The Spaceship experiment: Questions on Light's Inertia and Measuring Distance

[DonB]

Hello. Some years ago I came across an animated illustration of relativity, housed in what seems to be a classic use of a bouncing light within a spaceship. (Similar to that found in http://galileoandeinstein.physics.virginia.edu/lectures/srelwhat.html.) My education in physics is limited to one general physics course in college, so I'm not even up to novice level in this field. But after thinking on this (in my own limited way), two questions have haunted me that I am hoping someone can kindly explain to me.

One, the mental experiment is build upon the premise that besides light's own trajectory (at c) perpendicular to the path of the spaceship (SS), light is also traveling along within SS at the latter's velocity -- as if it is inerially bound to the vehicle. Before even attempting to deal with questions of whether this forces light to travel at faster than C (mathematically, the SqRt of c² + V_ss²), I am first curious as to what is the basis for believing that the light beam will be "pulled" along within the trajectory of SS? My assumption is that light is not inertially bound to SS, and thus once "started" that beam of light would travel in a straight line totally independent of SS's movement, non-movement, or change in movement. What am I missing? Can someone point me to experimental (or other) evidence that ties light to the movement of a material vehicle?

Second, I'm having trouble grasping the differences in distances (and thus, the warping of time/space to allow for light speed constancy) that this experiment is supposed to illustrate. According to what I've read, within the same fraction of a second the light beam travels different distances relative to the onboard and outside observers (arguably necessitating a warp in time/space to allow light to remain a constant speed as it travels these different distances). I do follow the explanation of difference in distance..., but I'm not seeing how that really describes the situation. Using a four-dimensional coordinate system (i.e., the spatial X, Y and Z axises within SS (or whatever coordinate system that light is bound to), plus a fourth dimension of where those axises are at T(1) and T(2) (Time 1 and Time 2) as the coordinate system (SS?) itself travels through space, it looks like the distances are identical for both observers -- in fact, for all observers wherever they may be. If that is right, then it seems to me to remove the necessity for time/space warp, at least within the illustration.

Please understand, I'm not approaching this as a challenge to Einstein or his theory. It's just that I'm hoping that someone more knowledgeable in this area can kindly explain what I'm missing.

Thanks.

 

[Doc Al]

One, the mental experiment is build upon the premise that besides light's own trajectory (at c) perpendicular to the path of the spaceship (SS), light is also traveling along within SS at the latter's velocity -- as if it is inerially bound to the vehicle. Before even attempting to deal with questions of whether this forces light to travel at faster than C (mathematically, the SqRt of c² + V_ss²), I am first curious as to what is the basis for believing that the light beam will be "pulled" along within the trajectory of SS? My assumption is that light is not inertially bound to SS, and thus once "started" that beam of light would travel in a straight line totally independent of SS's movement, non-movement, or change in movement. What am I missing? Can someone point me to experimental (or other) evidence that ties light to the movement of a material vehicle?

The light does just travel in a straight line (as seen in any inertial frame). It doesn't care about the motion of the ship. Not sure why you think the light is being dragged along--the ship and the light are both just moving in straight lines. As far as experimental evidence: Shine a flashlight against the wall. You're in a spaceship now--the Earth is zipping along with respect to some other frame--yet the light just shoots out in a straight line as expected.

Second, I'm having trouble grasping the differences in distances (and thus, the warping of time/space to allow for light speed constancy) that this experiment is supposed to illustrate. According to what I've read, within the same fraction of a second the light beam travels different distances relative to the onboard and outside observers (arguably necessitating a warp in time/space to allow light to remain a constant speed as it travels these different distances). I do follow the explanation of difference in distance..., but I'm not seeing how that really describes the situation. Using a four-dimensional coordinate system (i.e., the spatial X, Y and Z axises within SS (or whatever coordinate system that light is bound to), plus a fourth dimension of where those axises are at T(1) and T(2) (Time 1 and Time 2) as the coordinate system (SS?) itself travels through space, it looks like the distances are identical for both observers -- in fact, for all observers wherever they may be. If that is right, then it seems to me to remove the necessity for time/space warp, at least within the illustration.

Not sure what you mean here. Each frame uses its own coordinate system. That's part of the point: When observing the moving ship, you measure its speed and time of flight using your own clocks and metersticks. (And those on the ship do the same--they use their own clocks and metersticks.)

 

[DonB]

The light does just travel in a straight line (as seen in any inertial frame). It doesn't care about the motion of the ship. Not sure why you think the light is being dragged along--the ship and the light are both just moving in straight lines. As far as experimental evidence: Shine a flashlight against the wall. You're in a spaceship now--the Earth is zipping along with respect to some other frame--yet the light just shoots out in a straight line as expected.

Let me explain: SS experiment, two scenarios. First, SS is "dead in space"; the light beam is shined from the ceiling towards the floor; observers both in the ship and outside see the light go straight down (i.e., it's path is perpendicular to the direction that SS is pointing).

Second scenario, same as the first, except that SS is traveling at a high rate of speed. The classic illustration says that along with its perpendicular travel, the light also travels at the same speed and direction of SS (i.e., within SS), giving a somewhat diagonal path relative to the outside observer. So..., what causes the beam of light to "add" this additional parallel movement just because it's on board the now-moving SS -- a movement that it didn't have in the first scenario?

