B Questions about the Speed of Light, Time, etc.

[phinds]

@John Mcrain, you are trying to learn Special Relativity by asking semi-random questions on an Internet forum. That is a TERRIBLE way to do it. As I told you in post #7, get a good book on Special Relativity (and quit wasting your time with these repetitive questions).

 

[Dale]

What is time for Einstein?

There are two meanings of time. One is proper time which is the time shown on a clock. The other is coordinate time which is the timelike coordinate in a reference frame.

Regarding Einstein, the terminology was not fixed in his day so he was not always clear. But usually he was talking about proper time.

Time tick slower in moving train only when we compare from clock that is at rest, but in train clock tick at regular rate.

Yes. The proper time on a clock in a train is slow compared to coordinate time in the ground frame and is regular compared to coordinate time in the train frame.

1.So when train stops and when we then compare these two clocks, they will again show same time?
If not, why not if indeed clock tick at regular rate in train?

2.if speed is relative, both clock can say I am at rest and you are moving and vice versa, so which one will show less time,once they meet again in rest frame

This is the standard twin paradox. You should read any of the hundreds of threads already on this topic. It isn’t really relevant to this thread. You shouldn’t hijack a thread even when you are the OP

 

[John Mcrain]

AGAIN, time for Einstein is what clocks measure. How many times are you going to ask that question?

This definiton is joke/useless, like you say speed is what speedometer meassure!

I ask because of this:
Albert Einstein once wrote: People like us who believe in physics know that the distinction between past, present and future is only a stubbornly persistent illusion. Time, in other words, he said, is an illusion.

 

[John Mcrain]

There are two meanings of time. One is proper time which is the time shown on a clock. The other is coordinate time which is the timelike coordinate in a reference frame.

If theory show that time is not absolute, can we state that time dont exist in some way?
What is official definiton of time in physics?

 

[Dale]

This definiton is joke/useless

It really isn’t. It is a very useful definition, called an operational definition. Operational definitions are actually the most important definitions in physics because operational definitions are the ones that directly connect our theoretical concepts to our experimental measurements.

Albert Einstein once wrote: People like us who believe in physics know that the distinction between past, present and future is only a stubbornly persistent illusion.

That was a personal letter to the grieving widow of a personal friend. In his actual scientific publications he said

“It might appear possible to overcome all the difficulties attending the definition of “time” by substituting “the position of the small hand of my watch” for “time.” And in fact such a definition is satisfactory when we are concerned with defining a time exclusively for the place where the watch is located; but it is no longer satisfactory when we have to connect in time series of events occurring at different places, or—what comes to the same thing—to evaluate the times of events occurring at places remote from the watch.”

Where he describes both proper time and coordinate time.

What is official definiton of time in physics?

I already told you the definition of both proper time and coordinate time. The unqualified word “time” could mean either in context, but like @phinds I would usually use it to indicate proper time.

 

[jbriggs444]

This definiton is joke/useless, like you say speed is what speedometer meassure!

If you get a bunch of devices together (water clocks, hour glasses, sun dials, wrist watches with hair springs, wheels and escapements, grandfather clocks with pendulums and escapements, quartz oscillators, atomically stablized quartz oscillators, astronomical observations, radioactive decay, gray hairs, etc and discover that they can all be made to tick at the same rate then one is inclined to attach a name to the shared quantity that they appear to be measuring.

That quantity is called "time".

For a single clock, what is measured is "proper time". This means the clock's own time. Proper time is somewhat limited. It is only valid at the clock. It is approximately valid in the neighborhood of the clock.

If one has an array of clocks spread out over space and a synchronization mechanism (or even a single clock and a method for remotely reading it), one can establish a time standard that applies throughout an extended region. This gives you a basis on which to construct a coordinate system. So this sort of time is "coordinate time".

What Einstein pointed out is that there is no one true and correct synchronization convention. Ordinary language assumes that there is such a convention. Naive reasoning using ordinary English constantly trips over itself due to this pitfall.

[Proper] time is indeed what a clock measures. By definition.

 

[Vanadium 50]

How many times are you going to ask that question?

I remember when I was right, my boss was wrong and it blew up in his face. He claimed it was my fault for being "insufficiently convincing".

Equally obviously, PF is being insufficiently convincing here.

 

[berkeman]

Equally obviously, PF is being insufficiently convincing here.

I have my "Convincing Stick" sitting here next to my desk in the PF Mentor Offices...

 

[Vanadium 50]

Is that the same as the handle of a BanHammer?

 

[berkeman]

Is that the same as the handle of a BanHammer?

