Space Time

[The Mad Monk]

Not space time, space-time, or even spacetime.

Space Time is series of youtube videos produced by PBS, featuring several astrophysicists (but mostly Matt O'Dowd). It's a really fantastic series that does it's damndest to explain difficult concepts in a fun way.

Have a taste:

[The Mad Monk]

[The Mad Monk]

[The Mad Monk]

...and one that's really out there:

[The Mad Monk]

...and that's it. I've seen a lot of these, and have yet to find a bad one.

[The Mad Monk]

While I called the last one "out there" (and it is), I personally found the first one mind-blowing.

[Lorizael]

I find O'Dowd to be a little dry but I do like that he tackles complex subjects.

[Bereta_Eder]

I found these videos a long time ago, they are very good. Althoughit's difficult to understand everything in english.

For example the difference between particle and event horizon.

Another thing of interest is how the observer can influence events

https://www.youtube.com/watch?v=v6fAcigk3Ys

(not the video I originally found about it)

this brings to my mind the antrhopocentric idea that the universe created man in order for him to recognize/validate the universe's existence. Otherwise it wouldn't "exist"


nevertheless unless such studies lead to immortality or a definite answer as to why there is something instead of nothing I'll keep considering them like the work of elaborate plumbers

[Bereta_Eder]

Another thing I'm curious about is quantum entaglement.

So if you entagle some particles whatever and then you set them apart, they will always spin in opposite directions. No matter how far apart they are. galaxies apart.


So far so good.

I don't get it. Why is there so much commotion about that? saying that they interact faster than the speed of light etc

Apparently the spin (let's call it + or -) of one particle is determined the moment you measure it!!!!

Before you measure it, it doesn't "pre-exists" it is determined the moment you measure it.


At that moment, you also know with absolute certainty that its entagled particle galaxies away is spinning in the opposite direction.


Dong

It doesn't have any practical use though because you can't pass information through this.

[Bereta_Eder]

BTW in english our galaxy is called milky way..

In greek gala is milk and galaxias is galaxy.
We don't have a separate name for our galaxy we just call it galaxy (milky way)

[Lorizael]

Another thing I'm curious about is quantum entaglement.

So if you entagle some particles whatever and then you set them apart, they will always spin in opposite directions. No matter how far apart they are. galaxies apart.


So far so good.

I don't get it. Why is there so much commotion about that? saying that they interact faster than the speed of light etc

Apparently the spin (let's call it + or -) of one particle is determined the moment you measure it!!!!

Before you measure it, it doesn't "pre-exists" it is determined the moment you measure it.


At that moment, you also know with absolute certainty that its entagled particle galaxies away is spinning in the opposite direction.


Dong

It doesn't have any practical use though because you can't pass information through this.

So, I can explain why there is such commotion, why physicists think quantum entanglement implies something spooky, but it is difficult to wrap your head around and involves a little bit of math.

There's another way to look at quantum entanglement (advanced by Einstein) which seems to completely remove the spookiness. Say a street magician has hidden a coin in one of his hands and asks you to guess which hand. If you guess left and the hand is empty (and there's no trick going on), then you know instantly that the coin is in his right hand, and you would know this instantly no matter how far apart the hands actually are. Einstein said this is what's going on with quantum entanglement, too. Nothing weird is happening; the spins are just decided beforehand.

But another physicist, John Stewart Bell, demonstrated a couple decades later that this can't be what's going on with quantum mechanics. What Einstein said is that the spins have some common cause which determines their correlation. What Bell said is that correlations follow pretty strict rules from logic and probability, and that quantum entanglement violates these rules.

Here's an example using a bakery. Say you've got a batch of cookies, and each cookie can be large or not, soft or not, or chocolate chip or not. If you close your eyes and randomly choose one, what are the odds it will have some particular combination of properties? Well, that's obviously directly proportional to the number there are. So let's ask a weird question such as, what are the odds you randomly select either a large, hard (not soft) cookie or a soft, plain (non-chocolate chip) cookie? Maybe we don't know the answer to that question, but logically there's a lower bound on the probability which is the fraction of cookies that are large and plain. Any large, plain cookie falls into one of the two sets above, so the probability of getting a cookie from the set large, hard or soft, plain must be at least as great as the probability of getting a large, plain cookie.

If you have many different bakeries across the world all with the same baking instructions, then the cookie batches in one bakery are correlated with the batches in another by the common cause of those cooking instructions. Testing the probability in one bakery gives you the knowledge of probabilities in another bakery, and this is not spooky in exactly the same way that the street magician example is not spooky.

The problem is that quantum entanglement doesn't work this way. The above example relies on cookies having three, independent, binary properties that describe them. Whenever you can cook up three properties like that, you can play the same logic game and show that there's one set of properties which must show up at least as often as some other set. You can find a set of three such properties for quantum entangled particles (measurement of spin at different angles), but if you repeat the experiment again and again (different bakeries), the rules of quantum mechanics say the results don't conform to basic, screamingly obvious logic and probability. And experiments have borne this out.

So the spins being decided beforehand is not what's going on, and there must be something spooky about quantum mechanics, instead.

[Dauphin]

While I called the last one "out there" (and it is), I personally found the first one mind-blowing.

This was up on my list at university. Before that my big wow moment was realising that photons don't know they exist and my high school physics teacher didn't get what I was saying. From a photons perspective (such that it can have one) they are created and destroyed at the same time, even if they cross the universe in its entirety over tens of billions of years.