Workings of the Universe

This is a thread for discussing and learning awesome things about the universe.

For earlier topics, see these links to the "Learn stuff + share it" thread:
Dark Energy
Electroweak Symmetry Breaking

Gravitomagnetism
I don't know many details about this, but it blew my mind when I realized that it exists.

Gravitomagnetism is not magnetism. But it is to "regular" gravity what magnetism is to electricity!

When electric current moves through a wire, a magnetic field appears around it, going round and round the wire. Basically, the magnetic field is the electric field's buddy, doing something in those remaining dimensions of space that the electric field isn't acting on. There's a super tight coupling between the electric and magnetic fields, because they are really just two sides of the same force, the electromagnetic force.

It turns out that gravity has such secondary effects too. They are usually too weak to make a difference, but they are very real. One effect is seen through a special motion in Mercury's orbit. Because the Sun is rotating, and because Mercury is very close to the Sun, Mercury's orbit is slowly being pushed around, like an uneven pebble pushed by an ant. This is called frame-dragging and it is a gravitomagnetic effect. Gravity as we think of it just attracts, but Mercury is being pushed sideways as well! So there is a gravitomagnetic field pointing around the Sun, just like there is a magnetic field pointing around an electric wire. (Einstein knew all of this and it's part of relativity.)

What really blows my mind, though, is that this behavior seems to be something all forces do!

In the 1860s, there was a breakthrough in our understanding of electric and magnetic fields, and the equations found are considered some of the most elegant laws of nature. When you see gravitomagnetic effects, though, the exact same equations can be used there too!

What this means is that the equations don't really have anything to do with the electromagnetic force at all. They describe how any force will act inside of our three-dimensional space, whether it be gravity or electromagnetism or even the last two, the strong and the weak force. There are magnetic-like parts to everything. It's not just electromagnetism that is weird!

Quote from Silent Lion

The uncertainty principle is meant to mean that there is true randomness in the universe. But I don't see how that can be verified, without rewinding the universe and seeing if the results repeat. I have yet to find a compelling argument (or be qualified enough to understand a compelling argument) that the universe is not deterministic. The uncertainty principle is supported by Godel's incompleteness theorems, which say that the universe is an incomplete, non-trivial computational system and therefore not all values and formulae can be deduced from any particular point within the system. But, more colloquially, all that means is that not everything can be deduced, referring to human knowledge. That's not quite the same as saying it isn't predetermined. It could well be, only that we are incapable of deducing it. So, the uncertainty principle holds, for now, as a practical experimental concept, but is it really some fundamental truth about the universe?

Great questions! It's very interesting how you combine this with Gödel's theorem. That basically ties in to the debate of hidden variables.

Basically, a hidden variable means that there is some underlying system, and when we do quantum mechanics we only see the surface, the result. The pilot wave theory, I believe includes hidden variables. But many physicists, especially those working in the quantum field, deny hidden variables and accept the uncertainty principle as a fundamental truth of the universe. Weird experiments seem to indicate that there is no actual physicality at the most basic level, only abstract superpositions of potential futures. See this video from PBS Space Time, for example:

There's also another way to see the uncertainty principle which I only found today. The principle is actually a wave phenomenon, rather than a purely quantum one. The link further down explains it best, but I'll give it a shot. Location and velocity are two properties that cannot be known at the same time due to the uncertainty principle. Well, in terms of quantum waves, position is encoded in the amplitude of the wave, whereas velocity is encoded in its wavelength:

A perfect sine wave, like the one pictured above, has a very easy wavelength, but it has the same amplitude an infinite number of times! Therefore its velocity can be known, but its position cannot. Now to illustrate a more general quantum, see the following animation:

This illustrates superposition, which is adding many simple waves to make up one actual particle state. As this is done, one peak dominates while its wavelength starts to vary a lot. This means its position is known well, but its velocity is very uncertain.
Source: http://physics.stackexchange.com/questions/2871…ainty-principle

So the uncertainty principle isn't actually that weird. You can say the same for a wave on the ocean: where is the wave really? You can see many peaks and troughs, so its position is not exact, but spread out. What is really weird, is how this wave will collapse into behaving as a point particle. The randomness is encoded here, and the indeterminacy of the universe lies in what happens to the wave.

Heisenberg, one of the founders of quantum theory, made the philosophical point that our universe is causal, but not deterministic. We know the reaction to every action, but that reaction is a predictable range of possibilities. Note that a computer simulation can have the same characteristics, albeit with a pseudo-random nature. Numerical simulations start with initial conditions and equations that strictly govern change. You cannot calculate the result without letting it actually happen. In that sense, is the simulation predetermined or not?

In short: in quantum theory, which is our most successful theory ever in terms of predicting experimental results, the uncertainty principle is a fundamental truth of the universe. There could still be an underlying theory that has greater determinacy. And as you said, even though there is, we might never know for sure!

To your last question - yes!

I haven't read Stewart's books myself, but they seem interesting. Personally, I stay away from the higher level math, it simply does not speak to me. I do have a university degree in Physics, and that helps, but I find it much more fun to get the essence of something, even if it is mathematics, without having to understand the equations. My point is that if you get stuck with the maths, don't let that deter your curiosity. But yes, if you got the math, you got the power to play with the universe.

Frank Wilczek writes about deep physics without equations, I liked his recent book A Beautiful Question. Sean Carrol, Robert Oerter and maybe Roger Penrose have written books giving very deep insights without requiring too much math.

I can add two relevant points of interest:

Atomic orbitals
The electron was much debated once the atom's shape was discovered. If the electromagnetic force functioned in the same way for atoms as for larger things, then the orbiting electron would be always accelerating (changing direction to stay in orbit). Doing so would make it emit photons (not sure why) and it would lose energy and spiral inwards in no time at all.

As a digression, I believe this is true for planets as well. The orbiting Earth emits gravitational waves. Imagine it like the Earth and Sun are floating in water and the Earth moves around. The energy in these waves, however, are so tiny that we can never hope to detect them*. It's a whole different story for the much stronger electromagnetic force.

The agreed solution to this paradox was that the electron does not move at all. It is now regarded as a standing wave in 3 dimensions. This is basically what you get inside of a violin case when it is played, as it resonates with the tone. If you search for pictures of atomic orbitals, you'll see our modern visualizations of the electron:

Of course, further progress could one day show that this is not the most fundamental way to picture the electron.

Chaos and Emergence
Really amazing subjects that cannot be done justice by me, but I can give some hints. They are related to your wonderings about statistical behavior at different scales.

When a large number of small things act together, this can lead to entirely new behaviors. That is emergence, and it describes how complexity can create new phenomena from simple ingredients like atoms.

My favorite example is gases. We can manipulate their pressure and temperature very well. We find that they follow the laws of thermodynamics, which coincidentally state that the universe will some day end. But there is no hint of this in the behavior of the atoms making up the gases! There is even no concept of temperature for atoms. Temperature is the amount of disorder in the motion of individual atoms. It is simply kinetic energy which is not acting collectively.

Temperature is a measure of chaos. We have simple gas laws we can use only if the temperature is low enough. When it is, all the atoms will more or less act as one, and you get the gas laws from statistics of great numbers. But the higher the chaos, the more erronous the laws are, and the lower the predictability.

Chaos in fact defeats emergence, and you have to start considering the individual atoms again. This is much more demanding. That is why, when simulating atomic bombs or the hot plasma of the sun, we need vast supercomputers. Interestingly, we also need them for the quantum fluctuations inside of protons. That is also a high-energy mess!

You'll find chaos and emergence everywhere, on all scales. Weather is sonewhat chaotic yet predictable. Climate is more certain for the reasons you describe. Consider birds or fish moving in great flocks, or humans moving in traffic or along streets. We have physics for this. Consider the interplay of animal populations deriving from biology and brains. Equations exist. So too for the stock market. That one has a higher portion of chaos. Some would say that free will exists in such a void of chaos. Consciousness might even be quantum.

So as you move across scales, uncertainty may fall and rise and fall again, and the governing equations will be correspondingly demanding. Our surroundings are astoundingly predictable and simple to calculate compared to the quantum realm. But perhaps deep down, the quantum is again an emergent phenomenon?

* for the Earth, that is. We did detect then for black holes orbiting each other, and predictably, they did spiral inwards and merge as they lost energy to their gravitational waves!
The Dirac Sea
I just read about this and it blew my mind again.

First: The Dirac Sea is most likely just one way to interpret the vacuum. This interpretation might have no physical reality, but whether or not it is real makes no difference for the end result. It might be that all of our interpretations are equally irrelevant for the true nature of the universe.

The idea is that the universe is basically teeming with energy, but at such a constant level that we cannot measure it. All we can measure is the deviation from this level. That is our particles. And not just the normal particles. Because if you are able to take energy away from the Dirac sea, making the energy lower, that is also measurable. This is what antiparticles are, and this is how Paul Dirac predicted antiparticles in 1930.

What this implies is that antiparticles are not really particles at all, but they are holes in the vast continuum of the Dirac sea. Despite this, there is nothing about antiparticles to say that they are not a type of particle, too. And that's actually not weird..

The Physics of Holes
In some branches of physics, holes are absences of particles. This is really important when you deal with electricity. In a computer chip, or in a solar cell, holes are just as important charge carriers as the electrons themselves. But they are really just bubbles where there is one electron less than there could be. As an electron moves to fill in that slot, the hole will move over to where the electron used to be. The holes are therefore stable and move around just like electrons do, except they have opposite electrical charge! What's more, they have all of the properties that a particle can have, which is why physicists can easily treat them just like particles. Even though they are really not.

So how can we be sure that any particle is really a particle, and not a hole in a sea of particles? This also means it makes sense to think about the Dirac sea.

Quote from Silent Lion

The idea of holes and antiparticles strikes me as similar to temperature. Only allegorically, but I watched a documentary about the early understanding of cold, and how it was considered to be a separate, physical thing. It wasn't until a hundred years or so ago that we finally understood it to be the absence of something. I can't help but draw a parallel with the way our understanding of quantum particles might evolve.

That's a great way to see it. Drawing parallels has been extremely useful for me in learning physics. It's also a way to transfer knowledge from one field to discover something in another.

Quote from Silent Lion

But now that I have free tuition from a physics graduate , behold my QUESTIONS!

It has been pointed out in Stewart's book that the orbit of the earth is not the effect of pulling from the sun, like the lead-ball-on-the-end-of-a-string-being-spun-round imagined in Newtonian physics. Rather, the planet is being deflected off the curvature in space caused by the sun, like the camber of a road. So, naturally, I got thinking about these things: https://i.ytimg.com/vi/X6e1y0WzzeE/maxresdefault.jpg

Why is it that the coin seems to exit the hole at the bottom with greater velocity than it began with? I can only assume it's an optical illusion, because no matter what exotic physics happens, the coin cannot gain kinetic energy unless that energy is provided to it from outside. As the coin makes a lot of noise, if anything it should be losing energy - the effect of gravity over the short vertical distance from the coin slot to the spiral can't add very much.

You're right, the short distance doesn't add much at all. But then, the coin doesn't weigh much at all either! Sound is actually super cheap in energy, and the coin does indeed gain energy from the gravitational field as its moves down the surface.

This is a good example of conservation of energy*. You can draw parallels between an orbit and a pendulum. A pendulum, like in a grandfather clock, swings back and forth. If you keep a close eye on it, you'll realize that its highest speed is at its lowest point. Meanwhile, at the ends of the swing it's not moving at all for a split second. What is going on in the pendulum is that the (small amount of) energy is perpetually being traded between potential energy (being high up in a gravitational field) and kinetic energy (moving fast). So following the parallel to the orbit of the coin, that fast speed it's got at the bottom is actually showing you the potential energy that was contained at the highest point of its path.

However, when I think about it some more, there might indeed be a little illusion at play as well. If it wasn't for the fact that the coin is constrained to a narrow tube, spinning like crazy, then its velocity might seem slower. When it is merely coasting on the top of the surface, we might see that as a slower speed than it really is, in comparison. However, the planets themselves do have higher velocities at the "lowest" points of their orbits. For objects like Sedna, or the hypothesized Planet Nine, this is quite a considerable difference. I would recommend trying out the mobile game Orbit for a relevant demonstration :D

Quote from Silent Lion

The other question is, when we measure electromagnetic frequency, what is it the frequency OF? A sound wave, for example, is a measure of air pressure relative to a background equilibrium, represented as the x-axis at zero. So, is electromagnetic frequency an oscillation in the 'energy pressure/concentration' relative to some rest state?

In short, the field strength. I will assume first we're talking about the frequency of a loose piece of electromagnetic field, a photon. It will oscillate around the value of the background field, whatever that is!

A digression about the broader perspective: In some interpretations of quantum field theory, there is an electromagnetic field filling the universe. Its default value is zero, and we can know this for sure because any other value would affect particles. That is not to say that is ever is zero, because every single charge in the universe will add its own dimple. Electromagnetism can be seen just like gravity in this regard, as a huge sheet of dents caused by charges instead of masses/energies. The dents can of course go in two directions, because of the +/- nature of electromagnetism. The reach of electromagnetism is infinite, so this field technically has a non-zero value anywhere in space. By and large, though, this is so small you can ignore it. Even though the electromagnetic force is much stronger than gravity, the charges in the universe largely balance each other up (such as in neutral atoms), there just aren't enough free charges to have a cosmic effect.

Going back to the photon, this little electromagnetic lass will dance to her own tune, and if she dances up the stairs and onto a higher dancefloor, her dance will be a "higher" dance, but she won't care about that! Photons are not charged and they will not interact with electromagnetic fields. Only an outside observer will count the floor as well.

Wait, how can the (electric and magnetic) field strengths vary, but the photon have no charge? What confines these fields so they can't reach out and affect anything, and how can we measure the frequency if that's the case? I don't remember any good explanation of this! I will keep looking, though.

* Of course, it's due to a small energy loss that the coin falls down at all, but its behaviour is still very well approximated by conservation laws. Almost all of our laws are approximations when it comes to the behaviour of the real world, where a near infinity of minor effects take place constantly. We understand pretty much all of those minor effects too and we could always make a better fit, that's just a question of utility. Approximations are more powerful than they might sound. But I do want to make the point that physics can be overly simplified compared to the sheer complexity and beauty of reality!

I really like that one too! It sheds light on how particle and wave can be complementary attributes, not exclusive.

Yes! There were some good points to retrieve from your text though.

- Since mass equals energy, all energy is some kind of mass. The photon has no rest mass, but it does have mass since E=mc^2.
- Gravity is fundamentally tied to mass. Another way to put that is, gravity is fundamentally tied to energy. So energy from anywhere creates gravity. The Higgs field contributes some of this energy.
- Before the Higgs field took its current non-zero value, a lot more particles were massless like the photon. It took way less energy to move them around. When the Higgs field took its value, most of these got a rest mass, which meant that energy was stored in them even when they weren't moving around or doing anything else wonky.*
- Higgs was the last missing piece of the standard model, which explains all of the particles and their forces except gravity. Also except dark matter and dark energy. So it explains how the electromagnetic, the weak and the strong forces can sort of combine, but gravity is still out there even then.
- So as you move back in time towards the hypothetical Big Bang, you should see everything turning into a quark-gluon plasma; then the electroweak force should become mended and several particles become massless; you might also see the strong force join in and more fun stuff happen; and then, maybe something really odd starts to happen with gravity, that is, with the fundamental structure of spacetime. This is where we need quantum gravity – if such a theory is possible.

I am currently reading an interesting book about quantum gravity. Haven't read enough to know if I recommend it yet, but it should have some interesting details:
https://www.goodreads.com/book/show/2976…t-what-it-seems

An intriguing idea for what drove the expansion of the early universe:
https://news.vanderbilt.edu/2017/10/13/fil…ee-dimensional/

* All massless particles must move at the speed of light, so they always have energy there. No matter your reference frame, they will move at the speed of light, just like photons do. I'm actually not sure why they have to do this...

That is an awesome way of looking at mass and the speed of light!

And I think you are right that the temperature in the interstellar medium would rise. Everything would become interstellar medium. And the overall temperature of the universe would rise too – all mechanical energy, chemical energy and even rest mass energy would be converted into heat energy. However, there is one caveat: the universe is expanding. Although this actually represents an additional energy "entering the universe"*, it expands space to the point that matter gets more spread out, and that reduces the temperature.

Additionally, the vacuum energy is not the same as the ambient heat energy. To measure vacuum energy you'd want to build a radiation-free box. It is related to the fact that the energy of anything can never be measure to be exactly zero, by the quantum uncertainty principle. So since we have determined that space itself is something, its energy must fluctuate randomly in accordance with quantum principles. Heat energy will come on top of that.

There is more here that I do not understand very well. Something about the phases of quantum fields, and how both real and virtual particles (quantum fluctuations) appear to exist because of these fields' wiggling. I saw a bit of this in PBS Spacetime's episode on Hawking radiation (link).

* I think this way of seeing is dissolves when you get deep into the Einstein equations, the curvature of the universe and whatnot. Energy is actually not conserved in the very large picture of the universe, because the conservation laws that we are used to only work for a specific spacetime curvature.