It is because in both cases, you can work out that you'll never make an observation that really proves it one way or the other, but you might come up with some laws of physics that are more conveniently expressed in one way than another. They're both ultimately questions about your coordinate system, and in fact it is easier to invert time than it is to work out how everything's supposed to function in a rotating+sun-orbiting+precessing+... coordinate system.
It isn't because we have the observations we have. It might be mathematically easy to invert time, but empirically it's not so simple. Mathematical symmetries and expressions are nice, but then the actual physical consequences and requirements (e.g., material, time, energy inputs) sort of force one's hand.
Also, geocentrism is actually disproven by numerous observations, which in fact guided the mathematical formulation of our current physics.
One thing is (almost) certain: the laws of physics as we know them (i.e. the Standard Model and General Relativity) are incomplete ("wrong"). That doesn't mean any old model is equivalent or as useful as either, though.
>Also, geocentrism is actually disproven by numerous observations, which in fact guided the mathematical formulation of our current physics.
The validity of a coordinate system with the earth stationary at the center is guaranteed by the general principle of relativity. To get the stars to circle around it you would add a radially increasing potential in classical mechanics, or some coordinate shenanigans in GR. These coordinate systems are used in aerospace engineering to get convenient expressions of L1 points, etc.
I know it sounds hard to believe, but if you're willing to bite the bullet that "the center of the universe" has no physical meaning, then earth can't be not the center of the universe any more than it can be.
To be honest, I have no idea what you are talking about.
Certainly, there is merit in practical calculations when we are all this close to the ground where it all seems either flat or at least geo-centric.
But... the movements of the other planets in our solar system were really strange to model in geo-centric models. In short, each of the other planets should rotate around an imaginary axis to compensate for their changing positions in the sky.
Of course, all this is talk about in the context of our solar system. As for us being the center of the universe, well, the same argument holds for any other point in the universe. So, I think it's less likely than me winning the lottery in the next five minutes :)
In the XVII century, the alternative to heliocentrism was Tycho Brahe’s model: Earth at the centre, the Sun and Moon and the stars revolving around it, and the other planets revolving around the Sun. It was basically equivalent to the heliocentric model with a different coordinate system.
It’s important to understand that astronomers chose it because it really seemed to provide a better explanation given the knowledge and technology at the time.
Tycho Brahe himself noted that his model could be disproved by observing the stellar parallax effect as the Earth orbits the Sun (if the Earth does move, then the stars would look slightly different throughout the year). This is a real effect, but so small that it couldn’t be observed until the XIX century.
There was even more to it. Cosmographers of the time were willing to consider the possibility that the stars were very far away, far enough to make parallax changes undetectable. The main stumbling point was that they assumed the stars would be similar in size to our sun, but based on naked-eye and telescopic measurements of the diameter of the “disk” of the stars (many taken by Brahe and Galileo themselves), any solution that explained the parallax effect would have resulted in enormous stars, and this they couldn’t accept. Eventually the measurements turned out to be an optical artifact.
I read an entire book on this subject, published by a Catholic press. There seems to be an active interest in the Catholic science establishment in rehabilitating some of Galileo’s critics.
> I know it sounds hard to believe, but if you're willing to bite the bullet that "the center of the universe" has no physical meaning, then earth can't be not the center of the universe any more than it can be.
I disagree.
The sentence «"the center of the universe" has no physical meaning» requires that Earth can't be at the center of the universe, because there isn't a center for it to be at.
You're still allowed any or many arbitrary zero-points, but that's only the center of a number line, nothing else.
It's basically a semantic issue but I don't think the negation of an undefined proposition is true, I think it is undefined. (Thankfully the universe does not run in JavaScript, where !undefined is true.)
But, Earth.x = 42 (in some arbitrary coordinate space), if I understand correctly there isn't a sub-light coordinate space in this universe where Earth.x = undefined.
That's not the reason why the centre of the universe is undefined.
By analogy, while spherical coordinates are arbitrary, latitude is defined. Arbitrary, but defined.
But the centre of the Earth has an undefined latitude, and a topological subspace consisting of just the surface of the Earth can't hand-wave past that by pointing out that's just a coordinate singularity that can be safely ignored — there isn't a center of the Earth anywhere in that subspace. If the universe is flat and finite (looping), this problem still exists.
And if the universe is unbounded (infinite), that has a different problem because you can't properly define a median of an infinite set[0], so no center exists.
If it just stops suddenly after a certain amount of space, then we get to have a center, but there's no sign of that.
[0] I think. Infinity is easy to get messed up with.
We may not be able to observed it, and maybe can’t. Just like time slowing down would be imperceptible to the person on the spaceship nearing light speed. It takes the mathematics
Or if I’m lucky enough to have the time to watch the moon move slowly, it feels natural to my senses to say it’s moving across the sky. The moon feels like it’s moving around me. But I can stretch my brain and imagine the reality.
