“Everyone has his dream; I would like to live till dawn, but I know I have less than three hours left. It will be night, but no matter. Dying is simple. It does not take daylight. So be it: I will die by starlight.” -Victor Hugo

Whether you're at rest or in motion, you can be confident that -- from *your* point of view -- the laws of physics will behave exactly the same no matter how quickly you're moving. You can move slowly, quickly or not at all, up to the limits that the Universe imposes on you: the speed of light.

But what if you're actually a *photon*? What if you don't move near the speed of light, but *at* the speed of light? As it turns out, the way any massless particle experiences time, distance, and the Universe in general is entirely counterintuitive, and there's nothing in our common experience that matches up.

How does it all work? Find out on this week's Ask Ethan!

- Log in to post comments

Ethan baby, I saw you on a news programme. There's an energy similarity with Tom Cruise. Maybe Cruise meets Ming the Merciless he he

Speed of light is slower in water for example.

So when a particle is passing through water, relativistic effects (time dilation, relativistic mass) must be calculated using speed of light in water or still speed of light in vacuum?

From its own perspective, a photon can hardly be said to exist at all.

I'm not so sure this is right. The photon oscillates as it travels. Thus if one were 'riding on' the photon, one would be able to observe a sequence of changes (oscillations) in the internal state of the photon during the passage...and the

numberof oscillations would be related to distance. Red shift caused by the universe's expansion would be another 'internal clock' (albeit a very slow one): a photon rider could hypothetically know how far they've traveled by measuring the frequency change during the traverse.I believe in an earlier thread I made basically the same argument that Ethan makes in his article, and got corrected by someone. I'm hazy on the details but IIRC the correction was along the lines of: Lorentz factors apply to objects with mass. They do not predict the behavior of massless c-traveling particles. For those, you use other equations, and these other equations allow for photons to change over (externally observed) time. Whether that means they 'experience' time is a somewhat anthropomorphically-tinged question, but in terms of physics, they can have time-dependent reactions with the outside world even as they travel at c.

My understanding is that the speed of light is slower in various materials because the photons are being scattered off the atoms of the material, not because the photons themselves are moving any slower. If you had to adjust relativistic effects to fit the slower speed of light in water, then you'd never see Cherenkov radiation because charged particles could never exceed the speed of light through water.

@Frank #2: The equations of special relativity use a fixed constant, which we label 'c'. The value of that constant is equal to the speed of light in vacuum (exactly 299 792 458 m/s). In transparent materials, electromagnetic waves travel more slowly than 'c' because the electrical and magnetic properties of the material affect the oscillations (you can derive this from Maxwell's equations in matter, where the permittivity and permeability enter).

Charged particles can travel faster than the speed of light in transparent materials, just as a bullet or aircraft can travel faster than the speed of sound in air. When this happens, the charged particle will radiate a cone of EM radiation, peaking in the violet/UV, analogous to the conical shock wave of a sonic boom.

@eric #4: You wrote, "The photon oscillates as it travels. Thus if one were ‘riding on’ the photon, one would be able to observe a sequence of changes (oscillations) in the internal state of the photon during the passage..." In fact, this is not only wrong, it is exactly the visualization which inspired Einstein to develop special relativity in the first place!

What you describe, and what Einstein visualized, was electric and magnetic fields oscillating in place, without travelling as a wave. That situation explicitly violates Maxwell's equations. It is, to put it succinctly, impossible.

If you travel along with the photon, the electric and magnetic fields in that frame have to be "static," in order to satisfy the laws of electrodynamics. In the context of relativity, this is exactly the situation Ethan laid out -- time is "frozen" and distances contract to "zero."

@eric #4 continued: You also wrote, " For those, you use other equations, and these other equations allow for photons to change over (externally observed) time. Whether that means they ‘experience’ time is a somewhat anthropomorphically-tinged question, but in terms of physics, they can have time-dependent reactions with the outside world even as they travel at c."

Here you're not exactly, wrong, but rather misled. What you are describing is, as you say yourself,

externally observedtime. You're not addressing the photon's internal frame of reference at all. Yes, photons can have time-dependent interactions in the frame of an outside observer."So when a particle is passing through water, relativistic effects (time dilation, relativistic mass) must be calculated using speed of light in water or still speed of light in vacuum?"

No, Frank. When light passes through a medium, it passes by making the electrons (or other charged bits) wobble in sympathy, which absorbs the photon and then re-emits it in tangent (a bit like how a laser works). The mass of the electron (modified for being bound to a bigger body, hence if it moves, the entire thing moves a bit too) means that there is a delay as the electron gets moved so that it can create the continuing wave, and this delay slows down the arrival of the photon at the destination, appearing to slow it down.

This is why things can move faster than the speed of light in a medium and you get cherenkov radiation from that happening.

"I’m not so sure this is right. The photon oscillates as it travels. Thus if one were ‘riding on’ the photon, one would be able to observe a sequence of changes (oscillations) in the internal state of the photon during the passage"

If you're riding on the photon, you see the same size electric field. That level of E moves at light speed in the same direction as the photon. You are sitting on the same spot and moving in the same direction at the same speed. So you see no change in the electric field. No oscillation.

What about if it is all the other way around, what about we don't travel through time and space, what about if the only constant thing is consciousness and time and space travel through us... What about if consciousness is the element that has been overlooked or missing in our physics? That would change a lot of things!

