Friday, December 3, 2010

Light Does Not “Race” Through Space (5/14/2010)

This was originally posted on a horrible site called Myspace. When Myspace underwent a redesign in Fall 2010, hundreds of insightful reader comments that had been left over the years were lost. I have since deleted my account there.

When it comes to the light from distant objects, most people think of the Universe as a giant cosmic shooting gallery: There’s a star over here, a star over there, maybe a galaxy that way, all spitting out tiny packets of light called “photons.” These photons careen through space at an amazing speed, we are told, and a few of them reach Earth after their lengthy travels, allowing us to see the stars and galaxies they came from. Even prominent science writers sometimes describe a photon “racing along” for billions of years, only to go splat against someone’s retina or a photographic plate on a telescope. This isn’t too surprising; things in our everyday experience go fast and go splat, so depicting bits of light like this makes intuitive sense.

But it’s just wrong. The “common sense” view of the Universe as a shooting gallery of light — while somewhat easy to grasp — has been out of date for over 100 years. It’s as incorrect as saying that life on Earth appeared fully formed from the Creator’s hand within the span of a week. Yes, creation is easier to grasp than evolution by natural selection. But just because something is easy to comprehend doesn’t mean it resembles the truth.

Photons cannot be said to “race,” “speed,” or “careen” through space like bullets, in any manner at all. They may seem to race, and we may get “splatted” by them — but they do not, themselves, race. In 1905, Albert Einstein showed that as an object’s speed through its environment goes up, its relationship with that environment changes: distances become shorter (a phenomenon known as Lorentz contraction), and durations of time also become shorter (known as time dilation). Special relativity turns the speed of light into a kind of “cosmic speed limit”: Nothing can go faster than that, because for anything that travels at that speed, the distances traveled contract to exactly zero, and the duration of travel similarly contracts to exactly zero. No time or distance is “experienced” by a photon, ever. It is therefore wrong to say that a photon races anywhere (“races” being an intransitive verb describing what it, itself, does), or that it “spent five billion years traveling” through space. Such anthropomorphisms are our own invention; they don’t reflect the photon’s reality, as defined by special relativity.

If a photon isn’t a little particle flying at a terrific speed, what is it, then? Here’s where it gets odd: A photon’s path is a line that connects all points in space and time that are equivalent as far as relativity is concerned. Starting from any point on the line, 186,000 miles away from that point, the cosmic clock is one second earlier. So, the line could connect the following equivalent points in space and time: Your eyeball right now, the Moon 1.2 seconds ago, the Sun 8 minutes ago, the star Sirius 8.6 years ago, and the Andromeda Galaxy 2.5 million years ago. A photon “traveling through space” could cross all five of these points. Except that it isn’t traveling. It’s a dimensionless “thing” which an observer at one of the similarly correlated points in familiar space and time will encounter. Because in the bigger picture of relativistic spacetime, there’s no difference between these points at all.

Photons are objects which all observers seem to witness moving — they are seen at different places in space at different times — but which don’t, themselves, travel anywhere or experience any time. This has profound implications for the way we view the Universe as a whole, but that’s a topic for another time. Suffice it to say, we should abandon the archaic idea that particles of light race ontologically through space for billions of years. They don’t.


  1. This is a very good article, Mr. Current. I think that you should do a Youtube video about this article.

  2. It is NOT wrong to say that a photon spent five billion years traveling between points A and B. Everything depends on the frame of reference. I am not a photon and I'm not travelling at the speed of light. Looking at it from my frame of reference it really did take the photon five billion years to get from A to B ...

  3. @Anonymous -- Ah, we see the problem when we try to describe relativistic things in ordinary intuitive language. It is inaccurate to describe a photon's ontology using intransitive verbs like "race" and "spent," when we are making the description from our non-photon reference frame. If I just watched someone take off for Alpha Centauri at 99.9% the speed of light, I wouldn't say they will be "racing along for years" before they get there...would you? For them it's a much quicker trip.

    When we try to describe a photon using the same words we use for everything else, it becomes an ordinary but really fast object that experiences proper time and distance, just like a bullet. But that is a fundamentally inaccurate description. It also disregards the fact that in special relativity, spacetime for the photon has an effective equivalence of 300,000 kilometers per second, similar to the way we have a length equivalence of 2.54 centimeters per inch. I think that's pretty fascinating, and that's why I wrote about it.

  4. @Eddie
    I think it would take an X-Amount of time for you, the observer. But for them, the travelers, the perspective of time and space is completely different at those speeds. They may in fact go from point A to point B a lot sooner than what they had calculated it would take.

  5. Yes, except a "photon" is a so called packet of light that acts as both a wave and a particle, and oscillating as such does, in fact, move. Moving is motion, which requires speed, traveling and time, all of which (the motion, speed, traveling and time) are really one and the same. A photon also appears to have the quantum property of being able to be in two or more places at the same time as it moves along its path. I would almost argue that a photon's version of traveling is more of a type or phase of existence that involves a form of motion that is unique to its own state.

    You said that to a photon it would experience no time elapsing in its travels due to time compression (dilation), the problem is that this appears to be a property that applies on an atomic level. Light, however, is at minimum subatomic and would not be affected by time compression at its native speed the same way we would as a normal physical traveler. If you were going to contend that the traveler were a "sentient photon", it's experience of time would not be zero because it seems to be a quantum level particle and is not necessarily affected by normal atomic time compression that normal/macro sized physical matter would experience at such speeds.

