If they get back together, then do they join to create the old photon? What if you stick one in a stronger gravitational field for a while and then later you stick them back together but maybe the phases of something are out of sync due to time dilation (or maybe they're in sync but just one has experience an extra second of life)? Does the resulting photon become entangled with the second-ago rendition of itself? Are we able to control an entangled photon well enough to run a process like this?

Sorry for the interested bystander questions.

Photons don't experience time dilation as they don't even experience time (since they're moving at the speed of light).

Why do we associate the speed of time with the speed of light? Is this even valid assumption?

Yes.

A better name for "the speed of light" might actually be "the speed of time" or "the speed of information transfer".

It's a mathematical limit and you can derive it using pretty much anything, not just light.

As an object goes faster, it experiences time dilation, length contraction, etc. The "Speed of Light" C is basically the point at which all of these things reach either zero or infinity.

It also "just so happens" that electromagnetic waves (such as light) travel at exactly this speed in a vacuum. They travel that fast because they travel at the maximum possible speed, and that happens to be the maximum possible speed.

It's like if you always traveled at the speed limit because you didn't want to break the law, and we called it "triplesex_ speed", but really it's just the maximum speed that anyone would go if they didn't want to break the law.

Except in this case it's not just a law- it is (to the best of our knowledge) physically impossible to go faster. It's not just that we haven't seen anything go that fast- it's that going faster than that doesn't even really make sense theoretically - for example, it would result in time-travel (this has to do with the fact that the order in which events happen and the speed at which time passes is different to different observers depending on the speed at which you're traveling).

It is not an assumption, in layman terms: "speed of light" is a bad name. It is actually speed on interaction. Top speed of any interaction. Everything which isn't slowed down by mass has this speed, like light.

I’ve heard it explained as the speed of causality (that is, the maximum speed at which cause and effect can occur) and, like you say, anything without mass to slow it down, like light, will travel at that speed. It’s the propagation delay for quantum states or whatever.

They do blue or red shift in response to gravitational gradients and the like.

Yes, but from your point of view, not theirs

Ack, can't believe I missed that! Still, the same question could be asked for other particles that can be split in the same manner ... if such particles exist.

I have never though about it but that makes absolute sense.

Thanks for pointing that out.

> do they join to create the old photon?

Fundamental particles don't have unique characteristics so they don't have identity. It doesn't make much sense to ask if it's the old photon. It's a photon just like any other photon.

I think I am trying to save locality by giving this split, entangled particle an identity. To the split particle, it's the rest of the world that's split and entangled. So when it collapses, there is no faster than light travel from its perspective. Basically extending the concept of locality to allow for being in multiple places at the same time, relative to your perception of the world.

Basically, trying out the thought experiment where, say the universe is two photons, and one splits, can we model it so that each photon thinks it is the other one who split? The same way general relativity did for rotating bodies.

But, entanglement isn't always just a single particle that splits. An electron can be entangled with a proton and such too. So probably this approach is silly. And physicists seem to be fine with ftl correlation so long as no communication takes place, so probably there is no problem to begin with.

Wouldn't interaction with the particles that modified that portion of spacetime destroy the entanglement?

What's the underlying physical mechanism? The photon is absorbed and reemitted as two?

I guess one could draw the Feynman diagram for this interaction, but I don't think it makes a whole lot of sense to talk about a physical "mechanism". If the quantum numbers are conserved (energy, momentum, angular momentum, etc.) then it can just happen.

Confirmed, Its magic!

Can't say about the physics, but with my recent approximate knowledge of algebraic structures I think it has to mean that QM is an OK division algebra, meaning something is invertible. If the invertible thing is a matrix then it might have nice computational implications.

Wow, that seems really interesting that you might not need a very expensive setup to demonstrate entanglement then. I'm just reading more about the bell test.

Edit: I just found this talk https://www.youtube.com/watch?v=tn1sEaw1K2k which looks really interesting 'Shanni Prutchi Construction of an Entangled Photon Source'. It looks like she's using SPADs for detection of the photons.

Yeah, the expensive/hard part is generating & measuring only a single photon at a time. Of course if you fire a laser through an SPDC crystal a bunch of photon pairs will come out entangled, but that isn't terribly useful for most entanglement experiments.

Ah Gotcha, I'm intrigued wiki says "SPDC allows for the creation of optical fields containing (to a good approximation) a single photon. As of 2005, this is the predominant mechanism for an experimenter to create single photons", do you know of a simple explanation on how single photons are generated using the crystal + laser?

Most quantum optics experiments are done by simply attenuating laser pulses until they have on average much less than one photon per pulse. This way the probability of having more than one photon is very low (at the cost of reduced count rates since most pulses don't contain any photons). These pulses can then be used to create entangled photon pairs using SPDC.

There are also several ways of creating pulses that contain exactly one photon, but this is way more complicated.

It's not enough to produce entangled photons, you also have to be able to detect them individually. That's the expensive part.

Could you get a similar effect shooting electrons at something? Like is there a known path to do that.