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Chasney913
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Joined: 28 Aug 2007
Posts: 117
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 Posted: 13 Jul 2008 07:01:42 pm Post subject: |
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One of my friends asked me to explain to him about the speed of light, and so I told him that time slows down for someone as they approach the speed of light. (Of course they don't perceive the dilation, which I think I mentioned).
However, he decided to try to find ways to prove me wrong, through scenarios where he says that the time thing won't work. I'd like some help explaining how it does work in his scenarios, which I'll paraphrase here.
Scenario 1:
"According to [basic] physics, velocity = distance / time. An outside observer will time an object going close to the speed of light, measure the distance and determine that, yes, it is going close to the speed of light. An observer inside the object will measure the time as being less [editor's note: I don't think so.] due to time dilation, and get a larger value for velocity."
Scenario 2:
[Note that he's a mechanical sort of person, so this next one makes little sense to me, but that's okay.]
"A car going from A to B requires that the engine turns at a specific speed. However, if time does slow down as you approach the speed of light, the engine will slow down, forcing the car to slow down. [editor's note: Again, I'm pretty sure that time doesn't actually appear to slow down for the observer in the car, but I believe he's saying the car's engine will appear to move slower to an outside observer] This means the faster you go, the more the engine slows down, slowing down the car, which would mean you couldn't ever reach point B."
Anyone have any ideas on how to explain these to him? He doesn't like me saying "Well, formulas say that this should work." and doesn't really accept evidence from the cesium clocks (one on ground level, one on a mountain, one moves faster, and so on) and other such experiments as proof. Stubborn, I know. Also, I'm paraphrasing, so I might have a couple things wrong.
Help would be appreciated. |
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Taricorp
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Joined: 09 Mar 2006
Posts: 188
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| Posted: 13 Jul 2008 08:23:59 pm Post subject: |
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Chasney913 wrote: Scenario 1:
"According to [basic] physics, velocity = distance / time. An outside observer will time an object going close to the speed of light, measure the distance and determine that, yes, it is going close to the speed of light. An observer inside the object will measure the time as being less [editor's note: I don't think so.] due to time dilation, and get a larger value for velocity."
The moving observer will not notice any time dilation, except that everything which is in an inertial frame of reference (that is to say, not moving) seems to be moving faster. Light will still move at c to them, and the inertial observer will also measure the light to be moving at c. The crazy part is, they're both right.
Quote: Scenario 2:
[Note that he's a mechanical sort of person, so this next one makes little sense to me, but that's okay.]
"A car going from A to B requires that the engine turns at a specific speed. However, if time does slow down as you approach the speed of light, the engine will slow down, forcing the car to slow down. [editor's note: Again, I'm pretty sure that time doesn't actually appear to slow down for the observer in the car, but I believe he's saying the car's engine will appear to move slower to an outside observer] This means the faster you go, the more the engine slows down, slowing down the car, which would mean you couldn't ever reach point B."
Again, the moving observer does not notice any time dilation. Moot point.
The standard example:
You leave Earth at 99.999% the speed of light, heading towards Proxima Centauri (4.22 light-years away). When you get there, you turn around and go back to Earth. When you get back, it has been 8.44 years since you left (4.22 years each way) according to the clocks on Earth, but your clocks say you have only been gone for a couple weeks (at most- I don't want to bother actually calculating how long). Both your clocks and the ones on Earth are accurate. |
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FocusedWolf
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Joined: 31 Aug 2008
Posts: 22
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| Posted: 31 Aug 2008 08:43:50 pm Post subject: |
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I remember some news where they said astronauts on some space station (i think mir) spent so much time in space traveling at high speeds that they travelled a few seconds into the future..
Also theirs a book called "The forevor war" that deals with this time dilation topic...
Basically these soldiers fight some weird aliens far far away and use some type of star phenomon "collapse-sar" (black hole?) to travel that close to the speed of light...
so they spend months in space, but show up, get their asses kicked and go home... on earth 20 years has gone by... some crazy ration war where all money is called Calories... and the world is full of crime and gay people.
Well i'm not through with the book but according to book summaries they leave earth cause it's to weird for them and go fight more for a few months... next time they come back from the war everyone's gay and the ration war is over because theirs no babies but plenty of food to go around... so they say the hell with it and go back to war for a few months and when they come back the world is populated with clones only... and the clones are able to talk with the aliens and the war is over...
so the few living humans from this decide to get a planet of their own (i think they named the planet "Middle Finger" or something), and they live like the good old days before the war... but then they get tired of that and use the time dilation to fast foward into the future... where they find everything is gone and theirs god (some reptile creature that created everything for no reason)... so they ask him to bring everyone back or something. |
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sgm
Calc Guru
Joined: 04 Sep 2003
Posts: 1265
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| Posted: 31 Aug 2008 10:19:35 pm Post subject: |
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FocusedWolf wrote: Also theirs a book called "The forevor war" that deals with this time dilation topic...
No, what it deals with is science fiction and flights of fancy. Not the realities of time dilation. |
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simplethinker
snjwffl
Active Member
Joined: 25 Jul 2006
Posts: 700
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| Posted: 02 Sep 2008 07:09:17 pm Post subject: |
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Chasney913 wrote: Scenario 1:
"According to [basic] physics, velocity = distance / time. An outside observer will time an object going close to the speed of light, measure the distance and determine that, yes, it is going close to the speed of light. An observer inside the object will measure the time as being less [editor's note: I don't think so.] due to time dilation, and get a larger value for velocity."
Scenario 2:
[Note that he's a mechanical sort of person, so this next one makes little sense to me, but that's okay.]
