
Do we really travel through time with the speed of light? - laurex
https://backreaction.blogspot.com/2020/08/do-we-really-travel-through-time-with.html
======
antognini
Here is another explanation. In Newtonian physics lots of common quantities
are represented by vectors: position, momentum, force, the magnetic potential,
etc. Other important quantities are represented by scalars: time, energy,
power, the electric potential, etc.

One of the deep insights of special relativity is that these scalar and vector
quantities are actually _unified_ into a single, new entity called a "four-
vector". Each four-vector has three spatial components and one temporal
component. So in the two lists I wrote above, each scalar quantity gets paired
with the corresponding vector quantity: time & position; energy & momentum;
power & force; the electric & magnetic potentials. In Newtonian physics a
rotation in three-dimensional space will transform one component of your
position vector into another (say, some of your x-component becomes a bit of
y-component). In special relativity observers moving at different velocities
are related by a similar kind of transformation of their four-vectors --- a
bit of the time component mixes with the spatial component and vice versa.

But what happens if you do this with velocity? Velocity is a vector. It's
corresponding temporal quantity is a bit weird: change in time per change in
time. But it's a little more subtle than that, because it's actually the
change in _coordinate_ time per change in _proper_ time. Basically this is the
ratio between how fast you observe a clock tick in an observer's reference
frame relative to how fast they see it tick within that reference frame.

But the really weird thing about this particular four-vector is that it
_always_ has a magnitude of exactly the speed of light. No matter how fast you
go, the magnitude of your four-velocity does not actually get larger. All you
do by going faster is just mix some of the time component of your four-
velocity into the spatial components. (This is why the clocks of moving
observers tick more slowly.) And when you slow down, you just mix some of the
spatial components into the temporal component. And when you're at rest the
spatial components are all at zero, so the only component of your four-
velocity is in the temporal direction. So at rest your four-velocity points
directly in the future with a magnitude equal to the speed of light. This is
what is meant by the statement that we travel through time at the speed of
light.

~~~
Lambdanaut
During a salvia trip I once saw the universe like this. I had been sitting on
this conveyor-belt-like structure my entire life, which was the 4th
component(time) in that 4-Vector. The conveyor belt moved me along
perpindicular to space, which I have been observing my entire life from the
constant speed of time. Only on this trip was I able to look "down" and
realize that I was riding on time.

The interesting thing was that I had this feeling that the other elements of
the vectors were all also conveyor belts, moving in a single direction. X, Y,
Z(and an infinite of other more subtle scalars) were each their own belts,
with entities of a very different hyperdimensional sort riding along them.
Essentially, they were each moving at a constant non-changing rate along a
single dimension(X, Y, or Z) and because of this, they used time as we do a
spatial dimension.

Think about it like this.

Imagine that a trillion years ago you were stationary, and time didn't exist.
SUDDENLY you and all the stuff nearby you got pushed in space along the X axis
at a speed of 100000000000 km/hr.

You kept moving along the X axis at a relatively constant rate with the stuff
around you, such that the X-axis became a constant for you, while the other
spatial dimensions were your degrees of freedom.

In this scenario, time is the X axis for you.

I noticed that there were entities moving perpendicular to our concept of
time, treating our X-axis as their conveyor belt moving them along at a
constant rate.

All a drug trip of course, and I don't know much about physics, however, this
experience has shaped how I concieve of time and space.

~~~
dreamcompiler
> I noticed that there were entities moving perpendicular to our concept of
> time, treating our X-axis as their conveyor belt moving them along at a
> constant rate.

Those entities exist: We call them photons (and a few other things). Photons
move through space at a constant rate, c, and their clocks never move. If a
photon were conscious it might ask "What is this _time_ you speak of?"

It gets weirder: Because photons don't experience time they can't "remember"
being in Place A "before" they were in Place B, so they might also ask "What
is this _space_ you speak of?" Photons are always everywhere they need to be,
from their point of view.

~~~
lovehashbrowns
Vsauce has a great video on this concept.
[https://youtu.be/ACUuFg9Y9dY](https://youtu.be/ACUuFg9Y9dY) (Would Headlights
Work at Light Speed?)

I recommend watching it!

------
Xlythe
If you like hard sci-fi, Greg Egan has a written a few books that cover what
life would be like if there were 2 dimensions of time
([http://www.gregegan.net/DICHRONAUTS/DICHRONAUTS.html](http://www.gregegan.net/DICHRONAUTS/DICHRONAUTS.html))
or if the dimension of time was sumed instead of subtracted when measuring
distance
([http://www.gregegan.net/ORTHOGONAL/ORTHOGONAL.html](http://www.gregegan.net/ORTHOGONAL/ORTHOGONAL.html))

It's amazing to think how the universe is ruled by such minute details.

