# EPR, Bell question

1. Aug 10, 2015

### theargosy

What is wrong with my scenario?

A traveler is about to embark on a mission to Mars. A technician is responsible for communications with the traveler. An entangled pair of electrons is prepared, one to make the trip to mars and one to remain on Earth.

When the traveler reaches Mars he changes the spin on his entangled electron. Simultaneously by measurement on both sides, the Earth-bound electron changes spin.

By prior agreement, the change in spin indicates the arrival of the traveler on Mars, by faster than light communication.

Brian Wachter

2. Aug 10, 2015

### eltodesukane

You can know the spin only if you measure it.
If you measure it, you will get up or down.
It doesn't tell you if the spin has changed or not.
Remember the 2 electrons' spins were entangled.

Last edited: Aug 10, 2015
3. Aug 10, 2015

### Staff: Mentor

The earth observer had no way of knowing that the spin has changed with only one measurement.

Say he measures his particle and gets spin-up. He has no way of knowing whether he's performed the first measurement on the pair, the astronaut hasn't yet arrived and will measure spin-down when he arrives; or whether the astronaut has already arrived, performed the first measurement, and gotten spin-down.

4. Aug 10, 2015

### theargosy

When the traveler leaves earth both particles are spin up. The earth-bound particle will simply be monitored. When the traveler reaches Mars he resets his particle's spin. The technician records this as the signal that the traveler has reached Mars.

5. Aug 10, 2015

### theargosy

You mean I can't have an initial spin and know what it is?

6. Aug 10, 2015

### DrChinese

Welcome to PhysicsForums, theargosy!

You can have an initial spin and know what it is. That particle will not be entangled with anything, however.

Entangled particles are in what is called a "superposition" of states. They do not have a fixed knowable value until measured.

7. Aug 10, 2015

### Staff: Mentor

There's no way to do that. Two problems:
First, you can't "monitor" a particle's spin without measuring it. And once you perform that measurement, they're no longer entangled so all subsequent measurements of the two particles will be completely independent and uncorrelated.
Second, there is no such thing as two entangled particles in a known spin-up state - You can only have entanglement when the state of the two particles is a superposition of the two possible outcomes for the pair, and you don't know which you have until after you've performed the one measurement that breaks the entanglement.

If we're going to use the language of wave function collapse, an entangled state is something like $|\psi\rangle=\frac{\sqrt{2}}{2}(|++\rangle+|--\rangle)$ where $|++\rangle$ and $|--\rangle$ are the states "both particles are spin up" and "both particles are spin down". Any measurement performed on either particle causes the wave function to collapse randomly to either $|++\rangle$ or $|--\rangle$ and then you just have two independent particles both of which happened to have the same spin when we first looked at them, but are now evolving independently of one another.

8. Aug 10, 2015

### Avodyne

In that case, the observer on Earth will always measure spin up, no matter what happens to the spin on Mars. So again no signal is sent.

9. Aug 10, 2015

### theargosy

Thanks!

10. Aug 10, 2015

### theargosy

Thank you.

11. Aug 10, 2015

### theargosy

Thank you a bunch.

12. Aug 10, 2015

### theargosy

I thank you!

13. Aug 10, 2015

### theargosy

Thanks.

14. Aug 10, 2015

Thank you.