Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Is cheating going on in Relativity?

  1. Sep 13, 2005 #1
    Is cheating going on in Einstein’s theory of relativity? For instance, the textbook “Fundamentals of Physics” (6th edition, published by John Wiley and Sons, jointly authored by David Halliday, Robert Resnick, and Earl Walker, on pages 925, 926 and 927, where the relativity theory was briefly introduced) said specifically and repeatedly that clocks would be used to measure time intervals. But I did not find any time interval that was read from the clocks. Instead all the time intervals used in the instruction were computed by means of Newtonian physics.
  2. jcsd
  3. Sep 13, 2005 #2


    User Avatar
    Science Advisor

    Computing time intervals simply tells you what to expect when measuring the time. Indeed, time is measured by clocks; computations do not measure time. Not to worry, relativity is alive ,well and very robust, and has been so for roughly a century.

    Reilly Atkinson
  4. Sep 13, 2005 #3


    User Avatar
    Science Advisor
    Gold Member

    Welcome to PhysicsForums!

    Not sure exactly what you mean about cheating. Relativity (both SR and GR) has been very well tested through the years. John Baez has some good stuff at his physics site, including this:

    What is the experimental basis of Special Relativity?
  5. Sep 14, 2005 #4


    User Avatar
    Science Advisor
    Gold Member

    The most important concept in SR is that all clocks are local. In other words, the only one that matters is strapped to your wrist. Clocks strapped to other observers wrists will not, and cannot be ever be expected to agree. There is no universal clock. Special relativity is basically a four dimensional solution to Pythagorem's theorem. Wrap your head around that and you've got it... at least until you woozily stumble into the GR minefield and realized why you always hated geometry deep down inside.
  6. Sep 18, 2005 #5
    Thanks to reilly, DrChinese, and Chronos for your replies to my question. I however think the problem was not so simple.

    I wish all of you have access to the said textbook and then let us look into the thought experiment therein. The textbook authors claimed that the clocks will produce 2 different time intervals. But I believe otherwise and let me show you how.

    If we simplify the graphics and ignore the light, the thought experiment can be shown as follows:


    where S, R, P, and Q are points on two reference frames. A clock will move from S to R. And at P and Q each there is another clock. When the clock at S departs, it must register a reading of time. Let me call it the departure
    time "b". I believe the same departure time "b" will be recorded on another clock located at P. Don't you agree? (The textbook said, it was a light beam that departed. But it was all the same departure time, and that was why it was perfect all right not to mention the departure of light.)

    Now the clock will record an arrival time when it arrives at R. Let us call it "e". And the other clock at Q certainly will record the same arrival time "e" according to the design of the thought experiment. To obtain the time interval t, it is to deduct the departure time from the arrival time:
    t = e - b. This means that all 3 clocks would measure one identical time interval t, not two different time intervals claimed by the authors of the textbook.

    Consequently the relativistic nature of time cannot be proven unless relativistic advocates resort to cheating.
    Last edited: Sep 18, 2005
  7. Sep 18, 2005 #6

    Doc Al

    User Avatar

    Staff: Mentor

    Why don't you scan it in?

    To say "A clock moves from S to R" is very different from saying "A beam of light is emitted at S and arrives at R". Which is it? (I'll assume the latter.)

    You have two frames in relative motion, I presume. I assume S & R are in the "moving" frame. At the moment that the light is emitted at S, the point P passes by point S. Let's define the time read by the clocks at S and P to read the same at the moment they pass each other, which is the moment that the light is emitted.

    Not true at all! I assume the experiment is that the light arrives at point R just at the moment that point Q passes R. Each frame has its own clocks, presumably synchronized in their own frame: the moving frame has clocks at S and R; the other frame has clocks at P and Q. When the light arrives at R and Q, those clocks will not read the same times.

    You seem to just assume that the time of flight will be the same in both frames. But that ignores what is known about how time and distance measurements depend on the frame doing the measuring.

