Double Slit Experiment and Delayed Choice Quantum Eraser - light and time In the double slit experiment, there's observing or not observing. You observe, and get the particle pattern. You don't observe, and get the interference pattern.
Light travels at the speed of ... uhm ... light. This makes me think as follows.
To an imaginary passenger on a photon, it would appear as if time stood still. It also appears as if the entire universe is squashed flat - into a two dimensional plane. The point from where the photon is sent, is the same as the point from where the photon is received. Also, the time at which the photon is sent, is also the same time it is received. All from the point of view of that imaginary passenger.
From this it seems logical for me to think that observing and thereby interfering with the particle at any point necessarily is the same as tampering with it along its entire path and history - because the path is just a point, and the history is just one moment.
I'm thinking that this can explain why to us it seems we're affecting the past in these experiments. Maybe we actually are in some way or another.
I think I see flaws to this line of thinking also, but I'll stop here.
So this was just a random thought from someone rather un-knowledgeable about physics - have no idea how naive I am - but I'd like to know if there's some real physics work that have some correlation with this.
 A: That is unlikely to explain anything in physics because the path of light is not "all a single point" or "all at the same time". That sort of idea is based on coordinates, but in relativity motion isn't about coordinates, but instead about moving between events. Light moves from event to event, forward through time, the same as anything else.
Also, the theory explains too much, because any quantum effects about observers that apply to photons traveling at light speed also apply to other quantum particles moving slower than light speed, so using light speed travel to explain the quantum phenomena is a dead end.

If you track a particle as it moves around, the path you get is called its worldline. If you take a particular point along a worldline, that's called an event. You can think of a worldline as containing infinitely many events just like a line contains infinitely-many points. As objects move around, they go from event to event on their worldline.
In order to do mathematical analysis of motion, we often set up a coordinate system to describe the events. Every event is given a set of space coordinates ($x,y,z$) and a time coordinate ($t$).
If you take two events right next to each other on a worldline, find how far apart they are (using the space coordinates) and divide by how long it took the object to go from one event to the next (using the time coordinate), you get the object's speed.
However, the speed is just an artifact of the coordinate system we created. If we had set up a different coordinate system, we would get a different speed. For example, I think I spaceship just flew past me at half the speed of light. But if you're on the spaceship, you think you're stationary and I was going half the speed of light in the opposite direction. That happens because we both just choose coordinate systems where we personally are stationary.
What really matters is not to coordinates, but the events themselves. And between any two events, there is not a distance, nor a time, but an interval. Objects move along worldlines from event to event, and as they do it, there is an interval between the events. This interval is like the concept of distance, but mathematically is not quite the same.
The worldlines of people, spaceships, etc. are called timelike. This means that the interval between any two events on their worldline is positive. The interval can be measured in seconds, and is equal to the time between the two events in a reference frame where they occur at the same place. In other words, set a clock going from one even to the other with no acceleration. The amount of time the clock ticks forward going between the events is the interval.
Light also follows a worldline and goes between events. The difference is that light's worldlines are lightlike. This means that the interval between events on the worldline is zero. A physical clock can never get from event to event on a lightlike worldline. A consequence of moving between events with zero interval is that you're always going the speed of light in any reference frame. So if light zips past me to the left, and you're in a spaceship also going to the left at half the speed of light, it still looks to you like the light is running away at the speed of light.
If we choose coordinates to describe the worldline of the light, then the distance between events divided by the time between events will always be $c$. The light is still going from event to event, though, just like anything else. If you have coordinates for the events in any reference frame, you can calculate the interval from the formula $\tau = \sqrt{(\Delta t)^2 - ((\Delta x/c)^2 + (\Delta y/c)^2 + (\Delta z/c)^2 )}.$ If something is moving at $c$, then this always works out to zero. Light is still moving between events. It's just going between events with zero interval between them. That doesn't mean they're at the same place or the same time.
So when you hear something like "from the point of view of light, everything happens in the same place", it doesn't really parse. "Happening in the same place" is a statement about coordinates. It's saying that the spatial coordinates are the same for the events. But in what reference frame? There is no reference frame where the spatial coordinates of the events are the same. The concept is taking language that applies to coordinates and trying to use it to describe something that doesn't exist. Things don't "all happen at the same time from the point of view of light" because "happen at the same time" is a statement about a coordinate system, but statements like this never have an actual coordinate system associated with them. It is just using the wrong language and exploiting a weakness to over-interpret statements beyond their realm of applicability.
Instead, we should think about the worldline of the light as the light moves from event to event. There's an event where the photon leaves the laser. The photon travels through many events, all of them separated from each other by space and time but with interval zero, and eventually gets to an event where the photon entangles with some measurement apparatus. This second event happens at a later time than the first event in any reference frame, and it happens at a later point along the photon's worldline. The events are as distinct as any other two events. They don't happen at the same place, or at the same time, and so nothing about "affecting the past" or "affecting the entire trajectory", etc. is explained just by looking at the events on a worldline.

Also, when we have a new idea in physics of the form "A explains B", it doesn't work if B happens even without A. For example, suppose someone says "the reason it rains is that worms are about to come out, and the worms use incantations to bring the rain so they don't dry up." That would explain why worms are often seen after rain. But in areas without worms, it still rains. Worms can't be the explanation for why it rains, then.
If there is some strange-sounding quantum phenomenon around observations and entanglement, and we say "these strange things happen because of photons moving the speed of light" it means that the strange things only happen for photons or other things moving the speed of light. But entanglement, quantum erasers, double slits etc. are just generic features of quantum mechanics. They all happen with electrons moving along timelike worldlines just as they do for photons moving along lightlike worldlines, so motion at the speed of light can't explain these phenomena.
A: Your conclusions are right. The spacetime interval of lightlike movements is zero. However, non-locality issues appear when the considered worldline is not lightlike. And also be aware of the fact that lightlike movements of particles are not possible in a universe with matter and gravity, lightlike propagation is reserved to fields. 
