How does faster than light information break causality?

[mistermack]

Does anyone have a good illustration, explanation, or link, for how information travelling faster than light breaks causality? 

I've searched and made an effort to follow various online offerings in the past, and maybe it's me, but never found one that I can go all the way with. It may be beyond my physics grade, which would be a shame, so I thought I would ask is someone could provide or link something that you don't have to be immersed in physics to follow.

[dimreepr]

10 minutes ago, mistermack said:

Does anyone have a good illustration, explanation, or link, for how information travelling faster than light breaks causality? 

I've searched and made an effort to follow various online offerings in the past, and maybe it's me, but never found one that I can go all the way with. It may be beyond my physics grade, which would be a shame, so I thought I would ask is someone could provide or link something that you don't have to be immersed in physics to follow.

https://en.wikipedia.org/wiki/Faster-than-light

http://www.bbc.co.uk/earth/story/20160429-the-real-reasons-nothing-can-ever-go-faster-than-light

https://cosmosmagazine.com/physics/why-can-t-anything-travel-faster-light

 

https://www.popularmechanics.com/science/a25800/impossible-physics-of-faster-than-light-travel/

[mistermack]

Thanks, but those don't really do it. 

I don't want the thread to get sidetracked discussing why matter or energy can't exceed c. I have no problem with that at all.  I'm just interested in the claim that causality is violated, if information can be transferred faster than light.

[MigL]

Information is any property of a system, say momentum, for example.
If I can transfer the momentum of photons to a solar sail faster than the photons can get to the sail, then it will move without cause, so breaking causality.
Is that simple enough ?

[mistermack]

9 minutes ago, MigL said:

Information is any property of a system, say momentum, for example.
If I can transfer the momentum of photons to a solar sail faster than the photons can get to the sail, then it will move without cause, so breaking causality.
Is that simple enough ?

That sounds like you are transferring energy faster than light. 

[MigL]

Energy is also a property and information.

So are the winning lottery numbers.
And if you know them before they're actually drawn, you are breaking causality.
( time travel also implies causality breaks, But GR, being strictly classical, does not forbid it )

[J.C.MacSwell]

It is all to do with simultaneity and past present and future in different frames.

If you took information and sent it faster than the speed of light to a distant point in your reference frame, this would be backwards in time wrt some other frames. It could then be relayed to you at a point in your past by doing this in other frames.

[dimreepr]

46 minutes ago, mistermack said:

Thanks, but those don't really do it. 

I don't want the thread to get sidetracked discussing why matter or energy can't exceed c. I have no problem with that at all.  I'm just interested in the claim that causality is violated, if information can be transferred faster than light.

Try this:

https://www.technologyreview.com/s/427174/einsteins-spooky-action-at-a-distance-paradox-older-than-thought/

[geordief]

Causality seems to me far more fundamental than any speed.Once you accept a universal  principle of causality (so breaking it is not a consideration)  does everything not just get into line?

 

From causality being fundamental  we must have a maximum speed of transfer of information  and from maximum transfer of information we have a maximum speed of information carriers(objects with or without mass)

 

It just so happens that c is the fastest speed clocked  and so it is probably also  the fastest speed possible.

 

Perhaps I am being simplistic?

 

It is not the speed of light that has a bearing on causality but the other way round.

[mistermack]

20 hours ago, J.C.MacSwell said:

It is all to do with simultaneity and past present and future in different frames.

If you took information and sent it faster than the speed of light to a distant point in your reference frame, this would be backwards in time wrt some other frames. It could then be relayed to you at a point in your past by doing this in other frames.

This is the sort of thing I've seen claimed, but I've never seen it properly illustrated or gone through step by step. The ones I've looked at have simply claimed that it can be shown that an observer in another frame can be shown to have OBSERVED events to happen in the wrong order, breaking causality. Which is worth looking at, but it's not the same as breaking causality. I can observe causality being broken, if I press rewind on a video. But it never actually happened.

[swansont]

22 minutes ago, mistermack said:

This is the sort of thing I've seen claimed, but I've never seen it properly illustrated or gone through step by step. The ones I've looked at have simply claimed that it can be shown that an observer in another frame can be shown to have OBSERVED events to happen in the wrong order, breaking causality. Which is worth looking at, but it's not the same as breaking causality. I can observe causality being broken, if I press rewind on a video. But it never actually happened.

No, that's not causality being broken.

But consider that scenario, where you see the effect before the cause. Let's say the cause was pushing a button, and the effect was something bowing up some distance away (so there is a signal delay) You could then send a signal to the button-pusher to tell them not to push the button, and they would get it before they pushed the button, and decide not to do it (or to someone else, who cuts the wire; in any case, the bomb would not explode)

Now you have the situation of having observed the explosion cause by the button being pushed, but the button was not pushed.

The actual scenario of causality violation involves two (or more) reference frames, and sending signals between them

http://www.askamathematician.com/2012/07/q-how-does-instantaneous-communication-violate-causality/

[J.C.MacSwell]

16 minutes ago, mistermack said:

This is the sort of thing I've seen claimed, but I've never seen it properly illustrated or gone through step by step. The ones I've looked at have simply claimed that it can be shown that an observer in another frame can be shown to have OBSERVED events to happen in the wrong order, breaking causality. Which is worth looking at, but it's not the same as breaking causality. I can observe causality being broken, if I press rewind on a video. But it never actually happened.

Here are examples

https://en.wikipedia.org/wiki/Tachyonic_antitelephone

http://dumbscientist.com/archives/any-ftl-signal-can-be-sent-back-in-time

[mistermack]

2 minutes ago, swansont said:

No, that's not causality being broken.

