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Two beams of light traveling in opposite directions=2c

  1. Mar 11, 2014 #1
    depending on your frame of reference, correct? If I am standing still, and at the same time (my time) shine 2 flashlights in opposite directions, after one second on my watch, the leading edge of the light beams will be 372,000 miles away from each other. (186k miles to my left, and 186k miles to my right) Or, if I look up in the sky and see two space ships pass each other going in opposite directions, after 1 second on my watch, they will be 372 thousand miles apart. (2c) However, on each space ship, no time has passed at all. Anyone on board would think they were in both places at the same time. Nor could anyone on board measure how fast they were going, or how fast the other ship was moving, because no time has passed for them. No one on board could even take a breath, or move to control the ship, because no time is passing, and any movement would take time. If you could look in the window of a space ship full of people going at light speed from earth, even if you looked at them for 1000 years of earths time, everyone inside would appear frozen. No one could move or do anything because no time is passing for them. So if you slow them down a little bit so that one year passes for them while 1000 years pass on earth, they could still only accomplish in one year what we did in 1000 years. Nothing is gained for them. They do not live longer in "actuality" than someone on earth, its just that time has slowed way down for them. Actuality I would mean by the amount of time they would live in their "local" time. Please let me know if I have this right. I think I do, and if so, its not that hard to understand. You just have to shift your thinking to understand time is not constant, and this would be "common sense" if you were born and raised in a time when traveling at all different speeds up to near the speed of light were possible. You would see and experience it happening every day, so it would be perfectly normal. Its only "strange" because we can not experience it yet in our everyday life.
  2. jcsd
  3. Mar 11, 2014 #2


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    wow mate, you definitely need to do some paragraphs. Format that better, so more people will read it. You want people to read it, don't you?
  4. Mar 11, 2014 #3
    You're mostly correct, except for the part where you talk about a ship moving at the speed of light. That is impossible.
  5. Mar 11, 2014 #4

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    Since the bulk of that single paragraph post was about spaceships moving at the speed of light, I would say that he's mostly incorrect.
  6. Mar 11, 2014 #5


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    See this recent discussion, in particular post #2:

  7. Mar 11, 2014 #6


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    This is an example of what I term as a "So when did you stop beating your wife?" question. It makes a critical assumption about something, and BEFORE it actually verifies if that assumption is correct, it makes a further extrapolation using that assumption. In other words, it assumes that I had been beating my wife, and it asks when I stopped doing that.

    In this case, the whole scenario rest of a critical assumption that it is meaningful to actually transform to the photon's rest frame. When it asks on what one would actually see when one is moving with the photon, that is exactly what one is doing. But as we have learned from basic understanding of Special Relativity, we can't do this. Such transformation has no physical basis and meaning as far as Special Relativity is concerned. The equations and description of SR is based on a set of postulates, which includes that c travels at a specific, constant speed in any inertial frame. When you transform to the rest frame of light, you are no longer in a situation described by SR! Therefore, you cannot use all the descriptions and equations of SR! The implication of zero time, zero length, etc. are not valid there, because these all made use of SR's original postulates!

    So here, the STARTING assumption, that it is meaningful to actually apply SR's equations to the rest frame of a photon, is faulty. It then renders all subsequent application of that assumption to be faulty as well.

    This error has already been addressed in one of our FAQ in the Relativity Forum. And yes, as had been mentioned, you really should pay attention to proper formatting of your post. One, single, long-winded paragraph makes it very difficult to read.

  8. Mar 11, 2014 #7
    Sorry guys, it was a theoretical scenario. I understand that nothing with mass can travel at light speed. One point of it was to help explain that because someone leaves earth and moves at speeds near c for x amount of time, does not mean he is living longer than his earth bound brethren. (although he returns to find himself much younger) Everthing is TRULY moving in slow motion for him compared to earths time. Example: Someone could not take a trip from earth and travel at near c for 3000 earth years, and come back having knowledge that would take 3000 years on earth to aquire. His "study time", like his aging, would be determined by the watch on his arm. The benefit would be he could come back, be only slightly older, and be able to enjoy advances the people on earth had aquired in all those years he was away.
  9. Mar 11, 2014 #8


