That recent meteor collision got me to thinking. Why is everything in the Universe moving so fast?
As I understand it, 10 miles a second is not uncommon. Maybe some things move even faster than that.
The planets. Meteors, asteroids. Our Sun even. As I understand it, everything is traveling really really fast.
What on earth set everything into motion to begin with? I realize momentum is what keeps it moving fast. Things in motion tend to stay in motion. But something had to get it started, either directly or indirectly.
It only seems fast relative to out puny human experience. We can barely grasp the speed of sound never mind that of light and most of the astronomical phenomena we see are nearer to the former than the latter.
Because gravity. Except for some movements, which are because fusion.
(Gravity moves stuff around–the bigger the gravity, the faster the movement. When orbiting objects collide and pieces fly apart, those bits of flying debris are still indirectly flying about because gravity. Slow movements can be caused by the push of stellar winds and fast movement can be caused by supernovas, both of which are because fusion.)
If things were wizzing about at 10s of miles an hour, in at atmosphere like earth’s, they’d slow down fairly rapidly as well as catastrophically.
Speeds are what they are. If you are a certain distance from an object, in order to not run into that object due to its gravity, you have to move fast enough not to hit it. That is usually a pretty rapid speed compared to our usual experiences.
Everything in the solar system used to be a big cloud of gas a light year or so across.
When the gas and dust clumped up, that gravitational energy translated into motion.
Whoa … the Earth itself is moving a bit faster than 18 miles/second around the sun … so even an object stationary with respect to the sun is colliding with the Earth at 64,000 miles/hour …
Anything that isn’t moving fast falls down into the nearest massive object. If satellites and space stations weren’t moving fast, they would fall down to Earth. If the Earth weren’t moving around the Sun, it would fall into the Sun. If the galaxy weren’t rotating, it would collapse into one black hole.
Some particular motions do. Groups of galaxies are moving farther apart because of that expansion. The Hubble constant measures that in terms of kilometers per second per megaparsec (3.3 million light years). The latest measurements put he constant at somewhere around 70. So a galaxy at 330 million light years is apparently moving away at 7000 kilometers per second and one at 3.3 billion light years is rushing at 70000 km/s or nearly a quarter of the speed of light.
But over smaller distances, gravity predominates. Many of the small galaxies in our local group are moving toward our Milky Way galaxy, because it’s so much bigger and its gravitational attraction overcomes any other influence. And everything inside those galaxies are moved by gravity, as said above.
To clarify, expansion is space getting bigger, not traveling. This is how objects outside of our Hubble sphere are gaining distance faster than the speed of light and why there is red-shift in light over time. At that point the universe is expanding faster than the speed of light from our viewpoint, but the objects are not traveling faster than the speed of light.
While it is useful to think of them moving away, it makes several phenomenon easier to understand once one can adjust to that idea. The increased causal distance is increasing, but the objects themselves may not actually be moving in relation to us at all.
This is why there is an almost universal red-shift with other bodies compared to us, or any other observer. The very fabric that is space grew longer as light traveled through space, decreasing the perceived wavelength.
The question is a bit too general to give a precise answer, but within a gravitationally bound system the speed of objects usually conforms to what one would expect to find by applying the virial theorem and assuming a thermal distribution of kinetic energies. In other words the speed objects travel in space can simply be explained by the statistical effect of gravity on them.
Expansion actually causes particles to decrease their kinetic energy, but this happens over a long time and it’s not clear how this effect applies in a gravitationally-bound system.
Everything on earth is moving very slowly because if it weren’t, it would burn up in the atmosphere. Novelty Bobble nailed it in the first response: the universe has a whole histogram of speeds, and our experience is only with the very very slow end of that histogram, so anything off-earth is going to seem like it is moving very fast - but only compared to our prior experience.
They are moving actually very slowly, so slow that they are effectively ‘trapped’ in a very small section of the assumed universe, never can leave it, never reach lets say a object a certain distance away, no matter how long it travels towards it due to the expanding of the universe, even if the object moving is light, which is also very slow at this scale.
