I’ve thought about this question for years. A FAQ. Imagine you’re traveling at the speed of light. Well, physics says that’s impossible. Mass’ gotcha. So, what can travel at the speed of light? Photons (not just the visible ones). So, imagine … does a photon “experience” space and time?
Some weeks ago I recall reading that photons do not connect with space or time. Maybe some other term than “connect.” Perhaps something written by Carlo Rovelli, but I cannot find the citation. Anyway, so I reacted when I ran across a Forbes article by Ethan Siegel  titled “Ask Ethan: How Does A Photon Experience The Universe?“ (December 22, 2018).
There’s a lot written on this topic (and likely covered in physics classes as well). Siegel’s article is a good summary of the material.
… relativity declares that the laws of physics are the same and invariant for all observers, regardless of their motion. So what does this mean for a photon, which itself moves at the speed of light?
Siegel addresses the question by characterizing three scenarios:
Let’s imagine what happens in three cases: for someone at rest, for someone moving close to the speed of light, and then that last leap, to a photon itself.
Case 1: Rest frame. The everyday case. In your surroundings, objects’ shapes and colors appear “normal” and any clocks in that scene appear to be running at the same speed as your smartphone’s (although Rovelli says this is only a blurred perspective, as the speed of time varies from point to point in space ).
Case 2: Relativistic frame. Typically a hypothetical case, like looking at the cityscape from inside an ultra fast railroad car. In your particular direction of motion, for things outside your frame, shapes (lengths) of objects appear contracted and clocks appear to run slower (time dilates). The color of objects directly ahead of you appear bluer and those behind you appear redder.
Things move through spacetime. Basically spacetime always balances to maintain an equality: the less motion in space, the more in time; the more motion in space, the less in time.
The effects on things become more and more pronounced — to an unreachable limit. Capture that with your smartphone, eh.
Case 3: Photon’s frame. Imagine you’re a photon. 100% motion in space. 100% contracted. 0% in time. No time, eh. Special relativity out the window. Mathematical infinities.
[All a photon] experiences are two “things” during its existence: the interaction that creates it and the interaction that destroys it. Whether there is a photon that persists after the destruction, such as via scattering or reflection, is immaterial. All that a photon experiences are those two events at the endpoints of the photon’s journey.
From an inertial frame of reference, we can calculate the distance between its emission and absorption point, but not from the photon’s reference frame. We can calculate its light-travel time, from any inertial reference frame, but not from the photon’s reference frame.
The problem is that the photon’s reference frame isn’t an inertial reference frame: In an inertial reference frame, there are physical laws which don’t depend on the motion of anything external to the system. Yet for a photon, the physical rules it obeys depend exclusively on everything going on external to it. You cannot calculate anything meaningful for it from the photon’s reference frame alone.
… rest mass is what’s required to live in an inertial reference frame … A photon cannot see the Universe at all, because seeing requires interacting with other particles, antiparticles, or photons, and once such an interaction occurs, that photon’s journey is now over.
According to any photon, its existence is instantaneous. It comes into existence with an interaction and it winks out of existence with another interaction. This could be emission from a distant star or galaxy and its arrival at your eye, and it doesn’t matter whether it’s from our own Sun or an object tens of billions of light years away. When you move at the speed of light, time ceases to pass, and your lifetime only lasts an instant.
Photons do not have a rest frame. Distance is irrelevant, whether within an atom or between galaxies. Existentialism out the window, eh.
 Using a constant proper time reference across an event which spans space is an approximation, eh.
Times are legion: a different one for every point in space. — Rovelli, Carlo. The Order of Time (p. 16). Penguin Publishing Group. Kindle Edition.
 Ethan Siegel is a Ph.D. astrophysicist, author, and science communicator, who professes physics and astronomy at various colleges. He has won numerous awards for science writing since 2008 for his blog, Starts With A Bang, including the award for best science blog by the Institute of Physics. His two books, Treknology: The Science of Star Trek from Tricorders to Warp Drive, Beyond the Galaxy: How humanity looked beyond our Milky Way and discovered the entire Universe, are available for purchase at Amazon.
 Examples of other articles (Google Search “how a photon experiences space and time”)
7 thoughts on “A photon’s frame of spacetime — no rest for the massless”
Here’s an interesting take on the speed of light, in an unexpected place: Business Insider, “The speed of light is torturously slow, and these 3 simple animations by a scientist at NASA prove it” (January 17, 2019).
