Relativity?

[Viveka]

What you have to remember is that special relativity describes what observers see when they pass each other with uniform relative velocity. If they continue with that uniform velocity, they will never know what the other's clock says, because they will simply be getting further and further away from each other.

So if I am looking at a clock that supposedly measures time, then when I close my eyes time suddenly stops?

There is no such thing as a clock that measures 'time' directly. All any clock can do is count events, be it the swing of a pendulum, or the 'vibrations' of atoms. It is completely arbitrary; it just happens that the Earth takes as long as it does to rotate, and that period, a day, has been divided up into 24 equal bits, hours, which then have been divided into 60 minutes, which in turn are divided into 60 seconds. It is these seconds, which are the basic unit of time and a second is defined as 9,192,631,770 wobbles of caesium, for no other reason than that if you multiply that by 60 twice and the product by 24, that is how many times caesium will wobble while the Earth completes one full rotation. (In fact it is even more specific, because the altitude has to be take account of, since these clocks are so accurate that they can detect time dilation due to gravity if they are separated by a couple of inches.)

The fact that they cannot see each other's clocks does not mean it has any physical relevance. If the train were made of glass, would that make a difference?

As I said above, special relativity describes what you will see when you pass another inertial frame with a fixed relative velocity. To be clear: velocity is speed in a straight line, so two inertial frames that pass each other in a straight line will never meet again, unless the conditions of special relativity are abandoned, they stop travelling at a fixed relative speed, one or both stop, turn around, and they get back together.
The bit that causes the confusion is that during the split second when the two frames pass each other. As they look at each other, the paths of any marble (see davidm), ball (read my blog) or light clock (that's Einstein) appears to be stretched out, regardless of who is actually moving. People (even some physicists) treat that split second as if it describes the whole of the universe. Which, as I'm sure you can appreciate, it doesn't.

If the conditions of special relativity are broken, so that the clocks stop their uniform motion and can be brought back together, then in all likelihood they will tell different times. That's the woefully misnamed 'Twin's Paradox', which was confirmed by Hafele-Keating and every subsequent test.
Are you assuming that there is some absolute time? Tell you what; read my blog: http://willijbouwman.blogspot.co.uk Better still, buy the book: https://www.amazon.co.uk/dp/1521884722

There HAS to be absolute time, otherwise the speed of light wouldn't be what it is. It's not the ticks of the clock that count, it's the speed of light!

Whether there is any such thing as "absolute time" is irrelevant. We cannot count absolute time, we can only count events. Seriously: this is way easier with pictures, which is why there are so many in my blog and book. There is a section called 'How does time work?' It's about half way down and it explains all of this as simply as I know how. Do yourself a favour and read it.

I read the section 'how does time work?' and I have a question. If stickgirl is on an embankment and stickman on a train, wouldn't stickgirl see stickman's light-clock take an extra distance? If so, would the reverse also happen for stickman looking at stickgirl? Or not? I think not, because if I'm on a train and watch a person bouncing a ball, the ball would stay with the person bouncing it!

[OuterLimits]

I read the section 'how does time work?' and I have a question. If stickgirl is on an embankment and stickman on a train, wouldn't stickgirl see stickman's light-clock take an extra distance? If so, would the reverse also happen for stickman looking at stickgirl? Or not? I think not, because if I'm on a train and watch a person bouncing a ball, the ball would stay with the person bouncing it!

What do you mean by see?

On one level, everyone "sees" something moving as, well, moving and their own items as stationary. So just from that perspective each "sees" the other's clock as "moving" and this might be perplexing if you thought that everything must either MOVE or NOT MOVE.

In Einstein's time, all kinds of theorizing and experimenting was done to figure out what light moved relative TO. For instance, ocean waves move relative to the ocean. Nobody could find what light was moving relative to, and so Einstein posited that for all unaccelerated observers, what if moved the same speed. This would create paradoxes with time, as everyone would not just "see" but measure (using certain techniques) that the moving person's clock must be slowed down. The techniques you use to determine that a moving clock is slowed down presume there is a such a thing as "at a remote distance but at the same time".

[thedoc]

thedoc, do you think viveka might actually be peacegirl?

I really don't think she's smart or savvy enough to create a sock, so it's highly doubtful. There is a similarity in the devotion to a failed idea.

[uwot]

Nice work on time on your blog/book, uwot.

Thank you.

