Is there anything in physics that’s random?

[Whipley Snidelash]

TL;DR Summary

Is there anything in physics that’s random?

Is there anything in particle or energy physics that is random? If yes why wouldn’t random effects destroy past information? I am asking in relation to the theory that no information is ever lost. If I understand it correctly, I’m not a physicist.

 

[kuruman]

Radioactive decay.

 

[anorlunda]

It is random when and if radioactive decay occurs. However, the event is time reversible so information is not destroyed.

See this PF insights article.
https://www.physicsforums.com/insights/how-to-better-define-information-in-physics/

 

[vanhees71]

If it's a weak decay it's not time-reversible ;-)).

 

[Andy Resnick]

Summary:: Is there anything in physics that’s random?

Is there anything in particle or energy physics that is random? If yes why wouldn’t random effects destroy past information? I am asking in relation to the theory that no information is ever lost. If I understand it correctly, I’m not a physicist.

Dissipative processes destroy information by increasing entropy.

 

[Whipley Snidelash]

I am asking because I was criticized on another thread for saying that in order to travel into the past the information from the past has to exist or the past has to exist and that would require an infinite amount of information, every state of everything in the universe for all past time. I think he was trying to say the information is encoded in the present. However if random events destroy information then the past is not encoded and infinities of infinity of information needs to be recorded elsewhere in order to travel into the historical past. (I said an infinity of infinities because if you can travel into the past you should be able to travel to any moment in the past therefore they all have to exist, The information for every single particle, it’s state and position, in the universe)

 

[russ_watters]

If yes why wouldn’t random effects destroy past information? I am asking in relation to the theory that no information is ever lost.

What theory is that? I've never heard of such a thing.

The universe doesn't have a hard drive. It does not explicitly store past information at all, much less permanently. We glean past information from current state and trajectory, only. Sometimes this provides quality insight into past states, and sometimes it doesn't.

I am asking because I was criticized on another thread for saying that in order to travel into the past the information from the past has to exist

Time travel to the past isn't possible, but it's not about information it is because the past itself doesn't exist. That's what "past" means.

I can't go or even see/get information from 50 years ago about what my parents had for dinner on September 17, 1970. That information, much less the actual scene, simply does not exist anymore.

 

[Andy Resnick]

However if random events destroy information then the past is not encoded and infinities of infinity of information needs to be recorded elsewhere in order to travel into the historical past.

It's not either-or. An analogy to your scenario are constitutive equations for materials with memory which can be expressed in terms of a Volterra equation:

https://en.wikipedia.org/wiki/Volterra_integral_equation

In the integral there is a 'kernel' that effectively determines how quickly the past is forgotten.

 

[Buzz Bloom]

How about the randomness of quantum mechanics measurements of particle properties?

 

[mpresic3]

brownian motion is random

 

[Nugatory]

brownian motion is random

We need to be careful about how we define ‘”random” here.

If we knew the exact position and velocity of every water molecule bouncing around a suspended particle then in principle we could calculate the exact motions of the particle; the randomness is an expression of our incomplete knowledge. Something similar happens with a tossed coin - we consider the heads-up/heads-down outcomes to be random even though we could calculate the exact trajectory and final result if we had complete knowledge of the initial conditions.

This is very different from the randomness that appears in quantum mechanics, in phenomena such as radioactive decay and spontaneous emission. No matter how complete our knowledge of the system, the outcomes of measurements are random with probabilities determined by the rules of QM.

 

[256bits]

if we had complete knowledge of the initial conditions.

A big IF.
Considering that the irrational numbers, or transcendental numbers for that matter, could / would be within the equations that describe the system, we could never know its the exact state to a complete certainty.

 

[Whipley Snidelash]

What theory is that? I've never heard of such a thing.

The universe doesn't have a hard drive. It does not explicitly store past information at all, much less permanently. We glean past information from current state and trajectory, only. Sometimes this provides quality insight into past states, and sometimes it doesn't.

Time travel to the past isn't possible, but it's not about information it is because the past itself doesn't exist. That's what "past" means.

I can't go or even see/get information from 50 years ago about what my parents had for dinner on September 17, 1970. That information, much less the actual scene, simply does not exist anymore.

I agree about the past, that was the point that I was trying to make. I thought it was a Hawking theory. It was mentioned in relation to information going into a black hole being lost.

 

[anorlunda]

I thought it was a Hawking theory. It was mentioned in relation to information going into a black hole being lost.

This is a difficult topic to discuss, because we all have so many meanings of the word "information" in our heads.

Let me resort to a metaphor to illustrate. An electron has spin up or down. That means it carries 1 bit of spin information. Now take a collection of 1000 electrons. Suppose we could color them to see which are up. Further suppose that they were arranged on a 2D surface so that they appeared to spell out a word dot-matrix style.

Your grandparents could have read that word 50 years ago. We can use the word information to describe that, and the word knowledge to describe what your grandparents knew. Obviously, that arrangement of electrons can be scrambled destroying that information. Your grandparents knowledge can also be destroyed. Nevertheless, the 1000 scrambled electrons still carry 1000 bits of spin information, so in that sense, information was conserved.

An electron may not evolve to carry 1.5 bits or 0.5 bits of spin information. That is a better way to look at conservation of information. In quantum terms, it is related to unitarity. On macro terms, it is related to Liouville's theorem. In human terms, it is hopelessly muddled because of our fuzzy definitions of the word information and the related word knowledge. Every time we have a discussion of this topic with 10 people, they have 10 or more meanings of "information" in their heads.

So @Whipley Snidelash , if you want to did deeper in this thread, you should begin with a much more precise definition of what you mean by the word information.

 

[mpresic3]

We should indeed be careful about using the word "random". I heard a story about astronomers, who for some reason needed a random star. One astronomer chose random right ascension and declination and found the star closest to this polar coordinate. Another astronomer listed the stars in the stellar catalog from 1 to N N very large. Then generated a random number from 1 to N and looked at the star corresponding to this N. Clearly, the definitiion of "random" leads to different results.
One can see from Bertrand's paradox for chosing a chord at random on a circle you can get three different probabilities for this seemingly well posed problems.
I agree that random when it comes to quantum mechanics is a different character, than random for a molecule buffeted where in principle, a complete knowledge would provide the momentum and locations of all particles but in practice, similar (but not identical) tools are needed for both problems.

 

[Buzz Bloom]

Hi mpresic:

We should indeed be careful about using the word "random".

I agree that some care is needed when discussing randomness, but I think that the real issue that creates confusion is insufficient care in defining what "equally likely" means.

I heard a story about astronomers, who for some reason needed a random star. One astronomer chose random right ascension and declination and found the star closest to this polar coordinate. Another astronomer listed the stars in the stellar catalog from 1 to N N very large. Then generated a random number from 1 to N and looked at the star corresponding to this N. Clearly, the definition of "random" leads to different results.

Here the search for a random selection involves that the selection is from a finite number of choices.
If one wants each of the finite possibilities to be equally likely to be chosen, then selecting from the stellar catalog gives the desired result, while basing the choice on closeness to random orientation clearly fails to achieve equal likelihood of choice.

One can see from Bertrand's paradox for choosing a chord at random on a circle you can get three different probabilities for this seemingly well posed problems.

These problems involves a transcendental cardinality. I think my interpretations are correct, but I am not strongly confident that I have avoided mistakes.

https://duckduckgo.com/?q=Bertrand's+paradox&atb=v203-1&ia=web​

If I were given the task to choose a method, it would be that the length of the chord would be randomly chosen with equal likelihood, and the angle of a radius bisecting the chord would be randomly chosen with equal likelihood.

Argument 1:

The "random endpoints" method: Choose two random points on the circumference of the circle and draw the chord joining them.​

This involves the choices of chords whose corresponding arcs have lengths which are equally likely to be chosen.

Argument 2:

The "random radial point" method: Choose a radius of the circle, choose a point on the radius and construct the chord through this point and perpendicular to the radius.​

This involves the choices of chords whose corresponding pair (r, a) of values are equally likely to be chosen. The value of r is chosen as a distance from center of the circle to a point on a line of length equal to the radius. The value of a is is a length along the circumference from an arbitrary reference point on the circumference.

Argument 3:

The "random midpoint" method: Choose a point anywhere within the circle and construct a chord with the chosen point as its midpoint​

This involves the choices of chords whose corresponding centers are equally likely to be chosen from all the points inside the circle.

Regards,
Buzz

 

[DennisN]

Summary:: Is there anything in physics that’s random?

Is there anything in particle or energy physics that is random?

I think that sequential measurements in the Stern-Gerlach experiment is a nice example of randomness.

Summary example:

1. Let's say you measure the spin of a particle in the z direction and get $+\hbar/2$.
2. You then measure the spin of the same particle in the x direction and get $+\hbar/2$ or $-\hbar/2$ with equal probability (50%).
3. Then you measure the spin of the same particle again in the z direction. The result will not be definitely $+\hbar/2$ as in the first measurement. It will be $+\hbar/2$ or $-\hbar/2$ with equal probability (50%).

See e.g.

• Stern-Gerlach experiment description on HyperPhysics.
• See the sections "6.4.2 Repeated spin measurements" and "6.4.3 Quantum randomness" in this texbook chapter called "Particle Spin and the Stern-Gerlach Experiment".

 

[BillTre]

Obviously, that arrangement of electrons can be scrambled destroying that information.

Makes sense to me.

Your grandparents knowledge can also be destroyed.

This would seem to me to be an independent event I am assuming (such as when the parents die and the knowledge dies with them (unless recorded in some way)).

Nevertheless, the 1000 scrambled electrons still carry 1000 bits of spin information, so in that sense, information was conserved.

So it is the conservation of the amount of information, but not the precise information associated with each particular electron?
Or do electrons lose their individual identity?

This seems to imply that when an electron encounters an anti-electron, they annihilate each other and create other things (particles, energy), and the amount of information conserved in the resultant products?

 

[anorlunda]

If your question refers to the 20 year debate between Hawking and others about information loss in black holes, then yes it refers to the quantity of information. Even though atoms might be broken up and particles can decay, the quantity of information in the products (not the precise information) is the same as the original.

All black holes must eventually evaporate into Hawking radiation (you may have to wait trillions of years). The Hawking radiation carries information. Now compare the quantity of that information with the quantity of information that went into the black hole in the past.

The mechanisms of black holes are way over my head. I learned from, this book. As I understand it after 20+ years of arguing (modest by Physics Forums standards), Hawking admitted that information was conserved.
The Black Hole War: My Battle with Stephen Hawking to Make the World Safe for Quantum Mechanics
by Leonard Susskind | Jul 7, 2008

 

[OCR]

Or do electrons lose their individual identity?

Electrons can have an individual identity ??

.

 

[sysprog]

Sometimes when I'm coding or setting up crypto-things I hope and trust that there's such a thing as 'really actually truly genuinely random' but I have to be truthful and admit that I don't for sure with 'epistemic-level certainty' know.

 

[BillTre]

Electrons can have an individual identity ??

Does not a single electron in some experimental set-up have an individual identity?
This identity, however, seems to be lost in thermodynamic considerations (involving large numbers of them).

 

[vanhees71]

Electrons are indistinguishable fermions, i.e., there is no "individuality" of any two electrons.

 

[BillTre]

Electrons are indistinguishable fermions, i.e., there is no "individuality" of any two electrons.

I am no quantum mechanic, but this makes sense to me in "normal situations" like electrons in molecules (particularly in clouds of electrons, like in benzene rings or lumps of metal), buzzing around in their quantum mechanical way (as defined by their probability wave function), would not have an identity.

However, single isolated electrons in special experimental set-ups would seem to be an exception, distinguishable from others due to their residing in a special experimental set-up.
From wikipedia (see confinement of individual electrons):

Confinement of individual electrons
Individual electrons can now be easily confined in ultra small (L = 20 nm, W = 20 nm) CMOS transistors operated at cryogenic temperature over a range of −269 °C (4 K) to about −258 °C (15 K).[64] The electron wavefunction spreads in a semiconductor lattice and negligibly interacts with the valence band electrons, so it can be treated in the single particle formalism, by replacing its mass with the effective mass tensor.

Perhaps my interpretation is wrong, or perhaps it involves a special quantum physics meaning of individual which I am not aware of.
Or perhaps I don't properly understand what is meant by "no "individuality" of any two electrons".

 

[vanhees71]

Also single electrons are not distinguishable. All states are completely antisymmetric wrt. interchange of any two electrons (Pauli principle).

 

[Lord Jestocost]

I am not sure whether that should be stated in such a way. Richard Fitzpatrick writes in his „Quantum Mechanics” (http://farside.ph.utexas.edu/teaching/qmech/Quantum/node60.html):

"Equation (462) shows that the symmetry requirement on the total wavefunction of two identical bosons forces the particles to be, on average, closer together than two similar distinguishable particles. Conversely, Eq. (465) shows that the symmetry requirement on the total wavefunction of two identical fermions forces the particles to be, on average, further apart than two similar distinguishable particles. However, the strength of this effect depends on square of the magnitude of $\left< x \right>_{ab}$ , which measures the overlap between the wavefunctions $\psi(x,E_a)$ and $\psi(x,E_b)$. It is evident, then, that if these two wavefunctions do not overlap to any great extent then identical bosons or fermions will act very much like distinguishable particles."

 

[OCR]

.
Well, this "identity" deal seems to be complicated, but I think you're correct about the

problem subject assumptions you made in this post, below. . .

Or perhaps I don't properly understand what is meant by "no "individuality" of any two electrons".

Maybe. . . this can help ?

. . . get this thread locked. . . ? . .

Lol, carry on. .

.