Seven earth-sized planets discovered circling a star 39 light years from Earth • Hacker News Board

Re: Seven earth-sized planets discovered circling a star 39 light years from Earth

#751

Post by blueprint » Mon, Feb 27, 2017, 8:03 PM UTC

Arizhel wrote: post #718

maverick_iceman wrote: post #702

Earlier quoted context omitted.

It's probably futile to argue with you but have you considered maglev trains?

Yes, that's an application of Maxwell's laws of electromagnetism. That's not manipulation of gravity, it's using the MUCH stronger EM force to overcome gravity. I can overcome gravity all by myself just using my muscles to lift things. That doesn't mean that I've manipulated gravity in any way. I'm really shocked that I seem to be the only one who groks the difference between observing a natural physical force and ac…

> observing a natural physical force

There's your misunderstanding. Gravity is not a force. The force is merely the effect you observe on your specific instrument. The cause and substance of gravity is not explained solely by its effect (force), and the force is not what generates gravity, et al. And, in fact, mass is not the only thing that generates gravity. It's flux of energy density. So it boggles my mind that no one considers EM energy as a subset of the energy that can produce "stress on space".

Conclusively, you've made some assumptions you don't realize resulting in an uncontrolled thought experiment. When terms are defined incorrectly, questions using those terms become wrong. If the questions are wrong the answers also always come out wrong.

Re: Seven earth-sized planets discovered circling a star 39 light years from Earth

#752

Post by saalweachter » Tue, Feb 28, 2017, 1:29 PM UTC

Arizhel wrote: post #748

saalweachter wrote: post #733

Earlier quoted context omitted.

That's size, not mass. Venus is about 81% Earth's mass, and Mars is only a little over 10%. Mercury gets you another 5.5%, the Moon (as above) another 1.2%, and the asteroids are negligible. In the outer solar system, it's basically all the Galilean Moons + Titan. ETA: Hmm, or maybe those are surface gravity numbers you're thinking of?

Yep, I was roughly equating mass with surface gravity, which I see now is incorrect. I do find it really interesting and a little mind-boggling that the Moon, as huge as it is, with a good 1/6 of Earth's gravity, only has 1.2% of Earth's mass. Same with Mars: a good 1/3 of Earth's gravity, but a mere 10% of its mass. I really thought there'd be more direct correlation between mass and gravity than that. Of course, su…

The densities do vary by almost a factor of 2 (~3 g/cc for the Moon, 5.5 g/cc for the Earth), but the larger problem is that surface gravity is a linear function of the radius, and mass is the cube of the radius.

Consequently, the mass of an object with a given surface gravity is inversely proportional to the square of the density. If the Moon were the same density as the Earth, and had the same surface gravity as it does now, it'd actually mass less than it does now -- and Uranus, despite have 15x the mass of Earth, has a lower surface gravity, because its density is so low.

The rule of thumb for space is "any way your intuition can be wrong, it will be wrong". :-)

Re: Seven earth-sized planets discovered circling a star 39 light years from Earth

#753

Post by DougWebb » Tue, Feb 28, 2017, 3:34 PM UTC

roninb wrote: post #363

DougWebb wrote: post #272

Earlier quoted context omitted.

I always thought that Firefly's scenario of a bunch of Earth-like worlds in close enough proximity to have regular trading ships flying between them was unrealistic. Now, not so much. This system even fits in with the "Abandon Earth en-mass, move to new set of worlds" backstory. So, what did J.J. know, and when did he know it? And for that matter, how did he know?

Just making sure, but I hope you know that JJ Abrams had literally absolutely nothing to do with Firefly and wasn't even tangentially related to its creation nor production. Also, I'm relatively positive that JJ (in the sci-fi works he actually, well, worked on), himself, knew nothing (see Armageddon for sources) and that his script supervisors dealt with any discrepancies with reality JJ had.

Man, I feel dumb. Yeah I meant Joss Whedon. Too many J's in sci-fi I like, I guess.

Re: Seven earth-sized planets discovered circling a star 39 light years from Earth

#754

Post by credit_guy » Thu, Mar 02, 2017, 8:12 PM UTC

e0m wrote: post #33

To get a spacecraft there it would take: Accelerating at 1g the 1st half, and decelerating at 1g the 2nd half, the traveler would experience 7.3 years of time. For observers it would take 41.8 years at a max speed of 0.998c If you had a near perfect hydrogen -> helium fusion engine, it'd take about 6 million tons of fuel (about the mass of the Pyramids of Egypt or 2,000 Saturn V rockets)

I didn't get the same result. I used the relativistic rocket equation [1], and the change in rapidity from 0 to 0.998c is about 3.5, which means a total change of rapidity of about 7 (static to full speed and then back to static). Assuming perfect hydrogen->helium fusion engine (specific impulse=0.12c) the ratio between initial mass and final mass is 10^25. For comparison the mass of the earth is about 0.6*10^25, or in other words, if you want to send only 2kg of load to this star, you need fuel as much as the mass of the earth.

[1] https://en.wikipedia.org/wiki/Relativistic_rocket