• Question: If you are in a car travelling at the speed of light, and you turn the headlights on, what happens?

    Asked by jacko552 to Hywel, Joseph, Patience, Poonam, Rachael on 25 Jun 2010 in Categories: .
    • Photo: Joseph Cook

      Joseph Cook answered on 18 Jun 2010:

      This is obviously a hypothetical question, as cars can’t reach the speed of light. In fact, anything with mass cannot reach the speed of light, only things like photons can as they have zero rest mass.

      The way that energy increase for things with mass can be thought of as their ‘effective’ mass increasing the faster they go – this is their mass multiplied by a factor given the symbol gamma. The effect is small for any speeds you’re likely to encounter, but becomes more pronounced the closer you get to the speed of light. And you could never reach the speed of light because you would need an infinite amount of energy to do so, because your mass would become infinite at the speed of light (gamma approaches infinity).

      But you could argue that the car driver sees the light travelling away from them at the speed of light (as normal) because the speed of light is constant to all observers, and someone standing in front of the car would be hit before they see the car because the light and car both travel at the speed of light so both reach them at the same time.

      I suggest you do a degree in physics to find out more!

    • Photo: Poonam Kaushik

      Poonam Kaushik answered on 21 Jun 2010:

      u will c the light goin ahead of ur car!

    • Photo: Dr Hywel Jones

      Dr Hywel Jones answered on 25 Jun 2010:

      This question has been asked many times and is the usual example used to try and show how weird the theories of general and special relativity are. Good answers can be found here:

      http://www.physlink.com/Education/askExperts/ae169.cfm:

      As you approach the speed of light with your headlights on, however, you would still measure the light beam racing away from your car at 186,000 miles per second (c). A ‘stationary’ observer watching this happen, though, would not then measure the beam’s speed at almost twice c. Relativity says that all observers always get the same measurement for c.

      While that may not sound logical or plausible, it happens because what we normally think of as fixed concepts–length and time–are both variable at high speeds. If you observed a car travelling past you at close to c, its length in the direction of travel would appear shortened and the passage of time on board would appear slowed down.

      Although these ideas sound strange to all of us not used to moving at relativistic speeds, they have all been confirmed experimentally.

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