Sometimes math arrives first. We have the new maths (or an problem in current math), and that points to some possibility. Because it’s not observable, ignoring our senses is a requirement to develop that in the model and measurements and experiments. Eventually we are able to observe it.
I’m not familiar with history of astronomy. Would it be the case where the observations that lead to heliocentric thought we nuanced and had to build on more obvious perceptions that things aren’t adding up? Was the wobble of Venus part of that?
And you’re right, old models are useful and remain relevant a lot! The model of time moving linearly will likely always be the most useful model for navigating our daily choices (if we have any at all!)
If we reverse time and this implies running everything backwards in physics, do we include gravity in the set of things that are reversed? Then everything would fly off the face of the earth in reverse-time.
Reversing time on an attracting force still gives you an attractive force. Velocity is reversed, but acceleration isn't.
Imagine a ball being thrown up and then falling down, in a parabola. Reversing a video of that still gives you a video of a ball in a normal parabola trajectory.
The history of the universe channel on YouTube has an episode called 'what is time' that goes over symmetries like this. Lots of atomic/quantum scale interactions are time reversible like this, but not quote all, and that may be where time arrives from.
Well worth watching that episode as it does far better explaining than I do.
If you stop there it looks like you're right, but you also must change the definition of velocity to account for the new time.
v = dx/dt = -dx/du
+F = m dx/du
So the direction of gravity (the force F) stays the same when you flip time. I can explain that without the math by pointing out that if you took a video of a ball being thrown up and caught and played it in reverse, it would still depict a ball being thrown up and caught.
> I can explain that without the math by pointing out that if you took a video of a ball being thrown up and caught and played it in reverse, it would still depict a ball being thrown up and caught.
That's amazing, thanks. The portion where you caught the ball in forward time is equivalent to throwing the ball in reverse time.
If we change the analogy of throwing a ball to firing a gun into the air - does the analogy still work? Since when we fire the gun up, the bullet will travel faster up than it will travel down due to terminal velocity in forward time. How is that phenomenon explained in reverse time?
Instead of predominately striking the bullet in a way that causes it to slow down, the molecules in the air will predominately strike it in a way that causes it to move faster, in what looks like an unbelievable (but still physically possible) run of good luck.
So it seems like if we reverse time, we reverse entropy and that as time approaches 0, we would effectively be reversing the big bang and instead have the big collapse.
Another thought experiment that comes to mind is compressed gas in a cylinder. When we open the valve, the gas in the cylinder comes out. In reverse time, the gas would go back into the cannister and the valve would close after the gas went back in. Very low probability of that happening in forward time, though not not 0.
Though it seems weird, because why does the gas go into the cylinder? Because further into reverse-time, something sucked it all out (in forward-time, this machine is the compressor that put the gas in the cylider.) This hurts my brain!
In the way down -sky to gun - the molecules in the air will give it energy to accelerate more than gravity alone would. Before that - in the way up - air molecules will cause it to move upwards at constant speed until conveniently they stop doing so.
> unbelievable (but still physically possible)
Physically possible - but in the same sense that the second law is not a physical law.
" it's literally like playing a movie in reverse" seems overly authoritative for something we haven't observed. I have seen Tenet, but it's a work of fiction.
Have we observed reflected gravitational waves? In reverse-time, where would they originate from if they presumably rippled out into space and didn't collide with anything in forward-time?
.... they would "originate" from all the locations the gravity waves spread to and converge on the source.
Tenet has nothing to do with this- I'm just explaining it as it was covered in my many physics classes that covered the nature of the arrow of time (https://en.wikipedia.org/wiki/Arrow_of_time) and how I interpret it in terms of what seems most likely/least unlikely.
In these "it's like taking a video of throwing a ball in the air and allowing it to land on the ground, then playing it in reverse" examples, I can't help but think of Newton's first law. If an object at rest stays at rest, how does the ball leap from the ground? Where does the impulse come from? Reverse time seems too far fetched for me, or at least the simplified naïve version of it does.
It also seems a bit misleading, since in that scenario a ball is intentionally thrown so that it comes down the same way.
Let's consider something else - imagine an accretion disk of space dust slowly pulling itself together to form a planet. Play that in reverse, and you have the a planet slowly coming apart piece by piece. Imagine reversing the impact that created the moon. The moon comes apart piece by piece, creating an accretion disk around the earth, which then all moves and hits one area of the earth, and there several parts of it (and part of the earth itself) move together to form a separate planet, which then launches itself from the earths surface into space, flies around the sun a few times, and then slowly breaks apart piece by piece into another accretion disk.
