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I cant even find words on how to ask this question. My father brought

  1. Jun 29, 2011 #1
    I cant even find words on how to ask this question. My father brought it up to me and I don't know how to answer it...............



    I am at a fixed location you take off at near light speed away from me, 5 years later I look up in the sky and you would almost be 5 light year away from me. Now because it takes time for the light from the ship to reach my eyes is the ship further away than it appears to be. By the time the ships light reaches my eyes from that location has it already moved on?

    Lets do this the other way around. The ship is coming towards me at near light speed. I see the light from it after it already happened. Lets make up a number and say its a 5 minute lag time. Does that mean the space ship lands next to me 5 minutes before I see it???



    I am not grasping something here...

    Also....

    Why does it appear too take so long for a space ship traveling away from me at near light speed to reach it destination.

    I watch you travel too a star 5 light year away. You don't stop and you make a U-turn at the star and come right back. You felt 10 years go by I felt 70 year go by. Why when you where traveling at near light speed did I see you take 35 year to get too the star.

    I usually grasp stuff like this pretty easily... That's probably a good sign that I don't understand it at all lol

    Hope I get some answers!

    Thanks!!!!!!!
     
  2. jcsd
  3. Jun 29, 2011 #2

    ghwellsjr

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    Re: Help...

    Let me take your last example because it is complete:
    First off, if I am traveling at near light speed for a distance according to you of something less than 35 light years and then returning, you will see that it takes me 70 light years to make the round trip but to me it would be 10 years and I would see the star as being somewhat less than 5 light years away.

    What you will see during this 70 year interval is that for most of the time, you will see me traveling away from you. It will appear to you that it takes me somewhat less than 70 years to get to the star. Then in a much shorter time, you will see me come back to you.
     
  4. Jun 29, 2011 #3

    ghwellsjr

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    Now for your first example:
    If I am traveling away from you a nearly the speed of light, you will see me at any given time at about one-half of the distance traveled (according to your rendition of the distance) so after 5 years I would be a little less than 2.5 light years away from you. Yes, my ship is further away than it appears to be, by almost double and yes, by the time the ship's light reaches your eyes, it has already moved on, or, maybe it is already on its way back, but you won't see that until a long time later.

    But on the return trip, you always see what has happened in the past so if I am going so fast that it appears to only take 5 minutes for me to return, then you will see me coming home very quickly (just under the speed of light) but you will never see me land before I actually do land. The closer I get to you, the less the lag time is until when I reach you, there is no more lag time.
     
  5. Jun 29, 2011 #4
    Re: Help...

    Everything is becoming much clearer. Thanks!

    If I am not mistaken it would appear to (to me) take 65 years for the ship to get to its destination and just around 5 year for it to return?

    Why do I see it take so long for the ship to get to its destination?
     
  6. Jun 29, 2011 #5

    ghwellsjr

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    Your example is somewhat loosely defined but there is a specific example for which it is true that it would appear to you to take 65 years for the ship to get to its destination and just 5 years to return.

    It seemed like you pretty much understood from your first example that it will take longer for you to see the outbound portion of the trip than the inbound portion. It's because you have to wait both for the ship to travel to any particular location and for the light to travel back to you. It will always talk longer than the round-trip time for light to make the trip.

    If you had a laser that you wanted to shine on a planet that was 30 light-years away, it would take 30 years for the light to get there and another 30 years for the refection to get back to you for a total of 60 years. If the space ship was traveling a little slower than light so that it took 35 years for it to get to its destination 30 light-years away, then it would take 65 years for you to see it arrive there from the time it departed.

    But for the inbound portion of the trip, you only have to wait for the one-way speed of light time. For example, if you are already observing a planet 30 light-years away and somebody turns on a laser there pointed at you, it would take 30 years until you can see it.

    In the case of my spaceship, as already stated, it would take 35 years for me to get to the planet but you won't see that for 30 more years, and it will take another 35 years for me to get back but you start to see me coming back just after I arrive and so your image of me coming back takes only 5 years.

    Does this make sense to you?
     
  7. Jun 29, 2011 #6
    Re: Help...

    Maybe I am not understanding but this dost seem to explain why I am viewing this event take soooo long to get to where it is going.

    If I shine a laser at planet-x 5 light year away it would take 10 years before I see the reflection off that planet. I get that.

    If I send you in a ship towards planet-x (Still 5 light years away) at 99% the speed of light you would experience just over 5 years too reach it and just over 5 years to return.

    If I watch you in this ship I would watch you take 65 years to reach your destination and around 5 years for you to return.

    I understand why I see you take around 5 years when you are headed towards me.

    Its the 65 years I cant get my head around.

    0 light years before you take off. I see you right now.
    When I see you at 1 light year. Your location is approximately 2 light years from me.
    When I see you at 2 light years. Your location is approximately 4 light years from me.
    When I see you at 2.5 light years. Your location is approximately 5 light years from me. That means you are just about to turn around and come back to earth.
    When I see you at 3 light years you are now already on your way back to earth
    At 5 light years I finally see you turn around now you have been traveling for 10 light years... That would put you back on earth...

    See where I am getting confused???

    I cant figure out why It is taking me 13 years to see you at 1 light year or 65 years at 5 light years.




    You can combine this next question with the question above if it helps LOL...

    I have 2 clocks 1 in my hand and one that I placed on the ship. I watch both of these clocks.

    At the one year mark (same scenario 5 light years away 99% speed of light) I look at both of these clocks. My clock has gone by 1 year. I look at the clock on the ship and just under 4 days has passed on that clock.

    1 year I should see that ship at around 6 light months away, but I look at the clock and it says 4 days. Am I actually seeing the ship at 4 light days away from me (64 billion miles) or am I seeing it at 6 light months away (2.9 trillion miles)?

    If 4 days is the case. That would mean I see the ship traveling at 4000 miles per second even though I know its traveling at 184,000 miles a second...



    There is a piece of the puzzle I don't know about, or I know about and don't understand...


    Help...

    PS Thanks a trillion for trying to help me!

    Are most of you stay at home scientist (like myself LOL) that likes to know about this stuff but hasn't gone too school, or are most of you trained in some scientific field? Is that too personal, or is my curiosity getting the best of me?
     
  8. Jun 29, 2011 #7

    ghwellsjr

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    Re: Help...

    In order to make sense of your first post, I divided it into two entirely different examples because that is what I thought you wanted. If you go back and look at my first two responses you will see that in my first one (your second example), the destination, according to you is something less than 35 light-years, but according to me it is less than 5 light-years. In my second response, the destination according to you is somewhat less than 5 light-years. I could not tell that you were intending to describe the same example of the destination being about 5 light-years away because you mentioned 70 years for the round trip and 35 years for the one-way trip, both of which are incorrect numbers for a trip at near the speed of light to a destination 5 light-years away.

    So now your new numbers of 13 years and 65 years are also incorrect. This explanation is almost correct:
    The only problem is your last statement "That would put you back on earth". You see the trip coming back to earth taking a very short period of time but not instantly. It would be more like a few days.

    I don't have time to respond right now to the second part of your post. I'll cover that later.
     
  9. Jun 29, 2011 #8
    Re: Help...

    "If you go back and look at my first two responses you will see that in my first one (your second example), the destination, according to you is something less than 35 light-years, but according to me it is less than 5 light-years. In my second response, the destination according to you is somewhat less than 5 light-years. I could not tell that you were intending to describe the same example of the destination being about 5 light-years away because you mentioned 70 years for the round trip and 35 years for the one-way trip, both of which are incorrect numbers for a trip at near the speed of light to a destination 5 light-years away."

    Let me simplify what I was thinking at the time when I wrote this.

    If you travel at 99% the speed of light 1 year in your time is about 7 years in my time. So that would take you 5 light years there and 5 light years back. That would take just over 10 light years for you to complete the trip. For me it would take 70 years before I saw you again. What I was thinking (me observing you) it would take 35 years to reach this location 5 light years away and 35 back. Obviously I was wrong.

    I miss understood what you said when you answered it. What I was thinking... Is that you coming towards me at near light speed it would take just over what light would take to reach me. I guess I was wrong on that too...



    Basically this is what I want to know...

    What is the stationary observer seeing when the person is traveling at 99% the speed of light. What is he seeing for the 5 light years there and the 5 light years back.
     
  10. Jun 29, 2011 #9

    ghwellsjr

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    Re: Help...

    You have the ratio 1/7 correct for 99% c but you are applying it incorrectly. If your example is that the target is 5 light-years away as measured and observed by you, then my round-trip time will be just over 10 years according to you but it will be just over 1.41 years for me and for my clock.

    Now, to answer your last two questions, look back at my comment concerning your quote in post #7. Here are some details: Assuming that I spend no time accelerating, turning around, or decelerating at the end, the trip will take 10.101 years (10/.99) or 10 years and 36.9 days. For exactly half of that time, 5 years and 18.45 days, I will be traveling away from you but it will take exactly 5 more years until you see me reach my destination, so for 10 years and 18.45 days, you will see me traveling away from you. Then you see me instantly turn around and start my trip back to you but it will only take 18.45 days for you to see me make the entire trip back.

    Does that make sense to you?
     
  11. Jun 29, 2011 #10
    Re: Help...

    I see what I was doing... I was making time go faster for me rather than time slower for you. Now I understand whats going on haha

    Just a thought I had... If your a human going on this ship, you know your traveling 10 light years round trip, yet you experience 1.41 years. Not sure how to ask a question out of that, but do you understand where I am going with it?



    Yes it does, I get it completely. Thanks for the details!

    I see 5 years 18.45 days pass by in 18.45 days, correct? Visually it would be a skinny blue dot, but would I see it going faster than the speed of light?

    Also, If I am not mistaken, each day after I see it coming towards me it would be slowing down (not actually slowing down but visually), right? Day 1 I see 2.5 years go by day 2 1.25 years until it gets to a point that it is no longer noticeable. I am not sure if it is 50%, but it seemed reasonable if this is the case.


    Am I being a nuisance? I am sure I could go on forever...

    I really appreciate you helping me!
     
  12. Jun 30, 2011 #11

    ghwellsjr

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    A human on a ship will experience the trip taking 1.41 years but it will not be a distance of 10 light-years, it will be just under 1.41 light-years, so the distance between the earth and the planet will be about 0.7 light-years. The traveler will experience his speed relative to earth and the planet at the same speed you measure it at, 0.99c. Is that what you are asking about?
    Yes that is correct for the return trip. Actually, the light would be blue-shifted out of the visible range but we assume you have an electronic telescope that can detect a wide spectrum of light. No, you would not see the spaceship approaching you at faster than the speed of light, it would be 99% of the speed of light.
    No, the apparent speed of the returning ship does not change at all. Why would you think that?
    No, you are not being a nuisance, I'm glad to help you understand. I'm only returning the favor that others have helped me here.
     
  13. Jun 30, 2011 #12

    ghwellsjr

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    I don't know where you are getting 4 days from but you are right in that when you look at the clock on the ship after one year has gone by on your clock, you will see the "ship at around 6 light months away". But remember the time ratio between the ship's clock and your clock is about 1/7 so you will see the time on the ship's clock at a little less than 26 days (not 4 days).

    In order for you to measure the correct speed of the ship, you have to factor out the light travel time so you would use six light months instead of one light year.

    I have no idea about anyone else, but I am not a scientist, just an electronic engineer, so although I was taught Special Relativity in school, I didn't really learn it until after I studied it on my own and thought I understood it, but got straightened out on this forum.
     
  14. Jun 30, 2011 #13
    Re: Help...

    Ok. If your looking at this planet at 5 light years away. You know its 5 light years away. you get on the ship and you arrive on the planet in .7 years. Once your on the ship does the planet appear to be .7 light years away?

    I am watching 5 years and 18.45 days worth of light in 18.45 days. I know the light coming from the ship is not going faster than c. I am seeing over 5 light years of time in 18.45 days. Do you see how I am thinking I am seeing this ship hurtling at me at faster than light?

    Ignore that LOL I was misunderstanding something (nothing to do with what you said its just what I was visualizing in my head).

    Good! I will pick your brain some more than!


    Anything from post 1-9 I was making time faster for me and not slower for the person traveling at near light speed. So I was getting all sorts of odd numbers.

    What is this referring to?
     
  15. Jul 1, 2011 #14

    ghwellsjr

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    That's kind of a tricky question to answer. The planet's appearance will definitely change as a result of the traveler's speed but how would he correlate its visible appearance to its distance? The traveler needs to actually measure the distance to the planet and any method that is used will give the answer of .7 light years.

    Keep in mind that any measurement that is not based on prior knowledge will take time to perform. Radar is one method but this requires that he send a radio signal to the planet and wait for the signal to be reflected off the planet and based on the round trip time he can calculate how far away it was at some point during the trip, but which point?

    So a better way is for him to actually just keep track of his speed relative to the planet (and his speed relative to the earth) and the time it takes for him to make the trip. This will give him .7 light years. At the same time, he will be measuring how far earth is from him when he gets to the planet. Then he can do it all over again on the way back.

    Let's say you are looking at a star exactly 5 light-years away and all of a sudden you see it go super nova in a massive violent explosion. Now would you say that you are looking at 5 years of light in zero time? I don't think so. I think you would say that the explosion actually occurred 5 years before it finally appeared to you.

    So in the same way, when you see the ship turn around 10 years and 18.45 days into the trip and start back home, you know that what you are seeing actually happened 5 years earlier, at the exact midpoint of the trip, correct?
    I was correcting your misconceptions from this post:
    I was just pointing out that to calculate the speed of the ship after one year, you need to use a time of 6 months, not the 1 year that you see on your clock because you have to remove the time it takes for the image of the ship at around 6 light-years to reach you.
     
  16. Jul 1, 2011 #15
    Re: Help...

    I am just having a hard time understanding how I see 5 light years of light get to me in 18.45 days.......

    If I see that star go super nova at the exact same time your ship turned towards me. In 5 years I would see the start explode and your ship just staring to head towards me. But just 24 hours after that, your ship would be 98 days closer than it was at the start of that 24 hours.

    Do you see how I am not understanding?
     
  17. Jul 1, 2011 #16
    Re: Help...

    I think I see it now lol

    Its because as the ship gets closer, the distance the light travels is shorter and shorter. Correct?

    Unlike the star whose light always takes 5 years to get here.
     
  18. Jul 1, 2011 #17

    ghwellsjr

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    That's a good way of putting it. I'm glad you figured it out.
     
  19. Jul 1, 2011 #18
    Re: Help...

    BAM! Now I am officially smarter than everyone! LOL! I kid.

    Man, thanks sooooooooooooooooooooooooooooooo frickin much for helping me understand this. I am indebted to you!
     
  20. Jul 2, 2011 #19

    ghwellsjr

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    Well, you are smarter than you use to be, that's for sure. Maybe some day you can help someone else who is trying to understand the same problem. That's what these forums are all about.
     
  21. Jul 3, 2011 #20

    ghwellsjr

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    Re: Help...

    Have you had a chance to discuss your new understanding with your father?
     
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