Confused about the logic of a Special Relativity problem

[Herman Trivilino]

What do you see in a telescope aimed on the earth? You may see for example the Earth on the year 3000.

Never and nowhere could you see light that left Earth in the year 3000 because it hasn't left yet.

So yes, knowing the future is possible, and it's also quite pointless as you can also see the future whenever you know the future.

No, no, and vacuous because you never know the future.

I guess the future of the Earth was predetermined as you found out that light from events existed before the events.

No, and no.

 

[jartsa]

Never and nowhere could you see light that left Earth in the year 3000 because it hasn't left yet.
No, no, and vacuous because you never know the future.
No, and no.

Do you perhaps mean that I made an error?

You are floating in space viewing the Earth through a telescope, seeing events from year 3000. Then you accelerate so that "now" on Earth becomes year 1000.

So it's year 1000 on Earth now, and you are seeing events from year 3000 happening.

 

[Nugatory]

Do you perhaps mean that I made an error?

You have.

You are floating in space viewing the Earth through a telescope, seeing events from year 3000. Then you accelerate so that "now" on Earth becomes year 1000.
So it's year 1000 on Earth now, and you are seeing events from year 3000 happening.

Try drawing a spacetime diagram of the situation you're describing. Can you find a timelike path that starts in the future light cone of the event "earth, year 3000" and ends on any possible line of simultaneity (spacelike straight line) through the event "earth, year 1000"?

 

[jartsa]

Try drawing a spacetime diagram of the situation you're describing. Can you find a timelike path that starts in the future light cone of the event "earth, year 3000" and ends on any possible line of simultaneity (spacelike straight line) through the event "earth, year 1000"?

The "earth year 3000" went to the future when "earth year 1000" became the current earth. But light from "earth year 3000" did not go to the future, because that light was not located near "earth year 3000".

The light source disappeared while the light from the light source remained, so clearly causality went out of the window. In block universe we don't need such thing as causality.

I'm interested to know why this is wrong. Maybe clocks never run backwards, no matter how you accelerate, because it seems like causality will suffer anytime clocks run backwards?

 

[Ibix]

Jartsa - "now on Earth" can never be earlier than the date you are seeing through your telescope. If it could, what you call now would be before something that you can see has already happened. You can see this from a space-time diagram, as Nugatory suggested, or from the Lorentz transforms - place the Earth at x=d in its rest frame, then insist that t'=0 and see what range of t you can get.

 

[DrGreg]

You are floating in space viewing the Earth through a telescope, seeing events from year 3000.

Let's suppose you are 100 light years from Earth (according to Earth) when this happens. Then, someone on Earth in the year 3100 would say your observation is occurring "now". (Because, according to Earth it takes 100 years for the light to travel 100 light-years.)

Then you accelerate so that "now" on Earth becomes year 1000.

That's not possible. Depending on your speed relative to Earth, the event on Earth that you describe as "now" must lie somewhere between 3000 and 3200 (3100±100) Earth time.

 

[Nugatory]

Try drawing a spacetime diagram...

The "earth year 3000" went to the future when "earth year 1000" became the current earth. But light from "earth year 3000" did not go to the future, because that light was not located near "earth year 3000".

Try drawing a spacetime diagram.

 

[jartsa]

I see. When I'm trying to reverse a distant clock, by a sharp acceleration of myself, the clock starts to resist further reversing at some time. Or rather: the more a clock is reversed, the more it resist reversing. Thank you guys. It's clear now.

Hmm ... If I make sharp motions back and forth, a distant clock advances a lot.

 

[Ibix]

I see. When I'm trying to reverse a distant clock, by a sharp acceleration of myself, the clock starts to resist further reversing at some time. Or rather: the more a clock is reversed, the more it resist reversing. Thank you guys. It's clear now.

Hmm ... If I make sharp motions back and forth, a distant clock advances a lot.

No - nothing happens to the clock. The only thing changing is your definition of "now" in the sentence "the time clocks on Earth are showing now is..." The point is that there are limits to what one can reasonably call "now". A time you can see, or have already seen, isn't sensible since you would be in the position of seeing things that happen after what you call now.