Yes, we are on spaceship Earth that is zipping through space..., but it's movement is so slow (as a ratio to the speed of light) that it has no material impact on light hitting a "moving target". But if the Earth's movement was increased to the speed of light (for discussion purposes), would the target move out of light's path before the beam actually got to it?

 

[DonB]

Not sure what you mean here. Each frame uses its own coordinate system. That's part of the point: When observing the moving ship, you measure its speed and time of flight using your own clocks and metersticks. (And those on the ship do the same--they use their own clocks and metersticks.)

Sorry for not being clearer. Suppose at T1 the light beam (B) begins it's trip from the ceiling to the floor, starting at the X, Y, Z coordinate of 0, 0, 0 (within SS). (Allowing the experiment's premise that the beam of light travels along with SS at SS's direction and velocity) at T2 B has covered 10m and is at the floor of SS. Both inside and outside observers see B at that same point -- 0, 10, 0 (within SS coordinates). Therefore, it seems to me, B has traveled from 0, 0, 0 @ T1 to 0, 10, 0 @ T2 for all observers. Thus, there is no difference in distance, and consequently no difference in time.

Again, not arguing that I'm right..., just asking where I'm wrong.

 

[PAllen]

Second scenario, same as the first, except that SS is traveling at a high rate of speed. The classic illustration says that along with its perpendicular travel, the light also travels at the same speed and direction of SS (i.e., within SS), giving a somewhat diagonal path relative to the outside observer. So..., what causes the beam of light to "add" this additional parallel movement just because it's on board the now-moving SS -- a movement that it didn't have in the first scenario?

From the point of view of the outside observer, a 'vertical' photon would hit the flashlight wall, since the flashlight is moving. Only one angled so as to hit the same spot as observed within the ship will clear the flashlight.

 

[Doc Al]

Let me explain: SS experiment, two scenarios. First, SS is "dead in space"; the light beam is shined from the ceiling towards the floor; observers both in the ship and outside see the light go straight down (i.e., it's path is perpendicular to the direction that SS is pointing).

There's no such thing as being 'dead in space', by which I assume you mean 'not moving'. All speeds are relative to something. That same ship may be at rest with respect to you, but zipping along at 0.99c with respect to some other frame.

Second scenario, same as the first, except that SS is traveling at a high rate of speed. The classic illustration says that along with its perpendicular travel, the light also travels at the same speed and direction of SS (i.e., within SS), giving a somewhat diagonal path relative to the outside observer. So..., what causes the beam of light to "add" this additional parallel movement just because it's on board the now-moving SS -- a movement that it didn't have in the first scenario?

The only difference between the two scenarios is the speed of the observing frame. To the folks in the ship there is no difference whatsoever. A ship moving at constant velocity cannot in any way tell that they are moving by doing an experiment with light beams inside the ship. Light beams work the same way no matter what their speed. This is a basic principle of relativity.

And to add to what PAllen stated, the angle that the light beam makes depends on the frame doing the observing. In the ship, the beam is vertical. (Why? Because that's where they aimed the beam.) Viewed from a frame in which the ship is moving, that same beam of light is now at an angle.

Yes, we are on spaceship Earth that is zipping through space..., but it's movement is so slow (as a ratio to the speed of light) that it has no material impact on light hitting a "moving target". But if the Earth's movement was increased to the speed of light (for discussion purposes), would the target move out of light's path before the beam actually got to it?

No, of course not. (And realize that with respect to something, we probably already are moving at close to the speed of light.)

There was a time (in the 1800's) when it was thought that light traveled through some medium--called the 'ether'--and thus would be dragged along with that ether. The famous experiment of Michelson and Morley was an attempt to detect such motion through the ether. (See: http://galileoandeinstein.physics.virginia.edu/lectures/michelson.html" ) But Einstein's insight dispensed with all of that.

 

[Doc Al]

Sorry for not being clearer. Suppose at T1 the light beam (B) begins it's trip from the ceiling to the floor, starting at the X, Y, Z coordinate of 0, 0, 0 (within SS). (Allowing the experiment's premise that the beam of light travels along with SS at SS's direction and velocity) at T2 B has covered 10m and is at the floor of SS. Both inside and outside observers see B at that same point -- 0, 10, 0 (within SS coordinates). Therefore, it seems to me, B has traveled from 0, 0, 0 @ T1 to 0, 10, 0 @ T2 for all observers. Thus, there is no difference in distance, and consequently no difference in time.

(1) The light doesn't 'travel along with SS'. It just travels. In the frame of the ship, its motion is vertical; viewed from the other frame, it slants.
(2) The point is not to measure things using the coordinates system of the ship. That would be trivial! We know what the ship sees! The point is to determine what the outside observer measures using his own clocks of course.

The point of the exercise is to use the principles of relativity to deduce how a moving clock would behave as seen from another frame. (And the bouncing light is a good clock for that purpose since it is easy to analyze.)

 

[DonB]

(1) The light doesn't 'travel along with SS'. It just travels. In the frame of the ship, its motion is vertical; viewed from the other frame, it slants.
(2) The point is not to measure things using the coordinates system of the ship. That would be trivial! We know what the ship sees! The point is to determine what the outside observer measures using his own clocks of course.

The point of the exercise is to use the principles of relativity to deduce how a moving clock would behave as seen from another frame. (And the bouncing light is a good clock for that purpose since it is easy to analyze.)