~~~
bollu
Thanks! Are there more Greg Egan books about relativity? And does he spell out
the mathematical details somewhere?

~~~
mherdeg
One of my favorites is (spoiler alert) Incandescence, which presents the
utterly novel concept of an alien civilization whose faculties, environment,
and thought processes are totally un-human. The characters in this story very
gradually discover the principles of relativity from scratch.

For me reading this book, the process of understanding what it is that they
are understanding ends up conveying very little useful information of how
humans understand physics -- a bizarre and thought-provoking experience.

~~~
rishav_sharan
Greg Egan books expand the brain in weird ways. They can be a bit of a chore
till you hit a point where the narrative has grabbed you, but once that
happens, it an amazing ride.

Reading his books make me feel stupid and clever at the same time.

------
nitrogen
_Today I want to answer a question that was sent to me by Ed Catmull who
writes:

"Twice, I have read books on relativity by PhDs who said that we travel
through time at the speed of light, but I can’t find those books, and I
haven’t seen it written anywhere else. Can you let me know if this is right or
if this is utter nonsense."_

I wonder, was this question asked by _the_ Ed Catmull
([https://en.wikipedia.org/wiki/Edwin_Catmull](https://en.wikipedia.org/wiki/Edwin_Catmull))?

\----

 _Edit:_ to make this comment less useless, I'd like to point any laypeople
like myself who are interested in the nature of the universe to the PBS Space
Time show/YouTube channel:
[https://www.youtube.com/channel/UC7_gcs09iThXybpVgjHZ_7g](https://www.youtube.com/channel/UC7_gcs09iThXybpVgjHZ_7g)

I'm pretty sure they have videos related to this question.

~~~
acidbaseextract
The moment I saw that name I wondered the same thing. It would be cool to be
running in the same internet circles as Ed Catmull.

I'm in awe of his contributions to both computer graphics and Pixar, and I
really liked his management book, even if he definitely did some sketchy stuff
with wage fixing.

------
moonchild
> A distance in space-time is now the square-root of minus the squares of the
> distances in each of the dimensions of space, plus c square times the
> squared distance in time.

In case anyone was having trouble parsing this, that's:

√(-(∆x² + ∆y² + ∆z²) + c²∆t²)

Or:

√(c²∆t² - ∆x² - ∆y² - ∆z²)

------
smusamashah
I think if we first understand what 'time' and 'space' really are than rest
becomes easy to understand.

To me time is the rate at which things change or its just a measure of change.
Everything is changing state, electrons are moving, bodies are moving from one
point to another and also the clock moves the same way.

The diagrams that explain gravity show 'space' as a 2d plane with planets
weighing it down making other small objects come toward it. That's a
misleading representation I think in a sense that it depends on
weight/gravity. I understand space as a sponge. Heavy objects with huge mass
are like a squeezed lump in that sponge effecting space around it. Larger the
mass, more it squeezes, more it effects what's around it.

When an object moves through space, it effects time as well i.e. rate at which
change happens within that object. At very fast speeds, speed of light, it's
already moving through space changing space, change within that object is
reduced to nothing making time move slow for it.

That all I think it is.

~~~
karmakaze
I like the squeezed sponge visual.

~~~
smusamashah
The cloth sheet example with heavy object in the center making other objects
come towards it never made sense to me.

This example would never work in zero gravity.

Someone gave this sponge visual on stackexchange I think and since then it
made so much sense. Also, this makes time travel totally senseless.

You can not travel back in time without reversing all input you ever received
from external factors. Only thing we can probably do is stop or slow down time
for something.

------
redshirtrob
Serious question: Is there a reason to preface this post with "This transcript
will not make much sense without the equations that I show in the video,"
rather than include the equations in the post directly?

I ask because I cannot view the video for some reason. The embedded video
errors, and so does the direct YouTube video. Which means this post cannot
function as standalone content, by the author's own admission.

Cynical me things this is a ploy to drive traffic to a YouTube channel. Non-
cynical me hopes the equations in the video include snazzy graphics and
animations a la 3b1b. Either way it left me frustrated.