    Your comments are insightful and food for thought, but your comments and the ones to which you are rebutting are neither completely wrong or right to say, other the than zero-time experienced comment regarding the perspective of a sentient photon is flatly wrong because it does not suffer from normal atomic compression (because it is subatomic) the way we (as normal people) would.

    My biggest personal issue with modern physics is there lackadaisically inept inability to define exactly what constitutes "space" and the possibility that other "space types" exist and need to be defined and categorised properly. One must define the terms and definitions of space(s) before one continues with trying to expand one's understanding of the universe and to help others also understand it as well.

    ~ D.E.

  6. First, technically the speed of light isn't actually a "speed limit," unless you are only talking about baryonic matter, because it requires an infinite amount of energy to cross that speed. Nothing in special relativity prevents the existence of "tachyonic matter, which *only* travels above the speed of light. Tachyons have interesting properties; they have imaginary (as in square-root-of-negative-one imaginary) mass, and they speed up when they lose energy, and approach the speed of light (from the upper end) when you add energy; again, an infinite amount of energy is required to cross the border.

    Second, the "racing" analogy is actually fairly good (bar weird quantum stuff happening) from the perspective of an observer. From the perspective of a photon, this analogy is nonsense, but the bottom line in all physics, from classical mechanics on, is to give the perspective, or reference frame, from which you make your measurements. It's just like how centrifugal force is nonsense from the perspective of the center of rotation, because acceleration is always toward the center, not away from it. However, from the perspective of someone who's rotating, it's the opposite: the acceleration is always away from the center, and the idea of centripetal force becomes nonsense.

    Also, I think you've got it exactly backwards. There is nothing particularly special about light; it's the speed that's important. The only reason light travels at that speed (at least, in a perfect vacuum) is because it has no mass. Gravity is the same way: if the sun were to suddenly disappear, it would take about eight minutes before we were thrown out of orbit, because we are eight light-minutes away from the sun. So it's not "light" that connects those points in space-time, it's what we call the "light cone."

  7. Ian -- Unfortunately, like many commenters on the previous incarnation of this blog, the (admittedly subtle) point of this essay seems to have eluded you. I am not trying to change anything about special relativity; it is all about the words we tend to throw around. "Race" is an intransitive verb, and in the English language, intransitive verbs express action from the perspective of the actor. When I say "I walk down the street," the verb "walk" is expressing the experience of me, the walker, traveling through space and time. Similarly, if someone writes that "a photon races through space," they are describing the experience of the photon, traveling through space and time. Except the photon doesn't have such an experience. It doesn't experience proper time, nor does it experience the travel of distance. In fact it cannot have a perspective or reference frame at all. So to assign an intransitive verb to it, as if it were ordinary object like a person or a bullet, is false.

    In describing light, to properly include the reference frame from which you take your measurements (as you put it), one would have to say "the photon is seen racing" or "is considered to race." To merely say the photon "races" is wrong, an oversimplification. It's a bit like saying that almost all cosmologists believe that the universe was created in the Big Bang. That sentence is roughly accurate, but the wording implies that a creator performed the action, and I doubt almost all cosmologists believe in a creator. So technically it is wrong.

    I disagree that there's nothing special about light waves, or gravity waves for that matter. Anything that is massless and travels at c, which cannot be assigned a reference frame, and which connects the points along null surfaces of spacetime -- including gravity waves/gravitons -- is different from the rest of the universe. In other words, gravity waves cannot be said to race through space, either.

  8. Let me just say that I hardly ever enjoyed reading on a blog like here. The discussions and the tone are civilized, well written and interesting. As I understand it Eddie is making a point that refers to incorrect grammatical use of the intransitive verb "race". Since quantum physics are so hard to understand I think it helps to correct these small misconceptions.I personally found this article helped me understand a bit more about the issue. Thank you

  9. Thanks tröpfel. I'd also point out that light does not exist in "particle" form unless it is interacting with something. A quantum of light is a bit like an expanding soap bubble that bursts upon encountering something, leaving a "droplet" of energy that we find in one specific location, which is what we call a photon or particle of light. The idea that this "droplet" experiences the act of racing through space at light speed, independent of any such interactions, is inconsistent with both special relativity and quantum mechanics. See also my post The Case for Metaphysics.

  10. I'm curious, do you actually have much background in Physics because it seems to me that you are more of a philosopher than a physicist ?

  11. I am not a physicist, just a writer who likes to write about science. I studied physics in college as part of a biology major and have kept up with it since then. You're right that this essay is somewhat philosophical in nature, in that it addresses the language that is typically used to convey concepts surrounding photons and the speed of light. However, I contend that my point is consistent with the prevailing understanding of special relativity, and that the words we use are often erroneous or misleading.

  12. Great article. Illuminating, I would say...

  13. 8/14/11: On there was a recent blog post addressing this issue. William Orem makes a similar argument about photons, extending the discussion to the Higgs boson: If the Higgs boson is responsible for mass in the universe, then it must also be responsible for time -- time could not exist without this particle.