I am not a physicist either, but I’ll play the role of dabbler.
- The rocket pushes on the hot fuel it’s dumping out the back end. Its not really ‘pushing’ on anything. Its relying on a neat property of momentum.
Demonstration: Pick up an object heavy enough that you can hold it at arms length but that is still fairly hefty. (Let’s say a heavy stone) Hold it against your chest. Push it away from you as hard as you can. You’ll go backward while it goes forward.
Without resorting, for the moment, to equations: The faster you push it away from you, the faster you will push yourself away from it. Alternately, the faster you push it away from you, the less massive it has to be to achieve a certain amount of velocity of you backward.
Fuel being dumped out of the back of a rocket is extremely hot and gaseous and made to point the opposite direction you don’t want to go as its expanding. It goes backward. It goes backward really really flipping fast. (And the faster you can make it go, the less of it you have to dump out the back). Consequence: You go forward.
This doesn’t require any surrounding ‘air’ or ‘ground’ to push against. You’re pushing against the fuel (also known as reaction mass).
- In classic Newtonian mechanics, nothing. There’s a basic relationship to the amount of force you generate in a particular direction, your mass, and velocity. And this gives a pretty good measurement of the ‘reality’ of accelerated motion even up to appreciable fractions of the speed of light.
Barring hitting a planet, nothing will slow you down.
The speed of light isn’t a magic wall you hit. What happens is that as you approach the speed of light relative to some point you’re measuring that speed from (frame of reference is important in these discussions) you end up having time dilation effects and it appears to an observer back at your spacedock that you’re either getting more massive (so your thrust doesn’t accelerate you as much) or that you’re not putting out as much force (time dilation?).
These effects only start being measureable when you get to a serious fraction of the speed of light. You, aboard the spacecraft, can’t tell anything’s changed, and as far as you’re concerned, you’re continuing to accelerate at the same rate. So you’re not feeling any resistance from ‘the universe’ really.
Welcome to the counterintuitive nature of relativity (and there are dozens of physicists here willing to give you a much more detailed analysis of this problem).
From a practical standpoint, you can’t keep constant acceleration forever for a few reasons. One, you don’t have unlimited fuel to produce the force that causes the acceleration.
Two, the longer you want to burn the engines the more fuel you have to have, which increases your mass, which causes you to need more fuel to cause the same acceleration, which increases your mass, etc. If you work backwards in time from an empty tank given a (fairly simplistic) linear relationship between fuel and thrust, and dictate you want to maintain constant acceleration, you find there is a point at which your ship cannot carry enough fuel to push itself and its own fuel at that fixed acceleration.
This last point is somewhat esoteric but its a neat mathematical anomaly I ran across designing a fictional spacecraft, oddly. Its moot… you’d probably design for constant thrust, not constant acceleration. But you still don’t have infinite fuel for an infinite burn.
And again, you eventually run up against relativitistic effects, which complicates understanding what speed you really can get up to.