What would we see if we approach the speed of light ?

Let's embark together on a spaceship which accelerates away from Earth moving faster and faster:

1. What would we see when constantly accelerating ?
2. What Optical effects would occur and if we try to exceed light speed by bending space-time like in Sci-Fi movies ?
3. What would we see ?

Answering these questions will lead us to explore many fascinating phenomena of our universe and immerse ourselves at the heart of special and general relativity.

Take-off

Our spacecraft is initially at rest. Once launched we begin to accelerate, imagine that its propulsion system allows us to accelerate constantly as long as we wish. As we gain speed we move away from Earth. By accelerating more and more, one would think that the speed would eventually become unbearable. However this is not the case and speed has no effect on our body. To understand this, image yourself in a situation whereby two trains cross. It is impossible to feel whether it is your train or the other which is at rest. Similarly inside the spaceship we cannot feel its speed but only its acceleration, the thrust of the reactors which presses us against our seats. If this thrust is reasonable, it will be perfectly bearable all along our journey even when approaching the speed of light. There is, nonetheless, a risk associated with very high speeds “a collision”. At High Velocity, a mere Speck of dust would cause colossal damage if it hit us. Fortunately, our ship is equipped with a force field that repels dangerous objects and allows us to roam freely through space.

Aberration of light

As we accelerate, a first Optical effect appears, the stars in front of us which we get closer to seem to gradually move away and the sky contracts before us. In our spaceship, the optical effect is perfectly analogous. The stars are light comes from a certain direction but as we accelerate, the light rays appeared to come more and more from the front and their Direction seems different when we are in motion. This is the aberration of light. As light focuses in front of us its intensity increases while behind us the sky seems to widen and becomes darker. We often hear that the further we look into space the further we look into the past. Indeed, the light we receive from a distant star must travel billions of kilometers. this journey is not instantaneous and it takes some time for light to reach us, as a result we see the star as it was in the past when it emitted this light perhaps several thousand years ago who knows how it might have changed since then and as we move away from Earth the planet's light takes longer and longer to reach us. If we could zoom in with a telescope we would see people on Earth evolving in slow motion this is the Doppler effect each tick of the clocks on Earth takes longer and longer to reach us we receive light rays in slow motion and their intensity seems to weaken as the image of the planet shifts towards red color. In front of us, it's the opposite effect the ship catches up with the light and the Stars seem to get brighter and they shift towards blue color and their clocks seem to take faster.

Time dilation

As the reactors push us ever faster, the ship begins to approach light speed and at such speeds some phenomena will no longer be mere optical illusions, instead real physical effects will kick in with irreversible consequences “Special relativity comes into play”. A first consequence of Relativity is known as time dilation. Our universe is a huge four-dimensional fabric called space-time made up of three dimensions of space and one dimension of time inside space-time. All bodies trace a trajectory since they all Advance towards their future. For takeoff our ship was moving in the same way as Earth towards the future but by picking up speed and zooming away from Earth faster and faster our trajectory gradually deviated from that of the planet and the axis we call time is no longer aligned with the time axis of people on earth. If we decided to turn around and come back our clocks would have measured a different time and we would have aged less than earthlings because our trajectories in space-time would have been different. Another key consequence of special relativity is called length contraction. When a body moves it close to light speed its length gets contracted in the direction of its motion. From the spaceship's perspective the entire universe is moving backwards. For us the universe is contracted along our direction of motion as such the length of the journey to reach our destination is shorter than expected, we are not talking about Optical effects anymore but of a physical and concrete phenomenon. If we tried to reach a distant star the journey would genuinely seem shorter to us and it would take less and less time the faster we move. Thus contrary to popular misconception it is in fact possible to travel thousands of light years in just a few seconds and in the case where we would move almost at light speed the journey would even seem instantaneous to us for the people on Earth however several thousand years would have passed.

Length contraction

Length contraction is a very real phenomenon, however, visually it would be difficult to see. To understand why imagine that we crossed the orbit of a planet, as it moves very quickly towards us the planet's length is contracted but remember the farther we look into space the farther we look into the past and from our perspective the back of the planet is located further in space than the front of the planet. The image that we receive from the back of the planet has taken longer to reach us so we see it as it was in the past even. If it is contracted the planet does not look like it because we receive the image of its back with a delay rather than contracted it appears to be rotated this is a phenomenon we have already seen, Terrell Penrose rotation, when we move very fast and look to our side the edges of the object around us seem shorter in the direction of motion. This can be interpreted either as length contraction or as a rotation as a change in perspective. For the same reason a person watching us Zoom by would not really see our ship contracted but rather rotated on itself. Over time our field of view continues to contract it intensifies in front of us while going completely black behind us. By accelerating more and more we would perhaps expect to reach the speed of light at some point. What would we see then? This question might be one of the most fundamental in relativity. The answer is no, even if the ship accelerates constantly we will never reach the speed of light; that is because the speed of light is absolute you may try to catch a light Ray but from your point of view it will always Escape at the same speed, the speed of light. You may accelerate as much as you wish even if from Earth you would seem to approach the speed of light from your point of view you are still motionless and light escapes inexorably. In short it is strictly impossible to exceed or even reach the speed of light. At best our ship will continue to accelerate forever and our field of vision will continue to shrink ever more until forming an infinitely bright spot in front of us surrounded by a completely black sky. Almost at light speed all Optical effects would become extreme as we catch up with light rays before us they all seem to come from the front. Meanwhile the Rays behind us can never reach us. In our frame of reference, the universe would be extremely contracted like a thin fabric that we would cross from side to side in a fraction of a second. For a person on Earth we'd be moving at the speed of light but for us the trip seems instantaneous we move infinitely fast.

Warp drive

To conclude nothing can ever move faster than light through space but there might be a way to circumvent this rule. In fact nothing prohibits space itself from moving faster than light. Indeed, the theory of general relativity teaches us that the fabric of space-time is dynamic it can bend in multiple ways. We could thus imagine creating a bubble around our spaceship which we would Propel faster than light. This is called a warp drive in reference to science fiction. It would no longer be a matter of zooming a vehicle through space but of propelling the fabric of space itself. Our ship would remain motionless surfing on a wave of faster than light space-time. Such a structure may seem absurd and indeed it is currently thought to be impossible to achieve. Producing a warp drive would require bending space time with huge amounts of negative Mass a form of matter that does not seem to exist in our universe. That said mathematics still allow us to model and calculate what we would see in such a situation. If we looked from outside the warp drive would seem to appear out of nowhere the curvature of space-time deviates the trajectory of light rays forming a moving lens which splits into two one part moving forwards and the other retreating in the opposite direction. Light travels slower than the warp drive itself when it approaches light has not yet had the time to reach us and after it crosses us we receive both the light which had not yet reached us and the rays which were emitted later. Looking from the inside of the warp drive we would see the sky contracted in front of us and very bright behind us the universe would look extremely dark and a whole patch of the sky would simply vanish from our view the light emitted from there is too slow to catch up with us a cone of the universe remains permanently invisible.