Is there such thing as an absolute measurement?

[nuuskur]

Naively speaking, no, because we're always measuring relative to something. Mathematically speaking the question should be formulated as "Definition for absolute measurement" and then proceed to find examples or giving arguments why there are none.

 

[Hacker Jack]

Can't wrap my head around the fact that the speed of light is just defined but not measured. So you are saying a thing called photon or light that exists in reality like an object with mass moves from one point in space to another point and it is simply defined without some form of measurement. So how then do we even know that it has moved 299 792 458 metres in 1 second without measuring of some sort. And how can you be sure that is exactly the distance it traveled even with experiment when there are no perfect tools in existence.

 

[etotheipi]

It's just semantics, nothing deep. It's just because now the metre is defined as how far a photon moves in a vacuum in exactly 1/299792458 seconds!

 

[Hacker Jack]

But how can you know that without some sort of measurement. How can you know that the photon moved that much of a distance at specific time without measuring something.

 

[Ibix]

But how can you know that without some sort of measurement. How can you know that the photon moved that much of a distance at specific time without measuring something.

We define 1m that way. Then we get a very accurate clock and see how far light travels in the specified time. So we measure time, and use this process to define a distance measure.

Of course, the 299792458 number is picked so that 1m (by this modern definition) is very very close to the length of the rod we used to use to define the metre. That was deliberate, so that only metrology nerds needed to worry about the change.

 

[Hacker Jack]

But experiment with clock is not perfect so result of speed of light from experiment not perfect. But we just accept it as theory says?

 

[etotheipi]

you don't measure definitions bro

 

[Hacker Jack]

Then what proof is there that light is certain speed in certain time if you don't measure. Can I define the speed of a car driving by and say that is certain speed it's going at. Science requires experiment to verify theory even though experiment always had uncertainty or percentage error whatever you call it. But guy mentioned accurate clock is used to measure so I guess that's that.

 

[russ_watters]

Can't wrap my head around the fact that the speed of light is just defined but not measured.

IMO, it's over-stated to say the speed of light can't be measured, and the history gives insight into this. The problem is accuracy/consistency. Any system of measurement has arbitrarily chosen units that need to be defined. Defining the length of a meter is different from measuring the distance to the sun. The distance to the sun is measured, whereas the length of a meter has to be defined...but if you really wanted to, you could define a distance scale using an approximate value of the distance to the sun; and we have. But the length of a meter doesn't have a physical manifestation absent human assignment. If you tried to measure the length of a meter, you'd really be doing that by comparing two different representations of the defined value.

There are of course devices for measuring the speed of light, that have the same problem as comparing two meter sticks. They use approximations of the defined units for things like length and time. But a century ago it was recognized that the speed of light was constant, so it makes more sense to use the speed of light as the measuring stick instead of the object being measured. Eventually it made sense/became necessary to make that the "meter stick".

 

[etotheipi]

Can I define the speed of a car driving by and say that is certain speed it's going at.

well no, the trick with the photon is that as far as known it always goes along in a vacuum at a constant speed (thought to coincide with the universal speed limit) and which is the same as measured by any inertial oberver. so light can be used to define a standard unit of proper length

speed of a car is not constrained to be constant, nor invariant. so it's silly to try and define a unit of length based on that

 

[Ibix]

Can I define the speed of a car driving by and say that is certain speed it's going at.

If you define a mile as "the distance my car goes in 2 minutes when my foot applies a certain pressure on the accelerator on a flat track in fourth gear on a windless day", then sure. Note that there's many caveats here that there aren't with light in a vacuum, which makes light speed a much more reliable measure. This car based one is nowhere near repeatable enough for metrologists, but it's not wrong in principle.

 

[PeroK]

Then what proof is there that light is certain speed in certain time if you don't measure. Can I define the speed of a car driving by and say that is certain speed it's going at. Science requires experiment to verify theory even though experiment always had uncertainty or percentage error whatever you call it. But guy mentioned accurate clock is used to measure so I guess that's that.

You can take a look at all the historical definitions of the metre here:

https://en.wikipedia.org/wiki/Metre#Timeline

You can't have everything done by measurement, because you can't get started. You can't measure something until you have a definition. You need a definition of the basic units.

The modern approach is to take the speed of light in vacuum as a basic unit, as it were, instead of defining the metre. That's ultimately a question of usefulness and practicality.

Using your car wouldn't be very practical or reliable - although, in the past, units have been dependent on who does the measuring. Horsepower must at one time been the power of an average horse. And "feet" would have been literally that and a measurement would depend on the size of your feet.

 

[f95toli]

You need to be a bit careful here.
In physics (well, metrology) an "absolute measurement" means a measurement where the result does not rely on a comparison to a reference. Or, in other words, you don't need any part of your measurement apparatus to be calibrated for you to get the correct result.
Absolute measurements a rare, but they do exist. An obvious example would be the realisations of the SI which for obvious reasons ideally should be absolute. A good example would be the quantum hall effect which is used to calibrate resistance standards.
Sometimes we do "bend the rules" a bit. It is quite common to call a measurement "absolute" if it only relies on one primary standard as a reference, usually time/frequency.
For example realisations of the Ampere using electron pumps are sometimes called "absolute" even though you need to have an frequency standard that is known to be accurate (say a hydrogen maser) for it to work.
If you know the frequency and you know that your pump only moves one electron at a time you have a "absolute" current standards where the output current is I=e*f. Once you have that current you can then use it to perform absolute measurements of other currents by using "nulling".

 

[WWGD]

The only thing that is arbitrary is the unit of measurement. Once that has been decided, agreed upon, the measurement is " absolute".

 

[jbriggs444]

Any system of measurement has arbitrarily chosen units that need to be defined.

If, for instance, one defined the meter as the distance between two particular scratches on a particular artifact then the measured distance between said scratches would naturally be one meter exactly, by definition.

 

[collinsmark]

I found this to be almost (but not really) on topic:

(Source: https://www.smbc-comics.com/comic/reference)

 

[nucl34rgg]

Philosophically, I don't think you can have such a thing unless you are "measuring" large scale quantities. Even then, there is always the possibility of some sort of uncertainty or error.