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#1
Posted 08 May 2011 - 09:07 AM

“One, remember to look up at the stars and not down at your feet. Two, never give up work. Work gives you meaning and purpose and life is empty without it. Three, if you are lucky enough to find love, remember it is there and don't throw it away.”

**Stephen Hawking**

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#2
Posted 08 May 2011 - 09:46 AM

~Jon

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#3
Posted 08 May 2011 - 10:36 AM

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#4
Posted 08 May 2011 - 01:50 PM

- wjfox, Shimmy and ddmkm122 like this

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#5
Posted 08 May 2011 - 02:11 PM

We make our world significant by the courage of our questions and the depth of our answers. - Carl Sagan

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#6
Posted 08 May 2011 - 05:21 PM

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#7
Posted 08 May 2011 - 08:12 PM

We make our world significant by the courage of our questions and the depth of our answers. - Carl Sagan

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#8
Posted 08 May 2011 - 09:38 PM

There is some circumstantial evidence proving that we have been "visited" in the past. Most of it points to alien visitations but who's to say that it wasn't humans from the future?I actually hope that backwards timetravel is impossible.

It probably is since there is no evidence of future beings now.

Though, it may still be possible, but you cannot go farther back than the time machine was created. Which is fairly funny.

http://www.history.c...s-image-gallery

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#9
Posted 16 May 2011 - 11:32 AM

This relates to the concept I posted earlier, I believe, limited by the fact the time machine would be a 'wormhole' between two points in spacetime and therefore could only connect to the point in time and space when it was opened.Though, it may still be possible, but you cannot go farther back than the time machine was created. Which is fairly funny.

Whatever time is and whatever our perception of it is, whether it's related to the way we interpret or observe it, there is no doubt that there seems to be an arrow of time through which entropy in the universe goes in one direction, regardless of the fact there's no framework to dictate that 'time' must pass 'forwards' or 'backwards'. Time Travel into the past is probably always going to remain impossible, even the idea of a wormhole being created, with one end taken into the future via relativity would allow someone to come back and cause a paradox to occur within the time that falls between each side?

~Jon

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#10
Posted 16 May 2011 - 03:24 PM

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#11
Posted 17 May 2011 - 01:15 AM

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#12
Posted 17 May 2011 - 03:19 AM

There are several theoretical methods of backward time travel -- Caiman's being the most feasible -- however, humanity would need almost 500 years of unhindered scientific growth at singularity rates to attain even one of them. We're talking about the very brink of possibility here.

As for future time travel, relativity dictates that all we need to go forward x years is a device which can approach the speed of light for y amount of time (y<<x). In other words, if you travel at, say, 99% the speed of light for 1 year, you'll "skip" (actually, speed through) the normal time experienced by everyone else and come out, to stationary observers, several years later (don't feel like doing the actual equation). My guess is we'll develop dark matter engines capable of such a feat in 100-150 years.

Just out of interest why would you guess dark matter engines? We still don't really have any real understanding of what it is, and the only way we know it reacts is by letting its mass mess up our gravity calculations. What possible reason would we have to assume it would make an efficient engine?

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#13
Posted 17 May 2011 - 07:22 AM

There are several theoretical methods of backward time travel -- Caiman's being the most feasible -- however, humanity would need almost 500 years of unhindered scientific growth at singularity rates to attain even one of them. We're talking about the very brink of possibility here.

As for future time travel, relativity dictates that all we need to go forward x years is a device which can approach the speed of light for y amount of time (y<<x). In other words, if you travel at, say, 99% the speed of light for 1 year, you'll "skip" (actually, speed through) the normal time experienced by everyone else and come out, to stationary observers, several years later (don't feel like doing the actual equation). My guess is we'll develop dark matter engines capable of such a feat in 100-150 years.

Just out of interest why would you guess dark matter engines? We still don't really have any real understanding of what it is, and the only way we know it reacts is by letting its mass mess up our gravity calculations. What possible reason would we have to assume it would make an efficient engine?

Yeah, sorry, I meant antimatter.

As for why antimatter could work:

The collision of matter with anti-matter is, as far as I know, the only work source which provides 100% efficient energy, and this source could, theoretically,be linked to an engine and used as a means of propulsion. A system of this nature could potentially provide enough energy to achieve speeds of .99c or greater.

I would be very surprised if fusion were powerful enough, and I think most other forms of energy will be too inefficient.

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#14
Posted 17 May 2011 - 08:23 AM

~Jon

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#15
Posted 17 May 2011 - 07:48 PM

My understanding was that technically even our current rockets could reach .99c... you could get there in just a few years by accelerating constantly at 1g. The issue isn't the fuel itself, but rather, the amount of fuel you have to carry, which means it takes more fuel to maintain acceleration, ad infinitum. The same would be true whether you're using chemicals or anti-matter? Sure, the energy output of anti-matter is much more efficient than any chemical reaction and so you'd need significantly less to achieve the same results, but unless we discover a totally unprecedented source of the stuff, it'll always be energy inefficient to create. Then you have to consider how we convert the energy created into something that can actually power an engine.

Our current rockets wouldn't work. As speed approaches relativistic levels, the mass of the ship increases greatly (moving mass = (mass at rest)/(sqrt(1-(v^2)/(c^2)))). As the mass continues to increase, the energy required to maintain acceleration will become enormous. Only antimatter collisions can create enough work fast enough to reach such a speed in under a few decades.

You're right, it is inefficient to create right now, but we don't know what kind of methods we might develop in the future. Then again, at that point, with all the energy we will be able to harness from other sources, we may be willing to take the net loss purely for the sake of rocket speed.

Now, it may be possible to construct a fusion engine with such a capability, but at this point it really isn't certain.

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#16
Posted 18 May 2011 - 07:55 PM

I think the point that Caiman makes still stands though? It's not necessarily that our current chemically powered rockets wouldn't work, it'd just be incredibly impractical and difficult to carry/convert enough fuel to maintain the acceleration as the speed increases. Mass increases exponentially as one approaches light speed regardless of whether you're carrying chemical or anti-matter/matter fuel, either way. Perhaps we'll give nuclear pulse a go...Our current rockets wouldn't work. As speed approaches relativistic levels, the mass of the ship increases greatly (moving mass = (mass at rest)/(sqrt(1-(v^2)/(c^2)))). As the mass continues to increase, the energy required to maintain acceleration will become enormous. Only antimatter collisions can create enough work fast enough to reach such a speed in under a few decades.

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#17
Posted 18 May 2011 - 11:39 PM

- Caiman likes this

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#18
Posted 18 May 2011 - 11:53 PM

http://math.ucr.edu/.../SR/rocket.html

I've quoted the relevant section. Happy reading

How much fuel is needed?

Sadly there are a few technical difficulties you will have to overcome before you can head off into space. One is to create your propulsion system and generate the fuel. The most efficient theoretical way to propel the rocket is to use a "photon drive". It would convert mass to photons or other massless particles which shoot out the back. Perhaps this may even be technically feasible if we ever produce an antimatter-driven "graser" (gamma ray laser).

Remember that energy is equivalent to mass, so provided mass can be converted to 100% radiation by means of matter-antimatter annihilation, we just want to find the mass M of the fuel required to accelerate the payload m. The answer is most easily worked out by conservation of energy and momentum.

First: conservation of energy

The total energy before blast-off is (in the Earth frame)

Einitial = (M+m)c2

At the end of the trip the fuel has all been converted to light with energy EL, so the total energy is now

Efinal = γmc2 + EL

By conservation of energy these must be equal, so here is our first conservation equation:

(M+m)c2 = γmc2 + EL ........ (1)

Second: conservation of momentum

The total momentum before blast-off is zero in the Earth frame.

pinitial = 0

At the trip's end the fuel has all been converted to light with momentum of magnitude EL/c, but in the opposite direction to the rocket. So the final momentum is

pfinal = γ mv - EL/c

By conservation of momentum these must be equal, so our second conservation equation is:

0 = γ mv - EL/c ........ (2)

Eliminating EL from equations (1) and (2) gives

(M+m)c2 - γmc2 = γmvc

so that the fuel:payload ratio is

M/m = γ(1 + v/c) - 1

This equation is true irrespective of how the ship accelerates to velocity v, but if it accelerates at constant rate a then

M/m = γ(1 + v/c) - 1

= cosh(aT/c)[ 1 + tanh(aT/c) ] - 1

= exp(aT/c) - 1

How much fuel is this? The next chart shows the amount of fuel needed (M) for every kilogramme of payload (m=1 kg).

d Not stopping, sailing past: M

4.3 ly Nearest star 10 kg

27 ly Vega 57 kg

30,000 ly Center of our galaxy 62 tonnes

2,000,000 ly Andromeda galaxy 4,100 tonnes

This is a lot of fuel—and remember, we are using a motor that is 100% efficient!