And yes, I'm still buying the book, not just reading it online. For the blog, it would be helpful to have some kind of search function for keywords; I realize you can't link the graphical words, but perhaps you could add html words off to the side with key word anchor links in them.

It's not really my forte, but I will be looking into ways to make the blog more interactive-links, videos, animations, flashing lights, popcorn. Dunno, it's a work in progress and you're right, it could be way better.

But will viveka read it?

Funny you should say that...

[uwot]

I read the section 'how does time work?' and I have a question. If stickgirl is on an embankment and stickman on a train, wouldn't stickgirl see stickman's light-clock take an extra distance? If so, would the reverse also happen for stickman looking at stickgirl?

Yes. Suppose Stickman looks straight out the window. As the train moves from left to right, Stickwoman and her bouncing ball first appear to his right. If he keeps looking straight out the window, she sweeps across his field of vision and as the ball bounces, it too sweeps across.

Or not? I think not, because if I'm on a train and watch a person bouncing a ball, the ball would stay with the person bouncing it!

Well yes, but if you were on a train, someone watching you from the embankment would see the ball stay with you.
The point about relativity is that you can't tell who is actually moving. We assume it's the person on the train, because the train is going from one place to another. But you could be on a space station, in such an orbit that the train appears stationary, and the Earth rotates beneath it.

[Viveka]

I read the section 'how does time work?' and I have a question. If stickgirl is on an embankment and stickman on a train, wouldn't stickgirl see stickman's light-clock take an extra distance? If so, would the reverse also happen for stickman looking at stickgirl?

Yes. Suppose Stickman looks straight out the window. As the train moves from left to right, Stickwoman and her bouncing ball first appear to his right. If he keeps looking straight out the window, she sweeps across his field of vision and as the ball bounces, it too sweeps across.

Or not? I think not, because if I'm on a train and watch a person bouncing a ball, the ball would stay with the person bouncing it!

Well yes, but if you were on a train, someone watching you from the embankment would see the ball stay with you.
The point about relativity is that you can't tell who is actually moving. We assume it's the person on the train, because the train is going from one place to another. But you could be on a space station, in such an orbit that the train appears stationary, and the Earth rotates beneath it.

Okay. I think I have an argument that works:
1, As the embankment observer sees this contraction and dilation of the train moving appreciably close to c, and the train looking at the embankment observer sees this contraction and dilation when moving appreciably close to c. However, how can both be at different rates of ticking on their light-clocks? Embankment>train, and train>embankment.  Thus there is a contradiction.

2. But then to the person in the train looking at the interior of the train it would not occur to them and the light-clock would be invariant due to them not observing their own length-contraction and time-dilation. Neither would the embankment observer see their own length-contraction and time-dilation.

3. Therefore, the observer outside of the train looking at himself on the embankment,and with the observer inside the train looking at himself, the embankment clock and the train clock would be synchronous at the same time! Therefore the clocks are synchronized with both but they do not know it because they are both looking at their own light-clocks.

4. But let's say it's a glass train, due to their own non-seeing of their own length-contraction or time-dilation, while also viewing their own invariance of time, and due to observing the other's time-dilation or length contraction, then both observers would both see and not see synchronicity ! A contradiction in terms.

5. Thus there is either contradiction or synchronization.

[davidm]

Light is invariant. Light clocks are not.

[Viveka]

Let us say that Embankment Observer and Train Observer had shared consciousness. Let us say that both have two cameras and tv screen connected with a live feed. The Embankment Observer had a TV,  with camera looking at the Embankment from the Train's own POV and a camera looking at the inside of the Train. And the Train Observer with a camera looking at the train from the Embankment's POV and a camera looking at the Embankment from the Embankment's POV, with the TV giving him the live feed. Thus they share a consciousness in a sense. Now, would both observe the same thing or not?

[davidm]

As uwot has explained at least twice, as is explained in his book, and as is explained in the two links I gave you that you refused to read, here is how it works:

The observer on the embankment judges the train’s clock to be ticking slower and his own clock to be ticking normally.

The observer on the train judges the clock of the observer on the embankment to be ticking slower than his own, and his own clock to be ticking normally. This is because the observer on the train thinks of himself as at rest and the observer on the embankment as moving with respect to himself.

Who’s right? They can’t both be right! (This is your apparent contradiction.)

But as uwot has tried to show you, the question of who is right has no meaning unless the guy on the ground and the guy on the train can meet up again and compare their clocks.

