Questions about the Speed of Light, Time, etc.

[Dale]

Where I am wrong?

Right here:

time is absolute

The derivation clearly shows that time is not absolute.

distance is changed in frame at rest.

Clearly. And that fact is why the time is not absolute.

In the end, the point remains: two events that are simultaneous in one frame are not simultaneous in another frame. This is the relativity of simultaneity. The fact that the distance changes in one frame and doesn't change in the other frame in no way implies that the conclusion doesn't hold. Quite the opposite, it is the reason that the conclusion does hold.

 

[John Mcrain]

Clearly. And that fact is why the time is not absolute.

Dont understand.

 

[Dale]

Dont understand.

Please work through the math in post 15. It is laid out there. If you don't understand a specific step, please let me know and I can expand on that step.

 

[John Mcrain]

Please work through the math in post 15. It is laid out there. If you don't understand a specific step, please let me know and I can expand on that step.

But this thing dont need math is it pure logic(obviusly my is wrong, but cant find why)

If I can explain why light need more time to hit right detector,only with "change in length", why do I need talk about time?

 

[malawi_glenn]

But this thing dont need math is it pure logic(

Math is logic. Nothing can be more logical than math.

 

[John Mcrain]

Math is logic. Nothing can be more logical than math.

I also notice that distance is changed looking from rest frame because the fact light has constant speed.
does that tell me something about time?

hm maybe like this:

c=d/t,
I first conclude d increase, so to keep c=const, I must increase t as well, that is proof that time also change.
Is this OK?

 

[John Mcrain]

Your understanding is historically inaccurate.
Einstein proposed special relativity in 1905 as the solution to a problem that had tormented physicists for almost a half-century: the laws of E&M, discovered in the early 1860s, suggested that the speed of light ought to be constant but no one could figure out how that could be.

Einstein is first one who tell time is not absolute?
He get it from fact that light has constant speed?

 

[Nugatory]

But this thing dont need math is it pure logic(obviusly my is wrong, but cant find why)

That is pretty much why I made that comment in my previous post about people "insisting that the math should conform to their misunderstandings instead of allowing it to guide their understanding."

Your use of natural language instead of math is allowing you to start reasoning from false premises about the nature of time; naturally even valid logic applied to false premises will lead to invalid conclusions.

To repeat something from an even earlier post of mine in the thread: Learn to use the Lorentz transformations and Minkowski spacetime diagrams. If you were taking a proper physics class, at some point your professor would make you go through the exercise of describing this train/source/detector problem by drawing spacetime diagrams and labeling each event with their x and t coordinates.

 

[Dale]

But this thing dont need math is it pure logic(obviusly my is wrong, but cant find why)

Logic is part of math. Please study the math in post 15. It includes the logic.

I feel like you asked me to do an example for you and then you have done everything you can to avoid studying the example you asked for. It is a bit frustrating. Why ask me to go through the non-trivial effort of working out an example of you didn’t want to study it in detail?

If I can explain why light need more time to hit right detector,only with "change in length", why do I need talk about time?

Because your “change in length”, together with the invariance of $c$, logically leads to the relativity of simultaneity anyway.

 

[John Mcrain]

That is pretty much why I made that comment in my previous post about people "insisting that the math should conform to their misunderstandings instead of allowing it to guide their understanding."

Your use of natural language instead of math is allowing you start reasoning from false premises about the nature of time; naturally even valid logic applied to false premises will lead to invalid conclusions.

To repeat something from an even earlier post of mine in the thread: Learn to use the Lorentz transformations and Minkowski spacetime diagrams. If you were taking a proper physics class, at some point your professor would make you go through the exercise of describing this train/source/detector problem by drawing spacetime diagrams and labeling each event with their x and t coordinates.

Lorentz transfomation show time is relative,so what Einstien discover new?

 

[Dale]

Lorentz transfomation show time is relative,so what Einstien discover new?

He discovered that the Lorentz transform could be derived from two postulates:

1) principle of relativity
2) invariance of $c$

The first postulate was already widely accepted. The second postulate was a good summary of existing experimental evidence. Together these were reasonable postulates on which to derive things.

 

[John Mcrain]

At 1:33:26 - 1:34:16, he explain why time and space change together

 

[Vanadium 50]

Dont understand.

I'm sorry.

 

[John Mcrain]

He discovered that the Lorentz transform could be derived from two postulates:

1) principle of relativity
2) invariance of $c$

The first postulate was already widely accepted. The second postulate was a good summary of existing experimental evidence. Together these were reasonable postulates on which to derive things.

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

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.

 

[phinds]

What is time for Einstein?

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

when train stops and when we then compare these two clocks, they will again show same time?

No. The two clocks have taken different paths through space-time and therefore have ticked a different number of times even though each tick was one second per second.

 

[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.