On the idea of time in physics-relativity

Neddy Bate said:
That is a good way of looking at it when you are first trying to understand the basic concepts. But it is an over-generalization of what relativity really does. For example, without time dilation or length contraction, the "average" you refer to above would not come out to be the same in both frames. Imagine that the train car has a standard length, but it is moving at almost the speed of light. In that case, it is going to take a very long period of time for a photon to travel from the center of the train car to the front wall. Then after the light reflects off the front wall, it travels very quickly back to the observer at the midpoint. This makes for a rather long round-trip time for that photon, as measured by the train frame. The platform frame would not measure that same long period of time for an identical train car parked at rest on the platform. It would measure a much shorter round trip time.

So, if you want to explain why the speed of light comes out to be the same in both frames, you need more than just the "average" concept. You need the full apparatus of relativity, including length contraction, time dilation, and relativity of simultaneity.
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You might be right but does light see the train totally length contracted and totally time dilated because it is going at the speed of light?

The Muon's half life was extend by the fact they go .98 c (just a guess for the purposes of discussion) and the Earth atmosphere is length contracted for the same reason. So extending that to photons has the train got no length, and hence takes no time?

Now why I am a little rattled is that no one else seems to include length contraction into these Relativity of Simultaneity of problems. I would like to get it right one day.
 
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Robittybob1 said:
You might be right but does light see the train totally length contracted and totally time dilated because it is going at the speed of light?

The Muon's half life was extend by the fact they go .98 c (just a guess for the purposes of discussion) and the Earth atmosphere is length contracted for the same reason. So extending that to photons has the train got no length, and hence takes no time?

Now why I am a little rattled is that no one else seems to include length contraction into these Relativity of Simultaneity of problems. I would like to get it right one day.
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Have a look at the spacetime diagrams, see if you can spot length contraction.

You might also be interested in the barn and pole / ladder and garage / train and tunnel 'paradox': Ladder paradox
 
Robittybob1 said:
Well you tell me how the ladder will fit in the length contracted garage? Whereabouts on the ladder is the observer who will see the doors close at the same time?
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The garage doors will open and close non simultaneously.
 
eram said:
The garage doors will open and close non simultaneously.
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My first explanation of thisparadox was provided by WaiteDavid
http://www.youtube.com/watch?v=GFV5vc9MVAE&list=UUdGGqx7vcufpNMDP280vC6Q "Length Contraction Paradox". There was no Minkowski diagram given with it but the explanation seems different to that which I read in the linked Wikipedia article http://en.wikipedia.org/wiki/Ladder_paradox.
I don't know if that is a very satisfactory definition of the word fit. Open and close the doors at different times, would mean you could park an ocean liner in the garage.
 
Neddy Bate said:
That is a good way of looking at it when you are first trying to understand the basic concepts. But it is an over-generalization of what relativity really does. For example, without time dilation or length contraction, the "average" you refer to above would not come out to be the same in both frames. Imagine that the train car has a standard length, but it is moving at almost the speed of light. In that case, it is going to take a very long period of time for a photon to travel from the center of the train car to the front wall. Then after the light reflects off the front wall, it travels very quickly back to the observer at the midpoint. This makes for a rather long round-trip time for that photon, as measured by the train frame. The platform frame would not measure that same long period of time for an identical train car parked at rest on the platform. It would measure a much shorter round trip time.

So, if you want to explain why the speed of light comes out to be the same in both frames, you need more than just the "average" concept. You need the full apparatus of relativity, including length contraction, time dilation, and relativity of simultaneity.
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WaiteDavid also provides a very good answer to the sideways light clock.
"Sideways Light Clock" http://www.youtube.com/watch?v=eqz5hPTefM4&list=UUdGGqx7vcufpNMDP280vC6Q
He gave me the full algebra too if you are like me and not strong on the math.
http://www.physforum.com/index.php?act=ST&t=43398&f=30&view=findpost&p=568422
 
eram said:
just draw a minkowski diagram.
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So have I got this basic concept right?
"NOW" are all those things that are happening "NOW" everywhere in the Universe. So the light from things happening "NOW" may take a long time to reach me, in the future.

So you can't see what is happening "NOW" but you see what has happened in the past. But when you see it, is always your "NOW".
So you can't tell what is happening "Now" till later.
 
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Robittybob1 said:
My first explanation of thisparadox was provided by WaiteDavid
http://www.youtube.com/watch?v=GFV5vc9MVAE&list=UUdGGqx7vcufpNMDP280vC6Q "Length Contraction Paradox". There was no Minkowski diagram given with it but the explanation seems different to that which I read in the linked Wikipedia article http://en.wikipedia.org/wiki/Ladder_paradox.
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Figures 4 and 5 of the Wikipedia article give the same explanation as WaiteDavid. But then, in the next section of the Wikipedia article entitled "Resolution", they start talking about what happens if the ladder is stopped by impacting a closed door of the garage. I don't know why they start talking about that, except to make the article more detailed, and to confuse some people.
 
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Pete said:
Have a look at the spacetime diagrams, see if you can spot length contraction.
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Spotted.

fd9hzlh.png


In the Platform frame, the length of the platform is 20 units, (which is its proper length), and the length of the train is also 20 units, (which is a length-contracted length.) In the Train frame, the length of the train is 25 units, (which is its proper length), and the length of the platform is 16 units, (which is a length-contracted length.)
 
Robittybob1 said:
So have I got this basic concept right?
"NOW" are all those things that are happening "NOW" everywhere in the Universe. So the light from things happening "NOW" may take a long time to reach me, in the future.

