According to the theory of relativity, as currently understood, it is not possible for an object with mass, such as a photon, to be accelerated to or travel at the speed of light. Photons themselves are massless particles that always travel at the speed of light in a vacuum.
However, let's consider a hypothetical scenario where we assume a photon is somehow accelerated to the speed of light and shot directly at the Earth. In this scenario, a few things can be discussed:
Time dilation: According to the theory of relativity, as an object approaches the speed of light, time dilation occurs. From the perspective of the photon, time would essentially stand still. However, for an observer on Earth, time would continue to progress as normal.
Length contraction: As an object approaches the speed of light, its length in the direction of motion contracts. From the photon's perspective, the distance between its point of origin and the Earth would appear to be zero due to length contraction. However, for an observer on Earth, the distance would still exist.
Energy and impact: If we assume the photon could somehow transfer its energy upon reaching the Earth, it would likely have an extremely powerful impact. Photons carry energy proportional to their frequency, and at the speed of light, a photon would have extremely high energy. The interaction between such a high-energy photon and the Earth could potentially have catastrophic effects, causing widespread destruction.
It's important to note that this hypothetical scenario of a photon traveling at the speed of light and directly impacting the Earth is not based on our current understanding of physics. The theory of relativity does not permit objects with mass to reach or exceed the speed of light, and the behavior of objects at such extreme speeds is not well-defined within our current scientific framework.