What happens when you travel at the speed of light?

You can not travel at the speed of light, therefore your question is meaningless.


Nobody can answer that from experience, because no person or thing has ever

traveled at the speed of light.


IF traveling at the speed of light was possible and risked, anyone who does will be slowed down of time. It distorts the fabric of time.

For example, if you're on Earth and someone your age is traveling in space at the speed of light, the other person will be much younger than you when he/she arrives back to Earth.

It's OK to treat it as a "what if" question, even though it's not possible. You can simply project the trend that is observed as you approach light speed. Thinking about it may give you some insight into why travel at light speed is not possible. As any massive object accelerates, it exchanges some of its at-rest spatial qualities for time qualities. It would take a lot of space to explain here, but one consequence is that as the velocity of an object with mass increases, its mass increases. Since mass is the meaure of inertia, it might be easier to understand if you think of it as an increase in the object's inertia. If an object with mass were ever to reach light speed, it's mass (hence its inertia) would become infinite, so no further accleration would be possible, which is why the speed of light is an absolute speed limit.

If you synchronize a clock at your starting point with an identical clock in your vessel, then accelerate to near-light speed, decelerate as you approach your destination, then return via the same pattern, the clock on your conveyance will show less elapsed time than the clock you left behind. Time and space are interchangeable with changes in motion, so part of your starting space became a part of your time as you accelerated, and part or your time became part of the rest frame space. At very high velocities, your travel clock measures elapsed time more slowly than a clock in the at-rest space where you started (but it's important to note that as the traveler, you will not observe that difference. Everything will seem to operate normally, including the passage of time, whatever your reference frame).

For other things, it's probably better to think in terms of approaching light speed to avoid some inconvenient infinities. Because of Doppler effects, electromagnetic waves approaching you from the front would be shifted toward the blue, or higher energy end of the spectrum, so you would observe visible light shifting toward ultraviolet, then to X-ray, then Gamma ray frequencies (a very uncomfortable way to travel without shielding), and EM frequencies approaching from behind shifted from visible all the down into radio wavelengths on the red end of the spectrum. Head-on impacts with the atoms and molecules of interstellar gasses would generate additional radiation, so living on your ship would be something like living at the collision point in a high energy particle accelerator. The EM spectral shift would be vector-proportional along your line of travel, so at certain positions around ship's equator, luminescent objects would take on rainbow hues, shifting toward blue to the front and red to the rear. With very many objects in the neighborhood, the effect might be something like a rainbow.

There's more, but that should give you a general picture. A ship traveling at near light speed would not be a hospitable place.