As most of us know, accelerating to relativistic speeds requires truly astronomical amounts of energy. I have internalized that a ship carrying enough energy/fuel to accelerate to near-lightspeed by its nature also carries enough energy to destroy a planet several times over, even ignoring the fact that the near-C ship could just ram a planet to explode it rather effectively. The trick, though, is carrying enough stored energy onboard the ship to accelerate to those preposterous speeds, and then decelerate again at some point. Fuel has mass, and mass requires more fuel to accelerate it at the same rate as less mass, and so there is no way, in my understanding, to store enough energy aboard a ship to reach speeds like 0.5C, let alone 0.99C as is popular in these types of discussions. The answer to this problem is to not store the energy on the ship at all, but to beam it into the ship remotely. This is called beam propulsion, and is stupidly easy if you have something like a Dyson Sphere. I'm sure many of you are already familiar with this, but for those who are not, basically, a ship is built with large sail-like structures (think solar sails) that are targeted and fired on by intense and focused beams of energy that have access to astronomical amounts of energy, pushing the ship at a constant and high rate of acceleration for prolonged periods.
In-universe context; and main question
In the sci-fi universe I am building, Dyson Spheres, or stellar engines or whatever you want to call them, are common, albeit very expensive installations, and human civilization overall is very adept at beam propulsion, aided in no small part by their prodigious energy budget. Ships are sent between star systems at speeds often exceeding half of lightspeed (0.5C), and civilized space is formed into a network of these beam-powered routes that many of you might know as a concept called an Interstellar Laser Highway system. This works fine for traveling between systems with established infrastructure to speed you up at one end and then slow you down again at the other, but what about traveling to uninhabited systems where no infrastructure has been built yet?
Technologies and limitations
In the setting, I have decided that energy can be stored as photons within chambers lined completely with a perfectly reflective substance, trapping photons indefinitely. This method of power storage would be extremely energy dense, and I have read that light confined in this way, at a certain point of energy density, will start behaving something like a gas, and exert pressure, establishing an upper limit to how much energy can be stored in these chambers. So, say we have a starship en route to an uninhabited system. It was propelled to about 0.5C by a stellar laser back in a nearby frontier system, and has been traveling for a few centuries now. I have been trying to figure out a way for this ship to decelerate at its destination, and have tried to avoid having it use the energy it has stored onboard, for fear that it will not be anywhere near enough, but I can think of no other way that does not involve magic or cartoon physics.
The question, expounded
This question has multiple facets:
- Exactly how much energy is needed to decelerate a ship from (or for that matter accelerate it to) 0.5C ? I am badly under-informed in all kinds of mathematics, and energy is among my weakest areas in math, so I hope someone smarter than me will see the way.
- Exactly how much energy could be stored in one of those photon-imprisonment chambers of a given internal volume, according to current understanding, ignoring the issue of how to get the photons in and out of the chamber? Would the energy stored in one of these chambers of a reasonable size be enough to decelerate a craft from 0.5C?
- I've heard that photons have mass, and so would the mass of the light trapped aboard the ship amount to anything that could alter the ship's delta-V? I am also assuming in this setting that they have futuristic engines that can essentially convert electrical energy directly into velocity, among the setting's only clarketechs. Would the photon-imprisonment chambers be superior propulsion to the magical momentum drives if you broke them open? If so, what level of acceleration could they provide?
- What sources or courses can I consult to try to learn how to work out these math problems for myself? where should I go to actually understand things like the standard Rocket Equation or Specific Impulse?