And for THAT to happen, one or both observers are going to have to get out of their inertial frames. If one or both enters a non-inertial frame (a non-inertial frame is one that is NOT in constant uniform motion in a straight line), then the symmetry of their situations will be broken — the observed mutual relativistic time dilation will stop applying!

When they meet again, one or the other will definitely have aged less (slower clock). In the case of the train thought experiment, if the embankment observer remains in his frame and the train decelerates, turns around, re-accelerates and then comes to a stop before the observer on the ground, and then debarks the train and compares his clock with that of the guy on the ground, the train clock will definitely have ticked slower than the ground clock. The train rider will have aged less than the ground observer.

[OuterLimits]

Let us say that Embankment Observer and Train Observer had shared consciousness. Let us say that both have two cameras and tv screen connected with a live feed. The Embankment Observer had a TV,  with camera looking at the Embankment from the Train's own POV and a camera looking at the inside of the Train. And the Train Observer with a camera looking at the train from the Embankment's POV and a camera looking at the Embankment from the Embankment's POV, with the TV giving him the live feed. Thus they share a consciousness in a sense. Now, would both observe the same thing or not?

The "shared consciousness" idea seems to imply that information can travel instantaneously. In fact it can travel at speed of light max. Einstein's initial question was suppose someone is moving at the speed of light and is looking in a mirror. Will the traveler see anything? The light leaves his face going toward the mirror, but the whole ensemble of traveler, light, mirror is already moving at the speed of light, so we think maybe he will not see himself in the mirror. Is that what you think makes the most sense?

[davidm]

Let us say that Embankment Observer and Train Observer had shared consciousness. Let us say that both have two cameras and tv screen connected with a live feed. The Embankment Observer had a TV,  with camera looking at the Embankment from the Train's own POV and a camera looking at the inside of the Train. And the Train Observer with a camera looking at the train from the Embankment's POV and a camera looking at the Embankment from the Embankment's POV, with the TV giving him the live feed. Thus they share a consciousness in a sense. Now, would both observe the same thing or not?

And when NBC or whatever tells you that they are broadcasting the World Series "live" from Houston, what do you think that means? That what is happening on the field, and what you see on the screen, are occurring simultaneously?

[davidm]

Let us say that Embankment Observer and Train Observer had shared consciousness. Let us say that both have two cameras and tv screen connected with a live feed. The Embankment Observer had a TV,  with camera looking at the Embankment from the Train's own POV and a camera looking at the inside of the Train. And the Train Observer with a camera looking at the train from the Embankment's POV and a camera looking at the Embankment from the Embankment's POV, with the TV giving him the live feed. Thus they share a consciousness in a sense. Now, would both observe the same thing or not?

The "shared consciousness" idea seems to imply that information can travel instantaneously. In fact it can travel at speed of light max.

Right. So it's not "live" after all, if by "live" we mean instantaneous.

[Viveka]

Let us say that Embankment Observer and Train Observer had shared consciousness. Let us say that both have two cameras and tv screen connected with a live feed. The Embankment Observer had a TV,  with camera looking at the Embankment from the Train's own POV and a camera looking at the inside of the Train. And the Train Observer with a camera looking at the train from the Embankment's POV and a camera looking at the Embankment from the Embankment's POV, with the TV giving him the live feed. Thus they share a consciousness in a sense. Now, would both observe the same thing or not?

The "shared consciousness" idea seems to imply that information can travel instantaneously. In fact it can travel at speed of light max.

Right. So it's not "live" after all, if by "live" we mean instantaneous.

If we could somehow build quantum entangled cameras and tvs. Doesn't matter because it's a thought experiment, the same method Einstein used!

[davidm]

The "shared consciousness" idea seems to imply that information can travel instantaneously. In fact it can travel at speed of light max.

Right. So it's not "live" after all, if by "live" we mean instantaneous.

If we could somehow build quantum entangled cameras and tvs. Doesn't matter because it's a thought experiment, the same method Einstein used!

Quantum entangled cameras would NOT broadcast instantaneous feeds!

[Viveka]

Right. So it's not "live" after all, if by "live" we mean instantaneous.

If we could somehow build quantum entangled cameras and tvs. Doesn't matter because it's a thought experiment, the same method Einstein used!

Quantum entangled cameras would NOT broadcast instantaneous feeds!

And why is that? Quantum Entanglement is a method of faster-than-light travel of information.