of about 300,000 kilometers per second.

This is because the gravity of the earth is too small for light.

million has gravity that works on all matter with electromagnetic force. Light is the category of electromagnetic force, and photons are the medium of electromagnetic radiation , and of course it is also affected by gravity. Moreover, although photons do not have static mass, they have dynamic mass. Gravity is a phenomenon caused by the distortion of mass on space and time, so it is natural to use light when it is gravity.

Einstein General theory of relativity believes that any mass object will distort the surrounding space-time, small mass, small distortion, and large mass, due to the movement of objects, the curvature of space-time formed by is diverse, but overall it is like a vortex or trap formed around itself, and any object will be affected by passing through this vortex or trap.

The larger the celestial body, the greater the curvature, the deeper the vortex or trap, and the closer small celestial body tends to fall into this trap or vortex. If the speed is not fast, it will fall into the abyss, which is manifested as being pulled by gravity and eventually falling onto the large celestial body.

The speed mentioned above refers to the faster the object, the greater the probability of escaping the gravitational vortex or trap. The speed at which a celestial gravitational trap is escaping speed. The formula for calculating the escape speed of is expressed as: v=√ (2GM/R). Here v is the escape speed. G is the gravitational constant of , M is the mass of a celestial body, and R is the distance between the escape object and the center of mass of the celestial body.

Earth's mass is about 6*10^24kg and a radius is about 6371km. According to the formula, we can calculate that the escape speed of the earth's surface is about 11.2km/s. In other words, on the surface of the earth, as long as the speed of 11.2 kilometers per second can be achieved, it can escape the gravity of the earth. The speed of light is 300,000 kilometers per second, about 27,000 times that of 11.2 kilometers. The gravity of the earth is almost negligible to light.

The mass of the sun is 330,000 times that of the earth, and the surface escape speed is 617km/s, which has little impact on the light of 300,000 kilometers per second. Otherwise, wouldn’t the sun’s light be circling around the sun? How could it come to the earth? But the gravity of the sun is much greater than that of the earth after all, so many scientists observed the starlight near the sun during the total eclipse of and found that the deflection was about 1.66", which basically coincides with Einstein's general theory of relativity.

Therefore, the light emitted by the flashlight into the sky will be basically close to a completely straight line. In quantum mechanics , visible light is composed of light quantum , and light quantum has wave-particle duality wave-particle duality , and light quantum has wave-particle duality l6. The lifespan of a photon is not determined, but most people believe that the lifespan is infinitely long, so the light emitted by this beam of flashlight will theoretically float in the universe forever.

Is this true? No. In fact, after this beam of light is emitted, it completely dissipates in just a few seconds and disappears. There are roughly three reasons for this beam of light dissipation: 1. The photon collided with other particles, and the interaction changed the trajectory; 2. The scattering of the flashlight, the photon is Dilution dispersed; 3. With the distance away and the cosmic expansion effect, the light wave is gradually lengthened and becomes an invisible electromagnetic wave .

Let’s discuss these three reasons below:

photons will scatter, diffraction, absorption and conversion when encountering other particles

Theoretically, after the light is emitted, even if you turn off the flashlight, this beam of light will be like a shell that is shot. If there is no blockage, it will always be Fly down. Because the shell is very low in speed, it will be pulled by gravity on the earth and blocked by air resistance. Therefore, its flight trajectory is a parabola and it will fall down if it cannot fly far.

But the photon speed is 300,000 kilometers per second, and the earth's gravity is almost negligible, so it will fly down until it encounters obstacles. In fact, after the light is emitted, there are indeed many obstacles along the way. In the atmosphere, it is mainly blocked by atmospheric molecules and dust.

When a photon encounters various particles of matter, reflection, diffraction, scattering or absorption will occur, and a beam of light will continue to attenuate.

The light emitted by the flashlight into the sky must first pass through a dense atmosphere. The surface atmosphere density is 1.293kg/m^3. Each cubic centimeter contains about 2.6875*10^19 atmospheric molecules, which is about 1.7 billion. Photons will certainly be absorbed and eliminated quickly when passing through such a dense atmosphere.

When it encounters reflection, refraction, or diffraction, the light changes direction and naturally does not follow the original route, so the light weakens; when the photon hits the atmospheric molecule or any atom electron, the energy will be absorbed by the electrons, and the electrons get additional energy and will be in an excited state. jumps to to a higher energy level. After that, there is no more energy replenishment, and it will jump back to its original energy level and release a photon at the same time.

But this photon is no longer the original photon, and the direction of emission is no longer the past line, so we can think that this past photon has dissipated.

Even if you reach space, it is not an absolute vacuum. There are still rare particles, and photons will interact with these particles and transform. As a result, the light will eventually disappear.

photons will also be diluted due to the diffusion of the torch spot

Although the torch has a light-concentrating device, the light-concentrating ability is weak, and the emitted light continues to spread, and is proportional to the distance. Different flashlights have different luminous energy and different light-concentration capabilities. Let’s take a flashlight with a luminous power of 10 watts and a luminous angle of 10° as an example to calculate it.