No, the BH is in the middle of all of the Mentor desks. We have our desks arranged in a circle, and there is a special ceremony involving fire for any BH applications. Greg hates the fire part (increases his insurance premiums on the Mentor offices)...

 

[phinds]

I remember when I was right, my boss was wrong and it blew up in his face. He claimed it was my fault for being "insufficiently convincing".

Oddly, I had exactly the same thing happen many years ago. You and I must sound basically untrustworthy.

 

[John Mcrain]

Does direction of train matters?
I am at rest at railroad and train moves directly at me, then there is no time dilatation, because I will see train light beam inside light clock goes verticaly up and down,so tick at same rate as my clock.
Is this correct?

 

[Ibix]

Does direction of train matters?

No. The front clock of the train will always be behind the rear clock assuming that they are synchronised in the train rest frame. The tick rate depends only on speed.

I am at rest ar railroad and train moves directly at me, then there is no time dilatation, because I will see train light clock tick at same rate as my.
Is this correct?

Where are you getting this from?

 

[John Mcrain]

Where are you getting this from?

If some one hold light clock and walk toward me,I see light beam inside clock moves verticaly up and down just as when clock is at rest.

I am compare my clock and train clock, not train rear and back clock..

 

[Ibix]

If some one hold light clock and walk toward me,I see light beam inside clock moves verticaly up and down just as when clock is at rest.

So you have no concept of depth? How do you not walk into walls in your every day life?

 

[John Mcrain]

So you have no concept of depth? How do you not walk into walls in your every day life?

I will see light clock bigger and bigger as train approching me..

 

[Ibix]

I will see light clock bigger and bigger as train approching me..

So you have the light following a diagonal trajectory, with components both vertically and coming towards you. Just as the usual depiction has a diagonal trajectory, with components both vertically and left-to-right. The direction doesn't matter.

 

[John Mcrain]

So you have the light following a diagonal trajectory, with components both vertically and coming towards you. Just as the usual depiction has a diagonal trajectory, with components both vertically and left-to-right. The direction doesn't matter.

I know that light beam has diagonal trajectory, but I cant see it from this position/case..

 

[Ibix]

I know that light beam has diagonal trajectory, but I cant see it from this position/case..

I'm staggered that you are struggling with this notion. If an open-topped car is moving laterally with respect to you its occupants feel wind on their faces. Do you think they would not feel wind on their faces if they were coming directly towards you, just because you find it harder to judge their motion in that direction? If yes, why would physics care where you personally are standing? If no, why would you expect a clock to care about your location when you understand that other physics doesn't care?

 

[Dale]

Does direction of train matters?

The best way to answer this question is to look directly at the math of the Lorentz transform. See this section of the Wikipedia entry on Lorentz Transformations.

For time the transformation is $$t'=\gamma\left( t- \frac{\vec r_{\parallel}\cdot \vec v}{c^2} \right)$$ so there are two terms in the transformation. The term $\gamma t$ does not depend on the direction. This term is what we call time dilation so time dilation does not depend on the direction. The term $\gamma \vec r_{\parallel} \cdot \vec v / c^2$ does depend on the direction. This term is what we call the relativity of simultaneity so the relativity of simultaneity does depend on the direction. The relativity of simultaneity occurs in the direction parallel to the direction of motion.

 

[jbriggs444]

I know that light beam has diagonal trajectory, but I cant see it from this position/case..

This is a bit like a batter saying that he cannot measure distance when watching an approaching baseball with one eye closed. Then complaining when the ball hits him in the face. "Hey Ump, that ball could not have been moving because I could not see it moving!"

Say that the train is moving at 0.6 c and the light clock is 4 feet in height. If the light pulse has a 5 foot diagonal trajectory, it will take 5 nanoseconds to follow that trajectory, not 4. Even if we look from in front rather than from the side. The train and light pulse will have moved 3 feet forward in this time.

 

[vanhees71]

So you have no concept of depth? How do you not walk into walls in your every day life?

No problem, in everyday-life only Newtonian spacetime is correct, and there are no such troubles as time dilation, length contraction, and all this...

 

[Ibix]

No problem, in everyday-life only Newtonian spacetime is correct, and there are no such troubles as time dilation, length contraction, and all this...

Yeah, but either @John Mcrain's visual function works different from every other person's on the planet and he's unable to conceive of perspective, or he's not thinking about what he's saying.

 

[phinds]

... he's not thinking about what he's saying.

Forum decorum prohits a response

 

[gmax137]

Forum decorum

I like that. Poetic, even.