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    yep. that's one lucky guy. but his family will be long gone. so it's kinda sad too.
  10. Mar 11, 2014 #9

    Not if it was my family. :rofl:

    KIDDING of course!!!!!
    I LOVE my family and they mean evreything to me. It was just there for the taking and I could not resist. :smile:

    (Thanks for the reply too)
  11. Mar 11, 2014 #10


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    No. That statement is pretty much flat-out incorrect (although you might be able to plea bargain down to the lesser charge of "totally misleading"). It is also not supported by your example below - one way of seeing this is to observe that as far as the rocket guy is concerned, he's at rest for most of the journey and it's the earth that is moving rapidly, first away from him and then towards him.

    This is true, although it doesn't lead to the conclusion you've stated, that everything is moving in slow motion for the rocket guy. On the contrary, for any reasonable definition of time dilation, time is passing more quickly for him and he's he's aging more quickly than everyone else throughout his journey - even though less total is elapsed according to his wristwatch. You might want to try working through the FAQ at http://math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/twin_paradox.html to see what's really going on here.
  12. Mar 11, 2014 #11
    I think we are in agreement. Maybe my wording was not clear.
    Everything is moving in slow motion for the rocket guy as viewed from earth. Both earth people and rocket man experience time as moving "normally" in their frame of reference.....

    I thought the guy in the rocket would see earth as moving slowly as well however, at least upon the outgoing part of his trip. When he turns around, it SEEMS like he should start seeing things speed up on earth, but I could never get a clear answer on that. At some point, he would have to see things speed up, because time on earth is way ahead of "his time" when he gets back. Can you give me the correct answer? It would be greatly appreciated.
  13. Mar 11, 2014 #12


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    That subtlety is covered in the "Doppler" section of fthe FAQ I pointed you at, and on this forum user ghwellsjr has posted very good explanations with more detail.

    Some of the confusion comes from the way that the word "see" is used with two very different meanings in discussions of special relativity. When someone says that an observer "sees" something, they may mean either of two things:
    1) That is literally what the observer sees; we're talking about the sequence of images that form on the retina of his eye as light, which may have been emitted in an event long ago and far away, reaches him. In this use of the word "see", there's no notion of things happening at the same time in different places - all we have is the events happening in the one place where the observer and his eyes are.
    2) That is the observer's understanding of reality; he understands that if light from an event ten light-seconds distant reaches his eyes at time ##t##, then the event must in fact have happened at time ##t-10##. He also understands that (for example) if light from some other event that happened at twenty light-seconds distance reaches his eyes at time ##t+10## it happened "at the same time" as the first event, even though the light reached his eyes at a different time so by the #1 definition he did not see them happen at the same time. (Before you reject this second definition of "see", consider that without t we couldn't construct airline timetables or anything else that assumes that we can talk about things happening at the same time in different places).

    By the #1 understanding of "see", rocket-guy sees Earth-guy's clock running slow on the outbound leg and fast on the inbound leg. Likewise, Earth-guy sees rocket-guy's clock running slow on the outbound leg and fast on the inbound leg. However, rocket-guy sees the turnaround happening at the midpoint of his journey while Earth-guy sees the turnaround happening much later in the journey, so Earth-guy sees rocket-guy's clock running slow for a longer time, so rocket-guy ends up younger. (I'll repeat myself - read the "doppler" section of that FAQ carefully, and listen to ghwellsjr if he weighs in).

    By the #2 understanding of "see", both earth-guy and rocket-guy understand the other clock to be the time-dilated one, on both the inbound and outbound legs.
  14. Mar 12, 2014 #13
    Thanks for taking the time to post that here!
    That makes perfect sense to me, and seems totally logical. I was told before it was incorrect (rocket seeing earths time faster on inbound trip) and left me stumped. Now it leaves me thinking about when someone (on say a merry go round moving near light speed) would see things speed up if the times were the same as in the above sample. It would be upon the slow down I take it? And the stationary person would see the slow down starting at a much later time I would think. That seems like the logical answer, but not sure if its the right one.
  15. Mar 12, 2014 #14


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    haha. For some reason, when I first saw this, I thought you were implying that you would have thousands of kids before you left for space, so that in a few thousand years, the earth will still contain a significant amount of people that are your family.
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