(1) I think you miss my point. Between the two scenarios the outside observer sees a strictly vertical movement, and then a slanted movement (as you say). The only difference in the two scenarios is that SS is moving, thus implying that SS's movement makes a difference in B's (light beam) movement, much the same as a train's movement will transfer inertia/momentum to a ball that a kid is bouncing inside the rail car. So..., if this presumed change in lateral movement is correct, how is that lateral movement effected upon B?

(2) Again, I see your point, but feel that you miss mine. My point is that in fact the two observers see light cover the same distance in the same amount of time -- and as such, there seems to be no need to warp time/space. Granted, the different observers could use different coordinating systems to measure; but that strikes me as being similar to my using the metric system while my neighbor uses the English system to measure the distance between our houses -- then assuming that there must some kind of "warp" that causes the difference in readings.

Are you telling me that a four-dimensional coordinating system will not give the same distance for all observers? It seems to me that it will allow the inside observer to see beyond the artificial limitations of his environment -- limitations that mask portions of B's movement as he moves along with it.

It seems that movement "relative" to an observer is being confused for "net" movement between the observer and B; and then such net movement must then be "warped" to make that net equal c.

 

[PAllen]

Let's try this. Withing train (or rocket) you shine a flashlight light at your feet and it hits your feet. A camera on your toe takes a picture of it. Now look from ground observer:

1) Do you think ground observer sees light *not* hit your feet? The picture comes out different as seen by the ground observer? If you think this, we have bigger issues than relativity to worry about.

2) If ground observer sees light hit your feet, since they move during emission and absorption, ground observer *must* see light move at an angle. There are various detailed ways to understand this (depending on the light source), but they must all come out the same.

Meanwhile, ground observer shines a flashlight at his feet. Train observer sees ground observer's light hit feet, so ground observer's light is slanted for train observer, just like train observer's is for ground observer.

So far, all of this is Galilean relativity. Newton, Galileo, Lorentz, and Michelson (before famous experiment) would have all agreed with this. Where there was an issue in the late 1800s was the belief that one of these beams could be measured to be slower than the other. Experiment showed otherwise.

 

[DonB]

There's no such thing as being 'dead in space', by which I assume you mean 'not moving'. All speeds are relative to something. That same ship may be at rest with respect to you, but zipping along at 0.99c with respect to some other frame.


The only difference between the two scenarios is the speed of the observing frame. To the folks in the ship there is no difference whatsoever. A ship moving at constant velocity cannot in any way tell that they are moving by doing an experiment with light beams inside the ship. Light beams work the same way no matter what their speed. This is a basic principle of relativity.

And to add to what PAllen stated, the angle that the light beam makes depends on the frame doing the observing. In the ship, the beam is vertical. (Why? Because that's where they aimed the beam.) Viewed from a frame in which the ship is moving, that same beam of light is now at an angle.


No, of course not. (And realize that with respect to something, we probably already are moving at close to the speed of light.)

There was a time (in the 1800's) when it was thought that light traveled through some medium--called the 'ether'--and thus would be dragged along with that ether. The famous experiment of Michelson and Morley was an attempt to detect such motion through the ether. (See: http://galileoandeinstein.physics.virginia.edu/lectures/michelson.html" ) But Einstein's insight dispensed with all of that.

¶1) "dead in space" -- specifically, not moving relative to both of the observers.

¶2) Yes, I realize that this is basic relativity; I'm just asking what is the basis for accepting it -- the evidence that supports this.

¶3) I'm trying to wrap my mind around this. As I'm understanding it, a light travels in a straight line relative to the angle of the light source -- for discussion purposes, let's suppose a flashlight. On board SS the flashlight is attached to the ceiling, pointing vertically, as SS travels a solely horizontal path at .99c. You're saying that the inside observer (O1) sees the light travel a solely vertical path following the direction that the flashlight is pointing, but O2 (outside observer) sees a near-45 degree slanting path. So, the question becomes: (1) is the slanted path truly a longer path, which means that the beam of light is not taking a straight line relative to its source (as described above); or (2) is the slanted path really nothing more than the light following the straight 10m ceiling-to-floor path from the flashlight, but only given the appearance of longer length as SS moves the floor and light-beam laterally?

Thanks for the link.

 

[DonB]

Let's try this. Withing train (or rocket) you shine a flashlight light at your feet and it hits your feet. A camera on your toe takes a picture of it. Now look from ground observer:

1) Do you think ground observer sees light *not* hit your feet? The picture comes out different as seen by the ground observer? If you think this, we have bigger issues than relativity to worry about.

I likely have significant issues, but no this isn't one of them. <ha>

Ultimately I question that, given sufficient speed in the rocket, if a light aimed at my feet would actually strike my feet (assuming the light path is perpendicular to the rocket path). As I understand the mental experiment, we're told that if the rocket was going .25c the light will hit my foot; if .5c, the light will hit my foot; same for .75c, .9c, and .99c. So that begs the question, from an outside observer's perspective, what causes the parallel-to-the-rocket-path portion of the light's movement increase as the rocket goes faster? Intuition inclines me to think that, from the outside observer's viewpoint, the light has no parallel movement, and given sufficient speed the rocket will move my foot before the light gets there. That the inside observer doesn't see the same thing I am inclined to think would be from distortion due to the movement of the rocket.

 

[Doc Al]

¶3) I'm trying to wrap my mind around this. As I'm understanding it, a light travels in a straight line relative to the angle of the light source -- for discussion purposes, let's suppose a flashlight. On board SS the flashlight is attached to the ceiling, pointing vertically, as SS travels a solely horizontal path at .99c. You're saying that the inside observer (O1) sees the light travel a solely vertical path following the direction that the flashlight is pointing, but O2 (outside observer) sees a near-45 degree slanting path.

Yes.

So, the question becomes: (1) is the slanted path truly a longer path, which means that the beam of light is not taking a straight line relative to its source (as described above);

Yes.

or (2) is the slanted path really nothing more than the light following the straight 10m ceiling-to-floor path from the flashlight, but only given the appearance of longer length as SS moves the floor and light-beam laterally?

I don't understand why you say 'given the appearance' of longer length. Example: you are in your car traveling with respect to the road at 30 m/s. You drop your coffee cup, which travels downward 1 meter before it hits the floor. With respect to you, the cup travels 1 m. But as measured by me, standing on the side of the road, you and your cup have moved about 13.5 m horizontally during the 0.45 seconds that your cup fell. Are you saying that the cup 'really' only moved 1 meter? Doesn't make much sense to me. The distance something travels depends on who is measuring that distance. (Note that this is true even in pre-Einstein physics of Galileo and Newton.)

 

[Doc Al]

Ultimately I question that, given sufficient speed in the rocket, if a light aimed at my feet would actually strike my feet (assuming the light path is perpendicular to the rocket path). As I understand the mental experiment, we're told that if the rocket was going .25c the light will hit my foot; if .5c, the light will hit my foot; same for .75c, .9c, and .99c. So that begs the question, from an outside observer's perspective, what causes the parallel-to-the-rocket-path portion of the light's movement increase as the rocket goes faster? Intuition inclines me to think that, from the outside observer's viewpoint, the light has no parallel movement, and given sufficient speed the rocket will move my foot before the light gets there. That the inside observer doesn't see the same thing I am inclined to think would be from distortion due to the movement of the rocket.

You seem to think that the light is 'really' moving with respect to some medium that gets left behind as the rocket moves forward. But no, there is no such medium. It is a principle of relativity that all inertial frames are equivalent and that light moves at the same speed in every frame.

Constant velocity doesn't cause 'distortion' of anything. Right now you are moving at one speed with respect to some frame and another speed with respect to some other frame. Does it make sense that what you see is 'distorted' by the mere fact that you are moving with respect to something? And which something?

 

[DonB]

Yes.

Yes.

So..., from the outside observer's perspective, what force alters the light's direction -- it was initially pointing solely vertical as it came out of the flashlight, but immediately "bends" as it leaves the flashlight and travels at a near 45 degree slant. But why? I have many flashlights, and not a one of them shines a beam 45 degrees off axis. So, what force forces the beam off it normal course (from outside observer's perspective)?

I don't understand why you say 'given the appearance' of longer length. Example: you are in your car traveling with respect to the road at 30 m/s. You drop your coffee cup, which travels downward 1 meter before it hits the floor. With respect to you, the cup travels 1 m. But as measured by me, standing on the side of the road, you and your cup have moved about 13.5 m horizontally during the 0.45 seconds that your cup fell. Are you saying that the cup 'really' only moved 1 meter? Doesn't make much sense to me. The distance something travels depends on who is measuring that distance. (Note that this is true even in pre-Einstein physics of Galileo and Newton.)

NOW we're getting somewhere. This illustration is exactly what I need (I think) to show what I'm thinking. First, no, I'm not saying that the cup only moved 1 meter; and yes, the distances measured depends on who is measuring and his frame of reference. On those, we are agreed.

However, would you not agree with me that I, as the inside observer, will see the coffee cup travel at one speed, while you as the outside observer will see it travel at a whole different speed? No one in their right mind (I would think) would attempt to argue for some warp in time and/or space to suggest that both of us see the cup travel at exactly the same speed as it covers the two different distance within the same amount of time. So, maybe the key to all of my questions (or a big part of them) lies in this: what evidence necessitates that this exact same thing is not possible when we substitute light for the coffee cup.

(Thanks Doc for hammering this out with me. I know it's got to be frustrating; but I do appreciate it.)

 

[DonB]

You seem to think that the light is 'really' moving with respect to some medium that gets left behind as the rocket moves forward. But no, there is no such medium. It is a principle of relativity that all inertial frames are equivalent and that light moves at the same speed in every frame.

Let's say that I have not yet seen anything that proves to me that light is not moving with respect to such a medium; or that possibly light is in some yet inexplicable way that medium itself. Nor do I see reason to discard these options and (without evidence that I know of) assume that the spaceship is the medium for light. If you know of such evidence, I'd really love to hear about it.

Doc, you regularly say that it is a principle of relativity that <whatever your point is>. I am not so much doubting that it is a principle, but that isn't my question. My question is why is it a principle of relativity? What is the proof that light moves at the same speed in every frame?

Constant velocity doesn't cause 'distortion' of anything. Right now you are moving at one speed with respect to some frame and another speed with respect to some other frame. Does it make sense that what you see is 'distorted' by the mere fact that you are moving with respect to something? And which something?

Sure it makes sense -- and that's exactly why I mentioned the distortion in perspective. To me is not dissimilar to driving down the road at 60 MPH and pull up to a friend on a motorcycle and match his speed. For the moment that your eyes meet and your mind runs through the recognition process you fail to notice that you are moving -- you get the feeling of sitting still. Why? Because your speed hides your reality from you. I'm not saying that this is identical to the spaceship scenario, but it illustrates the way that the movement of the vehicle in which one is 'riding' can distort his perspective on other things that are moving (e.g., a light beam).

But I'm still having trouble with light traveling laterally within the spaceship. Let me offer a couple of new experiments. First, suppose that along with the flashlight on the ceiling of the spaceship there is an identical flashlight mounted to a spacedock, and at an identical angle to the first one. The dock is not moving relative to O2 (outside observer), and the ship passes it at .99c. As the flashlights reach their nearest point, both are fired. What does O2 see? It seem obvious to me that B1 (the light from the stationary flashlight) is seen by O2 as taking a solely vertical path; agreed? But what about the B2 within the spaceship? The classic experiment holds that, from the perspective of O2, B2 takes a near 45 degree diagonal path. So, the question is, why does B2 take a diagonal path while an identical flashlight at an identical angle viewed from an identical perspective (O2's) take a whole different trajectory? Is it that, like a ball bounced on a train, the vehicle has transferred momentum to the "projectile" (B2) that is riding within it? If so, how is momentum transferred to B2? If not, then what force bears upon B2 to make it take a different trajectory than B1?

A second experiment: Suppose O2 is holding a flashlight aimed at the trajectory of SS (spaceship). At just the right moment (T1) he hits the momentary switch and a very short burst of light heads out into space. At T2 it reaches SS and enters the window on the starboard side, and from T2 to T3 passes through the inside cabin of SS. At T3 it exits a port window and continues through space until it strikes something and ends its journey at T4. The question is, what is the angle of trajectory (relative to the original path) in each of the trajectory segments. By definition from T1 to T2 the angle the angle of trajectory is 0º -- which is the direction that the flashlight was originally pointed. I also presume that T3-T4 is 0º. But what about T2-T3? Options and resulting questions:

1) T2-T3 is also 0º. This spawns the question of why does light here not "travel along" (left to right) within SS, but it (allegedly) does in the original experiment?

2) T2-T3 is approx. 45º (relative to T1-T2). If this is so, then what force 'bends' the light's path and momentarily gives it horizontal momentum. How is that force transferred to the light? And how is it removed from the light once the light exits SS?

 

[DonB]

I don't understand why you say 'given the appearance' of longer length.

Doc, I'm not sure that I answered your question in my earlier response. I will attempt that now.

Suppose I am on a ship that is cruising along at 10KmPH. I decide to cross the 100m width of the ship (perpendicular to the ships own movement in the sea), also at 10 KmPH. So, after I reach the other side, how far have I gone? Some argue that it is a matter of perspective (which is true, as far as it goes). I see that I've gone 100m; the pilot of the helicopter above us see that I've gone 141.4m as he factors in the ship's movement that I do not see.

But the answer to "how far have I gone?" is more than a matter of perspective, but it is a matter of definition as well. What is meant by the question? If the question is a matter of one's ability to propel himself through distance, then I can not include the portion of the distance that was not my own doing. I must differentiate my own propulsion through the 100m from the ship's movement through the perpendicular 100m. In short, I only walked 100m; the pilot saw me walk only 100m, although that 100m "gave the appearance" of covering the 141.4m that my body traveled during that time period.

The pilot would be foolish to call the local news media and report that I had set a new jogging speed record; nor would anyone believe him if he said that there must be some warp in time/space. Yet I'm feeling that relativity is asking me to swallow that very same kind of mentality -- and that without supporting evidence.

 

[Doc Al]

So..., from the outside observer's perspective, what force alters the light's direction -- it was initially pointing solely vertical as it came out of the flashlight, but immediately "bends" as it leaves the flashlight and travels at a near 45 degree slant.

From an outside observer's perspective, the light doesn't 'bend'--it's always moving on a slant. No force needed.

But why? I have many flashlights, and not a one of them shines a beam 45 degrees off axis.

Sure about that? I'd say they all do when viewed from a moving frame.

So, what force forces the beam off it normal course (from outside observer's perspective)?

Again, no 'force' is involved. When you drop your cup in the car and I see it move to the side (along with the car), what force is involved? None.

NOW we're getting somewhere. This illustration is exactly what I need (I think) to show what I'm thinking. First, no, I'm not saying that the cup only moved 1 meter; and yes, the distances measured depends on who is measuring and his frame of reference. On those, we are agreed.

OK.

However, would you not agree with me that I, as the inside observer, will see the coffee cup travel at one speed, while you as the outside observer will see it travel at a whole different speed? No one in their right mind (I would think) would attempt to argue for some warp in time and/or space to suggest that both of us see the cup travel at exactly the same speed as it covers the two different distance within the same amount of time. So, maybe the key to all of my questions (or a big part of them) lies in this: what evidence necessitates that this exact same thing is not possible when we substitute light for the coffee cup.

Yes, it's pretty strange that for low speeds like those of cars and cups, speeds add just like our normal everyday experience (with Galilean relativity) suggests. But it turns out that things don't quite work out that way for high speeds. Light itself moves at the highest speed possible, so nothing you can do by moving the source or yourself can change that speed.

As far as direct evidence for the invariance of light speed, look at the sticky at the top of the forum: https://www.physicsforums.com/showthread.php?t=229034")

The real clincher is that relativity works! The predictions of special relativity--which are based on the invariant speed of light--have been tested exhaustively and confirmed countless times.

 

[DonB]

From an outside observer's perspective, the light doesn't 'bend'--it's always moving on a slant. No force needed.