    Where exactly is the cheating? You seem to have given your version of the thought experiment; why don't you describe the version in the book.
  8. Sep 18, 2005 #7


    User Avatar
    Science Advisor

    What you are missing is the fact that in the rest frame of the clock which moved from S to R, the clocks that are resting at P and Q are out-of-sync, so the time interval between them is not the correct one in the first clock's rest frame (if you walked through a hallway and the clock on one end said 2:00 while the clock on the other end said 3:00, then you would not conclude it took you an hour to cross the hallway, you'd just conclude the second clock was ahead of the first one). This is known as "the relativity of simultaneity", and it's a pretty simple consequence of the idea that each frame should assume light moves at c. Just imagine a rocket going by you where a crewmember sets off a flash at the rocket's midpoint and synchronizes clocks on either end of the rocket by setting them to the same time at the moment the light from the flash hits them; since the rocket is moving forward in your frame, you will see that the clock at the front of the rocket is moving away from the point where the flash was set off while the clock at the back is moving towards it, so if you also assume light moves at c in all directions in your frame, you will conclude the light hit the back clock before the front clock, and thus that the two clocks are out-of-sync.

    You might want to take a look at a detailed example I worked out (with diagrams) on this thread, involving two rulers with clocks mounted at regular intervals sliding alongside each other, showing how in each ruler's rest frame it is the clocks on the other ruler which are running slow (and which are out-of-sync).
  9. Sep 19, 2005 #8
    Thank you, Al Doc. I am sorry that I did not give you enough information because I don't have the scanning device. But in my opening post I have made it clear between [clocks] and [equation]. We used clocks to measure time intervals, or we used equations to compute time intervals. These are two distinctly different operations, clocks and equations. If someone told me he would use clocks to do something and he switched to equations, of course he cheated me. This was exactly what had happened in the said textbook.

    In your reply to me you said: "When the light arrives at R and Q, those clocks will not read the same times."

    Why the clock at Q will NOT read the same arrival time as the clock at R did? You did not explain why. I believe otherwise, because this was a thought experiment. If all the clocks at S and P could be thought to read the same
    departure time, the same "thought" logic must apply to the arrival point. Hence when the light beam arrived at R and Q, the clocks there would logically read the same arrival time.

    Further, there could not be any synchronization problem either, simply because it was all a thought. In a thought an infinite number of clocks can be thought as synchronized perfectly. Imperfection can only happen in real experiments in the laboratory. If the clocks in different frames were synchronized in different ways, our thought can make compensations to them, adding or deducting a few nano-seconds to make difference disappear.

    As a last argument of mine, even if the clocks at R and Q would not read the same arrival time as you alleged, the cheating was still here, simply because the textbook authors produced the different time intervals by means
    of Newtonian physics. My logic was so glaringly clear that nobody could miss. Say I bought an orange and an apple from Mr. Newton's store, and I told my parents I bought them from Mr. Smith's store. There was no change
    to the fact that there were an orange and an apple. But I cheated my parents about the source.

    If my parents got sick after consuming the fruits, the blame would fall on Mr. Smith, not on Mr. Newton, as a result of my cheating.

    As to JesseM's reply, I believe he had the same argument since he mentioned synchronization. My point was how to identify the cheat, not to tell what the light would do in different reference frames. As to your relativity of simultaneity, I shall respond separately after I have studied your details.
  10. Sep 19, 2005 #9


    User Avatar
    Science Advisor

    But whose clocks do you add or deduct time from? Each observer has a set of clocks which are at rest with respect to himself and which he defines as synchronized, while seeing the clocks of moving observers being out-of-sync. And each observer uses his own set of clocks to mark the time of events--so if I see an event happen next to clock A in my system when it reads 2:00, and another event happen next to clock B in my system when it reads 3:00, I'll say that the time-interval between the two events was 1 hour, even if other observers using their own set of clocks get a different answer. So if I say that my clocks are synchronized and your clocks are out-of-sync, while you say that your clocks are synchronized and my clocks are out-of-sync, the situation is perfectly symmetrical, there's no objective way to decide which set of clocks is measuring time-intervals "correctly".