Well...if light experiences everything collapsed to a 1 dimension dot, then a photon of light is perceived coming from all directions.

"What about if it is all the other way around,..."

And what about if it isn't all the other way around, hmm? What if it were all jam? What if teddy bears had picnics? Would ants have pants?

Why would it "be" the other way round, in any sense other than sophistry? It our description of reality has time going on, things being separated by space and events separated by time and space, why would we invert it all just to pretend of a different way of explaining it?

If you're going to say "What if...", there are an infinite number of "otherwise"'s to do, so you need to ask yourself why anyone would pick the infinitely smaller set of one otherwise you picked out?

I thought the red shift is light stretching wavelength as it accelerates away from us the observer. And space is of course a remarkable phenomenon. There is a phase-state associated with it [Barrow] - as to whether it stretches is an interesting idea-theorem-hypothesis- as it has been suggested that there may be actually holes in space which is the real 0. Absolute [if that is possible]. But after hyper-inflation space expanded exponentially ahead of light itself-which means space traveled faster than light! yipee.

@eric #4, @Michael Kelsey #7+#8, @Wow #10

If time is frozen in the frame of a photon, the electromagnetic field must be frozen (static) as well. Maybe the frequency of photon is not a property of the photon itself, but a property of a photon moving through spacetime: if, from our POV, we imagine the photon as a particle moving through space, it creates an electromagnetic disturbance in spacetime. The more energetic the photon, the more 'sudden' the disturbance, so the higher the frequency we experience?

Another question is if space really only contracts in the direction of motion. When the photon leaves from a very distant galaxy and instantanuously hits earth from its point of view, earth may have moved an incredible distance orthogonal to the direction of motion in that instant. How can the photon 'explain' this to have happened in zero time? To me, this seems only possible if the whole universe is a dimensionless single point from the point of view of the photon?

"If time is frozen in the frame of a photon, the electromagnetic field must be frozen (static) as well."

Since time doesn't exist, it isn't FROZEN, you can't tell what is static, since there's no duration to show it is static.

Indeed this isn't the case, anyway, since the photon is the result of an oscillating charge and its reception end point is where another charge is made to oscillate.

This oscillation still occurs, there's just no distance between them.

The problems here, like in every attempt to put *someone* "on a thingy" that isn't doing classical mechanics things is that you are inherently mixing up frames and assumptions of reality that don't necessarily apply.

So the claims of the E field being the same as you travel along with the photon DO NOT mean that the field is static, since from the event view of the lightlike observer, we shouldn't be talking about "travelling along".

Hence the statement is made in a VERY *VERY* restricted domiain: that of the E field at a point moving at light speed. NOTHING about how it is moving in time.

"I thought the red shift is light stretching wavelength as it accelerates away from us the observer"

In geometric GR, it's more that the wavelength is written on a band of elastic and this elastic is then stretched when you bring it "out" to the distant observer, whose same elastic band hasn't been stretched, so the "unit length" is the same length as the stretched band was before being stretched.

But having now been stretched, the unit length occupies a subset of the stretched band.

Or red shitted to longer wavelengths.

Just because I don't feel like doing the math (and would scarcely know where to begin anymore); how much does a neutrino age on its journey? Say, one of the neutrinos from the 1987A supernova.

A bit. Depends on the energy of it, and an entire spectrum of them were emitted, and the mass of the neutrino, which we only have an upper bound on.

The maths isn't hard, though, just plug in the values you figure apply and it's a simple equation for anyone who is leaving their teenage years.

I just don't feel like doing the maths for you.

If photons had consciousness, to follow the conceit of this post, I think it would find itself in a constant state of surprise. For example, a photon emitted 12 billion years ago headed in a particular direction would simultaneously experience the entire path of its course, but at the moment it set out, the universe had not evolved that course. Therefore, when it is absorbed by the earth-orbiting telescope that didn't exist when it set out, that must have been a real shock for the photon. "Oops, where'd that come from? Wasn't expecting that."

A photon leaving the nest might well say its goodbyes to friends and family, "I'm striking out in this direction." To which the sophomoric friend-photon might reply, "Not a good idea, there's a planet in your way." The departing photon would answer, ''Don't worry, it'll be gone by the time I get there," and it may have been correct, but unfortunately it's path got altered by some gravitational well it didn't expect and it crashes (seemingly instantly) into something else it had no idea was there. Meanwhile, the father-photon is wondering why his insurance rates go up at an astronomical pace.

I am not sure this is correct:

"[The photon] simply is emitted and then instantaneously is absorbed, experiencing the entirety of its travels through space in literally no time."

The classic coupling of a far field of traveling photons to an antenna through a transition region and a near field is not instantaneous. [ https://en.wikipedia.org/wiki/Near_and_far_field ] I haven't studied QFT so have to rely on this naive illustration: "near-field effects are due to a mixture of real and virtual photons." And as Matt Strassler describes it, virtual particles are not particles at all. [ http://profmattstrassler.com/articles-and-posts/particle-physics-basics… ]

So I assume photons experience instantaneous dispersal into vanishing "virtual particle" ripple modes, but the whole absorption process is somewhat time consuming. (I should estimate the time it takes for a signal to couple out through the near field and check if it is slower than the ultimate speed limit. Ironically _I_ am on a time limit...)