Why Is Light So Fast?
profmattstrassler.com
profmattstrassler.com
The best demonstration of this I've ever seen is on "If the Moon Were Only 1 Pixel - A tediously accurate map of the solar system" [1].
If you've not seen it before, I recommend opening it, using your mouse wheel to scroll from the beginning (near the Sun) to Earth. It should take about a minute, but there's some commentary on the way. Then, to save your mouse and your finger some work, try clicking the icon in the bottom right hand corner to auto-scroll the map at the speed of light.
[1] https://joshworth.com/dev/pixelspace/pixelspace_solarsystem....
These videos were super fun to make and kept me sane when I found myself with far too much free time and a bunch of world news to avoid. I never did the fifth (and final) walk but it's only about 100 meters long so I hope one day to do it in person (if I ever end up with that much free time again).
Yeah yeah dark matter blah blah, but it’s not a concise enough answer.
Dark energy, on the other hand, is the postulated cause of the expansion of the universe.
Edit: not a Physicist!
This is something I find incredibly counter-intuitive. At the photon’s reference frame (speed of light), time stops. In our reference frame, I’d expect some kind of “divide by zero” error in nature, resulting in the infinite speed of light. But it’s not infinite. It’s just… some constant.
The photon has zero velocity through time because its velocity through space is c. In its reference frame, no time passes between creation and destruction, but from all other reference frames it appears to be moving through space in discrete time but with high velocity.
Brains are bad at these scales. Maybe mine is worse than average. I can't fully believe how impossibly far away so many things truly are.
For example, if you've ever opened an electronic device and seen squiggly traces everywhere, it's because of light speed[0]. When signals get fast enough, you have to make sure the traces have the same length to within a fraction of a millimeter. Otherwise, the signals on two traces arrive at different times and nothing works.
[0](electrons move at ~c, more or less)
There's a significant delay in satellite communications, on the order of milliseconds. It takes several seconds to reach the moon. 20 minutes to Mars, a few hours to Jupiter. Famously 8 minutes to the sun. The nearest galaxy is 1.5 million years away.
Consider how stupefyingly large our solar system is. Then consider how insanely huge the galaxy is: 90,000ly. It takes ninety thousand years for a photon to go from one edge of the galaxy to another. Then compared to the universe at large, our galaxy is essentially a single point.
Light is extremely slow and the universe is vast beyond our meat brain's ability to comprehend.
Pedantic: electrons (in a wire) move at relatively ordinary speeds. Wikipedia says this [0]:
>The drift velocity in a 2 mm diameter copper wire in 1 ampere current is approximately 8 cm per hour. AC voltages cause no net movement. The electrons oscillate back and forth in response to the alternating electric field
Think of it more like a long row of billiard balls. You hit the first one, it moves a little, then it hits the next one, that one moves a little etc. Except the "hit" is just getting closer to the next electron to make it move (like if they were magnets).
At least that's my understanding of it.
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Maybe "hit" is the correct word? Isn't it the same forces at play that stop my hand from going through the table?
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So switching circuits need to take into account "transmission line" effects.
It feels more intuitive to me when thinking about it as causality always unfolding around you at the same speed, no matter your own frame.
The constant c was not named for causality, but it is a nice coincidence.
If a neutron disappeared, you'd NEVER noticed that causality.
* https://www.youtube.com/watch?v=ZbGxXyqlhbU (FloatHeadPhysics on this)
One physics convention just sets its value to 1. All of those Minkowsky diagrams that we see are measured in light seconds on the space axis, in order to make c have the value of 1 space unit per time unit; so all of the graphical sheep, spaceships, cats, people, torches and stuff that are placed upon them are very much not to scale. (-:
Interesting. So c² would also equal 1. Which (in those scaled units) implies E = m.
This not only greatly simplifies the relationship but actually makes much more sense that way.
Not only does this clarify the relationships, but in many ways, these "natural constants" are an artifact from our past ignorance. Boltzmann's constant, in a way, is nothing more than a "conversion factor" that we hold over from the time when we believed temperature and energy to be two separate concepts. In the same way, the speed of light is an artifact from a time when we considered time and space to be distinct concepts, measuring them in distinct units, and needing a conversion factor (in units of distance/time) to map between them.