    Did you look at the diagrams of the two ruler/clock systems I provided on the other thread I linked to? They make the symmetry of the situation pretty clear.
    Where do textbook authors use Newtonian physics when justifying the ideas of relativity? I've never seen that before, can you quote the section you're talking about?
  11. Sep 19, 2005 #10

    Doc Al

    User Avatar

    Staff: Mentor

    You do realize that this is a thought experiment, don't you? We can't actually read what the clocks say, we have to deduce what they say according to the accepted laws of physics. One principle that was probably applied was that light travels at the same speed ("c") with respect to any frame.

    The reason why the clocks at S and P read the same at the instant that they pass each other is because we agreed to set them to read the same time at that single instant. For two systems of coordinates, you can always set the origin to be the same. But what other clocks read at other times must be deduced from physics, not merely assumed. (If you just assume that all clocks read the same time, then you will get results that contradict known laws of physics.)

    You seem to have the wrong idea as to what a "thought experiment" means. It doesn't mean "just think whatever you want". It means that you set up a situation (one that may be impractical to do in real life) and apply real laws of physics to deduce what happens. You don't just assume whatever you want.

    A clear application of the assumptions of relativity (that the speed of light is invariant, for one) will show that the two frames must disagree as to how the other frame's clocks are synchronized.

    What "Newtonian Physics" was used? I suspect that what was used was something like: Distance = speed X time. That's just as true for Einstein as for Newton.

    Once again: Where's the cheating? To show "cheating" you should lay out, step by step, the actual argument made in the textbook (not just your version) and point to the exact statements that you question.
  12. Sep 20, 2005 #11
    Doc Al and jesseM, and everybody else who have responded to my charge, I am sorry I could not copy and transmit the textbook's full text over here due to my computer skills. I shall drop the matter for good. If my charge had hurt any body, I apologize to them all. Sorry again.
  13. Sep 20, 2005 #12


    User Avatar
    Science Advisor

    No one was asking you to transmit the full text (which would be illegal anyway), just to quote a paragraph or two where they mentioned Newtonian physics in connection to relativity, or perhaps scan a page if you don't feel like typing it.
  14. Sep 21, 2005 #13


    User Avatar
    Science Advisor
    Gold Member

    Not at all Sam, you asked an honest question... and got gang banged!! The best, and most honest part, was when you asked sincerely. I respect that, Sam. I rule in your favor.
  15. Sep 21, 2005 #14


    User Avatar
    Science Advisor
    Gold Member

    Footnote: I think you are profoundly wrong, Sam, but I like the way you think.
  16. Sep 21, 2005 #15


    User Avatar

    Staff: Mentor


    You may simply be one of those people who likes to learn things "by debate", so to speak, using an argumentative skeptical style. In many fields this is common, and you'll even find this sort of thing in physics, especially in true "cutting edge" areas.

    The problem is that on the Net there are many anti-relativity cranks and crackpots who are convinced that they are right and mainstream scientists are all wrong, and who argue their views very aggressively and persistently. Anyone who comes onto an Internet discussion group and tries to discuss relativity in a similar style is likely to be treated like one of those people, regardless of their actual motives.
  17. Sep 23, 2005 #16
    Thanks to everybody. As JseesM suggested, I shall quote some from the said textbook. On page 925 of the textbook, under the title The Relativity of Time, there were these words: “If observers who move relative to each other measure the time interval between two events, they generally will find different results.
    Why? Because the spatial separation of the events can affect the time intervals measured by the observers."

    The textbook then gives a thought experiment involving an observer Sally on a train moving at v relative to another observer Sam on the ground, represented in two figures. I do not know how to draw these figures. Let me describe the first figure and see whether you can visualize.

    Sally will send a light pulse from her clock toward the ceiling of the cabin where she is in. To her, the light travels along the height (D) of her cabin, and comes down along the same height (D) and arrives at her clock. The textbook says: "Sally measures a certain time interval t between the two events, related to the distance 2D from source to mirror by t = 2D/c."

    The quote immediately above has two parts. The first part means Sally would obtain information from her clock, reading a departure time and an arrival time; the second part means she needs not read, because the time interval is related to the distance traveled by light.

