# space travel

Discussion in 'Physics & Math' started by JimmyJames, Jan 18, 2002.

1. ### brucepValued Senior Member

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Probably Gene Roddenberry? The warp metric is a solution to the EFE so the concept of a warp drive is the 'child' of relativity. Miguel Alcubierre found the warp solution in the mid 1990's. After Star Trek began. LOL. The warp has amazing properties, such as the geometry of the warp spacetime can be manipulated for shielding the spacecraft. Then it has the unphysical 'thing' where the metric violates the weak energy condition requiring 'huge' negative energy to create the warp spacetime and keep it from collapsing. Because that part 'seems' like science fiction I like the relativistic rocket.

http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html

The science fiction about the relativistic rocket is how does your ship 'maintain constant acceleration g_earth' without pushing it? One idea is a sublight warp where you generate a localized gravitational field with electromagnetic charge. Where the ship would be constantly falling into a gravitational well] at whatever delta g the 'motor' is operating at. So how many nuclear reactors will we need, to tow along, to generate a big enough charge to do this [assuming the delivery technology is feasible] ? Duh, huge. Just a rough guess would be equivalent to 1/1000 the mass of the earth [hows this for some crank shit?]. Wonder what the local 'buzz' would be [LOL]. The author of the link below provides the derivation for a specific solution to the field equations which is associated with this idea. Looks good to me but what would I know. I don't worry to much about the authors conclusions since the metric component for mass, angular momentum, and charge are equivalent in GR. So what this was about is I'm not going to brush off any idea that was derived from GR. It's science fiction but there's a reason folks are still working on this stuff. I don't see the human race 'getting the will' to begin such a project, ever. Thinking we would is some other type of fiction. Still ...........?

http://www.modernrelativitysite.com/chap7.htm

3. ### ZMacZRegistered Senior Member

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Ok...I'm not gonna go quote for quote..

I bet you'd agree to an actual speed of 3000 KM/per sec ? (given that in my 'model' I'd use not just fuel for extra solar speeding up, but also some form of propulsion that would use electrical energy (hopefully) which through use of an ion thruster can use just about any mass to increase it's speed..)
Now let's see if we can get up to 300Km per second using only ion thrust...
Given that with the more mass you'd expel the smaller the entire mass becomes (of the vessel..)
But even with all that...yes..hard to do..I'd say about 1 millennium..
Given a starting mass of 10.000.000.000 kg and the vessel being constructed there in large parts..(to reduce building costs..and reduce transportation cost..)
90% of the mass being ''fuel' (ionisable substances), you'd technically be able to raise the speed of the vessel to the desired speed...
(if the ions leave the thruster at 50 km/s well..you'd already get almost 500Km/s for the entire ship....leaving 1.000.000.000 usable payload...)
With better ion thrusters...more payload....I'd say once the ion thrusters use single pulses that are being lead behind the ions over a longer trajectory..well..maybe 100Km/s can be achieved..and that would mean 10.000 Km/s for the ship at 100.000.000 usable payload..)

Oh wait..the target was 3000KM/s....

(the 0.1 C was not an actual number I had in mind..but merely used for the example..like in.."You don't NEED the speed of light to get somewhere..")

(Also....YOU MADE ME DO MATH !!!...- ish not happeh-...)

Umm... - just noticed-...this IS a math forum... : ) ....wth is space travel doing here ?

5. ### ZMacZRegistered Senior Member

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Umm..shields...

Ion shields ? (using the same principle as the ion thruster, to halt/decelerate micro meteorites..)
..which opens up possible avenues of 'fuel' reclamation..like using all that matter that needs to be diverted away as new propellant..

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8. ### originHeading towards oblivionValued Senior Member

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I had thought it was Asimov in his books from the 1940s but according to wiki one if the not the earlist references to FTL travel (jumps, hyperspace, etc.) was by John Campbell in 1931.

9. ### ZMacZRegistered Senior Member

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http://www.dailymail.co.uk/sciencet...ers-DO-work-halve-size-weight-satellites.html

And that is one way to generate some thrust without using propellant..

Combine with generational ark principle..(a spaceship that travels through space with a complete ecosystem on board..which would
also be very useful to set up shop at the destination.....grain's..bugs..flies...no mosquitoes (heh)...anaerobic bacteria...pigs..cows...cats...(heh)...
bunnies.....the whole shabang....everything you'd need to start a biosphere on any planet..and I'd make that an underground one, if it has no
atmosphere of it's own..

10. ### river

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Of course anti-gravity would be the best for space travel

That I know of

11. ### Billy TUse Sugar Cane Alcohol car FuelValued Senior Member

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Where you say: " You want to burn all the fuel you need to achieve escape velocity while you are as close to the Earth as possible."

