According to the theory of special relativity, as an object with mass approaches the speed of light, its energy requirements become infinite, making it impossible to accelerate to or beyond the speed of light for objects with mass. So, in reality, it is not possible for any object with mass, including spacecraft and humans, to travel at the speed of light.
However, we can explore the theoretical consequences of approaching the speed of light for a thought experiment. Let's assume you have a spacecraft capable of traveling at 99.99% of the speed of light (0.9999c). If you were to embark on a journey at that speed and travel for one day (as measured by clocks on the spacecraft), here's what would happen:
Time Dilation: As you approach the speed of light, time for you would pass slower relative to observers on Earth. This phenomenon is known as time dilation. The Lorentz factor (γ) that governs time dilation is given by γ = 1/√(1 - v^2/c^2), where 'v' is the velocity and 'c' is the speed of light. For 0.9999c, γ is approximately 70.71.
Time on Earth: For an observer on Earth, the time experienced by the spacecraft traveling at 0.9999c would be dilated. To calculate the time experienced on Earth, you multiply the time experienced on the spacecraft by the Lorentz factor:
Time on Earth = Time on the spacecraft × Lorentz factor Time on Earth = 1 day × 70.71 ≈ 70.71 days
So, if you were to travel at 99.99% of the speed of light for one day on the spacecraft, approximately 70.71 days would have passed on Earth when you returned.
This thought experiment highlights the fascinating and counterintuitive nature of time dilation at relativistic speeds. However, it's essential to reiterate that no object with mass can actually reach or exceed the speed of light according to our current understanding of physics. The predictions of time dilation and other relativistic effects become more pronounced as you approach the speed of light, but they remain theoretical as we cannot practically achieve such speeds with our current technology.