Does entanglement interfere with causality?

[student34]

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?

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.

Once you determine the state of say an electron, would that electron then go on to behave in a different sort of way?

 

[student34]

No. Measuring an entangled electron does not produce any measurable change on the other particle.

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?

 

[PeterDonis]

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?

No.

 

[PeroK]

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.

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.

I know you can argue semantically about these things, but physics is not semantics; physics is about outcomes. And, in this case, the outcome is independent of any action you take - for spacelike separated measurements.

 

[PeterDonis]

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?

If electron A and electron B are entangled, and you measure electron A, nothing measurable changes about electron B.

 

[PeterDonis]

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?

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.

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?

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.

You have asked the same question many different times now in this thread. The answer has not changed. It's not going to change no matter how many times you ask it.

Thread closed.