I have question: Assume you have very large fuel mass but rocket thrust at lift off is making 1.1G - G acceleration of the rocket. (Your weight at lift off, by on board scale, is 10% more than on Earth.) Assume also that the thrust of engine never changes. Thus, as fuel mass decreases, soon you will have acceleration (relative to surface of earth) at 1.2G less what ever fraction of the earth's 1G still exist at that distance from earth. Your on board scale then indicates a 20% plus that fractional decrease in Earth's gravity increase in your weight.*

However, when getting near speed of light and far from Earth (or other gravity fields) the mass a measured from earth get larger, but I don't think the acceleration felt by the space travelering ever stops increasing nor does he think his mass (plus rocket) is increasing. I. e. the on board scale forever show an increase in the traveler' weight. Is all that correct?

I.e. near C you can experience ever greater acceleration force on your body, but not go much faster, separate faster from Earth, every hour.

* Just for clarity, assume that when the thrust has increased the acceleration of the lower fuel mass (and rocket) to 1.2G - 0.4G, which is the gravity at that distance from earth, then the on board scale shows traveler's weight as 80% greater than it was on earth.

Last edited by a moderator: Feb 12, 2015
12. ### Billy TUse Sugar Cane Alcohol car FuelValued Senior Member

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Your first link is excellent.* Following quote from it is a quite general consideration for ALL space travel at large fraction of speed of light:

"One major problem you would have to solve is the need for shielding. As you approach the speed of light you will be heading into an increasingly energetic and intense bombardment of cosmic rays and other particles. After only a few years of 1g acceleration even the cosmic background radiation is Doppler shifted into a lethal heat bath hot enough to melt all known materials."

And of course space is not empty of matter either. Most of it, far from stars, is hydrogen atoms or ions but there a dust particles too. Most of them were blasted out of asteroids by cosmic ray local heating, especially those outside the orbit of Pluto in the Kuper belt, (or from "zillions to the zillionth power" of asteroids weakly bound to other stars) where they have very low "escape velocity" from them and sun/star. If traveling at 2/3 of the speed of light (200,000,000m/s) with space craft of only 1m^2 frontal area, and about 10 hydrogen masses per cm^3 or 10,000 per m^3 with much more in a "gas cloud" like our solar system formed millions of years later; then each second at that (2/3)C you run into: 200,000,000 x 10,000 = 2E12 Hydrogen mass equivalent

The mass of hydrogen atom is: 1.6605402 x10-27 kg. With neglect of any realistic mass increase, and KE = 0.5mV^2 that is, in watts (1.66/2)E-27(2E12)x (2E8)^2 = 4x1.66E^(12+16-27) = 6.64E1 = 64.6 watts but it is NOT EVENLY SPREAD OUT. I.e. It is hitting only 10,000 very very tiny (atom sized) spots. So is blasting away many dozen (>100)atoms of your space craft with each impacting particle. Conservatively your frontal erosion rate is MORE than a million atoms per second.

* I assume the average "dust particle" has at least 9,000 times the hydrogen atom mass, as many of its atoms may be more than 9 times heavier than hydrogen, to get conservative estimate of 10^4 "H Mass equaling" per M^3 you will be running into. As most of the mass is NOT ionized there would seem to be no way to prevent it fron striking the space craft and heating it very much more than the Doppler shifted cosmic rays do.

SUMMARY: Extention of your life into the future, as in the "twin paradox," is very, very, limited, and NOT by technology, but just by fact space is not empty vacuum. Just staying on Earth, and eating a proper diet, with modest exercise, and no smoking (or getting hit by bus, etc.) will let you, on average, postpone your death date dozens of times more distant into the future than the best possible "time dilation" high-speed space travel can.
posting now to not lose - more to come.

PS: If you don't space travel by fast rocket, but could do a "space warp" that most think is not possible and even if were would require continuous negatve enegy generation at power levels greater than the sun's total power output.
But assuming all that were some how possible by "advanced technology" the nature of space would still "cook you" more than red hot.

I.e. At the edge of the "warped space" you have extreme gravity gradients. They greatly accelerate the particle flux that was already very lethal, even before that acceleration of those particles, just due to your high speed thru the >10,000 Hydrogen equivalent mass per M^3 - I.e. your "space warp" is stronger than any other man-made accelerator and worse - it accelerate neutral partice which other man made accelerators can not.
----------------
* Now for a critical comment on burcep's first link: They assume 100% efficiency in conversion of fuel mass energy into thrust energy, by mass / anti mater production of gamma rays; but actually there is very little if any thrust as that reaction must conserve momentum.

For example, if electron and positron pair annihilate, you get two 0.511MeV gamma rays traveling in opposite directions. To get net thrust, you need to stop half of them (going the "wrong direction") in a thick hemispherical shell of some heavy metal, like lead. This not only greatly adds to the space craft weight but worse, if even a tiny fraction of G net thrust is achieved, that lead would be so hot that it would be liquid, if not vapor!

Last edited by a moderator: Feb 12, 2015

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There are many many many problems we will need to overcome to achieve effective relativistic stellar travel, but two things we do have in our favour, [1] Such methodology including space-time warpage is not forbidden by the laws of physics and GR, and [2] We have plenty of optimistic hopeful reputable smart people, looking at this and other means of achieving this ultimate goal for mankind.

14. ### brucepValued Senior Member

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Most the shielding can be accomplished by modifying the spacetime. You'd need shielding for non relativistic travel in the vicinity of stellar objects.

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Oh come on, where do we travel to? Are you serious?

Now, before WARP drives and Inertial Dampeners etc, we need to focus on what we can do right now. Where could we go if enough MONEY would be put to use? Could we get to Mars in under 3 months for instance?

16. ### cosmictravelerBe kind to yourself always.Valued Senior Member

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What we should be doing right now is getting our own planet in order. With all the problems we are creating we could destroy the only planet we know that supports life as we know it. Monies spent on journeys through space are better spent here on our planet to clean up pollution, stop the killing of almost extinct animals and better educating people as to what they are doing to destroy themselves by destroying their environment.

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I agree we need to take better care of our planet, but isn't that almost totally related to space travel and science in general? Why can't we have both? If we manage to convince investors we should go to mars or wherever, can't we convince them our planet is going "boom" sooner than expected because of pollution, and the same guys who will build the ultimate spaceship will probably be dealing with the climate too? Not sure, maybe this is too optimistic but almost every scientist I know, who really cares for space exploration, is also very worried about climate change and our inevitable end as a species if we keep ignoring science? I think they are very intimately related don't you think?

Edit: Sorry for so many question marks

18. ### cosmictravelerBe kind to yourself always.Valued Senior Member

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I'm for space exploration but doing so with robots and other non human means. As we see now the robots on Mars are doing a great job 24 hours a day learning thousands of new things about the planet. Robots cost about one twentieth of the cost to send humans into space and do a better job 24 hours a day. You do not need to worry either about a space craft blowing up or robots stop working because no one gets hurt. We can send 20 different spacecraft to many different cosmic places to study them as compared to just one if humans must go. Education is paramount in teaching others how to care for the Earth and building new things that are green and do not harm our planet.

We should devote more time and energy into making Earth a better place for everything to live both humans and animals as well as vegetation. So we need to prioritize what we are doing both here on Earth and in space by seeing that without a clean vibrant Earth there won't be any space travel because we will all be gone due to our own stupidity.

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Exactly......Just take the time out to realize how beneficial the very first aspect of space exploration has been......Satellites....I mean what would we do without them now!

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All very nice, noteworthy and commendable on face value.
And I seriously do not disagree at all. But as I have suggested many times, just think of all the money world wide that could be diverted to looking after Earth as well as continuing our space endeavours, if all militaristic spending were to cease.
On face value and in this immediate moment in time that isn't going to happen just yet.
And also what isn't going to happen, is that manned space exploration is not going to stop.
Our probes have done a remarkable job, but they are not perfect ambassadors, and do not have the remarkable qualities that a human may have with regard to any specific situation that may not have been envisaged.
Irrespective, human nature being what it is, man will always need to go where he has not been before. The advent of robots will not, nor should it stop that.

We at this time have at least two reputable private orginizations devoted to furthering our advances into the solar system and beyond, both led by reputable people.
"Tau Zero" led by Marc Millis, and the "Hundred Year Star Ship"company led by Mae Jemison.
Then we have "Mars One" and "Planetary Resources"

We should indeed devote much more time and energy into fixing up our own Planet, but not at the expense of manned Space exploration.
We were not born to stagnate on this fart arse little blue orb in the outer suburbs of a spiral galaxy with anywhere between 100 and up to 400 billion other solar systems, in our small observable Universe of around a hundred billion galaxies.

As long as humanity has their collective arses pointing towards the ground, robotics and manned space explorations and ventures, will continue hand in glove and for the benefit of all mankind.

21. ### cosmictravelerBe kind to yourself always.Valued Senior Member

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Yes military spending is by far the most useless thing we have going but it does happen and until those who are in charge change the way they think we will always be endowed by them and their ilk.

While I agree with what you say you must think about humans venturing into space that has nothing to offer them in order to live outside of Earth that we know of without human help of some kind. Radiation is very dangerous and as of the moment humans don't have a way to completely protect themselves from it or other dangerous things lurking in space and other planets. That is why more robotic craft should be used to gather as much information and try new ideas before sending humans far far away.

This planet is unique in this solar system so it should be the primary thing to save and help wherever needed. If we should travel elsewhere then need to return because there isn't a place where humans can survive and return to a totally destroyed planet humans will be extinct very soon after landing if they can land.

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Of course, but in time, man will be going far far away....that's the nature of the beast. Explorers have faced many dangers before, and countered many previously unknown situations. That will always be the case, and all those dangers will be countered or allowances made.
Agreed, but this planet still has a "use by date" that even man's ingenuity will not change.
And as our Sun matures and obtains red giant status, other planets [Mars?] might then be favourable for a billion years or so.
Then we need searching among the stars, if we have managed to outwit our own eventual extinction which I don't doubt we will not achieve.