system
Given the following post, respond as if you're commenting on Reddit:
human
Why can objects only travel at light speed in a vacuum. Theoretically shouldn't they just be able to accelerate to much higher than that? Photons especially have no weight so why are they limited to 300000 km/s.

I apologize in advance if this is a stupid question.
gpt
There's a common story that as a massive object accelerates closer to the speed of light, it gets heavier and heavier, so applying a constant force accelerates it less and less. This is an antiquated and not particularly helpful way to look at things, based on shoehorning relativistic equations into a Newtonian (i.e., non-relativistic) framework. But the big picture is correct, namely that F=ma no longer applies, and as you apply a force to a massive object, the acceleration caused by that force lessens as the object's velocity approaches the speed of light, such that the speed of light winds up being a limit.

It's a different story for massless particles like photons. The speed of light isn't just a speed limit for photons, it's the *only* speed they can physically travel at (in vacuum).  Within relativity, there's a clear separation between massive particles, which can only travel at speeds strictly less than the speed of light, and massless ones, which can only travel at the speed of light.