Existence

My advice would be to grapple with them by reading them, not by imagining them to say something completely different than they do. For instance: No, the speed of light is the same in all reference frames, as I've repeatedly told you. Because of this, time may not be the same between two reference frames. The light clock gedankenexperiment is an attempt to demonstrate this, but it's not the proof of it. (How could it be? Light clocks don't exist.) The light pulse travels on two different paths relative to the observations of two different observers; the observer traveling along with the clock sees the light pulse travel along a shorter path than the stationary observer. He has to, trigonometry proves it.If the pulse were not light, say it were a tennis ball instead, then the disparity would be resolved by the fact that the stationary observer would see the ball traveling at a greater speed to keep up with its longer path - the tennis ball would have its own speed relative to the vehicle (up and down) plus it would have the vehicle's speed as well. Therefore two observers making observations of a tennis ball clock would find their observations synchronized.But unlike the speed of tennis balls, the speed of light - being a wave that propagates in spacetime according to Lorentz mathematics - is the same for all observers regardless of their velocity. So the stationary observer continues to see the light pulse traveling along a longer path than the ride-along observer - trigonometry proves it - but he sees the light travel that longer path at the same speed, so it takes longer. So he's seeing the clock tick at longer intervals than the ride-along observer.The resolution of this paradox is the recognition that time actually slows down in the ride-along observer's moving reference frame. Things actually are happening slower. If the ride-along observer accidentally drops a tea cup as the vehicle shoots by at a great rate of travel, the stationary observer sees the cup break in slow motion. Everything is happening slower in that reference frame because time has slowed. The people in the vehicle observe events happening outside the vehicle like a VCR on fast-forward, because from their perspective time outside their reference frame has increased.As we've been trying to explain, it has nothing to do with the nature of clocks, not even of light clocks. Time dilation isn't something that happens to clocks, it's something that happens to time. And it's a necessary consequence of the fact that the speed of light has been proven to be the same for all observers regardless of their velocity, a fact that has been known about the universe since 1886.

No, because the tube is moving as well, because it's attached to the car. The image you're confused about is the path of the light pulse relative to an observer not moving with the car; by trigonometry, the path is longer, yet the light pulse has the same rate of travel to that observer as to the one in the car.Therefore the light clock "ticks" slower for the stationary observer than for the one in the car, proving that time dilates as a function of velocity.Edited by crashfrog, : No reason given.

Why would it matter if it is attached or not? How does not being attached to the car remove it from the car's reference frame?Does the light pulse undergo any kind of acceleration? Does its velocity change in any respect?No. Therefore it remains in the same reference frame it started in - the same frame as the rest of the light apparatus. Incorrect. The light pulse has the lateral velocity of the car relative to the salt flats, because the pulse is in the reference frame of the car and not in that of the salt flats.

Frames of reference aren't relative, they're what things are moving relative to. The frame of reference is the coordinate axis by which we measure position in over time. A frame of reference has its own X, it's own Y, it's own Z, and it's own t (time.)A moving frame of reference has coordinate axes that implictly have their own velocity vectors, but since everything in that frame of reference has that same vector component we can ignore it. But when we translate from one coordinate system to another - when we shift from the observer inside the car to the observer standing motionless outside it as it drives by - then, to chart the position of objects in the new coordinate system, we have to add in the velocity vector of the reference frame.Everybody knows that, instinctively - if you toss a tennis ball from a car moving 100 mph, then it hits the target at whatever velocity you tossed it plus whatever velocity the car had. And this is vector addition, not number addition, so both speed and direction get added in. That's why if you toss the tennis ball behind the car instead of in front, it hits at a lower speed than 100 mph - you've added a negative velocity with respect to the forward velocity of the car because the tennis ball has moved from the reference frame of the car to a new reference frame. Not because it's stopped being attached to the car, but because the tennis ball has been accelerated - its velocity has changed over time.

Then how can the speed of light be the same for all observers regardless of their velocity?

