According to our current understanding of physics, traveling at the speed of light is not possible for objects with mass, such as spacecraft or humans. As an object with mass accelerates towards the speed of light, its relativistic mass increases, requiring an infinite amount of energy to reach that speed. Additionally, time dilation effects would occur, causing time to appear to slow down for the traveler relative to an outside observer. From the perspective of the traveler, the journey may seem shorter, but for external observers, millions of years would indeed pass.
However, let's consider a hypothetical scenario where an object with mass could travel at the speed of light for millions of years. In that case, a few significant consequences would arise:
Time Dilation: As mentioned earlier, time dilation occurs as an object approaches the speed of light. Time would appear to pass more slowly for the traveler compared to someone at rest. Therefore, from the traveler's perspective, the millions of years of travel time may feel significantly shorter. However, upon returning to a reference frame at rest, the traveler would find that a much longer period of time has passed in the outside world.
Relativistic Effects: Relativistic effects, such as length contraction, would also come into play. Objects in the direction of motion would appear to be contracted or compressed in the direction of travel. This phenomenon is a consequence of the Lorentz transformation equations and the principles of special relativity.
Cosmic Background Radiation: As the traveler moves at such high speeds, they would experience a blue-shifted and highly energetic version of the cosmic microwave background radiation. The cosmic microwave background is a faint radiation pervading the universe that is a remnant of the Big Bang. At relativistic speeds, the wavelengths of this radiation would be compressed and appear more energetic.
Energy Requirements: The energy required to propel an object with mass to the speed of light is currently beyond our technological capabilities. It would necessitate immense amounts of energy and novel propulsion systems that can handle such extreme velocities.
It's important to emphasize that these scenarios involve significant departures from our current understanding of physics, and our understanding may change with new discoveries. As of now, the fundamental principles of special relativity indicate that objects with mass cannot reach or exceed the speed of light.