The Universal Speed Limit

Imagine you are in your car, traveling at the speed of light (about 300,000 km/s or 670,616,629 mph), and you turn on your headlights. What happens next? Well, that doesn’t matter, because it is impossible for anything with mass to reach the speed of light, as doing so would require infinite energy. But what if you had lots of energy available and were able to accelerate your car to nearly this speed? Your first guess might be that the light shoots forward like normal, and appears to an observer as if it is traveling nearly twice as fast as usual. But nothing can travel faster than the speed of light, so this can’t be the case. After some more thought, maybe the light appears slow from this perspective, oozing out of the headlights as it just barely outpaces your car. As it turns out, this is also incorrect. Perhaps counter-intuitively, you would still perceive the light moving forward at the same 300,000 km/s. A stationary observer, however, would also measure the light moving at the same speed. How is this possible?
It is at this point that our good friend Albert Einstein comes in to save us with special relativity. Einstein found that quantities like time and distance are not as fixed as one might believe. As an object moves faster, it experiences phenomena known as time dilation and length contraction. In your car moving at near-light speeds, you would not notice much different inside the vehicle. Things would look the same, and you would perceive time passing at the same rate. However, for an observer watching your car pass by, things would look quite strange. The vehicle would look shorter, and events within the car would take place in essentially slow motion. And what about the light? Well, while it would not move any faster, it would still receive a boost of energy from the car. Instead of speeding up from the observer’s perspective, the light would do what it always does at an increased energy: become bluer, transitioning from the visible spectrum into ultraviolet light.
From One Universe at a Time

2 thoughts on “The Universal Speed Limit

  1. This is such a fascinating post! I liked the way you walked through each thought process and then proceeded to explain why each scenario would not be the case. This also relates to our in-class discussion of traveling into a black hole; to the observer, the person “falling” into a black hole would appear to slow down and then at one point freeze entirely, whereas the person “falling” would experience their own version of time in which everything seems more normal. I would love to learn more about this concept (even though I will probably never be able to fully understand it). Great post.


  2. I especially like extending relativity to the perspective of a photon travelling vast distances, even across most of the observable universe. Because the photon moves at the speed of light, for our perspective it doesn’t experience any time in the millions of years we percieve it to be travelling. And by the photon’s perspective, it hasn’t spent any time moving and hasn’t moved any distance, even those vast distances become infinitely small as your perspective approaches the speed of light.


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