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Imagine you're at a party.
All your friends are crammed into a room, you're having a blast – but it's time to head home.
You start to make your way to the door, but every few steps you have to stop for just one more goodbye chat. One friend wants a hug. Another needs to tell you about a band she's discovered. By the time you're free, it's taken you an hour to get from the couch to the door.
Welcome to the life of energy inside the Sun.
It famously takes sunlight just over 8 minutes to travel the 150 million kilometers (93 million miles) from the Sun to Earth.
Before it can get there, though, the energy produced in the Sun's core has to fight its way through the Sun's densely packed interior.
It's repeatedly passed from particle to particle so many times that traveling the 695,700-kilometer distance from the Sun's center to its surface can take around 170,000 years.
That's according to a 1992 paper by astrophysicists Romas Mitalas and Kenneth R. Sills of the University of Western Ontario.
The pair had noticed that an assumption used to calculate the travel time of energy out of the Sun was incorrect.
It would be easy to imagine the energy simply streaming straight out from the core, but that would also be very wrong. A photon traveling in a straight line would take about 2.3 seconds to travel from the core to the surface.
Instead, it takes something physicists delightfully refer to as a "random walk".
In the dense plasma in the solar interior, a gamma-ray photon can only travel a short distance before interacting with a charged particle. It may be absorbed and re-emitted, or scattered off in a new, random direction.
That short distance – known as the step length – is the key ingredient in calculating how long sunlight takes to escape the Sun.
Previously, scientists assumed that the step length was roughly constant throughout the Sun's interior, and based their calculations on an estimated value of 0.5 to 1 centimeter. That gives a diffusion time of 3,000 to 30,000 years.
The problem with that, according to Mitalas and Sills, was that it did not accurately account for the much higher density of the star's inner layers.
OK, so we're back at the party.
You've left the house and walk to your car parked on the street.
A scientist sees only that final part of your journey. Watching from across the street, they measure how long it takes you to walk from the front door to your car, then use that speed to estimate how long it must have taken you to get from the couch to the door.
That scientist has not accounted for all the friends who stopped you along the way.
When the researchers recalculated the step length using a realistic model of the Sun's changing density, they discovered that it would be less than 0.1 centimeters for more than 50 percent of the Sun's radius.
That tiny correction stretched the Sun's photon diffusion time from tens of thousands of years to around 170,000 years.
Related: A Hidden Shift Inside The Sun Could Help Explain Weak Solar Cycles
Once you've finally reached your car, getting home is easy. You simply drive away.
That's kind of like what happens to sunlight.
Once the energy finally reaches the Sun's surface, it no longer has billions of particles blocking its path. It races across the vacuum of space, covering the remaining 150 million kilometers to Earth in just over 8 minutes.
So, next time you raise your face to the warmth of the Sun, spare a thought for how long that energy has spent getting to you.
It likely began its journey before the ancient Egyptians began building the pyramids – and maybe even before the start of the last Ice Age.
This article was fact-checked by Michael Irving and edited by Rebecca Dyer. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.
