Why can't we go to the center of the galaxy?

[billy_boy_999]

why aren't we spending more money on developing nuclear-powered rockets?

an interesting article about nuclear rocket propulsion

"...Nuclear-reactor rockets, like the ones that would be used in the Bimodal Nuclear Thermal Rocket, conduct nuclear fission reactions -- the same kind employed at nuclear power plants -- in which uranium atoms are split apart, releasing tremendous volumes of energy. In a nuclear thermal rocket, this energy is used to heat hydrogen propellant, which is stored aboard the rocket as liquid in supercooled fuel tanks..."

they're talking about creating fuel to travel to Mars but wouldn't it be merely an extrapolation to design an even more powerful rocket capable of traveling swiftly to the center of the galaxy? time dilation would be an inevitable hazard but i don't think that should prevent us from traveling a long way in space...maybe our space ships could even search around for nice planets to colonize...or look for extra-terrestrial civilizations in the fertile center of the galaxy...

i don't get why were not working harder at this...

 

[Grizzlycomet]

The center of the galaxy is very, very far away. Even if we could create a sattelite that traveled at the speed of light, it would take thousands of years to get there.

 

[billy_boy_999]

The center of the galaxy is very, very far away. Even if we could create a sattelite that traveled at the speed of light, it would take thousands of years to get there.

the "speed of light" relative to what? to earth? relative to earth, yes it would take a very long time but remember time dilation and the principle of relativity...as far as the crew of the spaceship is concerned there is no speed limit...

the time it would take to get to the center of the galaxy is only limited by the gradient of acceleration...the gradient of acceleration is only limited by the thrust of the rockets and the mass of the ship, if we can make nuclear thrust technology very efficient i think the journey is a perfectly plausible one...

 

[Janus]

To travel to the center of the Galaxy at 1g of acceleration (accelerate for half the distance and decelerate for the remainder) would take 12 yrs by ship time.
(and more than 33000 yrs by Earth time.)

Even if you had a ship that used matter-antimatter conversion to produce pure photons directed backwards at 100% efficiency for your reaction mass, (the absolute limit of efficiency for propulsion) You would have to start off with over 8 billion kg of matter and antimatter for every kg you want to deliver to the center of the galaxy.

Consider that the command/service modules of the Apollo mission massed 28,000 kg and only had to support 3 men for about 8 days, and you can imagine how large a ship you would need to support a crew for 12 years (Even with recycling of resources).

Multiply that by 8 billion and you have a rough estimate of how much matter-antimatter you'll need.

Now consider the fact that 100% efficiency is not attainable. If you drop it down to 90% then you need 101 billion kg per kg payload.

Also consider that half of that has to be antimatter, and that antimatter doesn't exist naturally in any significant amount. It would have to be made, and would take more energy to make it than you would get out of it.

The cost in natural resources would be astronomical, and this is for the most efficient propulsion system possible.

Remember, it is the stay at homes that have to expend these resources, and they couldn't even know if the mission succeeded or not for over 66,000 yrs, let alone derive any benefit from it.

 

[cragwolf]

To travel to the center of the Galaxy at 1g of acceleration (accelerate for half the distance and decelerate for the remainder) would take 12 yrs by ship time.
(and more than 33000 yrs by Earth time.)

What distance to the centre of the Galaxy did you use to calculate this time? Most estimates range around 8 kpc. If you input that value into the equations for a rocket accelerating at 1g for half the distance (and then decelerating at 1g for the remainder) then you should get approximately 19.25 years, at least according to my calculations. I might be wrong, though.

 

[Chronos]

at near light-speed it would take 50,000 years to reach the center of the galaxy, by local time. the crew may not age much, but, they would be waiting a very long time to hear back from mission control once there.

 

[Lonewolf]

the "speed of light" relative to what? to earth?

The speed of light is a constant relative to anything. That's one of the postulates of special relativity.

as far as the crew of the spaceship is concerned there is no speed limit...

Yes there is; the speed of light limits the speed at which they can travel.

 

[kokain]

If we went to the center of the universe, or even far away, it would not benefit anyone. No one would be able to follow, no message could be sent or recieved. It would be a waste of time and money.