Why is the Salt Lake Flats reference frame so special that it should intrude on the car's reference frame?What about the fact that the Salt Lake Flats is located on Earth, which is moving relative to the Sun's reference frame? And what about the fact that the Sun is moving relative to the reference frame of the Milky Way galaxy?If in fact the light pulse does miss the detector even though the emitter was pointed dead center at it, haven't you created a way to determine if you're in a moving reference frame from inside the reference frame? For instance - couldn't you use such a emitter-detector device to determine whether you were moving, or something was moving past you?And doesn't that, therefore, violate relativity? If you could build such an intrinsic motion detector, wouldn't that disprove Einstein's notions of relativity? How do you square that with the observation that our universe is relativistic?

Well, no. The car was introduced in many reference frames - the Salt Lake, the planet Earth, the Milky Way galaxy, the motion of the local supercluster, and so on - and you've chosen to ignore all of those except the Salt Lake reference frame, which for no apparent reason you assert has some kind of primacy over all the others. That is incorrect. The pulse will travel the distance from the emitter to the detector because it was aimed directly at the detector in all reference frames and the car is at constant velocity. Different observers in different reference frames will observe the pulse travel different paths but all observed paths begin at the emitter and end at the detector. I have not ever claimed that the car is not moving relative to the Salt Lake Flats. Why do you believe that I have? No, wrong. A universe where you can detect constant absolute velocity from within a moving reference frame - as you imagine the universe operates - is inherently non-relativistic because there's no need for anything to be relative. "Relative" means that velocity comparisons are reciprocal - if I observe that you're in movement past me, then you will observe that I am in movement past you. Velocity is relative in a relativistic universe.But in your universe, we can determine which one of us is moving, in what direction, and at what velocity relative to space itself, by means of observing the deflection of light beams produced within each of our respective reference frames. That makes velocity non-reciprocal and means that it is not a relativistic universe. And because the pole is fixed to the car, and the laser emitter is also fixed to the car, we know that the point where the laser is emitted - the emitter - is not ever moving relative to the pole. Therefore the pulse clearly hits the detector by your own definitions. Yes, you've hit on the inherent paradox involved in the fact that the speed of light is the same for all observers regardless of their velocity. And your instinct that something has to give, that something has to be "wrong" with reality, is absolutely correct.And what is "wrong" with reality is time dilation. That is how the paradox is resolved - we conclude (and this is borne out by direct observation) that observers in different inertial reference frames experience the passage of time at different rates. Since speed is a change in position over time, that results in the speed of light being the same for observers in different reference frames - as they use stopwatches or light clocks or chemical reactions or any form of time measurement to judge the speed of light, they arrive at the same observed speed of light regardless of their relative motion of travel to the light, because time in their reference frame slows down as a function of their speed. It's not traveling zero with regards to the car. It's traveling at C with regards to the car. To the observer on the flats, it is also traveling at C but taking a longer path. So the observer on the flats observes a longer interval between emission and capture than the driver of the car.That's time dilation, and it's a direct consequence of the speed of light being the same for all observers regardless of their velocity.

