# What can we scientifically measure directly?

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• democura
democura
TL;DR Summary
What can we objectively measure? It seems as if everything we measure is either mass or energy. Are there examples of when we measure something differently?
I am interested in finding out what we can objectively measure. How to define what we can measure directly, without (human) models getting involved, as in model-dependent realism as described by Hawking and Mlodinow? It seems as if everything we measure is either mass or energy (or spin?).

Time for example, cannot be measured without being relative or even subjective. How is time measured? We do not measure time directly. We measure the intervals between two events, for example between the emitting of a photon and the measurement of the returning photon. These can for example be measured with an atomic clock. Atomic clocks produce electromagnetic radiation with a precise frequency that causes atoms in the clock to jump from one energy level to another. What we in fact measure is energy.

LIGO, the Laser Interferometer Gravitational-Wave Observatory, explained in a somewhat simplistic way, can measure gravitational waves by measuring the distance two different light waves have traveled. The interferometer has two ‘arms’ which measure the time and distance of a laser (light) travelling from one point to another. Both arms are placed in an angle of 90 degrees. When a gravitational wave hits the interferometer, one of the arms will be slightly elongated and the other one slightly contracted because the gravitational wave will distort space. This will lead to a different travel time for each light wave. Again, we can only (for now) measure gravitational waves by using mass and energy, lasers and sensors in this case.

PeroK
It depends largely on your definition of “directly”. In your usage you state that time is not measured “directly”. From that I extrapolate that there is probably nothing that qualifies as “directly” according to whatever definition you are using.

Of course, using such a definition essentially renders the word useless. Since no measurement can be “direct”, then why should that bother us?

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democura said:
We do not measure time directly. We measure the intervals between two events
I think you are going to have a very difficult time (no pun intended...mostly) convincing people that there is a difference.

PF does not discuss philosophy, but if you take your position to its logical conclusion, how do you know you're not a brain in vat?

russ_watters and Dale
Dale said:
It depends largely on your definition of “directly”. In your usage you state that time is not measured “directly”. From that I extrapolate that there is probably nothing that qualifies as “directly” according to whatever definition you are using.

Of course, using such a definition essentially renders the word useless. Since no measurement can be “direct”, then why should that bother us?

Thanks for your reply. I am indeed looking for a definition of 'directly'. I think however that there is a difference in measuring photons for example, and time. What I mean with not being able to measure time directly, is that we always measure something else first, and then we use these measurements to model something called time, which is relative.

What is in your definition direct measurement? What role do mass and energy play, since these can be interpreted as fundamental building blocks?

I guess I don't understand the question. See below, what is being measured? Is this direct or indirect?

DennisN and phinds
democura said:
I am indeed looking for a definition of 'directly'.
Well, if you are going to make the claim that our measurements of time are not “direct”, then shouldn’t you already have a definition of “direct”? If you have no definition of the key descriptor then isn’t the claim itself literally meaningless?

democura said:
I think however that there is a difference in measuring photons for example, and time.
Why?

democura said:
What I mean with not being able to measure time directly, is that we always measure something else first, and then we use these measurements to model something called time, which is relative
First, by that definition then all measurements are indirect. I don’t know any measurement that cannot be cast in those terms.

Second, the time measured by a clock is called proper time. It is not relative. Proper time is an invariant quantity so all reference frames agree on its value. What is relative is coordinate time. Coordinate time is not measured, but is simply an arbitrary convention that is chosen based on convenience or whatever.

democura said:
What is in your definition direct measurement? What role do mass and energy play, since these can be interpreted as fundamental building blocks?
I don’t have a definition of direct measurement. I think “direct” is a useless distinction in this context. Also, if your concern is about relative quantities, you should be aware that while proper time is invariant, energy is relative. Different reference frames generally disagree on the energy. (Mass is invariant)

russ_watters, Lord Jestocost and Vanadium 50
Can't we say a direct measurement is a comparison to a standard? A measurement of voltage with a voltmeter is an indirect measurement inferred from Ohm's law.

hutchphd and Lord Jestocost
democura said:
TL;DR Summary: What can we objectively measure? It seems as if everything we measure is either mass or energy. Are there examples of when we measure something differently?

I am interested in finding out what we can objectively measure.
Here is such an example: we can count things.

DennisN and PeroK
gmax137 said:
I guess I don't understand the question. See below, what is being measured? Is this direct or indirect?

View attachment 334474
I'm pretty sure that calculator is a direct measurement of time.

Grinkle, hutchphd and gmax137
The SI is organized so that everything can be traced back to measurement of time (the realisation of the second), so if you say that time can't be measured directly then it follows that nothing else can either**There is a caveat, and that is we can of course also measure dimensionless quantities; in particular things like "the number of apples on a table". There is an ongoing discussion about how to correctly handle dimensionless quantities in the SI and you should be able to find articles on the subject.

f95toli said:
The SI is organized so that everything can be traced back to measurement of time (the realisation of the second), so if you say that time can't be measured directly then it follows that nothing else can either**There is a caveat, and that is we can of course also measure dimensionless quantities; in particular things like "the number of apples on a table". There is an ongoing discussion about how to correctly handle dimensionless quantities in the SI and you should be able to find articles on the subject.
Although @democura never provided a concrete definition for "directly", we can infer from what they did say that "the number of apples on a table" would also be an indirect measurement.

Looking at the table is receiving photons which, by their comments in posts 1 and 4, makes a measurement "indirect" in their usage. Also, the photons received are processed according to a model of what patterns of photons we expect to receive from apples, so that would also count as indirect by their comments in posts 1 and 4 about avoiding models.

Since time is an "indirect" measurement and even counting apples on a table is also "indirect", according to the OP's usage, I would be surprised to find any measurement that qualifies as "direct".

gmax137 said:
See below
That's a great picture. I don't know how we would get such pictures if we didn't have a man of your caliper.

phinds, pinball1970 and berkeman
That's a great picture. I don't know how we would get such pictures if we didn't have a man of your caliper.
Thanks! I think. You have a fine line between sarcastic and sincere.

If I had a do over, I would take more time. To get the calipers more perpendicular, to hide the magic eraser, etc.

The picture wwas great, but the pun was too good to pass up.

phinds, Nugatory, gmax137 and 1 other person
The picture wwas great, but the pun was too good to pass up.
Are you trying for a misinformation warning here?

phinds
gmax137 said:
I guess I don't understand the question. See below, what is being measured? Is this direct or indirect?

View attachment 334474
You used a calculator, so it must be indirect.

Thank you all for your replies, especially gmax137 for the picture of the calculator.
I am aware I have not given a clear definition of what a 'direct measurement' means and perhaps this is part of my question.

Perhaps I should have elaborated more on why I pose this question. I want to know how we scientifically determine what is likely part of reality, and what is not. The measurement of the calculator for example indicates some number (I can't clearly read it from the picture), but this number is nothing more than a convention. A centimeter does not exist in reality, it is a human construct of a distance. What I (intended) to describe with 'direct measurement' is that we measure what is really (likely) out there. Hence my intuition that particles like photons are something that we can measure and be pretty sure about that what we measure is part of reality. Length or distance is not. Is not the entire SI a convention in that sense?

@Dale: thanks for your answer. How does proper time fit into determining what is real? Perhaps I don't understand, but does proper time not also differ when measured near a black hole in comparison with a measurement between two events here on earth?

I can imagine the upcoming question: give your definition of reality. This is exactly what I am looking for. How do we know that what we measure is not a man-made convention but actually something out there? I am aware that this is a physics forum, but it becomes relevant to explain when (social) scientists for example believe they can measure things like happiness or fairness. As David Hume explained: many people mistake what 'is' versus what 'ought' to be. How does a physicist determine what 'is'?

democura said:
I am aware I have not given a clear definition of what a 'direct measurement' means and perhaps this is part of my question.
If you can't define what you mean, then the whole debate becomes meaningless.
democura said:
Perhaps I should have elaborated more on why I pose this question. I want to know how we scientifically determine what is likely part of reality, and what is not.
That's a philosophical question with little relevance to modern physics. In the past, even mathematicians terribly constrained themselves by worrying about whether things were "real". Complex numbers being the prime example. If they had got over their quasi-religious, philosophical hang-ups, then mathematics would have progressed more quickly.
democura said:
The measurement of the calculator for example indicates some number (I can't clearly read it from the picture), but this number is nothing more than a convention. A centimeter does not exist in reality, it is a human construct of a distance.
The fundamental background for all of physics is spacetime. That's about as real as it gets.
democura said:
What I (intended) to describe with 'direct measurement' is that we measure what is really (likely) out there. Hence my intuition that particles like photons are something that we can measure and be pretty sure about that what we measure is part of reality.
A photon is a useful concept, as the quantum of the electromagnetic field. If you knew anything about Quantum Theory, you might not be so confident of your intuition.
democura said:
Length or distance is not. Is not the entire SI a convention in that sense?
No. SI is a convention for units. Not for the physical quantities themselves. For example, mass is a physical quantity, for which the SI unit is the kg. But, again, if you are quibbling about this, then your notion that a photon really exists doesn't stand up to scrutiny.
democura said:
@Dale: thanks for your answer. How does proper time fit into determining what is real? Perhaps I don't understand, but does proper time not also differ when measured near a black hole in comparison with a measurement between two events here on earth?
No. Proper time is an invariant. You need to learn physics before you can philosophise about it.
democura said:
I can imagine the upcoming question: give your definition of reality. This is exactly what I am looking for. How do we know that what we measure is not a man-made convention but actually something out there?
That's not a question that makes much sense for modern physics. Quantum Theory is explicitly non-realistic in a definable way.
democura said:
I am aware that this is a physics forum, but it becomes relevant to explain when (social) scientists for example believe they can measure things like happiness or fairness. As David Hume explained: many people mistake what 'is' versus what 'ought' to be. How does a physicist determine what 'is'?
To paraphrase Hume, using the knowledge of modern physics: many people mistake what is versus what is part of the mathemetical model.

russ_watters, gleem and Dale
democura said:
This is exactly what I am looking for. How do we know that what we measure is not a man-made convention but actually something out there?
I don't think there is any way to be sure anything is really out there. This is what V50 was getting at:
PF does not discuss philosophy, but if you take your position to its logical conclusion, how do you know you're not a brain in vat?
In practice, IMO, it would be pointless to go through life as if nothing is real. Even if we can't tell for sure, we might as well act as if the world exists. Otherwise, why do anything? Why not just sit and starve?

There's a quote from some physicist (Bridgman, maybe?) where he says, all we know are the clicks on our detectors. I take this to mean that the rest of it (our theories and models) is just scaffolding that allows us to predict the next click.

democura, russ_watters, gleem and 1 other person
democura said:
How does a physicist determine what 'is'?
Why would a physicist do that?

russ_watters
I couldn't find the "clicks" quote. But here's Percy on the meaning of length:

Percy Bridgman said:
To find the length of an object, we have to perform certain physical operations. The concept of length is therefore fixed when the operations by which length is measured are fixed that is, the concept of length involves as much as and nothing more than the set of operations by which length is determined.

Lord Jestocost and PeroK
gmax137 said:
I take this to mean that the rest of it (our theories and models) is just scaffolding that allows us to predict the next click.
Exactly. Predicting the next click is what science is all about and all it CAN be about --- creating models that describe reality as best we can and in a way that makes useful predictions.

All of the OPs concerns just seem to me like metaphysical/philosophical dead ends.

Dale and Lord Jestocost
democura said:
I want to know how we scientifically determine what is likely part of reality, and what is not.
We cannot do this.

The tool that we use to do science is the scientific method (hence the names). In the scientific method we formulate a theory, which is a mathematical framework together with a mapping from the math to experimental measurements. We then perform experiments that either validate or falsify our theory. A valid theory can then be used to accurately predict the outcomes of other experiments in the domain of validity of the theory. That is it.

In particular, any experimental validation of any theory is compatible with both a realist philosophy (the experiment validated the theory because the math of the theory describes reality) or with a solipsist philosophy (the experiment validated the theory because my mind uses the math of the theory to organize my hallucinations). The scientific method, and hence science, is fundamentally incapable of addressing this philosophical question. If you want to know what is part of reality and what is not then you need to speak with a philosopher or a priest, not a scientist. Probably the priest will be a more productive conversation.

democura said:
How does proper time fit into determining what is real?
It doesn't.

However, proper time is an invariant quantity, meaning that all reference frames agree on the proper time read by any clock. As such, it is a pretty important quantity scientifically speaking. It is very useful in the scientific method and in constructing theories about time since it is the concept of time that can be measured by a clock.

democura said:
Perhaps I don't understand, but does proper time not also differ when measured near a black hole in comparison with a measurement between two events here on earth?
How you do that comparison certainly can differ, but the proper time itself is invariant. That comparison is a matter of convention and is not dictated by the physics.

democura said:
it becomes relevant to explain when (social) scientists for example believe they can measure things like happiness or fairness
That is a question you need to ask them instead of us. I imagine that they as a community are very aware of the difficulties in measuring such things. I think that we could only speculate here.

democura said:
As David Hume explained
David Hume was a philosopher, not a scientist. As far as I know he neither developed any scientific theories nor performed any notable scientific experiments. He could probably tell you a lot about reality, but not much about science. I could be wrong, and if so I would be glad to read of any influential experiments that he performed.

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@democura, this is a science forum. Your concerns seem to all be philosophical and are not things that can be addressed here. I suggest you join a philosophy forum where you are more likely to get the discussion you seem to want.

democura said:
The measurement of the calculator for example indicates some number (I can't clearly read it from the picture), but this number is nothing more than a convention. A centimeter does not exist in reality, it is a human construct of a distance.
Of course it exists. Just like a kilometer exists. And a lightyear exists. And so does every distance in between all of them, regardless of whether we've named and use them.

democura said:
What I (intended) to describe with 'direct measurement' is that we measure what is really (likely) out there.
All measurements measure something real. What makes something direct vs indirect is more in how many steps you have to take to get from the measurement to the quantity you want to know. Having to perform multiple measurements using completely different devices that each measure something different, and then having to use all those measurements in an equation to get another quantity you wanted would qualify as 'less direct' compared to, say, using a ruler to measure the distance between two lines on a piece of paper.
democura said:
Hence my intuition that particles like photons are something that we can measure and be pretty sure about that what we measure is part of reality. Length or distance is not.
What do you mean by 'measure a photon'? You can detect a photon. Through some sort of experimental setup you can 'measure' the various properties of a photon. But you can't 'measure a photon' because a photon is not a quantity. Things we can measure include, but are not limited to: counts (number of things), length, time, wavelength, frequency, energy, momentum, etc. So not only can length be measured, it is one of the only things that can be measured at all.

I think you are confusing how we perceive the world with what a measurement is. A radar pulse return transformed from an EM wave to a voltage/current signal, then amplified, then run through a process to show it on a screen, then saved as a series of 0's and 1's, then transformed into another series of voltages and currents, then transferred around the world and shown on another screen, then read by a person, then relayed by that person to another person who is blind, would still potentially be a direct measurement despite the long, long chain to get from the original EM wave signal to the blind person.

democura said:
How do we know that what we measure is not a man-made convention but actually something out there?
Because we have to have something 'real' to make a convention out of it. Or at least a useful convention. I suppose using 'the weight of 13 small fairies' would be a poor convention, so let's keep conventions to things that can actually be measured. The 'mass of 13 blocks of lead, which I have right here readily available for use for calibration', is a much better convention. Better still would be 'the mass of 10^26 carbon atoms', since carbon atoms don't lose mass over time like any macroscopic blocks would.

democura
phinds said:
@democura, this is a science forum. Your concerns seem to all be philosophical and are not things that can be addressed here. I suggest you join a philosophy forum where you are more likely to get the discussion you seem to want.
I am sorry to hear that, but I understand now reading most of the reactions. I was actually looking for the perspective of a physicist, not that of a philosopher. It believe that both fields can learn from each other, but perhaps not on this forum apparently.

Thank you all for your replies. They have been very helpful in understanding some of the basic physics in relation to more philosophical questions. As advised by Phinds, I will quit this forum and look for answers elsewhere.

berkeman
And so, it seems like a good time to close this thread off. Thank you to everybody who was trying to help the OP.

## What can we scientifically measure directly in terms of physical quantities?

We can directly measure physical quantities such as length, mass, time, temperature, electric current, luminous intensity, and amount of substance using appropriate instruments. For example, rulers measure length, scales measure mass, and thermometers measure temperature.

## Can we directly measure forces and energy?

Forces and energy are typically not measured directly. Instead, we measure quantities that are related to them and then calculate the force or energy. For example, we might measure the deformation of a spring to calculate force using Hooke's Law, or measure temperature change to calculate energy transfer.

## Are there limitations to what we can measure directly in quantum mechanics?

Yes, in quantum mechanics, there are fundamental limitations on what we can measure directly due to the Heisenberg Uncertainty Principle. For instance, we cannot simultaneously measure the exact position and momentum of a particle with arbitrary precision.

## Can we directly measure chemical concentrations in solutions?

Chemical concentrations are often measured indirectly. Techniques such as spectroscopy, titration, or chromatography are used to infer concentrations based on the interaction of the substance with light, reactants, or solvents, respectively.

## Is it possible to directly measure the speed of light?

Yes, the speed of light can be directly measured using experimental setups such as time-of-flight measurements, where the time taken for light to travel a known distance is recorded. Michelson's rotating mirror experiment is a classic example of such a direct measurement.

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