In physics, measurements help to understand the physical world. Two fundamental quantities we often measure are length and time. Length is defined as the distance between two points. It helps us quantify how far apart objects are, whether that’s measuring the size of a classroom, the height of a building, or the distance between two cities. On the other hand, time refers to the continuous progression of events, allowing us to determine how long a process takes. Time is essential for understanding motion, cycles, and changes in the physical world.

Units of Measurement
To ensure consistency in scientific communication, standardized units of measurement are used. The International System of Units (SI) provides the framework for this.
For measuring length, the SI unit is the metre (m). Smaller lengths can be expressed in millimetres (mm) or centimetres (cm), while larger distances, such as the space between cities or countries, are measured in kilometres (km). For instance, the length of a pencil may be 18 cm, while the distance between London and Manchester is approximately 260 km. Knowing how to convert between these units is important: for example, 1 kilometre is equal to 1,000 metres, and 1 metre is equivalent to 100 centimetres.

When measuring time, the SI unit is the second (s). This unit is widely used in science, particularly for measuring short intervals. For longer durations, minutes (min) and hours (h) are commonly used. For example, it may take you 5 minutes to walk to school, while a football match lasts 90 minutes, which is equal to 1 hour and 30 minutes. In scientific experiments, time intervals are often much shorter, measured in seconds or even fractions of a second. The relationship between units of time is straightforward: 60 seconds make up 1 minute, and 60 minutes make up 1 hour.

Measuring Length
Various tools are used to measure length, depending on the precision required. For everyday measurements, a ruler or tape measure is sufficient. For more precise scientific measurements, devices such as vernier calipers or micrometers are used. These tools allow us to measure length down to fractions of a millimetre. For example, a ruler may tell us that a piece of string is 12 cm long, but a vernier caliper could measure it more precisely, to the nearest tenth of a millimetre, like 12.3 cm.
In physics experiments, it’s crucial to ensure accuracy and precision when measuring length. This can involve repeated measurements and careful observation to minimize errors.

Measuring Time
To measure time intervals, we often use stopwatches or clocks. A stopwatch is particularly useful in experiments where we need to record the exact time something takes to occur, such as the duration of a pendulum’s swing or the time it takes for an object to fall. For instance, if you want to measure how long it takes for a ball to drop from a certain height, you could use a stopwatch to record the fall in seconds.
In modern physics, extremely precise instruments like atomic clocks are used to measure time with remarkable accuracy. These clocks can measure time intervals to a fraction of a second, and they are used for highly sensitive experiments, such as those involving the speed of light or synchronization in satellite systems.

Practical Applications of Length and Time
For instance, when studying the motion of objects, knowing how far something has moved (length) and how long it took (time) is fundamental to calculating speed. In technology, precise time measurements are important for synchronization in communication systems, while accurate length measurements are key in construction, engineering, and manufacturing processes.

Example Questions

1. Convert 5.5 kilometres into metres.

2. How many seconds are there in 2 hours?

3. You have a ruler marked in centimetres. If a pencil measures 14.5 cm, how long is it in millimetres?
4. Using a stopwatch, you record the time it takes for a marble to roll down a ramp as 3.2 seconds. How would you express this time in milliseconds?

5. A cyclist travels 24 kilometres in 2 hours. What is the cyclist’s average speed in kilometres per hour (km/h)?

6. If a pendulum takes 2.5 seconds to complete one swing, how many swings will it complete in 1 minute?

You can't get more direct than this in terms of proving that photons exist!

The particular case of the double-slit experiment will be discussed at: single particle double slit experiment.

Production:

Detectors are generally called photomultipliers:

Bibliography:

Maxwell-Boltzmann statistics, Bose-Einstein statistics and Fermi-Dirac statistics all describe how energy is distributed in different physical systems at a given temperature.

For example, Maxwell-Boltzmann statistics describes how the speeds of particles are distributed in an ideal gas.

The temperature of a gas is only a statistical average of the total energy of the gas. But at a given temperature, not all particles have the exact same speed as the average: some are higher and others lower than the average.

For a large number of particles however, the fraction of particles that will have a given speed at a given temperature is highly deterministic, and it is this that the distributions determine.

One of the main interest of learning those statistics is determining the probability, and therefore average speed, at which some event that requires a minimum energy to happen happens. For example, for a chemical reaction to happen, both input molecules need a certain speed to overcome the potential barrier of the reaction. Therefore, if we know how many particles have energy above some threshold, then we can estimate the speed of the reaction at a given temperature.

The three distributions can be summarized as:

A good conceptual starting point is to like the example that is mentioned at The Harvest of a Century by Siegmund Brandt (2008).

Consider a system with 2 particles and 3 states. Remember that:

Therefore, all the possible way to put those two particles in three states are for:

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GitHub forbade our China Dictatorship auto-reply bot, the reason given is because they forbid comment reply bots in general. Though it was cool to see a junior support staff person giving out what obviously triggered the action:before a more senior one took over.

Ciro was slightly saddened but not totally surprized by the bloodbath against him on the Reddit the threads he created:

So we observe once again the stupidity of deletionism towards anything that is considered controversial. The West is discussion fatigued, and would rather delete discussion than have it.

We also se people against you having freedom to moderate your own repositories as you like it, with bots or otherwise. Giving up freedoms for nothing, because "bot is evil".

Announcements:

I edited the VOD of the talk Aratu Week 2024 Talk by Ciro Santilli: My Best Random Projects about the CIA 2010 covert communication websites a bit and published it at: www.youtube.com/watch?v=QWL7l-5r1a4.

Announcements:

Materials paywalled: www.cs.ox.ac.uk/teaching/courses/2022-2023/grl/