I was just listening to some old recordings of some lectures on relativity, like you do, and realising the reason it can be hard to understand this fairly simple idea, is that it isn’t common sense. It’s ALL about frames of reference. Physics in your frame of reference is the same physics as someone experiences in their frame of reference. But how you observe them and how they observe each other are going to be different. Your space faring twin really does return 15 years younger than you.
However there is only one physics – one truth. Relativity can be hard to grasp because relativistic effects are outside of our sphere of experience.. our common sense. Yet particle accelerators, old school tellys and GPS wouldn’t work if these effects weren’t accounted for.
So it is critical for the world moving forward for everyone to realise the limitations of our experience, and accept that we might be missing something. That someone else’s experience of the world might be valuable.
Your common sense, for example, might tell you that there are two genders, male and female, and in your limited reference frame you would be right. To you. But you are nonetheless factually incorrect.
It is hard for us to step out of our experience bubble to see what else is going on. Einstein had a unique ability to do this. In physics at least. (In other regards he was, allegedly, a misogynistic knob like many of his contemporaries.)
Einstein did it by running thought experiments through his head sitting on a park bench. He stepped into the shoes of someone riding a bicycle at the speed of light holding a mirror in front of their face and wondering if their reflection would disappear.*
I think we could all do with taking time out just to sit and imagine… What would it be like if…?
* – the author does not recommend riding your bicycle while holding a mirror in front of your face.