    Don't you think these two actions, saying one thing and doing a different thing, constituted a lie?
  18. Sep 23, 2005 #17


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Your passage described a light-clock.

    The duration between emission and reception, t, represents "1 tick" of that clock... a perfectly good unit of time. One could regard the first emission as the time reading of "0 ticks".

    Upon the first reception, if the source reflects back or re-emits a light signal, waiting for a second reception yields a second tick.

    The duration of a tick is proportional to the separation, D, between the "mirror" on the ceiling and the source. That's what t=2D/c means... a relationship relating t and D.. not a definition of t. (If you want a clock with a finer resolution, reduce the separation D.)
  19. Sep 23, 2005 #18


    User Avatar
    Science Advisor

    Why is it a lie? It just says that the time interval she actually measures using her clock will in fact match up with what she would predict if she assumed light travels at c in both directions in her own frame. They're not saying that Sally "needs not read", they're just telling you what relativity predicts she will read on her clock. Predictions should match observations in any successful theory, no?
    Last edited: Sep 23, 2005
  20. Sep 23, 2005 #19


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I'm sorry, but I don't see anything wrong with this quote. All it says is that the time that Sally reads on the Clock and 2D/c will be the same amount of time. It's like saying that if I start a stopwatch as someone starts to drive away from me at 60mph, then that person turns around after 10 miles and comes back, my stop watch will read 20 min upon his return. If he turns around after 20 miles, my stopwatch will read 40 min, and if he turns around after 5 miles it will read 10 min. There is a direct relation to the time I read on my stopwatch and the distance he drives before turning around.
  21. Sep 25, 2005 #20
    It seemed that JesseM and Janus and most people believed that Sally's clock would produce a time interval equal to the one produced by Newton's equation.

    First of all my allegation had nothing to do with the equality between time intervals. My allegation was, if the textbook authors eventually used Newton's equation to compute the time interval, they should not in the beginning have said they would use clocks to do the job. They need not set up any clocks at all. Too much ado about nothing. They should not use the word "measure". They should just say they will compute. As a result the first paragraph I quoted in the former post of mine should read like this:

    "If observers who move relative to each other COMPUTE the time interval between two events, they generally will find different results. Why? Because the spatial separation of the events can affect the time intervals COMPUTED by the observers."

    The change of one action to another (from measure to compute) has illogical consequences. When computation has produced a shorter time interval, the explanation we can give is that we have put in (into Newton's equation) a shorter distance. I believe we cannot say, the reason for the equation to have produced a shorter time interval is because the clock ticked too slow. Can we accept such a logic?

    But in my view this was exactly what was happening in teaching the relativity theory. When a shorter time interval was obtained from Newton's equation, we were told the clock was malfunctioning, or time dilated.

    Such a practice (a shorter distance producing a shorter time interval) means time dilation has been used to prove the existence of time dilation. Is this logic sound?

    Secondly, if the textbook authors believed that the clocks would produce a time interval equal to the one computed from Newton's equation, they were obliged to demonstrate HOW. Or at least they should have said a few words
    about it.

    Let us represent your belief like this:

    (e - b) = t = 2D/c.....wherein e is the arrival time, b the departure time, shown on clocks; t is the resulted time interval, which equals the computed time interval 2D/c.

    As is customary that two reference frames are moving at v relative to each other. As a result when the light pulse departs and arrives at the clocks, the two frames have created a distance. Let us call this distance g. This
    means, to create the distance g the frames have taken the same time interval t as the light pulse has done. Don't you agree? If you do, the situation can be represented with this: t = g/v. This equation must be true especially if we
    refer to Janus example of stopwatches and cars.

    Since you believed that the measured time interval (e - b) is equal to the computed time interval 2D/c, it means
    (e - b) = t = 2D/c = g/v. This equation means that
    both frames have obtained the one and only time interval t, whether the observers used the measuring technique or the computation one.

    But the textbook told us that there would be two "different results." So, there must be something wrong? Don't you agree?
    Last edited: Sep 26, 2005
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?