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Detecting absolute motion

  1. Aug 3, 2010 #1
    I started posting to what seemed to be a related thread here:
    After concluding I was hijacking another thread, I am posting here.

    The concept is I think that, in theory, one can determine absolute speed. Am I the one moving or is that other object moving past me. I wrote up a theoretical experiment to demonstrate my concept. I decided to post it on a personal web site rather than post a bunch of text here. The location is: http://www.bkelly.ws/space_time/index.htm [Broken]
    Click on the link about absolute velocity to view or download the essay.

    Ich was kind enough to respond in the first thread and my response is rather general so I will put them in this post. For context, Ich raised the issue of simultaneity and the problems it would cause my experiment.

    Hello Ich,

    Regarding the trolley and the issue of simultaneity, while the essay stated that it makes the marks as it passed directly in front of Tom, I am claiming that given the goal of the essay it really does not matter if the marks are made when the trolley is a bit to the left of Tom, right in front, a bit to the right, or what the heck, a couple of hundred meters to the right or left. So exactly where the marking event(s) occur is not really important. That should get us down to two events.

    However, as I think on this a bit more, we can dispense with simultaneity. Worrying about the simultaneity provides no advantage in discussing this concept. Its sort of like saying we cannot have a theoretical discussion because no one knows how to build a trolley that can move at relativistic speeds. So, given that for our discussion we have a trolley that can move at near the speed of light, the trolley is declared to be 1 meter in width when stationary and has two paint cans, one at each end and they make two marks that are one meter apart, with respect to the trolley if you will, at the appropriate time and place. The concern is not in making and conducting the experiment for real, the concern is the concept of what would happen if we found a way to conduct this experiment.

    Lets go down your path a bit further. Assume that the fence goes past Tom at ½ C while Tom and the trolley are stopped, and the trolley makes its two marks. Then the fence is brought back to a center position and the marks examined. While moving, the fence is length contracted. When stopped then the fence will “un-contract” and the marks will be more than 1 meter apart.

    On the other hand, make the fence were stationary and the trolley moves along it at 1/2 C and the marks made. The trolley will be length contracted and the fence will not. The marks will be less than one meter apart. That is in contradiction to the marks being further apart when the fence is moved for the marking then stopped.

    That tells me that from the perspective of the trolley, we can detect which was moving. And that is the fundamental purpose of my thoughts.
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  3. Aug 3, 2010 #2

    Doc Al

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    What's important is when the two marks are made. You seem to think that everyone will agree that they are made simultaneously. Not so. Simultaneity is frame-dependent.

    If you don't care about when the marks are made and whether they are made simultaneously, then there's no point in discussing the distance between the marks or anything else.
  4. Aug 3, 2010 #3
    Are you trying to say there is no such thing as two events happening simultaneously? If not then what are you saying? Please frame your answer in the concept of the trolley.

    Our theoretical trolley may have a mechanical connection between the two markers one meter apart. It may have an electronic connection with all the relativistic calculation worked out in advance such that the marks are made at the same time. Lets assume we have the technology such that, when the trolley is in front of us, we can see that the two marks are made within one one millionth of a femtosecond of each other and within one billionth of a micrometer of being exactly 1 meter apart. If that is not accurate enough, I will declare it to be as accurate as needed.

    The trolley is a single item and moves all together. The trolley, all of the trolley, and all of its parts, should be within a single frame of reference. The two markers always move at the same speed. When I jump on the trolley and ride along at 1/2 C, my measurements continue to record a marking accuracy of a better than billionth of a femtosecond and millimeter.

    From the wiki page found here: http://en.wikipedia.org/wiki/Relativity_of_simultaneity

    All parts of the trolley are closer to all the remaining parts of the trolley than those that observe a car crash, and probably even closer that those that participate. The trolley does not have one end in London while the other is in New York City or New Delhi.

    To wrap it all up from another perspective, the trolley exists in theory only. If we can presume to discuss a trolley that we can accelerate to 1/2 C, which we can do (discuss the theory that is), then we can presume to have a method to deal with the simultaneity.

    So tell me please: Why can we not disregard simultaneity and discuss the concept I am trying to present.
  5. Aug 3, 2010 #4


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    That is correct.

    At the very crux of relativity is the relativity of simultaneity. IOW, simultaneity is relative to your frame of reference. It is not simply a wrinkle to be ignored or "dealt with".

    It is the very core.

    No it does not. Relativity forbids perfectly rigid materials.

    If you posit a perfectly rigid material, then you instantly and trivially have something that can move at infinite speed. You could have a radio that communicates with the moon in zero time.
    Last edited: Aug 3, 2010
  6. Aug 3, 2010 #5


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    According to whom? Who does this measurement? What is their velocity wrt to the components?

