Speed of Light Question

[danny8522003]

I've always wondered about this question but i've never had it clarified fully:

 

Hyperthetically;

 

If I'm in a car travelling at 99% of c, what do I, the driver, observe when i turn the headlights on?

 

Also, what would someone on the ground who appears at rest observe?

[danny8522003]

Yea that makes sense but that doesnt give the answer i was looking for.

 

Lets change it so im travelling at c...

 

Would i see the beam of light travel from my lights at c yet the person on the ground sees nothing being emitted?

[danny8522003]

Would it have something to do with:

 

                             v
                  u      -------> A
               -------> B
              C        w
               ----------------->

 

[math]

w = (u + v)/(1 + uv/c^2)

[/math]

[swansont]

your headlights would travel at c so relative to you, slowly. People would observe a path of light with you travelling through it so quickly, it looks as if your headlights are emitting "slow" light.

 

No, the people observing would observe the light emitted at c relative to the car, and either red-shifted or blue-shifted, depending on if the car was moving away or toward them.

[swansont]

Yea that makes sense but that doesnt give the answer i was looking for.

 

Lets change it so im travelling at c...

 

Would i see the beam of light travel from my lights at c yet the person on the ground sees nothing being emitted?

 

 

Objects with mass can't travel at c, so it's a moot question.

[danny8522003]

I know they cant but hyperthetically...

 

Even if i was travelling at 0.99c would i see the beam emerge at c as well as the person at rest?

[Kniteli]

Ok, at extreme speeds time slows down for the one traveling near c, but light does not, so light would look like it does now(this simply follows from the fact that as time slows down, though you are traveling near the speed of light, the slowlyness of your timelyness would cause the light to appear as if it was going to same speed no matter how fast you were watching it from.). If traveling AT the speed of light, time would theoretically stop, so you wouldnt see ANYTHING until you reached your destination(assuming you could devise a way to stop without crashing), which to you would seem like no time, but actual time would be however far you traveled in light years.

As for the observer, red shift blue shift, you can think of it as a light based sonic boom, just compressions of the spectrum. at least this seems logical to me, though since noone has ever gone that fast its up in the air as to how people observe light at those speeds.

[Halucigenia]

I've always wondered about this question but i've never had it clarified fully:

 

Hyperthetically;

 

If I'm in a car travelling at 99% of c' date=' what do I, the driver, observe when i turn the headlights on?

 

Also, what would someone on the ground who appears at rest observe?[/quote']

The driver would see the light from the headlights traveling at c.

The observer, who is not at rest - there is no such thing (remember the relative in relativity!), appears to the driver to be traveling at .99% of c.

The observer who sees the driver traveling at .99% of c sees the light from the headlights traveling at c.

 

i.e. all observers see light traveling at c.

Even if i was travelling at 0.99c would i see the beam emerge at c as well as the person at rest?

Hyper - Hypothetically Yes

 

As Kniteli states (I think, as it was a bit hard to follow) it is because of time dilation that the driver sees the observer's calculation of the speed of light to be incorrect and the observer sees the driver's calculation of the speed of light to be incorrect.

your headlights would travel at c so relative to you, slowly. People would observe a path of light with you travelling through it so quickly, it looks as if your headlights are emitting "slow" light.

ed84c, what medication are you on? "slow" light

Also I got to get me some of those headlights that can travel at c relative to the speed of my car, slowly?????? - they sound fun

[mmalluck]

So few people consider the human problems with traveling at near c. Do you realize that any light that happen to bounce up off the road and back at you would be blue-shifted to the point they'ed appear as gamma-rays. You wouldn't see them, but you'd feel them as you cooked.

 

Another one of my favorite question answer/pairs for quatum mechanics goes something like this.

 

Q: Your standing on a long road with two cars headed towards you. The first car speeds up to 65 mph. The second car starts a few seconds later but instantaiously speeds up to the speed of light. At the point the faster car overtakes the slower they both turn on thier headlights. Which car's headlights do you see first?

 

A. Niether. Your body gets smeared all over the cosmos when it's struck by the car with infinate mass traveling at the speed of light. The light speed car would reach you at the same instance the light did.

[ecoli]

that's a great answer, mind if I steal it?

[Daecon]

The only way you could travel at c would be if you *were* light, so if you were to re-word your question to that, my answer would be:

 

You would see *everything* all at once, because while you're travelling at c you're effectively frozen in time. Totally.

 

Therefore all the vision you would have would be of everything simultaneously.

[danny8522003]

The only way you could travel at c would be if you *were* light' date=' so if you were to re-word your question to that, my answer would be:

 

You would see *everything* all at once, because while you're travelling at c you're effectively frozen in time. Totally.

 

Therefore all the vision you would have would be of everything simultaneously.[/quote']

That happens when you fall through a black hole - although i cant remember which type

[Severian]

Good grief. If one more person asks this question, I think I am going to scream!

[Saint]

So few people consider the human problems with traveling at near c. Do you realize that any light that happen to bounce up off the road and back at you would be blue-shifted to the point they'ed appear as gamma-rays.

 

What would cause the blue-shift to occur? Are you travelling faster wrt to the beam of light coming toward you?

[danny8522003]

A blue shift occurs when you travel towards a light emitting source because the waves get 'squashed' together, reducing their wavelength towards the blue spectrum. Red shifts occur when you are travelling away from the source.

[Saint]

A blue shift occurs when you travel towards a light emitting source because the waves get 'squashed' together, reducing their wavelength towards the blue spectrum. Red shifts occur when you are travelling away from the source.

 

Thanks, and I agree that's the standard response to the question.

 

What's the squashing mechanism. That is, the beam of light has been emitted. Once you start moving toward the source, what is the mechanism that changes the wavelength/frequency. Are you simply moving through the wave peaks faster?

[danny8522003]

Are you simply moving through the wave peaks faster?

 

Yes.

 

It is important to realize that the frequency of the sounds that the source emits does not actually change. To understand what happens' date=' consider the following analogy. Someone throws one ball every second in your direction. Assume that balls travel with constant velocity. If the thrower is stationary, you will receive one ball every second. However, if he is moving towards you, you will receive balls more frequently than that because there will be less spacing between the balls. The converse is true if the person is moving away from you. So it is actually the wavelength which is affected; as a consequence, the perceived frequency is also affected.[/quote']

 

More information on the Doppler Effect.

[Saint]

In your example, what is realy happening is that you are running through a string of balls at a given velocity (your velocity wrt the source), and the string of balls is also moving toward you at some velocity (the velocity of the thrown balls). That should look like an old familiar idea to you. Additive velocities. We're not allowed to have that with a beam of light. So that goes back to the question. Without additive velocities, what is the mechanism for changing the frequency/wavelength of a beam of light due to velocity toward or away from the source?