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Highly relativistic spaceflight would be fatal for passengers and instruments

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Re: Highly relativistic spaceflight would be fatal for passengers and instruments

#61

Post by squires »

celerity wrote: post #20

You know what else is an obstacle to relativistic spaceflight? Being able to do relativistic space flight. Fun fact: people realized this a long time ago, and proposed different "shields" in front of the ship (it turns out that the most effective shape is a smoothed-out cone [of ice, say] -- see Asimov's Song of a Distant Earth).

This also reminded me of the "Archangel" class ships described in Endymion by Dan Simmons. FTL travel on these ships instantly kills all passengers but then conveniently resurrects them once they reach the destination.

Re: Highly relativistic spaceflight would be fatal for passengers and instruments

#63

Post by talmand »

ksmiley wrote: post #60
BjoernKW wrote: post #33

I can't help thinking that statements like these are merely meant to bog us down. I've heard and read statements like 'Interstellar travel isn't possible.' or my favourite 'We've enough problems down here on Earth already so let's forget about the more visionary ones altogether.' since the eighties. I mean, why does one become a scientist if one doesn't at least have some greater vision? This kind of statement seems…

I agree that both of the statements you gave are not productive. "Interstellar travel isn't possible" rejects all theories past and future without considering their individual merits, which is awfully arrogant. "We have enough problems down here on Earth" is short-sighted, in terms of a geological timescale. In a billion years, our most pressing problem on Earth will be that the expanding Sun will kill all terrestria…

"X isn't possible" statements have always held a special interest for me. To me it is that person stating that they, or possibly they think we, know everything there is to know about whatever the subject is they are describing. Many of the scientific advances from the past 100 years that we take for granted today were "impossible" 200 years ago.

As for the "enough problems here on Earth" statements, I always say that most likely the solutions to many of those problems will be solved directly, or indirectly, by visionaries chasing the dream of traveling to the stars. More than likely anyone who says they don't want to spend money on space exploration because of "problems" here is just saying they want to spend the money on their own preferred project, whether there's a benefit or not.

Re: Highly relativistic spaceflight would be fatal for passengers and instruments

#64

Post by Evbn »

noonespecial wrote: post #23
jessriedel wrote: post #22

Earlier quoted context omitted.

In short, because the Milky Way disk sets a preferred rest frame. Other matter in the galaxy all rotates with approximately the same speed around the galactic center. Here are some numbers to give you intuition. Tangential speed of the galactic disk, obtained by averaging over nearby stars: ~ 250 km/s Speed of sun (and other nearby stars, in random directions) with respect to the galactic disk: ~ 15 km/s Speed of Ear…

Ahh. Understood. Most enlightening. Thanks for that.

Accelerating relativistically greatly increases mass, so enlightening is indeed the safe way to avoid these problems.

Re: Highly relativistic spaceflight would be fatal for passengers and instruments

#65

Post by salimmadjd »

Strilanc wrote: post #58
salimmadjd wrote: post #7

I think once we are advanced enough to figure out how to propel anything that fast or able to pack the needed energy, this part might be easy to solve. Remember per special relativity mass increases as we approach C. Also I think there will be other physiological problems while accelerating to C. It would take like 34 days at 10g acceleration to get to C. I wonder how our bodies would handle 34 days at that kind of g…

You need to learn more about relativity. First, people on the ship will not feel as if they "have more mass". Their physical properties will be exactly identical, because they are at rest in their frame of reference. Flying through a galaxy at 0.999c is identical to "sitting still" while a galaxy flies by at 0.999c. Second, you're ignoring relativistic effects in your calculation of the amount of time it takes to "re…

>>Flying through a galaxy at 0.999c is identical to "sitting still" while a galaxy flies by at 0.999c.spaceship.

You're right if you were sitting in a train and watched the station go by, but not at .999c. You're ignoring relativistic mass.

Unlike General Relativity where we were able to measure the effects of sun's gravity against the position of emitted light from the stars, The truth is we have no empirical evidence of the effects on solid objects (not single accelerated particles) approaching C.

Re: Highly relativistic spaceflight would be fatal for passengers and instruments

#66

Post by salimmadjd »

schiffern wrote: post #21
salimmadjd wrote: post #7

I think once we are advanced enough to figure out how to propel anything that fast or able to pack the needed energy, this part might be easy to solve. Remember per special relativity mass increases as we approach C. Also I think there will be other physiological problems while accelerating to C. It would take like 34 days at 10g acceleration to get to C. I wonder how our bodies would handle 34 days at that kind of g…

>It would take like 34 days at 10g acceleration to get to C. So why not take 340 days at 1g? There are many problems with high relativistic travel. This is not one of them.

>So why not take 340 days at 1g?

Sure! My reasoning was why add 2 years just to accelerate and decelerate. And as I said, "once we are advanced enough to figure out how to propel anything that fast or able to pack the needed energy" there were other challenges to overcome.

