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I Questions about time dilation, special relativity, etc.

  1. Jan 6, 2016 #1
    I am a newbie to quantum physics but have been actively reading much about it for a couple weeks. However there are a few questions I simply cannot seem to find the answer to in regards to time dialation, the relationship between speed of light and time, etc. It seems like many sources repeat the same things over and over, with little proof that I can find as to why we believe them. I hope someone can help answer some or all of these things.

    1) What exactly made Einstein correlate the speed of light with the speed of time? What data did he have, or what evidence was out there to link these seemingly unrelated things? What made him think moving fast would change time? I'm aware of the light clock example, but this seems more like theory regarding a matter of visual perspective than something that could change the speed of a human aging.

    2) How could light possibly be the same speed for all observers? What a relatively stationary person fired a laser alongside a very fast moving person, and they both observed the same laser? If they both observed it at light speed, they would both be seeing the same laser in 2 completely separate places. What evidence is there to even make Einstein or anyone believe this is true?

    3) Even assuming the above is true, and the speed of light is the same for all observers, just how is that supposed to be correlated to actual "time"? Just because 2 people see something differently, like an optical illusion, I hardly see how a difference in perception of light, equates to an actual difference in the speed of passing time.

    4) What real proof is there of time dilation? Yes I have heard certain particles decay slower when moving fast, or that a caesium clock, or other clocks, reporting time a little different, but I hardly see how this is proof that "time" itself is changing for the clock. In fact, from what I had read about the Hafle Keating experiment, it may have disproved this, as clocks moving in opposite directions gained and lost time, which is inconsistent with the theory that a faster moving object slows down time. If that was true, both moving clocks would slowed at the same rate. This seems to indicate that even if moving clocks do experience an unexplained change in time, that there is some other force at play besides time itself changing.

    5) A related example - You could take a falling sand clock higher where the pull of gravity is less so the sand falls slower, or you could put magnetic fields next to a grandfather clock to change the speed of the pendulum. Just because some force (even if it is not yet known) changes the speed a clock works at, does not mean the clock is actually experiencing time differently. And just because a certain particle deteriorates slower when moving fast, I hardly see how that is a strong indication that actual time has changed for it, nor that this phenomenon could possibly scale to the level of the twin theory where one human would age faster than another.

    I want to have an open mind, I'm not trying to just argue, and admit I'm new to this realm with much to learn. I'm just not seeing where there is any proof or real evidence of these widely believed and highly unusual theories of changing time. Any input?
  2. jcsd
  3. Jan 6, 2016 #2


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    Lots of questions here. Will take multiple posts to address.
    He postulated it. "If this were so, what would we see?"
    Turns out, we see exactly what he described. This makes it a very good postulate.

    That is why relativity of simultaneity is a key concept.
    The solution to t he apparent paradox you mention is that they experience the passage of time differently. When you factor that in, the paradox goes away.

    Everything. SR is one of the most exhaustively tested theories in history, and passes with flying colours every time.

    Not an illusion. A relativistic traveler returning to Earth will actually be younger.

    Commercial aircraft experience it. It can be measured.

    A man drove up Mt. Ranier, tested and detected time dilation caused by GR (gravitational time dilation), from his camper van. With his family. On vacation.

    Your cell phone and any other GPS device will not work properly if it does not account for time dilation.
    Yes. That is exactly what it indicates.

    This likely requires further explanation. It may be the core question in your search for understanding.

    Do you own a smart phone with GPS? You are taking advantage of SR. There is an programming algorithm that must be applied the satellite signal processing to compensate for time dilation. If that were not applied, your phone would report your position with more and more error with each passing day.

    These are terse answers There's a lot more behind them, but there's a word-limit on posts. It'll take a while to cover all this off. Might be better to concentrate on just a few questions at a time.
    Last edited: Jan 6, 2016
  4. Jan 6, 2016 #3


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    If you believe that relativity is wrong, ask yourself the following question. How likely is it that thousands (millions?) of very smart people who have studied and expanded on the theory over the last 100+ years are wrong and you are right? The answer is that it is extremely unlikely. If you have looked at any decent source on relativity, it has the answers to the questions you posed, you are simply choosing not to believe the answers you are given. For example, you ask, "How could light possibly be the same speed for all observers?" The answer lies in the relativity of simultaneity, which is explained in virtually every text on relativity. Try putting your skepticism aside, accept that the theory is correct, and ask yourself what you are missing. Until you do this, you will not be able to make progress.
  5. Jan 6, 2016 #4


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    Frankly I thought the OP was pretty clear that he is grappling with the issue, not just thinking it's wrong:

    His mindset is admirable, in that it is open. That established, I am of a mind to grant him room to phrase his questions as "For the sake of discussion, let's say I don't believe it, please show me where I'm wrong."
  6. Jan 6, 2016 #5


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    For the experimental support, take a look at this FAQ: http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html (also linked from the top of the relativity forum). Look especially at the section on "Tests of Einstein's Two Postulates". The invariance of the speed of light is one of the more solidly proven ones, and the first experiments showing this were done long before Einstein proposed the theory of special relativity.