~~~
lovehashbrowns
I believe it's because the video is sponsored. Cynicism aside, this makes
sense because having a blog isn't as lucrative as having a successful YouTube
channel. This content (both the video and blogpost) is incredibly well-done,
so I tend to be okay with these type of things.

~~~
redshirtrob
Thanks. I was asking because I couldn't view the video and really wanted to
read the post. Looks like it's available now.

------
CGamesPlay
What really cemented this for me was actually looking at spacetime diagrams.
Light moves at 45 degree angles on a spacetime diagram, but EVERYTHING moves
at the same speed on these diagrams. I have seen several good YouTube
animations demonstrating the Lorentz transformation, here's one that might
help clarify what I mean:
[https://www.youtube.com/watch?v=k73psdcmzEY](https://www.youtube.com/watch?v=k73psdcmzEY)

~~~
quickthrower2
What I’m still confused about is if you travel through time at a certain speed
and time is like any other dimension doesn’t there have to be a underlying
concept of “this the real damn time” underneath it?

~~~
pas
There is, the damn spacetime, under all it's completely tangled together, you
can't separate them, because relativity. It's a very useful "illusion" for
everyday life, to think of time as something completely distinct and always
present... but it seems, our best experiments and thus models suggest
otherwise. Similarly with quantum stuff, there doesn't seem to be hidden
variables, it's truly a probability distribution.

~~~
quickthrower2
Thanks in this case I am trying to understand what is meant by “moving through
spacetime”

~~~
nybble41
> what is meant by “moving through spacetime”

AIUI the equations describe a _path_ through spacetime. There is no "speed" or
"movement" along this path, or at least none with an actual physical
interpretation. The path simply has a certain shape. The equations define the
path as a parametric function, and you could in principle calculate the change
in 4D spacetime coordinates with respect to the change in that parameter, but
it would have no physical meaning. To keep things simple, in a canonical
representation of the path this ratio (the "speed" through spacetime with
respect to the arbitrary scalar parameter—though "speed" is not really the
right term since the parameter does not represent time) is held constant and
_defined_ as the speed of light.

To put things in more familiar (or at least less spacetime-y) terms, these
very different parametric equations both describe a 2D unit circle with
respect to an arbitrary parameter r with range (-∞, ∞):

    
    
        p(r) = [cos(r), sin(r)]
        q(r) = [abs(2 - t) - 1, ((2 - t) / abs(2 - t)) * sqrt(1 - (abs(2 - t) - 1)**2)]
    

The "speed" of p(r) with respect to r is the magnitude of the derivative
[-sin(r), cos(r)], a unit vector, and thus equal to one for any value of r.
The magnitude of the derivative of q(r) is something rather more complicated,
and not even well-defined for most values of r. However, they both describe
the same circle. The parameter r is not part of the path; only the set of [x,
y] coordinates counts, and p(r) and q(r) describe the same sets of 2D points.

------
de6u99er
Ther was an experiment where scientists built two ultra precise clocks. One of
them stayed on earth while the other was sent to space orbiting the earth at
much higher speed than the one left behind. Once the clock from space was
returned scientist observed that time on the clock in space progressed slower
relative to the on the one from earth.

Which proves Einstein's predictions based on General Theory Of Relativity.

This is being considered when e.g. sending probes to Mars.

~~~
MichaelApproved
It was also done with planes flying around the world.

[https://en.m.wikipedia.org/wiki/Hafele–Keating_experiment](https://en.m.wikipedia.org/wiki/Hafele–Keating_experiment)

> _The Hafele–Keating experiment was a test of the theory of relativity. In
> October 1971, Joseph C. Hafele, a physicist, and Richard E. Keating, an
> astronomer, took four cesium-beam atomic clocks aboard commercial airliners.
> They flew twice around the world, first eastward, then westward, and
> compared the clocks against others that remained at the United States Naval
> Observatory. When reunited, the three sets of clocks were found to disagree
> with one another, and their differences were consistent with the predictions
> of special and general relativity._

