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

  1. Jul 7, 2011 #1
    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 physic 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 + Vss2), 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 spacial 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.

    Last edited: Jul 7, 2011
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  3. Jul 7, 2011 #2

    Doc Al

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    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.

    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.)
  4. Jul 7, 2011 #3
    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?
  5. Jul 7, 2011 #4
    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.
  6. Jul 7, 2011 #5


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    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.
  7. Jul 7, 2011 #6

    Doc Al

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    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" [Broken]) But Einstein's insight dispensed with all of that.
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  8. Jul 7, 2011 #7

    Doc Al

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    (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.)
  9. Jul 7, 2011 #8
    (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.
  10. Jul 7, 2011 #9


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    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.
  11. Jul 7, 2011 #10
    ¶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.
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  12. Jul 7, 2011 #11
    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.
  13. Jul 8, 2011 #12

    Doc Al

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    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.)
  14. Jul 8, 2011 #13

    Doc Al

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    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?
  15. Jul 8, 2011 #14
    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)?

    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.)
  16. Jul 8, 2011 #15
    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?

    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?
  17. Jul 8, 2011 #16
    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.
  18. Jul 8, 2011 #17

    Doc Al

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    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.
    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.


    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.
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  19. Jul 8, 2011 #18
    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.

    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?

    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.

    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.
  20. Jul 8, 2011 #19


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    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.
  21. Jul 8, 2011 #20

    Doc Al

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    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.
    And yet you seem to be clinging desperately to your 'common sense' Galilean relativity as if it were an absolute.
    Luckily, your personal comfort level is not the criterion used to evaluate a theory.

    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.

    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'.
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