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Question about time.

  1. Jul 22, 2008 #1
    ok so I was thinking about it and was wundering if anyone had an answer for me. the question is that: is something you shine a light at younger than something that shines a light at you? for example... I point a flashlight at a car that is 300meters away in order to see it. for the purpose of this example i will use simple numbers that arent necesairly true but serve the example btw... so lets say it takes 1 second for a particle of light to travel from my flashlight to the car that is 300m away (i know it would be quite shorter than that but you get the point). It then takes another 1s for the light to travel back to my eye so i can see it therefore I see the car the way it was two seconds ago. If i replace the car with a lamp that is also 300m away and turn on the lamp via remote then it will only take one second for the light from the lamp to contact my eye. does this mean that the car will be slightly younger than the lamp if they sit side by side (but there is a wall preventing light from the car and lamp from interfeering with their respective light)? if this makes sence than would that mean that a source of light is always older than something that neads to have light directed towards it in order to observe it?

    to explain what i mean by younger and older: you observe something that takes one second to reach by light. the light then travels back to you. this would cause the image to appear to be one second before some other object that is of equal distance yet the initial time taken to reach it of one second would be cut out becuase it would emit the light in this second case. so the image of the second one would be one second old and the image of the origional object would be two seconds old. the one:two ratio would not stay constant i.e. the origional case after 10 seconds of observation would still be only one second younger than the second case.
  2. jcsd
  3. Jul 22, 2008 #2
    No. You see the car the way it was 1s ago - when the light reflected off of it.

    Yes. You would see the lamp as it was 1s ago - turning on. Also, don't forget the 1s for the remote to activate the light.


  4. Jul 22, 2008 #3
    the image of the car would be one second old but the time it took for the light to get their should be taken into account. i.e. it takes one second for the light to get to the car, the photo is picked up and reflected so to speek which takes one second to get back to you. if the light was being emmited from the car than the photo of the car would be at your eye by one second but in comparison to the flashlight, the light would have just hit the car after that one second. so after one second in the light source being the car case, the light hits your eye. you dont see the light for two seconds if you shine a light at the car so that would mean that by the time you see the light bounce back once from the flashlight, the light could have bounced back twice from the car if it was the source of light. there would therefore always be a one second gap between the two situations wouldnt there?
    Last edited by a moderator: Jul 23, 2008
  5. Jul 22, 2008 #4
    oh and the time it took for the remote is irelivant. it was only there to serve the purpose of letting you know that both light sources wer activated at the same time... lets say the remote was hooked up to the lamop and flashlight via a split cable that was of equal length after the split.
  6. Jul 23, 2008 #5


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    You don't see the reflected light for two seconds after you turn on the lamp but you see the car as it was when that light hit it- one second before you are seeing it. And, by the way, this result depends only on the fact that the speed of light is finite. It does not depend on relativity in any way.
  7. Jul 23, 2008 #6
    The part you're missing is simultaneity. If you're standing next to the flashlight, you won't see both lights turn on at the same time (whereas an observer located between the two could). To illustrate, let's switch the lights back off after they have been "on" for one half second (you wouldn't have seen either turn on yet due to the matched delay of one second that you've added). One and a half seconds after "turning off the lights", you see a brief image of the car as the lamp light hit it. After a half second pause, you would see the car again as the light from the flashlight struck it.

    The hypothetical "observer in the middle" could (if he/she were stationary relative to the two lights) see the same half second pause between images (albeit a half second before you would).


  8. Jul 23, 2008 #7


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    Staff: Mentor

    You still have a math error there. If the signal travels through the cable in 1s (say it's a fiber optic cable) and the propagation of the light signals is 1 sec each way, from the instant you flip the switch, you'll see the emitted light in two seconds and the reflected light in three.
  9. Jul 23, 2008 #8
    i know... well obviously but i didnt want to re edit the origional post so just dont take that second it takes the electricity to travel into account i guess. I just wanted to make it clear that they were both turned on exactly at the same "time?" well not to the observor but i mean.. if light traveled instantly then they would be turned on at the same time. the split cable was just a thought expariment i used inorder to get around the whole uncertanty of time business.
  10. Jul 23, 2008 #9
    the interesting thing about this that none of you seemed to pick up was the question of what would happen if you shined a light at a lamp that was turned on? would you see the lamp in two states at once? if this is true then what would happen if the lamp was moved extreamly quickly to the left by one foot? then because light moves slowly in this thought expariment, would you not see two lamps for one second? you would observe both lamps at the same time.. which one would actualy "exist"? the concervation of mass principle states that both cant because this would be creating mass out of nothing. one of the lamps would be seen by the light you shine at it and the other one a foot to the left would be seen by the light it shines at you! the real question is... which one exists?
  11. Jul 23, 2008 #10
    oh.. if the lamp was moved to the left by one foot at the speed of light (or just under it) because i jsut reolized that the object wouldnt be able to break any rules of physics because i dumbed down the speed of light etc... the thought expariment would still work.
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