No, that's not correct. Relative to its own reference frame the car is motionless and the Salt Flats are shooting by underneath at .5c. Relative to the reference frame of the Sun the car is shooting around the sun at .5 c plus the Earth's motion around the Sun. Relative to the local supercluster (the neighborhood that includes the Milky Way galaxy) the car is moving at .5 c plus the Earth's motion around the Sun plus the Sun's motion through the galaxy plus the galaxy's motion through space, and so on.When we translate between these different reference frames we have to add the velocity of the reference frame we've left. That's what it means for velocity to be relative to something. We've not made any claim that light always travels in a straight line in the direction it was emitted. Haven't you heard of lenses? Haven't you heard of mirrors? These are all ways to change the direction of light. But the pulse is not traveling at a 90 degree angle relative to the salt flats. It's traveling at a 90 degree angle relative to the car. Relative to the salt flats, the pulse is traveling at an angled path that intersects with the moving detector.By trigonometry, we know that the path of the light pulse relative to the salt flats is much longer than the path of the light pulse relative to the car. (The hypotenuse of a right triangle is always longer than either of the two other sides.) If it were a tennis ball and not a light pulse, we would resolve this difference by observing that the velocity of the tennis ball relative to the salt flats is much faster than the velocity of the tennis ball relative to the car, because the tennis ball is imparted with the car's velocity (because it starts out at rest relative to the car.)But light is different than tennis balls. While the speed of a tennis ball depends on the relative velocity of the observer, the speed of light does not. The speed of light is the same for all observers regardless of their velocity.Hence the paradox - relative to the salt flats, the light pulse has a longer path to cover than relative to the car, but relative to observers in both frames of reference it has to have the same speed. It can't go any faster to cover the longer path relative to the salt flats, so the observer on the salt flats observes a longer period of time for the pulse to be captured by the detector than the driver does. In other words, relative to the observer standing still on the salt flats, things happen slower for the driver of the car. Not just for the driver, but time is actually slower within the reference frame of the moving car. Chemical and nuclear reactions happen slower. Clocks run slower. At rest, the engine of the car puts along at a "chug-chug-chug." But as the car drives by, the salt flats observer hears it go "chuuuug......chuuug.....chuuuuug" - despite the high rate of speed, the RPM of the engine actually appears to be lower, much lower, than when the car is at rest. According to the driver, it is faster - but a "minute" for the driver is a much longer period of time than for the stationary observer. Time is actually slowing down for the driver. That's time dilation. No, you will not. Motionless relative to you would mean that the distance between us was not changing, but since you are getting closer to me and then farther away when you pass, I am clearly not motionless relative to you. Because relative to the emitter the detector has not moved. The light pulse has moved at c in a straight line towards the detector. Since the detector hasn't moved relative to the emitter, and the emitter has always been pointed at the detector, the light pulse hits the detector. In the reference frame of the car, nothing has moved at all but the light pulse and the salt flats outside. In the reference frame of the salt flats, the emitter has moved along with the detector and the light pulse emerges from the emitter on an intercept trajectory to where the detector is going to be - two feet ahead, to use your numbers.The difference is the angle of the trajectory. Relative to the car it is 90 degrees. Relative to the salt flats it is less than that. Yes. http://en.wikisource.org/...Earth_and_the_Luminiferous_EtherThis experiment occurred in 1887 and proved, with no room for doubt, that the speed of light is the same for all observers regardless of their velocity. This is an explanation of the experiment: http://en.wikipedia.org/...chelson%E2%80%93Morley_experimentNo, this is not correct. Einstein quite clearly says that the speed of light is the same for "all inertial observers." An "inertial observer" is an observer whose velocity is constant. Therefore it's another way of saying that the speed of light is the same for all observers regardless of their velocity, as I've been saying. I'm simply restating Einstein's words in a simpler way for your understanding. That's a very perceptive question! And the answer is - all inertial observers. Everyone who measures the speed of light in a vacuum observes it to be moving at C, regardless of their own velocity relative to anything else. Directly vertical (a "90 degree angle" you would say.) The "point emitted" is the emitter, which has zero velocity relative to the car. If you're talking about a point in coordinate space, then you need to say which coordinate space you're talking about - the coordinate space of the car, or the coordinate space of the salt flats? Because relative to the salt flats yes, the pulse has motion in the forward direction relative to where the emitter was when the pulse was emitted.

No, this is incorrect. The pole is not moving at all in the reference frame of the car. Thus, in the reference frame of the car the pulse will strike the detector. No, this is incorrect. If the pulse were traveling at .5 c relative to the car or the car .5 c relative to the pulse, then an observer in the car would observe that the speed of light was .5 c.But we know what speed of light the observer in the car has to observe: c. Therefore we know that the pulse is moving at c relative to the car. Completely wrong. The pulse's path of travel in the reference frame of both the car and both the salt flats is straight, and it intersects with the detector in both reference frames. When it intersects with the detector in each reference frame - that's what's different, and that's the result of time dilation/ Everyone is agreed on this, ICANT, and no one has ever been able to exploit your "light doesn't take on velocity" position to construct a universal intrinsic velocity detector. Why do you suppose that is the case?

