# A question about "C" and causality.

#### Starstruck

Why is "C" (speed of light) so important? I think the number one reason why it is so important is because it is the fastest way to get information.

I recently read an article that explains that an FTL communications system would allow you to violate causality by sending information about an event before said event occurred. The example used was a spaceship between Proxima Centuri and Earth observes a phone call made from Earth to Proxima. Using their FTL communicator, they tell Earth not to make the phone call they just saw them make. Causality violated right?

Maybe not.

Consider the initial observation was made observing light from Earth. The fastest known method of communication currently known. However, the FTL communicator is now the fastest game in town. Lets say it transmits at 100x the speed of light. Would it not replace light as the only "super observer" in the Universe? Now, instead of C being the ultimate speed limit, the FTL is. Information received from light would now be outdated. Time would now be measured by the new speed limit and all clocks re-calibrated to it.

So now, using only the FTL measurements, the spaceship sees Earth pick up the phone, make the call, and hang up. When they send their message the events will have already occurred in the right order using the "new" speed limit provided by the FTL.

The speed of light would still remain a constant, the only thing that changes is that light is no longer the fastest game in town.

Time would still tick in the right order, because the FTL is the new "clock", not light.

Causality saved?

#### Catastrophe

##### "There never was a good war, or a bad peace."
Is there not a problem in your examples. Light leaves start. You intercept it en route but you have to read it, decide to tell the sender to stop, and then do so. By which time the original EM message would have reached its destination.

If the intended recipient were, say, 10 light years away, then you could receive the message and tell the sender not to send, but he/she will already have sent it. Thus you can receive the message before the intended recipient; you can tell the sender that you don't like his/her message; but it will already have been sent.

Am I making a mistake somewhere?

Cat

#### Slarty1080

The problem is that this example only considers the inertial reference frame of Earth and Proxima Centuri (although they move relative to each other this is a fair approximation), but this reference frame is not the only one in which causality must be observed. It must also be true for a near light speed observer moving in any direction relative to Earth / Proxima Centuri.

If we could say there was only one frame of reference where cause and effect needed to work then we could have FTL travel without violating causality. However there is no such special frame of reference and cannot be if relativity is true. And we have a lot of very good experimental evidence that relativity is true so there is no FTL travel.

In a nut shell you can pick any two:
Causality is not violated
Relativity holds true
Faster Than Light travel is possible

As far as we can tell nature has selected 1 and 2. If 3 is true then either causality is violated or relativity is false. Relativity is a cornerstone of modern physics and cannot be easily dismissed. The consistency of speed of light in a vacuum appears to be deeply imbedded in the fabric of the universe. As well as providing an absolute speed limit it also defines the equivalence of mass and energy and forms part of the definition of electromagnetism in Maxwell’s equations. Relativity might be wrong, but if it is then vast swathes of physics would need to be revised.

#### Helio

The example used was a spaceship between Proxima Centuri and Earth observes a phone call made from Earth to Proxima. Using their FTL communicator, they tell Earth not to make the phone call they just saw them make. Causality violated right?
No. Lightning has both a fast component (light) and a slow one (thunder). The ship should be able to receive both signals.

#### Atlan0101

Scenario:

You have two phones quantumly entangled (or rather the equivalent of quantum entanglement (to get technical)). One on Earth, one in the Centauri system four light years away It takes light four years to make the trip. The alien you are calling via the (equivalent of quantumly entangled) phone system is receiving at exactly the same time you are sending. Communication in this scenario is instantaneous across 24 trillion miles of space. Is faster than light travel -- say 186 million miles per second.... or even 186 billion miles per second -- any faster than instantaneous communication of information across 24 trillion miles of space contracted to zero distance (or even less than zero distance) between the two phones in the phone system?

If you had a super duper telescope and could "observe" the receiver receiving the call, when would you observe him / her / it receiving the call? A minimum of four years after making the call. How much time, how much activity, has passed in four years? How much older are you and the alien? How many ticks of your clocks have ticked?

And that traveler spoken of between Earth and Centauri. Where is he / she/ it in the observation of the Earth observer through his super duper telescope that can resolve light so superbly. It takes light a full second to cross 186,000 miles of space. If the traveler is half way to Centauri, that is 12 trillion miles, and two years, that light has to travel to get to the observer on Earth. Will the traveler be where in space the observer observes the traveler to be in two years in time?

