light speed

[mickbedford]

40yrs ago my physics teacher was insistant that you couldn't travel faster than light and drew a car on the blackboard doing a right turn, a right, a right turn then crashing into itself saying you cant be in two places at the same time and whilst i agree with that and i know others wouldn't the fact remains you wouldnt be crashing into yourself you'd be crashing into the light from yourself two totally seperate things, was that a poor analogy or have i got it wrong?

also is it right to say that if i syncronised 3 clocks seperated 2 by a light year and kept the third with me that no matter where i went all 3 would tell exactly the same time? obviously if i stood by one and looked at the other it would APPEAR to be 1 yr late but the actual clocks at that moment in time would be the same.

apparantly traveling at speed alters time, persoally i'm not buying it regardless of the maths and prefer to believe its simplier that that but i'm a 55yr old plaster so simple fits. is there a specific text or part of general relativity that would help me understand that nonsese cheers

 

[swansont]

23 minutes ago, mickbedford said:

40yrs ago my physics teacher was insistant that you couldn't travel faster than light and drew a car on the blackboard doing a right turn, a right, a right turn then crashing into itself saying you cant be in two places at the same time and whilst i agree with that and i know others wouldn't the fact remains you wouldnt be crashing into yourself you'd be crashing into the light from yourself two totally seperate things, was that a poor analogy or have i got it wrong?

Once you violate the laws of physics, you can't really reach a valid conclusion. There's probably a way of formulating the argument where your teacher was right, since you would have contradictions popping up.

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also is it right to say that if i syncronised 3 clocks seperated 2 by a light year and kept the third with me that no matter where i went all 3 would tell exactly the same time? obviously if i stood by one and looked at the other it would APPEAR to be 1 yr late but the actual clocks at that moment in time would be the same.

The synchronization of clocks takes into account the time it takes for the light to travel between them.

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apparantly traveling at speed alters time, persoally i'm not buying it regardless of the maths and prefer to believe its simplier that that but i'm a 55yr old plaster so simple fits. is there a specific text or part of general relativity that would help me understand that nonsese cheers

That's, in essence, rejecting science. GPS — which has relativistic corrections — works, despite your reservations.

[Bufofrog]

15 minutes ago, mickbedford said:

also is it right to say that if i syncronised 3 clocks seperated 2 by a light year and kept the third with me that no matter where i went all 3 would tell exactly the same time?

Mostly correct.  The implied assumption in your statement is that all 3 clocks are in the same inertial frame.  All that means is all 3 clocks are moving at the same velocity.

When you said, "no matter where I went they would all read the same time" is not accurate, I believe.  The problem is that you are now implying that you can change your velocity relative to the other clocks and the other clocks will still be synchronized with you, which is not true.  Once you change your inertial frame the clocks will not longer be synchronized.

[Strange]

39 minutes ago, mickbedford said:

40yrs ago my physics teacher was insistant that you couldn't travel faster than light and drew a car on the blackboard doing a right turn, a right, a right turn then crashing into itself saying you cant be in two places at the same time and whilst i agree with that and i know others wouldn't the fact remains you wouldnt be crashing into yourself you'd be crashing into the light from yourself two totally seperate things, was that a poor analogy or have i got it wrong?

I think it is a terrible analogy!

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also is it right to say that if i syncronised 3 clocks seperated 2 by a light year and kept the third with me that no matter where i went all 3 would tell exactly the same time? obviously if i stood by one and looked at the other it would APPEAR to be 1 yr late but the actual clocks at that moment in time would be the same.

They would probably not keep the same time. They would be in different gravity and, possibly, moving relative to one another.

39 minutes ago, mickbedford said:

apparantly traveling at speed alters time, persoally i'm not buying it regardless of the maths

It's not so much about the math but the evidence. We can measure this effect in many ways. We even use it in various technologies. 

 

[Phi for All]

1 hour ago, mickbedford said:

persoally i'm not buying it

Just make sure to satisfy your healthy skepticism. I don't know how long you've been questioning the science, but you should dig into it deep enough to either trust it or reject it completely. Sitting on the fence as a skeptic for too long isn't good, especially since you have no reasonable objections other than your own incredulity, which is based on an education from 40 years ago.

[Intrigued]

2 hours ago, mickbedford said:

apparantly traveling at speed alters time, persoally i'm not buying it regardless of the maths and prefer to believe its simplier that that but i'm a 55yr old plaster so simple fits. is there a specific text or part of general relativity that would help me understand that nonsese cheers

 What would your reaction be if I found you at work one day and told you that you were doing it wrong. Your plaster was too wet, your tools were inappropriate and your application technique was wrong? On enquiry you would learn that I had never plastered anything in my life. The closest I had come to it was nailing some plasterboard to a wooden framework. You would, I suspect, shake you head and dismiss my thinking as silly.

In reality I would not question your plastering tools or technique, since they would be based upon the evolved practice of thousands of plasterers, coupled with your own practical experience. I'm not sure why you would think it makes sense to doubt a theory that has been validated by experiments, observations and practical processes, carried out by thousands of experts. I guess you have the right to be wrong, but it won't change the facts.

