Stupid Questions Thread - January 2014

RomeoStevens

Haven't had one of these for awhile. This thread is for questions or comments that you've felt silly about not knowing/understanding. Let's try to exchange info that seems obvious, knowing that due to the illusion of transparency it really isn't so obvious!

On the Neil Degrasse Tyson Q&A on reddit, someone asked: "Since time slows relative to the speed of light, does this mean that photons are essentially not moving through time at all?"

Tyson responded "yes. Precisely. Which means ----- are you seated?Photons have no ticking time at all, which means, as far as they are concerned, they are absorbed the instant they are emitted, even if the distance traveled is across the universe itself."

Is this true? I find it confusing. Does this mean that a photon emitted at location A at t0 is absorbed at location B at t0, such that it's at two places at once? In what sense does the photon 'travel' then? Or is the thought that the distance traveled, as well as the time, goes to zero?

Getting this property for electromagnetic waves was one of the main things that led Einstein to develop Special Relativity: he looked at waves and thought, "If we do a Galileian transform so that light is standing still, the resulting field is an invalid electrostatic field"

12pragmatist12y

Other people have explained this pretty well already, but here's a non-rigorous heuristic that might help. What follows is not technically precise, but I think it captures an important and helpful intuition. In relativity, space and time are replaced by a single four-dimensional space-time. Instead of thinking of things moving through space and moving through time separately, think of them as moving through space-time. And it turns out that every single (non-accelerated) object travels through space-time at the exact same rate, call it c. Now, when you construct a frame of reference, you're essentially separating out space and time artificially. Consequently, you're also separating an object's motion through space-time into motion through space and motion through time. Since every object moves through space-time at the same rate, when we separate out spatial and temporal motion, the faster the object travels through space the slower it will be traveling through time. The total speed, adding up speed through space and speed through time, has to equal the constant c. So an object at rest in a particular frame of reference has all its motion along the temporal axis, and no motion at all along the spatial axes. It's traveling through time at speed c and it isn't traveling through space at all. If this object starts moving, then some of the temporal motion is converted to spatial motion. It's speed through space increases, and its speed through time decreases correspondingly, so that the motion through space-time as a whole remains constant at c. This is the source of time dilation in relativity (as seen in the twin paradox) - moving objects move through time more slowly than stationary objects, or to put it another way, time flows slower for moving objects. Of course, the limit of this is when the object's entire motion through space-time is directed along the spatial axes, and none of it is directed along the temporal axes. In this case, the object will move throug

9lmm12y

In the photon's own subjective experience? Yes. (Not that that's possible, so this statement might not make sense). But as another commenter said, certainly the limit of this statement is true: as your speed moving from point A to point B approaches the speed of light, the subjective time you experience between the time when you're at A and the time when you're at B approaches 0. And the distance does indeed shrink, due to the Lorentz length contraction. [...] It travels in the sense that an external observer observes it in different places at different times. For a subjective observer on the photon... I don't know. No time passes, and the universe shrinks to a flat plane. Maybe the takeaway here is just that observers can't reach the speed of light.