# Relativity and space travel - what's the problem?

1. May 10, 2004

### billy_boy_999

it seems to me that relativity (or at least the constant, impassable c) has had a stifling effect on science fiction and on our conceptualizations of space travel...i think the popular notion is that, because nothing can move faster than c, even with very advanced technology most of the galaxy is unreachable...

but this is all bull***t is it not? the speed of light is only impassable from another intertial reference frame...time and mass make the accomodation to maintain the appearance of an unbreachable speed of light...

in other words, it is perfectly possible for a space ship to cover a distance of 10 light years (as measured from earth) in what he feels is only one year...all he needs is constant acceleration, since there is no drag in space he doesn't have to worry about any 'speed barriers'...in fact, with constant acceleration, his time lengthens and space shrinks, i can't see that there would be any limit to how far in the universe he can travel in a certain amount of time...again, we watch him from earth and judge that he is taking longer (his clock ticks slower) and our constant and impassable 'c' is maintained...

so then, really the only obstacles that an interstellar traveller faces are a) a source of constant acceleration, and b) the willingness to say a last 'goodbye' to family and relatives as time dilation will mean he travels forward in time relative to the earth...

what is the problem here?

2. May 10, 2004

### chroot

Staff Emeritus
As a traveler approaches the speed of light, he will actually measure, by his own watch, that it only takes milliseconds to be anywhere in the universe. In the limit as he approaches the speed of light, it takes zero time to be anywhere. You could say that "time stops" aboard the ship.

The problem, of course, is that it's a one-way ticket. If you came back to Earth, you'd find that it had aged billions of years in the meantime.

- Warren

3. May 11, 2004

### billy_boy_999

right, i'm not denying that billions of years is not a big deal, i'm just saying that there is a common misconception that things like settling other star systems, for instance, is distance-prohibitive when in fact it's nothing of the kind...

and, speaking for myself, if i had the chance to gambol around the universe and settle some strange new space colony, i would be willing to say goodbye to everyone i know...and wouldn't it be neat to see what happened to the dear old earth in 10 billion years anyway?

4. May 11, 2004

### DW

I don't know about that. In 10 billion years you'd likely be thrown in a zoo as a lower life form as reckoned by whatever neosapien race dominates the planet if it even still exists as a living planet at that time.

5. May 11, 2004

### yogi

Remember also - the traveler does not get the benefit of living longer and enjoying a longer life - when he returns he will have eaten the same number of meals and enjoyed the same amount of time in contemplating physics problems as if he stayed home - his advantage is that he gets to see a lot more of the universe during his 75 year lifetime - it isn't much different than being frozen for a long trip - you don't get much enjoyment in an ice box - but when your thawed out you can explore some new world .

6. May 11, 2004

### Janus

Staff Emeritus
But is is distance prohibitive, but not from a time stand point, but an energy one. Before you even can close to that 10 billion years to 1 ratio you would need to use up the equivalent of the Solar system in mass for fuel.

7. May 11, 2004

### arildno

So what?
Since we won't see our folks again, can't we just use'm for fuel along with the rest of the solar system?

8. May 11, 2004

### billy_boy_999

janus, you're probably right that it is distance prohibitive at the moment from fuel...but surely appropriate fuels can be developed...cold fusion would be nice, or even more efficient use of solar energy...

9. May 11, 2004

### Staff: Mentor

To get even a single digit percentage of C in a spaceship with people on it will require a propulsion system unlike any yet concieved. 1% C is 6.7 million miles an hour or about 380x orbital velocity. Getting a human to Alpha Centuari before he dies (much less return him to earth) would require at least 10% C. Current propulsion technology is a good 3-5 orders of magnitude below what is required for interstellar travel. IMO, no amount of technology will make it possible: there just isn't anything available with the energy required to do it.

10. May 11, 2004

### arildno

11. May 11, 2004

### Janus

Staff Emeritus
Even with a fusion drive you would have to burn a mass of approx. 1/5 that of the Moon for every kilogram you would want to deliver at another star system , traveling at only .25c!

12. May 11, 2004

### billy_boy_999

if we, on the earth, were watching this spacecraft, then yes, i'm not checking your calculations but since the craft's speed cannot exceed c, apparent time and mass dilation are in effect...

but this mass increase is relative! it doesn't affect the ship as its own inertial reference point...if a ship can maintain a constant acceleration of, say, 9.8m/s^2 then it can continue speeding up in relation to the earth...if relativity holds, then there is no point at which - within the ship's inertial reference frame - acceleration becomes more of a burden...

am i misinterpreting einstein's theory?

13. May 11, 2004

### chroot

Staff Emeritus
Maybe you should check his calculations.

