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Relativistic Spaceship

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Re: Relativistic Spaceship

#211

Post by eru »

EA-3167 wrote: post #168
slow_typist wrote: post #66

Earlier quoted context omitted.

But accelerating by 1 g is very convenient for the crew. No muscle and bone degeneration, no space sickness, etc.

It would be cheaper in every sense to turn something like a giant asteroid into a rotating habitat ship than achieve a constant 1g to even the closest systems. Realistically interstellar travel is not a thing that (biologically modern) humans will ever be suited for, robotic probes don't need thousands of kg of food and water to stay alive, don't need artificial gravity, air, or entertainment. The fact that the trip…

You are right. One addendum: many humans would be bothered by one-way trips, but humanity is large, and in absolute numbers there are still plenty of volunteers for one-way trips to the planets and stars.

Re: Relativistic Spaceship

#212

Post by multiplegeorges »

robofanatic wrote: post #207
ryandrake wrote: post #12

Great visualization of the unintuitive nature of special relativity. It's kind of mind blowing to realize that if we could accelerate our spaceship at a mere 1G continuously, we could visit the center of the Milky Way in under 20 years (spaceship time), totally do-able in a human lifespan. Of course 27900 years would have passed on Earth, so you wouldn't be able to tell anyone about your vacation.

How many years will it take to accelerate from normal speed to that speed and decelerate back?

10.

Assuming a straight line and assuming 1G is your max acceleration: you accelerate for 10 years, reach your maximum velocity then flip and do the same thing in the other direction. You'll reach your destination with 0 velocity.

Re: Relativistic Spaceship

#214

Post by huytersd »

ryandrake wrote: post #12

Great visualization of the unintuitive nature of special relativity. It's kind of mind blowing to realize that if we could accelerate our spaceship at a mere 1G continuously, we could visit the center of the Milky Way in under 20 years (spaceship time), totally do-able in a human lifespan. Of course 27900 years would have passed on Earth, so you wouldn't be able to tell anyone about your vacation.

Time travel seems so easy and we could do it with the tools we have now for the most part. Maybe we’ll never go into the past but relatively contemporary humans are going to be all over the future in all kinds of places.

After the first time it happens people in the future can start to expect a human from the past to keep visiting every so often.

Re: Relativistic Spaceship

#215

Post by eru »

senectus1 wrote: post #196
timschmidt wrote: post #106

Earlier quoted context omitted.

Less complicated. The rocket exhaust literally forms a plasma shield in front of the decelerating rocket.

what about for the period of time that you're travelling sideways with a largely unshielded broadside facing not exactly empty space?

You wouldn't literally turn around.

You'd probably move your rocket engine over to the other side (or have both a front and back engine in the first place).

Most of the mass of your spaceship will be fuel (like 90%+). You can use that as shielding.

Re: Relativistic Spaceship

#216

Post by elihu »

whimsicalism wrote: post #118
bee_rider wrote: post #49

Earlier quoted context omitted.

You can’t “get to” light speed, that’s one of the big punchlines in relativity. If you pick an acceleration equal to the acceleration we experience on Earth (aka 1g, aka ~9.8m/s^2), you hit relativistic speeds (speeds at which you need to take into account the effects of relativity to do anything) surprisingly fast. On the order of hundreds of days. So, it is not really a matter of safe acceleration on a long space t…

dumb question - why does going faster make it more likely you encounter fast protons? couldn’t protons in any reference frame be going quite fast relative to you?

Nah, most of the random atoms floating around in space are going to be travelling very roughly as fast as the things (stars, planets, etc..) around them -- because anything travelling much faster is likely to eventually bump into something and lose some of its momentum.

There is the occasional weird exception though: https://en.wikipedia.org/wiki/Oh-My-God_particle

Re: Relativistic Spaceship

#217

Post by eru »

ryandrake wrote: post #78
bee_rider wrote: post #49

Earlier quoted context omitted.

You can’t “get to” light speed, that’s one of the big punchlines in relativity. If you pick an acceleration equal to the acceleration we experience on Earth (aka 1g, aka ~9.8m/s^2), you hit relativistic speeds (speeds at which you need to take into account the effects of relativity to do anything) surprisingly fast. On the order of hundreds of days. So, it is not really a matter of safe acceleration on a long space t…

> Instead you have to worry about the actual speed you are traveling at That's the other big punchline in relativity: There's no one "actual speed" because that implies that there is a single "important" frame of reference wherein those atoms are floating around waiting to be hit by a spaceship.

Oh, 'relativity' might not have a preferred speed. But our universe absolute does.

It's the inertial frame of reference that makes the cosmic microwave background look most uniform. See https://en.wikipedia.org/wiki/Comoving_and_proper_distances#...

> A comoving observer is the only observer who will perceive the universe, including the cosmic microwave background radiation, to be isotropic. Non-comoving observers will see regions of the sky systematically blue-shifted or red-shifted. Thus isotropy, particularly isotropy of the cosmic microwave background radiation, defines a special local frame of reference called the comoving frame. The velocity of an observer relative to the local comoving frame is called the peculiar velocity of the observer.

Re: Relativistic Spaceship

#218

Post by __MatrixMan__ »

PaulHoule wrote: post #98

I re-read this book recently https://en.wikipedia.org/wiki/Tau_Zero which has to be one of the most epic sci-fi novels ever since it is about a starship that gets its brakes damaged in a crash so their answer to every problem they face is to go faster!

I liked that one.

At many points it reminded me of The Freeze Frame Revolution, by Peter Watts (https://tachyonpublications.com/product/freeze-frame-revolut...), which was also quite good in a similar way, if perhaps a bit darker.

Re: Relativistic Spaceship

#219

Post by eru »

pests wrote: post #181
naikrovek wrote: post #158

Earlier quoted context omitted.

So depending on how you measure, you’re always stationary or moving near light speed, or somewhere in between, depending on your measurement reference (the thing you’re moving relative to)? How is there a speed limit at all, if that’s the case? You can accelerate to 0.5c and then toss an apple out the window and say you’re moving at the speed of an apple tossed out of the window, relative to the apple. You have all o…

There is no underlying reference frame. All motion is relative. Everyone, no matter how fast they are already going, will measure the speed of light as c. Accelerate to .99c and shine a flashlight in front of you. That light is moving ahead of you at the speed of c. Because to you, you are not moving.

That's true for the laws of physics, yes, but our universe does have a 'natural' frame of reference.

> A comoving observer is the only observer who will perceive the universe, including the cosmic microwave background radiation, to be isotropic. Non-comoving observers will see regions of the sky systematically blue-shifted or red-shifted. Thus isotropy, particularly isotropy of the cosmic microwave background radiation, defines a special local frame of reference called the comoving frame. The velocity of an observer relative to the local comoving frame is called the peculiar velocity of the observer.

From https://en.wikipedia.org/wiki/Comoving_and_proper_distances#...

Re: Relativistic Spaceship

#220

Post by eru »

richardw wrote: post #100
acchow wrote: post #95

Earlier quoted context omitted.

What if you define the "important" reference frame as the speed of light's (in the same direction you are going). Since c is universally constant, it seems like a reasonable privileged reference.

Not a physicist, but my impression is that you're always going 0 percent of the speed of light (in all directions) from your own frame of reference. All you notice is that our solar system is moving away, faster. I guess you'd notice a change in light frequency based on the light in front/behind. Redder behind, bluer in front. Edit: supposedly we can measure our speed compared to the cosmic microwave background which…

See also https://en.wikipedia.org/wiki/Comoving_and_proper_distances#...
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