I am going to stick my neck out here and try and discuss something that is on the edge of my knowledge. There are a couple of areas I know are just beyond my grasp...
One is the maths of public key encryption and sadly I had this explained and I remember getting it properly for a couple of days and losing it - that must be what Alzheimer's feels like. To be clear I do understand all of the principles and implications, it is just the maths that is that far beyond my everyday usage that I cannot retain it.
The other is relativity - mostly I get this but it is the time dilation that has me stuck, so I'll explain the bits I do grasp and where I fail. I am not really expecting a lot of answers - I will get some, and they will either be maths over my head or examples I don't find convincing. There was a recent case of some kid getting a prize for explaining relativity but the explanation had holes in it as I saw it, big holes.
So far I have grasped that gravity and acceleration are the same. This seems actually quite intuitive to me - in that they feel the same. Free fall is the same as simply travelling through space and zero gravity. Gravity at 1G on the surface of the Earth feels the same as 1G of acceleration in a rocket in space. It all fits and make perfect sense to me that they are the same.
Also, the fact that in gravity you experience time dilation - time goes more slowly compared to this not in gravity. The effect of a gravitational force is seen the same for all observers. If I am accelerating at 1G, people observing can see that, and so can I, and we all agree. The same as if I am sat on a planet with 1G force, every observer agrees. The idea that there is a time dilation due to gravity (or acceleration) is fine by me. At 1G time is slower than at 0G, and faster than at 2G. All observers see the same. Everything is fine. Let's accept that.
The other effect is speed - which is always relative. If two people are travelling at a speed away from each other, each see the other as going more slowly. This is an effect that I would see as logically an "illusion", in the same way the seeing someone at a distance they look smaller, but you look smaller to them - a paradox. If ever they change to coming towards you, you see them going faster than you, and they see you going faster to them. Ultimately this effect cancels out when you meet and have no time dilation that has accumulatively between you as a result. Whether the effect is "real" or an "illusion" is academic.
So far so good.
The problem I have is that there seems to be some effects that are speed related and do not fall in to either of the above.
The idea that someone whizzing off in a space ship and coming back later will appear to have some measurable cumulative time dilation effect. Now, this makes sense if they have some acceleration involved either way as that would slow their time compared to the person that had no acceleration and was left behind. But is that the only effect?
Apparently the ISS in orbit had two effects, one is due to lower gravity so faster time, and one is a speed effect and slower time, and they do not cancel out quite. It is the speed effect I cannot get my head around as surely to an observer on Earth they spend half their time travelling away (and hence seem slower) and half their time travelling towards (and hence seem faster), but the net effect of speed is always zero, only gravity ultimately has a cumulative effect.
So that is where I am lost - I hope one day to grasp it, and blog it.