I’ve been thinking a lot about special relativity recently, and wrote up a fun program for visualizing some of its stranger implications. Before going on to these visualizations, I want to recommend the Youtube channel MinutePhysics, which made a fantastic primer on the subject. I’ll link the first few of these here, as they might help with understanding the rest of the post. I highly recommend the entire series, even if you’re already pretty familiar with the subject.
Now, on to the pretty images! I’m still trying to determine whether it’s possible to embed applets in my posts, so that you can play with the program for yourself. Until I figure that out, GIFs will have to suffice.
Let me explain what’s going on in the image.
First of all, the vertical direction is time (up is the future, down is the past), and the horizontal direction is space (which is 1D for simplicity). What we’re looking at is the universe as described by an observer at a particular point in space and time. The point that this observer is at is right smack-dab in the center of the diagram, where the two black diagonal lines meet. These lines represent the observer’s light cone: the paths through spacetime that would be taken by beams of light emitted in either direction. And finally, the multicolored dots scattered in the upper quadrant represent other spacetime events in the observer’s future.
Now, what is being varied is the velocity of the observer. Again, keep in mind that the observer is not actually moving through time in this visualization. What is being shown is the way that other events would be arranged spatially and temporally if the observer had different velocities.
Take a second to reflect on how you would expect this diagram to look classically. Obviously the temporal positions of events would not depend upon your velocity. What about the spatial positions of events? Well, if you move to the right, events in your future and to the right of you should be nearer to you than they would be had you not been in motion. And similarly, events in your future left should be further to the left. We can easily visualize this by plugging in the classical Galilean transformation:
Just as we expected, time positions stay constant and spatial positions shift according to your velocity! Positive velocity (moving to the right) moves future events to the left, and negative velocity moves them to the right. Now, technically this image is wrong. I’ve kept the light paths constant, but even these would shift under the classical transformation. In reality we’d get something like this:
Of course, the empirical falsity of this prediction that the speed of light should vary according to your own velocity is what drove Einstein to formulate special relativity. Here’s what happens with just a few particles when we vary the velocity:
What I love about this is how you can see so many effects in one short gif. First of all, the speed of light stays constant. That’s a good sign! A constant speed of light is pretty much the whole point of special relativity. Secondly, and incredibly bizarrely, the temporal positions of objects depend on your velocity!! Objects to your future right don’t just get further away spatially when you move away from them, they also get further away temporally!
Another thing that you can see in this visualization is the relativity of simultaneity. When the velocity is zero, Red and Blue are at the same moment of time. But if our velocity is greater than zero, Red falls behind Blue in temporal order. And if we travel at a negative velocity (to the left), then we would observe Red as occurring after Blue in time. In fact, you can find a velocity that makes any two of these three points simultaneous!
This leads to the next observation we can make: The temporal order of events is relative! The orderings of events that you can observe include Red-Green-Blue, Green-Red-Blue, Green-Blue-Red, and Blue-Green-Red. See if you can spot them all!
This is probably the most bonkers consequence of special relativity. In general, we cannot say without ambiguity that Event A occurred before or after Event B. The notion of an objective temporal ordering of events simply must be discarded if we are to hold onto the observation of a constant speed of light.
Are there any constraints on the possible temporal orderings of events? Or does special relativity commit us to having to say that from some valid frames of reference, the basketball going through the net preceded the throwing of the ball? Well, notice that above we didn’t get all possible orders… in particular we didn’t have Red-Blue-Green or Blue-Red-Green. It turns out that in general, there are some constraints we can place on temporal orderings.
Just for fun, we can add in the future light cones of each of the three events:
Two things to notice: First, all three events are outside each others’ light cones. And second, no event ever crosses over into another event’s light cone. This makes some intuitive sense, and gives us a constant that will hold true in all reference frames: Events that are outside each others’ light cones from one perspective, are outside each others’ light cones from all perspectives. Same thing for events that are inside each others’ light cones.
Conceptually, events being inside each others’ light cones corresponds to them being in causal contact. So another way we can say this is that all observers will agree on what the possible causal relationships in the universe are. (For the purposes of this post, I’m completely disregarding the craziness that comes up when we consider quantum entanglement and “spooky action at a distance.”)
Now, is it ever possible for events in causal contact to switch temporal order upon a change in reference frame? Or, in other words, could effects precede their causes? Let’s look at a diagram in which one event is contained inside the light cone of another:
Looking at this visualization, it becomes quite obvious that this is just not possible! Blue is fully contained inside the future light cone of Red, and no matter what frame of reference we choose, it cannot escape this. Even though we haven’t formally proved it, I think that the visualization gives the beginnings of an intuition about why this is so. Let’s postulate this as another absolute truth: If Event A is contained within the light cone of Event B, all observers will agree on the temporal order of the two events. Or, in plainer language, there can be no controversy over whether a cause precedes its effects.
I’ll leave you with some pretty visualizations of hundreds of colorful events transforming as you change reference frames:
And finally, let’s trace out the set of possible space-time locations of each event.
Try to guess what geometric shape these paths are! (They’re not parabolas.) Hint.