A Spacetime Surprise: Time Isn’t Just Another Dimension
forbes.com
forbes.com
Of course there are nice little things to explore with this, like moving through running water, currents (localized loops), deep water (dense space), resistance on small vs large things, etc.
If we imagine on the small enough scale where we're looking at single water molecules, resistance (time) doesn't even mean too much. As long as your water is liquid, your little molecules are bouncing around in all directions, only tending towards a given direction with some probability. Otherwise each direction is near and damnit equally easy to travel in.
That's my two cents anyway.
> since it breaks the analogy
It was simply a suggestion as to why things might start behaving weirdly at the quantum level. My understanding is that very small things seem to act as if time doesn't really have any real favoured direction.
Of course the analogy is not perfect and does not measure correctly the scale of forces involved, or even how they behave at different extremes. It's just a rough and ready mental model to begin to see things in this framing.
If we measure a human body at the scale of seconds, then we can measure position of the human with precision to single digit meters. It will behave like a particle.
If we measure it at the scale of days, then it will start to blur, like a quantum particle: 30% at home, 30% at work, 30% everywhere else, including the Moon. We can use probabilistic math at this scale, to describe this weird behavior.
If we measure human at the scale of years, then the human body will demonstrate dynamics and predictable trajectory: home -> school -> work -> home. We can even invent a math formula, which will correctly predicts trajectory for a human at average.
If we try to measure a human at the scale of galactic year, then our detector will fail completely, because humans are short living objects.
When you measure your particles, you measure it at particle seconds, particle days, particle years, or at much higher scale?
For example, it's impossible to measure distances shorter than photon wave length using photon stream, but it's easy to do using beam splitter and interference.
https://www.scientificamerican.com/article/giant-molecules-e...
If we scale up infinitely, then we will see that our Universe is "forever young", frozen in time.
If we scale down, we will see more and more exceptionally stable "dead" objects, completely missing short living ones, so our Universe will look "forever dead" for us again. Anything interesting will happen at our own scale only.
Thank You.
Well of course, it's an analogy, it only needs to be roughly correct. The idea is simply to build a mental model that many people can picture.
> It's best understood with the reflecting-mirror-as-clock
> thought experiment.
It's good, but not something that really resonates with people's first-hand experiences.
"ether"!
(this is not meant to be substantive criticism of your idea; I just think it's funny how much this sounds like the 19th century concept of ether that was abandoned)
I personally think of the water as space, rather than spacetime. I think time is a byproduct of space-mass interactions. I could of course be very wrong!
«To justify giving mass to a would-be massless particle, scientists were forced to do something out of the ordinary. They assumed that vacuums (empty space) actually had energy, and that way, if a particle that we think of as massless were to enter it, the energy from the vacuum would be transferred into that particle, giving it mass.»
Temporal friction!
If you want to play around with time dilation and length contraction in special relativity, I wrote a web-based calculator to help with that [2].
You can enter the relative velocity of two aligned frames that are moving along each other's x axis, and it gives you a series of rows representing events. The frames are called "your frame" and "their frame".
Each row has 4 numbers, x and t of the event in your frame, and x and t of the same event in their frame. When you enter any two of those numbers, it updates the other two.
For example, you could enter x = 3 in your frame, t = 5 in their frame, and it will update t in your frame and x in their frame so that x and t in your frame and x and t in their frame refer to the same event.
The README gives an example of how you could use this to figure out the famous ladder paradox.
[1] https://www.pbs.org/show/pbs-space-time/
[2] https://github.com/tzs/Physics-special-relativity-calculator
The consequences of this simple rule are rather shocking, as the recent nobel winner Roger Penrose pointed out.
Take the case of two people on Earth, one stationary and one travelling past in a car. They each have different events which they consider the present. Now imagine that there is an alien government in Andromeda debating a possible invasion of Earth. To the person on Earth who is moving, the debate still going on, but in the reference frame of the stationary person, it's a day or so later in Andromeda and the invasion fleet is already on it's way. If there is no absolute reference frame, and both observers reference frames are equally valid, then the outcome of the invasion debate is a foregone conclusion. This is the crux of the Andromeda Paradox and the Reitdijk-Putnam-Penrose argument. https://en.wikipedia.org/wiki/Rietdijk%E2%80%93Putnam_argume...
Otherwise, the earliest time that the aliens can find out if they're going to invade or not is when they make the decision. Sure, the observers can find out, and try to tell each other and the aliens, but since it takes at least a round trip of light from the aliens to the observers, the observers always find out after the aliens have made up their minds.
If the aliens find out that they make the decision when they make the decision, it seems to me at least that they actually made the decision rather than having it be predestined
Or maybe more to the point: if you were watching a soccer game Earth from Proxima Centauri, you could only get the result about 4 years after the game took place on earth, so for you the issue is uncertain while for somebody on Earth "at the same moment" the result has been known for years. But of course then the whole concept of "at the same moment" is pretty ambiguous and fuzzy.
I don't really get what's paradoxical here.
Because of this disagreement, if you had FTL communication and wired it up so you could make a phone call to Andromeda, your phone call would arrive before or after the debate depending on your reference frame on Earth, and you could use it to send information backwards in time.
If I synchronize two clocks next to each other, and then move one away, they are no longer synchronized as one was moving. And we still don't have the speed of c one way, so you can't use that either.
You can in fact meaningfully agree. You put the clocks in similar reference frames (like “standing on the surface of the Earth”), and accept some small but non-zero margin for error. The fact that a spaceship traveling around the earth does not measure light pulses emitted by these clocks as simultaneous is irrelevant—because we know that the notion of simultaneity relies on reference frame. What we do get is that anyone standing on the Earth’s surface can measure the pulses as simultaneous, within the specified margin of error. This error must necessarily account for the the fact that time progresses differently at different altitudes and latitudes, if your clocks are accurate enough. Because General Relativity is an extremely accurate theory and we have accurate measurements of the relevant physical constants & the mass of the Earth, we can account for the different reference frames and run the clocks at the correct rates so they continue to be synchronized even when they are free running (again, with the appropriate margin of error).
If you are using ordinary quartz clocks, then the clocks probably have enough error that you can ignore relativistic effects. I’m assuming we want atomic clocks if we’re series about chronometry, especially considering how cheap they are these days.
The idea that synchronizing clocks is somehow impossible stems from an unrealistic idea of how people measure time in the first place.
It turns out there's some circularity in your definitions of "Move clocks at the exact same speed and acceleration in opposite directions" and "synchronized"; that's basically the definition of "synchronized", which is fine from a certain point of view, but from the point of view of examining the nature of space and time themselves, not adequate.
It made me gain a bit of empathy for creators using these tactics on Youtube and tolerate it as long as it's not being used to promote content that's devoid of any substance, which is pretty much never the case for Veritasium videos or any of the other educational channels I subscribe to.
Nevertheless, I commented about this one in particular because when I saw the title card, I legitimately thought, even having seen many of his videos, that he couldn't possibly deliver on that. He got me; he did. I was impressed.
I think you may be thinking of synchronization as perfect synchronization for ideal clocks. If you go down this route everything becomes impossible. Instead, think of synchronization as assurance that the maximum error is below some known bound. In general, the bounds can be made quite low.
If 2 clocks are out synch, but only to a small amount that doesn't affect my ability to meet someone for dinner, that doesn't mean they are in sync.
I agree that if you go down this route everything becomes impossible, because at some point you're talking about Planck time, which thankfully we don't need to meetup for dinner.
I think this is based on some false idea that error is somehow “not theoretical” which makes no sense. There is a rich set of about error and even an entire field dedicated to its study (statistics). The only reason we are interested in theories in the first place is because the theories correspond to the real world. If you have some theory about time which cannot be measured, or cannot explain errors, then that theory belongs in the garbage. (Think about this: if your theory cannot explain errors, but any actual experiment produces errors, then any actual experiment will demonstrate that your theory is “wrong”.)
Or consider other concepts like “flat”. What does it mean for a surface to be “flat”? No matter how flat a real surface is, it will never be mathematically flat. So either we have to throw away the word “flat” and stop using it outside mathematics (which is a stupid idea), or we define “flat” to mean that a surface is within some deviation from mathematical flatness.
You can’t escape error, even in theory, unless you leave the world of physics and do pure mathematics.
Its true that if two observers use their own local atomic standard to precisely measure elapsed time then they would have difficulty agreeing on when something happened. However, that's not how civil timekeeping works. One clock declares itself to be a master clock and broadcasts a time+position standard to everyone else. Other observers then construct their own PLL to compute a running estimate of the total phase shift between their local time standard and the one that is broadcast.
In effect, received civil time has a velocity relative to your local atomic time. If every observer can label their observations with civil time then the paradox disappears and you are left with information flow that is bounded to the speed of light. A fast-moving observer has a local atomic standard of time that moves at one second per second, and a received civil time that appears to be running much faster.
A peer-to-peer version of this is how we define international atomic time. Subtle variations in local gravity conspire to actually give the different clocks in the network a different local speed of time. They steer a global PLL to minimize the total squared phase error in the network. Each node in the network can make observations on the TAI timebase by adding its own clock offset to their local clock.
GPS is based on synced clocks. Problem is that the satellites suffer the rules of relativity. But the good news is that we know the rules of relativity, and therefore can compensate for them.
I always thought relativity was purely theoretical, but it is actually practically applied in GPS sattelites.
The difference is: in your two examples we know there's a delay, and all you did was say that some people will know the result before others who are receiving the delayed result.
What the original question is saying is that you can't ask "well, what is happening right now on Proxima Centauri?" since the very question of "now" is not specific. I think that's quite different.
On Earth, with Newtonian physics, we intuitively think that "now" is shared, so we really can say "I wonder whether someone has scored a goal now," and the answer shouldn't depend on whether you're in a car or not.
That's insightful, and actually it made me realize that it's probably true that nowadays we expect information to be available worldwide effectively instantly, but that obviously wasn't always the case. "Right now" we're having this discussion on Hacker News with people from around the world, but merely a few decades ago this type of simultaneity wouldn't have been possible. Nothing of what would could've done locally could've immediately influenced dozens of people around the globe.
This shared "now" is effectively a modern invention.
IIRC, the implication is that change predetermined and fixed, so is essentially static.
a) If you travel close to the speed of light, causality goes more slowly (e.g. a clock ticks more slowly) than it does for someone who is stationary. So, two clocks will diverge. This is true, AFAIK.
b) If you travel close to the speed of light, and somebody else is stationary, they are traveling "into the future" relative to you, whereas you are "in the past" relative to them. I believe this is nonsense. But, I believe this is required for the "Andromeda Paradox" to hold. Thus, I don't think it holds. (BTW, we know that paradoxes never hold, so there must be something wrong with it.)
The solution to the Andromeda Paradox that I am proposing is to say that there is one universal ordering of events (among both observers and the Andromedans), but the two Earthling observers have different times on their clocks at the particular point in the sequence of events where the Andromedans decide to invade.
Proposing this is fine, but proving it is quite a matter altogether - you have to replace special relativity. Special relativity pretty much explicitly states this isn't the case, making an exception for events that are causally connected, but there's been a slew of experiments over the past several decades that call into question even that exception at long distances, and more recently, even ones for local observers, e.g. https://advances.sciencemag.org/content/5/9/eaaw9832
Personally, I'm in the Rovelli and loop quantum gravity camp - time probably isn't actually a real thing on its own, and what we perceive as time is just an emergent property of thermodynamics.
Are you sure that's not compatible with special relativity? Genuinely asking. If not, is it possible to explain why it isn't? I'd like to know (but if I'm asking you to write a book, never mind).
When you say you're in the Rovelli and loop quantum gravity camp - is that compatible with special relativity? I'm guessing yes, but let me know.
I think time is an emergent property of causality. That sounds close to what you are saying (possibly a different way to say the same thing).
I believe you are correct in that the closest you can get to change the order of two events is to make them simultaneous (in the case of photons, which do not experience time) but you cannot change one event ordering beyond that since that would violate causality.
For a real world example, we can predict supernovas because we've already seen them from another trajectory of light: https://youtu.be/ljoeOLuX6Z4 so we can both see the remote event before and after local events
All that is changing with lensing is that you're seeing the light coming from it earlier looking in one direction than another. Either way, it happened in the past. The order of events is not changing.
It's like saying that a stock trader using a fiber connection figures out about a price movement more quickly than a stock trader using a dial-up connection.
At least, I think that's right; not a physicist.
The consequence you refer to is not a logical necessity based on relativity. It's just a particular interpretation, the "block universe" interpretation, which is not accepted by all physicists.
See here for a refutation of the argument referred to:
https://www.physicsforums.com/insights/block-universe-refuti...
Relativity does away with "now" period--"now" is a frame-dependent concept in relativity, and frame-dependent concepts, in relativity, have no physical meaning.
As I noted in the article I linked to upthread, the physically meaningful division of spacetime, at some particular event "here and now" (like you sitting in your chair reading this), is into three parts, not two: the past light cone, the future light cone, and the spacelike separated region (what Roger Penrose, in the very same book where he presents the Andromeda scenario, calls "elsewhere"). The past light cone behaves like our intuitive concept of "past", and the future light cone behaves like our intuitive concept of "future", but the spacelike separated region does not behave like anything our intuition is familiar with. The whole "Andromeda paradox" argument is based on failing to recognize that the spacelike separated region exists in relativity and doesn't match up with our intuitions about either "past" or "future".
I think it follows from just determinism; you don't even need Occam's Razor.
The reason it isn't logically required by relativity is that relativity does not logically require determinism.
I think about this like this. The universe is always in exactly one state but for each inside observer the knowledge about this state depends on its location.
The example you gave can be interpreted two ways and my intuition tells me that the example is only true in the case that the car is moving _away_ from Andromeda.
Is it also, weirdly, the case that if the car is travelling towards Andromeda that time slows down sufficiently that they perceive the outcome of the debate _after_ the person standing still?
The two people would perceive the changing of a traffic light on the same street at almost the same time - why would it be different for the Andromedans?
It allows me to think in ways others do not.
I made a video on a different way about thinking time slows near the speed of light:
∂F=J (barring constants) is a beautiful restatement, but I think it puts the cart before the horse to focus on it because that formulation is possible because of the invariances that hold and that comes from the raw Maxwell's equations .. at least historically.
The abstract formalisation is even harder to convey (at least for me, and so far) since it takes away the familiar "electricity" and "magnetism" and you need to think about the more complex F that combines both. One way perhaps is to start with circuits - which are discrete and circuit laws can be expressed with the same equation and then argue for the continuous case .. but speed of light invariance would still be a long way from that compared to the raw Maxwell's equations route.
Or maybe I misunderstood what you're suggesting.
Yes, that's called the Minkowski metric, and it's absolutely nothing new.
You can do the exact same physics with the opposite sign convention (called the signature of the metric), where time has a positive sign and all three spatial dimensions have negative signs; the only rule is that time has to be the odd one out, so rotations in a space-time plane obey hyperbolic geometry as opposed to Euclidean. You can get rid of the factor of c by moving to a different system of units, commonly called the natural units, where the speed of light is 1.
All of this is covered in any real introduction to Special Relativity, which, in turn, is at the beginning of any course on Modern Physics, as opposed to the Newtonian Physics.
Welcome to Spacetime:
https://www.av8n.com/physics/spacetime-welcome.htm
THE GEOMETRY OF SPECIAL RELATIVITY with the quote:
Lorentz transformations are just hyperbolic rotations.
http://sites.science.oregonstate.edu/~tevian/physics/paradig...
The math doesn't work unless time is different from the 3 spatial dimensions. In particular, distances in space-time can be negative, unlike distances in space.
Note that if the space intervals are all zero and the time interval is unit time or 1, the spacetime displacement is equal to C. Thus when at rest physically we progress though the time dimension at the speed of light. Conversely if two points are separated by an interval equal to C, their distance in the time dimension is zero.
The latter result isn't really a surprise, we all know time doesn't pass if you're traveling at light speed, but IMHO it's interesting to see how it arises from the geometry.
You seem to be saying that there has to be constant acceleration between observers for this effect to take place.
The x, y, z deltas are your displacement through space in those dimensions relative to some frame of reference (of an observer, presumably) and t is the time component. If the space deltas are all zero then you are at rest relative to that inertial frame. You are not accelerating or moving and your motion through the time dimension in unit time, according to this formula, is C. This is odd because C is normally thought of as a motion through space, but in this case your not moving through space (in the reference frame).
One of the bits of relativity that took me the longest to become aware of was that “right now” isn’t even meaningfully and universally defined within it.