By measuring its position over time in a coordinate system whose origin moves with the car.I can place anything in any frame of reference at any time, but an object's inertial reference frame is the one in which it is not accelerating (has constant velocity, frequently constant zero velocity, i.e. is at rest.) A frame of reference is a frame, a coordinate system, for reference, i.e. a convenient or simplifying way to describe where an object is at different points in time. No, that is incorrect. The car has velocity .5 c in only one coordinate system - the reference frame of the salt flats. Relative to the emitter+detector system, the car has zero velocity. Relative to the car, the pulse can have only one velocity - c. The speed of light is the same for all observers, remember? The point in what coordinate system? Remember we're talking about multiple frames of reference, and your continued effort to obfuscate which frame of reference you're speaking from is the source of your confusion. When you become appropriately diligent about making explicit the frame of reference from which you're operating when you make statements about this "point" or that, you'll see that we have been correct all along. I have explained why. It's a matter of verified fact that the speed of light is the same for all observers regardless of their velocity, so the observer in the car can only observe pulses of light travel at c. They can never go any slower than that in a vacuum. That's what I did say the observer in the car would observe. The observer on the salt flats, however, because he is in a different reference frame and therefore in a different coordinate system, observes the path of the pulse at a shallower angle than 90° to the motion of the car. It doesn't matter where the emitter is located on the car. Light pulses leaving it will only travel at c. It doesn't matter how fast the car is going or the direction in which the emitter is located; observers in any reference frame will observe that light leaves the emitter at the speed of c. Nothing. The light pulse never leaves its straight line direction, so it hits the detector directly above where it is pointed. How many times do we have to explain that?

As we've explained, where we would place that frame such that the pulse leaves the emitter at a lower than 90° to the direction of travel of the car is stationary with regards to the salt flats.That is why the observer inside the car will see the pulse travel in a direction 90° to the car's direction of travel relative to the salt flats, but the observer on the salt flats will see the pulse travel at a shallower angle relative to the car's direction of travel relative to the salt flats. They reason that they don't agree on the angle of travel is because these two coordinate systems are in motion relative to each other. You have the origin of your coordinate axis pointed out the back of the laser pointer instead of the front. Could you explain why that is?

Really? Here's an experiment you can do at home: point your laser pen at a mirror.

And what spot was that? Was the spot directly in a straight line with the direction the pen was pointed, or was it in fact somewhere on the wall behind you?

So the spot was on the pen, back where it started - not ahead of the pen in a straight line.So that answers your question about under what circumstances light won't travel in a straight line in the direction that the laser pen is pointed. In what reference frame? In the reference frame of the car, neither the laser pen nor the sensor have any motion at all. Do you see why? It's because of what motion is.What is motion, ICANT? It's a change in position over time. And the way that we measure position is relative to the position of something else that we refer to as the "origin", or sometimes "zero." That's the center of the coordinate system, the point (0,0,0). That point can be anywhere in space and have any intrinsic motion of its own that we decide.When we define an origin as being in motion, and having the same motion as another object (say, a moving car) then we describe that coordinate system as being the reference frame of the object. The reference frame extends in all directions to infinity, and it contains all objects in the universe as a result. Everything is in all reference frames at all time. When the reference frame has constant velocity, it is an inertial reference frame. When we say "observed reference frame" or "observers reference frame", that refers to the reference frame by which a certain observer is measuring the position of other objects in space. Practical concerns about how far observers can observe are irrelevant to that; an observer is defined as being able to observe the event in question.The same objects measured in different reference frames by different observers have different properties. Their speeds will differ. The direction of their velocity will differ. For reasons I've already explained, their masses and dimensions will differ as well, even though it seems like they shouldn't. This isn't any different from the fact that if you measure an angle from one side, it may be 30°, but if you measure it from the other side it may be 150°. Measuring always depends on what you're measuring from.The one exception to this is that any observer measuring the speed of light, in any reference frame, will measure the light to have speed c. This has been proven experimentally over and over again, and has been known for well over 100 years.

That is correct. As a result of its straight line motion it will hit the moving detector.