4 ms·

Well that depends on the ejection velocity. If you could shoot it out at close to speed of light, you'd need much less.

• LoganDark 3y ago
  If you were to shoot it out at close to the speed of light, you'd knock the Earth out of its orbit.
  • knodi123 3y ago
    aim a bit to the left.
    • mcswell 3y ago
      Sure, Han!
  • shadowgovt 3y ago
    "Sir Isaac Newton is the deadliest son of a bitch in space."
  • fouc 3y ago
    an ion thruster that shoots ions out at the speed of light probably won't affect Earth
    • LoganDark 3y ago
      Yes because ions are... ions. You don't see photons knocking the planet out of orbit, do you? They can exert force but they don't exert that much force. But if you're trying to avoid self-propulsion and want to launch from Earth, say, even a 200kg craft, anywhere "close to the speed of light", then that will most probably require a significant enough amount of force to knock the planet out of orbit.
      • eru 3y ago
        No, what makes you think so? If you can apply small force over a long time, that will get you up to speed, too. Someone did the math in the thread, and suggested that a constant 1g of acceleration would get you to the centre of the galaxy in 20 years (as measured by the clocks traveling on your spaceships). 1g of acceleration for 200kg is about 1962 Newton. (This back of the envelope calculation assumes you have eg someone fire a laser at your ship to give you the energy you need. If you need to bring your own fuel, the rocket equation increases the total mass needed. But the same principle still applies: something like an ion drive has very little force, even if the top speed it can reach can be enormous.)
        • deleted 3y ago
          [deleted]
        • LoganDark 3y ago
          I think everyone is collectively ignoring that I'm specifically talking about accelerating the craft from earth, not using an ion drive or similar form of propulsion on-board. As that's what is implied by the comment that I responded to: "If you could shoot it out at close to speed of light"
  • jvanderbot 3y ago
    That's silly. My flashlight shoots stuff at speed of light all the time. Mass matters. Shooting just enough mass at very high velocity is not much different than shooting a lot more mass at lower velocity, in terms of force.
    • LoganDark 3y ago
      > Mass matters. Exactly. You try accelerating 200kg up to anywhere close to the speed of light (say 80%). That is a lot of force. Technically even photons can exert force on objects, but they have such a small mass that it's a difficult effect to observe.
      • hotpotamus 3y ago
        From memory, photons are massless, otherwise they could not move at light speed. They do have momentum though.
        • LoganDark 3y ago
          Oh, thanks for the correction.
        • eru 3y ago
          Photons have mass, but no rest mass. (Or something like that.)
          • scubbo 3y ago
            Momentum but no mass, if I recall my physics correctly (low certainty). Wait, hang on, we have access to an appreciable-chunk of the world's knowledge at our fingertips... https://en.wikipedia.org/wiki/Photon https://en.wikipedia.org/wiki/Photon - "Photons are massless[...]In empty space, the photon moves at c (the speed of light) and its energy and momentum are related by E = pc, where p is the magnitude of the momentum vector p[...]Current commonly accepted physical theories imply or assume the photon to be strictly massless"
          • 3y ago
  • hwillis 3y ago
    no, the earth is actually big. 47 trillion times heavier than that mass. At 90% c that 160 billion kg would still be a small fraction of the planets (moving 10,000x slower, at 30 km/s) kinetic energy. 500,000x less. Of course that .06 m/s velocity change would be near instant, so bad things would happen. Probably a humanity-ending but not life-ending disaster. Global tsunamis and incredibly large tectonic changes, for sure. Imagine the entire water column of the marianas trench jumping up in the air and slamming into the ground below. If the energy was transferred at a single point, it'll be the worst extinction event ever (4000x worse than chicxulub) but I'd bet single-celled and maybe even some multicellular life would survive. Anything bigger than a mouse is fucked, though.