Block universe theory question

  • #1
frankrabbit
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block universe theory question
Hello,

can somebody help me out please? just watched this video
so far understood but if motion changes the now frame its logic if the alien cylcles to the guy sitting that his time is slower and the guys time
will be in the future (time delitation). but when the alien is moving away how can the motion direction which changes away from the guy sitting can bring the now frame to past. i didnt get that? or is this the minkowksi diagramm? you can only have one time line when moving to an non moving observer or moving away. from a nonmoving observer. directions doesn't count. or is this theory totally wrong? or maybe iam on a totally wrong path..

thanks
frank
 

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  • #2
PeroK
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Here's an alternative view. When a child is developing it has a clear sense of a local sequence of events. But, it struggles with the concept that there is a global "now". I.e. that there is some relationship between what is happening locally and what happens at some distant location. For example, they may be having something to eat in the UK while their grandparents are asleep in Australia. The idea that there is a concept of "now" that relates these two events takes some time for the child to grasp.

As we develop and get to understand global time zones and have telephones and the Internet to make communication across the world almost instantaneous, we lose any doubts about the absolute meaning of a global now and it becomes hard-wired into our thinking.

However, when we consider events at a far greater distances (even without considering relativity), perhaps this might shake our confidence in a global now. If we are thinking about a Mars rover, then we know it takes 20-30 minutes for communications to and from Mars. We can reason that the images from Mars are what happened 20-30 minutes ago and we can construct a more elaborate notion of a global now, based on the speed of light.

Then, we might consider a star or planet in the Andromeda galaxy. It takes 2 million years for information about what is happening "now" to reach us. And, perhaps at this point, we should rediscover some of the doubts we had as young children about what exactly "now" means for events 2 million light years away. And, to understand that what we mean by "now" over those large distances might be ambiguous and depend on the way we define "now".

If you can talk to someone in Australia on the telephone, there is little doubt about what "now" means over such short distances. But, we can't communicate effectively with a civilisation in Andromeda, so it's not immediately clear that there is only one way to define "now" there.

So, perhaps the video you have watched is "conning" you in a way. Playing on the ideas you have learned as an adult - that, ironically, were not innate "common sense" to you as a child!

Perhaps it doesn't matter whether you cycle round on a bicycle or not; or, more significantly, which way the Earth is moving on its orbit of the Sun. None of that has any physical significance to events in the Andromeda galaxy.
 
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  • #3
frankrabbit
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Thank you for the answer. I think I explained my question not clear enough.
 
  • #4
PeroK
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Thank you for the answer. I think I explained my question not clear enough.
Feel free to restate your question. To add a bit more:

Imagine a giant telescope is displaying pictures on a screen of light just arriving at Earth from Andromeda. What is on the screen is physical. Everyone agrees what is on the screen, whether they are on a bicycle or not.

But, if we take the usual convention for defining a global inertial frame in Minkowski spacetime, then two people on Earth in relative motion will define different events to be "now" on Andomeda. A quick calculation gives a difference of about 3.5 days for a relative speed of ##3m/s## (on a bicycle). This means that one will calculate that the images on the screen represent events Andromeda 2 million years ago; and the other 2 million years ago (plus or minus a few days).

But, we see that this is a non-physical convention, as the images on the local screen are the same for both. What they consider to be "now" on Andomeda doesn't change what they are seeing on the screen. There's no disagreement about the local physical observation.
 
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  • #5
frankrabbit
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Feel free to restate your question. To add a bit more:

Imagine a giant telescope is displaying pictures on a screen of light just arriving at Earth from Andromeda. What is on the screen is physical. Everyone agrees what is on the screen, whether they are on a bicycle or not.

But, if we take the usual convention for defining a global inertial frame in Minkowski spacetime, then two people on Earth in relative motion will define different events to be "now" on Andomeda. A quick calculation gives a difference of about 3.5 days for a relative speed of ##3m/s## (on a bicycle). This means that one will calculate that the images on the screen represent events Andromeda 2 million years ago; and the other 2 million years ago (plus or minus a few days).

