Larry Knight asked:
What would happen if you were driving at light speed and you switch your headlights on? What about a lit torch being carried at light speed? What would happen then?
We posed this question to Dominic Ford from the University of Cambridge...
Dominic: - I'm Dominic Ford from the department of physics at Cambridge. You've probably had the experience at some point that a fire engine has driven past you and as the fire engine has been driving towards you, you've heard it sound high pitched. And as itís driven away from you, you've heard it sound low pitched. That happens because sound is a wave and the Doppler effect says that the frequency has changed when you are moving relative to the source of the sound.
Now light is also a wave and so, it also shows the Doppler effect. But you have to be travelling very much faster before you see any effect. So if you were driving your car at close to the speed of light, an external observer would see that your front headlights would appear more blue than normal and your rear taillights would appear more red than normal. The driver himself wouldnít actually see anything different from normal because heís not moving relative to the source of the light. Just as the driver of a fire engine hears his siren at a constant pitch because heís not moving relative to the siren. Looking out the window though, the driver would be moving relative to any landscape that he was moving past, and he would see objects in front of him appear bluer than normal and objects behind him appearing redder than normal.
Of course if things move very much faster, for example, a plane moving at close to the speed of sound, you get a sonic boom, and you get a similar effect with light. But unfortunately, Einsteinís theory of special relativity says that you need infinite energy to make something move at the speed of light. And so, you can never actually get there and observe that effect.
There is one exception which is when a relativistic particle travels in a material like glass in which light travels at slightly less the speed of light Ė sounds ironic, but itís true! When the particle produces some radiation called Cherenkov radiation, which appears as a flash of light, the visual equivalent of a sonic boom.
Diana: - To the driver, if he or she had infinite energy and hadnít imploded by then, the lights would look perfectly normal. To an observer, there would be a colour shift with blue headlights and red tail lights becoming more red if thatís possible. And objects which do move faster than the speed of light will produce this Cherenkov effect, the sonic boom of the light world, and you can see it as a blue glow in some nuclear reactor chambers.
First, as far as we know, one can NOT drive at the speed of light, so the question is either meaningless or science fiction / fantasy.
What would the driver of the car see? chris, Wed, 9th Jun 2010
"what would the driver of the car see?"
My understanding is that the speed of light is always constant, regardless of the position and relative speed of the observer.
Everyone's right that the light you emit from your headlights is perfectly normal to you. It has same frequency and traveling with the same speed as if you were standing still. There are a few effects that happen to your view of the world around you as you move at near-light speed.
I watched the video, and I think it explains everything very well. diverjohn, Sat, 12th Jun 2010
Did you notice that at the beginning of the longer film, while travelling between 0 and 10% of the speed of light, it looked as though the viewer was actually moving backwards? (It was a bit like taking the handbrake off the car while pulling away on a hill. You sometimes roll backwards a bit before moving forwards). Why should that be?
I think what's happening in the movie is that because angles are getting squashed, everything in front seems to get squashed towards a point in the center of the view, which makes it look like it's moving further away. jpetruccelli, Sun, 13th Jun 2010
speed of light is always the same in a vacuum .
i have a simple question on this topic. suppose light is coming out from a source. we can consider the light consists of photon particles. now suppose i am sitting on a photon then my frame is photon's reference frame are same. then what i will see about the another photon. stationary or in motion. again i am saying that its a imagination. note that according to a Einstein - velocity of light is constant in every frame of reference. please make me it clear. bikash, Sun, 15th Aug 2010
In the first example, while traveling at the speed of light the car would shrink down to nothing, occupying no space. To an observer outside the car it would appear that time has stop inside the car traveling at velocity c. But, if I understand special relativity correctly (and that's a BIG 'if'), from the frame of reference of the observer inside the car they would not notice any difference. To them the space inside the car would appear unchanged and time would appear to be passing at the same rate as it was prior to the person and car traveling at relativistic velocities.
thank you JP and 'Creative Energy'. our physics is developing day by day. then is there any possibility to create a velocity greater then the velocity of light ? bikash, Tue, 17th Aug 2010
if you go at the speed of light you won't have time to open your lights. CPT ArkAngel, Sun, 19th Sep 2010
If you go at the speed of light, you won't have time to open the lights on because time has stopped. If you opened them before you reached the speed of light, observers who are at 0 speed relative to you (before you began to accelerate) will see the light (yellow-white for you) becoming yellow and then red, as you accelerate away from them, until it disappear into the infrared spectrum. When you will reach the speed of light, it will just vanished, because the frequency shift will be infinite. If you accelerate in their direction, the light will becomes green and then blue. If the observers can detect invisible frequencies of light, they will then detect UV light, then x-rays and finally gamma rays of always higher and higher frequencies...
Given the Einstein's ideas, the one and only thing that both of the observer and driver of the car would agree on is the speed of light. because speed of light is not relative to whoever is watching it. so far so good! But here comes the differences! As The observer would say that he saw the light and the shrunk zooming car both pass him at exactly the same time and it took both driver and the light, 5 ms (numbers given are only for the sake of simplicity) to pass him. But the driver would say that he saw the light far ahead moving faster than him reaching the observer earlier than himself. the driver would also say that it took him less than what the observer claims it did, something about 3 ms! It all has to do with relativity and the fact that Space and Time are different for different observers. that is to say everyone's space and time is different than the others'. Jsp_Fir1, Mon, 13th Dec 2010
If the velocity of light is gauged by an observer whose speed and direction are indeterminable relative to the motion of all things in the universe, which is moving, the light or the observer? Nothing can travel faster than light, yes? Think about everything the other way around. Nothing can travel slower than a thing that does not move. This may require a great leap of imagination but consider this, two trains are moving in the same direction at different speeds on parallel tracks. As one overtakes the other both switch on their lights. A mile down the tracks the beams of light strike an overpass simultaneously. What might be happening other than Einstein's explanation involving variable time? Please bear with my rebellious conjecture as I continue. The true motion of the trains is in the opposite direction and they are 'trailing' light behind them like a boat leaves a wake in water. An instant later the overpass which is also moving 'collides' with the light. If these 'light wakes' are being produced at equal rates the objects from which they originate must be moving at the same speed. For this to be possible another dimension of space is required. The slower train is in fact moving at the same speed as the apparently faster one but a component of its velocity lies at an angle outside our familiar three dimensional reality. The true nature of the universe will be discovered through Non-Euclidean geometry. Variable time is unnecessary. My humble apologies to Albert. Raoul Neppar, Sun, 26th Dec 2010
I have a doubt. If the light source is inside the vehicle, then what will a person standing outside see?( Supposedly everything outside is completely dark) R.Haricharan, Wed, 1st Jan 2014
so if you move at 99% of c, and turn on a laser in the same direction, the light will travel at c? If nothing can move faster than c, hows that possible? Its the same with Earth. It moves over 600km/s in space, but still we have a speed close to light in the LHC. Hows that possible? Special relativity makses sense in soem cases, but not all the time. Not in my head. Novaflipps, Fri, 4th Nov 2016