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Explaining Time Dilation and the Universe

  1. Oct 20, 2008 #1
    Im trying to explain to this guy why time dilation occurs and how asking what is beyond the edge of the universe is irrelevant.

    Ill start with his original question.

    "Forgive me if I'm wrong and my memory has faded, but consider this: what is empty space? What is there between matter? What if the fact it was traveling much faster meant that it was encountering some kind of external friction, stronger than the stationary clock, which reduced the energy available to the particles oscillating in the atomic clock? My thoughts exactly are hard to put into words, hope that made sense."

    I then explained best I could but he refused to accept time dilation and is stuck on the idea that space has substance(aether). He then said:

    "So if space in itself is nothing but void, what is the difference between the void in deep space and the void beyond the boundaries of the known universe? It doesn't makes sense the three dimensions are present "just cuz"."

    Sorry for the huge amount of writing but Im not really sure where to go, I dont think i can really explain it adequately, I dont even have my degree yet and Im no teacher.
  2. jcsd
  3. Oct 20, 2008 #2
    It should be noted that the person in question has supposedly taken mathematics up to linear algebra.
  4. Oct 20, 2008 #3


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    Well, firstly, one should dispel the notion of an 'edge of the universe.' There is nothing that says the universe must have an edge and, in fact, cosmologists do not entertain models which do. As an example, you should read up on the balloon analogy: we have a sticky thread in the cosmology forum on this.
  5. Oct 20, 2008 #4
    Keep in mind some people do not want to understand, others would like to but lack the creativity to understand, some will listen and try to learn....

    what is empty space?

    There is nothing "empty" about space....it fact gravity warps it, it appears to be quantized with a minimum length of planck length 10^-33cm, and there is constant production and annihilation of particles from quantum "foam"...the energetic uncertainty at sub Planck lengths....A theory of a "continuous creation" universe says the production of one atom per cubic mile per hour from such "foam" would sustain all!!! So a universe might, atom by atom, emerge from "empty" space!

    It appears that all may have begun in a quantum fluctuation, or if you prefer a big bounce or a big bang; in any event, energy,space, time and everything appear to have emerged from a tiny initial amount of energy...nobody understands the exact relationships and we may never know it all.

    "So if space in itself is nothing but void, what is the difference between the void in deep space and the void beyond the boundaries of the known universe? It doesn't makes sense the three dimensions are present "just cuz"."

    It is possible there is a "void" beyond the "space" of our universe; we'll likely never know because it's likely undetectable...In any case, space and time, for example, are required for a "universe"...us,stars, your pet, air, etc....without space and time there is nothing...a true "void" is likely beyond our understanding. Anyway, there is no space,no time, so there is "nothing"to see!

    Time dilation is also difficult to understand...after all, until Einstein in 1915, it was generally thought to be "constant". But time IS variable, space is variable just as speed is variable. We don't really know why they are related, but if you can visualize/observe changes in speeds, then the analogy is that space and time also vary; we are unaware of it most of the time due to our limited senses and everyday experiences whereas our everday senses are aware of acceleration and changes in speed.

    It's no crazier than understanding how we are all made of energy!!! If strings are elementary constituents of "matter" and are made of energy, then we are manifestations of that energy! Why can't you see radio waves; after all they are electromagnetic radiation, another form of energy, just like visible light??

    Our senses are limited, our intellect can take us further than our ancestral senses which likely were developed for survival. But the intellect must be utilized to "see" further.
    Last edited: Oct 21, 2008
  6. Oct 20, 2008 #5
    thanks for the responses but he now seems to have an issue with time itself

    "Given that the uniformity of "time" was tested with a clock, I don't see how you can immediately even accept the notion of time and and then just assume the clock runs unaffected.

    OHOOHOH BUT WAIT! It's an atomic clock. My answer: so wat?

    and then "but in essence I have a problem with our concept of time"

    So my new question is how would I go about justifying time as we currently define it.
  7. Oct 20, 2008 #6
    Why don't you work at leading him to understand relativity the way it was discovered historically?

    Light exhibits properties of a wave. With mechanical waves (sound, waver, etc), the speed of a wave is constant with respect to its medium.

    What does it mean for the speed to be constant with respect to its medium? Take a pool of water on a bus. Drop a ball in the middle of the pool, and waves will travel outwards away from the point the ball came in contact with the water. No matter how hard you throw the ball into the water or how heavy the ball is, the waves move the same speed.

    But the velocity of those waves is relative to the water in the pool. If the bus carrying the pool is at rest, then the waves will travel at however many meters per second in all directions. If the bus is moving, though, the waves moving in the direction of the bus will be moving faster, because you add the bus's speed to the speed of the waves. Similarly, the waves moving opposite the bus will move slower. Waves traveling perpendicular to the bus's motion will be unaffected.

    You have a problem with light. It can travel through empty space. What is the medium? There are no atoms. It's devoid of matter to act as a medium.

    Let's assume there IS a medium that permeates all of space. We call this the aether. Think of it as an invisible bus with an invisible pool on it. And instead of water waves, it's light. We should be able to watch the light waves and determine how fast our invisible bus (the aether) is traveling.

    Experiments were done exactly like this. Of them, the Michaelson Morley experiment is the most famous. And the result they found was the bus was also at rest. Always.

    This doesn't make sense. In our invisible bus experiment, you might measure the speed of the light waves sitting on the side walk. At the same, I measured the speed of light waves while riding in a car at 30mph. But we BOTH measured the invisible bus to be going as fast as we were. So you measured it to be moving at the speed of the sidewalk and I measured it to be moving the same speed as my car. At the same time. This is a contradiction. One thing can't move at two speeds at the same time.

    This is the real significance of the idea that "the speed of light is a constant". It's not weird that it is constant, because all waves move at constant speed. It's weird because it's constant relative to who's measuring. And two people moving at different speeds measuring the same thing at the same time will get two different answers.

    How can we reconcile this? Well, we can, but it breaks every fiber of intuition in our minds. Times slows down for fast moving objects. So when you see me flying by in my car, time has slowed down for me. But it doesn't end there. The situation is symmetrical. I could just as easily say that I'm standing still in my car and YOU are the one who is moving. So to me, YOU are the one slowing down.

    You might ask "well, then who's REALLY slowing down?" The answer is that it depends who is measuring. To each, the other has slowed down. Miraculously, though, although the idea that time is relative is weird, it doesn't lead to contradictions, as long as nothing moves faster than the speed of light. Another consequence is that moving things become heavier. The closer you get to the speed of light, your mass begins to grow very fast, and it requires infinite energy for matter to reach that speed.

    How you go about demonstrating these things is sort of neat, but you can find the details if you surf the forum a bit.
  8. Oct 20, 2008 #7


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    Our current answer is that we use the theory of general relativity to define space and time. Within that theory there is an observer independent quantity called the "proper time", which is measured by an "ideal clock". As you noticed, this begs the question "What is an ideal clock?". Luckily atomic clocks are experimentally known to be ideal clocks. Our concept of time would presumably change when general relativity is found to be an insufficient framework in which to fit our observations.

    There's a fascinating story about how we have used different notions of time at different :rolleyes: times, and how we cannot discard the old ones even after we have a new notion (Audoin and Guinot, Chapter 4 http://books.google.com/books?id=LqdgUcm03A8C).
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