Question: Lets say that i was in a car travelling at the speed of light (even though its impossible), what would happen if i turned my headlights on?
Keywords: car, light, physics, speed, travelling
Comments
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Just saw your name. Cmon Stephen, you know the answer to this one.
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if the car was travelling at the speed of light, what would you see? would it just be black behind you or…?
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You are testing us non-physicist-types, but let us continue for all those movie-watching young scientists, who wonder if the movie people have it right or almost right when the star ship Enterprise goes to warp speed.
We’d need to think about 4 effects, 2 due to classical (or Isaac Newton’s) physics and 2 due to relativity (or Albert Einstein’s improvements).
Firstly, the light from behind would change colour, which is the Doppler shift. This is just the ambulance-siren effect, in which sound (or light) coming towards us has higher pitch (frequency) and the sound coming from a source that we leave behind has a lower pitch (frequency). For light, this means that as we approach the speed of light, the light from behind gradually changes from visible to infrared then to microwave, to radio and eventually the wavelength gets so long and the frequency so low that it does not appear to be electromagnetic radiation at all.
Secondly we would see less and less light from the sides and most of the light would be concentrated towards the front and back. Again this effect for sound is experienced in fast-flying aircraft, which leaves the sound behind. It is like trying to pass a ball to someone who is running past. If the runner is running as fast as we can throw the ball, we’ll never get the ball to her from the side, only from the front. That’s why when we go really fast all the rain hits the front windscreen and hardly any on the side windows.
So now hopefully our brain doesn’t explode as we consider what Einstein has to say about this. My physics stops with very basic relativity, which says that there’d be two effects.
So the third effect is that an external observer (someone outside the car or the Borg and Imperial StarCruisers from which we are escaping) would see us getting thinner … but only in the direction of travel. We’d start looking like gingerbread people — still fat sideways, but flat looking front and back.
Finally, the external observers would see our movement inside the car getting slower and slower. Those Imperial Storm Troopers still see our car whizzing past, but we’d be moving very slowing inside the car. If our car were at light speed then we’d appear to have stopped as if someone had hit pause on the DVD player.
Of course we’d see everyone in the car as normal and our fellow passengers would see us as normal.
I think what we would see inside the car as we look out the windows is that most of the light would come from the front and the outside world would slow down. The light coming through the back windscreen would also get narrower as if we were trying to see open sky as we go into a tunnel – what we can see gets narrower – and the light changes colour and becomes dimmer and dimmer until we don’t see anything from behind. -
Hi students, Kieran and Mat,
Kieran’s got it pretty much summed up.
For some illustrative pictures and videos that explain these effects see http://www.fourmilab.ch/cship/
These two are especially good:
The Lorentz Contraction (Kieran’s third effect): http://www.fourmilab.ch/cship/lorentz.html
Relativistic aberration/beaming (Kieran’s second effect): http://www.fourmilab.ch/cship/aberration.htmlRelativity is the ultimate in ‘strange but true’. It’s truth is proved by the fact that GPS works, as Kieran said, as well as many many experiments that have been done to test it. (Incidentally, GPS requires more than Einstein’s Special Relativity, because Special Relativity only works when there’s no gravity around and nothing is changing its speed. His later Theory of General Relativity deals with these complications.)
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If you’re sitting in the car, going at the speed of light and you switch on your lights, then you see the lights coming on. If you look out the window, you’d see a freaky weird mess because all the stuff that’s stationary would look strange. If I am standing “still” and you drive past and switch your lights on then I’m not sure how you’d look to me. Need to think about that. [processing…]
While I’m thinking, just be aware that there is no universal “time” or “space”. It’s all relative. Space and time are actually features of the same thing, and physicists talk about “spacetime” – so moving really fast warps the nature of the space and time around you. So it’s not as easy as moving past me at the speed of light. The spacetime you’re in is very different from mine.
[still processing – gimme a sec…]
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OK, I took a walk, watched The Incredibles with my son, and I got it.
The crucial thing is that the speed of light, as a maximum, is never violated. Nothing goes faster than the speed of light. Not even Mr Incredible.
So you’re sitting in your super fast car. Let’s say it’s near the speed of light. You sit there and switch on the headlights. You see the headlights go on, no problem. Everything’s normal. The world out the window looks very weird. Time is slowed outside, but not for you, and because the light from the outside world is still hitting your eyes, the Doppler thing that Kieran talks about above means that the energy of the light is increased, meaning its frequency, and that means that you might not even be able to see it.
