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Optical Length Theory and Why Heavenly Bodies Appear to Move So Slowly

February 18, 2015

When you look at space-telescope pictures of a single region taken some decades apart, you see that not much has changes. As a matter of fact, if you keep stargazing long enough, you will notice that the farther objects are away from us, the less you can notice any actual motion among them. For example, even though you know that two distant galaxies are hurtling away from each other at fantastic speeds, or that for the Andromeda galaxy is actually on a collision course with ours, hurtling at us with a speed of 402,000 kilometers per hour, you can not see it getting any closer, and even if you lived for a few centuries. Why? Because of the universal speed limit, and and because Andromeda is so big.

Consider an ant. It may be a few millimeters long, but it can go many millimeters per second. Its speed to size ratio is quite great. A car, no matter from what distance, can go many more meters per second than there are meters in its length, so it’s speed to size ratio is even greater. A galaxy, however, is many billions of miles in length, but because it can only obtain a top speed of light speed, or in the case of Andromeda, just a fraction of that, its speed to size ration is incredibly low. Even if it were moving at a speed close to that of light, it’s speed to size ratio would still be infinitesimally small compared to that of an ant or an automobile. That’s why distant heavenly objects do not seem to move much: if it’s large enough to be seen at a such a distance, it can’t be moving fast relative to its size.


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