"A car going from A to B requires that the engine turns at a specific speed. However, if time does slow down as you approach the speed of light, the engine will slow down, forcing the car to slow down. [editor's note: Again, I'm pretty sure that time doesn't actually appear to slow down for the observer in the car, but I believe he's saying the car's engine will appear to move slower to an outside observer] This means the faster you go, the more the engine slows down, slowing down the car, which would mean you couldn't ever reach point B."
[post="125215"]<{POST_SNAPBACK}>[/post]
Scenario 1:
Velocities will be measured differently. When it comes to time dilation and the Lorenz transformations (or is it Lorentz, I always mix those two up) velocity is not an invariant, which means that different reference frames can measure two different velocities for the exact same object and timespan. That's why in relativistic mechanics 'v' is usually replaced by 'v/ sqrt(1 - v^2 / c^2)' (I think this is just as a first approximation, not an exact conversion).
Scenario 2:
First, time itself does not slow down, which is a common misconception. It's that an object's/observer's perception of time is different than another object/observer.
I had some trouble trying to put this into words, so I'm sorry if this isn't too clear:
The engine turns as a result of the pressure/heat (I don't know much about cars) resulting from the combustion of fuel (or as a result of an electric motor, etc...). Its performance has nothing to do with the velocity it's traveling. The velocity on the speedometer in the car is based on the distance traveled (determined by the number of tire rotations) and the perceived time in the car, so the speed of the engine is independent of the velocity it's traveling at.
Although, I suppose that the speed of the engine will appear to be slowing down as the velocity of the car approaches the speed of light, but the dilation would be the same as that for time within the whole car, so relative to the rest of the car the engine's performance wouldn't change.
One more thing, as can be inferred from the last sentence of my answer to scenario 1, the velocity itself can't approach the speed of light since the velocity used to calculate invariant quantities would diverge to infinity (this argument isn't exactly a proof, but the conclusion is valid). This means that I/you/your friend doesn't have to give ourselves headaches about what would happen to the engine if the car is traveling at the speed of light. |
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Chasney913
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Joined: 28 Aug 2007
Posts: 117
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| Posted: 02 Sep 2008 09:52:15 pm Post subject: |
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Alright, makes sense to me. However, I'm going to post exactly what he sent me, just in case I made an error while condensing it. [Beware; bad grammar and spelling].
Quote: Alright the theory about how time slows for an object the faster that object goes. now say that the object is traveling at the speed of light to an outside observer it would be traveling at the speed of light. but if a person in the object was to take a reading it would indacate that they are infact going faster than the speed of light. Dispite the theory that it is impossible for faster than light travel.
Why the different readings? think back to basic phyisics V= d/t Now imagine the object traveling a fixed distance if an outside observer measures the velocity of the object to be the speed of light. they time the object going the fixed distance so that they get time (T) for the certain distance (d). the observer on the object also measures the time it takes for the object to travel the same distance.
so if V=d/t then ("d" traveled)/ (time the outside observer measures "T") = ("d" traveled)/ (the time that the object expearces) this doesn't make sence, a object therefore has two velocities. If the outside observer measures more time than the obersver in the object, the observer has less time to cover the same distance therefore if the outside observer determines that the object is traveling at the speed of light then the observer in the object will calculate that they are infact traveling faster than the speed of light thus the speed of light is not a limit of velocity that cannot be passed.
Alright now that I am talking about time slowing down as an affect of speed I will once again tell you why that is not possilble. every time I start to explain why, people stop listening and saying that i am worng. so please here me out
to make things simple lets just say that a "car" is going to be traveling at some speed. the speed doesn't matter because if there is an affect on time do to speed then the number will show it with high or low velocities.
Say that the car is going to travel from point "a" to point "b" (100 km) and say that it takes 60min then its velocity is 100 km/h right?
now you know that the engine has to turn inorder for the wheels to turn to make things easy lets say that one full revolution of the engine is one full revolution of the wheels, and say that one revolution of the wheels equal 1m. So taking the 100km and convertert it into meters makes (100x1000= 100000m) 100000m for the wheels to travel. which means that there are 100000 revolutions in 100km so take 100000rev and divide by time makes it 100000rev/hour right? so the engine of the car is turning 100000rev/h to get to point "b", but if time is affected buy speed and slows down for the object means that the car engine has less time. If distance stays the same then there would still be 100000rev but less time. so if the engine was going 100000rev/h the car would not make it to point "b" 100km away it the car was set to go 100km/h. or think of it this way the faster the car goes the slower the engine will run. but if the engine runs slower the car will run slower, meaning that once an object moves it has to move slower than it was moving no matter what. (what ever makes the object move is also affected by the time effect thus making it not move the object as fast. In otherwords moving an object will only result in moving the object slower. so the faster the object the slower it goes. for a car to go by you at 100km/h the engine must be going 100000rev/h to in relation to you but if time slows down do to speed that would make the engine run slower than the 100000rev/h that it must have to get to point "b".
I'm not even going to start to correct that. If you have any idea how to respond, I'd love to know. I haven't sent him the other things that have been said, because he just moved into university, and has to get his email and stuff set up. But I'd still love an answer. |
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simplethinker
snjwffl
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Joined: 25 Jul 2006
Posts: 700
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| Posted: 04 Sep 2008 01:07:23 pm Post subject: |
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The fact that velocity is an invariant and so there can me different measured velocities depending on the reference frame answers most of that post.
Another thing, and this is a fine distinction, an object with mass can't travel at the speed of light, instead of past the speed of light.
For the car engine thing, you have to remember that the engine and the car are in the same reference frame, and that the passage of time experienced by the engine is not affected by it or the car's velocities. And again, I stress the point that time dilation only affects the perceived or experienced time of an observer, not the passage of time itself.
I hope this helps  |
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