But, we see that this is a non-physical convention, as the images on the local screen are the same for both. What they consider to be "now" on Andomeda doesn't change what they are seeing on the screen. There's no disagreement about the local physical observation.
so its like looking to the sky and seeing a star from the past. right?
 
  • #6
PeroK
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so its like looking to the sky and seeing a star from the past. right?
That's one point. All light is from past events. The question is more fundamentally what does "now" mean at some distant location? It's not as obvious and common sensical as you might at first think.
 
  • #7
frankrabbit
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That's one point. All light is from past events. The question is more fundamentally what does "now" mean at some distant location? It's not as obvious and common sensical as you might at first think.
the more the alien moves away from the observer the more he sees from the past of the observer because he leaves more distance. but while the alien is in motion his time moves slower so he will have a brighter frame of the observers past.
 
  • #8
PeroK
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the more the alien moves away from the observer the more he sees from the past of the observer because he leaves more distance. but while the alien is in motion his time moves slower so he will have a brighter frame of the observers past.
Sorry, that's not right at all. See my example with the telescope. If you are on your bicycle and your friend is not then you see the same light from the distant stars at the same time. Give or take a few nanoseconds.

You don't see anything different if you cycle around on a bicycle.

Brian Greene is (whatever his intentions) seriously misleading you here.
 
  • #9
PeroK
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Even if your friend came rocketing past you at half the speed of light, as he passes, you would both agree on a date-stamped image sent from a distant planet (with their date it was sent). He would see it blue-shifted if he is moving towards the source that quickly, but there would be no disagreement about the date-stamp. If that message says, when translated:

Greetings from planet X on the 100th anniversary of universal peace on our planet: 16th of Alien-July, Alien Year 2445.

Then you and your friend (if you are close to each other) receive that same message at the same time, regardless of your relative state of motion. That's an element of physical reality, as Einstein would have it.
 
  • #10
frankrabbit
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Sorry, that's not right at all. See my example with the telescope. If you are on your bicycle and your friend is not then you see the same light from the distant stars at the same time. Give or take a few nanoseconds.

You don't see anything different if you cycle around on a bicycle.

Brian Greene is (whatever his intentions) seriously misleading you here.
ok now i'am close to understand it...but the other way around its logical. so when the alien is cycling to the observer he will see a frame of future. this is time dilation if we have the situation Earth - andromeda distance plus motion and nonmotion observer.
 
  • #11
PeroK
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ok now i'am close to understand it...but the other way around its logical. so when the alien is cycling to the observer he will see a frame of future. this is time dilation if we have the situation Earth - andromeda distance plus motion and nonmotion observer.
No, no, no. That's all wrong. Special Relativity and frames of reference are not about what you see in terms of light signals.

The distant event you see at any instant is what you see and is independent of your state of motion. It may be blue or redshifted, but it still the same event.
 
  • #12
frankrabbit
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so block universum theory is totally wrong?
 
  • #13
PeroK
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so block universum theory is totally wrong?
No, your understanding of SR is totally wrong. Sorry!
 
  • #14
frankrabbit
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wait. when i have a atom watch here on Earth which is not moving and i put the same atom watch on a flying plan the watch on the plan is moving slower as the watch on earth. it is because motion slows time because the time is distributing through space when moving. so far right?
 
  • #15
PeroK
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wait. when i have a atom watch here on Earth which is not moving and i put the same atom watch on a flying plan the watch on the plan is moving slower as the watch on earth. it is because motion slows time because the time is distributing through space when moving. so far right?
No. The first postulate of SR effectively says that there is no state of absolute motion. In other words, all motion through space is relative.

In your example, the Earth is rotating, so in some inertial frame a plane on the ground is moving at approximately ##1000km/h##. A plane flying westwards is moving slower in that frame.

Likewise, we can find another frame where the plane is at rest and the ground plane is moving.

So, I would say that "motion slows time" is wrong and fundamentally disagrees with what SR really says.
 
