Now I know this isn’t true, but I don’t know how to explain why it isn’t…
Okay, if you’re driving down the road at 50mph, and you throw a tennis ball out the window at 20mph, when that tennis ball hits something, it has the force of going 70mph correct?
Well if thats true, then how come when you have you headlights on in your car, the light from them isn’t moving faster than the speed of light because you’re moving while the light is being projected infront of you?
Once again, I know that isn’t possible, just don’t know how to explain why it isn’t true… any ideas?
If you’re in a boat, and you toss a stone into the water, the ripples will move away from the stone at a constant speed. If you run the boats speed up to 75 knots, and toss a stone into the water, the ripples will still move away from the stone at that very same speed.
The speed of the ripples from a stone, or a photon through space is governed by the properties of the medium through which the wave travels.
Notice that the waves in front of the boat are higher and more frequent than the waves behind the boat because they can’t get out of the way fast enough. Same thing happens with light. Objects moving toword you are “blue” shifted while objects moving away are “red” shifted (or maybe its the other way around). It also causes the “WWRRRAaaaaaaaaaaaaaaaaa…” effect when a race car zooms by. (look up “Doplar effect”)
I’m afraid I’ll have to disagree Squink. The propagation of light does not depend on a medium. A boat can overtake the waves it creates but no matter how fast you travel light will always recede from you at c.
Let’s imagine for a moment that you could travel at c (traveling on light beam). What you’d see would be a non-time varying spatial waveform and since it’s non-time varying it couldn’t exist.
What I heard a while back, don’t have cite, is that if your in a spaceship traveling at close to the speed of light and turn on a light in it’s nose, the light will travel ahead of the spaceship at the speed of light.
The light’s speed is relative to the spaceship it came from, not the surrounding space. In the case of the tennis ball and car in the OP, the ball is immediately affected by the air. Not so in space. This is supposed to be a very simplified explanation of the “Theory of Relativity”. I could be way off base here, but that’s how it was explained to my simple mind.
I’m sure others will come along soon with a much better explanation than this.
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I’m afraid I’ll have to disagree Squink. The propagation of light does not depend on a medium.
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I didn’t intend to imply that the vacuum contains a medium in the usual sense of the word. However light would travel slightly faster in a vacuum if it weren’t for its interaction all those pesky virtual particle pairs.
The point of my post was that the propagation speed of a wave depends on the properties of what the wave travels through, be it water or hard vacuum. The method used to create the wave, rock or headlight, in a moving or nonmoving vehicle, has no effect on the rate of propagation.
Sure we can make things more complicated by dicussing sound waves on moving trains, or relativistic effects, but I don’t think thats strictly necessary when the OP deals with everyday automobiles and headlights.
I’ll do the best that I can, although this is a prickley subject.
Speed is a measure of distance over time. Light moves over an extremely long distance in an extremely short time, but let us consider for a moment a period of time in which a light particle moves a tangible distance; you used mph, so I will use feet. So our light particle, in the time we’re looking at it, moves one foot.
If you’re standing still, you can (not easily, but assume) measure the time that it takes this particle to travel this one foot (using, say, a stopwatch and a ruler).
If you’re moving, you can do the same (due to relativity - no vantage point has any advantage over any other), and get the same result. Your question is why.
The simplest answer that I have for you is that, as you move quickly, your ruler gets shorter. At low speeds (50mph, 1000 mps, whatever), the ruler doesn’t get much shorter; certainly not enough for you to notice. At great speeds (0.9999c), it gets much shorter. But again, you’ll probably not notice, because you (and your eyes, and brain) also got shorter by a comparable amount. What you will notice, is that the particle moves through the length of the (now shorter) ruler, in the same amount of time as it did originally!
At this point, you’re probably confused. I could offer a similar example using time as the scale which changes (and you would get the same answer), but that’s even less tangible than the ruler.
Incorrect. Ignoring friction, the speed will still be slightly less than 70mph. The diference will be so small as to be (almost) impossible to measure, but as you increase the speeds involved closer to the speed of light, relativistic effects become more pronounced, and the difference will be much more noticible.
Oh, and allow me to recommend a book. The Elegant Universe, by Brian Greene. Although the book is technically about string theory, a significant portion is dedicated to understanding principles of relativity and quantum mechanics, and has simple, understandable analogies, with an appendix for the relevant mathematics.
The speed of light relative to an inertial observer is always c. If the observer is on a spaceship coasting at .9c, the speed of light will be measured as c relative to the ship. If the speed of the same beam of light is measured relative to a “stationary” observer, the speed relative to the observer will still be c.
I thought I probably missed something. The second part of your quote helps explain to me, in simple terms, why relativity theory can be difficult. I sure ain’t no Einstein, but this helps my simple mind understand. Or, maybe not…
Velocity is distance divided by time (v=d/t). Einstein noticed that the velocity of light was always the same when measured by any observer travelling at any speed (300,000 km/s). If this is true, then the distance and time for the observers must not be constant (despite what “common sense” might tell you about these properties). I.e., as you approach the speed of light, your measurements of length and time adjust so that the speed you measure for light always remains the same.
Isn’t it easier to explain by flipping it around so that the car is going near the speed of light and the driver flips on the headlights? To the driver, everything is normal, but to an observer, they see something else…or the something else the driver sees near the speed of light -the observer - looks like he is in a slow time warP?
The guy in the car will see himself and the car at rest, and a beam of light going out in front of him at c. The guy on the road watching him will see the car going .9c (or whatever), and a beam of light going out in front of him at c. Both see the beam of light going at the same speed. There are differences in what they’ll see, of course: To the guy in the car, everything outside will be squished, and to the guy outside, the car and everything in it will be squished. Additionally, the guy outside the car will see the light from the headlights as very blueshifted.
And Squink, I’m pretty sure that virtual particle pairs can’t affect the velocity of light. The sea of virtual particles is Lorentz invariant, which means that the only fundamental speed you can associate with them is Einstein’s constant c. There’s no other speed available in the Universe: For everything that has a definite speed, that speed must be c. So if you ask the questions “What is the speed of light, considering virtual particles” and “What would the speed of light be, if it weren’t for virtual particles”, you have to get the same answer.
