It is distressing how many people have opted out of reality. One way is to declare that the Bible is true verbatim, and that the Earth was created about 6000 years ago. But there is rock-solid evidence that the Earth is much older than this. It has to be older than its oldest rocks, which can be dated by radioactive decay.
Radioactive elements decay by explosion of individual atomic nuclei, which occurs at a perfectly steady rate, measured as half-lives, i.e., how long it takes for half of the atoms to decay. Half-lives are different for each element, and range from days or even seconds to millions of years. We know exactly how fast each radioactive element decays because it is easy to measure. Explosion of a nucleus produces a burst of energy plus a new nucleus of a different element. The energy bursts can be precisely counted with a Geiger counter, and the appearance of the new element can be measured by chemical separation. When the rock is still molten, the newly created atoms are not fixed to the place where they were born, and separate from the other unexploded atoms. But after the rock has solidified, the new atoms and the unexploded ones are stuck together, so we can tell precisely how many half-lives have taken place since solidification. This gives us the exact age of the rock since it solidified, which is the minimum age of the Earth. The oldest known rock analyzed in this way was found to be 4.5 billion years old. Therefore, the Earth must be at least this old, and not 6000 years. This is unambiguous and certain.
The age of the Universe must be greater than this. It was discovered in this way: There are millions of heavenly bodies out there, traveling at sometimes small, sometimes enormous speeds relative to us. These speeds are easily determined by the Doppler effect. We have all noticed that when, say, an ambulance travels toward or away from us, its siren’s pitch is higher when approaching, and lower when going away. This is because, when coming toward us, each sound vibration is emitted closer to us than the one before. Thus we receive more vibrations per second than if the siren were stationary, which raises the apparent pitch to our ears. Similarly, the pitch of a departing siren is apparently lower. We can calculate the ambulance’s speed by measuring the change in pitch. The Doppler effect is detectable only when the ambulance’s speed is not too slow compared with the speed of sound.
If a star travels toward us at a speed not too far below the speed of light, the light waves reaching us are compressed via the Doppler effect just like the sound waves of the ambulance, so that we see more waves per second than the star actually emits. This shifts all colors toward the blue end of the spectrum. If the star is traveling away, all colors are shifted toward the red end. Thousands of stars have been examined for color shifts. The closest ones travel at random speeds toward or away from us, but all the others are traveling only away from us, i.e. their light is red-shifted. If the Universe were constant in size, we would expect random motion among all the heavenly bodies, close or distant. The occurrence of 99% red shifts means that almost all visible objects are traveling away from us in all directions, therefore away from each other as well. If there had been a gigantic explosion long ago this is what we would see.
We can measure not only the velocities of stars, but also how far away they are, by measuring their brightness. It has been found that each star’s velocity away from us is directly proportional to its distance. This means that if we imagine this explosion in the reverse direction, every thing would go back to the same point, i.e., there was only a single explosion which gave rise to the Universe we see. This has been dubbed the “big bang.” We can also calculate how long ago this explosion occurred, which was about 13.7 billion years ago. Although we cannot tell what came before, this date cannot be far wrong.