Two atoms
Posted: Tue Sep 08, 2026 11:59 am
This is a little thought experiment I came up with years ago which popped into my head for reasons I can't explain; quantum fluctuation perhaps. Anyway, it's a way to get your head around time dilation. In relativity, it's all tied up with the geometry of spacetime; this is simpler.
Imagine two atoms, one directly above the other. In this simple set up, for something to happen, a photon has to pass from one atom to the other.
Suppose the atoms are stationary, the photon takes the shortest route and travels straight up, or straight down.
Now imagine the two atoms start moving in parallel paths with equal velocity; two atoms, one above the other moving left or right.
If the photon goes straight up or straight down, it will miss, because the target atom has moved.
To hit the target atom, the photon has to take a diagonal path, which is longer than straight up or down.
In both cases the photon travels at the speed of light.
Therefore it takes more time if the photons are moving.
That's just a different take on Einstein's light clock thought experiment, to be honest, but I think it can be taken further. One of the main claims of relativity is that, if you are travelling in the same direction and speed as the two atoms, no matter how fast you are going, you will perceive the photon as taking the same time to travel from one atom to the other. It will look as if the photon is travelling straight up or down and the amount of time you measure it takes will be exactly the same as if you and the atoms were stationary. But we know that diagonal paths take longer than straight up or down. Why then can't we tell that it takes longer? Well, every atom that is travelling in the same direction and velocity as the two atoms is affected the same way, including those that make up your brain. Your thinking, and aging for that matter, takes exactly the same amount of extra time as the interaction of the two atoms.
Imagine two atoms, one directly above the other. In this simple set up, for something to happen, a photon has to pass from one atom to the other.
Suppose the atoms are stationary, the photon takes the shortest route and travels straight up, or straight down.
Now imagine the two atoms start moving in parallel paths with equal velocity; two atoms, one above the other moving left or right.
If the photon goes straight up or straight down, it will miss, because the target atom has moved.
To hit the target atom, the photon has to take a diagonal path, which is longer than straight up or down.
In both cases the photon travels at the speed of light.
Therefore it takes more time if the photons are moving.
That's just a different take on Einstein's light clock thought experiment, to be honest, but I think it can be taken further. One of the main claims of relativity is that, if you are travelling in the same direction and speed as the two atoms, no matter how fast you are going, you will perceive the photon as taking the same time to travel from one atom to the other. It will look as if the photon is travelling straight up or down and the amount of time you measure it takes will be exactly the same as if you and the atoms were stationary. But we know that diagonal paths take longer than straight up or down. Why then can't we tell that it takes longer? Well, every atom that is travelling in the same direction and velocity as the two atoms is affected the same way, including those that make up your brain. Your thinking, and aging for that matter, takes exactly the same amount of extra time as the interaction of the two atoms.