 

[Herman Trivilino]

I see. When I'm trying to reverse a distant clock, by a sharp acceleration of myself, the clock starts to resist further reversing at some time. Or rather: the more a clock is reversed, the more it resist reversing. Thank you guys. It's clear now.

Hmm ... If I make sharp motions back and forth, a distant clock advances a lot.

Making this kind of a thing "clear" is very difficult, as it's so far from what we experience.

You will make progress, I believe, by trying to understand that proper time is a relativistic invariant. For example, the clock you talk about keeps proper time as does your wristwatch. Just as the treading on your wrist watch will never jump or reverse, neither will that clock's reading. That's a relativistic invariant, meaning all observers will agree on that behavior.

When we talk about time dilation and relative simultaneity, we talk about what observers in motion relative to those clocks will observe when they compare them to the clocks they carry with them.

 

[jartsa]

I see. When I'm trying to reverse a distant clock, by a sharp acceleration of myself, the clock starts to resist further reversing at some time. Or rather: the more a clock is reversed, the more it resist reversing. Thank you guys. It's clear now.

When I'm trying to reverse a distant clock, the clock tends to become a nearby clock, nearby clocks behave less weird than distant clocks.

That's how it works. It's the length contraction phenomenon. Very simple actually.

 

[Herman Trivilino]

When I'm trying to reverse a distant clock, the clock tends to become a nearby clock, nearby clocks behave less weird than distant clocks.

You don't reverse distant clocks. You don't even change distant clocks. You change reference frames, but you'll never be in a reference frame that observes or even sees clocks running backwards.

 

[jartsa]

You don't reverse distant clocks. You don't even change distant clocks. You change reference frames, but you'll never be in a reference frame that observes or even sees clocks running backwards.

Maybe if I use scare quotes the error is not so severe.

So I'm looking at Earth that is 1000 ly away, I'm seeing year 3000 going on, and it is now year 4000 on Earth according to me. Then I accelerate to speed 0.99999999 c away from the earth. Now I'm seeing year 3000 going on, and it is now year 3000 on Earth according to me. As I'm seeing Earth without much delay, the Earth must be near.

The Earth "moved closer" and "reversed" as I accelerated.

 

[Nugatory]

Maybe if I use scare quotes the error is not so severe.

So I'm looking at Earth that is 1000 ly away, I'm seeing year 3000 going on, and it is now year 4000 on Earth according to me. Then I accelerate to speed 0.99999999 c away from the earth. Now I'm seeing year 3000 going on, and it is now year 3000 on Earth according to me. As I'm seeing Earth without much delay, the Earth must be near.

The Earth "moved closer" and "reversed" as I accelerated.

No that is not what happens. And you aren't paying attention to what was said in #53 and #57 of this thread.

 

[jartsa]

No that is not what happens. And you aren't paying attention to what was said in #53 and #57 of this thread.

It's not the same scenario as before, I curbed the "reversing".

How much can I tilt the line of simultaneity? Almost 45 degrees. As the wordline of light is tilted 45 degrees, the light that an observer is seeing left the light source about now, according to the observer, when observer's line of simultaneity is almost parallel to the wordline of light.

 

[pat devine]

Without going into all the detail, the explanation of what's happening during this "experiment" is badly worded. In fact it's just wrong. Comparing "times" is misleading because there is no shared moment, so shared now! Only when they are together do they share the same time and the same location, apart from that their times and locations are only truly comparable by reference to a combined space-time.

 

[pat devine]

Hey don't give up, most people even the experts only understand this particular "experiment" on a very superficial level. The explanation provided by your expert is misleading and in fact downright incorrect. Comparing the times between the two individuals is not possible, there is no shares moment, no shares now! Making time like comparisons is without reference to space is impossible. Only by reference to full space time coordinates is a comparison possible, and. It is neither a true time comparison nor a true space comparison but rather a blend of both.