The Picard ManeuverThe speed of light
Lumidaub
tomiant
TachyonTeleUhh, Jim. I don’t think…
He has no problem doing conversions, like anyone after about 1976
LenaThere’s a convention in theoretical physics to adopt so-called “natural” units. In the natural system of units, measures of length, time, and mass are chosen so as to make the speed of light and the gravitational constant 1. Or sometimes it’s the speed of light and Planck’s constant.
Anyhow, this makes the resulting measures of length, time, and mass completely nonsensical to any human scale problem. But it makes physics equations much shorter to write down, because you can drop all of the c’s, G’s, and h-bars and whatnot.
For example, the famous E = m c^2 becomes E = m. Energy is mass. Voila.
Explains perfectly well how I can eat a hot dog and then go do some things.
Also explains that after I do some things, I need to take a shit.
Don’t correct me, you all know I’m right and fifteen percent of you are currently on the toilet.
Beat me to it!
teftThe speed of light is a tautology. We define it via how many meters light travels in a second. And we define the meter by the same measure. It’s just the distance light travels in 1⁄299792458
c is a measurable constant, not come unit that is arbitrarily defined. Like Boltzmann’s Constant, or the ground state hyperfine transition frequency of the Cesium-133 atom… it just… Is.
Therefore, it is a useful tool to define units. You claim it is a tautology because we write it in units of meters per second, while the meter is defined based on c. This is easily disproven, as you can represent the speed of light in any unit of velocity. It is a fundamental constant, derivable through experiment without any unit a priori.
It’s not about the units i used. It’s about using something to define itself. The same problem happens when you use c to define empty space since empty space can define c.
Once you decide which units are used in maxwells equations then the electromagnetic permeabikity and permissivity pops out as a proportions of c.
Read more Feynman if you don’t believe me.
That may be, and I’ve been meaning to dig into my copy of the Lectures, but that’s moving the goalposts. You said that it was a tautology because it was defined by the meter, and the meter was defined on it. That statement is demonstrably false.
I used the meter because that’s generally what is used for measurement in scientific endeavors. There was no goal post moving if the statement applies for all SI measurements.
Literally the entire point of the comment that you’re responding to is that it isn’t true for the metre, and it isn’t true for any SI units.
Your entire claim of tautology rests on the assertion that the speed of light is defined by something external to light itself. That’s false. It remains false irrespective of which SI measurements you swap in.
Just because the speed of light can be expressed in terms of SI units, doesn’t mean its definition depends on them. Which is the point that wolframhydroxide was making.
This directly disproves your original assertion of tautology.
Every metric of speed of light is necessarily relative to other things. Even if you define as 1, now you must be able to know what one unit of time is relative to one unit of distance, and if you do not know that then you do not know that your speed of 1 means.
All fundamental units are defined relative to each other in physics, and all other units are defined relative to the fundamental units.
It’s not useful to tell somebody it is constant without a way to make use of it. Without knowing how it’s defined relative to other things we can’t use it.
The thing about all the absolute physical constants is that they are almost all based on units defined relative to other things. Unitless constants (defined only as a ratio) are extremely rare (like the fine structure constant) - but even then you have to make up units to measure them (although you can still agree on unitless values with somebody else who chose different base units for measurements).
I was unaware that the person to whom I was replying, who claimed to be intimately familiar with the complete works of Feynman, needed instruction in how to “make use of” a fundamental constant of nature. If that is something you think is necessary, perhaps you should see to their instruction in such matters, as you are so confident in your faculties of condescending instruction.
Furthermore, I am acutely aware of the existence and nature of dimensionless constants, thank you very much.
For somebody who claims to be acutely aware, you really seem to have no idea what goes into calibrating measurement devices to be able to measure physical constants. And for somebody claiming I’m the condescending one, you’re awfully rude yourself
Again, I think you’re replying to the wrong person. I never disagreed with any of this. I literally learned all of this years ago. I appreciate your attempt to educate, but I’m unclear on its purpose. The dude claimed that the speed of light is defined based on the meter, and that that makes it a tautology. That is simply, provably false. Then the dude tried to move the goalposts. Never did I say that our measurements are anything less than relative. Never did I suggest that our derived units are not based on fundamental constants. Now, you’ve said that the statement I made didn’t tell the dude “how to make use of” dimensionless units, which is a complete non sequitur. If you feel that that lecture is an important one when a dude demonstrates a fundamental misunderstanding of what c even is, that’s your own affair, and I invite you to give this lecture a few comment levels up to the guy who thinks that c is defined based on the meter.
The two constants - the speed at which light moves, and the unperturbed ground-state hyperfine transition frequency of cesium - can be combined to define every measurement of time, length, and velocity. They are the constants by which everything else is defined.
Throw in mass, which is easy - a certain number of atoms of a specific element will also have a universally constant mass. Combine it with the other two constants and you have force, energy, and work, and voila, you can describe nearly everything in classic physics.
…The Planck constant is a set distance. It’s being used here to define other things as a constant. The very article on the kilogram states outright that SI units’ foundation is three constants: “a specific transition frequency of the caesium-133 atom, the speed of light, and the Planck constant.”
Vacuum permeability is how much electric currents affect magnetic fields in a vacuum. As in, when there are no other interferences. It is not variable. The speed of light when there is no interference is not ‘affected by’ vacuum permability any more than a cartesian graph is ‘affected by’ the set of real numbers. Vacuum permeability describes vacuum, it doesn’t define it. Same goes for permittivity.
You ARE correct about the kilogram no longer being a number of specific atoms, though.
UndeariusThis got me thinking if we defined the metre to be a more round number, like 1/300000000.
It would shrink the metre by 0.6918mm.
Now I’m curious about what implications that would have.
π would be equal 3.
gandalf_der_12teπ doesn’t have the dimension “length”, it’s a dimensionless scalar
I know. The “also” refers to anything involving “length” besides the metre itself.
magza second is defined as the time it takes for a caesium atom to oscillate exactly 9192631770 times, at least according to the SI. a meter is then defined, as you said, as the distance light travels in 1/299792458 seconds, which corresponds to some large number of oscillations of those caesium atoms. these numbers are pretty much arbitrary though, we just picked them to match our previous, less precise, definitions of meters and seconds. but using oscillations of caesium atoms and speeds of light in your is completely equivalent to using meters and seconds, except that the latter units are more familiar to us
The first rule of tautology club is the first rule of tautology club lol
hansoloI’m a dumbass? I’m not the one deciding all speed should be scaled to a fraction of the fastest thing that exists, dipshit.
I mean, it’s better than trying to scale it to a fraction of the slowest thing that exists…
Because we tried to make the meter one 40millionth of the earths circumference, failed, and ended up at
Well, in our defense, we’re very small.
At least one second has a simple origin, and totally wasn’t back-defined in 1967
oh wait
The current and formal definition in the International System of Units (SI) is more precise:
The second […] is defined by taking the fixed numerical value of the caesium (Cs) frequency, ΔνCs, the unperturbed ground-state hyperfine transition frequency of the Cs-133 atom, to be 9192631770 when expressed in the unit hertz, which is equal to s−1.[1]
mercHaving said that, we’re pretty lucky that using those pretty arbitrary values we ended up with a speed that you can approximate as 300 million m/s and be off by less than 0.1%.
JackbyDevFor the simulation.
lumpenproletariatThe human eye can’t see over 1.
It’s deeper than that as we have instruments to measure things we can’t personally perceive.
From everything we know it’s quite literally the limit the universe can update. For the photon moving at light speed time quite literally doesn’t exist. When you look up at the stars, for the photons hitting your eye balls their experience is that creation to reception is instantaneous. The millions or billions of years we perceive it traveled doesn’t exist at that speed.
I just think that’s neat.
That’s beautifully written. I like the idea that I receive cute lil interstellar photons. The stochastic nature of the universe means I am being irradiated by an interstellar object thousands of years away. They started a journey from a star thousands of years ago, crossed the vast expanse of space without hitting anything, pierced our planet’s atmosphere as our planet and system hurtle through space, and was then absorbed by a single cone cell in my eye. It almost feels unbelievable.
Why would god use dumbasses as the standard unit of philosophy?
It’s measuring speed, speed of light is 1 dumbass.
Which proves God is American (anything except metric)
I learned recently that it was the Babylonians who invented the hour and the minute as a unit of time, and they used base 60, which I thought was pretty neat.
IIRC, they picked 60 because it could be evenly divided into 1/2, 1/3, 1/4, 1/5, and 1/6 which allowed them to stick to whole numbers more easily.
If you use your thumb to count the sections of 4 fingers you get 12.
Then you hold up a finger on your other hand. When all 5 are up you have 60.