But they have another characteristic which seems to melt our brains! They vary in brightness (in either or both of visible and radio frequencies). Why is this a problem?
An object which has immense brightness, say a million times the apparent radiation brightness of our Sun, seems to us to necessarily have to be rather large in physical size. Our Sun is nearly a million miles in diameter, and we assume that any object which radiates a million times as much radiation MUST BE a hundred million miles in diameter or larger.
Why is THIS a problem? Because light and all other known radiation can only travel at the speed of light, around 186,000 miles per second. Light and heat from the Sun's surface takes around 8 minutes to travel the 93 million miles to get to the Earth.
So say you have a giant ball that is 100 million miles in diameter. And the surface of it somehow changes in temperature rapidly (by some unknown process) every few seconds. The problem arises due to geometry! If the location on the surface which happens to be NEAREST the Earth and Sun gets hottest and brightest, then there is a CIRCLE of surface which is farther away from us. The object is large enough so that difference can easily be a few light seconds distance. So we might be getting MAXIMUM radiation from the surface area nearest us, but at that time we would be receiving MINIMUM radiation from that circle of surface area which was that few light-seconds farther away from us. And a MAXIMUM from another larger circle that was twice that differential distance away from us. And a MINIMUM from another even larger circle of surface that was farther away, and so on.
The objects are so far away that we cannot see any surface detail. All we are able to see is the AVERAGE RADIATION of the entire surface which is facing us. A very large object should then NOT create any evidence for us (of any change in the AVERAGE radiation) regarding very rapid variations in brightness (or average radiation output).
So there is clearly some major item which we do not understand correctly!
IF pulsars are as far away as we believe they are due to the red shifts that we measure, then they MUST be immensely bright for us to even see them at all. And so if they are that bright, we believe that they MUST BE extremely large objects (as we do not know of any process which could produce such immense amounts of radiation from a smaller radiating surface area). And then we should not be able to see any variations in brightness faster than over a period of minutes or whatever light-size the object actually is.
Self-Sufficiency - Many Suggestions|
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What is the solution to this poser? No one yet knows!
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C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago