[Swizec]

> Electromagnetic waves have no mass, they don't travel in time, so the entire portion of their travel takes place in space, so we say they travel at the "speed of light."

This part comfuses me. If they don't travel in time, how do they have a speed? Light is a type of electromagnetic wave right? And it takes many years to travel to us from a nearby star.

If we can measure or calculate the time it takes for light from some place to reach us, does that not imply traveling through time?

[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.

[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.

[TheOtherHobbes]

It's fine to be confused, because the idea that "photons don't experience time" is physically meaningless.

If you plug c into the Lorentz transformation you get an infinity, which doesn't tell you anything particularly useful.

There's no physical way to accelerate to light speed, so it's meaningless to make assertions about how the "experience" of travelling at light speed would be different to the (presumably simpler) experience of travelling at < c.

The problem is that relativity is a classical theory, and it says nothing about the underlying physical processes of photon creation/destruction and propagation.

Maybe one day a Theory of Quantum Gravity will fix that problem and provide a detailed low-level picture of what actually happens when things move through spacetime. But we're not going to get there for a while.

In the meantime, we'll carry on using concepts like "position" and "time" without really understanding the mechanisms that generate them.

And if that sounds obvious, it really isn't. It's astounding that the universe knows where everything is and where it's going. Not only does it somehow keep track of all those changing spacetime relationships within a self-consistent system, but it also generates the counterintuitive geometry described by relativity.

How does it do that? No one knows.

[cygx]

It's fine to be confused, because the idea that "photons don't experience time" is physically meaningless.

It's just a colloquial description of the fact that the time interval between two events along a null geodesic is zero.

[meric]

But some believe it's God who does this.

[TimTheTinker]

This sounds a lot like the common "God of the gaps" argument that Hitchens and others describe, in which a deity or deities are supposedly invoked to explain what we do not yet understand.

Yet it is fascinating that (1) any system of thought (including science itself) must rely on axioms; (2) by Godël's incompleteness theorem, no system of thought can prove its own axioms; and (3) thus it would seem that faith is inescapably required to believe in anything at all.

When evaluating world views, perhaps the best metric is to evaluate which of them requires the least faith.

For my part, when considering the known universe's mere existence, atheism seems to require a lot more faith than theism.

[e12e]

Another way to look at it, is a requirement to accept uncertainty - the existence of unknowns - or indeed "unknowables".

To each their own - but I don't see "There are some things we cannot describe in our system of knowledge" as a particularly strong proof for the existence of God.

Perhaps some of us have been imbued with an unhealthy and naive lust for certainty, in part by an education system that put an emphasis on right and wrong answers rather than on the quest for better questions?

[TimTheTinker]

There's a lot of difference between "there are some things we cannot know" and "all systems of belief rest on axioms that must be taken on faith".

To me, the latter is an encouragement to rationally evaluate existing belief systems against each other, since it reduces all of them to a level playing field. From there, one can apply a simple twofold truth test to each system: correspondence to reality and internal consistency.

[tasty_freeze]

> atheism seems to require a lot more faith than theism.

There is a world of difference between thinking something caused our universe to exist and perhaps giving it the name "god", and believing in a specific god or specific claims about any god.

[TimTheTinker]

Acknowledged. Advocating for a more specific form of theism would require more specific arguments.

[meric]

1. We give capital letters to the Big Bang, certainly we can have capital letters to the word to describe the cause of the universe, God?

2. God is not part of this universe.

3. God, having been the cause of this universe, can be described as all powerful - at least in the same way Big Bang was powerful.

4. God is outside of time.

The above requires no faith at all - to me it's obvious God described in points 1-4 is true.

Now the following requires a degree of faith:

5. God is one, indivisible, at least in the same way a quantum (by definition) is indivisible.

6. God, rather than having zero consciousness, is completely conscious, having equal or more consciousness than all the consciousness in this universe.

If God is all powerful by definition of having caused the universe then I find it harder to believe the specific claim that God in fact, is not conscious, even less than a starfish. And when we are talking about God, it's more likely to be all or nothing, so my specific claim is that God is completely conscious, and being outside of time, all-knowing.

[azag0]

Very trivialized: in some sense, you could say that for light itself, there is no time. In the same sense as there is no space for things that do not move (in space).

[erikpukinskis]

Think about a wave on a lake. It may appear to be moving in time. The water particles certainly move up and down. But if nothing is in it's "path" is the wave really moving? It's actually just there, the wave undulates and that creates the perception of motion, but really the thing you see moving is just a visual effect on the surface of a field the size of the entire lake. A field which is not moving at all.

Photons are similar. You see the peak of the wave moving around, but the wave itself is everywhere and eternal... until other forces get involved anyway.

[bramen]

I'm not sure about this analogy. You can argue that the apparent motion of the wave crests is an illusion being pieced together by our brains when we see the totality of the elliptical movements of water particles at the surface.

But at the moment when you drop a pebble into a pond, there are definitely parts of the surface which are moving and parts which are not, and the influence of the energy you introduced with the pebble can clearly be seen to spread outward over time.

Granted this doesn't map directly onto electromagnetic waves because the mechanisms involved in wave propagation are different.

[Koshkin]

This picture is incorrect: the electromagnetic wave has a mechanical momentum in the direction of its propagation, which means that something is moving in that direction.

[erikpukinskis]

Does it have momentum before we measure it? I thought momentum was a property of the collapse event, not a property of the wave?

[Koshkin]

According to the classical electrodynamics - it sure does. From the quantum mechanical point of view, it also does - in the sense that we can always measure it (i.e. it is an observable). The "property" in this case is not so much a particular outcome of such measurement as much as the expectation value; actually, I'm afraid that the use of the word "property" in this context can only lead to confusion as it effectively conflates several different things: the (quantum-mechanical) state, the observable, and the particular value observed.

[neom]

This might help: https://simple.wikipedia.org/wiki/Space-time

[jfjdiriri73737]

Basically to establish time a measure has to be taken. Either by a human with our units for time, or by interaction with some force or object to establish that "this happened then".

We commonly think of time in the linear time line sense.

It's more accurate to think of it as a big mesh of points of interaction.

Think more Cartesian space than left->right