How does faster than light information break causality?

[dimreepr]

7 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.

Still digging? "How low can you go... BASE" 

Edited June 15, 2018 by dimreepr

[mistermack]

1 hour ago, Silvestru said:

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

I didn't start this thread as an argument. I asked for good clear reasons why causality is contravened, if faster than light information can be sent. I'd be perfectly happy for that to happen. I'm just responding to the offerings so far posted. 

[J.C.MacSwell]

28 minutes ago, mistermack said:

I didn't start this thread as an argument. I asked for good clear reasons why causality is contravened, if faster than light information can be sent. I'd be perfectly happy for that to happen. I'm just responding to the offerings so far posted. 

So...you now have your answer?

[swansont]

3 hours 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.

It's a given in your problem. Given by you. It's the event.

2 hours 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."

That's a solution for science fiction writers, not for discussing the scenario you have offered.

Quote

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.

No, it's not built on that. It's a conclusion of it. Causality being real is the foundation, since we have independent confirmation that relativity is an accurate description of nature.

As some people put it:
Relativity
Causality
FTL

You get to pick two.

[mistermack]

2 hours ago, swansont said:

That's a solution for science fiction writers, not for discussing the scenario you have offered.

It's from your own link.

3 hours ago, J.C.MacSwell said:

So...you now have your answer?

No, all I have so far is assertions. 

[StringJunky]

52 minutes ago, mistermack said:

It's from your own link.

No, all I have so far is assertions. 

I'm guessing, having no expertise myself, that this is the point where the maths needs to come in. There's only so far one can get with a verbal explanation. I'm sure that if someone laid it out mathematically and you understood it that way, all would be clearer. I suspect swansont is dumbing it down to make it palatable for you without using maths.

"Things should be explained as simple as possible but no simpler" - Einstein..

Edited June 15, 2018 by StringJunky

[J.C.MacSwell]

5 hours ago, mistermack said:

 

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.

Theres no circular reasoning. Relativity is assumed to be correct. There is no attempt to prove it. You asked how causality could be violated by FTL information travel.

[mistermack]

Ok, try looking at it from the reverse angle. 

Imagine that alongside the matter and energy that we are familiar with, there was today discovered a previously unknown form of matter and energy. It behaves in just the same way as all of the stuff we know at present. The only difference is that the speed of the "new" light is  half of c. 

All of relativity applies to this new stuff in exactly the same way. The only difference is the c/2 figure. There's absolutely no reason to suppose that an equivalent of SR and GR would not apply equally to this new material. And the beings made of this stuff would be claiming exactly the same thing. That no information can travel faster than C/2. And they would argue that if any information COULD be passed at a speed of c, that would break the causation rule, and replies could be received from messages that were never sent.

We happen to know that messages CAN be transmitted at c, so where did they go wrong?

[J.C.MacSwell]

11 minutes ago, mistermack said:

Ok, try looking at it from the reverse angle. 

Imagine that alongside the matter and energy that we are familiar with, there was today discovered a previously unknown form of matter and energy. It behaves in just the same way as all of the stuff we know at present. The only difference is that the speed of the "new" light is  half of c. 

All of relativity applies to this new stuff in exactly the same way. The only difference is the c/2 figure. There's absolutely no reason to suppose that an equivalent of SR and GR would not apply equally to this new material. And the beings made of this stuff would be claiming exactly the same thing. That no information can travel faster than C/2. And they would argue that if any information COULD be passed at a speed of c, that would break the causation rule, and replies could be received from messages that were never sent.

We happen to know that messages CAN be transmitted at c, so where did they go wrong?

 

So the new light would measure as c/2 in all frames? That would be mathematically inconsistent with our frames. They would have to live in a different universe.

Edited June 15, 2018 by J.C.MacSwell

[swansont]

2 hours ago, mistermack said:

It's from your own link

Yes, it is. It remains a solution for science fiction writers. I was clarifying this for your benefit.

20 minutes ago, mistermack said:

Ok, try looking at it from the reverse angle. 

Imagine that alongside the matter and energy that we are familiar with, there was today discovered a previously unknown form of matter and energy. It behaves in just the same way as all of the stuff we know at present. The only difference is that the speed of the "new" light is  half of c. 

All of relativity applies to this new stuff in exactly the same way. The only difference is the c/2 figure. There's absolutely no reason to suppose that an equivalent of SR and GR would not apply equally to this new material. And the beings made of this stuff would be claiming exactly the same thing. That no information can travel faster than C/2. And they would argue that if any information COULD be passed at a speed of c, that would break the causation rule, and replies could be received from messages that were never sent.

We happen to know that messages CAN be transmitted at c, so where did they go wrong?

There is no physics construct that has anything moving at c/2 in all frames, or that limits information to that speed. You might as well be postulating perpetual motion for all the good it will do. Once you assert violations of physical law, you can pretty much conclude whatever you want.

[mistermack]

15 minutes ago, J.C.MacSwell said:

So the new light would measure as c/2 in all frames? That would be mathematically inconsistent with our frames. They would have to live in a different universe.

I don't see why. It would require an unknown type of space time to exist alongside our own. 

[J.C.MacSwell]

2 minutes ago, mistermack said:

I don't see why. It would require an unknown type of space time to exist alongside our own. 

It would have to have it's own set of frames. Ours can't measure something something as c/2 in all frames.

[mistermack]

24 minutes ago, J.C.MacSwell said:

It would have to have it's own set of frames. Ours can't measure something something as c/2 in all frames.

No, that's not right. The frames are just imaginary constructs and would work equally well for both sets of materials. All it would need for c/2 to be measured in all frames is for an equivalent symmetry to exist in the new material.

[J.C.MacSwell]

6 minutes ago, mistermack said:

No, that's not right. The frames are just imaginary constructs and would work equally well for both sets of materials. All it would need for c/2 to be measured in all frames is for an equivalent symmetry to exist in the new material.

 

 

[swansont]

57 minutes ago, mistermack said:

No, that's not right. The frames are just imaginary constructs and would work equally well for both sets of materials. All it would need for c/2 to be measured in all frames is for an equivalent symmetry to exist in the new material.

Materials? A vacuum is not a material.

No such symmetry exists. If you propose one, you have to show it can coexist with relativistic symmetry. You can't just wave your hands and say it exists.

[mistermack]

1 hour ago, swansont said:

Materials? A vacuum is not a material.

No such symmetry exists. If you propose one, you have to show it can coexist with relativistic symmetry. You can't just wave your hands and say it exists.

I'm talking about an unknown form of matter and energy, similar in all regards, except that the speed of light is c/2.

I'm not saying it exists. I'm saying "what if?"  

If there was such a world, the argument would be made that no information can ever be passed faster than c/2, and causality would be violated if it was. It's exactly the same situation that we have here.

Ok, here's another what-if.

Imagine that there are blind creatures living deep in the ocean. There is no light available to them, and no land to get a fix on. 

All they have is the speed of sound in water, and their clocks operate using sound waves. As they move through the water, their clocks slow, (because of the extra distance the waves have to travel) and if they could reach the speed of sound, their clocks would stop altogether. You basically have relativity under water. Anyone, in any frame, will measure the same number for the speed of sound, for the same reasons that we get the same number for the speed of light in any frame. 

If particles of matter were composed of sound waves, then no matter could exceed the speed of sound, by being accelerated using similar matter. 

And in the same way, these hypothetical water beings, using relativity, would conclude that information could never be transferred faster than the speed of sound, and if it was, causality would be violated. 

You could then come along with your torch and prove them wrong.

[Janus]

26 minutes ago, mistermack said:

I'm talking about an unknown form of matter and energy, similar in all regards, except that the speed of light is c/2.

I'm not saying it exists. I'm saying "what if?"  

If there was such a world, the argument would be made that no information can ever be passed faster than c/2, and causality would be violated if it was. It's exactly the same situation that we have here.

Ok, here's another what-if.

