I thought about making the title for this week’s Idea simply “Quantum,” since one-word titles are kind of cool, but ultimately decided that accuracy was a higher priority than coolness. That’s why I’ll never be cool.
Anyway, quantum mechanics! This installment runs down the basic ideas; we’ll leave the conceptual heavy lifting for next week.
The Biggest Ideas in the Universe | 7. Quantum Mechanics
And here is the Q&A video. Mostly I talk about the double-slit experiment, and how it implies that the wave function is something other than simply a stand-in for our lack of knowledge about where the particle is.
The Biggest Ideas in the Universe | Q&A 7 - Quantum Mechanics
Hi,Sean! What is the diference between the wavefunction of the universe and the quantum fields? Thanks!
Hi Sean! I’m a psychologist from Brazil that studies quantum mechanics in my free time and you are absolutely wonderful! Thanks a lot!
Please, there’s a thing I never understood:
Let’s take many worlds interpretation and delayed choice quantum eraser…
We shoot entangled photons p0 and p1, and their waves spread through the multiverse…
We measure p0, and all the outcomes spreads through the multiverse…
We shoot photons q0 and q1, and their waves spread through the multiverse…
Now we measure q0, and the outcomes spreads through the multiverse…
And now we choose at random to use photons p1 and q1 to swap entanglement to photons p0 and q0…
Now it comes the tricky part:
Let’s suppose we swapped the entanglement. There are some universes that the data from the measurement of p0 and q0 is coherent with the entanglement swapping and others that are not…
Does the universes that the data is incoherent just vanishes away?
Thanks!
If the wave-function is physically real what could it be made out of? Waves aren’t fundamental things but an organised behaviour of smaller entities e.g. sound waves are an organised behaviour of atoms. Could the Shrodinger equation be a statistical approximation like the Navier-Stokes equations or gas laws.
How can an electron be both a particle and a wave ? Has it something to do with mode/method of observation ? Thank you for an insightful talk.
Dr. Carroll,
I’m sorry this question is about spacetime. I learned about your videos too late. I have wondered about the following: Let’s assume events in the observer’s present, but absolute elsewhere are real. Since observers at a large enough distance and with large enough speeds can move events in the absolute elsewhere into their own future or past by changing the direction of travel, does this imply that all events in the block universe and the associated web of causal connections in some sense came into existence simultaneously (I know that that verb construct isn’t right, because it implies some kind of embedding in an exterior time. We don’t seem to have words to describe the situation.) E.g. nutrinos shortly after that big bang will have events that are in other observers’ far future located in nutrino’s past but absolute elsewhere region. Does that make sense? Thank you!
[OT] your articles on Discovery Magazine appear now to be locked behind paid subscription. Are you allowed to republish them? Thank you prof. for your efforts in spreading knowledge and share your insights.
Could you please say something about the nature of “force-carrying particles” and how these relate to fields, particles like the electron/proton/neutron, and wave functions?
Sean,
in the first frames of the video, if you place your head off center, you won`t appear everywhere with a big START BUTTON right in the face… I`m just saying..
Thanks for the videos
Marcel,
Is the Born Rule a necessary axiom or can it be derived?
Firstly, thanks Sean for these fascinating video chats.
Is it possible when observing a particle that rather than the wave function collapsing or an infinite number of particles are generated into an infinite number of universes, instead the particle is observed once it has interacted with a field, something similar to Higgs boson?
So the probability of a particle being in one place is like a lottery and only makes an appearance once a ‘field’ has interfered with the wave function which could have been caused by the observation.
Two of the last and oldest questions to be answered are still pending; What is the Universe made of and what makes it evolve in a spontaneous way. These are the “stuff” and the logical reason for the working of the Universe which are the metaphysics concepts of “substance” and “cause”. These concepts are the only ones that can give us a true picture of what the Universe is really like without the observer’s influence being in the way.
These concepts could possibly be found a priori, but we don’t need to. Physics already have the answers to these metaphysical questions. Let’s start with the best (In my view) ontological description of gravity according to general relativity from physicist Bill Unruh:
‘ .. A more accurate way of summarizing the lessons of General Relativity is
that gravity does not cause time to run differently in different places (e.g., faster far from the earth than near it). Gravity is the unequable flow of time from place to place. It is not that there are two separate phenomena, namely gravity and time and that the one, gravity, affects the other. Rather the theory states that the phenomena we usually ascribe to gravity are actually caused by time’s flowing unequably from place to place… “ arXiv:gr-qc/9312027v2 17 Dec 1993
We see here that the rate of time may have different values here and there, whether we measure it or not. This Time qualifies as a SUBSTANCE; a substance-process since it has a rate of evolution. The CAUSE is described here as a local difference in the rate of evolution of this substance-process. This substance-process has a variable (rate of evolution) and a property or constant which is the Planck (above post).
The Universe obeys the rule of non-contradiction, which makes it easily described by mathematics and a bona fide logical system. This means that the Universe was (had to be) created and is operating according to simple rules of logic. Such a system can only be functional on logic if it is all made of a single type of substance and is “motivated” by a single type of cause. Therefore, the whole Universe is made of various forms of this single type of time substance-process and of various forms of this single type of cause.(at all scales) Such a Universe made of the time process substance not only allows change but also makes it compulsory; everything vibrates giving us a new Universe every .. second.
There are many implications to this. But one interesting implication is that Dark Matter has to be this time process substance; Dark Energy has to be a difference in the rate of evolution of this time process substance. But, as stated above, this substance, in various forms, also makes all our instruments we use to try to detect it…
Since the time process substance is spontaneously dynamical, it follows that it represents, in itself, an illogical state of affair (not a stable state) which is in the process of resolution and that, somehow, does not obtain. (Similarly, an object spontaneously falling in a gravitational field is an example of an illogical state of affair in the process of its logical resolution). So, the Universe is essentially a spontaneously dynamic system driven by its fundamental illogical state of affair. Why? Since “being” coming from “nothingness” is impossible and a contradiction, the Universe could only spring out from a loophole in this contradiction, in between these two opposites, as “happening” or dynamical, which is neither of them (or a mixture?)… (Next?: Two sparks for One Universe: Symmetry Breaking.)
This talk was really brilliant. I never saw a more concise introduction to quantum physics.
In the Copenhagen interpretation, a measurement M collapses the wavefunction irreversibly. Even if we start with an ensemble of equally prepared particles that allows us to repeat the same measurement over and over again, the density matrix is deprived of all the components that are non-diagonal with respect to M’s eigenstates. Is this loss of information somehow related to an increase of entropy? Do measurements introduce an “arrow of time” into quantum physics?
I have several questions related to what information the wave function is supposed to contain. I fully understand that it simultaneously contains position and momentum information (which are Fourier transforms of one another). But is the same function also contain the spin state information for example? Or is that a separate function. When you have more complex particles is there a single function that evolves with the schrodinger equation that contains all the quantum information of the particle?
Another question has to do with the “wave” like properties of a photon. In classical mechanics we talk about the waving of the electric and magnetic field positions. And in QM what is waving is a probabilistic description of where the particle is. It is sometimes confusing as to what is “waving” field potentials or probability distributions.
Great questions, Douglas. I too would like to know. Thanks.
When are you doing your next Teaching Company course?
Hi Sean,
I’m trying to understand superposition in dual slit experiment, but asking about single slit experiment to simplify the answer:
When you have just one slit (instead of two slits) and you shoot a photon through, the wave function gives a probability for where the photon will hit the screen. Supposedly the exact location is unknown prior to hitting the screen due to the Uncertainty principle. Q: in this case, why would the concept of superposition come into play? (assuming no entanglement) Why can’t we think of the final position as unknown (like flipping a coin due to incomplete data), and the wavefunction is simply a probability and the measurement on the screen is just the outcome (like the outcome of flipping a coin)? Meaning that we don’t know the exact path the photon took (until it hit the screen), but why should we believe that it could be in all those paths at once??
I ask you a question about physics and you give me Wittgenstein? Smh.
From the QM q&a: “Once you set (the wave function for the electron) to zero, you get a discrete set of solutions with definite energies. That’s where you get the discreteness. QM is not a theory about discrete stuff. QM is a theory of smooth stuff that sometimes appears discrete, because of the boundary conditions of the equations we are solving.”
But if you go to Wikipedia the emphasis is on the discrete (edited):
In physics, a quantum is the minimum amount of any physical entity involved in an interaction. The fundamental notion that a physical property can be “quantized”…This means that the magnitude of the physical property can take on ONLY DISCRETE VALUES consisting of integer multiples of one quantum. For example, a photon is a single quantum of light…
Was Quantum Mechanics misnamed? Is the emphasis on “quanta” misleading? Are “quantum jumps” in energy wrong headed? Or would the Heisenberg matrix approach emphasize the discreteness? Is this all outdated thinking?
Thanks so very much for doing these presentations …. and you patience!
From above. I am saying that an electro-magnetic wave (EMW) is a travelling variation of the variable of the medium which is the time process substance. What about Electro and Magnetic? First of all, there is no place in the universe logical system for some other substance; E and M are forms of the time process substance.
Superimpose the classic EM model over the wave of variation in the rate of the time process. You see right away that, within the wave, the magnetic field is in fact the time rate variable in the process of changing, increasing or decreasing. The electric field is a line along which this variation of the rate of the time process changes direction, from increasing to decreasing and vice-versa… following the known laws of induction. So, magnetism and electric field are emergent? Not really. They are the dynamic derivatives (first and second) of the rate of the time process we have directly access to.
Remember, only one type of cause is allowed in the Universe. So, magnets sticking to your fridge are subject to a higher probability of existence resulting from a difference in the rate of the time process.
Incidentally, if we could SEE the time process substance … we would be technically blind, just like a goldfish in a glass of milk. Luckily, we only see variation in the rate of the time process i.e. EMW.
(Take note: No goldfish was actually subjected to the “glass of milk” procedure)
Many thanks for posting these videos Sean. I have a 16 year old at home in the UK waiting to embark on studying A-levels at college this September (this is 16-18 year old education prior to university etc.. in UK, not sure what the US equivalent is). He wishes to study maths and physics.
Due to the coronavirus lock down, my teenager will be missing several months of education, however your videos are going to be an essential day time study component in the mean time.
I’m very grateful to you for your time and passion in putting these videos together. You have a great skill in presenting material without condescending or patronising your audience.
Many thanks again.
Hi Colin, I think you will also find this most helpful. Be well.
https://www.youtube.com/user/DrPhysicsA