Question of the Week

How fast does a moving light source go?

Sun, 13th Jun 2010

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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.

1966 Ambassador 990 station wagonNow 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.


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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.
However, if you were driving at 99% of C and you turn on your headlamps, the lights will exhibit a colour shift to a much higher frequency, but will not exceed the speed of light.
The confusion arises when people compare photon to bullets. Here's an example from the air forces.
If a machine gun which fires bullets at a velocity of 1,000 feet per second is fitted to an aeroplane which flies at 1,000 feet per second, then the bullets will be moving at 2,000 feet per second as measured by a ground observer, even though the pilot will observe the bullets moving at 1,000 fps, and the oncoming target, an enemy jet-fighter flying towards the first pilot, also at 1,000 fps, will see the bullets coming towards him at 3,000 fps. Thus, the bullets may be moving at 1000, 2000, or 3000 feet per second, depending on the observer and reference.
However, light behaves differently from bullets. Bullets are particles both when in motion and when at rest. Light, on the other hand, behaves as a wave when travelling, not as a particle.
Comparing light to sound is more meaningful. We're familiar with the shift in sound frequency as speed increases, so imagine a car moving at the speed of sound (about 750 mph) when the driver sounds his horn to warn a pedestrian on the road ahead. Will the pedestrian hear the claxons before the car strikes him? No - the car is traveling at the same speed as the sound is, so the unlucky pedestrian will hear the warning only at the moment of impact.
C is constant as far as we know, though some research and speculation is ongoing about whether C had a different value in the past.
A quick note about bullets being particles while in motion, I meant Newtonian motion. When an object starts moving, relativistic effects come into play such that moving object emit energy and gain mass, but these effects are negligible until objects reach 10% or more of C.
Finally, invoking sci-fi writer's license: if the vehicle in question actually reaches the speed of light (in violation of known laws) and the lamps are turned on, the light will be moving at the same speed as the vehicle and so will not illuminate the road ahead. However, light spreads not only forward, but radiates in all directions. So, if the reflecting surface were removed from the headlamp shell, allowing light to shine through the sides, then an observer watching the car pass may see the headlamps as long as he's not directly in front of it.
Bottom line, the question is meaningless until exceptions to relativity are discovered.
Consider "tachyons", as-yet-unproven particles which exist only at speeds beyond light. diverjohn, Wed, 9th Jun 2010

What would the driver of the car see? chris, Wed, 9th Jun 2010

"what would the driver of the car see?"

Good question.
if I recall effects of relativity correctly, I believe the driver would see the headlamps operating normally. Using sound waves as a paradigm, a man yelling from a moving car does not hear the doppler effect. diverjohn, Thu, 10th Jun 2010

My understanding is that the speed of light is always constant, regardless of the position and relative speed of the observer.

That means that not only would the headlights look normal to the driver but, if he passed another vehicle coming the other way, the headlights of vehicle 2 would look normal to the first driver too. And if at the same time there was also a third vehicle parked up but with his headlights on, then those headlights would also look normal. In fact, all three drivers would each perceive the light coming from all three sets of headlights as travelling at the normal speed of light.

This sounds counterintuitive and is not the same as diverjohn described. So how can the speed of light be the same regardless of the speed in which either the light source or the observer are travelling? Well, speed is equal to distance divided by time so, to make this impossible thing happen, time and space will apparently both become distorted to accommodate it.
John Chapman, Thu, 10th Jun 2010

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.

First, things in front of you will appear more blue and things behind you will appear more red.  The speed of light stays constant, but it gets Doppler shifted.  Just like an ambulance passing by you has a higher pitched sound when it's heading towards you and lower pitched as it heads away from you, things you are flying towards will look more blue and things you are flying away from will look more red.  In fact, at high speeds much of the light won't even be visible to you anymore, since it will be ultraviolet and beyond.

Next, things ahead of you tend to be squashed in angularly towards the center of your view.  This is hard to describe, so you should probably check out some pictures (see links below).

Finally, things become brighter in front of you and dimmer behind because of a variety of effects (the angular change, blue shift and photon flux rates).

While looking up some pictures of these effects, I came across a couple of interesting links, so I thought I'd share them.
1)  This is a paper (originally published in the American Journal of Physics, which is a teaching journal) about a computer program that shows pictures of what scenes would look like under relativistic motion.  It goes through the effects in detail.

2) Here's a link to the program they use:

3) Here's a nice video of what a city would look like if you were driving at relativistic speeds: jpetruccelli, Fri, 11th Jun 2010

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 always travel between 0 and 10% of the speed of light in my car and I've not yet experienced this. 
John Chapman, Sat, 12th Jun 2010

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 .

Using your example, it depends on the density of the medium or in other words on the refractive index of the medium. donchiragjain10036, Mon, 14th Jun 2010

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.

The same would apply to the torch. To an outside observer the torch would appear to have disappeared into thin air while to an observer traveling at c along with the torch, the torch would appear unchanged. CreativeEnergy, Sun, 15th Aug 2010

Hi bikash,

Your question illustrates a very important point about relativity.  You can't use the theory to predict what happens if you're riding around on the back of a photon.  The reason for this is that asking the question supposes that the photon isn't moving in your frame of reference (so long as you're not accelerating).  However, special relativity assumes from the start that the photon is always moving at the speed of light, no matter what your frame of reference is.  In other words, riding around on the back of a photon is impossible to describe because no matter how fast you go, the photon is still moving away from you at the speed of light.  You simply can't catch up to it. jpetruccelli, Mon, 16th Aug 2010

What JP wrote is correct, bikash. It's funny that you should think of that question, because in the thought experiment that Einstein devised while working on special relativity, he imagined that he was riding on a train traveling at the speed of light. In the thought experiment, Einstein wondered if he held a mirror in front of his face while traveling at the speed of light if it would be impossible for him to see his reflection because he would have caught up with the photons being reflected from his face.

It was at this point that he realized that the speed that light travels must be a constant to all observers regardless of their frame of reference, because if the velocity of light were cumulative with the velocity of other moving objects, then we would receive an unintelligible stream of information going into our eyes from many different sources of light. In that event the light from many different sources, moving at different velocities would also in turn be traveling at different velocities and this simply could not be the case, since this is not how we experience the world.    CreativeEnergy, Mon, 16th Aug 2010

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

There is a theoretical exotic particle known as the tachyon that always travels faster than light. If the particle actually exists it may one day be possible to manipulate the particles in such a way that would enable us to travel faster than light in the distant future.

The warping of space is another possible way in which we may one day achieve faster than light travel, but that requires tremendous amounts of energy which currently isn't available to us. 

Also, if worm holes really do exist, it may be possible for us to create a stable worm hole in which we could bypass large distances of space rather quickly. But again this is something in our distant future, if at all, bikash.

Eric CreativeEnergy, 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...

For the driver the beam of light has no change. CPT ArkAngel, Sun, 19th Sep 2010

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

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