Imagine that there are blind creatures living deep in the ocean. There is no light available to them, and no land to get a fix on. 

All they have is the speed of sound in water, and their clocks operate using sound waves. As they move through the water, their clocks slow, (because of the extra distance the waves have to travel) and if they could reach the speed of sound, their clocks would stop altogether. You basically have relativity under water. Anyone, in any frame, will measure the same number for the speed of sound, for the same reasons that we get the same number for the speed of light in any frame. 

If particles of matter were composed of sound waves, then no matter could exceed the speed of sound, by being accelerated using similar matter. 

And in the same way, these hypothetical water beings, using relativity, would conclude that information could never be transferred faster than the speed of sound, and if it was, causality would be violated. 

You could then come along with your torch and prove them wrong.

All your arguments here just display a fundamental lack of understanding about Relativity.

Regarding your first argument, if light at c/2, then its speed would not be invariant.  But this would not change the fact that c is the invariant speed of the universe.  In such a case, there would not be any restriction on information traveling faster than light, nor would this violate causality. Causality would only be violated by speeds in excess of c, or twice the speed of light.

You must give up on this idea that it is light that is responsible for Relativistic effects. The fact that light travels at c is a consequence of Relativity not its cause.

Your second argument is equally misguided.   Such creatures would not conclude that their clocks run significantly slower as they neared the speed of sound in water, nor would they arrive at the conclusion that it was theoretically impossible to exceed the speed of sound.  They would however, if they had accurate enough clocks, note that clocks rate measurements were effected by relative motion and that this becomes more and more noticeable as the relative speed approaches c.  They would also come to the conclusion that c is a limiting speed for the universe. They would not need any knowledge of the existence of light or of anything that traveled exactly at c. They would conclude that anything that had zero rest mass would be required to travel exactly at c, but they wouldn't have to actually directly measure the existence of such a thing.

[pzkpfw]

mistermack, you are missing half of the point of the speed of light: it's not just a limit, it's invariable. It's constant.

You keep wanting to treat it as merely a speed limit that things get compared to, but it's more than that.

Every observer must measure the speed of light to be c - no matter what they are doing. If you and I are travelling towards a light source, even though we travel at different speeds towards it (which we can tell by a growing or shrinking distance between us) we both would measure the speed of light coming from the source as being c. (That's what shows us that it's time and distance that are not - relatively - constant).

No other speed works like that. If we were both travelling towards a source of sound, even your blind underwater case, we'd measure different speeds for the waves of sound travelling towards us.

Same with your hypothetical c/2 thing. There's no way it can have truly relativistic effects, as the constancy of c does, and be in the same Universe with the same laws.

(NB: your creatures might all agree on the speed of sound - but that's relative to the water, not relative to themselves.)

Edited June 16, 2018 by pzkpfw

[mistermack]

6 hours ago, Janus said:

Regarding your first argument, if light at c/2, then its speed would not be invariant.  But this would not change the fact that c is the invariant speed of the universe.  In such a case, there would not be any restriction on information traveling faster than light, nor would this violate causality. Causality would only be violated by speeds in excess of c, or twice the speed of light.

I don't think you've understood what I wrote. I'm proposing a totally new universe alongside ours, previously undetected, with it's own forms of light and matter and energy. The only difference is that THEIR light travels at c/2. Otherwise, relativity works the same as here. They will produce SR and GR in the same way, the only difference being the speed of their light.

6 hours ago, Janus said:

Your second argument is equally misguided.   Such creatures would not conclude that their clocks run significantly slower as they neared the speed of sound in water, nor would they arrive at the conclusion that it was theoretically impossible to exceed the speed of sound. 

They would conclude that the clocks in their own frame never vary, but moving clocks would appear slow, and clocks moving close to the speed of sound relative to them would appear almost stopped. And others in other frames would conclude the same about them. All of the arguments for relativity would apply in the same way. Because you are observing using sound, and your clocks operate on sound. 

