The Learn Physics Thread - Space and Time - The Bear and Badger

SpaceGazelle

@Cinty

Do you understand the arguments made by Newton and Einstein regarding this absolute space thing? Have I got that across?

Minnesänger

Yeah, i felt that part was explained in an interesting and accessible manner. The concept itself is a bit of a headfuck as it seems counter-intuitive and my brain instantly defaulted to the Newton position, but the explanation made sense and I thought it was taught well.

GooberTheHat

Minnesänger wrote:

I’m reading it, I’m enjoying it - I’m also unlikely to engage directly with it, just casual lurking.

Same

Lurch666

Very much enjoying this.
I knew some of this but you are filling in lots of gaps in my knowledge.

b0r1s

I'll ask questions when I'm stumped, but also just taking it all in at the moment, enjoying the historical context stuff too.

yourfavouriteuncle

I’m following it and love it. It all makes perfect sense so far. Don’t stop.

RamSteelwood

yourfavouriteuncle wrote:

I’m following it and love it. It all makes perfect sense so far. Don’t stop.

this

SpaceGazelle

Cool. As long as people understand it so far. That's the main thing. It's hard to know if I'm making myself clear.

SpaceGazelle

Minnesänger wrote:

Yeah, i felt that part was explained in an interesting and accessible manner. The concept itself is a bit of a headfuck as it seems counter-intuitive and my brain instantly defaulted to the Newton position, but the explanation made sense and I thought it was taught well.

Good, it should be a bit of a headfuck but it'll make understanding general relativity easier later on.

SpaceGazelle

The Universe is a complex place because there's so much stuff in it. Under a few simple laws, atoms do this complex dance and after 13 billion years they form themselves into bodies, brains and forums.

If we take a pack of cards and shuffle them, they're almost cetainly in an order that has never been seen before and will never be seen again, and that's just 52 things. Imagine the mind-boggling complexity the universe can offer with all the atoms it has at its disposal.

We tend not to think of these things, and for good reason. It's easy to assume we know what things like forces are because we push doors open every day. But did you open the door or did the Universe open the door? We are after all made of atoms, and our brains and hands are subject to the same few laws that any atom is.

A physicist would argue you haven't chosen to open the door any more than a passing meteorite that hits a door chooses to. This leads to all kinds of awkward questions about free will so you might say, fuck you Universe, I'm going to spend all day opening and closing a door for no good reason. At the end of it the Universe will shrug and explain you were always going to do that, and to prove me wrong about free will you should've just opened the door once. At which point you kick the door in and down a bottle of whisky.

It's good that we don't think about these things because we don't want to be paralysed by wonder every time we need to get into the kitchen to get something to eat. Thinking about physics can be hard but the thing I love about relativity is that we can sort of understand it, or perhaps we should say the Universe can understand itself. It's not exactly intuitive but it's a damn sight easier than thinking about thinking about opening a door.

Tonight when I get some time we'll get into light properly. As long as people understand the argument each guy is making we'll be ok. Most should be on Newton's side for now (like everyone was at the time) because our brains are hardwired that way.

SpaceGazelle

Ok. So finally we get to light. The two greatest discoveries in all of physics (apart from Jupiter having moons obvs) both concern light.

The first was made by a brilliant Scot called James Maxwell in 1865, who unified electricity and magnetism and discovered that the speed of light was a constant. The second was made by Einstein in 1905, who upset an awful lot of people (including himself) when he discovered that light was in fact a particle. We won't be going down that road in this thread, that way madness lay.

Einstein had an absolute banger of a year in 1905. He published his paper on special relativity thus transforming our ideas about time and space, he discovered light was a particle thus birthing quantum mechanics and just for good measure he proved that atoms existed by staring at some pollen. What a guy.

Btw his paper on special relativity was titled "On the Electrodynamics of Moving Bodies" - which is just a fancy way of saying the theory of magnetism, and when it was published it was pretty much ignored. Partly because he published too much cool stuff in one year and partly because he gave the paper such a silly title. In fairness it does explain magnetism but a better title could've at least prepared the reader for the radical ideas contained within it.

This was 1905 after all and any hapless physicist that came across it hoping to learn a little more about magnetism was instead confronted by these crazy ideas about space and time, and they didn't sign up to be a physicist just to be intellectually assaulted in such a violent and abusive manner. It did eventually get the recognition it deserves but it took a while.

Anway, back to Maxwell and his equations. There are only four of them and they look kinda funny so we won't be going into the details except to say they they concern electricity and magnetism, and people were really getting into these new phenomena. Metals were being dug up and formed into wires and some cool experiments were happening. Volta had invented the first battery back in 1800 and they'd had time to try and figure out this electic force and magnetic force and finally Maxwell could write it all down in the form of equations, and that means an equals sign.

So similar are the equations for electricity and magnetism (they share this beautiful symmetry) that Maxwell played around with them using the equals sign and combined them into what we now know as electromagnetism. And the most curious thing happened. He had an equation that had some mysterious constant in it but he knew the values of all the other terms (by people doing experiments and measuring stuff to find those values out), and the value of this constant could be calculated - and when the units of the constant were looked at it was a speed!

A speed relative to what? And what was this constant all about anyway? Maxwell suspected it was the speed of light because he strongly suspected that this light phenomena that had puzzled people for all these years was in fact an electromagnetic wave. Someone had finally discovered what light was (it's not 1905 yet and this disturbing business about waves being particles is not something we'll need to worry about anyway).

Next time we'll have a deep dive into this constant c.

poprock

For me, this is where it all stops being great entertainment (because it has been entertaining - rattling through the basics of modern physics by treating it as a narrative of who thought what and who discovered what is a brilliant approach.) and now it starts getting challenging. This is around about the level where I stopped really understanding it at school and started just learning the equations brute force style without truly grasping what they meant.

Wave-particle duality absolutely fucked my teenage brain.

SpaceGazelle

It fucks eveyone's brain. Nobody understands it even remotely and we won't be covering it in this thread.

poprock

Phew.

hunk

Matter are waves and waves are matter? (Like a sinusoid derived from a vibrating particle?)
Imagine if everything travels at the speed of light; would light still be able to reach us and would we be able to observe our surroundings and experience "time"? What is time anyways?

davyK

Funnily enough , time was discussed by Cox earlier this week. Time is wrapped up in entropy - that's the best explanation of what time is that I've heard. At least - one that can be grasped.

It's just the sequence that things happen in. Sand on a beach is more likely to just blow about. While sand in theory could blow about and form sandcastles, it doesn't. Build a sandcastle it will end up just as sand blowing about. That's entropy. And it only goes in one direction.

hunk

Entropy is a good one.

What is light? I know an electric current induces a magnetic field and vice versa a moving magnetic field induces an electric current (think dynamo). Is light a similar phenomenon in electro-magnetic terms?

Why does nature behave in discrete quanta on certain stimuli. Why did Einstein hate the incl. of probability mathematics in physics? Does God like gambling?

SpaceGazelle

hunk wrote:

Why did Einstein hate the incl. of probability mathematics in physics? Does God like gambling?

The real problem with QM is the gambling. The paricle wave thing is troubling but it's just an inconvenience compared to the gambling. Light is always a particle when you look at it but when you don't it travels like a wave. This upsets people but we can explain this away by saying we don't know what it is, it's some sort of 'wavicle'. It's very alien to us but it's fine, we just don't know what particles and waves really are. Probably just constructs in our head to make understanding easier and at some point we could assume we'll get to the bottom of it.

The gambling however, is outrageous. It implies there is no reason anything happens, no actual mechanism at play. Science is built on cause and effect. Something hits something that causes it to do something that affects something else and so on. There is a chain of events and each event has a cause.

QM says there is no cause. There's only a probability you'll find (say) an electron in one particular place. The probabilities are well figured out but where you'll find it can never be known, just the probability you'll find it at the spot you'll search for it.

Another example, when an excited electron in an atom falls down an energy level and gives off light, it might just fall to one energy level below it and emit a low frquency of light, or it might drop a few energy levels in one go and emit light of a higher frequency. Eventually it'll emit light to get to the lowest energy level, whether in small jumps (2, 3, 4?) or in one go.

There is no reason or mechanism that determines how many jumps it'll take, only that it'll get there. Einstein thought this was insane, science is founded on mechanisms - reasons. Just because we don't have the understanding or equipment to determine what it'll do - the number of hops it'll take, doesn't mean there isn't some reason. Maybe we just don't understand atoms or electrons or energy well enough. He called them local hidden variables - stuff we just haven't figured out yet.

Remarkably, a guy from ~~Dublin~~ Belfast called John Bell figured out there can't be any hidden variables. The proof is quite amazing (proving something isn't there) but that's where we're at. Just probabilities and no scientific reason.

SpaceGazelle

Here you go.

https://en.wikipedia.org/wiki/Hidden-variable_theory

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

Lurch666

There's been a question about the wave/particle thing I've been wanting to ask and this seems the best place.
So in the double slit experiment you get an interference pattern of multiple slots projected showing the light travels in waves but if you set up a method of detecting which photons of light go through which slit you don't get an interference pattern,just two lines of light-one for each slit.
My question is how do we know the method of detecting the photons isn't causing them to behave differently?

SpaceGazelle

It absolutely is making then behave differently. The question is how? Nobody knows. As long as someone is watching they change behavior. You could get your pet cat to observe and they'll suddenly act like particles. You can do all sorts of sneaky things - just put a detector at one slit, put it before the slit, after the slit, down the hall using wires, use a monkey. The universe will always figure out something is watching and it'll change behavior.

Lurch666

So you have no way of knowing if you get an interference pattern with detectors that aren't being observed?

SpaceGazelle

Strange but true. Supposing you put the detectors on and walk away from the experiment. As long as you don't look at any results it'll always be a wave pattern. The moment you get curious and look at the pattern on the back screen it'll be particles. The screen pattern remains in some hazy reality that hasn't been decided until you look. It's called a superposition. I'm aware how mad this sounds.

acemuzzy

What if, instead of a screen, it was a.... cat?

Lurch666

So what puzzles me is how can we know there's an interference pattern if detectors are set up but no one is looking at it?

SpaceGazelle

Einstein wasn't thrilled either. He asked if he being asked to believe the moon didn't exist if nobody was looking at it.

Lurch666

Thanks. That's cleared it up for me.
Wasn't sure if it was just the presence of detectors that caused the light to act as particles or if they had to be observed.
Although it might be the detectors are causing the particles but we have no way of knowing.
That's probably why I couldn't find the answer in a general google search.

RamSteelwood

This is where physics starts getting too clever for it's own good.

"does a tree falling in a forest make a sound if nothing is there to hear it?"
Me - yes
Physics - well....

Lurch666

That's what makes it so interesting. If other people managed to think in a different way and figure out some of this stuff who's to say one of us might figure out something no one else has.

Just found the book that got me interested in this:Quarantine by Greg Egan.
Not the best scifi novel ever but it introduces a nice concept for a quantum 'superpower'.

SpaceGazelle

RamSteelwood wrote:

This is where physics starts getting too clever for it's own good.
"does a tree falling in a forest make a sound if nothing is there to hear it?"
Me - yes
Physics - well....

Physicists don't like QM but what can you do? Nature is how she is, and it turns out she's a mentalist.

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