I’m always glad to see visualization used in communicating science.
Check out the videos of his animations. Tick-tock, ping-pong.
How fast light travels relative to Earth
How fast light travels between Earth and the moon
How fast light travels between Earth and Mars
And here’s another Space.com article by Paul Sutter on the topic: “Does Light Experience Time? And Other Riddles” (March 16, 2019).
My posts Imaging a light pulse? and A photon’s frame of spacetime — no rest for the massless discussed the landscape of light speed. Live Science recently posted an article “Here’s What the Speed of Light Looks Like in Slow Motion” (March 29, 2019) about a visit to the Caltech lab by The Slow Mo Guys where light pulses can be visualized at 10 trillion frames per second. The article includes Gav and Dan’s video “Filming the Speed of Light at 10 Trillion FPS” — a quick overview of the complex equipment and visualization, each of which involved hours of processing — far from instant photography.
The Science Asylum’s YouTube video “Where Does Light Come From? (Electrodynamics)” (published on Apr 15, 2019) is an excellent overview (visualization) of classical electrodynamics.
Quote from video:
So, I’m looking for a quantum explanation of these EM laws:
Vocabulary: Field arrows (vectors)
So, this is a question about photons that’s always fascinated me: Can photons interact? This Live Science article “Inside Giant Atom Smasher, Physicists See the Impossible: Light Interacting with Light” by Paul Sutter (April 25, 2019) discusses some recent experiments by the ATLAS collaboration at the Large Hadron Collider.
Well, I don’t know about that. The arxiv.org PDF paper discusses the experiments for 7 pages and then has 21 pages of acknowledgments, references, and the ATLAS Collaboration list. It’s a fascinating and quite technical presentation of the selection criteria for detection / photon reconstruction and the methods used to exclude background and other signatures. One has to trust the “high priests” of physics on that score.
The paper also references two other processes in which photon-by-photon scattering has been measured.
In fact, I recall reading here that photons can interact in the presence of (nearby) atomic matter.
So, it’s not like two ultra pure (coherent single frequency) laser beams (in an ultra clean vacuum cavity) were directed at each other and a few photons interacted / scattered — something that might be simpler to visualize.
Wiki: Two-photon physics
 Wiki: In technical terms, QED can be described as a perturbation theory of the electromagnetic quantum vacuum. Richard Feynman called it “the jewel of physics” for its extremely accurate predictions of quantities like the anomalous magnetic moment of the electron and the Lamb shift of the energy levels of hydrogen.
Sean Carroll answered some questions about light in this recent video.
YouTube > Think Inc. > “‘The Speed Of Light’ Is Wrong! Ft. Sean Carroll | Think Inc.” (June 29, 2020) – [Caltech physics] professor Sean Carroll dives into why “The Speed of Light” has stuck around for so long when it should really be called “The Speed of Causality” [and] why we use meters and seconds when measuring the speed of light, and much more!
I was glad Carroll talked about the pitfalls of explaining the twins paradox scenario:
How did the fact that light has a speed arise? Something that (hopefully) all of us take for granted. Evidently in ancient times the question was debated .
So, when was the speed of light first quantitatively measured? The credit goes to astronomer Ole Rømer in 1676. But, as Wiki notes, the “theory” was controversial, a saga about more than just collecting data and calculation.
In 1865 Maxwell’s theory of electromagnetism framed light as an electromagnetic wave, traveling at the associated speed. But how likely is a Jay Leno-like “Jaywalking” survey to find that commonly understood?
Paul Sutter discusses the more modern frame of reference in this article:
Space.com > “Why is the speed of light the way it is?” by Paul Sutter (July 16,2020) – Why does the speed of light have the value 299,792,458 meters per second? Why isn’t it some other number? And why do we care so much about some random speed of electromagnetic waves? Why did it become such a cornerstone of physics?
Then, because units of measure are arbitrary, Sutter considers how we might explain the speed of light to an alien civilization.
 In contrast to the ancient consensus about sound as a movement of air; and any movement requiring time. Yet without our modern frame of reference of speed (or waves).
Stack Exchange > History of Science and Mathematics > “When was it first realized that sound travels with finite speed?”
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