# How does quantum world affect us and why should I care about it? [duplicate]

We live in a world which is much larger than quantum world. The laws of quantum physics are not valid. While I am pressing the keys on my laptop, I have 100% certainty that I am writing what I really want.

Then how is such microscopic world going to affect me? The quantum world is entirely different, perhaps best suited for those Aliens.

Then why should I worry about uncertainty and other stuffs happening at infinitesimally small scale?

## marked as duplicate by John Rennie, Brandon Enright, Kyle Kanos, Qmechanic♦Mar 19 '14 at 17:16

• Well, I guess the answer is that you shouldn't. Unless you start asking why things are happening that is. As soon as you want to know why the caps-lock LED on your keyboard is emitting light and not just are happy that it does, you should however start caring. As long as you are happy when things work and you don't want to know why: just move on, don't worry about quantum physics. You will be fine. – André Mar 19 '14 at 11:50
• Why negative vote? This is clearly a question anybody beginning with QM might ask. – gpuguy Mar 19 '14 at 12:07
• @gpuguy I'm giving this question the benefit of doubt for that reason, but phrases as contentious as "the laws of quantum physics are not valid" are argumentative and not really conducive to a constructive discussion. While the spirit of the question is definitely valid, there's quite a bit of work to be done on its language. – Emilio Pisanty Mar 19 '14 at 12:19
• I used to think that the reason to study QM, might be more than mere curiosity. – gpuguy Mar 19 '14 at 13:01
• One of the main reasons that science goes forward is curiosity. – Wojciech Mar 19 '14 at 13:45

For one thing, without quantum mechanics you would be vaporized by light from the sun. Actually more realistically there couldn't be a sun or anything else, everything would be boiled into a highly energetic plasma.

Quantum effects are tied to Planck's constant $h$. We are used to thinking that the 'classical limit' $h\rightarrow 0$ is always a good approximation of the world we live in. And usually that is true. However there are a handful of cases where actually taking this limit really would be catastrophically bad.

The example I have in mind is blackbody radiation, which describes the spectrum of light emitted by a perfect absorber/emitter in thermal equilibrium. The CMB is a blackbody, the sun is approximately a blackbody, you are sort of a blackbody, any time you heat something and it glows you are directly observing blackbody-like behavior. The spectrum of blackbody radiation is described by Planck's famous distribution. [To read more see http://en.wikipedia.org/wiki/Black_body].

The key point is that the maximum frequency emitted by the blackbody is given by $$\nu_{max} = \frac{\alpha kT}{h}$$ where the irrelevant constant is $\alpha = 3 + W(-3/e^3)\approx 2.8$ where $W(x)$ is the Lambert W function.

Notice that if I try to send $h\rightarrow 0$, $\nu_{max}\rightarrow\infty$!! This is known as the ultraviolet catastrophe. If we made $h$ a bit smaller, the max frequency emitted from the sun would be pushed deep into the UV and cook us.

More examples of this: (1) if we send $h$ to zero then atoms could not be stable (for example the Bohr radius $a_0 \sim h$ would simply shrink to 0), (2) semiconductors (and thus the laptop you are presumably reading this on) could not exist because you could not have band structure without $h$, (3) the decay rates of nuclei would become zero and so we couldn't have nuclear reactors, and the earth would be cooler because it is partly heated due to nuclear decay.

Of course, even in situations where $h\rightarrow 0$ is a good first approximation, precision is its own reward and quantum mechanics is crucial to being able to have a quantitative understanding of any object past a certain level of precision.

If I may say, I am so glad you used the example of typing on your keyboard. The laws of quantum physics may not be so readily apparent to you, but they have a massive impact on your daily life and are quite valid at the level of our world. It is only through understanding and exploiting those laws that you can type at your computer. The transistor and the diode - two of the most integral parts of a common microchip - are based on the principles of quantum mechanics. Lasers, MRI machines, USB drives, even to some degree light switches all rely on quantum mechanics. But don't worry, you don't have to worry about uncertainty if you don't want to. That's what we're here for

When you press the key on your laptop, lots of thing are happening that could not happen but for quantum physics. The key sensor requires it. Every electronic gate from the internal clocks to the keyboard scanner to the CPU to the display screen would not work without it. Even the very fact that your chair supports you, and your fingers push the keys down instead of just passing through them is due to quantum physics. (Oh, by the way, your self wouldn't even exist without QM, never mind your eyes not being able to receive quantum photons as you read this.)

The ubiquitous electronics only exist because someone took the trouble to learn quantum physics and apply it. But to just use the toys, you don't need to know.

The laws of quantum mechanics are always valid, but in everyday circumstances those laws can be well-approximated by those of classical mechanics.

Your question seems to be, "why should I care about quantum mechanics if its irrelevant to my everyday experiences?" Quantum mechanics is fascinating, largely because it permits phenomena that are so at odds with our everyday experiences!

I should add that quantum mechanics is necessary to account for the observation of such phenomena.

There are an increasing number of technological devices the development of which depended upon quantum mechanics. If you use any of these devices, then the underlying quantum physics theory affects you.

Please see this previous question:Are there any practical applications of the uncertainty principle.

Also here is list from a popular science site that may help you: 10 Real-World Applications of Quantum Mechanics