# How can we assume that reaching the speed of light slows down time? [duplicate]

How can we assume that gaining the speed of light slows down time?

• Though the question is phrased in an unclear way I'm guessing it's asking about the physical reality of time dilation. In that case there are several questions that address this issue. – John Rennie Oct 15 '14 at 9:02
• It is probably also worth pointing out that we don't assume this, we deduce it from more basic assumptions ie. relativity and the universal speed of light. – m4r35n357 Oct 15 '14 at 9:06
• We don't assume this, we measure it's effects. Every day. – dmckee --- ex-moderator kitten Oct 15 '14 at 14:39