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

Question about Sticky

  1. Feb 21, 2008 #1
    I have just read the "Read Before Posting" sticky thread and am wondering. Relativity is a difficult theory to understand. How is a person supposed to learn of its validity if they are to automatically assume it is correct.

    How do I know it is absolutely correct? If people can't question it how can they learn? Also why would it matter if people argued for or against it? People who don't care about discussing its validity would just ignore such a thread. Or is it a subject that evokes personal insults and gets out of hand?

  2. jcsd
  3. Feb 21, 2008 #2
    Mabye because there is very little to no evidence that Einstein is wrong and therefor, give it that this is internet we are talking about, people could write whatever they want and that never has a good end.

    But main reason is probably that this forum is supposed to help people learn and solve problems. If every second thread was an attempt to disproove this theory, students wouldnt be amused would they?
    Oh and that doesnt mean you cannnot ask why is it correct when according to [your piece of evidence] it should be incorrect. You are right that alot of learning comes from questioning the theory, but people often overreact.

  4. Feb 21, 2008 #3


    User Avatar
    Science Advisor

    This is your first time online?
    There are hundreds of cranks out there who question the validity of relativity. They all share at least three characteristics:
    1. Discussions with them get out of hand.
    2. They have insufficient knowledge of relativity.
    3. They don't want to learn about it.
    That's why there are specific rules.
  5. Feb 21, 2008 #4
    Ok thanks. So I can discuss the difficutly I'm having and not be dismissed. Hopefully.

    I'm currently studying relativity and my teacher doesn't seem to grasp the problem I'm having and continuously gives me the same answers. As if that will help my confusion, if you catch my drift.

    Einstein pointed out that events aren't observed simultaneously from all frames of reference. I see a lightening bolt 3 miles away at a different time to a person 6 miles away. In order to know this we might create an artificial absolute frame of reference, calculate the speed of light, time observed and distance. We can know when the event occured and when each observer saw the event.

    That is to say our single frame of reference is incomplete. We could create a model of the entire visible universe and understand each frame of reference, it's distance to one another, it's time dilation and when each frame observes every event taking place. This model in my mind would be a collective, yet limited, absolute reference point within its own context.

    My teacher keeps telling me that there is no absolute reference point. But within reason we can create one. Is this right?

    The reason this is confusing to me is why Einstein makes the claim that if I am moving it appears as if the whole universe is moving. On the one hand you could state this is true. But on the otherhand we can know by calculating time dilation which body is travelling at which speed comparatively to another. You could suggest that both references were correct but I'm more inclined to accept the artificial absolute as acceptable and my single point of view is an illusion, or in appearance only.

    Why is this a problem for me? I seem to gather from Einstein that he understood that according to the experiments in water that light the speed of light was constant. He then extrapolated it to mean that the speed of light was constant for all frames of reference. This is a big leapof faith. If you remember the mental experiment of the passenger on the train observing the speed of light compared to the observer on the station, he uses an artificial absolute reference point to suggest that time dilation occurs for the passenger so the speed of light is constant. That makes sense.

    He then eliminates this absolute reference point to suggest that for the passenger on the train it appears that time dilates also for the observer standing at the station. If that were true then the theory is definitely counter-intuitive. But we can calculate the time dilation of the observer on the train. So the reciprocation that Einstein suggests, based on the hypothetical absolute frame of reference, is true for only one observer and an illusion for the other.

    But this is the reason, I believe, that Einstein makes the claim that the speed of light is the same for all observers. Seems to be half true and half in appearance only.

    See if I can make this clearer. The observer sees the man at the station apparently moving and the man at the station sees the train apparently moving. Both believe the other to be experiencing time dilation but when they meet up only one observer did.

