Do we really travel through time with the speed of light?

https://backreaction.blogspot.com/2020/08/do-we-really-travel-through-time-with.html

by laurex • 6 years ago

354 201 6 years ago

backreaction.blogspot.com

CGamesPlay2• 6 years ago

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

Zamicol2• 6 years ago

Nice find on that YouTube link. Thanks.

quickthrower22• 6 years ago

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?

pas2• 6 years ago

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.

CGamesPlay2• 6 years ago

There is, it's called the "spacetime interval". It's relative to the observer, because everything in spacetime is, but it's the "true" measure of separation in spacetime.

nitrogen• 6 years ago

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)?

----

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

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

acidbaseextract• 6 years ago

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.

antognini• 6 years ago

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.

vnorilo• 6 years ago

This is great, thanks! This is the first time I hear it explained like this and it just somehow clicked immediately.

I now have this (probably wildly incorrect, total layman here) vision of the time dimension of velocity being "a normal" simultaneously to all the spatial dimensions and acceleration as rotation of the four-vector away from the time axis...

sorry for any terminology errors.

mjfisher• 6 years ago

That's exactly it - it's not wildly incorrect at all.

TeMPOraL• 6 years ago

Thank you for clear explanation. I wonder how changes in 4-vector velocity relate to "energy & momentum" 4-vector? Does that vector also have fixed magnitude?

mpcjanssen2• 6 years ago

There is indeed also a relation between momentum and (rest) energy describing the conservation of energy, In the rest frame this relation reduces to the famous E=mc^2.

cygx4• 6 years ago

Does that vector also have fixed magnitude?

It's magnitude is called 'mass' (or 'rest mass'/'invariant mass' if you want to differentiate it from the concept of 'relativistic mass', which probably should be retired).

This means we have

    four-momentum = mass * four-velocity

whereas

    three-momentum = gamma * mass * three-velocity

cletus• 6 years ago

Yeah, I'm not a physicist but hearing a good explanation of the time-space four-vector being constant as an explanation for time dilation (among other things) was a real light bulb moment for me.

It's also why I'm basically convinced that any sort of FTL (including wormholes and compressing space), time travel or cheating causality is essentially impossible, no matter what results you contrive by putting things like negative mass and/or energy into various equations.

How this all gets reconciled with quantum mechanics is another matter. Part of me wonders if the problem isn't just that space and time are ultimately discrete (at the Planck unit level) and our equations describing it are continuous. But again, I'm no physicist.

pdonis7• 6 years ago

the really weird thing about this particular four-vector is that it always has a magnitude of exactly the speed of light

This isn't weird. The magnitude of any four-vector is frame invariant in relativity. Lorentz transformations in relativity are the analogue of rotations in ordinary Euclidean space, and have the same property of preserving the magnitudes of vectors.

All you do by going faster is just mix some of the time component of your four-velocity into the spatial components.

I think this is a misleading way of putting it. In a frame in which you are moving, the time component of your four-velocity is larger than it is in your rest frame, not smaller. So it's not the case that some of the time component in your rest frame got "moved" to a space component.

dheera• 6 years ago

It should be stated somewhere here that relativity operates in Minkowski space and not Euclidian 4D space.

What this means is that the length of a relativistic four-vector [ct, x, y, z] is sqrt((ct)^2 - (x^2 + y^2 + z^2)).

It is not the same as the Euclidian magnitude, i.e. NOT sqrt((ct)^2 + x^2 + y^2 + z^2)

The article does mention this, although with slightly confusing wording, but it should be emphasized that Lorentz invariant quantities need to be computed with the Minkowski metric.

andi9992• 6 years ago

At first it seems suprising that this invariant is always the same for different objects and setups (that it is the same after Lorentz transformation is the meaning of invariant as you pointed out). So why is it for all objects equal to c.

If you think for stationary examples it just means that the eigenzeit has a different pace, so two clocks next to each other going at a different pace. But this is ruled out by another definition, such that clocks are references by light clocks. And now it is also clear why this invariance is c.

garmaine• 6 years ago

It's very weird to someone who has never been exposed to this before, who has only ever worked with vectors in a context where they certainly can be of varying magnitudes, like velocity normally is in Newtonian mechanics.

tgb2• 6 years ago

This isn't weird. The magnitude of any four-vector is frame invariant in relativity. Lorentz transformations in relativity are the analogue of rotations in ordinary Euclidean space, and have the same property of preserving the magnitudes of vectors.

The weird part isn't that two observers agree on the vector magnitude. The weird part is that one observer calculates the same value for any two objects. Frame-invariance isn't the relevant property.

yamrzou• 6 years ago

Thank you for a clear explanation. I'm a total layman here, so forgive my questions if they don't make sense:

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"

On which basis was this pairing between position and time "decided"?

But the really weird thing about this particular four-vector is that it always has a magnitude of exactly the speed of light

Why is that? Is it by definition?

it's actually the change in coordinate time per change in proper time

How does this translate to what we call "time" in our daily lives?

jiggawatts7• 6 years ago

It's a tautology. You cannot observe time slowing down, because anything that would affect the "speed of time" affects your brain also in the same proportion.

Hollywood movies are not how physics works. The plucky protagonist can never look down at their hand and see a stopwatch run slower or faster! There is no such thing. If some sort of science-fiction field existed that would affect a clock, it would affect the protagonist also.

If the clock runs at half speed, your brain runs at half speed, so you see it ticking forward at twice the rate, and 2.0 * 0.5 = 1.

Now, of course, I tell a small lie: There can be differences in the tick rate over some distance. But these differences are necessarily small. Big differences in the rate of temporal flow create gravity, as per Einstein's General Relativity. To get an observable difference in time flow over the space of, say, a meter would require a fantastically huge gravitational field that would instantly crush the poor scientists into a pancake.

Conversely, in the gentle field of Earth's gravitational field, if you hold an atomic clock in your hand a meter from you head, and an atomic clock in line with your head, then yes, you can see a tiny difference in the tick rates, on the order of nanoseconds per century or somesuch (I'm too lazy to work it out).

However, as the distance decreases between the clock and the observer, the difference gets smaller. Extrapolating down to atoms and the like the conclusion is inevitable: time locally always ticks forward at one second per second.

dkersten3• 6 years ago

I find using the speed of light confusing as it has little to do with light other than that light happens to travel at that speed [1], but rather the speed of propagation of cause and effect. If you stay absolutely still, cause and effect still propagate at the same speed and so time ticks forward at this speed. If you physically move (have velocity), cause and effect still ticks forward at the same speed, but some of it appears to tick into moving you physically. You can still only propagate at the speed of propagation, but some of that propagation is happening on the physical position axis, so less happens on the time axis.

[1] because light is massless, so in a vacuum there is nothing slowing it down

gus_massa• 6 years ago

On which basis was this pairing between position and time "decided"?

Three partially correct answers of the same question:

1) Nobody decided it. It is just how the universe is.

2) You have the formulas where the space-something appear, and you randomly try propose to replace something with things you know, like mass, time, color, ... It is actually more easy, because using the units you know that something must be measured in seconds, so it discards a lot of possibilities.

3) You have the "classic" formulas where space appears https://en.wikipedia.org/wiki/Lorentz_transformation that were actually discovered almost 20 years before the work of Einstein. And you rewrite them in the equivalent of the "vectorial" notation. In the usual space it is like replacing (x,y,z) with a big X and never looking inside X. If you try it, the only way you can success is if you group (ct,x,y,z).

The trick is to look carefully at the calculation and try to think what you can arrange it to be able to rewrite the calculation in the correct form.

For example with momentum, you look carefully and realize you must add energy as the fourth component.

If you choose to group space and time, and you choose to group momentum and energy, then you must use the same choose in all the formulas. You can not choose a different grouping for each formula.

But sometimes it is not so easy. With speed you realize that it is too bad to be fixed, and you must define another speed-like-thing and then give another formula that transform the "real speed that you measure in a lab" and the "abstract speed-like-thing that you use in the calculation".

With the electric field, ... well you can't put it in a 4-vector, you must use something more weird that is a 4x4-matrix.

im3w1l• 6 years ago

The reason for creating these pairings is that they make calculations easier. Maxwells laws of electromagnetism in particular become very beautiful when defined in terms of these 4-vectors.

jules3• 6 years ago

What matters is your path through spacetime. The four velocity is just a tangent vector of this path. The magnitude of the tangent vector has no physical meaning, only its direction does.

moralestapia• 6 years ago

Thanks antognini, it made so much more sense now.

vchak1• 6 years ago

What is so special about the speed of light? As a thought experiment, if everyone on the planet was blind, would c have been replaced by the speed of sound?

Chilinot2• 6 years ago

The speed of light is just the speed of any particle without matter. We just use "speed of light" since its simpler to understand and talk about rather than saying "speed of matter-less particles".

The speed of sound is the speed of air-molecules bouncing into each other (~300m/s). Since air-molecules have matter, they dont travel at the speed of light.

dkersten3• 6 years ago

As chilinot said, it’s the speed of any massless particle in a vacuum. A massless particle has nothing slowing it down, so it moves at the maximum possible speed. It’s actually the propagation speed of cause and effect, or put another way, how long it takes for a quantum event to affect whatever is in the adjacent point one Planck length away. It’s simply how quickly these things ripple forward when there is nothing slowing these ripples down.

Every other massless particle moves at the same speed. “Sound” is not a massless particle, it’s propagation of the compression of matter, so therefore moves much, much slower and would not replace c. It has nothing to do with what we can observe and rather to do with its properties. Personally, I find referring it to as “the speed of light” is confusing since it rarely has anything to do with light/photons other than that light happens to move at that speed.

tsimionescu6• 6 years ago

The first very special thing that was observed about the speed of light is that it is NOT relative. That is, if I fire Alice light beam at you from a moving train, while Bob fires a beam at you from a platform, both beams will reach you at the same time. Sound does not behave the same way, light was the first thing that we observed like this.

This was a gigantic problem, an experiment contradicting one of the most fundamental laws of nature as we knew them at the time - Galileo Galilei's principle of relativity.

Note that this observation has nothing to do with our eyes's ability to perceive light. The same observation will not happen with sound waves; and it will hold even for frequencies of light that we can't directly observe with our bodies, such as radio waves.

As others note, it was later discovered that this is not a special property of light itself. It is in fact a special property of the universe, and it applies to any particle without mass; the photon happens to be the only massless particle that we can directly observe, so it was the one which gave the name to the physical quantity.

davrosthedalek3• 6 years ago

The difference between sound and light is that sound needs a medium. This medium breaks the symmetry: There is a special system, the one in which the bulk of the medium doesn't move. For light, you don't have that. All reference systems are equal, independent of the speed they move with respect to each other. Light appears at the same speed in all of them. That's not possible with Newtonian velocity addition.

The experimental "proof" of the frame-independence comes from the michelson morley experiment. (Scare-quotes, because you can't prove things in physics, only disprove)

rocqua• 6 years ago

It is the speed of cause and effect. If something changes at location X, that cannot cause anything to change at location Y faster than the distance between X and Y divided by the speed of light.

Light (in a vacuum) goes as fast as the speed of cause and effect. And it just so happens that the speed of light is pretty easy to measure (as compared to other possible things).

So if we were all blind, we would still be affected by this speed.

rubatuga• 6 years ago

Sounds like the time cube is relevant once more.

Viliam12342• 6 years ago

To be pedantic, it's time-space tesseract, but yes.

browsergap• 6 years ago

THat's probably the best fucking explanation in a HN comment I have ever read. Mind expanded. I actually feel like I get this stuff, which I didn't study at college, a little bit. I took chem... physics always seemed impossibly strange.

throwaway_pdp09• 6 years ago

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".
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 seems strongly to be confusing the map and the territory. The maths is a model, the description of vectors has not link to reality but happens to model reality in some useful way (reality and the model correspond to some useful approximation), but it isn't reality.

at rest your four-velocity points directly in the future with a magnitude equal to the speed of light

may just be an artefact of the maths. IDK though. (and no offence intended, just my POV)

BunsanSpace• 6 years ago

exactly the speed of light *causality

The speed of light is variable, the speed of causality isn't.

Lambdanaut2• 6 years ago

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.

dreamcompiler2• 6 years ago

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.

layoutIfNeeded• 6 years ago

What kind of "entities"? Beings?

shoes_for_thee2• 6 years ago

My question -- and I am not being glib -- is there something to the idea that we can "raise our vibration" and would doing so decrease our speed through time?

jerf• 6 years ago

"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."

For what it's worth, time is distinguished in physics. It isn't just "another dimension" that "happens" to be used as time, but if you could just twist correctly it would become a spatial dimension. It is truly a "time dimension". The simplest way of looking at it is in the Minkowski metric, where distance is SqRt(x^2 + y^2 + z^2 - t^2), note the minus sign on the "t" which distinguishes it.

If you're really up for a solid math-based mind-screw, Greg Egan worked out the shape of physics in a universe that works on essentially Einsteinian relativity, but with two spatial dimensions and two temporal dimensions: https://www.gregegan.net/DICHRONAUTS/DICHRONAUTS.html It turns out that A: sensible things can be said about this and B: it looks nothing like what I expected two temporal dimensions to look.

(And if that's not enough, he's also got a universe of four spatial dimensions: https://www.gregegan.net/ORTHOGONAL/ORTHOGONAL.html)

dfalzone• 6 years ago

Source(s):

Saw this in a salvia trip once

Edit: Why the downvotes? This was clearly supposed to be a joke.

thr0w__4w4y• 6 years ago

That is a great explanation. I remember you, or somebody else here on HN, provided a similar explanation when this general topic came up within the last year or so... and I thought the same thing then -- finally this makes some sense to me. But then I'd lost the post and recently couldn't find it when a colleague could have benefited from it. Thank you.

zackees• 6 years ago

The funny thing about this theory and it’s speed of light proposition is that fails to explain why things are observed to travel faster than light.

https://www.google.com/amp/s/www.sciencealert.com/faster-tha...

mpcjanssen2• 6 years ago

Faster than light in a medium. The last part matters. There is currently no reason to doubt relativity.

ta12345678902• 6 years ago

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.

ta12345678902• 6 years ago

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• 6 years ago

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.

guerrilla6• 6 years ago

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.

Xlythe• 6 years ago

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) or if the dimension of time was sumed instead of subtracted when measuring distance (http://www.gregegan.net/ORTHOGONAL/ORTHOGONAL.html)

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

bollu• 6 years ago

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

mherdeg• 6 years ago

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.

Zecc• 6 years ago

minute details

Pun unintended?

moonchild• 6 years ago

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)

kaetemi• 6 years ago

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

db48x2• 6 years ago

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

JPLeRouzic• 6 years ago

> Light travels through space at the speed of light.

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

drran• 6 years ago

For light, speed of time is zero, because it travels at speed of light. :-/

alextheparrot2• 6 years ago

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-...

wlesieutre• 6 years ago

Parker Solar Probe is supposed to get up to 690,000 km/hr on a close flyby of the sun!

ShamelessC• 6 years ago

The movie Interstellar explores these concepts quite well. it's not 100% accurate but Kipp Thorne was consulted for the visuals on the black holes in the film at least.

jazzyjackson2• 6 years ago

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• 6 years ago

Sounds like https://en.wikipedia.org/wiki/Voices_of_a_Distant_Star

renewiltord• 6 years ago

It's probably the Makoto Shinkai film the other guy posted but another book that uses time dilation is The Forever War. And there's the classic Childhood's End.

rotoradiator• 6 years ago

in shorter version Brian Mays song '39 has similar plot

pdonis7• 6 years ago

* How fast can something orbit the earth?*

If you mean the actual Earth and a free-fall orbit, the orbital speed of something in low earth orbit, like the ISS, about 8 km/sec, is as fast as anything can orbit the earth.

To get really fast orbital speeds (as in, speeds that are any significant fraction of the speed of light), you need objects much, much more compact than the Earth, objects whose actual size is not that much larger than the size of a black hole with the same mass. The only non-black hole objects we know of that are anywhere close to that compact are neutron stars.

Aerroon• 6 years ago

I wonder if black holes or neutron stars in a solar system are necessary for a civilization to start exploring the universe outside of their own solar system. It could be quite a boon to have something like that in the outer solar system for gravity assists.

MayeulC2• 6 years ago

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.

Hmm, this hasn't really fully clicked with me yet (so take what I am saying with a grain of salt, I'd be glad if anyone could clarify/prove me wrong), but speed is relative.

If your station moves fast from the perspective of earth, earth moves fast from the perspective of the station. So, from the perspective of earth, time moves slower on the station, and from the perspective of the station, time moves slower on earth.

If you were to travel back and forth, that would even out (something about having to accelerate in one direction, then another). There is no absolute time reference in the universe.

That can work, however, if you spend some time inside a strong gravity field.

alextheparrot2• 6 years ago

The general idea was that we can orbit a fixed reference (In this case the earth) at a high relative speed. At high earth orbit (35km), orbiting the circumference (Simplifying assuming a perfect circle) would mean one orbit done per second would be 2/3c.

The time delta math (Sqrt(1-v^2/c^2)) puts us at 3 earth days per two space days.

The vacationing was more cheeky, in that in this case a 2 week vacation would appear 3 weeks on earth. If there would be some way to quickly switch between modes than 1 week, you would spend more self-referential time doing something than your home clock would count.

austincheney4• 6 years ago

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.

pdonis7• 6 years ago

According to the Special Theory of Relativity space is curved

No, it's the General Theory of Relativity that says that spacetime is curved. Spacetime is flat in Special Relativity.

curvature varies according to the forces acting upon it

No, in GR spacetime curvature varies according to the matter and energy that are present. Gravity is not a force at all in GR.

cygx4• 6 years ago

Gravity is not a force at all in GR.

Rather, the gravitational force is a pseudo-force like the Coriolis force.

irjustin3• 6 years ago

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

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

scarygliders• 6 years ago

Upvoted you and thanks. I was hoping someone would mention "the speed of causality", because to my layman's mind, I've always found thinking about "the speed of causality" makes vastly more sense of the universe to me and makes spacetime/relativity/and-all-that-jazz far easier concepts to comprehend.

For example, if a powerful laser beam was shot off in a vacuum towards an observer who is far away, it's not the "speed of light in a vacuum" which dictates the time at which the observer detects that laser light, it's the speed of causality in that volume of spacetime which determines when that light beam reaches them/is detected.

austincheney4• 6 years ago

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)

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.

milchek3• 6 years ago

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?

XorNot• 6 years ago

This is in the same vein as the thought-experiment about the One-Electron Universe [https://en.wikipedia.org/wiki/One-electron_universe].

The idea is, maybe everything in the universe is just 1 electron looping through 4D space over and over, interacting with itself at various points of the loop. Comes about from the observation that antimatter can be described perfectly as just regular matter moving backwards in time.

milchek3• 6 years ago

Thanks for linking that, I had not heard of it before - very cool. The theory from Yoichiro Nambu about production and annihilation of particle-antiparticle pairs really struck me as interesting! That said, I'm a real novice with all of this so I need to do some further reading.

FeepingCreature• 6 years ago

Non-physicist answer:

If you switch to viewing the universe as purely a mathematical construct, all these questions will fall away. :) The notion of ontologically privileged time or even space is based on us elevating our model of the universe into objective fact for no reason. Anything that the brain would notice even in a timeless, purely mathematical universe is thus an unneeded addition purely on Occam grounds.

For instance, we may ask the question: "would my brain, if you computed its output given its inputs, output data like 'I exist, I am conscious, I notice the flow of time passing, the present moment is ontologically privileged to me'? It seems obvious that it would; that is, the mathematical answer to the question of "what does my brain output" is that sentence. But a mathematical question cannot be ontologically privileged. We see thus that these notions of ours are necessarily vacuous; any conception of 'time', 'now' and 'me' must be relative, depending purely on the surrounding mathematical/causal structure, or else not exist at all - since even if they did exist, our noticing them is not dependent on their existence, and thus Occam bids us to discard them.

kanzenryu2• 6 years ago

You might find this explanation interesting https://www.lesswrong.com/posts/rrW7yf42vQYDf8AcH/timeless-p...

milchek3• 6 years ago

Thank you for sharing.

sooheon• 6 years ago

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."

Viliam12342• 6 years ago

What if time is 'solid'?

You can adopt a perspective where time is solid. But then the question is, why are particles arranged in this solid time exacly as if they were moving in a real time.

As a metaphor, imagine that you are a god, and you are trying to create a 4D universe with solid time, where the same laws of physics (rephrased in the solid-time language) apply. How would you do it? Ultimately, you would have to create the earlier parts of the solid time first, then the following parts of the solid time, etc, until the end of the universe. It wouldn't work any other way, essentially because of chaos theory; the state at T+1 depends on the details of state at T, so you would have to create the T slice of solid time first, and the T+1 slice later, depending on it.

If you remove the time from the universe naively, you are sweeping it under the rug... into the process that created the universe. But isn't this just adding an extra complexity? If the "process that created the universe" can have real time, why not the universe itself? Isn't this just an extra epicycle?

The only way out is if you used your godlike powers to create all possible states of universe simultaneously -- not just our universe, but literally every possible combination of particles -- and then connect them causally in a way that respects the laws of physics. You don't have to create time T first and time T+1 later, if you create all possible realities first, and then just draw an arrow from each T into the corresponding T+1.

So, instead of the frozen version of our universe (where time = linear movement from the beginning of the movie towards its end), you get a frozen version of everything possible (where time = tracing a path across the everything, following the local currents of causality).

The advantage of the latter model is that you get the parallel universe for free; it just means that local causality is not a single arrow pointing from the current place, but rather a collection of arrows.

But you still need to think (in both models) how to make them compatible with known quantum physics. How to model quantum interference, entanglement, etc.

meroes• 6 years ago

I think you're adding an assumption that isn't justified, that this god would have had to start at what we call t=0, i.e. big bang.

Part of timless physics is that there is no privileged moment in time.

The laws of physics allow god to start at any point in the configuration space of the universe and evolve toward any direction.

This isn't prevented by chaos theory. Chaos theory happens regardless of where you start your t=0. The laws of physics are 100% symmetric (CPT symmetry).

This does not mean we will necessarily recurr either. The laws of physics are symmetric, but due to the Past Hypothesis (prior condition of the universe was very low entropy), we experience an emmergent direction in time, and if the universe is infinite and unbounded it seems like it wont recurr.

drdeca• 6 years ago

I don’t see the obstacle of making that compatible with QM. Just have the state space which you described (which would be the Hilbert space), and then the Schrödinger equation specifies which curves through the state space are “progression through time”.

sheerun• 6 years ago

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).

jonny3832• 6 years ago

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?

ganzuul• 6 years ago

It might be a limit to precision too if infinite accuracy requires infinite space.

For low energy an approximation might suffice, and that would go some ways to explain quantum effects.

zemnmez• 6 years ago

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?

jonny3832• 6 years ago

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.

mensetmanusman• 6 years ago

But then it doesn’t matter, because what’s at the top of the simulation tree?

foreigner• 6 years ago

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• 6 years ago

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.

koonsolo2• 6 years ago

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• 6 years ago

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

koonsolo2• 6 years ago

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 . Basically when you accelerate and decelerate, your time gets influenced, and so the person experiencing the most acceleration will have the slower time.

kovacs_x• 6 years ago

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

smusamashah2• 6 years ago

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• 6 years ago

I like the squeezed sponge visual.

smusamashah2• 6 years ago

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.

de6u99er• 6 years ago

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• 6 years ago

It was also done with planes flying around the world.

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• 6 years ago

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

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

toxicFork• 6 years ago

It's also considered with the GPS :)

http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps....

lapetitejort• 6 years ago

Atomic clocks are so sensitive now that it's possible to desync them while they're in the same room [0]

[0] https://www.nist.gov/news-events/news/2010/09/nist-pair-alum...

Ericson2314• 6 years ago

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.

shoes_for_thee2• 6 years ago

Okay so

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

Is positive kinetic energy the direction opposite time?

ngcc_hk• 6 years ago

Try this: 1 dim relativity https://www.youtube.com/playlist?list=PL04B3CE7DCE67CA5D

redshirtrob2• 6 years ago

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.

lovehashbrowns2• 6 years ago

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.

redshirtrob2• 6 years ago

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

Zamicol2• 6 years ago

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.

username505• 6 years ago

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?

Do we really travel through time with the speed of light?

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