Say the traveler arrives at Centauri and his ship was the starship Enterprise of Star Trek fame, which could travel at warp speed. Say it took the ship four hours to make the trip, warping space and time to get there in four hours. It takes light four years to travel from the Centauri system to Earth. The observer makes his observation of Enterprise's arrival in the Centauri system four years and four hours after the Enterprise left Earth. That is four hours short of the speed of light across four light years of space-time. The observer observes a near light speed travel for the Enterprise, but no light speed travel as such, and certainly no faster than light speed travel. The observer, though, through his super duper light resolving telescope, along with its x-ray vision into the ship, observes that the crew didn't age a day during the trip of four years and four hours he lived and observed to have passed watching light and light speed information travel back to him at light speed across increasingly longer stretches of space. Nor did that clock aboard ship clock a time greater than four hours in all that four years and four hours.

Captain Kirk upon walking into the observatory the day the observation arrives to Earth -- via light speed -- of his arrival in the Centauri system watches his four year younger self arrive in the Centauri system approximately four years before. He has made many trips to many places in the galaxy during that four years. He could show up to be in all those places while standing in the observatory alongside the observer, like the particle in QM that if you can tell its velocity precisely you can never tell its position precisely / if you can tell its position precisely you can never tell its velocity precisely.

There are two travelers going away from the observer on Earth, the real traveler ahead in space and time and the relative traveler always pliably stretching out and dragging behind in space and time (always a history) in the light. Oncoming to the observer on Earth, the relative traveler in the light is always positioned closer in space and time to the observer than the real traveler, the two -- the real and the relative -- contracting space and time between them (and between them and the observer) and becoming one only upon arrival at the observer.

As the observer who observed Kirk's four year and four hour trip across four light years of space-time could tell you, there is no such thing as faster than light travel. The fact that Kirk was standing beside him when he observed Kirk to arrive in the Centauri system is irrelevant. Causality was never violated.

The only way causality could possibly be violated is on the immediate spot of an event occurrence. And at no time did Kirk in the scenario travel faster than the speed of light locally to him, or even come closer to it than 186,000 miles per second. And at no time was he observed by any observer distant from him to have traveled faster than the speed of light. More, regarding the Centauri system ahead of him, he observed the Enterprise to be fast forwarding in time travel to the Centauri System. Not traveling backward in time. Why? Because he and his ship and his clock were all observationally behind the Centauri System regarding time in the first place. Four years behind it at the beginning of his voyage (it being four light years distant from Earth to begin with). He started out backward in time, relatively speaking, and had to travel forward in time just to reach the Centauri System. He did it in four [forward] hours in time, contracting a bubble of space and time, warping a bubble of space and time, playing the principle of uncertainty to the hilt, getting the job done.

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#### Catastrophe

##### "There never was a good war, or a bad peace."

Quote
These restrictions are consistent with the constraint that mass and energy that act as causal influences cannot travel faster than the speed of light and/or backwards in time. ... Consequently, the relativistic principle of causality says that the cause must precede its effect according to all inertial observers.

Causality (physics) - Wikipedia

Quote

Cat

Slarty1080

#### Slarty1080

Quote
These restrictions are consistent with the constraint that mass and energy that act as causal influences cannot travel faster than the speed of light and/or backwards in time. ... Consequently, the relativistic principle of causality says that the cause must precede its effect according to all inertial observers.
Causality (physics) - Wikipedia
Quote

Cat
Yes! Cause must precede effect according to all inertial observers. If there was only one inertial reference frame then faster than light travel would be possible. Bu there isn't, all inertial reference frames must be considered including those traveling at near light speed in different directions.

Catastrophe

#### Catastrophe

##### "There never was a good war, or a bad peace."
Has anyone yet found a fault in my post #2?

Cat

#### Slarty1080

Has anyone yet found a fault in my post #2?

Cat
These sort of questions can be difficult due to missing information and different assumptions by different people. I believe that both you and Starstruck and basing your discussion on a single inertial reference frame (but I may be wrong as it was not clear to me if the Spaceship in the original question was moving).

In a single inertial reference frame it is possible hypothetically to exceed the speed of light without violating causality. But the universe does not have just one inertial reference frame. In other words there is no master cosmic clock to order events.

Catastrophe

#### Catastrophe

##### "There never was a good war, or a bad peace."

However, I cannot see that, once the message has been sent, it is possible to go back and stop it. To do so, imho, it would require the frame of reference of the middle person to include only those FORs where the message had not been sent, which invalidates the whole 'point' of the question?

Cat

#### Slarty1080

@Cat, yes rereading what you wrote again, you are correct. Paraphrasing:

Assuming that this whole system is a single inertial frame of reference we can assume a common clock for all events. In this case the original phone message is by definition the first event, the middleman might then detect the signal almost immediately, regardless of the fact that he is light years distant and might call the sender almost immediately to say stop, but it’s already too late by then.

The key issue was assuming the middleman event was the first event. It can't be.

Catastrophe

#### Catastrophe

##### "There never was a good war, or a bad peace."
"The key issue was assuming the middleman event was the first event. It can't be."