    You also may find the proposition that the speed of light is the same for all observers less surprising if you know that we can calculate the speed of light from the laws of electricity and magnetism (discovered by Maxwell in 1861) and the speed of the observer doesn't appear in these laws. These laws of E&M are the same regardless of the speed of the observer (otherwise, electrical appliances on opposite sides of the earth would behave differently because the earth's rotation is moving them at several kilometers per hour in different directions) so unless there's something else involved the speed of light ought to be the same for all observers. The great unsolved problem of physics between 1861 and 1905 was figuring out how this could be, and Einstein's great contribution was to demonstrate a consistent physics that matches experimental results and answered that question.
  7. Jan 6, 2016 #6


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    This is very cool. This is the guy that detected time dilation from his van, with his kids, while on vacation. (i.e. very easy to do, if you have access to accurate clocks)
  8. Jan 7, 2016 #7
    As far as I know, everyone who's introduced to this idea initially doubts it. The fact that you are doing so indicates that you are thinking deeply about it. That's a good thing, and it is the first question you need to address. Everything follows from this premise, or postulate.

    How would you establish that one observer is moving slowly and the other is moving fast? The Principle of Relativity tells us that there is no experiment we can do to distinguish between two states of uniform motion, so it's just as correct to say that the fast moving observer is the slow one. All you can really say is that they are moving with respect to each other.

    No, they wouldn't. If the light beam hit a wall they would both agree that it hit that wall. What they wouldn't agree on is how far it traveled to hit the wall, or how much time it took to get there. One observer would say it went farther and took more time, but each observer, when dividing the distance traveled by the time taken, will get the same result for the speed of the light beam.

    The amount of experimental evidence supporting these seemingly crazy ideas is overwhelming. And when you study these ideas you see that they really do make sense.
  9. Jan 7, 2016 #8


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    But the planes, weren't moving at the same speed. One was flying West against the direction of the rotation of the Earth, and one was flying East with the rotation of the Earth. So one plane was moving at its ground speed - the speed of the rotating Earth, while the other was moving at it's ground speed + the speed of the Rotating Earth while the ground clock was moving at just the speed of the rotating Earth. Thus as measured with respect to the center of the Earth, all three were traveling in circles at different speeds and should ( and did) end up recording different times.
    Let's take the sand clock example. A sand clock on a mountain top runs a bit slower than one at sea level. Relativity say that an atomic clock on that same mountain runs faster than one at sea level. With the sand clock this can be attributed to the difference in gravity force at the two locations. With Relativity, it cannot. It predicts that the difference in clock rate is due to a difference in gravitational potential. The mountain clock runs faster because it is higher in the gravity field and that it would run faster even if there were no difference in the gravity felt by the two clocks, just by the virtue of it being higher.
    Experiments have been done to test this and they all agree what Relativity predicts. They have even used centrifuges to simulate gravity of extremely high values and found that it had no effect on how the sample aged.

    As far as the effect "scaling up" to a human being. We are, in effect, a collection of subatomic particles, and at the basic level it is the interaction of those particles that control our metabolism, movement, aging etc. So anything that effects these subatomic particles that make us up is going to effect us on a whole.

    I don't see were it makes a whole lot of sense to postulate some mysterious unknown force that retards the actions of clocks ( and all clocks,no matter what the design, equally), in such a way that it perfectly mimics the effects predicted by Relativity (which does not posit such a force) just so one can hold on to the idea that "time" is immalleable .
  10. Jan 7, 2016 #9


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    Why not learn SR properly? Your post is brimming with popular misconceptions. Forget everything you think you know and start learning SR properly. There's a free online book here.

  11. Jan 7, 2016 #10


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    In the OP's defense, he does seem to have read up on it, he's struggling with reconciling what he's read with real world scenarios.

    (I defend him because he seems to be a rare case of exploring correctly. He's done the reading, he's pretty certain it's correct, he just can't reconcile it. So he's come here for elucidation.)

    At this point, it's appropriate to address specific questions directly.

  12. Jan 7, 2016 #11


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    I think the speed of light is the maximum speed of causality, rather than the speed of time. I'm not sure what the latter would mean. Einstein postulated that the speed of light was constant in all frames, which was consistent with an apparently crazy prediction of Maxwell's equations, and explained the null result of the Michelson-Morley experiment among other things. A logical consequence of requiring that the speed of light be invariant and requiring that the principle of relativity hold is that simultaneity, length and elapsed time must be frame dependent quantities. The light clock is a simple way to show that. You can only make it work if the speed of light is variable (Newton), if time is not an absolute thing (Einstein) or if the principle of relativity doesn't hold (no evidence consistent with this).

