According to the Special Theory of Relativity, the speed at which light travels is the same in all reference frames. In other words, you will always observe that light will travel at 2.99*10^8 m/s, regardless of how fast you’re going. At first, one might think that this number is so high that it would make little difference in how we observe the universe. In reality, however, cosmic entities are so far apart that we use the distance light travels in a given time frame to describe astronomical distances. Below is a chart that describes the distances, in such units, of familiar objects in space:
Because we use light to observe these distant objects, we see them the way they were in the past. For example, if we could see Jupiter in the night sky, then we would see how it looked 43 minutes ago.
The speed of light is not only constant, but it is the fastest an object with mass can move. In fact, it would take an infinite amount of force to accelerate a massive object up to the speed of light. That is to say, even with our best efforts, it would take us much longer than 4.35 years to get a probe even close to Alpha Centauri. Moreover, the Special Theory of Relativity says that strange things happen for objects that travel near the speed of light. For instance, an object moving near the speed of light will appear to age more slowly to us, as an observer at rest; this is called time dilation. In addition, that same object will appear to be shorter in length; this is called Lorentz Contraction. A picture below shows what this might look like for a baseball travelling at fractional speeds of light:
These physical rules that define the speed of light make it one of the most interesting things in our universe.
Other sources:
The Solar System 
The fact that time dilation and length contraction occurs is really weird but so interesting! I wonder if there is the possibility of seeing these phenomena in action, or whether or not we can take pictures of those effects in the process. Maybe likely candidates would be hypervelocity stars that get thrown out of galaxies by large black holes, or the massive matter jets that black holes eject.
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How light speed interacts with physical objects (especially time dilation) is just one of the coolest things to me. If you were to actually travel at the speed of light, not only would time appear to be slower, but time would actually appear to be stopped in your frame of reference. And I mean stopped in the sense that you wouldn’t actually notice that any time had passed while you were traveling at light speed. This means that if you were to travel at light speed for a year, you would think that not even a second had actually passed. It’s really just like a one-way time machine!
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Does this mean humans are not able to travel at speed of light?
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That’s right. Any object with mass would be unable to travel at the speed of light, according to the Theory of Special Relativity. That is because the mass of an object approaches infinity, as its velocity approaches the speed of light. It would take an infinite force to accelerate it further. Here is a web page with a graph explaining this theory.
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Grader here, Great discussion and really good post giving a brief overview of some of lights cool properties. Its interesting to think about the speed of light even more as a manifestation of space-time itself. Any thoughts on tachyons?
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