- Warren

14. May 12, 2004

### Nereid

Staff Emeritus
Reduces the energy requirement - for propulsion anyway. All sorts of other problems of course, but perhaps more to do with maintaining a stable 'on-board' society.

In addition to the problems to do with a source of propulsion energy for a relativistic interstellar manned ship, there's the small matter of avoiding collisions. While the density of interstellar dust grains is low - at least in our solar neighbourhood - it's not zero; how to protect the ship and inhabitants against a collision with milligram dust grain?

15. May 12, 2004

### Staff: Mentor

Thanks. I soooo hate doing the math myself, I was hoping someone would do it for me..

16. May 12, 2004

### billy_boy_999

russ and janus - i don't see how these figures apply at all! they have nothing to do with the inertial reference frame of the travelling spaceship...they have to do with our earthbound observers watching that spaceship and using the equivalence of mass and energy and the pliability of time as a spacial dimension to maintain a local, observed constant of c...that's all...

if i've misinterpreted einstein's theory, please tell me how!

nereid has pointed out something i think is more relevant...

first of all, this addresses a fundamental challenge to the philosophical implications of the principle of relativity...the cosmic microwave background radiation issues a similar challenge...if we can make quantitative measurements of increased incidents of exposure to 'cosmic dust' and measurements of motion in relation to the tiny assymetries of the background radiation, then does the principle of relativity hold?

the practical implications to high-speed space travel are even more salient - at very high speeds, say in relation to an earthbound measurement of distance over a spacebound measurement of time giving 10x the speed of light, tiny dust particles (possibly even neutrinos, if they have mass) would become so massive that there would need to be some sort of shield - perhaps this is the technological requirement that is farthest beyond our abilities at the moment...

Last edited: May 12, 2004
17. May 12, 2004

Staff Emeritus
Yes, you have misinterpreted Einstein. Relativity doesn't give anybody a free ride (well, until you get into wormholes and Alcubierre warps!). Your space ship has to accelerate up to its speed - in this case c/4. To do that it has to throw mass out the back and rely on Newton's reaction force. There is no other way known to produce acceleration in empty space. No the Lorentz tranformation doesn't give you any help on the mass. This has nothing to do with "fuel" incidentally, it is reaction mass. And to get to c/4 requires a reaction mass of about 1/5 the mass of the moon (according to Janus' calculation, which I haven't checked).

18. May 12, 2004

### billy_boy_999

are you telling me that the faster you accelerate, the more proportional mass (in rocket fuel, or whatever) you need to further accelerate? this completely contradicts the principle of relativity! if we could figure out how much more mass we now need to maintain a constant acceleration if we are travelling at high speed we could easily calculate our absolute motion!

the increase in mass is a relative effect! e=mc^2 and time dilation balance the books to maintain both a constant c and an apparently impassable c...

19. May 12, 2004

### Janus

Staff Emeritus
The problem is like this:

Imagine that you have a rocket with A 1 kg payload, and it takes 1 kg of fuel/reaction mass to give it a delta v of 1 m/s. How much fuel would it take to get it up to 2 m/s? 2 kg ? No, more like 3.

To understand why, think about the state of the rocket at the point when it is 1 m/s short of its 2 m/s final velocity. It consists of the 1kg payload and the 1 kg of fuel needed to give the payload that extra 1 m/s of velocity. If it takes 1 kg of fuel to accelerate 1kg 1 m/s, it will take 2 kg of fuel to accelerate the above 2kg up to 1 m/s. giving a total of 3 kg of fuel you need to start with.

To get up to 3 m/s, you need the 3kg of fuel to accelerate the last 2 m/s, and 4 more kg to acclerated this fuel and payload the first 1 m/s. This gives a total of 7 kg of fuel.

Following the same pattern, it takes 15 kg of fuel to reach 4 m/s, 31 for 5 m/s, 63 for 6 m/s, etc. Notice that the fuel usage goes up almost exponentially.

This example gives you a rough idea what you are up against. And that is just 1/2 of the problem. In a situation where you want to deliver a spaceship to another star system for example, you are going to have to slow down when you get there. So you must provide the fuel to accelerate the fuel you are going to use to slow down

Thus, using the 5 m/s figures, It will take 31 kg of fuel to come to rest at the end of your voyage, which means you will have to accelerate 32 kg of mass up to 5 m/s to start. At 31 kg of fuel per kg, this means that you need a total of 992 kg of fuel to deliver 1kg of payload to its destination.

Generally, the higher the velocity of your rocket exhaust, the better this fuel to payload ratio will be.

