The discussion centers on the implications of quantum entanglement and causality, particularly in the context of two particles, E and S, where E is observed on Earth and S is located one light-year away. Observing particle E determines the state of particle S, but the interaction is constrained by the speed of light, preventing instantaneous causation of events like a supernova. The conversation highlights the distinction between quantum mechanics (QM) and relativity, emphasizing that interpretations of QM do not universally agree on the nature of entanglement and causality, and that the events in question must maintain a consistent time ordering across all reference frames.
PREREQUISITESPhysicists, students of quantum mechanics, and anyone interested in the philosophical implications of quantum entanglement and causality in modern physics.
Yes I understand that you can not send useful information to people.PeroK said:It works like this. Suppose you and I are a long way apart and we each have one of a pair of entangled particles. You want to send me a message. Our code is spin-up = yes and spin-down = no.
You measure your particle and if you get spin-up, then you know I get spin down and hence the message is "no". And if you get spin-down, then I get spin-up and the message is "yes".
Now, you want to to send me the message "yes". So, you measure your particle and if it's spin-down, then bingo I get the message "yes". But, if you get spin-up, then I get the message "no". Which is not what you intended.
And, in fact, even if you forget to measure your particle (or you don't want to send a message that day), when I measure mine I still get one of spin-up or spin-down and have no way to know that you didn't actually want to send a message that day.
As you have no way to control the result of your measurement and therefore no way to control the result of mine, you cannot influence what result I get or what message I receive. That means it's not a message from you at all, it's just some random result that is independent of what you choose to do.
There are literally dozens of threads on here with the same question: why can't I use quantum entanglement to send a message FTL?
Once you determine the state of say an electron, would that electron then go on to behave in a different sort of way than if its state weren't fixed?Dale said:No. Measuring an entangled electron does not produce any measurable change on the other particle.
No.student34 said:But if it is evidence for Bell's theorem for networks, isn't that evidence that events can be connected faster than the speed of light?
There's no causality - which is the important thing. In fact, it's not possible to say who measured the system first. In the above example, you can equally say that I measured my particle first and tried to send a message to you. And, the same in your supernova/bomb example. The remote particle was measured and the bomb either went off or not. You can choose a frame of reference where that happened before you measured the particle on Earth. Hey, that's the relativity of simultaneity again in a new guise.student34 said:Yes I understand that you can not send useful information to people.
But what about just the idea that you have changed something on the other side that could somehow have a physical impact.
If electron A and electron B are entangled, and you measure electron A, nothing measurable changes about electron B.student34 said:Once you determine the state of say an electron, would that electron then go on to behave in a different sort of way than if its state weren't fixed?
Not faster than light. If a measurement on Earth is going to cause a supernova one light year away, it can't cause the supernova faster than light.student34 said:I am not sure about this, but wouldn't fixing an entangled electron on Earth potentially cause something physical to happen with the other electron in some very delicate contraption?
The other electron one light year away can certainly interact with other objects in its vicinity. But nothing measurable about those interactions changes if the entangled electron on Earth is measured.student34 said:For example, if we observe an electron on Earth that is entangled with an electron one light year from here, then doesn't that enable the electron to interact with objects, such as emit photons or the ability to interact with nearby electrons?