Interstellar travel is possible?

From a French Web site

Yes! Proof, I offer here a very clever solution, which uses only current physical knowledge.

The problem of interstellar travelThe stars are very far! The nearest star (other than the Sun!), Alpha Centauri, is 4.3 light years from us, ie it is so far away that its light takes more than four years to reach us. Expressed in kilometers, this represents forty trillion kilometers, or 107 million times the Earth-Moon distance, or was about two hundred thousand times the distance between Earth and Mars. And other stars are even further ...

Thus the space probe Voyager 2 (which is now, after having been accelerated by Jupiter, the object fastest manufactured by mankind), its maximum speed of 40 km / s, 32 000 years would reach Alpha Centauri .Firstly, it does not move toward it .... And from any nearby star actually.

To get to travel to a nearby star, say 10 light years in humanly reasonable times, say twenty years should be an average speed of about half that of light, ie 150 000 km / s or 540 million km / h. (I neglect here the relativistic effects, that is to say that I consider 20 years of earth time. For the occupants, the journey would actually be slightly reduced, 17.3 years)

Relativistic effects

Indeed, a time interval delta.t (read: "delta-t") measured by a clock in the vessel corresponds to a time interval ₀ delta.t measured on land using the formula:

due à Lorentz. due to Lorentz. In this formula, v is the speed of the vessel, and c is the speed of light.For a vessel that would run half the speed of light, v ² / c ² = 1 / 4 and root factor ... It was so 20 years (on earth) x 0.866 = 17.3 years (in the Navy)

That said, even if this reduces the time travelled perceived by occupants, it is not very interesting for us earthlings, because what interests us is the ability to send a spacecraft to another star in an acceptable time for us !

The top speed of rockets: Tsiolkovsky formula

But back on topic. So, can we reach such speeds with rockets (and why not with atomic rockets)?

As strange as it sounds, the maximum speed that can reach a rocket, only depends on two factors: speed Ve gas exhaust (or particles) emitted by the nozzle, and the relationship between the initial mass mid ship and saw mf his final mass on arrival. Well, yes, whatever the engine architecture, cycle selected thrust, fuel, or no use of nuclear energy, only these three factors are needed to determine the "delta V", the increase maximum speed that can reach a rocket from its initial velocity (0 in general). Tsiolkovsky's formula (which dates from 1903) gives us this delta V:

Delta_V = Ve. Log (mi / mf)

For example for a chemical rocket that ejects its gas to 3 km / s (Ve), and whose mass is made up of 99% by fuel (so far is to know how), the final mass is more than 1% of the initial mass, and we delta_V = 3 x Log (100) = 13.81 km / s. only 50 000 km / h! A little better than Apollo. This is not the way to go in the stars!

For any fan of maths, this formula is calculated very simply from the principle of action and reaction, from the equation dv = V dm / m, it suffices to integrate.

So a rocket to go faster, there are only two solutions (one can do both at once for that matter!)

• • It happens to increase V, ie that increases the pressure in the nozzle and therefore the energy required, for example by finding a better fuel

  • Ratio is increased or mid / mf, which amounts to a ship which is a large tank (something like our current rockets, for that matter)

Good, but then with a rocket atomic is what we can gain? Yes, but only slightly: it can use nuclear energy in effect to further speed up the gas output. Hopefully with atomic rockets reach Ve = 10 km / s, but is still insufficient for our needs. Another alternative is that ion engines: these engines not pop around, but charged particles (ions), which are much faster (29 km / s or 290 km / s for the project VASIMR). But it is far from the 50 000 km / s we would have to go to the stars within a reasonable time ....

There is however a way out of the framework imposed by Konstantin Tsiolkovsky: raise fuel open space. It's possible with what is called Ramjet, vessels that catch using powerful magnetic fields that surround the particles and use them to propel themselves. The first of these ships was designed (on paper) by Robert Bussard in 1975. To capture enough particles in the almost empty interstellar ramjet one needs a huge magnetic collector, covering thousands of square kilometers. I do not explore this path over here, but I give you a drawing, it looks nice:

The trip to constant acceleration

The writers of science fiction are often also the limitations of Tsiolkovsky (After all, it is their right), and they sometimes describe vessels (were not saying how) accelerates at an acceleration of 1g (the equivalent of Earth's gravity) for months. Now as all know the students and science students, 1 g = 9.81 ms ^-2 (read: 9.81 meters per second per second), meaning that such a vessel has a speed s' increases of 9.81 m / s every second.To arrive at half the speed of light, such a vessel would thus a time of 177 days (and ten times longer if the acceleration is one-tenth of g, or roughly 1 ms ^-2) could be say that it is sufficient to double the time to reach the speed of light ... In reality, the calculation should involve relativity. It is a bit complicated, but we can find the details here (the site of Benoit).

In summary: The final speed of a ship of rest mass m ₀ F undergoing extensive (thus accelerating "classic" F / m ₀₎ for a time t (measured in the vessel) will be:

We see that this speed can not exceed the speed of light (c), regardless of t.

From the perspective of the starting point (the Earth!), To get to a distance x, such a vessel will put a time

. . For example to get to Alpha Centauri (x = 4 10 ¹⁶ m) with an acceleration of 1 g (m ₀ / F = 1 / 9, 81 = 0,102), the calculation "classic" gives t = sqrt (2 x m ₀ / F) = 1045 days.But the relativistic calculation above gives 1864 days, which is much longer. From the perspective of land, the ship "punishment" to accelerate as it approaches a nearly the speed of light. In fact, its inertia does increase at these speeds! Note that our ship moving at speed off to Alpha Centauri, and that he would actually stop in mid-term if it intends to explore its potential planetary system ...