[Janus]

2 hours ago, mickbedford said:

also is it right to say that if i syncronised 3 clocks seperated 2 by a light year and kept the third with me that no matter where i went all 3 would tell exactly the same time? obviously if i stood by one and looked at the other it would APPEAR to be 1 yr late but the actual clocks at that moment in time would be the same.

  As already pointed out, we take the light travel time into account when we determine simultaneity.  So judging the synchronization of another clock would involve not just what you see, but how far away the clock is.   If we assume that the clock never moved between the light leaving it and the light arriving, it is one light year away, and you read a time 1 year behind your clock, you will say that they are synchronized.  (This would not be true for someone moving with respect to the two clocks, but I'll get to that later.)

 

2 hours ago, mickbedford said:

apparantly traveling at speed alters time, persoally i'm not buying it regardless of the maths and prefer to believe its simplier that that but i'm a 55yr old plaster so simple fits.

There is a quote by Robert Heinlein that may be applicable here:

"You can go wrong by being too skeptical as readily as by being too trusting."

2 hours ago, mickbedford said:

is there a specific text or part of general relativity that would help me understand that nonsese  cheers 

There are a lot of resources you can try, but no matter which one you choose, you are going to have to approach it with an open mind, and be aware that you will likely have to discard some notions you already have about how things work. 

I'll try to get you started.

First off, we need to start with Special Relativity.   This is basically General Relativity when you assume no gravitational fields.

SR is based on two postulates:

1. The laws of Physics are the same in all inertial reference frames.

2. The speed of light in a vacuum is a constant in all inertial reference frames and is independent of the relative velocity of the source.

An example of an inertial reference frame would be a spaceship coasting in space.   If it fires its engines to change speed, while doing so it would be considered a non-inertial reference frame.  Once it starts coasting again it will once more be a inertial frame, just a different one than it was earlier. 

What the first postulate mean is if you had a lab in your spaceship and used it to do any conceivable experiment,  you would not get any different results between before you accelerated the ship and after.  If you hadn't been aware of the acceleration, there would not be any way for you to tell that there had been a change in velocity in the lab between the two sets of experiments. 

The second postulate basically means that this also applies to any measurement of the speed of light.  Measuring the speed of light always gives the same answer: c ( 299,792,458 m/s), and it doesn't matter if the source is moving relative to the lab or not.  In other words, we would measure light as moving at 299,792,458 m/s relative to the lab, both before and after the acceleration and it would not matter if the source of that light was moving or at rest with respect to the lab. 

This also means that two different labs moving relative to each other would each measure the same light as moving at c relative to themselves.

So for example, if you have two labs, A&B, in relative motion with respect to each other, and as they pass each other a flash of light is emitted from where they meet, Lab A, will measure events like this:

With itself in the center of an expanding sphere of light as B chases after one edge of that expanding sphere.

However, B would measure this as happening:

B would remain at the center of the expanding sphere and A would be chasing after one edge.

And as counter intuitive this may seem, countless experiments and measurement have confirmed this. 

So how does this effect how we measure simultaneity?

Imagine you have a set of train tracks with an observer along side it, there is also a railway car on the tracks, moving relative to the tracks.  the track observer is halfway between two light sources that emit flashes of light that meet at the track observer at the same moment the railway car passes him.  For the track observer, events unfold like this:

with the expanding circles representing the light flashes.  Both flashes are emitted at the same time, and reach both observers at the same moment.

For the railway car observer however, things occur differently.  He agrees with the track observer that the flashes arrive at the same time as they pass each other, but not that they were emitted at the same time.

Here are events as they unfold for him:

Unlike the track observer, who remains a fixed distance between the two sources, the railway car is only halfway between them when he sees the flashes. When either of the two flashes is emitted, he was closer to the left source than the right source.    Since each flash must travel at c relative to this observer in his frame, the only way for the two flashes to meet when he is half way between the sources is for the right flash to be emitted before the left flash.   Thus according to the track observer, the emission of the two flashes are simultaneous events, but according to the railway car observer, they are not.

If we were to put clocks at the sources and each flash carried the information of the time stamp for the clock reading when the light left, then if the track observer reads identical time stamps when the flashes arrive, he can conclude that the clocks at the sources are synchronized and read the same at all times. 

The railway car observer will see the same two identical time stamps, But for him, the light from one flash left earlier and took longer to reach him then the other, So the two clocks can not be synchronized with each other and don't read the same at the same time, and that one clock always reads ahead of the other.

This is known as the "relativity of simultaneity".  

This is a fundamental concept in Relativity, and one you really need to come to grips with if you wish to understand it (many of the "contradictions" people think they have uncovered in Relativity stem from not grasping this concept.)

Other concepts such as Time dilation and length contraction build from here.

I'll leave it here for now, but first I want to make one thing clear.  In these examples we use light.  But Relativity isn't really about light itself.  It's really is about the nature of time and space.  Light behaves the way it does because of this nature.  Light doesn't dictate the rules of Relativity, it adheres to them.  Because of the nature of time and space, the speed c is special.  Light travels at c in a vacuum, and since it is something we can detect and measure, it is a good tool for examining those rules that govern time and space.  Light is convenient for use in these examples, but is not required for Relativity to hold true.