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Time and Time again.

  1. Aug 31, 2007 #1
    Hello Everyone.

    There's a thread going on here now about time but I didn't want to interrupt their conversation as my questions are a little bit different....and the fact I have questions and not really anything to add.

    Just some questions about time some answerable, others maybe not so much First of all this is my favorite thing to think/talk about in the whole world. But all I ever do is talk about it with my friends and none of them are physicists or philosophers so I don't get the feed back that is going to help me. With time there are so many questions to ask it's hard to know which on to ask first. I find when describing one answer to a question another question pops up. When talking about time it’s hard to stay on one sub-topic without deviating to another. But I guess that’s the fun in talking about time (also the crazy feeling you feel after giving some thought to it). Here it goes...

    1. What happens (or is speculated to happen) with time at the speed of light?
    ie: Would time stop? Could we perceive anything? Would the fact we are going that fast prevent any light from getting to our eyes to perceive it?

    2. If you go at the speed of light you would be covering distance as in km; as well as traveling a greater distance in time. Is distance traveled connected to time in some way?
    I'm Confused between the speed of objects and the speed of time. When you say go fast, is this fast in the sense of 23,425,883km\hr, or traveling through time at some fast rate. We can say 10km/hr shouldn't we be able to say we go through time at 1hr/something? Like the distance of time so to speak. Not how much distance in km you cover in 1hr but how much "Time Distance" you cover /1hr. So is distance covered in kms equal to distance covered in "Time distance"?

    Sometimes it feels like I’m going through life so fast I’ll die earlier older. Like, I’m aging in proportionally to the allotted time that has gone by. And I’m not just talking about the bio-degradation of everyday life (like too much sun gives wrinkles) it feels as though I’m literally jumping frame to frame in life. I attribute this to always wanting time to go faster at work. I spend so much time thinking “I wish time was sped up” that I think it’s actually happening. Days, weeks go by and I come out of my little time warp I aged but it didn’t take that long. Like my physical body is now older in the sense its material has gotten more degraded, but it seems like I didn’t travel that far in the “Distance of Time”………arg…….You know what? I’m sure some of you’ve felt the same way, I can’t really explain it that well. But something to think about I guess.
    Last edited: Aug 31, 2007
  2. jcsd
  3. Aug 31, 2007 #2
    First, it is not possible to travel exactly with the speed of light. If you travel with the speed, say, 99.9% of the speed of light (relative to Earth) you will not notice anything unusual inside your spaceship. This is the principle of relativity: a moving laboratory is undistinguishable from the laboratory at rest. However, looking from your spaceship at Earth you'll see that all processes there become much slower. From your point of view, things on Earth will almost stop moving.

    Observer on Earth that can look inside your spaceship (suppose that you have a videoconference between your spaceship and Earth) will see that everything inside your spaceship is moving very slowly.

  4. Aug 31, 2007 #3

    light does...I assume something else might as well. Actually maybe this thread would be better in metaphysics or somewhere as I am speaking hypothetically. (I'm trying to get this time dilation thing down) is there a site that explains this really well? and simple...:S I spent a lot of time on this time dilation thing and I still can't grasp it...thats why I asked the speed and time question earlier I wanted to know if time is going by according to the speed of us then the faster we go the more time we advance. So if a group of people set out at light speed they would look fast to us (assuming we could see them pass) and we would look slow to them. Does that mean they look out the window and see barley a glimpse as they fly buy, but if they had seen it, it would have been rotating slow and everyone on it would be moving slow?
    And if they came back to earth 10 mins from when they left they would be older than their twin. This to me points to that the speed of which you travel accelerates your personal time line the faster you go the more "Time Distance" you loose. But this brings me back to time being relative take particles for instance, when a heavy particle forums like a muon or something it's existence is really short...to us. But for it, it's his whole life. The earth barely rotates for the whole existence of this things life, not one day goes by, but it "experiences" all of it. If you could experience that life would it seem as short then as it does now? If we could ask the particle if he knew how short his life was , he may look at us and say, "What? I've lived a lifetime". Is there a relationship between our speed km/hr and our advancement through time?

    . and oh i have another question to add too:
    though not all that possible what would be the ramifications if the speed of light changed? (also hypothetical)

    I know this is a lot of F'ed up stuff to reply to or dismiss but thanks for at least helping to set me in the right direction.
  5. Aug 31, 2007 #4


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    As the sci.physics.faq says, it is not possible for a massive particle to go at the speed of light.

    See for instance http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/headlights.html

    Note that "massive" and "massless" are being used here in a particluar manner, in particular we are talking about the so-called "invariant mass" when we say massless, not the so-called "relativistic mass".

    This is another common source of confusion. If you want more information on this topic, try for starters:

    http://math.ucr.edu/home/baez/physics/Relativity/SR/light_mass.html [Broken]

    I don't want to digress into this unless you are interested, I just want to point out the issue.

    So your question doesn't have a direct answer. We can say that as you approach the speed of light, the time for a trip of a fixed distance (distance as measured in the non-moving lab frame) becomes smaller and smaller.