 

[Integral]

If you consider the numbers given by Janus you will see that there is insufficient mass in the solar system to power this journey. Such a trip is pretty much impossible according to the current state of Physics.

 

[kokain]

Janus's answer to every question is the same. Not enough fuel. Then we need to find a different way to power ships. The question was "why don't we" not "can we". We can't do any of the things in most of these posts, but it is fun to think about.

 

[Integral]

Some of us are constrained by the physical universe we live in. Others are not, thus we have science fiction.

 

[meemoe_uk]

12 years to travel to the centre which is thousands of light years away? are you sure.. if so...
that means as far as the travelers are concerned, they are moving towards the centre faster than light. It's a funny bit of SR. first you get your head round the idea that nothing can travel faster than light so you'd think it'd take thousands of years even from the travels point of view, then you realize space contraction effect cancels it out so things can appear to travel faster than light, in this case, the centre.

Does this case work for any old particle flying towards the earth?

 

[kokain]

There is most likely not a center of the universe

 

[billy_boy_999]

couple things...

janus - I'm not following your calculations - are you taking relativistic mass dilation into account?


Lonewolf -

The speed of light is a constant relative to anything. That's one of the postulates of special relativity...the speed of light limits the speed at which they can travel.

forgive me but this is baloney, your two statements are contradictory...

in a sense, the speed of light does indeed limit your speed, if you measure you're speed against the speed of the light traveling past you, your speed will always be 0...but this has nothing to do with your speed relative to the Earth or relative to the centre of the galaxy...if there was something limiting your speed then that would mean there is an absolute reference frame against which to measure your absolute motion - completely in violation of the principle of relativity!

or think of it like this - if your mass increased in proportion to your speed relative to an observer on the earth, then the Earth would be the center of all absolute motion in the universe! the mass increase is a relative effect, it applies to the observers on the earth, not to the spaceship...

kokain - as far as this hypothetical trip not being of any benefit i beg to disagree...the people on the spaceship would certainly derive some benefit...whether or not we earthlings would be willing to undertake such a massive effort knowing we could not gauge its success is an interesting sociological question...if we could find a way to send a dozen spaceships to the center of the galaxy would we not feel some satisfaction that we have quite possibly furthered the seed of the human race? it's interesting to think about...


meemoe_uk -

...that means as far as the travelers are concerned, they are moving towards the centre faster than light. It's a funny bit of SR. first you get your head round the idea that nothing can travel faster than light so you'd think it'd take thousands of years even from the travels point of view, then you realize space contraction effect cancels it out so things can appear to travel faster than light...

exactly, many people misunderstand the limits SR applies to space travel...as long as the travelers are measuring distance from the Earth and time from the spaceship, there is no limit to how fast they can travel...of course now we are using measurements from two different frames so a purist can say - well, you're not really going faster than light at all...in fact, if you then measure your speed as a fraction of local light speed, you will find that you're not actually in motion at all...

the old 'mass increases as you approach the speed of light' routine only works when you are observing an object in a different inertial reference frame, so it applies when people on Earth are watching our very fast ship as well as when scientists are watching very fast particles in accelerators...

 

[Janus]

What distance to the centre of the Galaxy did you use to calculate this time? Most estimates range around 8 kpc. If you input that value into the equations for a rocket accelerating at 1g for half the distance (and then decelerating at 1g for the remainder) then you should get approximately 19.25 years, at least according to my calculations. I might be wrong, though.

Your right, I went back a re-checked my time calculation, and I get an answer in the same range as yours , I used a slighty larger number for distance, which gave me an answer of 20 yrs.) The MR calculation still checks out okay.

 

[Janus]

couple things...

janus - I'm not following your calculations - are you taking relativistic mass dilation into account?

I'm using the Relativistic Rocket Equations as found at:

http://www.ksc.nasa.gov/facts/faq04.html

Which take time and length dilation.

These actually help you.

If you were to do the same calculation using the non-relativisitic equations for distance, time and acceleration, and plug the same exhaust velocity(c) into the classic rocket equation, you find that the same trip at 1g would take 242 yrs and use more more fuel/reaction mass than exists in the entire visible universe. ( something in the order of 10¹⁷⁸ kg of reaction mass for every kg of payload.

 

[billy_boy_999]

janus - absolutely, time and length effects are critical...i was just checking because there was some confusion on the SR, GR forum about whether or not to apply mass dilation to the inertial reference frame of the ship...

the numbers are very interesting...a 20 year journey could take you to the center of our galaxy, 1g acceleration/deceleration, but you'd need something like 8 x 10^9 kg of fuel-mass even for 100% energy conversion...

if we are talking about a journey of that duration/speed we cannot obviously store enough mass in a rocket...but perhaps we could convert spacebound energy into fuel, stellar radiation, cosmic rays, vacuum energy...vacuum energy seems a bit far-fetched...cosmic rays have a lot of kinetic energy but probably too sparse and too difficult to harness, stellar radiation would be a boundless energy supply if we could figure out how to use it efficiently...cold fusion of waste supplies would be handy...

even with all of this, perhaps a 20 year journey is indeed unrealistic...perhaps we should be thinking about generational ships...or, firstly, how about closer destinations - alpha centauri would be nice, i here it's nice in the summer, if a little touristy...

i wonder how long it would take us to travel to alpha centauri using nuclear thermal rocket technology...

 

[energia]

travelling 20 lightyears using rocket propulsion is out of the question
rocket propulsion can never achieve the relativistic velocities needed

it will also never work using nuclear propulsion

nor is it possible with any current or near term propulsion technology

not even matter/antimatter anihilation would be enough (as a source of thrust)

however matter/antimatter anihilation as an energy source to drive an electromagnetic coil large enough to warp space is a possibility

 

[Janus]

if we are talking about a journey of that duration/speed we cannot obviously store enough mass in a rocket...but perhaps we could convert spacebound energy into fuel, stellar radiation, cosmic rays, vacuum energy...vacuum energy seems a bit far-fetched...cosmic rays have a lot of kinetic energy but probably too sparse and too difficult to harness, stellar radiation would be a boundless energy supply if we could figure out how to use it efficiently...cold fusion of waste supplies would be handy...

All these sources are too dilute to provide any kind of thrust worth mentioning.

There is one possible energy source you could make use of: interstellar hydrogen. The idea is to scoop up the hydrogen (possibly with a large magnetic field), funnel it to the ship, fuse it, and exhaust it out the back. The good part is that the faster you go, the more hydrogen you collect.

This idea was first suggested a few decades ago and is called a "Bussard ramjet".

The down side is that all that hydrogen you are scooping up creates drag on your ship, eventually the drag equals the thrust of your engine and you quit accelerating. Since nuclear fusion doesn't generate exhaust velocities greater than 10% of c, this is pretty much the limit of how fast a Bussard ramjet can go.

 

[billy_boy_999]

interesting, Janus, thank you for your illuminative reply, as always...the hydrogen collection idea is fascinating..

one thing i don't understand though...

...Since nuclear fusion doesn't generate exhaust velocities greater than 10% of c...

were you trying to suggest that 10% of c is the speed at which hydrogen 'drag' counteracts thrust to further acceleration? or are you suggesting that the fusion rocket's thrust itself is limited by a speed?

energia -

travelling 20 lightyears using rocket propulsion is out of the question

why? please be more specific - perhaps you meant to indicate a time frame as well? or do you have a different take altogether?

rocket propulsion can never achieve the relativistic velocities needed

this follows logically from our argument if you're talking about conventional fuelled propulsion but could you be a little more specific?

it will also never work using nuclear propulsion

why exactly would that never work? perhaps some figures might help - are you still talking about a journey of 20ly? in what time frame? what are the limitations? (i'm sure there are limitations to nuclear propulsion but your statements don't elucidate them very well)

nor is it possible with any current or near term propulsion technology

i don't see that this statement really means anything in particular...

not even matter/antimatter anihilation would be enough (as a source of thrust)

again, you're going to have to be a lot more specific if we are going to be able to judge the merits of your arguments...

