They say all roads lead to Rome, but in space, all roads will lead to the sun and be made of lasers.

The funny thing about light has no mass, and yet it can still shove things around.

Every photon carries momentum, a tiny kick that would multiply by trillions upon trillions of photons,

and applied steadily steadily becomes a real

force. And unlike chemical exhaust, that light doesn't need fuel tanks or reaction mass.

The universe provides all the thrust you want, as long as you can make or borrow enough photons.

Today we'll be talking about interstellar relays as a way of moving around not just information,

but also people and cargo, and we'll have a few relay methods to discuss, such as black holes as galactic regional

hubs. One of our most compelling methods would involve setting not just signals by communication

lasers, but propelling ships with them too. One of the most frustrating things about rockets

is the faster you want to go, the more fuel

you need, and the more fuel you add, the more you have to push that fuel itself.

It's an exponential trap, the infamous tyranny of the rocket equation.

If you want to reach even one tenth of light speed and slow down again, nearly all your ship's

mass must be fuel.

Even by fusion, you can't pack enough

in without touring the vehicles into a flying gas tank. That's why for interstellar travel,

the best fuel or propellant is one that you don't have to carry with you. The other important

thing about an ideal fuel or propellant is its exhaust velocity, how fast the exhaust of your

rocket comes out the back

side relative to your ship. The faster they go, the less fuel you need to get the same

final speed for your ship, and usually it is impractical to achieve a final speed more than

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