Slarty, thank you. You have improved my understanding too. Cat

#### voidpotentialenergy

Light C speed travels through space/time at the min interval of the distance to the next smallest thing or chunk of space/time etc etc.
Quantum entangled information travel through nothing avoiding time and space like gravity's duality of doing both leaving a wave on one and instant travel on the other. (if we can call it travel)
Spooky action at a distance information is instant since it had no distance or time to travel.
Light whatever distance it traveled at in normal space/time.

JMO

Et phoning home and his gravity signature can never interact in two different mediums.

#### Slarty1080

Light C speed travels through space/time at the min interval of the distance to the next smallest thing or chunk of space/time etc etc.
Quantum entangled information travel through nothing avoiding time and space like gravity's duality of doing both leaving a wave on one and instant travel on the other. (if we can call it travel)
Spooky action at a distance information is instant since it had no distance or time to travel.
Light whatever distance it traveled at in normal space/time.

JMO

Et phoning home and his gravity signature can never interact in two different mediums.
According to relativity no meaningful information can travel faster than light. Quantum entangled "information" does not actually provide any meaningful content to people receiving that signal at either end.

#### Atlan0101

According to relativity no meaningful information can travel faster than light. Quantum entangled "information" does not actually provide any meaningful content to people receiving that signal at either end.
Of course nothing travels faster than the speed of light anywhere locally to anything. As genius mathematician and logician Kurt Godel pointed out to his best friend Albert Einstein, who could not dispute it, an anomaly existed in the Theory of Relativity. If arrows of time exist, traveling faster than the speed of light would mean going against the flow and traveling backward along the 1-dimensional line of the arrow, violating causality at every point [shattering] and [annihilating] the arrow, thus all the time at every point, all at once, of that 1-dimensional line-stream flow. Blackholes, in or inside their event horizons; and more generally, more gradually, 'entropy' (it does the job on the Gordian Knot by the simple method of always cutting through its entanglement rather than even trying to unravel it: Ultimately the same thing in the end, but not exactly the same path to it); can do that, but so far as we know nothing else can.

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#### Helio

According to relativity no meaningful information can travel faster than light. Quantum entangled "information" does not actually provide any meaningful content to people receiving that signal at either end.
Yes, it seems that when one forces one particle to be in a chosen mode that entanglement ceases. So, so far, no info can be transferred.

Catastrophe

#### voidpotentialenergy

Of course nothing travels faster than the speed of light anywhere locally to anything. As genius mathematician and logician Kurt Godel pointed out to his best friend Albert Einstein, who could not dispute it, an anomaly existed in the Theory of Relativity. If arrows of time exist, traveling faster than the speed of light would mean going against the flow and traveling backward along the 1-dimensional line of the arrow, violating causality at every point [shattering] and [annihilating] the arrow, thus all the time at every point, all at once, of that 1-dimensional line-stream flow. Blackholes, in or inside their event horizons; and more generally, more gradually, 'entropy' (it does the job on the Gordian Knot by the simple method of always cutting through its entanglement rather than even trying to unravel it: Ultimately the same thing in the end, but not exactly the same path to it); can do that, but so far as we know nothing else can.
IMO gravity and spooky action at a distance are the same or properties of the same thing.
If the universe does have a limit to small then it will be filled with lots of nothing.
Like you say traveling (if we can call it that) through no/space no/time can be instant speed and not violate any physics law.

Catastrophe

#### Atlan0101

The speed of light is 186,000 miles per second (300,000 kilometers per second). Space gained (gaining) per time passed (passing). As long as you are not inside a closed systemic particle accelerator, natural or otherwise (such as the LHC), it is what you will measure yourself and your vessel doing, open systemically, regardless of any relative velocity whatsoever. You can't even be observed at any distance to be doing anything like faster than light travel. That is why it is called a "constant."

Space is not a rigid absolute, nor is time. Four-dimensional space-time is not a rigid absolute. The only thing physicists forget is that it is four-dimensional rather than one-dimensional. And a non-rigidly absolute four-dimensionality means be a contractive / expansive bubble of SPACE and time. You have to contract the space dimensions to contract the time dimension. You have to expand the space dimension to expand the time dimension. No physicist I've ever read, including Einstein and Hawking, knows that. All of them have taken the observed traveler in the observed light arrived to the observer to be the traveler at any distance (in other words, the speed of light is mathematically instantaneous across all distances). Meaning, that in the math of the physics, the map [is] the territory because the territory itself can never be observed to be mapped in fact. The speed of light is not instantaneous -- and can never be instantaneous -- with the territory. The territories are simultaneous four-dimensional territories, but the speed of light can never make them one and the same four-dimensional box. It can never quantum entangle our Solar system and the Centauri system, nor any traveler and any observer. Nor any observer and any other observer.

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