Sure about that? I'd say they all do when viewed from a moving frame.

Point well made. I admit that I default (often unknowingly) to a I'm-standing-still-so-that-must-be-the-absolute-point-of-reference perspective, and that no doubt plays into the this.

Again, no 'force' is involved. When you drop your cup in the car and I see it move to the side (along with the car), what force is involved? None.

Ah, but there I disagree. Prior to dropping the coffee -- as I stopped by McD's and picked it up -- you and I were in the same frame of reference. Had I dropped the coffee at that point, we would have both seen it fall straight down. But what gave it the diagonal path that you later saw after I was moving was the momentum that was transferred to it when I accelerated the car up to cruising speed. Without the kinetic energy of that transferred momentum, you would see the cup fall straight down, right? So..., where does that same kinetic energy come from in the light/spaceship experiment, and how is it transferred?

Yes, it's pretty strange that for low speeds like those of cars and cups, speeds add just like our normal everyday experience (with Galilean relativity) suggests. But it turns out that things don't quite work out that way for high speeds. Light itself moves at the highest speed possible...

I hope you will forgive me (no insult intended), but absolutes like that always make me cringe. The history of science (and other fields) have regularly shown that the absolutes of yesterday are only the thresholds of tomorrow. How can we know that this (or any other) absolute is true? Obviously "hasn't been" is a far cry from "can't be." I'm left to believe that it is based upon speculation (again, forgive the term); and if this concept is really based upon speculation instead of irrefutable evidence, then what other "givens" are presumed as well. Not what I feel comfortable with, personally.

, so nothing you can do by moving ... yourself can change that speed.

"...can change that speed", relative to what? If my car stalls on the highway, and a trailer-truck is heading straight for me and the car at 70 MPH, my bailing out of the car and barefooting it down the highway at 80 MPH doesn't change the speed of the truck..., relative to the car or the ground. But it does change it relative to me. I'm not yet seeing why the same thing won't happen with light.

The real clincher is that relativity works! The predictions of special relativity--which are based on the invariant speed of light--have been tested exhaustively and confirmed countless times.

I understand that point, and it is not without merit. Yet, pre-Einsteinian physics worked, too -- at least for a long, long time. As I wrote in the paper that I mentioned earlier, being "a" answer doesn't mean that something is "the" answer.

 

[Mentz114]

I hope you will forgive me (no insult intended), but absolutes like that always make me cringe. The history of science (and other fields) have regularly shown that the absolutes of yesterday are only the thresholds of tomorrow. How can we know that this (or any other) absolute is true? Obviously "hasn't been" is a far cry from "can't be." I'm left to believe that it is based upon speculation (again, forgive the term); and if this concept is really based upon speculation instead of irrefutable evidence, then what other "givens" are presumed as well. Not what I feel comfortable with, personally.
"...can change that speed", relative to what? If my car stalls on the highway, and a trailer-truck is heading straight for me and the car at 70 MPH, my bailing out of the car and barefooting it down the highway at 80 MPH doesn't change the speed of the truck..., relative to the car or the ground. But it does change it relative to me. I'm not yet seeing why the same thing won't happen with light.

This has been answered in this thread. Light is a wave, and that makes the difference.

https://www.physicsforums.com/showthread.php?t=503207 starting at post#63.

There's even a movie.

 

[Doc Al]

Ah, but there I disagree. Prior to dropping the coffee -- as I stopped by McD's and picked it up -- you and I were in the same frame of reference. Had I dropped the coffee at that point, we would have both seen it fall straight down. But what gave it the diagonal path that you later saw after I was moving was the momentum that was transferred to it when I accelerated the car up to cruising speed. Without the kinetic energy of that transferred momentum, you would see the cup fall straight down, right? So..., where does that same kinetic energy come from in the light/spaceship experiment, and how is it transferred?

Rather that worry about accelerating frames (which are a bit tricky to deal with), just stick to frames that were always in relative motion. That's typically the stipulation in most elementary relativity thought experiments, such as the light clock.

The cup is already in a car that happens to be moving with respect to me.

I hope you will forgive me (no insult intended), but absolutes like that always make me cringe.

And yet you seem to be clinging desperately to your 'common sense' Galilean relativity as if it were an absolute.

The history of science (and other fields) have regularly shown that the absolutes of yesterday are only the thresholds of tomorrow. How can we know that this (or any other) absolute is true? Obviously "hasn't been" is a far cry from "can't be." I'm left to believe that it is based upon speculation (again, forgive the term); and if this concept is really based upon speculation instead of irrefutable evidence, then what other "givens" are presumed as well. Not what I feel comfortable with, personally.

Luckily, your personal comfort level is not the criterion used to evaluate a theory.

"...can change that speed", relative to what? If my car stalls on the highway, and a trailer-truck is heading straight for me and the car at 70 MPH, my bailing out of the car and barefooting it down the highway at 80 MPH doesn't change the speed of the truck..., relative to the car or the ground. But it does change it relative to me. I'm not yet seeing why the same thing won't happen with light.

Because you insist on applying a Galilean version of the addition of velocity, where if you are walking forward in a train car at 5 mph and the train itself is moving at 20 mph, the speed of you with respect to the tracks is 20 + 5 = 25 mph. Well, sadly, that rule was found wanting and has been replaced by something that works for all speeds, including that of light.