20. May 12, 2004

### billy_boy_999

yes, of course, janus, i perfectly understand the difficulties regarding fuel...but you're not addressing my point about relative speed...suppose we ignore the fuel difficulty, we have a future technology that allows us constant acceleration without fuel, say cold fusion or elf-power, doesn't matter...at what point in our acceleration curve does the energy we need to put in to maintain our uniform acceleration become greater than it was? at what point do we need more elves? we don't, if the principle of relativity holds...if ever we found we were going too fast and needed more elves, the elves could say "blimey, we are now going at 7-elf speed" - which is a measure of absolute motion...

i'm saying that if we had a source of propulsion that gave a certain mass (the mass of our spaceship) a constant acceleration there would be no speed limit and no local mass dilation...

21. May 13, 2004

Staff Emeritus
i'm saying that if we had a source of propulsion that gave a certain mass (the mass of our spaceship) a constant acceleration there would be no speed limit and no local mass dilation...

That would be a one-way force: F = ma. Violates the conservation of momentum. Elves indeed! It would only work in OZ.

22. May 13, 2004

### Staff: Mentor

The energy you need to put in increases exponentially all the time. The point at which it becomes noticeabe is around 10% C (IIRC). Even using 25% C as the example though, time dilation hasn't given you much back (time hasn't slowed for you all that much).
Its not about technology. Its the laws of physics you're disregarding here. The only way to accelerate a mass is through Newtonian action-reaction (note: that "something" can be light energy, it doesn't have to be mass).
'Cosmic dust' is a phenomenon local to our section of the galaxy and would only imply a preferred reference frame for calculating trajectories in our galaxy, not a universal preferred reference frame.

The fact that you can calculate a velocity with the respect to the CMB doesn't necessarily imply a preferred frame either: Relativity simply says that the laws of physics work the same in all frames. A preferred frame would be one where the laws of physics worked differently. The main reason people want to overturn Relativity (it seems to me) is they don't like the idea that C is constant, but time is not. The goal of ether theories is to find a frame in which time is constant and C is not. The various tortured permutations of ether theory attempt to show that so far every time we've measured the speed of light, we've measured it in that frame and that by getting outside that frame (with a light speed experiment performed in space for example), we'd see that C really is only constant in the ether.
That seem like the opposite of what relativity says. I'm not sure where you are getting the idea from except maybe f=ma. F=ma quite simply doesn't apply at relativistic speeds. A particle accelerator (for example) accelerates a constant mass particle (an electron carries no fuel) to a high fraction of C in our reference frame and the energy required to accelerate it is easily measurable. From the perspective of the electron, it is also moving at a high fraction of C, but time and space have dilated (the particle accelerator is smaller).

Last edited: May 13, 2004
23. May 13, 2004

### Staff: Mentor

Ok, here's the one we can give him though: Using something like solar sails or ground-based laser propulsion, you can do Newtonian acceleration without carrying fuel. In that case, the energy is just e=1/2 mv^2. With a velocity of 75 million m/s, thats still a lot of zeros on your energy level.

24. May 13, 2004

### arildno

Just a few questions:
1. I have gleaned from popular (non-scientific) sources that "vacuum" should not be regarded as "nothing" from a quantum mechanical perspective but rather as a "seething soup" of particles which do not quite succeed in remaining in stable forms over time, but "pop in and out of existence" (by transforming into other types of particles or something(?))
Is this, scientifically speaking, an unreasonably inaccurate popularization?
2.
I've heard some rave about the possibility of harnessing vacuum energy.
Are there, today, any indications that this type of "harnessing" is in fact, possible, and if so, will this harnessing give a net energy gain for the harvester (a spaceship, for example)?
(If I have to use more energy to extract the energy from the vacuum I want to use as "fuel", this method seems rather futile..)

25. May 13, 2004

### billy_boy_999

hi russ, setting aside for a moment your thoughts on CMBR etc.,
are we still talking about spaceships here? or are we talking about electrons in particle accelerators? of course if we accelerate a particle we (the observers) are stationary, as is the particle accelerator (in relation to the particle)...as such, the faster that thing goes the more energy we have to put into accelerating it...but this is not the same as what the particle experiences!

exactly, space and time have changed for the particle, not for us...if that electron is measuring its speed against the speed of light it will measure its speed to be zero, so will everything...but if it measures its speed using the distance we give it for the circumference of the accelerator and using the time it itself measures to cover that distance it can certainly breach the speed of light...

as well, the particle's mass has not increased in its own inertial reference frame...is this wrong? if this is wrong please explain how this doesn't contradict the principle of relativity! (our increased mass is directly proportional to a measurable absolute motion velocity - this is unacceptable!)

Last edited: May 13, 2004