The doppler effect works the same way for light and sound. So would time dilation. The reason we don't observe time dilation in sound waves, is that our clocks are not governed by sound, they are independent of it.

Edited June 16, 2018 by mistermack

[J.C.MacSwell]

3 hours ago, mistermack said:

I don't think you've understood what I wrote. I'm proposing a totally new universe alongside ours, previously undetected, with it's own forms of light and matter and energy. The only difference is that THEIR light travels at c/2. Otherwise, relativity works the same as here. They will produce SR and GR in the same way, the only difference being the speed of their light.

 

So...another Universe superimposed on ours that works essentially the same as ours except their light is c/2?...but c/2 relative  to what? It can't be relative to all our frames and be consistent with relativity for us. c/2 in some of our frames would be FTL in others, so this is just a more complicated version of saying "what if FTL travel/communication"....

Relativity or causality would have to break if there was any interaction between our Universe and this hypothetical universe. If there is no interaction then it is just as welcome to exist as the undetected pink unicorns that no doubt dance in front of our noses everyday.

3 hours ago, mistermack said:

 

They would conclude that the clocks in their own frame never vary, but moving clocks would appear slow, and clocks moving close to the speed of sound relative to them would appear almost stopped. And others in other frames would conclude the same about them. All of the arguments for relativity would apply in the same way. Because you are observing using sound, and your clocks operate on sound. 

The doppler effect works the same way for light and sound. So would time dilation. The reason we don't observe time dilation in sound waves, is that our clocks are not governed by sound, they are independent of it.

I am trying hard to find something in this that isn't wrong, but with regard to the bolded:

A clock that relied on round trips of sound waves in a stationary medium would run fastest if it was at rest with respect to the medium, the water...yet that would be a moving clock with respect to other frames..

Light doesn't work that way...if it did we would still have the aether theory, but it doesn't so we have Relativity.

Edited June 16, 2018 by J.C.MacSwell

[mistermack]

1 hour ago, J.C.MacSwell said:

A clock that relied on round trips of sound waves in a stationary medium would run fastest if it was at rest with respect to the medium, the water...yet that would be a moving clock with respect to other frames..

If YOU are made from sound waves, and so is your clock, you would have no way of identifying the medium, or whether you are at rest or in motion. 

[J.C.MacSwell]

10 minutes ago, mistermack said:

If YOU are made from sound waves, and so is your clock, you would have no way of identifying the medium, or whether you are at rest or in motion. 

That's quite an assertion. 

[Mordred]

quite an assertion without once applying any modelling or mathematics which is also the same problem with properly understanding the river model. However further care must be taken as there is several versions of the River model with specific dynamics being examined for example this variation is specifically treating a system of particles as a vector field.

https://www.researchgate.net/profile/Oyvind_Gron2/publication/222095679_A_river_model_of_space/links/0a85e53bf2467ed98d000000.pdf?disableCoverPage=true

it certainly does not describe another universe interacting with our own, so that is strictly your own speculation.

 

Edited June 16, 2018 by Mordred

[Janus]

7 hours ago, mistermack said:

I don't think you've understood what I wrote. I'm proposing a totally new universe alongside ours, previously undetected, with it's own forms of light and matter and energy. The only difference is that THEIR light travels at c/2. Otherwise, relativity works the same as here. They will produce SR and GR in the same way, the only difference being the speed of their light.

If everthing else was the the same, then c would still be the invariant speed of this universe.  If light traveled at c/2, then its speed would not be invariant.  In this case, the speed of light would not be special in any way, and light would be like anything else that does not travel at c.   Light itself is not important to Relativity, and your continued belief that it is, is totally misguided.

7 hours ago, mistermack said:

They would conclude that the clocks in their own frame never vary, but moving clocks would appear slow, and clocks moving close to the speed of sound relative to them would appear almost stopped. And others in other frames would conclude the same about them. All of the arguments for relativity would apply in the same way. Because you are observing using sound, and your clocks operate on sound. 

The doppler effect works the same way for light and sound. So would time dilation. The reason we don't observe time dilation in sound waves, is that our clocks are not governed by sound, they are independent of it.

No, they would not measure any such slowing of clocks at they approached the speed of sound.  You are coming to this conclusion due to you misinterpretation of Relativity. 

Doppler shift for sound and light do work differently.

For sound, the Doppler shift formula is

[latex]f = \left ( \frac{S \pm vr}{S \pm vs} \right ) f_s[/latex]

Where S is the speed of the sound for the medium

vr is the velocity of the receiver with respect to the medium

vs is the velocity of the source with respect to the medium.

f_s is the source frequency

What this means is the frequency measured by the receiver depends on both the source and receiver's velocity with respect to the medium.  The source being stationary and the receiver moving at v gives a different result from the source moving at v and the receiver being stationary.

On the other hand the Doppler shift for light is found by

[latex]f = \sqrt{ \frac{1+\Beta}{1-\Beta}}f_s[/latex]

where Beta = v/c

in this case v is the relative velocity between source and receiver, and there is no distinction between movement of the source and receiver, nor does it matter which one you consider to be moving.

Both equations have a component that is due to the changing distance between source and receiver and the propagation time delays involved.  With sound, if you factor out these propagation delays, you are left with the source and received frequencies being equal. (using sound, an observer at the receiver will not conclude that a clock at the source runs slow compared to his own).  With light, there is still a frequency difference after factoring out the propagation delays.  This frequency difference will be equal to the time dilation factor, and the observer will conclude that the clock at the source runs slower by this factor.

This is further demonstrated by the "transverse" Doppler effect.  This is the Doppler shift measured during the moment when a source and receiver in motion with respect to each other pass and they are neither receding or approaching each other.  With sound, there is no measured Doppler shift at this moment.  With light there is still a Doppler shift that is equal in magnitude to the time dilation factor.

Edited June 16, 2018 by Janus

[Mordred]

3 minutes ago, Janus said:

If everthing else was the the same, then c would still be the invariant speed of this universe.  If light traveled at c/2, then its speed would not be invariant.  In this case, the speed of light would not be special in any way, and light would be like anything else that does not travel at c.   Light itself is not important to Relativity, and your continued belief that it is, is totally misguided.

No, they would not measure any such slowing of clocks at they approached the speed of sound.  You are coming to this conclusion due to you misinterpretation of Relativity. 

Doppler shift for sound and light do work differently.

For sound, the Doppler shift formula is

[latex]f = \left ( \frac{S \pm vr}{S \pm vs} \right ) f_s[/latex]

Where S is the speed of the sound for the medium

vr is the velocity of the receiver with respect to the medium

vs is the velocity of the source with respect to the medium.

f_s is the source frequency

What this means is the frequency measured by the receiver depends on both the source and receiver's velocity with respect to the medium.  The source being stationary and the receiver moving at v gives a different result from the source moving at v and the receiver being stationary.

On the other hand the Doppler shift for light is found by

[latex]f = \sqrt{ \frac{1+\beta}{1-\beta}}f_s[/latex]

where Beta = v/c

in this case v is the relative velocity between source and receiver, and there is no distinction between movement of the source and receiver, nor does it matter which one you consider to be moving.

Both equations have a component that is due to the changing distance between source and receiver and the propagation time delays involved.  With sound, if you factor out these propagation delays, you are left with the source and received frequencies being equal. (using sound, an observer at the receiver will not conclude that a clock at the source runs slow compared to his own).  With light, there is still a frequency difference after factoring out the propagation delays.  This frequency difference will be equal to the time dilation factor, and the observer will conclude that the clock at the source runs slower by this factor.

This is further demonstrated by the "transverse" Doppler effect.  This is the Doppler shift measured during the moment when a source and receiver in motion with respect to each other pass and they are neither receding or approaching each other.  With sound, there is no measured Doppler shift at this moment.  With light there is still a Doppler shift that is equal in magnitude to the time dilation factor.

fixed the latex lol ya got used to another forums command braces.

Edited June 16, 2018 by Mordred