——

Also, GPS wouldn’t work without understanding relativity because the clocks in
the satellites move faster (I think?) than the ones here on Earth.

~~~
imglorp
And even locating a clock to a higher elevation, because gravitational
curvature has the same effect as relative motion.
[https://arxiv.org/pdf/1710.07381.pdf](https://arxiv.org/pdf/1710.07381.pdf)

Can't find the reference now but even moving the clock a few meters is a
measurable drift.

------
milchek
Non-physicist here, so excuse the layman rambling, but to me this was always
something I just assumed when asking myself how fast is time? I just figured
it was a constant because it is, it doesn't slow down or speed up - well,
except for when you're doing really tedious work of course!). So, I just
always assumed that if it's a constant, then it must be equal to the fastest
or slowest constants we know.

That always leads me to this (probably) crazy rabbit hole I can never quite
articulate or resolve that no doubt countless others have wondered about -
what if it's not moving at all? What if time is 'solid'?

I guess the best way to explain what I mean here is like a flip book. On each
page there is a drawing, if you look at one page you just see a static
drawing. If you look at the book, it's just a static book. But if you flip
through the pages, it animates and appears to be moving.

What if our experience of time is just like this 'flip book' experience, where
in actuality, our experience of time is that we are moving through it, but
time is not moving at all?

I'd love to know if there have been any other studies or discussions about
this? Because if time is a constant, it could also be 0, no?

~~~
kanzenryu2
You might find this explanation interesting
[https://www.lesswrong.com/posts/rrW7yf42vQYDf8AcH/timeless-p...](https://www.lesswrong.com/posts/rrW7yf42vQYDf8AcH/timeless-
physics)

~~~
sooheon
Bravo. I get why this guy has a following now.

BTW, I think Vonnegut had this right in 1969:

"I am a Tralfamadorian, seeing all time as you might see a stretch of the
Rocky Mountains. All time is all time. It does not change. It does not lend
itself to warnings or explanations. It simply is. Take it moment by moment,
and you will find that we are all, as I’ve said before, bugs in amber."

------
alextheparrot
How fast can something orbit the earth?

My standard model of “Thing going really fast” has been some nuclear powered
craft that just shoots out into the abyss, but after a bit of googling
apparently our fastest craft is/will be Juno, which is staying within the
solar system. This will apparently go 250,000km/hr or 0.000232c partially
based on Jupiter’s gravity.

It is interesting to think about just “living fast”, where our lives are still
only ~80 years, but the time elapsed on earth is on the order of many
multiples of that. We would exist in a way that we orbit the earth at a much
larger fraction of light speed, while still allowing us to interact with the
earth’s resources on normal time scales. Imagine taking a vacation back to
earth for a year, but popping back to the space station where it has only been
a few hours.

Maybe I’ll write a novel on that or find one already written, there seems to
be a lot of depth in exploring that.

[0] [https://www.missionjuno.swri.edu/earth-flyby/story/how-
fast-...](https://www.missionjuno.swri.edu/earth-flyby/story/how-fast-can-
juno-go)

~~~
jazzyjackson
I saw a really moving animated short that used this as part of its plot, but I
forget the name (japanese) and haven't been able to google it since.

A couple of high school sweethearts promise they'll love each other forever,
but there's a war on and the boy quickly gets recruited to fight at the edge
of the solar system (I remember a scene where they stand at some railroad
tracks and watch a monumental silver space craft fly overhead, one of the
battle cruisers), but before he gets there the boy and girl continue their
relationship as pen pals, at first sending messages back and forth from mars
base (45 minute delay, but not aging any faster), but as the boy travels
closer to the front of the war, his velocity increases and he ages less than
the girl waiting for him back on earth, and of course more time passes by
between messages.

Eventually it's the boys 20something birthday and he still has love for the
girl, while the girl has already grown up and moved on with her life.

~~~
zapu
Sounds like
[https://en.wikipedia.org/wiki/Voices_of_a_Distant_Star](https://en.wikipedia.org/wiki/Voices_of_a_Distant_Star)

~~~
jazzyjackson
You have solved my puzzle, and I am thankful for it.

Edit: hah, figures my bias would mix the genders up, the girl went off to
fight the war.

------
Zamicol
I hope someone knowledgeable is still reading comments this late:

I was thinking about this last night. How does this fit into GR? I understand
how it fits into SR, but in GR space is just curved. Unlike velocity time
dilation, under Einstein space isn't moving. In GR it's the curve that's
"equal" to gamma, the dilation factor in SR.

This is evident where escape velocity (root((2GM)/r)) replaces the (v^2) term
in the Lorentz factor to give gravitational time dilation in the Schwarzschild
solution (t' = t(root(1-(2GM)/(rc^2))).

Mind you, I've always thought everything is moving at c through spacetime, but
I understood that from SR. I just don't see how it fits with GR.

------
ta1234567890
> Now, of course there is a difference between time and space, so that can’t
> be all there is to space-time. You can move around in space either which
> way, but you cannot move around in time as you please.

Not really. The instant you move in space, you also move in time, and since
everything is moving and there is no fixed frame of reference, technically you
can never move back to the same spot where you started.

So even though it might appear to us, at our perception scale, that we can
move back and forth in space, in reality whenever we move, space changes and
so it's just like moving in time, there is no going back.

------
Ericson2314
The thing that annoys me a bit about these explanations is that one can do a
classical spacetime too, where every timeslice is just that, a slice through
the space.

I would rather spend a bit of time formalizing that classical spacetime, and
_then_ get into how special relativity is different. Much more apples to
apples.

------
username505
Imagine still thinking ‘You’ travel anywhere - as opposed to the distance
between objects and the matter there within expanding and contracting to
infinitismally small and large (real-time/life-size) proportions... hmmm lol
begs the question do you move through life or does life pass you by?

------
sheerun
Little misleading title: each particle moves with exactly speed of light
through spacetime; the time is just the direction in which it moves through it
(relativity says time is individual for each particle).

------
ngcc_hk
Try this: 1 dim relativity
[https://www.youtube.com/playlist?list=PL04B3CE7DCE67CA5D](https://www.youtube.com/playlist?list=PL04B3CE7DCE67CA5D)

------
koonsolo
Which is the reference point for speed? Is it the centerpoint of the universe
mass? Is it relative and calculated with acceleration and deceleration?
Something else?

~~~
lalaithion
There is no reference point for speed. It's all relative. Hence the theory of
"relativity".

~~~
koonsolo
I still don't get it. Let's say you jump on an airplane with an atomic clock,
and fly around the earth. According to the theory, your time will go slower.
So the clock on the plane will be behind the clock on earth.

But what if you fly against the turning of the earth. That way the clock on
the earth goes slower, and so when arriving back, the clock on earth is
behind?

Every time I look this up, I never seem to be able to wrap my head around
that.

EDIT: Found the answer to my question:
[https://en.wikipedia.org/wiki/Time_dilation#Reciprocity](https://en.wikipedia.org/wiki/Time_dilation#Reciprocity)
. Basically when you accelerate and decelerate, your time gets influenced, and
so the person experiencing the most acceleration will have the slower time.

------
ta1234567890
> If you want to calculate a distance in space, you use Euclid’s formula. A
> distance, in three dimension, is the square-root of the of the sum of the
> squared distances in each direction of space. Here the Δx is a difference
> between two points in direction x, and Δy and Δz are likewise differences
> between two points in directions y and z.

I really dislike it when explanations about reality become explanations about
a model of reality.

Can the original question be answered without assuming some model is the
perfect reflection of reality, or just without using the model?

~~~
lokedhs
What would that even mean? The model is a description of reality. When you
talk about the model, you are referencing a mathematical description of
reality.

In a similar way, the ratio π is the mathematical description (or model, if
you like) of a circle's circumference relative to its diameter. I'm not sure
how to describe this relationship without invoking the mathematical
description of said relationship.

~~~
guerrilla
> The model is a description of reality

No, not neccessarily. It's purpose is to make predictions. It need not be an
accurate description of reality. We know this from our own internal model of
reality and from classical mechanics, neither of which seem to be accurate
descriptions of reality but both of which are very good models for predicting
outcomes.

~~~
coldtea
As expressed by ta1234567890, that's a misunderstanding of what models are,
and what "the map is not the territorry" means...

~~~
guerrilla
Are you responding to the wrong comment? ta1234567890 and I don't seem to
contradict eachother, at least in the part of the thread I'm responding to.
Your comment is too vague for me to respond to and doesn't contain a clear
position or an argument for it: what did you mean to say?

~~~
coldtea
You seemed to disagree with your parent, which corrected ta1234567890.

I happen to disagree with ta1234567890 too, and think the parent is correct,
and ta1234567890's comment is non-sensical.

He says:

"I really dislike it when explanations about reality become explanations about
a model of reality. Can the original question be answered without assuming
some model is the perfect reflection of reality, or just without using the
model?"

My point is, you cannot talk about reality without using a model, except in
some obscure philosophical sense (like e.g. Kant's ding an sich).

Models are not some bizarro abstractions that have little to do with reality,
they are exactly our way of talking about reality. Any talk about reality is
about a model of reality.

So ta1234567890 inquiry is bogus, we can't have "explanations about reality
[NOT] become explanations about a model of reality".

That models are abstractions, and that 'the map is not the territorry" (two
ideas that might have informed ta123456... comment) is correct, but it just
means that no model is perfect. Not that model-less talk about reality is
possible...

Plus, the idea that no model is perfect, is problematic in itself, often usef
for a tin-foil rejection of any model. Some models can be perfect for their
domain, or perfect for up to some very small degree of measurement error...

~~~
guerrilla
> Models are not some bizarro abstractions that have little to do with reality

They're tools to predict experience. Anything beyond that is philosophy,
positive or negative (i.e. that they are only that or that they are more than
that). Your philosophical position is that the reason that they are able to
predict future experience is because they accurately describe reality to some
degree. This is just the opposite position of Hume. If you think that the
models give us knowledge of reality, then this is the opposite position of
Kant.

Your position is held by only 57.1% of professional philosophers. The Humean
position is held by 24.7%. [1] My point here is that Kant's position is hardly
obscure being that one more extreme than it is held by a quarter of
philosophers and it's not wise for you to take your position as being
objectively correct and anything else a "misunderstanding." Even if you only
mean that it is more common, it's firstly only barely so and secondly however
reality is or is not is not going to be decided democratically.

I think additionally that your position is particularly naive considering that
QM doesn't even pretend to tell us how reality actually is. That's what the
various interpretations do. Or will you claim that QM is not a physical model?

1\.
[https://philpapers.org/surveys/results.pl](https://philpapers.org/surveys/results.pl)

~~~
perl4ever
>QM doesn't even pretend to tell us how reality actually is. That's what the
various interpretations do

Isn't that backwards? QM, without "interpretation" tells what outcomes of
experiments should be. That's reality.

The interpretations are interchangeable, so even if there was some oracle to
tell us one of them was better, it wouldn't tell us anything about reality.

~~~
guerrilla
No, this is a non-standard definition of reality [1]. QM does not give us an
ontology, i.e. a specification of what exists. That's what the interpretations
do.

1\.
[https://en.wikipedia.org/wiki/Reality](https://en.wikipedia.org/wiki/Reality)

~~~
perl4ever
A well known definition of reality is "what doesn't go away when you stop
believing in it".

If you stop believing in QM, everything happens just as before and if all
knowledge was lost, the theory could be recreated.

But if you stop believing in the Copenhagen interpretation, and start
believing in many-worlds, or you say "the hand of God directs things", nothing
changes. If all knowledge of those ideas vanished, nothing would ensure it was
recreated.

~~~
guerrilla
Your new explicit definition of reality is different than your old implicit
definition of reality. There are presumably more things that "don't go away
when you stop believing in them" than "what outcomes of experiments should
be."

What people want to know is _what_ doesn't go away when you stop believing in
it. What you've claimed so far is that the results of experiments (i.e.
effects) don't go away if you stop believing in them. This isn't
controversial; however it is incomplete. It doesn't tell us what caused the
effects. If many-worlds, for example, is true, then when you stop believing in
it then its branching and branches will not go away. If pilot-wave theory is
true, the pilot waves do not go away when you stop believing in them. If
objective collapse theories are true, wave functions spontaneously collapse
whether you believe so or not. You, on the other hand, are only accounting for
effects and models, not causes and what is modeled.

~~~
perl4ever
>There are presumably more things that "don't go away when you stop believing
in them" than "what outcomes of experiments should be."

If so, I don't know what.

Anyway, I think you're implicitly privileging one abstraction, called an
"interpretation" over "just the plain math". But if they're equivalent, what
is real about choosing one or the other, let alone between different
"interpretations"?

Why is an "interpretation" a real thing and not the equations?

This seems structurally similar to a classic theological argument between
theist and atheist; ok, you believe in a god, but why that _particular_ god?
And if there's no reason for a _particular_ god, why bother with any?

~~~
guerrilla
> If so, I don't know what.

Like the things I mentioned. Please read more carefully.

> Anyway, I think you're implicitly privileging one abstraction, called an
> "interpretation" over "just the plain math".

One doesn't need to privilege one over the other to ask what the math is
_about_.

> Why is an "interpretation" a real thing and not the equations?

Nobody thinks the math is not "a real thing." They want to know what the math
is about.

------
shoes_for_thee
Okay so

Movement through space (in any direction) is movement _against_ time.

Is positive kinetic energy the direction opposite time?

------
kaetemi
So, we travel through time at the same speed _that light travels through
time_? Did I get that right?

~~~
db48x
No. Light travels through space at the speed of light. Physical objects travel
through time at the speed of light.

~~~
JPLeRouzic
>> Light travels through space at the speed of light.

