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**Is Intergalactic space travel possible?**

The Andromeda Galaxy is around 2.5 million light years away – a distance so large that even with the speed of light at traveling as 300,000,000m/s it has taken 2.5 million years for that light to arrive. The question is, would it ever be possible for a journey to the Andromeda Galaxy to be completed in a human lifetime? With the speed of light a universal speed limit, it would be reasonable to argue that no journey greater than around 100 light years would be possible in the lifespan of a human – but this remarkably is not the case. We’re going to show that a journey to Andromeda would indeed be possible within a human lifespan. All that’s needed (!) is a rocket which is able to achieve constant acceleration and we can arrive with plenty of time to spare.

**Time dilation**

To understand how this is possible, we need to understand that as the speed of the journey increases, then time dilation becomes an important factor. The faster the rocket is traveling the greater the discrepancy between the internal body clock of the astronaut on the rocket and an observer on Earth. Let’s see how that works in practice by using the above equation.

Here we have

t(T): The time elapsed from the perspective of an observer on Earth

T: The time elapsed from the perspective of an astronaut on the rocket

c: The speed of light approx 300,000,000 m/s

a: The constant acceleration we assume for our rocket. For this example we will take a = 9.81 m/s^{2} which is the same as the gravity experienced on Earth. This would be the most natural for a human environment. The acceleration is measured relative to an inert observer.

Sinh(x): This is the hyperbolic sine function which can be defined as:

We should note that all our units are in meters, seconds and m/s^{2} therefore when the astronaut experiences 1 year passing on this rocket, we first need to convert this to seconds: 1 year = 60 x 60 x 24 x 365 = 31,536,000 seconds. Therefore T = 31,536,000 and:

which would give us the time experienced on Earth in seconds, therefore by dividing by (60 x 60 x 24 x 365) we can arrive at the time experienced on Earth in years:

Using either Desmos or Wolfram Alpha this gives an answer of 1.187. This means that 1 year experienced on the rocket is experienced as 1.19 years on Earth. Now we have our formula we can easily calculate other values. Two years is:

which gives an answer of 3.755 years. So 2 years on the rocket is experienced as 3.76 years on Earth. As we carry on with the calculations, and as we see the full effects of time dilation we get some startling results:

After 10 years on the space craft, civilization on Earth has advanced (or not) 15,000 years. After 20 years on the rocket, 445,000,000 years have passed on Earth and after 30 years 13,500,000,000,000 years, which around 1000 times greater than the age of the Universe post Big Bang. So, as we can see, time is no longer a great concern.

**Distance travelled**

Next let’s look at how far we can reach from Earth. This is given by the following equation:

Here we have

x(T): The distance travelled from Earth

T, c and a as before.

Cosh(x): This is the hyperbolic cosine function which can be defined as:

Again we note that we are measuring in meters and seconds. Therefore to find the distance travelled in one year we convert 1 year to seconds as before:

Next we note that this will give an answer in meters, so we can convert to light years by dividing by 9.461×10^{15}

Again using Wolfram Alpha or Desmos we find that after one year the spacecraft will be 0.563 light years from Earth. After two years we have:

which gives us 2.91 light years from Earth. Calculating the next values gives us the following table:

We can see that as our spacecraft approaches the speed of light, we will travel the expected number of light years as measured by an observer on Earth.

So we can see that we would easily reach the Andromeda Galaxy within 20 years on a spacecraft and could have spanned the size of the observable universe within 30 years. Now, all we need is to build a spaceship capable of constant acceleration, and resolve how a human body could cope with such forces and we’re there!

**How likely is this?**

Well, the technology needed to build a spacecraft capable of constant acceleration to get close to light speed is not yet available – but there are lots of interesting ideas about how these could be designed in theory. One of the most popular ideas is to make a “solar sail” – which would collect light from the Sun (or any future nearby stars) to propel it along on its journey. Another alternative would be a laser sail – which rather than relying on the Sun, would receive pin-point laser beams from the Earth.

Equally we are a long way from being able to send humans – much more likely is that the future of spaceflight will be carried out by machines. Scientists have suggested that if the spacecraft payload was around 1 gram (say either a miniaturized robot or digital data depending on the mission’s aim), a solar sail or laser sail could be feasibly built which would be sufficient to achieve 25% the speed of light.

NASA have begun launching continuous acceleration spacecraft powered by the Sun. In 2018 they launched the Near-Earth Asteroid Scout. This will unfurl a solar sail and be propelled to a speed of 28,600 m/s. Whilst this is a long way from near-light speeds, it is a proof of concept and does show one potential way that interstellar travel could be achieved.

You can read more about the current scientific advances on solar sails here, and some more on the mathematics of space travel here.

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