It's as if we would all collectively agree that the "up" direction, from now on, would be measured in floops, and the slope of a hill would be measured in floops/meter.
From a philosophical point of view, it's not just saying "measure time in units such that c = 1". It's saying "let's consider time to be a distance, and measure it in the same units as we do the other ones".
Here is an example. The charge of an electron is exactly the opposite of the charge of a proton, to within measurement error (like ten digits). This is simply something which has been measured, but physics has no explanation for why they are the same. Getting to the point, what is more likely: that a God created it that way in order to achieve His goals, or that there is some reason connecting the two charges such that they must be of exactly the same magnitude but we just haven't figured it out?
I'm putting my money on the latter. If there is an all-powerful creator, there is no reason to have fine tuning at all -- He could just force the desired behavior and outcome.
If the basic laws of the universe change, we'll be disincorporated before we have a chance to know it's happening, because it will happen at whatever the speed of light is in the new balance, and the chemical processes that make us tick will change to other chemical processes that don't actually work anymore.
In the new universe silicon based life might function. Or stars might not work anymore.
So I expect it would change a lot.
I don't think the numbers independently are valuable, but together the constants of physics are tuned to support life. To be honest, I dislike the Anthropic principle as a generalizable cop-out, but it nonetheless works.
The fact that time even exists is implied by / a result of causal actions having some finite propagation time.
I think I know what you're getting at, but somehow the phrasing bothers me, as if there is meta-time or as if cause and effect have time between them... for the photon at light speed, time isn't passing, it's emitted and then zero "time" later it hits something very far away.
It's more like we somehow need to think of cause and effect chains that have orderings without time.
I wonder if future generations will ever look back and casually quip something about "well they believed X existed, that was their problem, it all makes intuitive sense if you just..."
However, you have to be careful with terminology. There is no inertial frame co-moving with the photon. All we can say is: as a massive particle gets faster relative to an observer's frame, the time it experiences relative to the observer's frame becomes shorter, and in the limit, as it approaches the speed of light (but never reaches c), the experienced relative time approaches zero (but never reaches 0).
This is well explained by Don Lincoln on the Fermilab YT channel:
Do photons experience time?
So what exactly are you talking about?
he’s talking about the photons frame of reference
Imagine a universe simulated in a computer with “ticks” where the entire state is updated.
It would be different to ours, but it would work just fine.
The fact that causal effects happen in the next tick means some minimal time has passed. So in your definition causality can cover the entirety of a finite universe in an incredibly short amount of time (one tick). But it seems like that's not the same as covering the entirety of a finite universe in zero time. In that case, every result would happen within the same tick as its cause.
A simulation could use particles with attributes like position, velocity, and also a "sum of forces". Then each update has the following steps:
1. Reset the sum of forces for all particles to zero.
2. For all particles, add the contribution to the forces on it from all other particles.
3. For all particles, update the next particle position based on the collected forces.
In the above, there's no intermediate state between updates, everything moves to the next position synchronously from the perspective of in-universe observers. (The external simulation can update particles one-at-a-time, but this is not an "observable" inside the simulation.)
If there’s some universal limit for causality itself, then light just happens to be the fastest thing among all the other things subject to causality.
Other similar particles, like the W and Z bosons, are manifestations of the weak field. Since that field breaks the symmetry, those particles have mass and move slower.
BTW, that symmetry breaking is the very same one that physicists talk about when we discuss the Higgs boson.
Can you unbox this a little? I think I may just have Friday brain, but I'm having some difficulty convincing myself in the moment that infinite-speed causality development would prevent time.
The best definition I think I've seen is to view the universe as a partially ordered set of events, meaning that you can only order events (in time) if they're within each other's cones of causality. Outside of that you cannot say which happened first. That's the partially ordered part.
But even that is incomplete and arguably even self-referential. What's a "cone of causality" (without relying on causality)?
Also, there's the issue of what exactly time is and whether events are time-symmetric or not. Many physicists seem to view time as an emergent rather than fundamental property of our Universe.
Time is a relationship between clocks.. beyond that, yes, it's hard to say exactly.
Time seems to be what prevents everything from happening at once.
I can't even describe 'happen' without using verb tenses, which represent time.
If you want to simulate a giant world with millions of players, you either have to slow down the frequency at which you update the world to give the computer enough time to do the computation, or you have to introduce some sort of speed of causality in the game in order to be able to distribute the computation across multiple nodes.
If you have an infinite speed of causality, a server may need to receive data from all the servers. In the worst case, all servers will need to receive data from all the servers.
With finite speed of causality, a server only needs to receive data from the servers that simulate parts of the world that are nearby.
It's kind of an anthropic principle argument.[1] If the fundamental constants had substantially different values, the resulting universe would be boring. All the mass collapsed into one black hole, or evenly distributed as fundamental particles. Or atoms don't work. Or stars don't work.
This leads to the usual problems - many-worlds theory, gods, etc. Strassler hasn't gone there, but others have.
[1] https://profmattstrassler.com/2024/10/03/why-is-the-speed-of...
In natural units, in all such theoretical universes, c is 1; and all that this argument really states is that humans and similar atom-based things have to move at very small fractions of 1.
I think this could be an instance of falling prey to the same sort of assumptions that the anthropic principle is intended to expose. For example, imagine theoretical universes where the speed of light is non-constant, or varies depending on where you are, or changes over time, etc.
Hypothetically you could go faster if you used fission or fusion energy but practically the chemical bonds get in the way. Even
https://en.wikipedia.org/wiki/Nuclear_thermal_rocket
is limited by the strength of chemical bonds of the reactor so it gets a factor of 2 or 3 or so on exhaust velocity compared to a chemical rocket.
We could build a decent fusion pulse drive today if we had higher temperature more compact superconductors and super-efficient compact lasers that could fit in a spacecraft and ignite a strongly net-positive inertial confinement fusion pulse.
Our superconductors are almost good enough, but our high-power lasers are way too inefficient and bulky. We can't even make economically viable ICF on Earth with current lasers.
The biggest un-realism in The Expanse is the lack of huge heat-sinks, at least in the show. (They aren't mentioned in the books but I assumed they'd be there.) Without heat-sinks even if the drives were >95% efficient the ship would melt. Also in the show the thrust plumes from the engines are portrayed as looking like grill flames. In reality they'd look more like beams of light fading off into space.
https://en.wikipedia.org/wiki/Heavy_ion_fusion
the argument is that "highly efficient laser" might be an oxymoron and if you were serious about commercial fusion you might trade lasers for much more efficient particle accelerators that run at a viable shot rate. Trouble is that you need heavy ions (lead) at 8GeV and it takes multiple barrels that are a km long or so... A huge machine that might be competitive with lasers for a commercial power plant but that can't be built in a subscale prototype. It might not be compatible with the magnetic nozzle though as 8GeV is not relativistic for lead ions.
A heavy ion power plant is possible in terms of the physics but needs to be the scale of a fission power plant to work at all and is projected to cost maybe 2x what an AP1000 costs assuming everything goes well and we know things usually work worse than you expect. So nobody is interested in funding a full-scale prototype, a reasonable development plan is you build several linac barrels and a test fusion facility and expect to rebuild that and add more barrels. It probably costs about what Musk spent on Twitter in the end.
K. Sandner and H. Ritsch, Physical Review Letters (2012); Temperature Gradient Driven Lasing and Stimulated Cooling
Humans can travel around the world (our domain) in a matter of hours (on rockets, our fastest mode of travel thus far). Similarly, the fastest waves can cross the ocean in a matter of hours. Light, on the other hand, takes billions of years to cross the visible universe. It's downright glacial at those scales!
More seriously: Your very point is already made near to the beginning of the headlined article, in the book quote. You might want to read beyond the headline question, otherwise you're just repeating what the article already says.
If you now want to make the point that humans are fast on human-centric scales, which you did not really make above, you enter a whole other discussion that involves biology, and humans not really being very fast at all compared to some other creatures. You have, after all, to introduce non-human entities, rockets, to show examples of humans being "fast".
And really fast rockets aren't examples of humans being fast, as they are good examples of humans becoming dead, from the accelerations involved for starters. Take a human out of a rocket system, and it can go much faster. Human-ridden rockets are in fact slow, too, even on human-centric scales, compared to the things that are extremely fast in the human-centric world. So that argument falls down.
Which leads to part 2, pointed out in a top-level comment by Animats, at https://profmattstrassler.com/2024/10/03/why-is-the-speed-of... which goes on to explain that humans are by necessity slow.
There are even biological discussions of the same idea: why humans are slower than, say, houseflies. We're slower than c for physics reasons, and we're slower than things on our own scale for biological reasons. We actually are not extremely fast. We aren't as fast as our machines, nor even as fast as some other creatures.
If you were to travel at nearly the speed of light, you could cross the universe in a matter of minutes. Of course an external viewer on say Earth would disagree and say it took billions of years, but who’s counting?
n.b. I obviously lack the vocubulary to communicate properly about this, help needed!
Meanwhile we measure time in hundreds of millions of meters and space in nanoseconds. Something causes humans to be slow.
Which isn't that surprising, life is basically a diffusion process gone haywire and while we're more efficient than just a big rock being pushed by small particles we still rely on statistical physics to push molecules around and it takes a while for those statistics to average out.
The speed of light being "slow" in cosmic terms is almost necessary for our existence in that we need a relatively long period of relative stability in order to evolve into sentient life. And that becomes a whole lot harder if, say, the Milky Way was only one light day across.
Bacteria can exhibit doubling times on the scale of tens of minutes. We know of trees that live for thousands of years.
On the other hand, we know of chemical reactions that can propagate significantly faster that the speed of sound (high explosives) and nuclear reactions that propagate even faster that this. At the other end of the scale, we have mildly radioactive elements with half-lives measured in billions of years.
This is all to say that everything is relative and no matter what constants you choose for the universe, they’re going to seem arbitrary.
Right, and that's just the visible universe. The full extent of the universe is much larger -- I think that the most cautious lower bound estimate is that it's 250x larger. It could be 10^10x larger, or even infinite/unbounded. In such a vastness, the speed limits for light and baryonic matter are perplexingly slow.
Well... That's to be expected. It's right there on the definition.
The speed of light isn't that fast. The website "the moon is one pixel" is a webpage where our solar system is represented at scale if our moon were 1px in diameter : https://joshworth.com/dev/pixelspace/pixelspace_solarsystem....
You can scroll through it. It's so long to go from one planet to another. So much empty space.
At some point you're tempted to click on the C button which you see on the bottom right of the page. Speed of light! Surely that will autoscroll fast! … Nope, to scale, the speed of light is waaaay slower than your scrolling was! And then you realize, at the size of the universe, how even light isn't that fast.
- make the matrix as rough as possible while still enabling interesting events (ie, try and maximize the Planck length)
- make the maximum speed at which events propagate across the matrix as slow as possible to save the CPU (ie, try to minimize the speed of light)
- limit the size of the simulated universe
But our Planck length is tiny and the universe is probably humonguous unless we’re being deliberately deceived by This Simulators.
So despite the suspicions aroused by the slow speed of light, we might live in the mother / “real” universe after all.
"The Planck length does not have any precise physical significance, and it is a common misconception that it is the inherent “pixel size” or smallest possible length of the universe.[1] If a length smaller than this is used in any measurement, then it has a chance of being wrong due to quantum uncertainty.[2]"
https://simple.m.wikipedia.org/wiki/Planck_length#:~:text=Th....
How would that save CPU time?
The Game of Life does have a maximum speed of propagation of causality, but it's not designed in, it's just a consequence of the basic rules that define the simulation.
But if you limit the speed at which events propagate, my feeling is the rate of events occurring will be lower overall, since one event triggers another and each event will trigger fewer secondary events per second if it propagates less distance per second.
You could now also have islands of stability so that a cataclysm on one end of the simulation will take a long time to spread to the rest of it.
IE, Andromeda can explode and we won't even know for a long time. In that time, we will continue doing interesting / entertaining things, or continue calculating the answer to life, the universe, and everything.
With a slower speed of light, I think you’ll get qualitatively different events, not just fewer events.
But you're right in that you'd get different effects, because effects like gravity depend on the distance between objects, which is invariant on the speed of light, so a "human" in a universe where c is halved but everything else is the same wouldn't just be larger, but be completely different.
But if you're changing the behavior of the simulation then it's not even an optimization anymore. You just changed the simulation. It may as well be something entirely different.
"Ok, then, what’s a hydrogen atom?
It is the simplest example of what physicists call a “bound state” — the word “state” basically just meaning a thing that hangs around for a while, and the word “bound” meaning that it has components that are bound to each other, as spouses are bound in marriage. In fact, the image of a married couple, especially one with one spouse weighing a lot more than the other, is probably the one you want."
The ratio of the Hydrogen atom's ground state electron binding energy to the electron's mass-energy is one half the fine structure constant squared. That implies the nuclear forces don't have much to do with it - electromagnetism is simply, and dimensionlessly (i.e. independently of any arbitrary units or scales), a weak force.
A real demo is to talk via VOIP with someone on a satellite internet connection. The old, geostationary satellite kind. It takes so long for the audio to get there and back that you have to practically say "over" when you're done talking.
Fast is relative but astonishment is a human reaction based on our brains being slow because evolution gave rise to large molecules which have to jiggle around before things work so the light is already at the moon by the time we've registered much. If instead you use a computer processor it only goes about 10 cm per clock cycle or not so fast in practical terms.
Seems to me that we should be asking the question why is the speed of light so slow!
We're forever discussing the Higgs and how matter gets (some of) its mass but we spend precious little time discussing this relationship:
c0 = 1/(μ0 ε0)^0.5
That is why vacuum permittivity and vacuum permeability 'conspire' to slow down light to its known value.
Is the universe 2D?
And it's E=mc^2 because the only 'consensus' value in the universe is c? Why is that? And so the 'mass' (whatever that is) must be moving at the speed of light for the equation to make sense, even though it's stationary?
The blogs demonstrate great factual knowledge and 'mastery', but don't really explain anything IMHO
As to the internal/intrinsic energy of a given object, think of it as "hidden potential energy". It is essentially the energy that was required to turn photons into the matter that you, and I, and everything else are made of! The equation itself is mc^2 simply because that is what you get when you rearrange and simplify the experimentally-verified equations which it was drawn from. Likewise, for c is nothing more than the measured value of the speed of light in vacuum for any observer. Of course the choice of units is completely arbitrary. Whether you state it in miles per hour, kilometers per second, or whatever, the ratio remains constant.
We are just making more and more detailed observations and then creating mathematical models of these behaviors. For example, we observe that space is curved around mass. We can model that and it helps us understand what's going on, so it's useful.
We don't, however, understand what exactly is curved and what is this empty space that curves.
When you make a wave with a rope or on a water the speed of the wave is formed by the speed of the interaction of the molecules in it. I believe similar thing happens with the light in subatomic level. There, may be the spped of light is limited on how fast an higs boson interacts with the neighbpring higs bosons.
Regarding E=1/2mv*v, where does the 1/2 come from?
Consider constant force acting on a mass initially at rest, for time t producing final velocity v and distance travelled x.
F = ma [Newton's 2nd Law]
v = at = (F/m)t, so mv = Ft [momentum transfer is force times time]
x = (1/2)vt [area under v-t curve: triangle formula]
E = Fx = F(1/2)vt = (1/2)v(Ft) = (1/2)mv²This rhetorical trick is so common in physics, that's why I wanted to mention it. The trick is to define the same word twice with its opposite meanings and use the word with both meanings sometimes even in the same sentence. I wonder what is the name of this trick in logic.
[1] This quote is from a different articla: https://profmattstrassler.com/waves-in-an-impossible-sea/wav...
The whole concept of measurement (which you will need to describe something as stationary) depends on an observer travelling at speed velocity less than speed of light. If you Try to assign a inertial frame of reference to a photon you will break laws of special relativity
- photons always travel in speed of light from perspective of any inertial observer whether it is moving or at rest (verified by experiment)
- relativity prohibited mass less particles (like photon) to be stationary.
E = mc^2 doesn't apply in photon and apply only on stationary objects. The complete equation is
E^2 = (p^2 c^2) + (m c^2)^2
Where m is the rest mass. In photon case the second terms vanishes and the energy is merely the first term.
-Time and space behave differently for photons. In relativity, time dilation and length contraction become extreme at light speed. A photon does not experience the passage of time in the way objects with mass do. Therefore, trying to define a reference frame for a photon would lead to contradictions in our understanding of spacetime.
So in general it doesn't mean much absolute vs relative on photon case.
I added my comments to your sentence. Obviously, if photon appears to be stationary, this means relative to something else. So relative and absolute are not meaningless.
I study motion not special relativity. Motion is a fundamental concept. We don't need to use Einsteinian lingo to study motion.
As a result, of all the speeds, when discussing the speed of light (which conventionally means "in a vacuum" unless otherwise mentioned), you can in fact ignore the question of reference frame, with the exception of you don't want to use a reference frame itself moving at the speed of light. But other than that, an article exclusively confining itself to the discussion of the speed of light in fact doesn't need to worry itself about the relativity of reference frames. For that speed alone, you can't level the complaint against it that it ignores the issue of different frames, because it uniquely doesn't matter.
(Relatedly: It is frequently given as the reason you can't reach or exceed the speed of light is some stuff about masses rising. While mathematically true in its own way, I think there's a cleaner reason to explain why you can't reach or exceed it, which is that you can't even get closer to it. No matter what you do, the speed of light is c. You accelerate to a thousand miles a second in some direction, and how much closer are you to c? The answer is, none. Light continues fleeing from you, in all directions, at c. There is no "get really close to c somehow and then just push yourself over really hard" because there is no "get close to c" in the first place. No matter how hard you accelerate, in what direction, in what order, in what manner, you not only can't get "close" to c, you can't even get closer. For similar reasons, in this paragraph, I don't need to qualify in which reference frame you go a thousand miles per second different than before, because it doesn't matter for c. You can't even get slightly closer to it, let alone "exceed" it somehow. It is an absolute.)
If we're in an airplane going 250 m/s - it also takes me 2 hours to fly to NYC. The air around the plane is windy, the air inside is still.
Now we're in a spaceship headed from the earth to the moon & it's going to take us 3 seconds. We've agreed ahead of time it takes 1.5 seconds for light to go from the earth to the moon. C is the speed at which light moves, and we're taking 2x the amount of time light would. The air inside is still. But we're still going fast.
How could it be that we are not approaching the speed of light?
What do you make of time dilation at higher speeds if we aren't approaching the speed of light at all, ever?
There are many ways to grasp this, and one has to try several of them to find an intuitive explanation that works for onesself; but one way is to consider that you, in your spaceship (as long as you are coasting along with no rockets firing, and aren't performing orbit/deorbit burns) are, in your frame of reference, at rest. Your speed is zero. You haven't approached anything at all, and light is still whizzing away from you at c. Indeed, it's Terra and Luna that are experiencing time dilation as far as you are concerned, because they are the ones with the high speeds.
* https://www.youtube.com/watch?v=Zkv8sW6y3sY (FloatHeadPhysics addressing this in another way: there are a other approaches still)
Accelerate at the beginning and end of your trip over to Luna, and of course general relativity comes into play and things get more complicated. What many gedankenexperiments get wrong is that usually there's only a short period of burning the rockets, in the real world. So for most of your trip in the rocket you aren't burning propellant and are in an inertial frame of reference. No inertial frame of reference approaches the speed of light/causality, by postulate 2 of special relativity, and one is always at speed zero in one's own inertial frame of reference.
In the historical context I'd classify it as a "clue" but not a proof. Proof really came from observations.