So you can't see what is happening "NOW" but you see what has happened in the past. But when you see it, is always your "NOW".
So you can't tell what is happening "Now" till later.
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Yes, that's right.

EDIT - but remember that "NOW" elsewhere in the universe is a relative thing.
 
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Pete said:
Yes, that's right.
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The Holographic Universe (Part One) Why did I pick such a weedy part of the hologram to call home?

Back to the the Minkowski diagrams - Only those events in our past light cone have an effect on our "now". I see that events in our future light cone could have been influenced by events which are outside of our past light cone "now".
In the world around us where is the light cone? Where are and what are the events not in our light cone? It is not the past as such, but events that have happened but further than than what light can bring to us in the time since it happened. So where are they? they are all around us just like our past (my thoughts keep turning them into things but they were events that have happened to those things, some have reached us, some haven't).
Every second we get another second older, but the things that happened more than 1 light-second away can't affect me in the next second. Not withstanding that these same events could affect me in the future seconds.
 
Robittybob1 said:
The Holographic Universe (Part One) Why did I pick such a weedy part of the hologram to call home?

Back to the the Minkowski diagrams - Only those events in our past light cone have an effect on our "now". I see that events in our future light cone could have been influenced by events which are outside of our past light cone "now".
In the world around us where is the light cone? Where are and what are the events not in our light cone? It is not the past as such, but events that have happened but further than than what light can bring to us in the time since it happened. So where are they? they are all around us just like our past (my thoughts keep turning them into things but they were events that have happened to those things, some have reached us, some haven't).
Every second we get another second older, but the things that happened more than 1 light-second away can't affect me in the next second. Not withstanding that these same events could affect me in the future seconds.
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This reminds me of a thought experiment I came up with involving a lottery number. Imagine there is a random number being selected in the caboose of the train. There just happens to be an observer on the platform looking into the window of the caboose, and he sees the random number just as it is selected. He sends a light signal containing the winning number to his friend who happens to be located at the front of the train. Does the information get there in time for his friend to quickly buy a ticket with the winning number on it?

The train is length contracted, (according to the platform), so that works in their favor. Because of relativity of simultaneity, the train clocks located at the front of the train are behind the train clocks at the rear of the train, (according to the platform), so that works in their favor. So it seems like their scheme might work. But if we look at it from the train frame, none of those things are true. The train is not length contracted, and the train clocks are all synchronized. So the scheme cannot possibly work when analyzed from the train frame. A little math would show that it would not work when analyzed from the platform frame either, if anyone cares to work it out for themselves.
 
Neddy Bate said:
This reminds me of a thought experiment I came up with involving a lottery number. Imagine there is a random number being selected in the caboose of the train. There just happens to be an observer on the platform looking into the window of the caboose, and he sees the random number just as it is selected. He sends a light signal containing the winning number to his friend who happens to be located at the front of the train. Does the information get there in time for his friend to quickly buy a ticket with the winning number on it?

The train is length contracted, (according to the platform), so that works in their favor. Because of relativity of simultaneity, the train clocks located at the front of the train are behind the train clocks at the rear of the train, (according to the platform), so that works in their favor. So it seems like their scheme might work. But if we look at it from the train frame, none of those things are true. The train is not length contracted, and the train clocks are all synchronized. So the scheme cannot possibly work when analyzed from the train frame. A little math would show that it would not work when analyzed from the platform frame either, if anyone cares to work it out for themselves.
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The truth of the matter is that the organisers of Lotto in England had done their calculations and to them it was evidently possible to send a light signal around the world 7 times in just one second, so just in case one counter in some part of the country had not adjusted their clocks for daylight saving correctly they decided to close the ticket booths at least one hour before the draw is done.
This way the confusion that they had discovered when they tried to draw the Minkowski Diagram for the Flying Scotsman was masked and their ignorance of the Theory of Special Relativity did not need to be revealed.
 
Neddy Bate said:
This reminds me of a thought experiment I came up with involving a lottery number. Imagine there is a random number being selected in the caboose of the train. There just happens to be an observer on the platform looking into the window of the caboose, and he sees the random number just as it is selected. He sends a light signal containing the winning number to his friend who happens to be located at the front of the train. Does the information get there in time for his friend to quickly buy a ticket with the winning number on it?

The train is length contracted, (according to the platform), so that works in their favor. Because of relativity of simultaneity, the train clocks located at the front of the train are behind the train clocks at the rear of the train, (according to the platform), so that works in their favor. So it seems like their scheme might work. But if we look at it from the train frame, none of those things are true. The train is not length contracted, and the train clocks are all synchronized. So the scheme cannot possibly work when analyzed from the train frame. A little math would show that it would not work when analyzed from the platform frame either, if anyone cares to work it out for themselves.
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It's quite straightforward. On board the train, buying closes as soon as the number is drawn.

Even if the man on the ground was at the caboose when the number was drawn, he would have to send a faster-than-light signal to inform his partner in time.
 
eram said:
It's quite straightforward. On board the train, buying closes as soon as the number is drawn.

Even if the man on the ground was at the caboose when the number was drawn, he would have to send a faster-than-light signal to inform his partner in time.
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In the end you are dealing with humans, and they are known for making mistakes, or even being involved in corruption. Could the ticket seller be bribed to keep the booth open for just few seconds so the scam will work? It would be so silly to have the draw being so close to the end of ticket selling. There are so many people looking for a loophole that it would soon be found.
They did that with horse racing in NZ (50 years ago?). The race was run and the results were sent to another town by phone where bets were still being taken on the race. They placed bets with bookkeepers on the winners after the race had been run, for the bookkeepers were relying on the radio broadcasts (delayed) for the results.
 
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