Everything here is built on two ideas: gravity is a force that pulls, and light travels in straight lines without changing on the way. Both are wrong. They are wrong by so little, for everything this site draws, that using them is the right choice — but "good enough" is not the same as "true", and it is worth knowing which one you have been given. Here are three places where the cracks show, and what is behind them.
This site pulls planets around with Newton's gravity, which has one job: every mass attracts every other mass, and from that you can work out where everything will be. It is astonishingly good. It got us to the Moon. It is also not what is really happening.
The place it first visibly failed is Mercury — and the evidence is sitting in this site's own data files.
Mercury's orbit is not a fixed ellipse. The ellipse itself slowly turns, and the file says how fast:
"rates": [ …, 0.16047689, … ] —
degrees per century, for the direction Mercury's closest point faces.
0.16 degrees a century sounds like nothing. In the units astronomers use it is about 578 arcseconds per century — and at that rate the whole ellipse comes right round in roughly 225,000 years. Now split that number up:
| the tug of all the other planets (Newton can do this) | ~532″ |
| left over, and unexplained for sixty years | ~43″ |
| what is actually measured | ~578″ |
That leftover was a genuine scandal. It was measured carefully in 1859, and Newton's gravity simply could not produce it. Astronomers assumed something must be tugging that they had not found, and went looking for an undiscovered planet closer to the Sun. They even named it in advance — Vulcan. People reported seeing it. It is not there.
In 1915 Einstein finished a new theory of gravity, and one of the first things he did with it was Mercury. It gave 43 arcseconds a century, with nothing to adjust and nothing to tune. He said it gave him heart palpitations.
The turning is drawn enormously exaggerated: six steps of 30 degrees here, where the real thing takes about 225,000 years to go round once — and Einstein's share of it, on its own, would take about three million. It is a tiny effect. It is also the one that showed Newton was not the last word.
Here is the thing worth being straight about. There is no relativity anywhere in this site's code. It moves Mercury's ellipse round at a fixed rate that it reads out of a table, and that is all it does.
But that table was measured, not worked out from Newton's laws. So the number in the file — 578 arcseconds — already has Einstein's 43 baked into it, put there by the people who measured the real Mercury. This site therefore draws Mercury in very nearly the right place, for a reason it does not contain. It is right by copying. If it had calculated that rate from Newton's gravity instead of copying a measurement, Mercury would be visibly in the wrong place within a few thousand years of simulated time.
Not that gravity pulls harder than Newton thought. That gravity is not a pull at all.
Mass bends space and time around it, the way a heavy thing dents a stretched sheet. Nothing is being tugged. Everything simply travels the straightest line available — and near a mass, the straightest line available is a curve. The Earth is not being held in its orbit; it is going straight, in a place that is bent. When you fall, nothing is pulling you. The floor is what is pushing.
Relativity is quietly everywhere on this site without ever being named:
Over on the life of a star there is a sentence about how a star holds itself up: the middle gets to around fifteen million degrees, and hydrogen slams together hard enough to stick.
That sentence is not true, and it is worth knowing exactly how untrue it is.
Two hydrogen nuclei are both positively charged, so they push each other apart, and the closer they get the harder they push. To touch, they have to get over that hill. At fifteen million degrees the protons in the Sun's core are moving fast — but nowhere near fast enough. They are short by a factor of about a thousand. Not "just barely fail". Hopelessly fail.
Do the sum with ordinary physics and the answer is not that the Sun shines dimly. The answer is that the Sun does not shine at all, no star ever ignites, and nothing whatsoever on this website exists.
What rescues the Sun is that a proton is not a tiny ball at a definite place. It does not have one position. It has a spread of possible positions — and part of that spread reaches through the hill and out the other side. Every so often, without ever having gone over the top, the proton is simply found on the far side. This is called tunnelling, and it is not a trick of the maths; it is how the world turns out to work when you look closely enough.
Tunnelling is rare. Any particular proton in the Sun's core waits, on average, something like a billion years for it to happen. That sounds like a problem and it is the opposite of one. If fusion were easy, the Sun would have got through its hydrogen in a geological eyeblink and gone out long before the Earth had cooled. The Sun shines because of a loophole, and it lasts because the loophole is very narrow. Both halves of that are needed, and both are quantum mechanics.
Like relativity, it is all over this site unnamed:
This one is not a theory the site leaves out. It is a question that sounds childish, has no obvious answer, and needs both of the other explanation pages to settle.
Suppose the universe went on forever, had always existed, and had stars sprinkled fairly evenly through it. Pick any direction at all and look. Your line of sight goes out and out — and sooner or later, it must land on the surface of a star. There is no direction in which it could keep going forever without hitting one, because there is always another layer of stars further out.
So the entire sky, in every direction, should be as bright as the surface of the Sun. Not a scattering of dots on black. A solid blaze, everywhere, all night.
The same thing happens in a big enough wood. Look horizontally in any direction and you do not see out — you see a trunk. Every line ends on wood. An infinite universe full of stars is a wood with no edge, and every line should end on fire.
The obvious escape is that something is in the way — dust and gas, soaking the light up before it reaches us. It does not work, and the reason it does not work is neat: anything sitting in an infinite blaze absorbing light for an infinite time gets hot, and anything hot glows. The dust would end up shining exactly as brightly as what it was hiding. You cannot hide from a sky like that. You can only fail to have one.
Both of those are on how the universe began, and neither was known when the question was first properly asked. A question about why it gets dark at night turns out to be answerable only if you know how old the universe is.
Here is the part that makes this the best question on the page. Look far enough in any direction and you do hit a hot surface — the wall of fog from the universe's first few hundred thousand years, which no telescope can see past. It covers the entire sky, in every direction, with no gaps. Exactly as the paradox demanded.
It is just that the stretching has pulled that blaze out of visible light and down into microwaves, and your eyes do not work there. The night sky is not dark. It is dark to you. Point a radio dish at any patch of empty black sky and the glow is sitting there, filling it, at 2.7 degrees above absolute zero. That is the microwave background from the previous page, and it is also the answer to a question somebody asked about their own back garden two hundred years earlier.
You need trees. A wood, a park with a decent clump, an orchard — anything with a lot of trunks and some depth to it.
Now the useful bit. When you can still see out, it is because the wood ran out before your line of sight did. That is exactly our situation looking at the night sky — except that what runs out is not trees, it is time. Look far enough and you are looking at light that left before there were any stars to see.
Because for everything this site shows you, the difference is far smaller than a pixel.
Newton's gravity puts the planets where a telescope will find them tonight, and will keep doing so for a very long time. Light that travels in straight lines is a perfectly good description right up until you are next to a black hole. Using the simpler idea is not laziness — it is the normal way physics is done, and the harder theory has to agree with the simpler one everywhere the simpler one already works, or it would have been thrown out.
What is not fine is not saying so. A site that quietly used Newton and let you think that was the truth would have taught you something false without ever writing down a false sentence. So: this site uses Newton's gravity and simple light throughout, and both are approximations. Very good ones. Not the truth.