That seems a tautology isn't? Perhaps you wanted to say something else?

~~~
db48x
>> Light travels through space at the speed of light. > That seems a tautology
isn't? Perhaps you wanted to say something else?

Very nearly, but it is exactly what I wanted to say. Light travels at a fixed
speed _through space_ which we happen to call "the speed of light". A physical
object which is motionless in space travels at a fixed speed _in the time
dimension_ which happens to have the same size as the speed of light.

This has consequences. A physical object which has some speed in space still
has a 4-velocity vector whose length is the speed of light. That means the
space velocity and time velocity must combine into a vector whose length is c.
Naturally, if the space velocity is non-zero, the time velocity must be less
than c. This is called time dilation.

> Therefore, if I got this right, for an external observer, light does not
> travel trough time, only space. However, in its own reference frame, light
> should travel trough time at the speed of light (and doesn't move in space
> in that reference frame).

Not quite.

Taken to the extreme, an object with a space velocity equal to the speed of
light would have no velocity at all in the time direction. This can be a
confusing statement, because we must make a distinction between times as
measured by an observer and times measured by the moving object. If a photon
could measure events (and most cannot), it would measure no time elapsing
between it's own creation and it's own destruction. It will be emitted by an
electron in one place (perhaps the coronasphere of the sun) and simultaneously
absorbed by another electron in another place (perhaps a rhodopsin molecule in
someone's retina). An observer, such as the owner of the retina, would of
course still measure some time between the emission and the absorption; the
events would not appear to be simultaneous. This is called relativity of
simultaneity.

~~~
criddell
I always get messed up when I try to think about the experience of the fast
moving entity.

Imagine a photon that is emitted by the sun and travels through space bouncing
off the moon and onto my retina. In my frame I can trace the path of that
photon and see a sequence (sun->moon->eye). For the photon, all of that
happened simultaneously, right? It would have no experience of having traveled
from the sun to my eye because it was in both places at once. Is that right?

------
kovacs_x
there is no time. only the infinity of now. :)

------
foreigner
What a terrible bit of teaching.

>physics is all about equations

No. Physics is about the nature of reality. Mathematics is a tool we use to
describe the world.

~~~
geertj
At present math is the only tool that works though. I would even go a bit
further and say that math isn’t only a tool but it’s a device that provides a
deeper understanding and intuition. A metaphor that jumps to mind is that of
woodworking. Woodworking tools are not just necessary to shape the wood. They
also allow you to feel the grain and other properties of the wood, providing
for a deeper understanding.

------
jonny383
I'm becoming more and more convinced that we are living in a simulation, and
time dilation (as you approach c) is an effect of the limits of computing
power.

Consider that the faster you travel, the more collisions (or potential
collisions) between particles need to be decided.

Is the limiting factor of c directly related to the available computational
power?

~~~
zemnmez
If we were living in a simulation, why would our perception of relative time
be absolute relative to the simulation? Why wouldn’t the simulation just
simulate slower at whatever level of abstraction it operates on, causing us to
be none the wiser, like the AI in a frame-locked videogame?

~~~
jonny383
Human perception isn't a consideration of the simulation, we are just a side-
effect.

You can either simulate the entire universe in infinite detail taking infinite
time, or you can optimize to focus on the more important bits in finite time.

------
austincheney
The speed of light, _c_ , is the speed of light in a vacuum. The speed of
light is variable given the medium by which that light traverses.

Also, when we speak of traversal the implication is closing distance across a
space. Space is also variable. According to the Special Theory of Relativity
space is curved and that curvature varies according to the forces acting upon
it. The general assumption is that such a curvature is uniform according to
the force imposed upon at a given point relative to that force, but there is
no reason to make such an assumption. More simply space is curved and that
curvature may vary in unexpected ways regardless of whether an observer is
outside or effected by the force imposing the curvature.

That said the safest prediction is to say we all travel at various speeds of
light given enough space and duration compared to a competing observation.

~~~
irjustin
downvoted because time traveling through a medium vs traveling through curved
space are not anywhere near the same underlying principles[0].

The implication being suggested is - if I was trapped in a block of plastic
or... underwater, I would experience time different compared to someone
standing on land just because "c" is slower.

Clearly not the case.

To be specific, when "the speed of light" is discussed like this, it usually
means the "speed of causality"[1] which is the underlying meaning of "c" in
relativity.

The speed of causality through a medium (i.e. water) is the same as a vacuum
if spacetime curvature is the same.