    We can't say what happens at the speed of light - it's like division by zero, it's not defined.
    Last edited by a moderator: May 3, 2017
  6. Aug 31, 2007 #5
    Ok fair enough then replace all the "light speed" with "almost light speed". then can it be discussed?
  7. Aug 31, 2007 #6

    Nothing special, at least from your own point of view. From your own perspective, everything will be perfectly normal. Others observing you, however, will perceive time slowing down for you, and you will observe time slowing down for them.
  8. Aug 31, 2007 #7


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    Yes - and I tried to discuss it fairly specifically, when I said that one could cover an arbitrarily large distance (as measured in the lab frame) in an arbitrarily short time, by travelling "fast enough".

    This is perceived as time "running slowly" on the ship by people in the lab frame. People in the ship view things a little differently (I'm not sure if you want me to get into that?).
  9. Sep 1, 2007 #8
    hell yea, thats my problem i just cant visualize it...so any additional description would be helpful.
  10. Sep 1, 2007 #9


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    OK. From the point of view of the traveler, what happens is that the destination appears to be much closer. This effect is known as Lorentz contraction. Since the distance can be made arbitrarily small, it can take an arbitrarily small amount of time to reach it.

    So far we've talked about two of the three main effects of relativity, which can find a mathematical expression in terms of the Lorentz transform. The third effect is a bit harder to describe than the first two, but it's necessary to gain a coherent picture of what's going on in this example (and in any other). This effect is the relativity of simultaneity.

    Let's go back to our traveler, again. Suppose Earth (assumed to be the starting point) and the distant planet both have clocks, and so does the distant destination planet, 1000 light years away. They both synchronize their clocks in the Earth frame - it is assumed that any velocity difference between Earth and the destination planet can be ignored.

    Suppose our traveler travels at a very high rate of speed, so that it takes him only 10 years by his onboard clock to make the journey When he arrives, his clock will read 10 years, and the clock at his destination will read something greater than 1000 years.

    How does our traveler explain this? He explains the trip time by saying that the distance contracted. Time dilation, though, does not explain the large reading of the destination clock. To him, the Earth clocks were actually ticking slowly. What is needed to explain his observations is a third effect, the relativity of simultaneity. According to the traveller, the Earth clock and the clock on the destination world are not synchronized in his reference frame. So the Earth observer thinks the clocks are synchronized, while our traveller does not think they are synchronized. In relativity, synchronizing clocks is observer-dependent, it is not something that can be done in any absolute sense. This is called 'the relativity of simultaneity'.

    The three effects, taken together (time dilation, length contraction, and the relativity of simultaneity) are sufficient to understand relativity, and have a mathematical expression as the Lorentz transform.

    To recap the Earth POV, the distance remained unchanged, the trip time was seen as 1000 years, and the low reading on the traveler's clock is explained by time dilation.

    To recap the traveller POV, the distance shrunk, the trip time was seen as 10 years (which was just the shrunken distance divided by his velocity), and the high reading on the destination clock is explained by the relativity of simultaneity.
  11. Sep 1, 2007 #10


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    In your frame, the reason you can get to distant locations quickly is that objects moving relative to you shrink in length (something that's true in all frames, it's known as 'Lorentz contraction'). For example, say you're traveling at 0.8c between Earth and a star 10 light years away as measured in the Earth's frame. In the Earth's frame this will take you 12.5 years (because 12.5 years * 0.8 light years/year = 10 light years), but in this frame your clocks are slowed down by a factor of 0.6 (the length of your ship is also shrunk by a factor of 0.6, but this isn't important), so your clocks only advance forward by 12.5*0.6 = 7.5 years between the moment of leaving Earth and the moment of arriving at the distant star.

    In your own frame, your clocks are running normally while the clocks on Earth and the star are slowed down by 0.6, and the distance from Earth to the star is shrunk by 0.6 due to Lorentz contraction, so it's only 6 light years from Earth to the star. Since the star is moving relative to you at 0.8c, the time for it to reach you is 6 light years / 0.8 light years/year = 7.5 years, so this explains why your clocks have advanced 7.5 years between the time you leave Earth and the time you arrive at the star.

    You might ask why it is that, if the clocks on Earth and the clocks at the star are ticking slowly in your frame, the clock at the star reads an elapsed time of 12.5 years when you arrive, which is greater than your own time. This has to do with something called the "relativity of simultaneity", which means that different frames disagree about whether two distant events happened "at the same time" or not, and so they likewise disagree about whether two clocks at different locations are synchronized or not. If the clocks on Earth and the clocks at the distant star are synchronized with each other in the Earth/star frame, then in your frame they'll be out-of-sync by 8 years, with the clocks at the star ahead of the clocks on Earth. So if the trip from Earth to the star takes 7.5 years according to your clock, and the clocks at Earth and the star appear to be slowed down by a factor of 0.6 in your frame so they only advance forward by 7.5*0.6 = 4.5 years in this time, then if the clock on Earth read a date of 2010 when you departed, the clock on the star would have read a time of 2018 (8 years out-of-sync), so when you arrived at the star the clock there would read 2018 + 4.5 = 2022.5, 12.5 years more than 2010 which was the date you left Earth in the Earth/star frame.
  12. Sep 3, 2007 #11
    a doubt about time

    sorry interrupting this way, but can u tell me if we sit in a space ship traveling with a comparable speed of light then will the biological process going in our body slows down if yes why?
  13. Sep 3, 2007 #12
    If I am traveling in a fast spaceship, then neither I nor my travel companions would notice any changes in our biological processes. However, people on Earth will appear to us as incredibly slow. There is no paradox here. The apparent speed of physical and biological processes depends on who is observing them.

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