What if we prefer to stop at the destination? We accelerate to the half way point at 1g and then immediately switch the direction of our rocket so that we now decelerate at 1g for the rest of second half of the trip. The calculations here are just a little more involved since the trip is now in two distinct halves (and the equations at the top assume a positive acceleration only). Even so, the answer turns out to have exactly the same form: M/m = exp(aT/c) - 1, except that the proper time T is now almost twice as large as for the non-stop case, since the slowing-down rocket is losing the ageing benefits of relativistic speed. This dramatically increases the amount of fuel needed:

d Stopping at: M

4.3 ly Nearest star 38 kg

27 ly Vega 886 kg

30,000 ly Center of our galaxy 955,000 tonnes

2,000,000 ly Andromeda galaxy 4.2 thousand million tonnes

Compare these numbers to the previous case: they are hugely different! Why should that be? Let's take the case of Laurel and Hardy, two astronauts travelling to Vega. Laurel speeds past without stopping, and so only needs 57 kg of fuel for every 1 kg of payload. Hardy wishes to stop at Vega, and so needs 886 kg of fuel for every 1 kg of payload. Laurel takes almost 28 Earth years for the trip, while Hardy takes 29 Earth years. (They both take roughly the same amount of Earth time because they are both travelling close to speed c for most of the journey.) They travel neck-and-neck until they've both gone half way to Vega, at which point Hardy begins to decelerate.

It's useful to think of the problem in terms of relativistic mass, since this is what each rocket motor "feels" as it strives to maintain a 1g acceleration or deceleration. The relativistic mass of each traveller's rocket is continually decreasing throughout their trip (since it's being converted to exhaust energy). It turns out that at the half way point, Laurel's total relativistic mass (for fuel plus payload) is about 28m, and from here until the trip's end, this relativistic mass only decreases by a tiny amount, so that Laurel's rocket needs to do very little work. So at the halfway point his fuel:payload ratio turns out to be about 1.

For Hardy, things are different. He needs to decrease his relativistic mass to m at the end where he is to stop. If his rocket's total relativistic mass at the halfway point were the same as Laurel's (28m), with a fuel:payload ratio of 1, Hardy would need to decrease the relativistic mass all the way down to m at the end, which would require more fuel than Laurel had needed. But Hardy wouldn't have this much fuel on board—unless he ensures that he takes it with him initially. This extra fuel that he must carry from the start becomes more payload (a lot more), which needs yet more fuel again to carry that. So suddenly his fuel requirement has increased enormously. It turns out that at the half way point, all this extra fuel gives Hardy's rocket a total relativistic mass of about 442m, and his fuel:payload ratio turns out to be about 29.

Another way of looking at this odd situation is that both travellers know that they must take fuel on board initially to push them at 1g for the total trip time. They don't care about what's happening outside. In that case, Laurel travels for 28 Earth years but ages just 3.9 years, while Hardy travels for 29 Earth years but ages 6.6 years. So Hardy has had to sit at his controls and burn his rocket for almost twice as long as Laurel, and that has required more fuel, with even more fuel required because of the fuel-becomes-payload situation that we mentioned above.

This fuel-becomes-payload problem is well known in the space programme: part of the reason the Saturn V moon rocket was so big was because it needed yet more fuel just to carry the fuel it was already carrying.

Other fuel options

Well, this is probably all just too much fuel to contemplate. There are a limited number of solutions that don't violate energy-momentum conservation or require hypothetical entities such as tachyons or worm holes.

It may be possible to scoop up hydrogen as the rocket goes through space, using fusion to drive the rocket. This would have big benefits because the fuel would not have to be carried along from the start. Another possibility would be to push the rocket away using an Earth-bound grazer directed onto the back of the rocket. There are a few extra technical difficulties but expect NASA to start looking at the possibilities soon :-).

You might also consider using a large rotating black hole as a gravitational catapult but it would have to be very big to avoid the rocket being torn apart by tidal forces or spun at high angular velocity. If there is a black hole at the centre of the Milky Way, as some astronomers think, then perhaps if you can get that far, you can use this effect to shoot you off to the next galaxy.

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.

~Jon

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#19
Posted 23 May 2011 - 07:44 PM

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#20
Posted 23 May 2011 - 11:08 PM

### Also tagged with one or more of these keywords: time travel

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