[speeder]

Another way to think about it, is how time effects the object itself.

Photons are completely immutable, while they travel they don't change at all, if a photon was a "smergsboard" it would remain "smergsboard" during the whole trip.

One of the most interesting ways I saw explaining this, is imagine 'spacetime' as a cartesian space.

You have 4 axis, X, Y, Z and time.

EVERYTHING has speed of 'c', so you use trigonometry and rotations to figure the values, light, that have a speed of 'c' in the 3 space axis, then obviously have speed of '0' in time axis.

----

Now, one interesting application of that knowledge is how they figured the speed of neutrinos... As I just wrote, if something is travelling at speed of light, it is 'frozen', never changing...

But 10 years or so ago people figured that neutrinos change mid-flight, there are 3 (or more... people are unsure yet) 'flavors' of neutrinos, and during tests people noticed that even if you make a machine that generates only one specific flavor, what reaches on the other side is not necessarily that flavor, meaning they changed mid-flight...

But if they change, then they have some speed in 'time', this means then that the speed in space must be smaller than light.

Right now there are couple experiments where people are trying to use the changes in neutrinos to calculate their speed in 'time', and then by elimination figure their speed in space. I find it quite interesting, how people can use math to figure physics when our instruments aren't precise enough.

[platz]

photons don't freeze in time in their own reference frame, and one doesnt get to priviledge any particular reference frame including those that are different from the photon's

[cygx]

The (proper) time interval between any two points along a null geodesic is zero. I see nothing wrong with calling photons frozen in time.

[platz]

no,the "Spacetime Interval" along a null geodesic is 0, but a null geodesic "does not have a Proper Time associated with it". Undefined is not the same as 0. "For lightlike paths, there exists no concept of proper time and it is undefined as the spacetime interval is identically zero. "

[cygx]

Tomayto, tomahto. You can't parametrize a null geodesic by proper time, sure, but I don't see anything particularly wrong with calling the arc 'length'/spacetime interval between events along any particle trajectory a proper time, even when it's 0.

[platz]

https://www.reddit.com/r/askscience/wiki/physics/light_frame...

    (Δs)2 = 0

        It is wrong to then equate this to some sort of proper time Δτ, find that

    Δτ = 0

    for a photon, and thus conclude that photons always "experience" zero proper time. No. Remember that Δτ is defined as the 
    time difference between two events as measured by an observer (i.e., an inertial frame) that actually travels between the events. Photons 
    have no reference frames! So the definition of Δτ doesn't apply to null paths.

[cygx]

I disagree: While you cannot have a clock travelling alongside the photon, you can have a family of clocks travelling along light-like trajectories that have the photon trajectory as its limit.

But as I was alluding to with my 'tomayto, tomahto' remark, this is a question of semantics, and we're not really arguing about physics, but labels.

[platz]

actually i'm reading now that photons do not even have their own reference frame simply by definition/axiom. Interesting..

[sscarduzio]

I wish HN had "reddit gold". Thanks for jotting this down for us, super clear and interesting.