    Change the viewpoint and you get a different answer.

    Unlike the streetcar that can move at .5c (which can be easily granted as it does not violate any laws of physics), the issues of measurement are not trivial and not technicalities to be waved away by positing advanced technology to circumvent them; they are critical.
  7. Aug 3, 2010 #6
    My first reply is, ok, they are not simultaneous. But they are close enough to test the theory. Work it that way.

    Second, I feel like you are endeavoring to avoid the main crux of my concept. None of the responses have even addressed the core concept much less shot it down.
  8. Aug 3, 2010 #7


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    No, the simultaneity issue will corrupt the effect you are trying to measure.

    The core concept is a non-issue because you are trying to take measurements while ignoring any problems with calibrating them.

    As a similar example, I can http://www.math.toronto.edu/mathnet/falseProofs/first1eq2.html" [Broken] - and by extrapolation that any number equals any other number - as long as you don't worry about the niggling little detail of dividing by zero. If I presented that as my core idea, and insisted that you ignore the detail, how long would you let me go on insisting that 1=2?
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  9. Aug 3, 2010 #8
    So following that concept we can never measure the speed of anything, particularly light because a photon leaving the laser is one event and a photon hitting the receiver is the other and we can never tell if the photon emission really has any relationship to the reception. Heck, we don't really know for sure its the same photon.

    Its clear that the respondents don't want to address my concept. I will stop arguing and wait a while to see if anyone else will offer a decent explanation or discuss my concept.
  10. Aug 3, 2010 #9


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    Of course we can. If they are comoving - in the same frame of reference - this is a trivial task. Even if they aren't in a comoving FoR, we can still make accurate measurements. But they will be relative measurements - relative to our FoR. They say nothing about any absolute velocity.

    I'm sorry you don't like the answers you are getting. At the risk of sounding sarcastic, are you expecting that the laws of physics will change so that they operate in a manner more to your liking?

    You have an opportunity to learn here, to break free from the constraints of your preconceptions about how the universe works. But learning will require you letting go of those preconceptions. Instead of insisting your experiment will work and demanding we discuss it the way you've laid it out, try asking where it is flawed. You will get answers that will enlighten you.
  11. Aug 4, 2010 #10
    OK, I'll try.

    What results would we get if we repeated your experiment on a large spacecraft containing Tom, Sally, trolley, fence, and markers, with identical relative speeds between Tom, the trolley, and Sally, with the spacecraft stationary relative to the sun? Then with the spacecraft moving at 0.5c relative to the sun? at 0.99999c relative to the sun?

    The results would be exactly the same.
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  12. Aug 4, 2010 #11


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    bkelly, let's examine the special case when the trolley is moving at 1 m/s wrt the fence, so that relativistic effects are negligible. Now, let's say that the timing is off such that the front mark is made 0.5 s before the rear mark. In that case the marks will be 0.5 m apart. Will we conclude that the length of the trolley was 0.5 m? Obviously not. Even in Newtonian mechanics the marks must be made simultaneously in order to be a measurement of the length of the trolley.

    Now, in relativity there will only be one frame in which the marks are simultaneous. In all other frames not only will they disagree about the length of the trolley, they will disagree about whether or not the marks are related to the length.
  13. Aug 4, 2010 #12


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    it appears that you're under the impression that you have a valid argument here, and that all participants here just want to dodge your logic.

    If so, you better drop that attitude immediately. PF is a place where you can learn about physics. If you think you don't have to learn, you're wrong here.

    One more time I offer to help you learn why this statement is wrong. It has to do with - surprise - relativity of simultaneity. If you're not interested, just say so, and we're done. No need wasting anyone's time.
  14. Aug 4, 2010 #13
    You guys are saying that simultaneity will make the events of my theoretical experiment unusable. I don't understand why so allow me to ask a few pertinent questions.

    Given the description of the trolley, how much do you expect the problem of simultaneity to change the marks made by the trolley? When I calculate the expected distance of 0.866 meters at a velocity of 1/2 C, what do you say the results will be? Can you give me a tolerance? Do you say the results will change from test to test or will the results be predictable?

    In post #3 above, I quoted from a Wiki page about simultaneity and provided a link. In that quote the page clearly implied that observers to a car wreck would all be within a single frame of reference. It referenced two crashes in London and New Delhi indicating that they would be in separate frames of reference. This further implies the immediate wittiness to the crash would all be within the same reference.

    Two witnesses to a car wreck can be on opposite sides of the street and be in the same frame of reference. Why is it that two markers on a 1 meter long trolley, with a direct physical connection between them, cannot be in the same reference?