A 10 watt power light bulb produces 10J/s (joules/second). photon energy E=hc/λ, which is equal to Planck's constant multiplied by the speed of light divided by the wavelength.

visible light is a very narrow band in electromagnetic spectrum , with a wavelength between about 380nm (nanometers) and 760nm. We take an average value of 570nm. According to the photon energy formula, the photon energy with a wavelength of 570nm is obtained as approximately 3.5*10^-19J. In this way, the total amount of photons of 10 J/second of energy emitted by this flashlight is about 2.86*10^19, which is 2.86 billion photons.

The light emitted by the flashlight continues to spread at an angle of 10 degrees, the spot will continue to expand and the photons will be diluted. The radius of the spot when the 30-meter ejects is about 2.5 meters, and the radius of the spot is 250 meters when the 3 km ejects; the radius of the spot is 25 kilometers when the 300-meter ejects; the radius of the spot is 25 kilometers when the 3000-meter ejects.

At this time, even if all the photons have not attenuated, how many photons are there per square meter? We calculate it according to the circle area formula , and we learned that when we were 3,000 kilometers away from the flashlight, the spot area was about 196349540849 square meters, and there were about 146 million photons per square meter. If the area of the light spot captured by the human eye is 1 cm, then 145 photons will enter the retina every second. Although it is already very weak, it can still be seen.

But on earth, it is impossible for flashlight to go so far. The dense air will attenuate this beam of light long ago. Even if there are individual photons left, it is difficult for human eyes to perceive it.

But even in space, this beam of light cannot travel for 1 second, because the speed of light is 300,000 kilometers in 1 second. At 300,000 kilometers, the diffusion radius of this beam of light reaches 25,000 kilometers, and the spot area is 1963495408493621 square meters, the number of photons per square meter is only 14,566, and the number of photons per square centimeter is less than 0.015 photons.

In fact, when the flashlight travels for 0.1 seconds and travels for 30,000 kilometers, there are less than 1.5 photons that can enter the human eye per second. For the human eye, it is generally necessary to have 6 photons to sense photos. A particularly good vision also requires 3 photons, and 1.5 photons can no longer be seen.

The nature of ordinary light sources is to diverge in all directions. People can install a light concentrator on the light source so that the light can be transmitted in one direction. The flashlight light source is generally an ordinary light source, so it cannot spread further. Lasers are light sources that are naturally propagated in one direction, with very little divergence, about 0.001 arc, so they can shoot further.

During the last century, when landing on the moon, astronauts placed several laser reflection devices on the moon. Scientists launched lasers on the earth to this reflection device, and then received the reflected lasers. According to the time spent on emission and return, they could accurately measure the surface distance between the earth and the moon.

Of course, both the transmitting and receiving devices must use telescopes, which cannot be accomplished by relying on the human eye. Theoretically, the larger the main mirror area of the telescope, the more photons it will be focused, and the further you can see. I won't go into details here.

The speed of light is far away from us and the expansion of the universe causes the light wave to grow into invisible light

We know that the universe is expanding, and the farther the distance, the faster it expands. When the speed of light is far away from us and the expansion of the universe, the wavelength will have Doppler effect . The so-called Doppler effect of light means that the light source moves towards us, and the frequency and wavelength will be compressed, and when it runs contrary to us, the frequency and stretch wavelength will be reduced.

visible light is composite light, that is, light composed of multiple colors, and can be dispersed through prism . The wavelength from long to short is roughly divided into red, orange, yellow, green, blue, blue and purple colors and other colors. The wavelength length is to move towards the red end, and the wavelength shortens and move towards the blue end.

In this way, the light away from us will form redshift . With the expansion of the universe, this redshift will become larger and larger. Finally, it will move out of the 760nm wavelength range that the human eye can see, becoming infrared or radio waves. infrared and electromagnetic waves at wavelengths above the human eye cannot see.

This is also a telescope that people make in addition to optical structures, but also in different electromagnetic wave bands such as radio, infrared, ultraviolet and X-ray , gamma-ray . In this way, observing distant dark and weak celestial bodies can make up for the lack of light sensing in human eyes.

Therefore, the light emitted by the flashlight will disappear quickly in the universe. In theory, although some photons may exist permanently, it is difficult for people to capture it. In a very distant place, even if a photon is captured, it is difficult to tell where it came from and what object emitted it.

But if a huge star or galaxy emits too much photons and contains very strong energy X-rays and γ-rays , it will be seen by humans even if it is more than 10 billion light years away from us. However, it is not just based on the naked eye, but relying on various large and precise telescopes. It also requires gravitational lenses in the universe to be observed.

The photons in the cosmic microwave background were emitted after 's Big Bang .38 million years. It has been 13.8 billion years. Although it is very weak, it is still captured by scientists, which shows that the lifespan of the photon is extremely long.

That’s all, welcome to discuss and thank you for reading.

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