Agreed, but "photon rockets" are pretty cool, because their specific impulse is better than any matter-propelling rocket. The bit problem with "photon rockets" is how to produce enough of them aimed at a specific direction, without producing things like neutrinos, which can also carry away momentum, but are much harder to direct. At 07:05 AM 11/13/2011, Warren Smith wrote:

There's a lot of garbage science fiction out there that has nothing to do with science, especially in hollywood... but in this post I want to demolish two sci-fi ideas that occur in works of Larry Niven (et alia) which maybe did sound achievable to the naive. But a third such idea (super strong materials) will also be considered and argued to maybe have hope.

2. "Bussard Ramjet." A magic interstellar travel ship which somehow attracts interstellar hydrogen (with a "magnetic field"?) into its inscoop, pipes it thru a fusion reactor, and spits the hot helium out the back. "Keeps accelerating forever" (preferably at 1G) need not carry fuel, and if you wait long enough you'll achieve speeds arbitrarily near lightspeed enabling you to visit the whole universe in your lifetime thanks to time dilation.

Actually: Even if this could be made to work, the impinging hydrogen would slow the ship down, it's a jet not a rocket, and the ultimate speed would be upper-bounded by the ejection velocity of the helium, no matter how long you waited. That is way below lightspeed since the mass loss (hydrogen-->helium) is very small fractionally. Meanwhile, if you just carried a tank of hydrogen fuel massing e times your unfueled mass (genuine rocket), you could achieve slightly greater speed without all the technical difficulty... and if it were deuterium you'd get a good deal faster speed.

THIS WHOLE "BUSSARD RAMJET" IDEA IS JUST IRREDEEMABLY STUPID.