[0]
[https://www.youtube.com/watch?v=CiHN0ZWE5bk](https://www.youtube.com/watch?v=CiHN0ZWE5bk)

[1]
[https://www.youtube.com/watch?v=msVuCEs8Ydo](https://www.youtube.com/watch?v=msVuCEs8Ydo)

~~~
austincheney
Space is a medium of variable density that impacts the speed of light and
everything else. That is clear with regard to solar winds and termination
shock.

The curvature of space is directly related to time travel. Strange how you
completely ignored that much larger portion of my comment to punch a straw
man.

> is the same as a vacuum if spacetime curvature is the same.

If that were true particle physics wouldn’t need dark matter/energy to balance
conditions that are irregular upon observable matter alone.

[https://en.m.wikipedia.org/wiki/Causality_(physics)](https://en.m.wikipedia.org/wiki/Causality_\(physics\))

Causality, at least in physics, is an abstract notion and not an empirical
notion. In causality effects occur in the same order as their respective
causes which is substantive for logical consider, but is not necessarily
measurably accurate or expected.

~~~
irjustin
While there are a lot of big words being used, there appears to be a gap in
understanding the core principles related to the article.

> If that were true particle physics wouldn’t need dark matter/energy to
> balance conditions that are irregular upon observable matter alone.

You responded to my vacuum statement as if we agree the vacuum is perfectly
empty. In relativity, the vacuum is not considered empty[0]. Maybe I should
have used "patch of space", but more importantly I said "if the spacetime
curvature is the same". That literally defines how causality moves through it
so that would be inclusive whatever was there including dark matter/energy,
which brings us to...

> Causality, at least in physics, is an abstract notion and not an empirical
> notion... but is not necessarily measurably accurate or expected.

No it's not... From the wiki you quoted: "In Einstein's theory of special
relativity, causality means that an effect can not occur from a cause that is
not in the back (past) light cone of that event. Similarly, a cause cannot
have an effect outside its front (future) light cone."

We're talking about relativity here, not philosophy or another topic where
causality isn't well defined. It is not abstract. It is explicitly defined as
"c" in Einstein's equations. I'm not sure what you're trying to prove/disprove
here.

You can say I attacked a straw man, but the original comment was edited. So
now, I don't know.

I recommend PBS SpaceTime[1] - I still don't understand a lot of things in
this space even after watching many videos multiple times, but it really
helped put pieces together.

[0]
[https://en.wikipedia.org/wiki/Vacuum_state#:~:text=According...](https://en.wikipedia.org/wiki/Vacuum_state#:~:text=According%20to%20present%2Dday%20understanding,into%20and%20out%20of%20existence).

[1]
[https://www.youtube.com/channel/UC7_gcs09iThXybpVgjHZ_7g](https://www.youtube.com/channel/UC7_gcs09iThXybpVgjHZ_7g)

~~~
austincheney
There were no big words or any form of exotic vocabulary.

> No it's not

Causality states that the order of effects must match the order of events. The
Wikipedia articles states that almost verbatim. The mention of a light cone
binds the general use of the word to its application of physics without
changing the definition. No where does the article extend that definition to
anything vaguely measurable.

> You can say I attacked a straw man, but the original comment was edited.

It was most likely edited hours before your reply. When I completed the edit
there were no replies. I am living on the otherside of the world from the US
in a far away timezone.

~~~
irjustin
> No where does the article extend that definition to anything vaguely
> measurable.

Ah, you're right, I didn't realize you dropped the "speed of" in the "speed of
causality" (because the "speed of causality" can be measured and expected...
what does "expected" even mean in this context?!). So going 2 replies up you
linked to Causality and made a statement about causality but ignored the
"speed of causality". So nothing was said?

So you agree with me!

> It was most likely edited hours before your reply. When I completed the edit
> there were no replies.

Ah! Don't worry, thankfully we know that I replied to you 35 minutes later
thanks to Hackernew's API[0][1] and that I'm half a world closer in SGT, and
that I was originally replying to something that was edited away.

[0] [https://hacker-
news.firebaseio.com/v0/item/24328890.json?pri...](https://hacker-
news.firebaseio.com/v0/item/24328890.json?print=pretty) [1] [https://hacker-
news.firebaseio.com/v0/item/24329038.json?pri...](https://hacker-
news.firebaseio.com/v0/item/24329038.json?print=pretty)

~~~
austincheney
> Ah! Don't worry, thankfully we know that I replied to you 35 minutes later

I don't find that as reassuring. Instead of an editing/time conflict the
problem is just poor reading comprehension.

