[FrancescM (OP)]

Hi Naked Scientists,
Is it possible to establish two-way communications from Earth to a spaceship travelling at near the speed of light?
Say I am travelling to the recently reported exoplanet K2-18b, I would spend over 110 years on my way, while on Earth thousands of years would pass. Would it be possible to relay instructions from an Earth-based control base to the spaceship? Does it make sense at all to try to interact with an object travelling at near the speed of light using electromagnetic radiation?

Thank you!
Francesc M.

[Colin2B]

Is it possible to establish two-way communications from Earth to a spaceship travelling at near the speed of light?

It is possible, but the delay between you sending a message and receiving a reply is so great that you wont be able to have a normal conversation.
If you look at the moon landings the roundtrip delay over that short distance was 2.5s (assuming the astronaut replied instantaneously - that is manageable. By the time a spaceship is out at Alpha Centauri you will have a delay of around 8.8 years, so communication is possible, but slowly.

[Origin]

Is it possible to establish two-way communications from Earth to a spaceship travelling at near the speed of light?
Say I am travelling to the recently reported exoplanet K2-18b, I would spend over 110 years on my way, while on Earth thousands of years would pass.

If a space ship is traveling to a star 124 light years away and the ship was traveling at 99% the speed of light then it would take a little more than 125 years earth time for the ship to reach the star.  It would take about 18 years space ship time to reach the star. Communication would be kind of useless.  A message sent 2 years after the ship left would not reach the ship until it had reached the star.

[Halc]

Is it possible to establish two-way communications from Earth to a spaceship travelling at near the speed of light?
Say I am travelling to the recently reported exoplanet K2-18b, I would spend over 110 years on my way, while on Earth thousands of years would pass. Would it be possible to relay instructions from an Earth-based control base to the spaceship? Does it make sense at all to try to interact with an object travelling at near the speed of light using electromagnetic radiation?

It is quite possible to exchange messages, but as Colin points out, it takes time for the message to get there, a function of the separation of the sending of the signal and the receiving of it.  The speed is what you're asking about, and that would result in both a relativistic (small) and doppler (large) change in the frequency of the signal, but electronics can compensate for that.

K2-18b is 124 light years from Earth.  If it takes 110 subjective years to do that (your astronaut isn't going to live that long), accelerating continuously half the distance and decelerating the 2nd half, then the ship need pull only about 0.031g (3% of Earth gravity) for 55 years to reach top speed of about 0.94c.  During this time, 176 years would pass on Earth, not thousands.  Messages would, at top speed, be red shifted by a factor of about 50, or about 17x for the doppler and 3x for the dilation.  Messages on the return trip would be blue-shifted by a factor of about 6.

If you bump up the power of the ship to 1G (Earth gravity), the trip might be done before the passenger dies of old age.  At 1g, it takes 9 years 5 months to get there, 126 Earth years, with a top speed of nearly .9999c which has a dilation factor of about 65.

[Outcast]

Is it possible to establish two-way communications from Earth to a spaceship travelling at near the speed of light?Say I am travelling to the recently reported exoplanet K2-18b,

Yes, but Earth will have to do it in the dark. To accelerate even a small spaceship to near the speed of light, you will need to commandeer the entire energy of our sun...
P.S. You're gonna need to commandeer the entire energy of another star to stop you when you get there...prolly piss off the locals...LOL