I understand that point, and it is not without merit. Yet, pre-Einsteinian physics worked, too -- at least for a long, long time. As I wrote in the paper that I mentioned earlier, being "a" answer doesn't mean that something is "the" answer.

You are well over 100 years behind the times. And no one says anything is THE answer; that's not how science works. Newtonian physics worked great and still is the workhorse of everyday physics and engineering. But we found that it didn't work under certain conditions. It was replaced by Relativistic physics, which has proven itself to work better than Newtonian physics. Even if it defies 'common sense'.

 

[DonB]

Rather that worry about accelerating frames (which are a bit tricky to deal with), just stick to frames that were always in relative motion. That's typically the stipulation in most elementary relativity thought experiments, such as the light clock.

The cup is already in a car that happens to be moving with respect to me.

As you wish... So, the the cup is already in a car that is moving in respect to you, and thus already as momentum transferred to it from the car. So..., how did the momentum from the already moving lightbeam in the already moving spaceship transfer from the latter to the former? Surely there is an answer to this, and the absence of it is a major hindrance in me wrapping my mind around how this thing works.

And yet you seem to be clinging desperately to your 'common sense' Galilean relativity as if it were an absolute.

Forgive me if I was wrongly taught that the scientific process involves moving from the known to the unknown. I do not desparately cling to anything, but rather I'm starting where I am. I suppose some of us just evolve slower than others.

Luckily, your personal comfort level is not the criterion used to evaluate a theory.

Never said it should be. Just trying to explain where I am in the process.

And no one says anything is THE answer...

Sorry, but that isn't the way that it sounds. I ask a question of "why", and get something like "principles of relativity state..." as if it were the end in itself. So much given in definitives that it doesn't sound to me to be understood as less than that.

You are well over 100 years behind the times.

The Neanderthal apologizes. I sincerely appreciate your forbearance; but I feel I've taken too much of your time and pressed you to the limit, and for that I am sorry.

 

[Janus]

Ah, but there I disagree. Prior to dropping the coffee -- as I stopped by McD's and picked it up -- you and I were in the same frame of reference. Had I dropped the coffee at that point, we would have both seen it fall straight down. But what gave it the diagonal path that you later saw after I was moving was the momentum that was transferred to it when I accelerated the car up to cruising speed. Without the kinetic energy of that transferred momentum, you would see the cup fall straight down, right? So..., where does that same kinetic energy come from in the light/spaceship experiment, and how is it transferred?

Let's put it this way: If I was holding a coffee cup also, and dropped it after after you left McD's, you would see my cup fall at a diagonal with respect to you, but my cup didn't change momentum between you stopping and you driving off.

My cup drops straight down according to me and yours falls at the diagonal.
Your cup drops straight down according to you and mine falls on the diagonal.
It doesn't matter which of us accelerated to get to that state or what velocities we had before we dropped our cups, all that matters is that we have a relative velocity.

 

[DonB]

Very enlightening point, Janus. That was very helpful. I'm going to have to chew on that for a while to see what all the ramifications (in my thinking) are.

Thanks!

 

[DonB]

Let's put it this way: If I was holding a coffee cup also, and dropped it after after you left McD's, you would see my cup fall at a diagonal with respect to you, but my cup didn't change momentum between you stopping and you driving off.

Janus, I've been giving your explanation quite a bit of thought. Let me just throw out some of what has crossed my mind, and you (or anyone else) can feel free to interact with any of it.

1) As I understand the discussion, it's basically that one's perspective/observations determine his reality. The one man sees the cup drop straight down, therefore that is what is real to him, and what he must account for; the other man sees the cup fall at an angle, and so that is the reality that he has to measure and calculate from. So..., what about a third observer that is on a ladder directly above the Z axis as the cup falls? He sees nothing, right? Is that, thus, the reality that he has to deal with?

2) Take the same concept and put it back into the spaceship experiment. We're told O1 on board the ship sees the light beam take a solely vertical path from ceiling to floor -- say, 10m. O2 is outside looking into the starboard window (perpendicular to the path of the ship) and sees the beam cover the longer ~14m in a slanted/diagonal path in the same period of time, and relativity insists that time/space must be warped in order for this happen. But suppose there is yet another observer (O3) in another spaceship (going a different speed) who is looking into the first ship through its ceiling window, somewhat similar to the ladder man mentioned above, but at an angle that when the light beam is triggered he is looking at the "back side" of the light beam as it travels along his line of sight. He thus only sees a simple dot in space, and no movement at all. If relativity insists that light is moving a c relative to all frames of reference, how can O3 perceive light as moving at all let alone moving that fast?

3) Back to the cup experiment... If the person that sees the cup drop in a diagonal has to account for the full diagonal distance -- as opposed to, say, measuring the point the cup is at any given point in time to where the hand that held it is at that same point (since the hand is also moving laterally while the cup is moving diagonally) -- then doesn't that wreak havoc with the "constant" acceleration due to gravity? If a longer distance is covered from O2's perspective as compared to O1's (the 'dropper'), then it seems that we are forced to conclude that even at a mere 20 MPH this 'constant' is seriously warped. Right?

 

[Janus]

Janus, I've been giving your explanation quite a bit of thought. Let me just throw out some of what has crossed my mind, and you (or anyone else) can feel free to interact with any of it.

1) As I understand the discussion, it's basically that one's perspective/observations determine his reality. The one man sees the cup drop straight down, therefore that is what is real to him, and what he must account for; the other man sees the cup fall at an angle, and so that is the reality that he has to measure and calculate from. So..., what about a third observer that is on a ladder directly above the Z axis as the cup falls? He sees nothing, right? Is that, thus, the reality that he has to deal with?

