![]() The gravitational force inside an atom is entirely negligible relative to the gravitational force – it's about 40 orders of magnitude smaller – and fixing the altitude prevents gravitational redshift caused by the Earth’s gravitational pull. second and the speed of light.Ītomic clocks rely on the constancy of atomic transition frequencies. The atomic transition from the 6S orbital, Delta_f1, is the transition that defines the meter. You would find that the time-intervals are always the same. Say, you bounce a laser-beam back between the ends of the block, at fixed altitude, and use atomic clocks to measure the time that passes between two bounces. That’s pretty idiotic – and yet nobody could stop you from doing this.īut now, consider that you make a measurement. You could simply insist on using units of length in which tomorrow a block down is two miles, and next week it’s ten miles, and so on. Just as you could insist on defining space so that the universe doesn’t expand, by willpower you could also define space so that a city, like Brooklyn, does expand. We really shouldn’t be talking about space to begin with. This brings us to the relevance of clue #2. If we so demanded, we could define space so that this city shrank, grew, or remained stationary, but it isn't particularly meaningful. (Though these solutions are usually only dealt with by computer simulations due to their mathematical complexity.)ĭenver, Colorado, USA, exhibiting the street grid typical of large cities in the southwestern USA. ![]() #Space in time that connot be explained fullThe full solution, then, is both the cosmic and the local solutions stitched together: expanding space between non-expanding galaxies. ![]() It’s not that galaxies expand unnoticeably, they don’t expand at all. But the solutions that describe galaxies are different – and just don’t expand. The solution of general relativity that describes the expanding universe solves Einstein's equations on average it is good only on very large distances. This is a key point, and missing it is the origin of much confusion about the expansion of the universe. The cosmological Friedmann-Robertson-Walker model, therefore, just does not describe galaxies. The Milky Way is disk-shaped with most of the (visible) mass in the center bulge, and this matter isn’t distributed homogeneously at all. Colvin, via Wikimedia Commonsīut clearly, that’s not the case on shorter distances, like inside our galaxy. scales, the Universe is uniform, but as you look to galaxy or cluster scales, overdense and underdense regions dominate, and the Universe appears very non-uniform. The various galaxies of the Virgo Supercluster, grouped and clustered together. ![]()
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