Delusions

They appeal to new physics. Wormholes, ships that carry with them their own space-time, use of parallel universes ... All this is perhaps not so delusional than it looks, but it's not for tomorrow, and maybe forever!No, we need something more solid.So what? Let us look at a technology that was not designed to go in the stars but with some nudges and a few ideas ....

LightSail Ships

The solar sail spacecraft are to be propelled by the pressure of sunlight. It already exists, and the idea to organize a solar sail race to Mars is "in the air."The solar sail vehicle is a typical large (1000 km2 for example) and very light (typically a few tens of kilograms). it consists mainly of a large "sail" of a few microns thick, usually mylar. The "radiation pressure" from the sun pushes the sail with a force very low but steady and guiding it properly it could go anywhere in the solar system (and even closer to the sun than the Earth: it Just slow down the speed of rotation around the sun).I can not resist the pleasure to give you the detailed calculation of the force experienced by a solar sail:The momentum p of a photon energy hv, so the wavelength λ = c / ν on the sail is p = hv / c = h / λTo simplify calculations, suppose that the sun emits not quite a rainbow of colors, but only the yellow-green (λ = 500 nm). It was so p = 1.32 10 ^{-27 -1} and kg.ms energy Ep = hv a photon will be 3.957 10 ^-19 J

Our sun has a total capacity of 3.9 Es x10 ²⁶ W. It emits therefore np = Es / Ep = 9.85 ⁱⁿ October 1944 photons per second. Now the Earth is 149 million kilometers from the sun. All these photons are distributed on a sphere, therefore the same radius R, whose surface is S = 4πR ² 2.79 10 ²³ m ^2. What makes us end up with a density d = np / S = 3.53 on October ²¹ photons per square meter per second. But each photon carries a tiny pulse (p above). The impulse received by one square meter solar sail will be i = pd = 4.66 10 ^-6 kg.m ^-1. S ^-1 In other words, a veil of 1m ^{2 and} weighing 4.66 milligrams will accelerate 1 ms ^-2, about one tenth of the g. If we want to accelerate a mass of 1 kg with the same acceleration, the sail area to be weaned by 1 / i = 214 456 m2, a square of 463 m side (and therefore weighing 1kg!). It requires extremely lightweight materials, but it is not utopian.

You can learn to navigate the solar sail here .

Interstellar Laser sailboat

Obviously, if you want to use a solar sail to travel to another star system, there is a problem: solar energy decreases gradually as one moves away from the sun ... and the acceleration becomes completely negligible. Caramba, Alone!But all is not lost: you can replace the sun! Imagine that we build on land (or better: on the moon) a very powerful laser and very directional. Is the beam of laser that will push our ship to the stars!Come a little: Suppose that our laser has a power W (say 100 megawatts to fix ideas) Suppose it is fairly well focused to water continually 100% of the sail even when the vessel will be light years away from the soil (calculation shows that we should install a focusing lens of nearly 1km in diameter somewhere between Mars and Jupiter. But theoretically possible). Our laser emits therefore n = W / Ep = 2.5 October ²⁶ photons per second. But unlike those emitted by the sun, all these photons will be received by the sail! This represents a luminous flux hundred thousand times brighter than the sun. The total impulse received by the sail is then i = pn = 0.33 kg.ms ^-1. In other words, if our boat weighs 330 grams in total, it will accelerate at 1 ms ^-2, about one tenth of g, which is not so bad. With a thousand times more powerful laser, we can accelerate a 330 kg probe to the stars!In this age of miniaturization advanced, we can accommodate many things in such a mass. But within two decades, in the era of nanotechnology, 330 kg will be more than it takes to stay in the probe, in addition to the sail and payload, something even more useful: a .. .cannon.

What? A gun?

But not just any gun, a cannon (electromagnetic railgun) capable of firing a projectile tiny, the size of a grain of sand at extreme speed. But what the hell is that going to serve?

Let us remember: the laser accelerates interstellar yacht . But he can not slow down. Our yacht will spend a few hours in the system of the target star. What a pity!

if the gun shoots back his tiny projectile at a speed almost equal to that of the vessel, the projectile in question will find himself at once stopped. It will then slowly drop onto the planet in the system. As it is small, it will not have time to warm up too when traversing the atmosphere of this planet.It will therefore be almost intact on its soil, and there.

Let's take a closer look on this little grain of sand. Nanotechnology is a jewel: it consists of a shell very durable which includes a core asset: a replicating assembler.

Miniaturization: Nanotechnology replicants and robots

An assembler is a replicating device extremely small, consisting of hundreds of millions of atoms only. This device contains a complete computer, and a set of "tweezers" that can manipulate individual atoms. All permits, under the control of the computer, making any object (a very small object anyway) ... including a copy of itself. Then the two assemblers in turn make two copies each. Then four of eight, sixteen ... In no time there are millions and billions, destroying the shell around them and continue to play outdoors, using local materials. When their total mass reaches a ton (which takes less than 24 hours), computers that control ever changing program, and assemblers are beginning together to make ... Including vehicles that can explore the planet, a giant radio antenna that will send the results of exploration to Earth, and if necessary will receive new orders.humanity has a mission of exploration of the automatic system of the target star!

Return

But we can do better: ask the assembler to produce a new laser, which will slow down another solar sail from the Earth! This time, instead of bringing a tiny grain of sand in the system of the target star is a world we can ship to drop!

The mission as I moved

To take heavier payloads to the stars, however, and especially to take a manned spacecraft, weighing hundreds of tons, solar sails are insufficient. The laser power required would be too huge. But let us remember the formula of Tsiolkovsky. A laser on Earth (or Moon) could provide the energy needed to ship the engine to accelerate the expenses that it projects rearwardly and thus achieve fantastic Fr. There is even need nanotechnology to do so!

Conclusion: the stars are within our reach now!

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