The interesting thing for me is what the observer sees, watching the car WHOOSH past.
[It’s important to realise how difficult it would be to actually do this. Light goes round the Earth 7 times each second, right? So we’re talking fast. But let’s imagine we have some expensive camera that takes care of this. Let’s also ignore the fact that you and your car would hugely increase in MASS by travelling so fast – that’s a feature of the maths you’ll appreciate when you study this. Which you totally should by the way.]
You would see the car. I think. The energy of the light coming off it would be altered, so you would have to shift your camera to work in frequency ranges you might not be able to see. Look up blue and red shifting to understand this, but it’s like the ambulance thing. But let’s say you altered your camera to detect the light coming off the car. You see the car approaching. At the exact moment it goes by, the headlights go on. Awesome timing, but let’s imagine you see that. You see the light coming out the headlights when they come on. If you could measure the speed of light coming out the headlights, you would measure it as going away from the car at some speed that is a lot slower than the speed of light. You’d think – wait, what? The light’s moving slowly? What gives? So then in that nanosecond you notice the clock on the dashboard of the car, and you notice that – wow – time is passing really slowly. Clunk … clunk … clunk go the seconds. Like the Matrix in Bullet Time, but less cool. So the light looks like it’s emerging from the headlights slowly, but actually it just seems that way because time (for the car) is passing a lot slower than it is for you. Time has slowed in the other frame of reference.
I LOVE that.
So you can make sense of:
a) Light being a maximum and
b) There being no contradictionsif you remember that the speed of TIME can change.
How’s that?
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i love it! thank you!
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stephenhawking
Edward
*Nice*
If you were travelling at the speed of light and switched on the headlights, YOU would see that the headlights are on. That’s your “frame of reference”.
What someone else sees is the difficult thing. It’s not as though someone watching you do this sees what you see. When you travel that fast, space and time literally change. Very weird. You would look very compressed, for example, even if I could see you. It would look to me that time is travelling very slowly for you. Because time changes, you can get round some of these problems with light seeming to travel faster than light.
That sounds like a cop-out. But the special theory of relativity, which describes these weird things, is all about relativity – that everything works within one frame of reference, but not between frames of reference as you’d expect it. If I run at 5 kph down a moving train that’s moving at 50 kph, I’m just running at 5 kph. I’m not super running at 55 kph – that’s nuts. The way to get around this at really fast speeds is to say that time and space are different. Time can slow down and space can shrink. This prevents faster-than-light stuff.
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You’re starting us off on an easy one aren’t you stephenhawking? 😉 I’m only going to answer this if you take me in your car travelling at the speed of light, (even though it’s impossible).
Matt is right, if you switched the headlights on while travelling at the speed of light, from inside the car, they would look like they were on. In fact, everything that happens from your point of view would probably seem normal even moving around.
Everyone outside however would not see things as normal. From their point of view, the car would appear be squashed like an accordion and if you slowed down from the speed of light, (though why would you if you got that fast), your sports car, (it was a sports car wasn’t it?), would appear normal sized again. This is centred around the Theory of Relativity where things just work from one point of view not from all points of view which is different from how events in our lives work so things start to get a little different. Part of relativity is that when, (let’s just drop the “if” and “impossibilities), you travel at the speed of light or get closer to it, time starts to slow down and space shrinks. Time and space are treated as different things.
If you’re ever looking for a movie filled with physics and science jokes along the way, look for Young Einstein with Yahoo Serious in it. It’s a fairly old movie but it should be at your local video store. The best part about the movie is that the science mentioned in it is spot on. The explosions are kind of neat too.
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this is why einstein called it relativity — what is observed depends on things happening RELATIVE to the observer. you would “see” the light from the headlight moving away from the car. an external observer would not see that because the time and distance observed by you and by the external observer are different? we know that this is so, because for observers on earth, we see clocks on satellites run slower than clocks on earth. GPS satellites send out signals and GPS units essentiall time how long it takes for these signals to reach the unit. so the signals need to be sent from the satellites at precisely known and fixed tie intervals. the scientists and engineers who programmed the GPS satellites had to take this different in clock time into account. so the fact that GPS works, means that time goes slower and that you in your car and an external observer see different things that makes sense in each frame of reference.
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