But consider that scenario, where you see the effect before the cause. Let's say the cause was pushing a button, and the effect was something bowing up some distance away (so there is a signal delay) You could then send a signal to the button-pusher to tell them not to push the button, and they would get it before they pushed the button, and decide not to do it (or to someone else, who cuts the wire; in any case, the bomb would not explode)

Now you have the situation of having observed the explosion cause by the button being pushed, but the button was not pushed.

 

That doesn't sound right at all. I don't see how you have established that you see the explosion in that case.

[dimreepr]

1 minute ago, mistermack said:

That doesn't sound right at all. I don't see how you have established that you see the explosion in that case.

Oh please, how simple do you want it?

If I punch you, you won't feel it till my fist gets there.

 

[mistermack]

3 minutes ago, dimreepr said:

Oh please, how simple do you want it?

If I punch you, you won't feel it till my fist gets there.

 

That doesn't really add anything. I see how you press, and see the explosion. I see how you cancel, and don't see the explosion. What I don't see established, is the case where you cancel and still see the explosion.

[dimreepr]

Just now, mistermack said:

That doesn't really add anything. I see how you press, and see the explosion. I see how you cancel, and don't see the explosion. What I don't see established, is the case where you cancel and still see the explosion.

Because it already happened, so you can't cancel. 

 

[mistermack]

1 minute ago, dimreepr said:

Because it already happened, so you can't cancel. 

 

Nope, that adds nothing either. You would need to be a lot more specific to establish anything. That's just hand waving.

[dimreepr]

 If time-travel is possible (and it might be), it wouldn't be possible to change the past because it's a closed loop, therefore, if you could time-travel then whatever action you take to avoid my punch only ensures it's connection.

IOW you always push the button and see the explosion.

[mistermack]

3 minutes ago, dimreepr said:

 If time-travel is possible (and it might be), it wouldn't be possible to change the past because it's a closed loop, therefore, if you could time-travel then whatever action you take to avoid my punch only ensures it's connection.

IOW you always push the button and see the explosion.

Well, that's just stating the obvious. The question was did Swansont's post establish the mechanics of how causality would be broken in his example. 

[dimreepr]

1 minute ago, mistermack said:

Well, that's just stating the obvious.

I thought so, it also renders the question moot:

22 hours ago, mistermack said:

Does anyone have a good illustration, explanation, or link, for how information travelling faster than light breaks causality? 

But if you still think it's a valid question, then... No, because it's impossible.

 

[Silvestru]

Was reading the links from J.C.MacSwell and this could also add to the topic if we are discussing closed timeline curves.

https://en.wikipedia.org/wiki/Novikov_self-consistency_principle

[StringJunky]

24 minutes ago, mistermack said:

Well, that's just stating the obvious. The question was did Swansont's post establish the mechanics of how causality would be broken in his example. 

There are frames in which B happens before A, when A should precede B. If that's not breaking causality then I don't know what is.

[Silvestru]

It's like if I stopped your parents from going to the cabbage patch, what would have caused your existence?

[mistermack]

There's a telling paragraph at the end of the article that Swansont has added the link to, and it goes like this :

"

There is a “cure” for faster-than-light communication causing causality violations.  There isn’t really a problem with signals going back in time, if they only go back in time somewhere else.  For example,imagine there was a magic post office in the year 1500 that sent letters from Rome (Rome) to Tenochtitlan (Mexico City) and one week back in time.  Since it took 5 weeks to cross the Atlantic, there’s no risk of paradoxes and causality violations (“Dear Ahuitzotl, in a week Giovanni Borgia is going to be killed.  Nothing you can do about it, just thought you’d like to know.”).  The real problems crop up when you can send instantaneous messages in two or more reference frames.  That allows you to bounce signals back and forth, and thus send a message to yourself in the past.  So, the fix is to have only one frame with instant communication (magic post offices only send letters in one direction).

But this cure; picking a special reference frame (a special speed) in which communication can be instantaneous, isn’t really in keeping with the spirit of relativity or observational evidence; that all speeds are equivalent."

 

Since relativity is BUILT on the assumption that no information can travel faster than light, it's hardly surprising that you can use the principles of relativity to argue for a false result. It's a circular argument. Relativity is right in all frames. Nothing can move faster than light in relativity. If something moves faster than light, you can use relativity to show a break in causality. 

In fact, if something COULD be transmitted faster than light, then relativity is clearly at fault on that subject. Definitely circular.

[Silvestru]

4 minutes ago, mistermack said:

There's a telling paragraph at the end of the article that Swansont has added the link to, and it goes like this :

"

There is a “cure” for faster-than-light communication causing causality violations.  There isn’t really a problem with signals going back in time, if they only go back in time somewhere else.  For example,imagine there was a magic post office in the year 1500 that sent letters from Rome (Rome) to Tenochtitlan (Mexico City) and one week back in time.  Since it took 5 weeks to cross the Atlantic, there’s no risk of paradoxes and causality violations (“Dear Ahuitzotl, in a week Giovanni Borgia is going to be killed.  Nothing you can do about it, just thought you’d like to know.”).  The real problems crop up when you can send instantaneous messages in two or more reference frames.  That allows you to bounce signals back and forth, and thus send a message to yourself in the past.  So, the fix is to have only one frame with instant communication (magic post offices only send letters in one direction).

But this cure; picking a special reference frame (a special speed) in which communication can be instantaneous, isn’t really in keeping with the spirit of relativity or observational evidence; that all speeds are equivalent."

 

Since relativity is BUILT on the assumption that no information can travel faster than light, it's hardly surprising that you can use the principles of relativity to argue for a false result. It's a circular argument. Relativity is right in all frames. Nothing can move faster than light in relativity. If something moves faster than light, you can use relativity to show a break in causality. 

In fact, if something COULD be transmitted faster than light, then relativity is clearly at fault on that subject. Definitely circular.

I don't understand what value this brings to your side of the argument.