Re: Highly relativistic spaceflight would be fatal for passengers and instruments

#67

Post by monochromatic »

jessriedel wrote: post #22
noonespecial wrote: post #12

There's one thing I never quite got about these speculations. When you consider a ship leaving a solar system and accelerating, and the speed gained turning the "ambient hydrogen" into deadly radiation, why does it always seem to assume that hydrogen atoms are just kind of hanging out at low velocities relative to the home solar system and only turn into a problem when the bald captain says "engage"? Its a big univer…

In short, because the Milky Way disk sets a preferred rest frame. Other matter in the galaxy all rotates with approximately the same speed around the galactic center. Here are some numbers to give you intuition. Tangential speed of the galactic disk, obtained by averaging over nearby stars: ~ 250 km/s Speed of sun (and other nearby stars, in random directions) with respect to the galactic disk: ~ 15 km/s Speed of Ear…

The other take-home message is that all of the speeds in question are small relative to c. So trying to go with the flow won't get you anywhere very fast.

Re: Highly relativistic spaceflight would be fatal for passengers and instruments

#68

Post by noonespecial »

Nursie wrote: post #38
noonespecial wrote: post #12

There's one thing I never quite got about these speculations. When you consider a ship leaving a solar system and accelerating, and the speed gained turning the "ambient hydrogen" into deadly radiation, why does it always seem to assume that hydrogen atoms are just kind of hanging out at low velocities relative to the home solar system and only turn into a problem when the bald captain says "engage"? Its a big univer…

'a few rouge hydrogen atoms' It's rogue. Sorry, pet hate, can't help myself. That and people typing 'tounge' when they mean tongue.

Yes, sorry. I'm severely dyslexic. There's a limit to mechanical spelling help.

I meant the particles that are more like Han Solo and less like Darth Maul.

Re: Highly relativistic spaceflight would be fatal for passengers and instruments

#69

Post by Strilanc »

salimmadjd wrote: post #65
Strilanc wrote: post #58

Earlier quoted context omitted.

You need to learn more about relativity. First, people on the ship will not feel as if they "have more mass". Their physical properties will be exactly identical, because they are at rest in their frame of reference. Flying through a galaxy at 0.999c is identical to "sitting still" while a galaxy flies by at 0.999c. Second, you're ignoring relativistic effects in your calculation of the amount of time it takes to "re…

>>Flying through a galaxy at 0.999c is identical to "sitting still" while a galaxy flies by at 0.999c.spaceship. You're right if you were sitting in a train and watched the station go by, but not at .999c. You're ignoring relativistic mass. Unlike General Relativity where we were able to measure the effects of sun's gravity against the position of emitted light from the stars, The truth is we have no empirical eviden…

Read the Wikipedia article on mass in special relativity (http://en.wikipedia.org/wiki/Mass_in_special_relativity).

The summary of the article mentions that, in the mass' frame of reference, relativistic mass is the same as rest mass:

> As seen from the center of momentum frame, the relativistic mass is also the invariant [rest] mass [...]

Actually, the whole idea of relativistic mass is misleading to intuition. It makes more sense to think of relativistic energy. This is mentioned near the end of the article:

> Many contemporary authors such as Taylor and Wheeler avoid using the concept of relativistic mass altogether: > > > "The concept of "relativistic mass" is subject to misunderstanding. That's why we don't use it. First, it applies the name mass - belonging to the magnitude of a 4-vector - to a very different concept, the time component of a 4-vector. Second, it makes increase of energy of an object with velocity or momentum appear to be connected with some change in internal structure of the object. In reality, the increase of energy with velocity originates not in the object but in the geometric properties of spacetime itself."[6]

Remember: the laws of physics are invariant with respect to absolute velocity. If you add 5 m/s along some direction to all velocities, all the same interactions will occur. Relativity does not break this invariant.

Re: Highly relativistic spaceflight would be fatal for passengers and instruments

#70

Post by juiceandjuice »

linuxhansl wrote: post #24

There's also the risk of hitting small dust particles, etc. A quick back of the envelope calculation reveals that hitting a spec of dust that weighs just 1 ug (a millions of a gram) at 90% c produces ~36 mega joule. Not a huge amount of energy (about the energy of burning 1kg of coal), but still. Hitting a grain of sand (10mg) at 90% c produces ~360 giga joule (energy of a small lightning)

You are making a fundamental error by assuming classical kinetics. At .9c, your first interactions would be electronic (ionization), then nuclear and radiative. The complete and initial ionization of all the atoms of 10mg of, say carbon, would be a lot less than 360 GJ. Then you'd have different interactions depending on your hull material, magnetic shielding/Bremsstrahlung losses, etc...

With some complicated magnetic fields, you could probably do something smart to take advantage the different charge/mass ratio of an ionized atom and positron and minimize interaction with a steady/confined positronic cloud. You might be able to annihilate enough electrons of the dust particle to ionize it enough to subsequently deflect it from the ship. Then you'd still have to worry about gamma ray radiation and back pressure and maybe some other interactions.

I don't really know about all of this for sure, just a thought.

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