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$I$ has a clear physical meaning if $I\lt 0$ – which is a significant percentage of the spacetime, so to say: $$I = -c^2\Delta t_{\rm proper}^2$$ where $\Delta t_{\rm proper}$ is the time measured by clock that moves by a constant velocity (without acceleration); and that visits the point $(x_1,y_1,z_1)$ at time $t_1$ and $(x_2,y_2,z_2)$ at time $t_2$. ...

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For online sources, there are some good introductions to special relativity here and here. For a print book An Illustrated Guide to Relativity seems like a good intro. Another good one is Spacetime Physics by Wheeler and Taylor, which I think for the most part just requires algebra though there may be some sections/problems that use some basic calculus. ...

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There are loads of things you can do with lasers. However, maybe this one might be of interest? It's called "Laser Audio Interferometer" I tried this and it's simple. The mirror reflects the laser beam back down the bore of the HeNe laser tube, forming a second optical cavity external to the laser. Or use a laser pointer instead; I've been told ...

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Though the question is off topic, its a rare opportunity for a young person to connect with more senior physicists, so I'll share my thoughts: There is a vast, beautiful mathematical world waiting for you to discover - you haven't even seen a deep treatment of mechanics yet. It takes a lot of work and a lot of dedication, to grasp, but we're all here ...

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It's much simpler than you think! The analogy is meant to underline that if you look the wire from far away it appears a 1-d path, while if you zoom you can see that the path is actually taking place in 3-d, because the wire is an extended object rather than really a line, even though two directions are far smaller than the third one. Now the ...

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There's "learning" and there's "understanding". The two are often completely separate. If you understand what you have learned you can apply it to real problems. Do the exercises which often come with textbooks

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