Light ~ young and old

May 4, 2003 on 11:57 pm | In Ad Rem | 1 Comment

In past articles I’ve discussed the progression of fusion as a star gets larger, and directed you to a wonderful site which visually explains hydrogen to helium fusion. To go into more detail we’ll discuss the speed of light, and the distances between objects on galactic scale. This will open up for future discussion on distances between galaxies and red shift.

At its speed, 100% of light’s energy is being put into the first, second and third dimensions. The fourth dimension, time, has no remaining energy, and so the actual photon of light does not pass through time, only space. To the outside observer light travels at 299,792,458 meters (186,000 miles) per second.

The measurement of a light-year is for distance. To be even more specific it is the distance light travels in a year. That is 229,792,458 meters by 31,536,000 seconds, or over 7 quadrillion meters. It takes light approximately 8 seconds to reach us once it leaves the sun. The distance from the sun to the earth is 149,600,000 kilometers average, 1 AU, or 8 light-seconds. The distance from the sun to Pluto is 5,913,520,000 kilometers average, 39.5 AU, or 5 light-minutes. Our nearest star is 4.3 light-years away. Our galaxy is approximately 100,000 light-years in diameter. Our nearest large galaxy, Andromeda (M31), is 2.9 million light-years away. I’ll talk about its double nucleus at another time.

Light has to travel a long way in this Universe. Sometimes it even loses the battle. At the extreme distances between galaxies the expansion of the universe is evident. As the actual fabric of space is accelerating outwards it is actually possible for the rate to exceed light’s capability to overcome it. There are many galaxies receding at well over the speed of light. Once a galaxy reaches this point, in relation to us, we can no longer receive updates on what is happening in said galaxy. It will appear frozen in time before it fades from view.

To come back to our solar system, and specifically the structure of the sun, I’ll discuss the rate at which light can escape the sun. In matter on earth you have lots of space involved. There is a great distance between atoms in your body. The great pressures involved in the core of a star compress the hydrogen to where the atoms bump into each other rapidly and fuse into helium. At this density exiting photons easily hit hydrogen nuclei, which absorb the photon, and send it back out in a completely random direction. This “random walk” can easily make the progression out of the star take 40,000 years. While the light you see may be that old, it still takes only 8 minutes to reach us, so we still see the sun as it was 8 minutes ago. With all this going on the photon hasn’t aged at all.