34student

I am told on this forum that each particle has only one world line each. Do you agree?

Is the world line of the particle 1 meter away from the world line of a particle at the other end of the train?

So if there are 10 cars to the train, are you saying that there actually are 20 cars?

Ok, but objectively speaking, does more than one world line exist for the particle? I think that I understand what you are saying. Now here is my question that will either put my issue to rest for me or keep me wondering. If the particle is at x1 for Bob, and x2 for people on Earth, does the particle necessarily have to go through x1 and x2 (not necessarily in this sequence)? Then that would seem to imply that the train does not have a single world line. With world lines in mind, this means that there are at least 2 trains in existence (and probably many more from other frames of reference). Do you agree?

That is what I thought, and this is why I am so confused. Going back to the example in my OP, let's think about a particle sitting at one end of the train. The train lies on the x axis. The particle has one "String"/world line for Bob and another for Earth observers, (and possible an infinite more creating a plane). Which world line is real, or do they both exist? If both particle locations exist, why can't one of the two observers detect both particles?

I addressed your issue about Bob observing the contraction. Why does it matter whether Bob sees the contraction or not. I do not know what this has to do with my issue.

I want to learn, always. I always thought that the past was one 4 dimensional static block, literally a block of static particles that would look like like strings because of the dimension of time. But now I am being told that there is a number of different pasts. One past universe where the train is 1 meter long, another past universe where the train is 100 meters long, etc. This is quite shocking.

Interesting. So are you saying that the train does not have one absolute size in the past? It doesn't really matter if Bob can see the contraction or not. My argument is against the idea that the train contracts for Bob whether he can see the train or not. And as a thought experiment, if I go fast enough, the sun should turn into the shape of a dinner plate. Wouldn't that cause a lot more fusion and other changes to its gravity and overall behavior?

Now how is that logically possible. How can one length be two lengths?

During the year 2050, was the train 1 meter or 100 meters?

Bob and Earth begin with the same t value. When Bob returns, his t value is the same as Earth's t value when t was 2051. For some of that year, according to Bob's clock, the train was 1 meter. But from 2050 to 2051 on Earth, the train was never 1 meter.

When he comes back, it is 2100 on Earth. To him, it's (I am not doing the actual calculation) 2051. So let's say one year passed for him on the ship. Then you are on my side of the argument. The train becoming 1 meter is just an illusion for Bob.

I did not mention BH's.

Thank you for your link to the physicist, but I am looking for an understanding of SR through my OP.

I just explained this to you. These are Newtonian terms for simplicity.

I have to speak in a "Newtonian language" to explain the block universe. But the answer to your question is "Nothing moves in a block universe". Don't you notice that nobody else is raising this issue except you?

If you want to help me understand, please address my OP. Where is it flawed? Yes, Bob is travelling relative to the Earth/train. What's the problem?

F For every t (time) there is a corresponding function S(t) (static universe). That is all there is in a block universe. If you agree, continue reading. For some t's between 2050 and 2100 Bob sees the train as 1 meter. But from Earth, people see the train as 100 meters for all t's. There cannot be two S(t)'s for the same t in a block universe in this example.

In what circumstances are you referring to?

Zero angle.

You didn't explain why; please explain why. Note that a God's eye view of the block universe would have no effect on it. This statement supports my argument that length contraction is not actually happening and is just an illusion.

Why not? Please read up on the block universe.

He isn't. Things do not really move in special relativity either. My op explains the contradiction with the way we think about special relativity today.

Nothing moves in a block universe. Exactly. My point is that I do not believe length is relative.

The year is 2050. Bob goes close to light speed in his ship. He sees a 100 meter train contract to 1 meter. He gets back from his trip in the year 2100. Aliens from another dimension are looking at our block universe from 2050 to 2100. Will they see a 100 meter train or a 1 meter train or something else? This is meant to support the argument that length contraction is an illusion and not actually happening.