  • #16
PeroK
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wait. when i have a atom watch here on Earth which is not moving and i put the same atom watch on a flying plan the watch on the plan is moving slower as the watch on earth.
In fact, in the Hafele Keating experiment, the clock that flew westwards ended up showing more time that the ground clock. The difference when all clocks are reunited is called differential ageing and is not directly related to time dilation.

Popular science sources often conflate the two for the sake of simplicity, but they are fundamentally different concepts.
 
  • #17
Ibix
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wait. when i have a atom watch here on Earth which is not moving and i put the same atom watch on a flying plan the watch on the plan is moving slower as the watch on earth. it is because motion slows time because the time is distributing through space when moving. so far right?
You keep talking about time dilation (and differential aging in this example). This is not what the video is talking about. It is talking about the fact that "now" is not defined in relativity - it is something we humans add in, and there's actually quite a lot of flexibility in how you do it.

In pre-relativistic physics, the whole of history is divided into "past" and "future". If I have a car accident, there are events that could have caused it, and there are events that could be caused by it. The first set of events are called the past, and the second set are called the future. And there's a curious fact that we can all agree on this - wherever we are, there is one instant in your personal history when what you call the past is exactly the same as what I called the past at the time of my car accident. That instant is simultaneous with my accident - they happen at the same time.

This view of the universe is wrong. On the every day scale you need extremely precise measurements to detect the flaws, which is why we don't usually notice and most people live their whole lives without knowing.

In relativistic physics, no influence can travel faster than the speed of light. So there are events that could have contributed to my crash and events that my crash could influence, but there is also a lot of spacetime that can neither have contributed to my accident (no information from that event could have reached me) nor been affected by my crash (no information from my crash could have reached the event). This concept completely replaces "now". There is no dividing line between past and future - there is this region of events that aren't firmly in the past (they couldn't, even in principle, have influenced me) and aren't firmly in the future (they couldn't, even in principle, be influenced by my crash). People often add a notion of "now" to this by synchronising clocks and saying that "now" is the set of events with a clock that reads 12:44:30 GMT on the 26th January, but it turns out that there is more than one way to synchronise two clocks. Worse, if I have a friend who is stationary with respect to me and we synchronise our clocks by some process, and you have a friend who is stationary with respect to you and you synchronise your clocks by the same process, if you and your friend are moving with respect to me and my friend, we will not agree that you and your friend have synchronised clocks, and vice versa. We do not have the same definition of "now".

That is the point that the video is talking about. Not that the alien's clocks tick slow or fast, but that you and the alien are currently in each other's "region between the definite past and the definite future", and so you are both free to use different definitions of "now". There are no physical consequences from you doing so.

The reason that you don't notice this on the everyday scale is that the speed of light is so fast compared to us. If you and I are a meter apart, that's about the distance light travels in 3ns. So while I have some flexibility in defining now, there's only 6ns difference between the earliest event at your location and the latest event at your location that I could call now - which is way too small a gap to notice any flexibility.
 
  • #18
sophiecentaur
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so its like looking to the sky and seeing a star from the past. right?
Everything you see is also "from the past". It's just a matter of whether the delay is perceptible or not. Our brains have not had to evolve with the ability to cope with the short time delays involved with life more than a few hundred years ago.

Clocks which used to be set to local Noon were far enough 'out' as to upset the early railway timetables so that the trains setting off from Penzance (far west of England) needed to start 'earlier' so as to arrive at Paddington 'on time'.

Jules Verne's "Around the world in eighty days" gives another example of the flexibility of 'Now'.

The speed of sound is 'so' finite that we are very aware of the difference between the 'now' when we see the flash and the 'now' when we hear the gun. On a foggy day, our 'now' is dominated by what we hear.

We have observed delays in our seeing events in space and related that to the finite speed of light. Rømer managed to work out the speed of light from the apparent variations of the timings of orbits of moons of Jupiter by assuming the orbits are regular. That disturbance of 'Now' is not an SR phenomenon, only a classical one.

A bit of serendipity: the teams studying the outputs from the Mars lander missions have been working according to the Martian day so their 'now' (their alternative 24 hour clock) is marching through that of the rest of us and allows them to be working during the martian day so they don't miss anything during our night.
 

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