It is said that energy and matter can neither be created nor destroyed. Then why light rays from stars?

Question

If energy cannot be destroyed then why the distant stars look less bright and nearer stars look more bright? Does this means that light rays emitted from a source will continue to move on and on in a vacuum ad infinitum?

Answer 1

Because a given amount of light moves out from a star in all directions, like spokes from a wheel. When you're close to the hub of the wheel, the spokes are close together and you can see a lot of them. But when you're far away from the hub, the same number of spokes are still coming out, but they're farther away from each other. So from any given point you can see less light than if you were close to the star. It doesn't mean the energy is destroyed. Eventually when you get far enough away, the light will be so faint you can't see it anymore. I don't know if the photon travels forever or what, that's a good question. I wonder what eventually comes of it.

Answer 2

Because even though the light is traveling in a straight line, the energy is not in the the ray itelf, but in a spherical wavefront.

The best way to see this is by analogy with a wave in a pond made by throwing a stone in it. The total energy of the wave, which is circular, is contant. But the energy is distributed over a larger circle as the ring expands. Total energy: contant. Energy per unit length of the wave: decreaing.

The same thing happens to all waves, including light waves. And with light, the les energy, the less bright a light source will seem.

Furthermore, there are some very bright yet far away stars thatseem brighter that fainter, nearer stars.

Answer 3

You are correct in that the 3rd law of thermodynamics states that energy cannot be created or destroyed, just converted to another type of energy.

There are 2 factors that will cause the phenomena that you have described.

1. Light as well as all other energy decreases in magnitude at the ratio of the inverse square of the distance. For instance if you were 10 feet away from a light source and your photometer reads 100 foot candles, the same light source from 110 feet away would read .01 foot candles.

2. Some of the light will be absorbed by clouds of cosmic dust or gasses and turned into thermal energy.

Answer 4

We only see a very small portion of the energy emitted by a distant star. Imagine a theoretical sphere around a star. If you take a 1° square, and move away from the star, the area of that square will increase as you move away. The energy (assuming it's not being blocked by any planets, accelerated by singularities, etc) will be constant for as far as you go. However, being finite beings, we are not capable of directly observing equal 'squares' from every star that we see. More distant stars will only have very small 'square' areas that are visible to us, while the sun, for instance, has a pretty high area.

I would assume that as long as light is not absorbed by anything, it will continue for eternity, it's intensity (light/area) will just decrease over distance.

Answer 5

Light rays do move ad infinitum. The dimness of distant stars arises from the same one-over-r-saquared rule that applies to radiation emitted from any source: you go twice as far away, and the radiation is distributed ove a sphere with four times as much surface area.

Answer 6

This is because the light spreads out as it gets farther from the star. This occurs by an inverse square law, that is to say, the difference in brightness between a star x light years away and a star yx light years away is equal to y^2. This same principle is what makes a noise sound quieter when it is farther away; the sound is spreading itself more thinly as it travels away from its source. It's the same thing with light; it spreads itself more thinly as it travels away from the star that made it.

If, hypothetically, all stars shone their light directly at Earth rather than out in all directions, then they all would appear the same brightness (and the Earth would soon be destroyed by too much light). But they don't.

Answer 7

The light from a star moves outward equally in all directions. So, at any given distance the light energy is spread out over the surface of a sphere. The further away you move, the larger the sphere and hence the larger the surface area over which the light is spread.

Answer 8

It is true that energy can nrither be created nor destroyed,the fact that stars more distant from us look dimmer is because of their intensity decreases as distance increases (Newton's law of inverse squares).The energy from the stars remains in the universe in one form or the other so that any given point in time the total energy of the system(universe)remains constant.

Answer 9

You are absolutely right. light would go on ad infinitum.
Energy is only altered, but never destroyed.
Light is just one form of energy, no less and no more, and we are designed to be able to notice this energy and we call it "light". That's it.

The fact that more distance (if we compare to stars that are originally the same) causes a seemingly less bright light is due to the density of particles in space: on average one atom per cubic meter.

If there was no matter like one atom per cubic meter and no stellar or even interstellar clouds, then we would see even the stars from all the other galaxies as bright as from our own galaxy. Fortunately we don't, an fortunately our eyes aren't sensitive enough to see the light really arriving (copmpare with pictures from hubble), otherwise, the nbights would be really bright for us and we could hardly sleep ... :-)

Answer 10

the energy is NOT being used up as it travels through a vacuum. Instead, due to dispersal the energy is becoming less CONCENTRATED. essentially, when you increase the radius of the spherical observation zone around an object eminating light/energy in complete spherical shape (ie, a star or the sun), your observation zone corresponds to a smaller percentage of the sphere of total energy. that's why distant stars appear to be more dim than similar but closer stars.