Shape of Spacetime: Exploring Relativity & Simultaneity

[NoahsArk]

Thank you for the explanations.

 

[Ebeb]

I've also seen space-time described many times as like a block.

Yes indeed. Einstein himself considered it a Block. Some Einstein quotes will help you:

<< From a "happening" in three-dimensional space, physics becomes, as it were, an "existence" in the four-dimensional "world". >>

<< Since there exists in this four dimensional structure [space-time] no longer any sections which represent "now" objectively, the concepts of happening and becoming are indeed not completely suspended, but yet complicated. It appears therefore more natural to think of physical reality as a four dimensional existence, instead of, as hitherto, the evolution of a three dimensional existence. >>

<<...for us convinced physicists the distinction between past, present, and future is only an illusion, although a persistent one." >>

Karl Popper about his encounter with Einstein:

<< The main topic of our conversation was indeterminism. I tried to persuade him to give up his determinism, which amounted to the view that the world was a four-dimensional Parmenidean block universe in which change was a human illusion, or very nearly so. He agreed that this had been his view, and while discussing it I called him "Parmenides".)... >>

 

[NoahsArk]

I love those Greeks:)

 

[NoahsArk]

Yes, but they would tick at the same rate. All you would see is that the clocks (and all events too) at different places become shifted relative to each other

Suppose there are two clocks in my reference frame which read the same time, and I go to sleep and wake up and measure that one clock is now ahead of the other. Assuming that no one or nothing changed the mechanism of the clocks, then I would have to conclude that I switched reference frames right? If space time switches are possible, then it also seems like we can conclude that all reference frames exist at once. To switch into a new frame, it seems like that frame already has to exist or else there'd be nothing to switch into.

 

[sweet springs]

Suppose there are two clocks in my reference frame which read the same time, and I go to sleep and wake up and measure that one clock is now ahead of the other.

After you go into sleep one of the clocks moved in any way and come back to the same still position before you wake up. Frame of reference of yours and the clock's coincide when you fall in sleep and wake up.

 

[Dale]

it also seems like we can conclude that all reference frames exist at once.

Of course all reference frames exist at once. They are just mathematical abstractions used in the analysis and have no physical consequences. Any and all of them can be used as desired.

 

[Nugatory]

Assuming that no one or nothing changed the mechanism of the clocks, then I would have to conclude that I switched reference frames right? If space time switches are possible, then it also seems like we can conclude that all reference frames exist at once. To switch into a new frame, it seems like that frame already has to exist or else there'd be nothing to switch into.

A reference frame is a convention for assigning time and space coordinates to events, so what you're calling "switching" frames is just choosing to use a different one. If I say that the top of my chimney is 10 meters above the ground, then a moment later I say that it is 130 meters above sea level... that's what switching reference frames is like.

 

[Ibix]

Suppose there are two clocks in my reference frame which read the same time, and I go to sleep and wake up and measure that one clock is now ahead of the other. Assuming that no one or nothing changed the mechanism of the clocks, then I would have to conclude that I switched reference frames right?

No. You might reasonably conclude that you'd undergone some acceleration.

Switching reference frames is (very nearly literally) like rotating a map on Google maps. Nothing changes physically - your description of things changes, that's all. If you've turned round it makes sense to rotate the map to match, but you are not obliged to do this.

Similarly, if you've accelerated it makes sense to use a frame wherein you are at rest, but you are not obliged to do so. Or you can use a frame where you aren't at rest without accelerating. But nothing changes until you accelerate.

Tl;dr - you keep using "switch frames" as jargon for "accelerate". This is like using "rotate a map" as jargon for "turn round".

If space time switches are possible, then it also seems like we can conclude that all reference frames exist at once. To switch into a new frame, it seems like that frame already has to exist or else there'd be nothing to switch into.

It's the easiest interpretation and the one with the least philosophical baggage, to my mind. But it is an interpretation, and not a testable claim.

 

[NoahsArk]

Thank you for the responses. When I accelerate, though, doesn't that automatically mean I switched reference frames?

