Willowz
Hello,

Can someone explain the http://demonstrations.wolfram.com/TheAndromedaParadox/" [Broken]? What does it mean, philosophically/etc? Local and non-local events. Time? Was this paradox an invitation for an explanation of the unknown concept at the time called "spooky action at a distance"? Which we can now call QM?

Last edited by a moderator:

## Answers and Replies

Homework Helper
Suppose the Andromedans launch a fleet of spaceships to invade Earth. The launch happens on April 4, Earth time--in Earth's reference frame, the launching of the spaceships is simultaneous with the clocks striking midnight on April 4. When the light from that event reaches Earth, Earthlings take into account the finite speed of light as well as the distance to Andromeda, and deduce that the spaceships were launched on midnight of April 4, 2011.

For somebody who starts traveling towards Andromeda on April 4, Earth time, the launch of the spaceships happens on a later date, say April 5. That is to say, when he receives light from the event and calculates when it was emitted, he will get April 5.

Neither the Earthlings nor the traveling observer will know about the launch for millions of years. If our traveller decides to stay on Earth instead of continuing towards Andromeda, he would receive information about the launch at the same time as everybody else. If he continues his journey, he would obviously receive news of the launch sooner. However, he cannot convey this information to Earth until his radio signals actually reach Earth, and this will not happen before light from Andromeda reaches Earth directly. The fact that the launch happened on April 5 in the observer's reference frame results in absolutely no observable difference.

Willowz
As I'm reading in other threads on this topic. Some claim that the paradox is only apparent, or even not really a paradox. Do you have any idea why Penrose labeled this thought experiment as a "paradox"?

Willowz
"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."
So, does this paradox arise because of the fact that "simultaneity" is observer dependent but not in respect to "objective" reality, at the same time?

Last edited:
Gold Member
As I'm reading in other threads on this topic. Some claim that the paradox is only apparent, or even not really a paradox. Do you have any idea why Penrose labeled this thought experiment as a "paradox"?

You realize this is a "paradox" of relativity, not quantum mechanics? And also that there is no violation of locality involved.

This is a common source of discussion and this thread handles it well...

And checking Penrose's Emperor's New Mind, where he gave the example, he does not call it a paradox. Instead he is trying to illustrate that what seems strange is true.

Willowz
Thank you. Got lost in my own ramblings.

jposs
Curses, foiled again.

I would have thought the paradox was why are we going to crash with it in 5 or so billion years when everything in the universe is moving away from us. But I have little education and know less.