viscence t1_jd0ehya wrote

Every experiment you can do to find out will tell you the same thing: "right here". If you measure how fast a galaxy is moving, you find that they all tend to move away from "here," and moreover they tend to move away from "here" faster the further they're away.

So did the big bang happen here and we're just spectacularly privileged to live in exactly that place? No. It doesn't matter where you perform the experiment, the answer is always "here". The explanation for this is that space itself is expanding. We're not seeing debris being thrown away from a central location where an explosion happened into some pre-existing space, what we're seeing is the metric expansion of space itself. All the distances between things are just increasing, not because the things are actually moving but rather because the space between them is literally growing. If you extrapolate time backwards far enough, all the distances become zero, and we call that singularity where our understanding of physics breaks down "big bang".

If you have two objects that are too far apart to affect each other, but that randomly happen to be stationary with respect to each other today, then tomorrow they will be further apart. There is no centre to this. The universe isn't expanding away from one spot, it's expanding everywhere like an infinite sponge soaking up water. And really, with the universe being infinite as far as we know, there can't really be another answer. It's not like the centre could have been "three quarters of an infinity" away from here.


viscence t1_jcrg718 wrote

Imagine you have two boxes. In each of them you have a perfect, frictionless rubber ball, labelled A and B. You've shaken the box with A inside a lot, and the ball is bouncing around inside it... high energy! The other, B, is just sitting at the bottom of its box, low energy.

Now you put the boxes together and remove the wall between them. What happens? Soon the bouncing ball hits the stationary one. It's a glancing blow, but B now has a tiny bit of energy, and A has a little less. The total energy is the same. Soon, it happens again! This time a lot of energy transfers. Now B has a little more than half the energy, and A a little less than half!

As you watch, this keeps happening. The balls keep trading energy between them. Sometimes A gets a bit more of the energy, sometimes B, most of the time it's about even. It IS possible for all the energy to go back to A... but the balls have to hit JUST RIGHT for that, and there are far more ways they can hit where that doesn't happen.

Now repeat the experiment with 1000 rubber balls in each box. Again the ones in box A start with all the energy. The same thing happens, when the wall goes down the A balls slam into the B balls and everything just reaches an equilibrium quickly. Sure, sometimes one rubber ball gets a huge kick, maybe because two others slam into it at once, but on average there isn't really a difference between the balls labelled A and the ones labelled B anymore. It's even more unlikely to spontaneously arrange itself into a state where the A balls have all of the energy again -- ALL 2000 balls would have to collide perfectly at once for that to happen. But the total energy remains the same.

That's "what decides", on the microscale -- chance. On the macroscale, the resultant statistics. Things don't really tend to low energy, they tend to equilibrate. If you throw a hot rock into a pot of cold water, the water gets hotter and the rock gets colder, but the water doesn't get hotter than the rock (on average) and the rock doesn't get colder than the water (on average)... they come to be the same temperature... and if you have a perfectly insulated pot, the total energy inside doesn't change even if the water and rock are changing.


viscence t1_jci1l90 wrote

Oddly enough you can somewhat argue about both of these, as billions of years ago the gravitational effect of what would become the sun had a significant impact in getting that energy into what would become the earth... or that nuclear energy comes from isotopes formed from previous stars. It just becomes a matter of definitions at some point though.


viscence t1_jchzsoa wrote

At body temperature, you emit around one or two times ten to the power of 22 photons per sterradian per second per meter squared, at around several to tens of microns of wavelength. The atmosphere doesn't let all of these through, but a lot! if you have a surface area of 1 square meter and lived 80 years under an open cloudless sky of 2*pi sterradians, you've probably sent more than 100000000000000000000000000000000 photons into space.


viscence t1_jchxkmb wrote

The sun manages to transfer heat through a vacuum just fine -- radiatively, by emitting photons of light. That is also how the earth loses heat. Like the sun, the earth has a temperature and therefore glows, radiating away heat. The earth is a lot colder than the sun, so it radiates much less, and invisibly in the infrared.