If d = c, I don't think it determines the values of x, y, z, and t. What if x = 2c, y = z = 0, and t = 1? Unless I'm not following your logic correctly. In my recollection, for two given points in spacetime, d is invariant but the values of x, y, z, and t depend on the observer's frame of reference.
"No matter who is doing the observing or how quickly they’re moving, the combined motion of any object through spacetime is something all observers can agree on. "
Without the theory of spacetime, it would seem that the two mirror systems would somehow get out of sync over time, since it appears, from the stationary system, as if the photons in the moving system are doing fewer rounds in the same amount of time. And from the moving system the photons in the stationary system appears, likewise, to be doing fewer rounds. The theory reconciles it by saying that time itself is not fixed, but dependent upon relative motion. And the relative motion is something that observers at both systems can agree upon. So, by accepting that time is dependent upon relative motion, they can agree upon the motion of photons in both systems.
Without any metric, your 4D array entries are conceptually independent and you can't make rules that depend on a neighbor entry (as you haven't defined what a neighbor is!).
In the Euclidean metric, which certainly is the most intuitive, the entries with indices closer to each other in each dimension are also closer to each other physically.
But in the Minkowski metric, the x=0,t+1 or x=0,t-1 entries are actually not closest to your x=0,t=0 entry for example! Instead, x=1,t=1 , or x=-1,t=1 are closer (indeed, with distance zero). Generalizing to more dimensions this maps out what is called the "lightcone" as it's a cone. So you can in some sense say that all points along the lightcone are "the same" or "on top" of the cone tip, at least as seen from your rules that depend on the distance. This difference in metric is what causes all the time dilations and other artifacts of Special Relativity. Light can be seen as a connection between points in the array that are already the "same" point.
You can go one step further and say that the lightcone originating from each array entry can tilt in 4D differently in each point. This is General Relativity and causes all the effects within, if you also postulate that the tilts originate from mass-energy in the region.
If all the motion of surrounding objects is in one direction only, then no time passes.
That’s why I find it easier to stop thinking of the speed of light, and thinking of it as percentage of speed of information transfer in the space-time medium. Light (or rather electromagnetic waves) travels at 100% the speed of space, but it has no special treatment in the medium, and does so by travelling at 0% the speed of time.
It's a bit unclear to me how light or a photon could have a perspective, but I guess it's not really relevant? This kind of thing is part of why I haven't taken a deeper dive into physics. The metaphors and allegories always break down if you look at them too hard, you really need to wade into the underlying math to get a real understanding I think.
It seems so simple to me now!
The presenter I’m thinking of offers us a self aware photon that is born in a star and then is immediately absorbed by a planet in the next galaxy. Or put another way, from the photon’s perspective it isn’t a vector so much as a line segment between two points.
> you can’t put space (which is a measurement of distance) and time (which is a measurement of, well, time) on the same footing without some way to convert one to the other.
This idea about 'equal footing' is really a hazy way of asking if these dimensions can be considered 'independent', which the article then describes as being interdependent.
> The key idea is that we’re all moving through the Universe, through both space and time, simultaneously. If we’re simply sitting here, stationary, and not moving through space at all, then we move through time at a very specific rate at which we’re all familiar: one second per second.
This second/second rate is 'c' the speed of light, but through the 'time' dimension of spacetime.
> However — and this is the key point — the faster you move through space, the slower you move through time. The other dimensions are not like this at all: your motion through the x dimension in space, for example, is completely independent of your motion through the y and z dimensions.
This point is misleading. These dimensions are like time in that if I was traveling at the speed 'c' in only the x direction, then to move in y or z directions, movement in the x direction would need to be reduced. The main difference is that we're observing these x, y, z motions in non-relativistic scales but since 1 sec/sec is already 'c', any observation of change in motion through time is relativistic.
> But your total motion through space, and this is relative to any other observer, determines your motion through time. The more you move through one (space or time), the less you move through the other.
Hopefully, now you can see that we're always moving at 'c' through spacetime. The only reason spatial dimensions seem qualitatively different is due to our observed motion through spacetime being timelike. A photon would have reversed qualitative characterizations of space vs time dimensions.
This video[0] (at ~1:55) illustrates this pretty well.
Also, I never understood the grandfather paradox. If I travel in the past and change something, wouldn't the change propagate in time at the speed of light? And it would take a long time before it overwrites the original time.