    How close must two events be in distance or maybe time in order to be considered one frame of reference or to be one event?
  15. Aug 4, 2010 #14


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    You haven't specified what mechanism the trolley uses to decide how to drop the marks "simultaneously". For example, suppose a flash is set off at the midpoint of the trolley, and at each end there is a device programmed to drop a marker as soon as it detects light from the flash. From the perspective of the trolley observer, if light is assumed to travel at the same speed in both directions in the frame where the trolley is at rest, this procedure should guarantee that both ends drop their markers simultaneously. However, from the point of view of the observer on the track, if they assume light travels at the same speed in both directions in their own frame, then since the back of the trolley is moving towards the position on the tracks where the flash went off while the front is moving away from that position, this observer will say the light catches up with the back before it catches up with the front, so the markers are dropped off non-simultaneously, and thus the distance between the markers is not the same as the length of the trolley in the track frame.

    On the other hand, we could set off a flash at a position on the track closer to the front of the trolley than the back, at just the right distance so the track observer would calculate the light should reach both ends simultaneously in the track frame. In this case, the distance between the markers is equal to the length of the trolley in the track frame, but then the trolley observer will just say that this procedure caused the light to reach the two ends non-simultaneously in his own rest frame.
    Unclear what you mean by "in" the same reference frame. Each reference frame is just a coordinate system which can assign coordinates to events throughout space and time, so all events in the universe can be analyzed from the perspective of the same single reference frame. Sometimes authors may talk about observers being "in" a given frame as shorthand for "at rest in", but this only makes sense for objects that exist for an extended period of time, instantaneous events don't have a rest frame since you can't talk about their position at different times in order to determine whether the position is changing or constant.
  16. Aug 4, 2010 #15
    I think the wikipedia article was getting at that when a car crashes into another car, it doesn't really matter what reference frame you're in, the cars were at the same place at the same time because it is a single event in spacetime. What might have confused you about the New Dehli part of the wiki article is that the article talks about two car crashes happening instead of just one. In one frame they might appear to happen simultaneously, but because they are space-like separated another frame can disagree as to which crash happened when. If two car crashes happen on the same block it might appear to happen at the same time to all the people on the block, but the two crashes wouldn't appear to be simultaneous to an observer in a frame that is within a block of both crashes (as seen from the frame of those same pedestrians who witnessed the crash) but was moving significant fractions of the speed of light (like if a cosmic ray happened to be within a block of the crashes and could actually observe anything). It isn't really a matter of how close you are to the events that determines if they appear to be simultaneous or not, it matters how you are moving relative to another frame of reference, thus simultaneity is frame dependent and we call that the relativity of simultaneity.
    Last edited: Aug 4, 2010
  17. Aug 4, 2010 #16
    That is getting to the core of the present discussion. I cannot find any reference or reply that says two events must be within X and Y parameters in order to be in the same frame of reference. (You pick the quantities noted by X and Y. Let it be distance, time, or what ever you know to be blocking this discussion.)

    The respondents are saying that two markers one meter apart are not in the same frame of reference therefor my theoretical experiment is invalid. So I ask again:
    Question 1: What is needed?

    Regarding how the two markers are triggered: They are only one meter apart. They are not kilometers apart, not event tens of meters apart. Just one. Hence the question just above, what is the threshold?

    So if that question goes unanswered try this.

    configuration A: There is a solid rod between the two markers. At some point something in the middle of the rod tweaks it and the rod activates the markers.
    Question 2: Are the markers in the same reference? Can the marks be considered to be made simultaneously. if not, why not?

    configuration B: Lets make the marks closer together. There is a single spray can. It sends out paint, or light, or what ever your might like, in the usual format that we expect from a spray can. In between the spray can and the fence there is a somethings that blocks the spray. I declare that that something causes a precisely one centimeter wide blockage when the trolley is at rest. There is one event of one pulse out of the spray can, and one centimeter of the spray is blocked when at rest.

    Now send that trolley flying by at 1/2 c and have it make a mark. I calculate that the mark will be 0.866 centimeters wide.

    Questions 3: Is that all in a single frame of reference? Will that work for my theoretical experiment?

    Condition C: Lets go crazy with this. The trolley is a meter stick of standard proportions. Lets make it 2 centimeters wide and one meter long at rest. The meterstick goes flying by right next to the fence such that an observer sees the 1 meter length as along the path of travel, and the 2 centimeter width as a vertical dimension. Make the distance between the meter stick and the fence to be microscopic. Its as close as it can be without touching. The meterstick is also microscopically thin. Considering the past discussions, say it is one atom thick, yet remains optically opaque. And it is within two atoms distance from the fence. As the meter stick goes by at its 1/2 C velocity, a stationary light flashes. The fence has a photo sensitive surface. An image of the meterstick is made on the wall and persists long enough to observe and measure. One light source sent out. One beam of light from a single point in space. One event. The meterstick is close enough to the fence and thin enough, and the light is far enough away that there is no paralax problem of diverging light rays. I don't care if the image is to the left of exactly where we want it, or to the right. Its just there.

    Now: My understanding is that the image will be 0.886 meters long. Is that okay for a theoretical discussion?

    If not, then I don't understand what your are trying to say. Please explain why not?

    One last request: Please excuse any typos. My fingers are getting old and sometimes type what they want and not what I want. Lets not get hung up on a truly minor detail such as that.

    Thank you.
  18. Aug 4, 2010 #17


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    bkelly, let me ask a question back.

    Let's say we set up your experiment exactly as you describe, and it all works out tehnically perfectly. We get the measurements and we determine a length contraction of, say, x, leading to conclusion y (whatever conclusion you want to make).

    We now take the entire experiment - trolley, observers and all - and put a rocket behind it and accerelate the entire thing at 1g for a few days. The entire experiment is now moving away from Earth at relativistic speeds.

    We now perform the exact same experiment as before.

    What do you think we will observe? Do you think we will get different results for x and/or y?
  19. Aug 4, 2010 #18


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    Hi bkelly,

    I read what is posted on your website and I'm baffled. What I don't get is, what is the absolute velocity with respect to ? In the experiments you seem to be measuring the velocity of the metre stick wrt the fence.
  20. Aug 4, 2010 #19


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    Again, two events can be analyzed from the perspective of any frame of reference--in that sense they are "in" all of them. However, objects (not events) are sometimes said to be "in" a frame of reference if they are at rest in that frame, so we might say two objects are "in the same frame" if they are at rest relative to one another.
    Who said that? I don't see any comments on this thread saying such a thing, can you quote the comment you're talking about so I can see if maybe you're misunderstanding?
    There's no "threshold", events at different locations which are simultaneous in one frame are always non-simultaneous in others. If two events occurred simultaneously in a frame where the distance between them was L, then in another frame moving at speed v relative to the first frame, there will be a difference in time between the two events of gamma*v*L/c^2, where gamma = 1/sqrt(1 - v^2/c^2). So for example, if the second frame is moving at v=0.6c relative to the first, then the time between the events in the second frame is 1.25*0.6c*L/c^2 = 0.75*L/c. So if L=1 meter, then with c=299792458 meters/second, the time between the events in the second frame is about 2.50173 nanoseconds (0.00000000250173 seconds). Small, but if we're talking about a trolley dropping markers it makes a difference, since if the trolley is moving at 0.6c, it will have moved forward a distance of 0.6*299792458*0.00000000250173=0.45 meters in that time. So if the trolley is length-contracted to 0.8 meters in the track frame, then if the back marker is dropped 0.0000000025 seconds before the front one in the track frame, the front marker will be dropped at a distance of 0.8 + 0.45 = 1.25 meters from the back one in the track frame, which is greater than the trolley's rest length, not shorter.
    In relativity it's impossible to have perfectly rigid rods, you can only have more realistic rods where if you push on one end, the other end doesn't move until a sound wave moving through the material of the rod has had time to travel from one end to the other. If the rod is at rest in the trolley frame, then presumably if you jiggle the middle the sound waves going in either direction will travel at the same speed in this frame, so the markers will be dropped simultaneously in the trolley frame. But again, this means the markers are not dropped simultaneously in the track frame.
    Again I don't know what you mean by "in the same reference". Are you just asking if the markers are dropped simultaneously in the track frame? If that is what you're asking, the answer is no.
    Is the "something" at rest relative to the trolley, or at rest relative to the track? If at rest relative to the trolley, isn't it in front of the spray can at all times? If at rest relative to the track, then won't it always leave a "shadow" of the same size on the fence regardless of how fast the spray can moves past it?
    OK, so all photons are treated as traveling at a right angle to the plane of the fence in the fence's rest frame, and they all reach the fence at the same instant in the fence's rest frame? In this case, the image of the meterstick will indeed be 0.866 meters in the fence rest frame. But keep in mind, in the rest frame of the meterstick the same photons are not traveling at a right angle to the plane of the fence and they do not all hit the surface of the fence simultaneously, so you can explain this result in the frame of the meterstick too.
  21. Aug 4, 2010 #20
    I will answer your question, but first: Now that you have me all worked up about simultaneity and the impossibility of my experiment, and now that I have provided some conditions that I think resolve the simultaneity problem, please respond to the my last post and we can work our way back to the original concept.

    Start with my condition C last. Will that suffice to run a theoretical test of contraction due to relativistic speeds?

    You baited the hook by refusing to consider the merits of my theory using simultaneity. Now that I have bit the hook and resolved that problem, after considerable effort, it would be rude of you to just say "oh never mind about that." Lets layout a solid conclusion to this problem of simultaneity.
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