    The only reason we know of time dilation occurs is because of Einstein's artificial absolute point and experimentation to prove it. When I reverse the experiment and use the same artificial absolute I get reprimanded and told there is no absolute, that I don't understand reciprocation and that I can't understand it logically. Not just by my teacher either. I've taken this issue to others aswell. It seems that I can understand what others are saying but they can't quite grasp the actual reason I'm confused and explain it back to me. Seems more like dogma, in reference to anyone I've ever spoken to about it, than actual understanding.

    On one hand you can create an artificial absolute and on the other it doesn't exist, seemingly depending on what works for the theory that the speed of light is constant fot all observers to be true.

    That is my current understanding and have not yet received adequate explanation. Thanks
  6. Feb 21, 2008 #5
    I'll dumb it down a bit.

    An observer on a moving train sees a light travelling between the floor and the ceiling. An observer on the train station sees the same light travel a different distance yet they perceive the light to be travelling at the same speed. How is that possible? Time slows down for the observer on the train. Right? Basic relativity example used by Einstein based on an artificial absolute frame of reference.

    Now try reverse the experiment. Pretend the light is still on the train, the train is stationary. The train station, with the observer still standing there, is actually moving. How is it possible that both observers see the same speed of light now? This is where my teacher throws the artificial absolute frame of reference out the window and the theory sticks.

    Seems like a contradiction to me.
  7. Feb 21, 2008 #6
    Huh I figured it out.

    So it's a simple answer because in the first eg. the moving observer sees the light travelling a shorter distance than it really is and in the second example the moving observer sees the light travelling at a longer distance (and the light is moving between two moving points in the first eg. [add time dilation] and two stationary points in the second eg. [negative time dilation]) then both observers will see it travelling at the speed of light. Double negative equals positive.

    So this is neither counter-intuitive nor has it got anything to do with reciprocation. Duh! Why don't people explain this properly? Make me go nuts with this illogical frigging reciprocation.
  8. Feb 21, 2008 #7
    Go to sci.physics.relativity on usenet if you want to see what happens when people are allowed to post whatever they want with no evidence.
  9. Feb 21, 2008 #8


    User Avatar
    Staff Emeritus
    Science Advisor

    From my POV, what needs to happen is that you need to listen more to your teacher. I'm sensing a certain hostility here between you and him or her, which is unfortunate.

    No, you can't create an absolute reference point.

    You are making some false assumptions about the way time acts, apparently they are so deeply ingrained into your worldview that you don't realize that they _are_ assumptions (incorrect assumptions, to boot) rather than facts.

    A litmus test for how time acts is the twin paradox. If you have two twins, each with their own clock, a twin following an accelerated trajectory will have a lower elapsed time on their clock than a twin following an unaccelerated trajectory.

    Assumptions: the twins are in the flat space-time of SR, and the twins start out at the same location in space-time and eventually re-unite.

    see for instance.
    http://math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/twin_paradox [Broken].

    The twin "paradox" is fundamentally incompatible with any notion of "absolute" time.
    Last edited by a moderator: May 3, 2017
  10. Feb 21, 2008 #9
    I thought the speed of light was the problem.

    This idea that if i'm moving faster than another that I will observer their clocks going slower is silly.

    To explain it properly let's just say that observer one is stationary and observer two is moving. Let's compare them side by side. Observer one sees his own clock travelling normally and observer two is in slow motion and so is his clock. Observer two's clock is travelling normally however he sees observer one in fast forward and so is his clock. Then the speed of light is equal for all observers and the Theory of Special Relativity makes perfect LOGICAL sense.

    Reverse the lorentz transformation to understand a slower moving bodies actual time and velocity. No wonder people don't understand it.

    It may seem perfectly acceptable that if I am moving it appears as if I'm stationary. But I won't see slower time dilation in others. It's a simple optical illusion like escher drawing that is physically impossible, not some freak of nature phenomena. Had me going for months.
  11. Feb 21, 2008 #10
    I see you figured it out yourself, but just incase I posted this very useful link containing 24 lectures mainly about special/general relativity.
    Check them out, I remember this exact problem being discussed in there, I belive it could have been lecture 5 or something near that number.

  12. Feb 21, 2008 #11
    No your misunderstanding the problem like my teacher.

    There is an ARTIFICIAL absolute frame of reference. That is not the same thing. By calculating all reference points you do get an absolute of sorts. Whereby we can tell who's time is moving faster of slower or who's velocity is faster. If there was no artificial absolute then we'd still be in the dark ages.

    Between you and me who is travelling faster hypothetically? How would you know? Without some reference to make an accurate judgement then we both are! Illogical explanations. Einstein's theory could have been so much easier to learn.
  13. Feb 21, 2008 #12

    Doc Al

    User Avatar

    Staff: Mentor

    Open your mind, grasshopper.

    OK. Observer one certainly observes his own clocks running normally, and he measures Observer two's clock and concludes that it operates more slowly than his own.
    Half right. Observer two sees his own clock running normally. But he also measures Observer one's clock as running slow--according to his clocks. The relativistic effects are completely symmetric.
    If what you said were true it would contradict the fact that the speed of light is the same for all observers.


    You will appear stationary to yourself.
    Sure you would, if they were moving fast enough with respect to you.
    Not quite. You haven't seen the light yet.
  14. Feb 21, 2008 #13

    Doc Al

    User Avatar

    Staff: Mentor

    To use your terminology, the idea of telling who's velocity is actually faster is silly. Forget Einstein for the while, you need to study Galileo.

    The answer is that it's a meaningless question to say who's travelling faster unless you specify with respect to what. And the "what" is completely arbitrary.
    Perhaps because you missed the point?
  15. Feb 21, 2008 #14
    What I believe happened was that Einstein developed an accurate theory that is rather complicated and after he explained time dilation he couldn't quite figure out how to reverse the situation. So instead he went off on a long tangent of reciprocation (or symmetry as you put it) which has no basis in logic.

    Therefore for the next hundred years this theory got repeated by a whole bunch of people whose specialty is mathematics as opposed to mental visualisation, which is the process by which Einstein's brain worked.

    Now you honestly try to tell me that if my time is going faster than yours (based on a previous experiment whereby only your clock experienced time dilation, if you will) that you could possibly see my clocks running slower.

    I'm now not so sure that the "cranks" as they were so endearingly termed are necessarily nuts. They are just trying to get their heads around a concept that Einstein fans admit can't be logically understood.

    I have just offered my theory of the situation based on the best of my knowledge, so please anyone, give me one good reason for the validity of reciprocation, when it makes no sense and Einstein's theory functions perfectly without it?

    Just one good reason and i'll accept it without question.
  16. Feb 21, 2008 #15


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    You have demonstrated here in this very thread that you haven't the faintest grasp of the theory, yet you're completely confident in dismissing it, and even putting words in Einstein's mouth! Don't you see the irony in your position?

    Special relativity is incredibly simple; all you need is a decent explanation and high-school algebra. The fact that you do not yet understand it means nothing about its veracity. You're simply ignorant. If you wish to remain ignorant, it's a personal decision -- you can fault no one but yourself.

    - Warren
  17. Feb 21, 2008 #16
    Hey I said his theory functions perfectly. I offered an opinion on a possible mistake in his theory no words were put in anyone's mouth. But if you would like to put words in my mouth and make personal insults then that is a choice too.
  18. Feb 21, 2008 #17


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I haven't read a single coherent explanation of any "mistake," Joanna. All I've read are misconceptions that indicate you don't know what you're talking about.

    - Warren
  19. Feb 21, 2008 #18
    I am giving you the solution.

    Offer me one good explanation for reciprocation and I'll accept it without question.

    If einstein was right on this point, then I'm wrong. If not the Theory of Special Relativity makes complete sense to me.

    It's your call.
  20. Feb 21, 2008 #19


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Please define "reciprocation."

    - Warren
  21. Feb 21, 2008 #20

    Doc Al

    User Avatar

    Staff: Mentor

    You seem to think that because something makes no sense to you, that that is somehow an argument against it. (It's actually a famous fallacy of informal logic: The argument from personal incredulity.)

    Have you even lifted a finger to actually learn something about relativity and why both observers must see the other's clocks run slow?

    Nothing wrong with not understanding something. It's the ranting and railing that gets tedious fast.

    You are sadly mistaken if you think Einstein's theory "functions perfectly" without the "reciprocity" of time dilation (and other relativistic effects). In fact, one of the premises underlying relativity states that the laws of physics are the same in all inertial frames. So if it's true that a moving clock "runs slow" in one frame, that same principle had better apply in every frame.

    In order to properly understand how it could possibly be true that both observers see the other's clocks run slow, you first have to understand what it means to measure the rate of a moving clock. You'll see that a full appreciation of time dilation requires understanding length contraction and the concept of simultaneity, as well as just "time dilation". It's tricky, but not that hard. And it's perfectly logical.
    It's never a good idea to accept anything "without question".

    There are many books out there that do a great job of teaching the basics of relativity from the ground up. (If you are interested, just ask.)

    Here's a link to one in a series of lectures covering the basics: http://galileo.phys.virginia.edu/classes/252/time_dil.html" [Broken]. This particular lecture explains by example how the symmetry of relativity is consistent and perfectly logical. (To understand it, you'll have to go back and read the earlier lectures in the series.)

    If you're interested in learning, by all means stick around. But if all you want to do is rant against your professors or against relativity--take it elsewhere.
    Last edited by a moderator: May 3, 2017
  22. Feb 21, 2008 #21
    Let's just say you're seeing me off at the station (which you might well wish to do right now :tongue2:) and I'm on the train and my clock is running slow. Why would I also observe your clock to be running slow? That has had me up for nights on end over the past few months. I just figured out this morning that if I didn't see your clock running slow, rather the opposite it was fast, then everything falls into place and alleviates all the confusion I had experienced.

    Give me one good reason as why my understanding is misguided and I'll graciously accept it. Otherwise I think I'll get a few good nights sleep.
  23. Feb 21, 2008 #22


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    If you see a traveler's clock as running slowly (because he is moving quickly with respect to you), then he must see your clock as running slowly, too (because you are moving quickly with respect to him).

    - Warren
  24. Feb 21, 2008 #23

    Doc Al

    User Avatar

    Staff: Mentor

    Why is your clock running slow?

    Why don't you crack the books or the link I gave and find out? If you expect a one sentence answer that explains relativity using "common sense", stop wasting time.
    It eliminates confusion by ignoring the complications of relativity. Not a good deal.

    Summarizing your logic: If I think X, then I conclude Y and Y makes sense. Sadly, Y has nothing to do with relativity (and doesn't really make sense).

    I think you should just relax and get some sleep.
  25. Feb 21, 2008 #24


    User Avatar
    Gold Member

    Your clock is not running any differently than it would if you were not on the train. You would not be aware of any effect of your motion. Only when your clock is measured by an observer in motion wrt your frame will it appear to be slower than the observers clock.

    Also you seem to be confusing the situation where two inertial observers measure each others clock rate, and the round trip situation where clocks are synchronised, and one of them takes a round trip. This is not a symmetrical situation.

    [simultaneous with Doc Al's post]
  26. Feb 21, 2008 #25
    Well good explanation meaning that I agree with it. Just to clarify.

    That's an answer. So you are saying that if you were at a stationary or inertial train station and I was travelling at ANY constant velocity in any straight line (towards you or away from you or even parallel to you) my clock would run at the same rate as yours? I was led to believe that in this case time dilation would still occur for me and my clock would still be behind yours. So if my speed being non-inertial and you are stationary or inertial might I see your clocks running fast?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook