Relativity of Simultaneity

[@PretzelBS]

I love how Mary sends the signal when she’s 24 in both reference frames and botan receives the signal at 28 in both reference frames

[@ShopperPlug]

Excellent explanation. Relativity of Simultaneity is just wild in physics and wonder how quantum entanglement communication would result about it.

[@rheiagreenland4714]

This is truly a bit of a magic moment. Realizing that all of this relativity stuff, something that at times seems so bizarre and unintuitive and paradoxical, makes such perfect, splendid sense with nothing more but the fundamental speed limit of the universe, c, is brilliant. Great work!

[@PrairieKass]

this explanation was so good that it is the first one to actually make me understand this concept.

[@slawless9665]

So when we say "some of the stars we see from Earth have actually already exploded long ago," is that an attempt to apply simultaneity to events in different positions? Would it make just as much sense to say "a star that I am watching explode right now IS exploding right now in my reference frame" - even though I know the light from it must have traveled several lightyears before reaching me?

[@Kuvina]

Good question! Simultaneity is not usually defined that way, and if we know the light has traveled x lightyears to get to us, then we would say the supernova happened x years ago in our reference frame. So basically we retroactively designate the time at which the event happened even though we didn't know about it at the time. If you say x years ago was when you turned 15, then the events of you turning 15 and the supernova are simultaneous in your reference frame, but not necessarily in a different one.

[@PeterNerlich]

Hmmm... I see. But what if the thing we send isn't a person, but light itself? Imagine a mirror 1 light year out, floating in space, reflecting the light back. There is still a little exchange of force somehow, but the light comes back at c basically without acceleration... right...? So is it still an inertial reference frame or not because it changes direction? Does it matter?

[@Kuvina]

Thank you for the question! So to address the main point, even if you could change direction instantaneously, with seemingly no acceleration, it still would not be an inertial frame. It's not the acceleration per se that makes a frame non inertial, but rather the fact that its velocity (including speed and direction) differs. Also, when light is reflected, what's actually happening is that the photons are being absorbed by atoms (increasing their electrons' energy) and then another photon is emitted. So photons themselves can't actually change direction like that. Another issue arises due to the fact that our formulas kindov break down at light speed. Using the time dilation equation with something going at exactly speed c would give us tm = 0*ts, which basically means that photons don't age at all. So for that reason, it doesn't really make sense to consider it a reference frame at all, because time doesn't pass, so nothing else really works. This will of course be covered in a lot more detail in the twin paradox episode, but a good way to think about for now is that acceleration causes the reverse effect to regular time dilation, meaning you see others aging more quickly than you. The faster this acceleration is, then the more dramatic the effect will be. So the traveling twin will indeed see the other one aging slowly for both the outbound and return trip, but during the turnaround (acceleration), they will see the other one aging a lot more quickly, which is more than enough to make up for both the outbound and return trip, and so the moving twin will ultimately see that the earth twin is older, which matches the earth twin's own observations. Eventually, in the limiting case where you turn around instantaneously, it's basically equivalent to switching to a reference frame in which the other observers are already older.

[@DrDeuteron]

suppose it's a photon with energy E. That has momentum p = E/c. Now it reflects to have momentum -p, for a total change of dp = -p - p = 2E/c. How long does it take to reflect? Maybe 2 periods dt = 2T = 2/f = 2h/E. (h is Planck's constant), so the force is: dp/dt = (2E/c)/(2h/E) = E^2 / (hc)

[@thalianero1071]

Of course you could also throw away the concept of inertial frames altogether and break literally everything

[@Nolrai12]

Mary and Botan, hmm. Hmm. I wonder where those names could possibly come from. You should use Yukari if you ever get to GR.

[@Kuvina]

Mary and Botan were both characters that I had made up. I named Mary simply because I like the name, and I named Botan after a brand of rice.

[@dreamwork69]

i just happened to find ur channel & my God this enlightened me ❤ thank u so much!!!

[@FishSticker]

The whole video i was just screaming "WHAT IF SHE FLIES BACK"

[@MichaelDarrow-tr1mn]

this is called the twin paradox

[@Flemmi]

I got an idea to get around the acceleration of our mover Mary, what if she travels through an area of curved space that leads her without acceleration back to earth?

[@hedgehog3180]

So think of an example of curved space time. You'll probably think of a gravity well, and that is the only way to curve space time. Basically Mary would have to orbit in the same gravity well as Botan but somehow travel 0,6c faster. This is obviously impossible the additional speed would move her to a higher orbit and in order to return she's still have to decelerate. Plus you have to account for the time dialating effects of a curved spacetime itself, Mary would experience time pass slower because she is higher in the gravity well.

[@Alice-iv3or]

So cool! Love your channel!

[@techconbd7661]

Love from Bangladesh 🇧🇩🖤🖤

[@vaclavtrpisovsky]

Love from a spaceship heading towards you at 0.6 _c_ (you better move, I can’t possibly stop in time)

[@elskx443]

@@vaclavtrpisovskynow I see your reply

[@walter_mayer]

The explanation of Twin Paradox is a misleading one. Do the Twin Paradox with two star ships (one flying to the right and the other to the left) then you cannot argue with inertial and non inertial reference frame. It is no paradox - it is reality, that both see the other younger. Its the same with distance. If two are looking from far distance to the other, both see the other smaller. Is this a paradox - NO!

[@dreamwork69]

he said there are more to come before the twin paradox episode, so we don't judge before that. Please wait and be patient.

[@walter_mayer]

@@dreamwork69He shows a complicated explanation. Make it easy. Both have equal age. Before Mary starts, they come to the commitments: a) When Mary is 24 in her reference frame she will Mary will stop flying away. b) When each of them is 24 in her/his own reference frame, they will send out there birthday date. And then everything is clear symmetrical. And both will get the same time difference, when the signal arrive.