Why is the Andromeda Paradox flawed according to Penrose's book?

[ppppppp]

i just realized, Andromeda paradox is flawed. I feel good, as compared with penrose, I am really just started in this subject.

it is flawed becuase penrose mistakendly consider the"each day" for the person in moving (although very slow) and the person are the same. They are not same. the person in moving car has days slightly slightly shorter than the person in rest. So they cannot see events in andromeda differently if they compare notes! they are seeing the same things happen in andromeda, but in their frame of references, just the number of days for person in moving car is just 1-2 days shorter than the number of days for the person in rest!

 

[JesseM]

i just realized, Andromeda paradox is flawed. I feel good, as compared with penrose, I am really just started in this subject.

it is flawed becuase penrose mistakendly consider the"each day" for the person in moving (although very slow) and the person are the same. They are not same. the person in moving car has days slightly slightly shorter than the person in rest. So they cannot see events in andromeda differently if they compare notes! they are seeing the same things happen in andromeda, but in their frame of references, just the number of days for person in moving car is just 1-2 days shorter than the number of days for the person in rest!

The Andromeda paradox is about how each one defines simultaneity in their own frame (see info about the relativity of simultaneity here and here), it has nothing to do with what they see visually. If there were clocks on Earth and Andromeda which were synchronized in the Earth/Andromeda frame, then the event of the Earth clock showing a date of 2011 would be "simultaneous" with the event of the Andromeda clock reading 2011 in the Earth/Andromeda frame, even though the person on Earth couldn't actually see the light from the Andromeda clock reading 2011 until 2.52 million years later. Then if you had another observer traveling past the Earth at relativistic speed v in 2011, in that observer's frame the event of the Earth clock reading 2011 would instead be "simultaneous" with the event of the Andromeda clock reading 2011 + v*(2.52 million light years)/c^2, so for example if the observer was moving at v=0.0001c then in the observer's frame the Earth clock reads 2011 simultaneously with the Andromeda clock reading 2011 + (0.0001)*(2520000) = 2011 + 252 = 2263. But this observer also wouldn't see the light from the Andromeda clock reading 2263 until about 2.52 million years later (actually slightly less than the amount of time for the Earth observer because of the slight length contraction of the distance from Earth to Andromeda in this observer's frame).

It sounds like you're just getting started learning about relativity, it's great that you're interested but try not to go down the path of jumping to the conclusion that standard results are wrong before you understand it better! For some good online intros. to the basics of SR you might take a look at this thread.

 

[ppppppp]

when you go to wiki, the explanation, quoted originally from Rogers Penrose is flawed.

I agree with your explanation, which is exactly what I mean Penrose mistakenly assume each day is same for observer in moving car and observer at rest.

 

[PAllen]

when you go to wiki, the explanation, quoted originally from Rogers Penrose is flawed.

I agree with your explanation, which is exactly what I mean Penrose mistakenly assume each day is same for observer in moving car and observer at rest.

It is ridiculous to assume Penrose is mistaken about something like this (he may have odd ideas about the direction of future physics, but is one of the greats in history understanding relativity, having been the first to understand some features of special relativity, and many features of general relativity). Either you misunderstand him or your source does and misrepresents him.

[EDIT: I just looked this up in Wiki where Penrose is quoted. Nowhere does the article or the quote say or imply that days are the same for two observers moving relative to each other. It's all about simultaneity. It seems you have misunderstood the explanation. ]

 

[ppppppp]

if you do the math, you will know. Just because penrose is well-know, not everything he said is all right.

 

[JesseM]

when you go to wiki, the explanation, quoted originally from Rogers Penrose is flawed.

I agree with your explanation, which is exactly what I mean Penrose mistakenly assume each day is same for observer in moving car and observer at rest.

The quote on the wiki page is:

Two people pass each other on the street; and according to one of the two people, an Andromedean space fleet has already set off on its journey, while to the other, the decision as to whether or not the journey will actually take place has not yet been made. How can there still be some uncertainty as to the outcome of that decision? If to either person the decision has already been made, then surely there cannot be any uncertainty. The launching of the space fleet is an inevitability."

This agrees completely with my explanation, it's just saying the date on Andromeda that's simultaneous with the event of the two people passing is different in each person's rest frame (it's not talking about what either of them sees visually). In one person's frame they pass each other simultaneously with a date on Andromeda before the fleet has launched, in the other person's frame they pass each other simultaneously with a date on Andromeda after the ship has launched (just like how in my example, at the moment the two observers pass one another, one observer says it's currently 2011 in Andromeda, the other says it's currently 2263 in Andromeda). If you think this somehow differs from what I said, please explain what specific part you're talking about.

 

[ppppppp]

Penrose says, the observer in car will see things a bit ahead ( 1 day earlier) than the observer at rest (they just cross at same point) NO! That is a mistaken.

The observer in car will count more days in his reference frame but that is because his One day is just slightly shorter than the ONE day for the observer at rest.

 

[JesseM]

Penrose says, the observer in car will see things a bit ahead ( 1 day earlier) than the observer at rest (they just cross at same point) NO! That is a mistaken.

You are mistaken, Penrose didn't say anything about the observer in the car "seeing" anything different, he was just talking about how simultaneity is judged in each observer's frame. It is true that if event A is the event of the observer in the car passing by the standing observer, then in the car's frame the date in Andromeda that's simultaneous with A is somewhat ahead of the date in Andromeda that's simultaneous with A in the standing observer's frame. Again this is exactly like my example, which you said you agreed with.