When I rotate myself on a map or in space, what was on my left before now becomes on my right, etc. But when I do that I'm still seeing the same physical objects. When I switch reference frames in space time, clocks that were simultaneous go out of sync, so I am actually seeing things different physically (different readings on the clocks, different appearances of things due to more time having elapsed, etc. Also, this seems like more than just a situation where I am changing units of measurement because in that scenario I will still be seeing things the same. If there is a way to understand this better geometrically please let me know. I started to try and learn about how to read space time diagrams (I've read them before but they can get confusing for me), so hopefully that will help.

 

[PeterDonis]

When I accelerate, though, doesn't that automatically mean I switched reference frames?

No. You can choose whatever reference frame you want.

When you get in your car and go to the grocery store, you accelerate. Do you change your reference frame? If you're like me and most people, you think of yourself as moving and the Earth as fixed; you go to the grocery store, you don't cause the grocery store to come to you by pushing the gas pedal of your car. So you use an Earth-fixed reference frame even though you are not at rest in this frame.

 

[PeterDonis]

When I switch reference frames in space time, clocks that were simultaneous go out of sync, so I am actually seeing things different physically

No, you're not. Switching reference frames doesn't change anything you actually observe or measure. It only changes things you calculate from your observations. You don't directly observe what a clock far away from you is reading "right now". You only observe what you see in light signals from that distant clock; and what is contained in the image of the distant clock that you receive at a particular time by your clock is the same no matter what reference frame you adopt. The only thing that changes if you change reference frames is what coordinate time, according to the reference frame, you assign to that distant event after doing a calculation.

In other words, it's best to think of reference frames as "bookkeeping" devices, allowing you to keep track of events by assigning abstract coordinates to them, not as things you actually observe or which affect things you actually observe.

 

[Dale]

When I accelerate, though, doesn't that automatically mean I switched reference frames?

No. Switching reference frames is something you do in your analysis, not physically. You are free to switch reference frames or not after acceleration, as you please

 

[Nugatory]

When I accelerate, though, doesn't that automatically mean I switched reference frames?

No. You only switch reference frames when you decide to stop using one reference frame for your calculations and start using another.

We often choose to do our calculations using a reference frame in which we are at rest, but there's no reason why we have to do it that way and we sometimes don't. For example: if we're talking about the solar system, we generally choose to use a reference frame in which the sun is at rest and we (standing on the rotating and orbiting earth) are moving and accelerating. We choose to keep on using this reference frame instead of switching even though (actually "because" not "even though" - not switching simplifies the calculations) our velocity is constantly changing.

Other times we switch reference frames without noticing that we have. For example, because of the Earth's rotation the reference frame in which you are at rest at noon is different than the reference frame in which you are at rest at midnight, and chances are you choose to use one at noon and the other at midnight without thinking about it (unless you're navigating a spaceship or calculating satellite orbits). But that's "automatic" only in the sense that you weren't thinking about it, and there's no reason why you have to use those particular reference frames at noon or midnight or any other time.

 

[NoahsArk]

Maybe a better way for me to have phrased it, instead of saying that "I'm switching reference frames", would be to say that, once I start moving relative to things that were previously at rest in my reference frame, those things are now in a different reference frame. If I'm in an air plane that's on the run way waiting to take off, I am in the same reference frame as the ground, but when I start flying, the ground and people on it are now in a different reference frame. Everything on the plane moving with me is in the same reference frame still. Is that a better way to phrase it?

Thank you for the responses and Happy Thanksgiving.

 

[PeterDonis]

Maybe a better way for me to have phrased it, instead of saying that "I'm switching reference frames", would be to say that, once I start moving relative to things that were previously at rest in my reference frame, those things are now in a different reference frame.

No. Everything is always "in" every reference frame; reference frames are just different descriptions of the same events.

 

[Mister T]

Thank you for the responses. When I accelerate, though, doesn't that automatically mean I switched reference frames?

You are using "reference frame" to mean "reference frame in which I'm at rest". We usually just call this the rest frame.

When I switch reference frames in space time, clocks that were simultaneous go out of sync, so I am actually seeing things different physically (different readings on the clocks, different appearances of things due to more time having elapsed, etc.

When you utter a phrase such as "a clock located over there reads such-and-such right now" you have not made a statement about anything physical at all.

The time that elapses on any clock is a relativistic invariant, meaning it will be the same for all observers.