He sees the cup recede from him. Changing the viewing angle doesn't really change anything he still sees the cup fall straight down (down being the direction of gravity).

2) Take the same concept and put it back into the spaceship experiment. We're told O1 on board the ship sees the light beam take a solely vertical path from ceiling to floor -- say, 10m. O2 is outside looking into the starboard window (perpendicular to the path of the ship) and sees the beam cover the longer ~14m in a slanted/diagonal path in the same period of time, and relativity insists that time/space must be warped in order for this happen. But suppose there is yet another observer (O3) in another spaceship (going a different speed) who is looking into the first ship through its ceiling window, somewhat similar to the ladder man mentioned above, but at an angle that when the light beam is triggered he is looking at the "back side" of the light beam as it travels along his line of sight. He thus only sees a simple dot in space, and no movement at all. If relativity insists that light is moving a c relative to all frames of reference, how can O3 perceive light as moving at all let alone moving that fast?

Again, from his perspective the light recedes from him at c. If I point a light at a mirror and have it bounce straight back to me, just because I didn't see the light move from side to side does not mean that it didn't move from my perspective.

3) Back to the cup experiment... If the person that sees the cup drop in a diagonal has to account for the full diagonal distance -- as opposed to, say, measuring the point the cup is at any given point in time to where the hand that held it is at that same point (since the hand is also moving laterally while the cup is moving diagonally) -- then doesn't that wreak havoc with the "constant" acceleration due to gravity? If a longer distance is covered from O2's perspective as compared to O1's (the 'dropper'), then it seems that we are forced to conclude that even at a mere 20 MPH this 'constant' is seriously warped. Right?

No, the acceleration due to gravity only deals with the vertical component of the motion. The horizontal component has no effect on this. If you drew a horizontal line through the cup and measured its acceleration as the ball fell, you would measured it as 32 ft/sec/sec.

The difference between a cup and light is the following:

Imagine the cup is moving at 20 mph to the right and at a given moment is falling at 32 ft per sec. To find its velocity at that moment with respect to one at that moment you add these two components of its velocity together to get 29.6 mph, which is the velocity it is moving relative to you, this is greater than either its horizontal speed or its vertical speed. IOW, while the path it takes is longer than that it would take it it fell straight down, its speed along that path is also greater. It has an over all faster speed to travel a longer distance to cover it in the same time..

Light can only travel at one speed relative to any frame: c. If I see the same light take a longer path than you do, it had to take a longer time by my clock to cross that path. Light travels a longer distance at the same speed vs. the cup which travels a longer distance at a greater speed. (though technically, the cup will take a slightly longer time from my perspective to travel the longer path, because of the way velocities add in relativity. This serves to make the falling cup scenario exhibit the same time dilation as the light example. It's just that at cup falling speeds, this dilation is really really small.)

 

[DonB]

He sees the cup recede from him. Changing the viewing angle doesn't really change anything he still sees the cup fall straight down (down being the direction of gravity). Again, from his perspective the light recedes from him at c. If I point a light at a mirror and have it bounce straight back to me, just because I didn't see the light move from side to side does not mean that it didn't move from my perspective.

No, the acceleration due to gravity only deals with the vertical component of the motion. The horizontal component has no effect on this. If you drew a horizontal line through the cup and measured its acceleration as the ball fell, you would measured it as 32 ft/sec/sec.

The difference between a cup and light is the following:

Imagine the cup is moving at 20 mph to the right and at a given moment is falling at 32 ft per sec. To find its velocity at that moment with respect to one at that moment you add these two components of its velocity together to get 29.6 mph, which is the velocity it is moving relative to you, this is greater than either its horizontal speed or its vertical speed. IOW, while the path it takes is longer than that it would take it it fell straight down, its speed along that path is also greater. It has an over all faster speed to travel a longer distance to cover it in the same time..

I am totally with you to this point -- exactly what I anticipated.

Light can only travel at one speed relative to any frame: c.

Janus, what is the experimental evidence or mathematical basis for this conclusion. (I know that it's commonly stated, but I'd like to know why -- it is one of the major bumps that I have to get over.)

If I see the same light take a longer path than you do, it had to take a longer time by my clock to cross that path. Light travels a longer distance at the same speed vs. the cup which travels a longer distance at a greater speed.

I've probably already said this, but I don't yet see why it's like that -- specifically why light itself travels the different distances. My mind concocts two scenarios:

1) Light move independent of any spaceship movement-influence. To go back to an earlier experiment I described, I have to wonder if light triggered on board the spaceship will take the same path as the light triggered from the space dock -- even in spite of it's being on a moving vehicle. From the outside observer, both beams travel a solely vertical path. (But come to think of it, this doesn't really change the overall argument because the on-board observer now sees the light taking the longer path.)

2) If the inside light does move long laterally within the spaceship -- influenced somehow by the ship itself, going faster or slower as determined by the ship speed -- I can't help wondering why it's overall speed isn't broken down into components of it's own movement (cf. to gravity on the cup) which is c, and the ship's additional movement (cf. the movement of the car upon the cup). I know Doc says I'm reverting back to the old way of looking at it -- and while I don't necessarily argue that he is wrong, I'm not understanding what this can't be right.

But I go down the same path I've already gone. Sorry to be so dense.