According to our current understanding of physics, an object with mass cannot travel at or exceed the speed of light. The laws of physics, as described by special relativity, prevent massive objects from reaching or surpassing the speed of light.
However, let's consider a thought experiment where you are hypothetically traveling at a speed very close to the speed of light and then suddenly turn on your engines to decelerate. In this scenario, several things would occur:
Time dilation: As you approach the speed of light, time dilation becomes significant. Time would appear to pass more slowly for you relative to an observer at rest. This means that while you might experience a short amount of time to decelerate, more time would have passed for an observer at rest.
Energy and momentum: As you decelerate, your kinetic energy and momentum would decrease. According to special relativity, your energy and momentum would increase significantly as you approach the speed of light, so when you decelerate, those values would decrease accordingly.
Increased mass: As you approach the speed of light, your mass would also increase. This effect is known as relativistic mass. When you decelerate, your mass would decrease back to its original value.
It's important to note that this thought experiment is purely hypothetical since objects with mass cannot reach or exceed the speed of light. The behavior of objects traveling at or near the speed of light is not intuitive and is governed by the principles of special relativity, which predict various unusual effects such as time dilation, length contraction, and the increase in relativistic mass.