 

[ppppppp]

"Two people pass each other on the street; and according to one of the two people, an Andromedean space fleet has already set off on its journey, while to the other, the decision as to whether or not the journey will actually take place has not yet been made. How can there still be some uncertainty as to the outcome of that decision? If to either person the decision has already been made, then surely there cannot be any uncertainty. The launching of the space fleet is an inevitability." (Penrose 1989).Penrose assumes two people see differently. (one actually already see/know the attack by alien started), which is mistaken. I think you will also agree it is mistaken, if he assumes two people see differently as below

Two people pass each other on the street; and according to one of the two people, an Andromedean space fleet has already set off on its journey, while to the other, the decision as to whether or not the journey will actually take place has not yet been made.

 

[PAllen]

"Two people pass each other on the street; and according to one of the two people, an Andromedean space fleet has already set off on its journey, while to the other, the decision as to whether or not the journey will actually take place has not yet been made. How can there still be some uncertainty as to the outcome of that decision? If to either person the decision has already been made, then surely there cannot be any uncertainty. The launching of the space fleet is an inevitability." (Penrose 1989).


Penrose assumes two people see differently. (one actually already see/know the attack by alien started), which is mistaken. I think you will also agree it is mistaken, if he assumes two people see differently as below

Two people pass each other on the street; and according to one of the two people, an Andromedean space fleet has already set off on its journey, while to the other, the decision as to whether or not the journey will actually take place has not yet been made.

Read more carefully, and with less preconception. Does Penrose say "see"? No. He says "according to" which is a reference to a notion of simultaneity. Remember, this quote is a snippet of some longer discussion, which you should assume is similar to the explanation in the rest of article, which is similar to JesseM's description, which is similar to any reasonable book on special relativity.

 

[JesseM]

Penrose assumes two people see differently.

He never uses the word "see".

Two people pass each other on the street; and according to one of the two people, an Andromedean space fleet has already set off on its journey, while to the other, the decision as to whether or not the journey will actually take place has not yet been made.

He's just talking about their judgments of simultaneity, he doesn't say they actually know about what's going on in Andromeda simultaneously with their passing one another, in either frame. In fact they won't learn about the fleet taking off until millions of years later!

 

[ppppppp]

see does not mean see with eyes! You are completely disqualified as "contributor" all you talk is just circling like yoyo..

it is simply wrong to write: "according to one of the two people, an Andromedean space fleet has already set off on its journey, while to the other, the decision as to whether or not the journey will actually take place has not yet been made." in the illustration (with picture as shown in wiki.)

 

[JesseM]

see does not mean see with eyes!

So do you understand he was talking about simultaneity, not what either observer actually could see visually at the time they passed one another?

it is simply wrong to write: "according to one of the two people, an Andromedean space fleet has already set off on its journey, while to the other, the decision as to whether or not the journey will actually take place has not yet been made." in the illustration (with picture as shown in wiki.)

It's a correct statement about simultaneity. Do you disagree?

 

[PAllen]

see does not mean see with eyes! You are completely disqualified as "contributor" all you talk is just circling like yoyo..

it is simply wrong to write: "according to one of the two people, an Andromedean space fleet has already set off on its journey, while to the other, the decision as to whether or not the journey will actually take place has not yet been made." in the illustration (with picture as shown in wiki.)

Contributor just means I give money to this site, because I like it, nothing more.

What Penrose wrote is perfectly defensible. JesseM's explanation of it is almost word for word the same as mine (we apparently wrote simultaneously but mine posted first). The diagram is showing presumed lines of simultaneity. It has nothing to do with lines of sight or path of light or seeing in any way; in fact, by definition, light cannot follow such spacelike paths. Also, note, the diagram is not part of what is quoted from Penrose. To see the rest of Penrose analysis, you would have to consult wherever the quote came from.

 

[JesseM]

I looked it up, the quote is from p. 303 Penrose's book The Emperor's New Mind which I have a copy of, and Penrose actually makes it quite clear that he is talking about simultaneity and not what either observer actually sees visually or knows about at the moment they pass one another:

Two people pass each other on the street; and according to one of the two people, an Andromedean space fleet has already set off on its journey, while to the other, the decision as to whether or not the journey will actually take place has not yet been made. How can there still be some uncertainty as to the outcome of that decision? If to either person the decision has already been made, then surely there cannot be any uncertainty. The launching of the space fleet is an inevitability. In fact neither of the people can yet know of the launching of the space fleet. They can know only later, when telescopic observations from Earth reveal that the fleet is indeed on its way. Then they can hark back to that chance encounter, and come to the conclusion that at that time, according to one of them, the decision lay in the uncertain future, while to the other, it lay in the certain past.

Also, in this section he was actually referring back to an earlier discussion of the same scenario from p. 201, and on that page he includes a diagram which is much clearer than the one on the wiki, it shows the worldlines for two observers A and B along with lines labeled "A's simultaneous space" and "B's simultaneous space", showing that these lines of simultaneity intersect the wordline of Andromeda at different points (one intersection happening before the launch of the fleet, the other intersection after).

edit: a quick google search reveals that someone actually posted Penrose's diagram from p. 201 on this thread, here it is: