# Why does SETI require this?

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#### billslugg

The time of the change is controllable only by the first person to do it. The other person has no way to tell when the first person did it. You cannot see whether a photon has collapsed until you measure it. When you look at your photon and observe it collapse to a particular state you don't know if the other guy did it first or if you were the first one to do it. Thus timing information cannot be exchanged.

You cannot remove the random choice for input. When you look at a photon, its state collapses randomly, you have no way to influence its final state. You must take what you get.

#### Classical Motion

If detection has to be queried, how do they know the change in state is instant?

#### billslugg

They check it instantly in the laboratory. The check them at time t=0, t=1, t=2 and it is always the same result.

#### Classical Motion

Well then, I have been mis-led with these explanations about it. This is worthless.

What do you think they mean when they say a photon collapses? What measurement do they do that causes that, what stimulus is used? And in this case is the photon mass or field?

#### Turtle

I probably missed out on something here. I thought that if an entangled photon was found to be polarized vertically than its corresponding photon would be polarized horizontally. If it just hit the wall then what?

#### Classical Motion

So I guess entanglement is two queries at the same time and always getting opposite results.

#### billslugg

Photons exist ina "superposition" of all the possible quantum states right up until the time they are measured. At that point they chose one state or the other. If you take two photons and "entangle" them, then they will occupy opposite states but only after one is measured. You take the first photon and measure it and its state collapses to a value. Since the two photons are, in essence, one particle, the second collapses at the same time. When measured, its quantum state will be complementary to the first.

Do not fret at your inability to understand quantum mechanics. No one understands it.

#### billslugg

I probably missed out on something here. I thought that if an entangled photon was found to be polarized vertically than its corresponding photon would be polarized horizontally. If it just hit the wall then what?

You can take an elementary particle and have it decay into two smaller particles. In order for angular momentum to be conserved, one of the tinier particles must be oriented with spin up and the other particle must be oriented with spin down. They are "entangled". In a manner of speaking they are but one particle spread out over a very large distance. Measure one and they both collapse. One has to be "up" and one has to be "down".

#### billslugg

So I guess entanglement is two queries at the same time and always getting opposite results.
Yes, in a quantum state that has two opposite values, one particle will be the opposite of the other. Take a big particle with no spin, break it into two tinier particles with spin, in order to conserve angular momentum, one must be spin "up" and one must be spin "down".

#### Turtle

OK, so now take a stream of trillions of entangled photons and send the outgoing ones out there to a receiver person. Next run the home stream through a polarizer and eliminate all the photons which are not say vertical. Those vertical photons then are unscathed by a detector. Untouched, fresh as a daisy.

Now these photons must remain in existence long enough for the corresponding horizontal photons to be detected. So send em out into a clean vacuum. The receiver person will then get a preponderance of horizontal photons. The rest is Morse Code.

#### billslugg

What you are proposing is to tag some photons and send them to someone. That is conventional speed of light communication.

We are talking about a photon that is sent to a second person and after it gets there then the first one is measured. The second one takes the opposite quantum state instantaneously. Since we have no way of controlling what quantum state the first one assumes, no information can be transmitted faster than the speed of light.

#### Turtle

Well, can't we filter the home stream of photons for vertical polarization and send the cross polarized ones to the receiver and maintain the entanglement? If that were the case then another equally far away receiver could get our vertically polarized ones and they could communicate.

#### billslugg

Once a photon is measured, the entanglement is broken.

You cannot tell which quantum state the photon will assume until you measure it. There is no way to filter them ahead of time.

If you send someone a group of filtered photons, they can only travel at the speed of light. Nothing is gained. Might as well use radio.

#### Turtle

But if they go through the filter it's because they haven't touched anything. That's because they were polarized the same way as the filter. Like going through a window. The rest of the photons smacked into the filter and were done for. How would they lose their entanglement?

And I was talking about two receivers at roughly the same distance away. They could then communicate with each other. The sender of the entangled photons couldn't.

#### billslugg

A photon with a superposition of quantum states loses that superposition when it is observed. Passing them through a filter counts as an observation. Once that superposition is lost, so is any entanglement it had. After you filter the photons they are useless for any attempt at faster than light communication.

#### Turtle

So if one entangled photon out in space went through an unknown polarization process along its way without being observed it would maintain it's entangled state???? Once its state is observed (by some intelligence?) it loses its entangled state???

#### billslugg

Yes, anything that interferes with a particle in a superposed state will cause it to collapse. This is why it is so difficult to make a quantum computer. The qubits must be kept very cold, in a vacuum, isolated from all external influences or they will collapse.

#### theglassisempty

I've been wondering about SETI's requirement that a signal must repeat before it is considered as proof of an alien or artificial source.

If a signal, and we have apparently received several, does not repeat could it not be worth further investigation?

These non-repeating signals were investigated to death. Nothing else was found.

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