    You can't just "observe a laser". Einstein is quite careful to explain what you can do, which involves mirrors or light sensors and clocks and synchronisation procedures. And the end result of all the time dilation and length contraction and relativity of simultaneity is that, in any frame, there is a self-consistent story of what happened. The stories will be different in different frames, but they will be self-consistent and predictable.

    This isn't an optical illusion. A closer analogy is how you can turn a square into a diamond by rotating it 45°. "Square" and "diamond" are just different descriptions of the same thing - what I call vertical, you call diagonal. Similarly, the stationary light clock and the moving light clock are the same thing seen from a different angle (a hyperbolic angle, in fact), and what I call simultaneous is not what you call simultaneous.

    Again, the speed of light isn't the "speed of time", whatever that might be. It's just an invariant speed. Declaring it invariant ends up implying that time cannot be an absolute quantity, something that is universally agreed upon. It must be a frame dependant quantity.

    The first part of your question isn't answerable unless you can tell us what you mean by "time"? Einstein's answer to that question was purely pragmatic: time is what clocks measure. You can easily show that if one clock is affected, all must be - you simply gather an arbitrary collection of clocks of many designs in one place and set them ticking in synchrony. Then you accelerate yourself to high speed relative to the clocks and look at them. By your own admission, a caesium clock will be running slow. But the others must also be running slow or they'll drift out of sync - and if you've left a colleague stationary next to the clocks, how will she explain that they are drifting? Either you've got two contradictory stories (the clocks are co-located and in sync to your colleague but not in sync to you), or the clocks drift out of sync for both of you (but then why did your acceleration affect the clocks?), or they all stay in sync. The last one is the only one that makes sense, so time (by Einstein's definition) is different. No one has come up with any other definition of time so far...

    Others have commented on your misinterpretation of the Hafele-Keating experiment, so I won't.

    As my last paragraph, you could use your colleague's heart rate as a (poor) clock and add it to the collection of clocks. Either your colleague's heart is beating slowly (which, taken to extremes, will kill her if all her metabolic processes, including her perceptions, are not also slowed down) or you get one of the contradictions above. Note, however, that because your colleague's thought processes are slow by exactly the same rate as her pulse and the clocks, she will not experience time differently. Time always passes for you at one second per second - time dilation always happens to someone else.
  13. Jan 7, 2016 #12


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    I think it's important to avoid any confusion, and be clear that his colleague's metabolic processes are not slow - that is an artifact of MMX's frame of reference. His colleague really is (as you go on to say) experiencing time on her ship at one second per second, as are all her clocks.
  14. Jan 8, 2016 #13


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    The complete statement is that each of them experience their own thought processes (heartbeat, etc.) and their own wrist watch as normal. They both measure (it's not strictly "perceive", I would say, because they need to subtract out the Doppler effect before they can get at the time dilation) the other to be thinking and ticking slowly.

    I guess my point is that we should probably both be avoiding "are" in favour of "are measured to be in some frame".

    Aside: there's an old Star Trek episode ("The Naked Now", if memory serves) where the Enterprise starts going back in time. The crew can tell because the clocks are ticking backwards. That's (an exaggerated form of) the kind of silliness that relativity ducks by insisting that clocks stationary relative to you always tick at one second per second.
  15. Jan 10, 2016 #14
    Thanks for all the replies. I have read them all and checked the references in brief, and plan to check the references in more detail later. Also many of the keywords pointed me towards useful info as well. I still have a few questions I couldn't quite find the answer to.

    1) It comes up very often, people saying there is exhaustive proof of special relativity. Yet I seem to find very little. Most of what I find is just mathematical theories on paper, thought experiments, animated demonstrations of things like the common train and platform example, light clock examples, or examples of length of objects contracting at speed. These all make sense "if" the speed of light was truly constant to all observers, but I'm not seeing too much proof actually confirming this is true. Are there any solid experiments, physically recreated with scientific instrumentation, preferably video documented, that confirm this?

    For example, have train and platform type experiments been physically performed in any way with strong documentation? For example actually setting up a high speed rocket platform with lasers, sensors, cameras, etc to actually see this phenomenon happening? I know MIT had a demo where they photographed individual photons, so it seems we should have equipment fast enough to potentially photograph or otherwise collect a disagreement in the speed of light from different perspectives.

