Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Tangled Up in Spacetime

scientificamerican.com

Post reply on HN
Print view View original

21–27 of 27 posts

Re: Tangled Up in Spacetime

#21

Post by anindha »

soVeryTired wrote: post #18
anindha wrote: post #8

Earlier quoted context omitted.

Space and time are very macro concepts according to general relativity. It doesn't actually explain how time elapses differently on one atom vs. another atom only that it does. I am traveling close to the speed to light how is my watching running slower? How do the atoms in my watch know to run slower? I am not suggesting that current quantum mechanics is more than a model, but I think we are along the right path to…

>I am traveling close to the speed to light how is my watching running slower? How do the atoms in my watch know to run slower? I think in some sense this is the wrong question to ask. Your watch isn't running slower. The point is that if you're moving, you must be moving relative to something else. Their watch will appear slower to you, and your watch will appear slower to them. The counterintuitive part is that you…

Yes, I should have been clearer. Running slower compared to a stationary observer.

Your last point doesn't explain why or how time dilation occurs, just that we know that the speed of light is a constant to all reference frames.

There is a famous experiment with three synchronized atomic clocks. Two were flown in opposite directions around the globe the other stationary. The eastward flown clock lost 59ns and the westward flown clock gained 273ns, both relative to the stationary clock. The reason is that the clock flown eastward is travelling faster since it is in the direction of the earth's rotation. Measurements are consistent with the theory of relativity.

https://en.wikipedia.org/wiki/Hafele%E2%80%93Keating_experim...

Re: Tangled Up in Spacetime

#22

Post by pavel_lishin »

soVeryTired wrote: post #18
anindha wrote: post #8

Earlier quoted context omitted.

Space and time are very macro concepts according to general relativity. It doesn't actually explain how time elapses differently on one atom vs. another atom only that it does. I am traveling close to the speed to light how is my watching running slower? How do the atoms in my watch know to run slower? I am not suggesting that current quantum mechanics is more than a model, but I think we are along the right path to…

>I am traveling close to the speed to light how is my watching running slower? How do the atoms in my watch know to run slower? I think in some sense this is the wrong question to ask. Your watch isn't running slower. The point is that if you're moving, you must be moving relative to something else. Their watch will appear slower to you, and your watch will appear slower to them. The counterintuitive part is that you…

Isn't part of it the spacetime geometry, specifically your path through it?

It makes absolute sense for my gas tank to be lower if I take a long roundabout path from A to B while you take a short one, even though our cars have no idea about what the other one is doing.

Re: Tangled Up in Spacetime

#23

Post by johncolanduoni »

pavel_lishin wrote: post #22
soVeryTired wrote: post #18

Earlier quoted context omitted.

>I am traveling close to the speed to light how is my watching running slower? How do the atoms in my watch know to run slower? I think in some sense this is the wrong question to ask. Your watch isn't running slower. The point is that if you're moving, you must be moving relative to something else. Their watch will appear slower to you, and your watch will appear slower to them. The counterintuitive part is that you…

Isn't part of it the spacetime geometry, specifically your path through it? It makes absolute sense for my gas tank to be lower if I take a long roundabout path from A to B while you take a short one, even though our cars have no idea about what the other one is doing.

Yes, general relativity is all about the "metric tensor" of space, which is just a fancy way of saying it's about assigning distances to paths. For paths you can actually physically take (i.e. movement slower than the speed of light relative to local space) this ends up corresponding to the time experienced along that path.

Re: Tangled Up in Spacetime

#24

Post by empath75 »

ianai wrote: post #4

I thought quantized spacetime had already been disproven below a testable range? I.e. Any theorized particles wouldn't be seen by particle accelerators smaller than say a solar system or drastically larger.

Sort of depends what the theory predicts, as to whether that's relevant.

Re: Tangled Up in Spacetime

#25

Post by empath75 »

jsprogrammer wrote: post #20
ianai wrote: post #4

I thought quantized spacetime had already been disproven below a testable range? I.e. Any theorized particles wouldn't be seen by particle accelerators smaller than say a solar system or drastically larger.

The end of the article states that everything it discussed prior is only part of a "toy". It is known that the toy being studied does not fully correspond to our universe.

Think of coming up with one of these theories like writing a complicated computer program. Sometimes the whole task is too big to even think about, it you sort of understand one part of it, so you work on getting that part sorted out as best you can, and hope you can somehow figure out the rest of the program later, and it'll be useful.

Re: Tangled Up in Spacetime

#26

Post by hyperliner »

"In the AdS/CFT correspondence Maldacena showed that one can completely describe a black hole purely by describing what happens on its surface. In other words, the physics of the inside—the 3-D “bulk”—corresponds perfectly to the physics of the outside—the 2-D “boundary.”"

Very interesting!

Re: Tangled Up in Spacetime

#27

Post by soVeryTired »

anindha wrote: post #21
soVeryTired wrote: post #18

Earlier quoted context omitted.

>I am traveling close to the speed to light how is my watching running slower? How do the atoms in my watch know to run slower? I think in some sense this is the wrong question to ask. Your watch isn't running slower. The point is that if you're moving, you must be moving relative to something else. Their watch will appear slower to you, and your watch will appear slower to them. The counterintuitive part is that you…

Yes, I should have been clearer. Running slower compared to a stationary observer. Your last point doesn't explain why or how time dilation occurs, just that we know that the speed of light is a constant to all reference frames. There is a famous experiment with three synchronized atomic clocks. Two were flown in opposite directions around the globe the other stationary. The eastward flown clock lost 59ns and the wes…

> Your last point doesn't explain why or how time dilation occurs, just that we know that the speed of light is a constant to all reference frames.

But that's what the light clock experiment [1] is all about. The two observers see the light moving at the same speed, but one observer thinks the light takes a longer path than the other observer does. If both observations are equally valid, the inevitable conclusion is time dilation.

[1] http://www.emc2-explained.info/The-Light-Clock/#.WBxO8vnLdPA

Post reply on HN