• Question: Lets say your travelling at the speed of light in your car. What happens when you turn on the headlights?

    Asked by msummers to Amy, Drew, Julia, Kimberley, Sara on 22 Jun 2011. This question was also asked by jackietang.
    • Photo: Drew Rae

      Drew Rae answered on 20 Jun 2011:

      This is a fun thought experiment. Light moves at a constant speed, so it doesn’t get added on to the speed of the car. The light from your headlights would be going at exactly the same speed as your light-speed car, so it wouldn’t go away from the car, let alone bounce and come back for you to see.

      On the other hand, the collisions of air particles and your car would be happening at light-speed too, so you’d have bigger problems than not being able to see the road!

    • Photo: Julia Griffen

      Julia Griffen answered on 20 Jun 2011:

      HAHA… funny one.. I reckon you wudn;t see them.. or anything around you for that matter, no light would be able to reach your eyes.

    • Photo: Amy MacQueen

      Amy MacQueen answered on 21 Jun 2011:

      Ha brilliant!!! 🙂 your car is amazing!

    • Photo: Sara Imari Walker

      Sara Imari Walker answered on 22 Jun 2011:

      Hi Msummers and Jackietang! This is an excellent thought experiment – in physics we call them gedanken experiments (which comes from German since many early physicists who were the first use these thought experiments, including Einstein, were German).

      First off, we can’t ever get to light speed. I’ll get to that below. But let’s assume that we are traveling very near the speed of light. Then, Drew is partly right, no matter what “light always travels at a constant speed” relative to the observer. However, this last part I added “relative to the observer” is a crucially important and makes all the difference. It is the foundation upon which all of the rest of Einstein’s special theory of relativity is derived. What this means is that even if you are traveling near the speed of light you won’t EVER catch up to the light beam when you turn your car lights on. Why? Because it will still be moving away from you at the speed of light!! This is the essence of relativity – the speed of light is ALWAYS RELATIVE to the observer. This sounds completely weird and unintuitive but that’s how most of modern physics is! We are forced to accept this weirdness based on experimental evidence. Special relativity is extremely well held up by experiment. The speed of light is a constant of nature and that’s a fact no matter what your reference frame (including traveling at the speed of light!).

      What’s crazier is all of the things that we can derive from this one fact. For one, time slows down the faster you travel – the limit is that photons (the particles that actually travel at light speed) don’t experience time at all! You read it right – anything traveling at light speed does not experience time. This is a neat trick – we are all actually traveling through “spacetime” at light speed, but most of our effort is put to traveling through time! For photons they travel through just space at light speed so they are stuck forever at a single moment in time. It gets even weirder! Photons travel at light speed because they are massless. Any massless particle will travel at light speed (the photon is the only one we know of however). Massive particles (basically anything but photons) can never get there. The mass of an object actually increases the faster and faster and object the object is traveling! The closer it is to the speed of light the harder it is to accelerate it further due to the increasing mass. So massive things can never travel at light speed! Your car would never get there!

      There is a whole other host of weird things one can get out of special relativity! Its super interesting to study. I highly recommend it!!