 

[Janus]

interesting, Janus, thank you for your illuminative reply, as always...the hydrogen collection idea is fascinating..

one thing i don't understand though...


were you trying to suggest that 10% of c is the speed at which hydrogen 'drag' counteracts thrust to further acceleration? or are you suggesting that the fusion rocket's thrust itself is limited by a speed?

A limit on the exhaust velocity. The most efficient way to use fusion in a rocket is thus:

You allow the hydrogen to fuse creating a vast amount of kinetic energy. This kinetic energy is displayed by the motion of Helium formed by the fusion process, This helium is now dead weight, so you use it as your reaction mass.

Fusion converts about 1% of the hydrogen's mass to energy. This is the energy available to "throw the helium backwards" Considering the mass of the helium molecules to the energy generated you get a top exhaust velocity of 10% of c.

 

[billy_boy_999]

okay, Janus...i'm still having trouble with the 'exhaust velocity' idea...what does thrust have to do with velocity? if a hydrogen fusion engine can give a certain amount of thrust and can continue firing with the same energy indefinitely then the ship will continue accelerating...what prevents the ship from continuing to accelerate past 10%c?

and also, just to clarify, when you say 10%c, you are talking about an earthbound observer's measurements?

thanks again...

 

[Doc Al]

exhaust velocity

and also, just to clarify, when you say 10%c, you are talking about an earthbound observer's measurements?

Exhaust velocity is with respect to the rocket.

 

[energia]

you can find answers to a lot of your questions at NASA's http://www.lerc.nasa.gov/WWW/PAO/warp.htm site

go through all the links and read the various proposals
and you will see that relativistic speeds needed for distances of even a few light years are nothing more than a dream by current or even near term propulsion technology

 

[Janus]

okay, Janus...i'm still having trouble with the 'exhaust velocity' idea...what does thrust have to do with velocity? if a hydrogen fusion engine can give a certain amount of thrust and can continue firing with the same energy indefinitely then the ship will continue accelerating...what prevents the ship from continuing to accelerate past 10%c?

and also, just to clarify, when you say 10%c, you are talking about an earthbound observer's measurements?

thanks again...

With a standard rocket where you carry your fuel with you it doesn't. What the exhaust velocity does control is the mass ratio of fuel to payload. For the lower the exhaust velocity, the higher this ratio will be, and the more fuel you need to reach any given velocity.

With the ramjet however, where you gather your fuel from surrounding space, it does present a limit on ship velocity.

Imagine a pair of hydrogen atoms hitting your collector at .1 c. they will have a certain momentum with respect to your ship that it will have to absorb, this subtracts form your ship's forward momentum.

The hydrogen is fused releasing energy that shoots a helium atom out the exhaust at 0.1c, producing forward momentum for your ship. But the helium has less mass than the two hydrogen atoms, so the forward momentum they produce is less than the momentum the ship lost in collecting the hydrogen.

Thus the ships slows down until it the momentum absorbed by each pair of hydrogen atoms equals the momentum of a helium atom traveling at 0.1c. It then just mantains this velocity.

Now in real life, not all of the energy produced by the fusion process is going to be used in generating exhaust. Some will be used to maintain the magentic "scoop", some for life support, and some will be lost as waste heat. So you will have to nudge even that 10% figure down.

At 10% c, relativistic effects only amount to a 2% difference in elapsed Earth time and elapsed ship time. (You just shave 2 1/2 months off for every 10 lys traveled.)

 

[Växan]

At 10% c, relativistic effects only amount to a 2% difference in elapsed Earth time and elapsed ship time. (You just shave 2 1/2 months off for every 10 lys traveled.)

it's even worse than that..

at 14% c you have only a 1% time dilation (99,49874371% true time)

here is my handy Geometrodynamics Table as a reference

which includes the Formula for calculating time dilation

at 10% c there is only a 0,5% time dilation (half of 1%) so you only save 433 seconds per day of travel (more accurately - 0,50125629% time dilation)

so 1 day would equal 85966,91 seconds rather than 86400 s

a relative calendar year would equal 31398770,7931 seconds
rather than 31556952 seconds - (158181,2069 seconds less)

one would save: 1 day - 19 hours - 56 minutes - 0,35 seconds per year of travel

so in relativistic terms 10% c is not much faster than a school bus

to make matters even worse - the Bussard Fusion Ramjet must accelerate to 6% c - using conventional impulse propulsion before the Ramjet can even be engaged

a spacecraft with humans onboard can only accelerate at 1g for long durations - and accelerating at 1g (9,80665 m/s/s) would require 21 days 5 hours 30 min to reach 6% c - and 35 days 9 hours to reach 10% c

a 40 TW Fusion Ramjet would be lucky to achieve 50% efficiency
so it's doubtful that even 10% c would be reached using this method of propulsion


(the maximum period of time a spaceship can accelerate at 1 g is 30570322 seconds - or 353 days 19 hours 45 minutes 22 seconds - at 30570323 seconds the spaceship would reach c - which is impossible according to The Special Theory


the only proposed propulsion system capable of velocites approaching c
is warp drive - which expands the space behind the spaceship whilst compressing the space in the direction of travel

incidentally this is the propulsion system used in the Star Trek series

using this method of propusion - theoretically - it would be possible to achieve the effect of FTL velocity without actually reaching c

 

[Stingray]

Your ship would probably get destroyed from collisions with random space junk and highly blueshifted radiation.

 

[meteor]

John Baez gives a duration of the trip to the center of the galaxy (first acelerating at 1g, and then at the middle of the trip decelerating by the same amount) of 21 years for the people of the ship. Well, I think that I'm nitpicking here
http://www2.corepower.com:8080/~relfaq/rocket.html
Surprisingly using the same technique, the voyage to Andromeda galaxy is only of 29 years for the people of the ship!. Suddenly, Andromeda galaxy doesn't seem so far away

 

[billy_boy_999]

Doc Al - how can the exhaust velocity be 10% purely in respect to the rocket? ...10% c relative to the rocket and what else? relative to the local speed of light? that would contradict the michelson-morley experiments for sure...

Janus - okay, it sounds like you are talking about a drag effect here that limits velocity, countering the momentum of 'stationary' hydrogen ...this is very interesting in that it offers some practical limitations to the intra-galactic relativity principle, i.e. what is our absolute motion in relation to the interstellar hydrogen?

Växan -

(the maximum period of time a spaceship can accelerate at 1 g is 30570322 seconds - or 353 days 19 hours 45 minutes 22 seconds - at 30570323 seconds the spaceship would reach c - which is impossible according to The Special Theory

don't forget that you're now involving the reference frame of a 'stationary' observer...this limitation has no effect on the inertial reference frame of our traveling spaceship where time dilation and the shrinking of space accommodate the constant speed of light...my point was, to the passengers on board the ship there is no speed limit in relativity, the only speed limit is set by the limitations of fuel for acceleration...

meteor -

Surprisingly using the same technique, the voyage to Andromeda galaxy is only of 29 years for the people of the ship!. Suddenly, Andromeda galaxy doesn't seem so far away

exactly...the article you gave was a little vague about the practical matters of fuel/acceleration but the point that relativity really sets no speed limit within a discrete inertial reference frame is an important one i think...

 

[Doc Al]

Doc Al - how can the exhaust velocity be 10% purely in respect to the rocket? ...10% c relative to the rocket and what else? relative to the local speed of light? that would contradict the michelson-morley experiments for sure...

I really don't understand your question. Why is there an issue with an exhaust velocity of 0.1 c? Exhaust velocity is the speed at which fuel is ejected--relative to the rocket. I don't know what you mean by "purely in respect to the rocket". I don't know what you mean by "and what else". What do you mean "relative to the local speed of light"? And what is the connection with Michelson-Morley?

Can rephrase your question?

(You're not using your personal definition of speed again, are you?)