> I thought that time is like a film, where nothing moves except the observer.
We are not merely observers of the universe - we are part of it. When we change where (or when) we are in the universe, we change the universe itself.
> Also, I never understood the grandfather paradox.
Instead of the "go back in time and kill your grandfather" idea, lets keep the scale smaller. Let's go back in time 1 minute, and then smash our time machine. Now, there are two "me"s in this timeline. But energy and matter can't be created out of thin air! Maybe it took a lot of energy to send me back, but that was in the future, and that future is gone, now that our time machine is smashed. We could very well send gold bars back a minute, and double our money!
The way you typically resolve this paradox is through keeping infinite timelines. In our example, there's a timeline where we disappeared off the face of the earth, and a timeline where there are two "me"s. They each continue independently. But if there is only a single timeline, we can do all sorts of paradoxical things.
> If I travel in the past and change something, wouldn't the change propagate in time at the speed of light?
The speed of light is measured in meters per second. Time is measured in seconds. I'm not sure I'm clear on what your suggested propagation looks like.
I’m not trying to talk about ideas of self or anything. Just that the same thing can be described in two ways: as a low dimensional thing changing with respect to another dimension, or a slightly higher dimensional object that just is.
Viewed as points changing over time, a solution to the grandfather paradox would be a remarkable coincidence, with everything happening to match up exactly as it had.
Viewed as curves in time, a loop is just a loop. There’s nothing remarkable about a circle. You don’t look at a circle and think “gee what a remarkable coincidence that this infinitely fine path crosses itself perfectly. You just go “yup, it’s a circle.”
Neither of these is a better picture than the other. Everything (things described as 3D things) does move in time. Everything (described as 4d things) just exists.
It's not news, it's like a short physics chapter for motivated high school students.
Not that I'm complaining at all, it's just a surprise. Is this part of a series of articles or something with a unique purpose? Curious if Forbes is on some kind of educational mission or something.
I thought this series by Greg Egan was really good by any metric.
Everything has to work differently-- not just biology. Thermodynamics, electromagnetism, etc. The speed of light depends on wavelength.
Sometimes a fictional work is written as a "single point of departure" from another work or some real event and the author explores the consequences of this change. Orthogonal merely changes a minus to a plus but the consequences are extremely far reaching.
Well yes, that's why biology has to work differently. Biology is just (close to) the end of the chain of consequences of the change. ;-)
.. but it "just" ended up describing the regular Minkowski space-time.
In classical physics, forces that act in the same direction, add up. If you forcefully throw an object out of the back of a moving car, its speed at the moment you throw it would be the speed of the car + the speed of the throw. Simple and intuitive enough.
But when you involve near-light speeds... So, if you're moving at 90% of the speed of light, and you have a lightbulb in your vehicle, what would be the speed of the photons coming out of it? Logically, it would be 190% of the speed of light. But apparently that's not how it works. So how the hell does it work then?!
Edit: from an external observer, the forces also do not add linearly becasue they are not acting on the same spacetime direction.
When you create waves on a water surface with your hands, the speed in which they propagate along the surface is the same regardless of whether you just tap the water straight down, or make a sweep with your hand.
But at the edges of those circumstances, we see discrepancies not predicted by classical physics. That lets us know that classical physics is an approximation. The truth is that even in your car + thrown object example, the velocities do not actually add linearly -- it's just that the difference between the linear approximation and the reality is tiny.
One analogy that might be helpful: consider every object in the universe to be moving at c, but for most objects that you encounter, the vast majority of that motion is forward in time rather than space. As you add energy to the object, it transfers that velocity from time-motion to space-motion. Because c is so large compared to our usual experience, we have to look really hard to discover the change in time-motion... but it's real. GPS uses satellites that are in a sufficiently different frame from the surface of the Earth, and requires sufficient time precision, that relativistic time difference calculations are required.
So, for me, in a vehicle moving at 90% of the speed of light, the time itself slows down such that I observe the photons coming out of my lightbulb still at the speed of light, right? Basically, it would adjust the t in v=s/t because the speed and the distance are to remain constant. And because me and my spaceship or whatever have mass, there will always be a bit of difference between my speed and the speed of light to accommodate this adjustment. That's just so weirdly backwards to think about.
Now, I wonder about redshift and universe expansion deduced from it. Wavelength is a frequency, right? And frequency, by definition, is how many times something (wave period) happens per unit of time. So maybe the universe isn't expanding after all, maybe it's just that time runs faster in the parts of the universe where this light comes from, and the photons just keep oscillating with the frequency that was "correct" in whichever reference frame they were emitted? Besides, the idea that universe expands seems silly tbh. There must be a more sensible explanation. Maybe time slows down over time (?!) and it just ran faster when the universe was younger?
How does this hypothesis account for the CMB?
Wikipedia has a nice summary of alternative cosmological theories (and why they are often dismissed by experts):
https://en.wikipedia.org/wiki/Non-standard_cosmology#Proposa...
Edit: on further reading perhaps I haven't even addressed your question...
If I had to hazard a guess I would say it's to do with light having no mass. Once it's emitted it's no longer in your frame of reference so your supposed speed is irrelevant to it.
In our daily life we have a handy stationary frame of reference - the Earth; it's therefore intuitive and normal for us to think about speed in an absolute sense. In reality though, speed can only ever be measured relative to some other point, so if you're an astronaut freely floating in space, you can't make any definitive statement about your absolute speed because there is no such thing.
Another thing that we are used to experiencing is a uniform flow of time - you and I can both synchronize our watches and believe that we're reading the same time even if we're on opposite sides of the globe. This is in fact only true in a very limited set of circumstances, and generally speaking not the case. Just as there is no such thing as absolute speed (or more properly, velocity), there is also no such thing as absolute time.
Special relativity stipulates that light always travels at c in every reference frame; this is not a discovery of relativity but a prerequisite of it, and a key insight that allowed Einstein and friends to explain things that had previously been inexplicable. The explanation for this, as others have commented, comes from general relativity, and is due to the interconnection between space and time - space-time - such that every non-accelerating thing is moving at the speed of light through time (whatever that means). The speed of light is therefore not so much the speed of photons, as it is that photons move at the speed of time. Something like that I think - this is beyond the scope of my brain to explain!
So why aren't your photons moving at "190% the speed of light"? In our normal life, we think of time as fixed, and velocity as variable; in reality, the speed of light is fixed, and time is the variable. What changes for a 'stationary' observer is not the speed of the incoming photons, but some time-related factor - their frequency. This is (a simplified explanation) of redshift, that led Hubble to theorise that the Universe is expanding. Everything is moving away from us emitting light, and since the velocity of light is fixed, it's the colour that appears to change.
A note about the notion of velocities adding up - in fact they don't ever, exactly. That they appear to is an approximation due to the enormous velocities needed before you would notice any difference. This is another consequence of relativity. Likewise synchronizing clocks - we can get so close that we couldn't measure any difference, but it would be insanely impractical to set and keep totally synchronous time.
(source: physics-related undergrad some years ago - I stand to be corrected on any of the above :-)
The important point here is that this is is not "logically" but "intuitively", and it turns out our intuition is simply wrong. Historically, this was in fact the motivation for special relativity: we noticed that the speed of light does not obey Newton's relativity principle: if you fire two beams of light, one from the ground and one from a moving train, they will both arrive at a detector at the exact same time.
Having observed this concerning fact about the world, we now had to come up with a theory that explained it. The one we landed on was that speed is a number that goes not from 0 to infinity, but from 0 to c. This then must mean that accelerating an object from 0 to 1/3c is easier (requires less force) than accelerating that same object from 1/3c to 2/3*c, and that this difficulty increases the closer you get to c. The final theory in fact predicts that an object with mass can't even reach c, it's speed can only grow infinitely close to c. However, massless particles (such as the photon) do move with speed exactly c.
Note that we have since repeated this experiment with other particles of non-0 mass and confirmed that photons themselves are not special, c is indeed a limit. For example, electrons have some mass and don't move with speed c, and an electron fired from a moving train will arrive faster than an electron fired from the platform, but the speed of the speedier electron will NOT be V_e + V_train as Newton would predict, it will be (V_e + V_train)/sqrt(1-(V_e+V_train)^2/c^2) if I remember the Lorrentz transform correctly.
In the vehicle, you'll measure the photons moving at 100% c.
Someone outside the vehicle as it speeds by will see the photons moving at 100% c - as you approach and as you move away. Other parts of spacetime change to accommodate this, effectively.
The author of the article we all comment here wrote a book which, if I remember, covers that story in more detail (I linked to it in another post and I highly recommend it to anybody who wants to learn a bit more than when reading an article here or there or read some comments).
Just to give you some hints, the real story is not "reasoning" something but the experiments -- the experiments practically produced the results that, when analyzed, are still the best explainable by accepting that the speed of light doesn't change. See:
https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_exper...
Michelson is such a genius, that even the most currently advanced experiments today use the same ideas that he used then -- LIGO is also an interferometer:
https://www.ligo.caltech.edu/page/what-is-interferometer
Learning from these experiments, even before Einstein, some important formulas we know today as related to special relativity were already developed by others:
https://en.wikipedia.org/wiki/History_of_Lorentz_transformat...
Just consider that you have this object, light, that will always travel at the fastest speed possible. You can approach that speed. But throwing light in front of you can not increase light's speed.
That's wrong. It's still a straight line, just along the geodesic of the local spacetime. It looks curvy if you bend and flatten spacetime into a Euclidean space, but that's because you bent it! Light in a vacuum always travels in straight lines. Gravitational lensing isn't gravity bending the light, it's the light traveling in straight lines through highly curved space.
> “...you can’t put space (which is a measurement of distance) and time (which is a measurement of, well, time) on the same footing...”
...two parentheticals followed by dippy do metaphor 'footing?' Feet? I’m only half kidding. The article's first 50% starts in the best of ways, ‘shortest distance between two points’. After ‘feet’ it’s all down hill--
> “the faster you move through space, the slower you move through time.”
Well. Of course. Every school child knows this is true. When the kickball is heading for your face, time stands still. But slow down a minute, walk me through this as carefully as you did for the two-points...
> "But as you approach the speed of light"
Never going to happen
> "— or rather, ..where the relative speed between you and it is close to the speed of light — you’ll observe that it’s contracted along its direction of relative motion..."
It relative that it and it's--WHat? There was distance between two points and now I have 'relative speed'. What the heck is that that I should be able to call it an it so quick??
I could go on, but my point--Why, once the subject becomes actually difficult to understand, does the author's explanation becomes worse? More parenthetical. It and that. And tries to put me in a position to imagine moving at the speed of light or a clock "defined by light bouncing back and forth between two mirrors". Huh. What?
Great start. Poor finish. Maybe if you thought less about name dropping Einstein, I could appreciate what is relative speed between two anything. I agree, this curious piece of math and science could use some popular updating.
/END RANT
This reminds me of Quake (and many other games), where moving diagonally is faster than moving either forwards, as the game simply adds the two vector components independently based on which keys are pressed.
Of course we're very familiar with the motion through one dimension being dependent of the others. My car has a top speed, regardless if I'm driving north, northwest or whatever.
The weird thing about spacetime thus does not seem to be that we're racing through it at top speed, but rather that we can't slow down.
A Spacetime Surprise: Time Is The Easiest Dimension To Waste With Misleading Titles.
So, out of curiosity, I switched over to Android Firefox and turned on "reader view." Much better!
https://support.mozilla.org/en-US/kb/view-articles-reader-vi...
https://worldscienceu.com/courses/special-relativity-world-s...
But it does, if you're moving at a constant speed... just like you're moving through time in...
and 2) books on spacetime math/geometry/tensor algebra and its applications beyond modern physics.
For a more recent treatment, the works of Carlo Rovelli are all about more human scale applications or implications of the theory.
https://www.google.com/books/edition/Einstein_s_Theories_of_...
https://m.youtube.com/watch?v=WIKOFLRQuc8
That's not advanced undergrad certainly :) .. but I found the following great to learn from. Especially the treatment of momentum and the stress energy tensor really worked for me -
https://www.amazon.com/First-Course-General-Relativity/dp/0521277035https://www.amazon.com/Beyond-Galaxy-Humanity-Discovered-Uni...
“Beyond The Galaxy: How Humanity Looked Beyond Our Milky Way And Discovered The Entire Universe”
There is no dimensions outside your head in the first place.
It is a superimposed abstraction, like a coordinate system.
And the best part of your phrasing is its consistency with simulation theory.