    2) Right now I'm not counting the the cesium clock, or particle decay experiments as proof of actual time changing, but rather a tidbit of possible evidence. Since cesium clocks seem to be based on a combination of radio waves and subatomic particles jumping back and forth, I feel there are probably at least 10 other possible theories why they could change its operating pace, rather than simply assuming it experienced time differently. Not to mention there is also possibly gravity dilatation, as demonstrated by that guy's van experiment that could skew results of speed based dilation. (For simplicity right now i'm focusing my understanding on the speed based dilation). Have there been other clocks this has been measured with? If there were many types of clocks (not just cesium clocks), which all operate differently, experiencing identical time loss at identical speeds, that could be more evidence. So have tests with other clocks been done?

    3) What exactly made Einstein believe this back in the day? They didn't have equipment back then to measure things this fast did they? To my knowledge, the speed of time was determined for observers on earth by observing an eclipse of one of jupiter's moons, and documenting the difference at different points of earth's orbit. That's just one reference frame though for observers on earth. Without having fast moving objects back then, what gave anyone back then the idea of a constant speed of light for all observers?
  16. Jan 10, 2016 #15


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    GPS for example.
    The structure of Maxwell's equations.
  17. Jan 10, 2016 #16


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    See Nugatory's link in #5.
  18. Jan 10, 2016 #17


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    Why do you discount the Michelson-Morley experiment as confirming the constancy of the speed of light? If the speed of light were not constant but depended on motion with respect to some absolute state of rest, then the Michelson-Morley experiment would have to show a positive result, which it doesn't. This seems inescapable to me. There have been many repeats of this experiment with increasing levels of accuracy, and all show a null result.
  19. Jan 10, 2016 #18


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    Yes. But you discount them:
  20. Jan 10, 2016 #19
    Then you haven't been paying attention to the replies in this thread. Read Post #5.

    Everyone, scientists of the day included, every student I know of, and probably every other student since, has doubted its validity. Einstein's thought experiments tend to leave most of us with the thought that we're being tricked or fooled, like we feel when confronted with a riddle. For this reason and many others, people have devised numerous tests of the theory and it has come thorough with flying colors. And in addition, after expending the required amount of mental effort, we find that Einstein's theory does make sense. It's not a riddle after all. It's an insight into the very nature of space and time itself.

    But to end the story there leaves out the most important point. To the modern day engineers who calibrate and coordinate the various atomic clocks aboard GPS satellites Einstein's theory is a basic fact of life. The same is true for lots of other engineers, scientists, and technicians working with things like the high speed of electrons in circuit boards. Or accelerators, in devices used for things like proton therapy. It’s encountered every minute of every day in many different places across the globe. Einstein’s relativity is a fact of modern day life.

    Just like the modern day puzzles of dark matter and dark energy, or the now-solved solar neutrino problem, the puzzle of the day then was how to reconcile the way electric and magnetic fields interact with each other and with matter (something fundamental to the structure of the matter that makes up you and I and everything else we see, hear, smell, taste, and touch) and the way objects made of that matter interact with each other. The former is known as Maxwell's Laws and the latter Newton's. Einstein was trying to solve this intellectual and physical puzzle and he succeeded!
  21. Jan 10, 2016 #20
    I've reviewed post 5 once again, and the Michaelson-Moorley experiment. Maybe I am missing something, but I'm just not finding this strong evidence.

    1) The Michaelson-Moorley experiment is an experiment to test whether there is a flow of aether which the waves of light propagate through. It indicates there was no aether, and supposedly shows that from our reference point, the light in both perpendicular directions travels at the same speed. But I only see where it has tested our reference point. An apparatus like this could only test one reverence point at a time, the people's who are operating it. For this to prove any special relitivity 2 people would have to watch the same test on the same apparatus, one of them operating the apparatus, and one of them zooming by at nearly the speed of light who would in theory see that same test appear differently. I don't see anywhere this has been done, nor do we have the technology for it. I just don't see where this experiment, as it has been done, comes anywhere close to proving time dilation.

    2) I continue to hear so many things about exhaustive evidence, but then just pointing back to the GPS example. I don't discount this, but I consider it only a tidbit of possible evidence, as there could be many other potential explanations besides simply time distortion. If there is so much overwhelming evidence, why is the GPS example and muon particle decay examples the only ones I can find? If the evidence is so exhaustive, there should be many many more, well laid out and photo/video documented examples (not just thought experiments or mathematical theories).

    3) There are some excerpts from other tests in the post made in #5, but the details are extremely brief. I know, I could independently google each and every test in hopes one offers more proof, but I've already done that with a couple and didn't find it. It started to feel like a wild goose chase. Especially when the top listed one, Michaelson-Moorley, does not initially appear to be proof. Is there any other modern, and well documented evidence including video/photo of the tests or scientific instrumentation use/setup, besides simply the GPS or Muon example?
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