 

[MonstersFromTheId]

This is an incredibly interesting conversation. But a few q's from a complete neophyte here if you don't mind.
Three actually.
1) Even in REALLY deep DEEP interstellar space, the "vacuum" of space is no "vacuum" right? I mean there are ALWAYS at least SOME stray molecules floating around in every cubic foot of space right?
So even if the "vacuum" of space looks like a "vacuum" at mach 18, at .1c wouldn't the "vacuum" of space start to look a lot more like a kind of soup?
2) Wouldn't hitting just random molecules along the way essentially irradiate the living piss out of any potential crew?
3) What would happen if you tried to tear ass through the Sagittarius Nebula at something like .1c? Wouldn't that wind up being a bit like diving head first into a shotgun blast?

 

[selfAdjoint]

You are describing the "relativistic ramjet" idea. This was popular a couple of decades ago but detailed studies showed that the drag would exceed the reaction generation. So we're back to carrying your reaction mass along.

Of course there are "new physics" (i.e. crank) proposals all the time. "Vacuum propellers" or coupling to the zero point energy was one. And there are always the weird solutions of Einstein's field equations, like wormholes and the Alcubiere warp drive.

 

[Nereid]

This is an incredibly interesting conversation. But a few q's from a complete neophyte here if you don't mind.
Three actually.
1) Even in REALLY deep DEEP interstellar space, the "vacuum" of space is no "vacuum" right? I mean there are ALWAYS at least SOME stray molecules floating around in every cubic foot of space right?
So even if the "vacuum" of space looks like a "vacuum" at mach 18, at .1c wouldn't the "vacuum" of space start to look a lot more like a kind of soup?
2) Wouldn't hitting just random molecules along the way essentially irradiate the living piss out of any potential crew?
3) What would happen if you tried to tear ass through the Sagittarius Nebula at something like .1c? Wouldn't that wind up being a bit like diving head first into a shotgun blast?

The ISM is (mostly) a plasma, so, being charged, there is a way - in principle! - to reduce the multiple hazards of colliding with lots of ions and electrons (really intense and well-designed magnetic fields). However, this doesn't work so well for neutral particles.

I'm sure all readers realize that denizens of the ISS, Shuttle, etc are doing a real-life experiment with the effects of impacts with relativistic particles ... right now. Cosmic rays are comprised of both neutral (few) and charged particles (mostly protons), and the Sun emits copious quantities of them too (though not really 'relativistic'). We here on the surface of the Earth are being hit in the head by ~6 CRs per second (IIRC); effect? And that's with a few hundred kms of air.

I suspect the infrequent collision with micron-sized ISM dust grains would be far more hazardous than even the ISM neutral gas at 0.1c :tongue2:

Anyone like to suggest a back-of-the-envelope calculation? (It's not all that difficult)

 

[russ_watters]

No one addressed this one...

12 years to travel to the centre which is thousands of light years away? are you sure.. if so...
that means as far as the travelers are concerned, they are moving towards the centre faster than light. It's a funny bit of SR. first you get your head round the idea that nothing can travel faster than light so you'd think it'd take thousands of years even from the travels point of view, then you realize space contraction effect cancels it out so things can appear to travel faster than light, in this case, the centre.

Does this case work for any old particle flying towards the earth?

Not quite. The length contraction cancels out time dilation in such a that they would not percieve themselves travleing faster than C. They wouldn't travel 33,000 ly in 20 years, but rather they'd travel 17 ly (guess) in 20 years (according to them), never exceeding C.

 

[billy_boy_999]

DOc Al - okay, i think i understand the 'exhaust velocity' idea a bit better, but i don't see how the exhaust velocity would limit accrued speed, it simply limits the gradient of acceleration...

nereid - very interesting, could we really design a magnetic 'shield' to neutralize charged particles? what about the dust grains? would there be any (theoretical) way to use the energy and rest mass of these dust grains to our advantage in the same way that we can funnel interstellar hydrogen into our (again, theoretical) fusion drive?

russ - this is a very important point, length contraction and time dilation adjust themselves in such a way that no one can ever measure his speed to be greater than c...but i think, too often, the popular impression is one that ignores length contraction, and time dilation, and insists on using simple earth-based measurements of distance to prohibit travel to the rest of the galaxy...within the bounds of relativity there is no limit to where you can travel in a certain amount of time...