Not to nitpick, but nothing but massless waves/particles like photons can actually go at light speed. And conversely, masless particles/energy can't go any slower either.

Anything with mass, even a single electron or a single neutrino, which masses almost, but not quite nothing, can never reach 100% light speed, even if you gathered up all the energy in the Universe to push it.

Even 1g of constant acceleration will get you to 99% of light speed, or whatever percentage we decide is "good enough" though. And do it relatively quickly.

A constant 1g thrusting spaceship that will reach that "close enough" 99.whatever% of light speed in about 1 year.

This is a good writeup of the math involved.

There's big practical problems with this though.

Mainly, to produce that constant 1g of acceleration, you really can't gather enough fuel or energy together to do it. Because when you do the math for The Rocket Equation, and start plugging in numbers, a nasty exponential curve shows up. As you need more more fuel to go faster and thrust longer, you need more fuel to push the extra fuel, and so on.

And adding in much more powerful and efficient forms of propulsion, like nuclear power, fusion, or even matter/antimatter annihilation doesn't help as much as one would hope.

And your limited as to how fast you can go by the specific impulse/thrust velocity, the total thrust, and the thrust/weight ratio of the ship.Because both the total mass and how fast what your spaceship squirts out the back end to push it forward in space has to be taken into account.

The most aggressive matter/antimatter annihilation rocket ideas, like a hydrogen/antihydrogen rocket, gets very good specific impulse/exhaust velocity, it's crazy efficient, you're getting exhaust at around 94% the speed of light, but the total thrust is low, and there's no way to get even close to 1g of acceleration out of it.

And I am intentionally overlooking the massive challenge of producing that antimatter, and how to store it on the ship. Best we've done so far is to hold 0.00000000000167 grams of antiprotons made by the CERN particle collider for 204 days in a magnetic field.

You'd need exponentially more for an interstellar mission.

Specific Impluse vs. total thrust is kinda-sorta like having a 10 speed bike with gears. High thrust is like first gear, you start accelerating quickly, but soon you're just spinning your feet, high Specific Impluse is like tenth gear, you accelerate slowly, but it's much more efficient over time.

And physics, chemistry, and nuclear science points heavily to the conclusion there's no magic engine that does both high thrust and high Specific Impluse.

You can compromise and sort of combine both, using nuclear power or antimatter to get some kind of reaction mass or working fluid much hotter than any chemical burning or reacting can get it, but there's drawbacks to that too. The specific impulse is far higher than chemical reactions, double or more, but orders of magnitude lower than the specific impulse purely nuclear or antimatter thrust provides. And by carrying some sort of reaction mass along on your ship to heat up and squirt out the back for higher thrust, brings the rocket equation of needing ever more reaction mass to push the reaction mass you have right back into play.

Also, the savings from time dilation for the ship and any crew, that makes the trip seem shorter, meaning they get there alive, and don't need as much food, water, air, books to read etc. from their reference frame, don't get significant until 80-90% the speed of light. Which even the most powerful and efficient energy source, antimatter, won't get you close to.

And if you could go that fast, we haven't addressed the problem of shielding the ship from cosmic rays, interstellar hydrogen (same thing as cosmic rays at that speed), dust, or an occasional unlucky micrometeor, which would carry the energy of a small atomic bomb if/when it collides.

To make things easier, and at least somewhat achievable with relatively known technology, an Orion style nuclear bomb or possibly a fusion pulse pusher ship could conceivably get to the nearest star system, Alpha Centauri 4.3 light years away, at somewhere around 5-10% of light speed. Adding in acceleration and deceleration time, it would take somewhere around 50 to 100 years to get there.

And that's assuming lots of international cooperation. That SpaceX and other companies make good on their goals of making spaceflight and space industry routine. That nobody complains about manufacturing 300,000 nuclear bombs. And it's probably going to be a purely robotic mission, because life support, consumables, or even if we invent cryogenic freezing/hibernation that works and isn't just a fancy way of being dead that's just as permanent as "regular dead".... The ship will probably be too heavy and complicated to make it.

This is starting to look like a sensible discussion.

Someone's gonna need to poke one to "truth" them.

We'll just end up like
But anything could happen in the next half hour....Dom dom domdedom....Stingray!

I was just thinking that about what AJ_Dual posted. It's laid out well, in terms that people who know nothing about it can *mostly* follow, but if he made the whole thing up as he typed I wouldn't know it.