Veritasium hauls an atomic clock up Mount Blue Sky to test Einstein
Lasers that chill atoms, an 80-kilometer beam of light and a clock flown in a helicopter all serve one question: does time on a mountain run exactly as Einstein said?
Veritasium
Story of No. 19 on the daily chartNo. 1 on the Science chart
Why this story? One of the most watched videos since yesterday, with 1,495,420 views.
No device could prove our theory of gravity wrong... until now
Things we can learn from the video
Press to jump
- 0:00The plan: compare a clock in Boulder with one 4,000 meters up, where Einstein says time runs faster.
- 0:24Jonathan Oppenheim explains that gravity's main everyday effect is slowing time, and why that challenges quantum mechanics.
- 7:23NIST physicists show why ytterbium optical clocks beat the cesium clocks that define the second.
- 17:07Laura Sinclair explains the 80-kilometer laser link from the mountain to Denver, then fiber to Boulder.
- 19:00In Western Australia, a team tracks a laser beacon on a helicopter carrying an optical clock.
- 32:07The researchers discuss whether the mountain measurement agrees with general relativity.
Two days after asking whether anything moves faster than light, Veritasium has gone back to Einstein. This time it brought a mountain.
The new documentary compares an atomic clock in a NIST lab in Boulder, Colorado, with a portable one in a tent at an observatory 4,000 meters up Mount Blue Sky. General relativity says the higher clock should tick slightly faster. Physicist Jonathan Oppenheim of University College London explains why that matters: "It's not as much that space is curving as that time is stretching." A clock precise enough to catch that stretch with no room for error could one day show where gravity and quantum mechanics part ways. About 1.5 million people watched it in its first nine hours.
Derek Muller sets up the question, and producers Gregor Čavlović and Emilia Gyles carry the film in three chapters. The Clock covers ytterbium atoms cooled by lasers to almost absolute zero and held in a lattice of light. The Climb covers the drive up the mountain and an 80-kilometer laser link to Denver. For the last stretch to Boulder the signal travels through fiber optic cable, the technology Veritasium explored a fortnight ago in a video we reported on here.
In Western Australia, Gyles watches a team beam a clock signal to a helicopter. "Certainly no one's put a clock on a helicopter before," says Sascha Schediwy. The same team also ran tests on Mount Fuji. The final chapter, The Count, uses tuning forks to explain the optical frequency comb, the invention that won Theodor Hänsch and John Hall the 2005 Nobel Prize, before Čavlović asks the researchers the title's question. The film also carries a sponsored NordVPN segment.
In the comments, a physics PhD student from one of the Boulder labs says it is surreal to see work from lab meetings become a Veritasium film. Former students of the University of Western Australia recognize the physics building roof where the helicopter was tracked. Other viewers point out that the video began life under a different title, one that promised a clock that might break physics.
Published on Trari
Where next?
What viewers say
"Cooling down atoms with lasers to a millionth of a degree above absolute zero, while simultaneously trapping the aforementioned floating atoms in one spot, all to measure time so accurately we may change the understanding of gravity" is not something I was prepared to hear today.
Science is freakin' amazing. This is exactly why I'm not ashamed to be called a nerd. Actually quite proud of it.@SingularisFoxSee it on YouTubeI’m a Physics PhD student in one of the collaborating labs in Boulder. Really surreal to see what I have heard updates on in meetings turn into a Veritasium video!!
@DarioTringaliSee it on YouTubeWhat’s incredible is not just that time passes differently depending on gravity, but that this difference has always been there. What changed wasn’t the universe, but our ability to measure it.
@FER2000-z3vSee it on YouTube
6 more comments
I just realised, if i want to tell someone who is absolutely out of touch, how an atomic clock works (not scientifically, just casually) i need to tell them at least 4 entirely different stories about 4 scientific areas, before i start to speak about clocks and how lasers are involved.
Science is fascinating.@rivájSee it on YouTubeThe fact that optical atomic clocks are precise enough to measure gravitational time dilation over tiny height differences is genuinely mind-blowing
@gysghost5126See it on YouTubeOppenheim is an incredible name for a physicist
@pacman1pSee it on YouTubeI did my honours thesis at the physics department at UWA, and I learnt to ride a unicycle on that roof.
@mostlyunreliableSee it on YouTube5:20 Fun fact: "Coordinated Universal Time" has the abbreviation UTC because the abbreviation isn't for the English words: It's for a compromise between the English abbreviation (CUT) and the French abbreviation (TUC for Temps Universel Coordonné)
@VAXHeadroomSee it on YouTubeatomic clock researchers be like: the answer to everything is lasers.
@YMandarinSee it on YouTube
Public comments from the video's page on YouTube
Is this your channel?For the channel's owners and team: a ready picture, text and badge for your audience
Share the story with your audience. Everything is ready:
For an Instagram story or a WhatsApp status
For a community post or your channel's description
For your website
You may use the picture, the badge, and link it to this story.
- Channel
- Veritasium
- Subscribers
- 21,400,000
- Channel views, last 7 days
- 19,014,367
- Channel views, pace per day
- 3,079,489 (on a normal day: 3,176,604)
- Video published
- October 23, 2026
- Video views so far
- 1,495,420 (less than a day online)
- Video views, pace per day
- 2,656,812
The numbers
For a channel of more than 21 million subscribers, the numbers show a documentary off to a quick start, in step with the channel's usual busy days.
💡 Test your knowledge
Which invention earned Theodor Hänsch and John Hall a share of the 2005 Nobel Prize in Physics?
Hänsch and Hall were honored for laser-based precision spectroscopy, including the optical frequency comb technique, which lets scientists count the oscillations of light.