Can information travel faster than light?

[Razzor7]

subatmoic particles are points; they have no physical dimension. They influence each other by their fields. Changes to their fields propogate at the speed of light.

Ahh... I still suffer from Classical Syndrome.

 

[Sam G]

Mechanical forces ultimately come down to forces between subatomic particles in atoms, which are still, ultimately, fundamental forces governed by c.

The point that the poster was making was he thought that the only way we could transmit information was by light, which is limited by c, and that a stick might bypass that.

This is quite intriguing, please tell me if the following would be correct:

I have a laser pointer with a mechanical switch/lever attached to a rod that is exactly 1 light minute long. This rod would run the same direction that the laser points to and is a two piece apparatus, or a rod inside a pipe, which would allow for an on/off indicator at the opposite or long end. Essentially this would be a long mechanical button that pushes and pulls the lever of the laser switch.

Now this thing is up in outer space with a person holding each end. It would seem to me, that if I were on the long or indicator end of this thing, I could push the rod/button to the 'on' position and see the light from the laser after 1 minute.

Then, the other person on the laser end could switch the lever off -moving the rod/button into the off indicating position on my end- allowing me to know that I would see the light go out in 1 minute. The actions by the other person would change the information of the on/off indicator on my end and it would be received exactly 1 minute sooner than seeing the light, right?

What I think you're going to tell me is that when the person on the other end manipulates the switch -I will see both the light and the indicator on my end change 1 minute later or at the same time on my end. It would also be true then that when I push the rod/button to the 'on' position and even though the laser is only 1 light minute in length -it would actually take 2 minutes for me to see the light from the laser.

Is that right?

 

[DaveC426913]

This is quite intriguing, please tell me if the following would be correct:

I have a laser pointer with a mechanical switch/lever attached to a rod that is exactly 1 light minute long. This rod would run the same direction that the laser points to and is a two piece apparatus, or a rod inside a pipe, which would allow for an on/off indicator at the opposite or long end. Essentially this would be a long mechanical button that pushes and pulls the lever of the laser switch.

Now this thing is up in outer space with a person holding each end. It would seem to me, that if I were on the long or indicator end of this thing, I could push the rod/button to the 'on' position and see the light from the laser after 1 minute.

Then, the other person on the laser end could switch the lever off -moving the rod/button into the off indicating position on my end- allowing me to know that I would see the light go out in 1 minute. The actions by the other person would change the information of the on/off indicator on my end and it would be received exactly 1 minute sooner than seeing the light, right?

What I think you're going to tell me is that when the person on the other end manipulates the switch -I will see both the light and the indicator on my end change 1 minute later or at the same time on my end. It would also be true then that when I push the rod/button to the 'on' position and even though the laser is only 1 light minute in length -it would actually take 2 minutes for me to see the light from the laser.

Is that right?

I don't really understand your apparatus. I don't understand what a 'long end' is, or how many lengths the light or the rod is moving.

But the upshot is that the light will take one minute to travel one length of your device. The switch will actually take much longer than one minute - it will take as long as the speed of sound travels through that material, which might be only a few thousand miles per hour.

 

[yuiop]

information can travel faster than light. but it is never useful information.

Can you give an example?

It might be that not everyone uses the same definition for the term "information".

I know what thomasxc is getting at here and George is right that we have to evaluate carefully or even redefine exactly what we mean by "useful information". When quantum entanglement is carefully evaluated it seems nature can communicate at a sub operating level at speeds that are effectively instantaneous and infinitely faster than the speed of light. This quantum communication channel does not seem to be available at a level that can be used by sentient beings to communicate faster than the speed of light. Now this statement requires quite a lot of elaboration so I hope you will bear with me as I try to explain better. When I stated that entangled particles can "communicate" instantaneously with each other I mean that that a measurement made on one entangled particle has an instantaneous causal effect on it entangled partner over great distances and much faster than the speed of light. Despite this instananeous causal connection between the entangled particles it can not be used by two spatially separated observers to communicate useful information to each other at superluminal speeds. For this discussion I will define "useful information" as information about an event in space in time. For example the statement "Bob knows Anne has a red sock in her box" is useless information while "Bob knows Anne opened the box and saw a red sock at 12.30 pm on Saturday 26/07/2008" is useful information.

Care to elaborate? Or were you just kidding?

Even "useless" information is quite useful. At the very least, its arrival can tell you that the sender is sending. Beef that up and you've got Morse Code. Now you can send an Encyclopedia.

Information cannot travel faster than light.

Imagine two distant civilisations that are very advanced but have not learned to live in peace with each other. They each have powerful laser weapons that can destroy each others home worlds and there is a cold war of mistrust. It would be very desirable to have some sort of early warning system in this situation to know if they are under attack but once one of the the lasers is activated, any early warning system would require superluminal communication. Can quantum entanglement be usefully employed here? Anne and Bob are peace loving and they decide to give it a go. A red sock in placed in one box and a blue sock is placed in the another box. Bob picks a box at random and Anne has the other and neither know what colour sock they have in their box. In fact these are magic quantum socks and they each have a grey sock in their box but as soon as one box is opened it decides to to be blue or red and sends an instaneous signal to the other box that turns it into the opposite colour. Anne travels off to infiltrate the enemy civilisation while Bob stays at home. Anne is successful in her infiltration and finds out the enemy are about to activate the death laser in a pre-emptive attack :O She quickly opens her quantum box and finds she has a red sock inside. By opening the box and seeing a blue sock inside, Anne has caused the sock in Bob's box to turn red instantaneously. Anne now knows Bob has a red sock and if Bob happened to open his box he would have the information "Anne has a blue sock". Unfortunately Bob has no information about when Anne opened her box so the information is useless as an early warning that a laser attack has been launched. Bob is about to be fried to a crisp and as far as he is concerned having an early warning that an attack is underway so he can take avoiding action is useful information, while knowing instaneously what colour sock Anne will see if and when she opens her box is useless information. It does not matter what pre agreed code system they have for colour of socks or even if they have many boxes and open them at pre-agreed times they can not use the "magic quantum socks" to communicate information about events in space and time faster than the speed of light. This is despite the fact the socks CAN communicate causally with each other faster than the speed of light. (I justify this below)

...
Here's what I think you're trying to say.

Two boxes are side-by-side. Put a cat in one of the boxes and a dog in the other. Separate the boxes by some distance. Two people, Ted and Bob, the boxes open the boxes simultaneously (in some inertial frame). If Ted finds a dog in his box, then he knows instantaneously that Bob found a cat in his box. Ted knows this before there is enough time for Bob to send a signal to Ted, if signals propagate more slowly than the speed of light.

But, in order to know Bob's result, Ted already had information before he opened the box. He knew that one box contained a cat and the other a dog. No information traveled faster than the speed of light.

The example I gave of the blue or red socks or your example of dog or cat does not really illustrate the truly weird quantum entanglement is or why it is necessary to conclude that the entangled particles CAN communicate faster than the speed of light or indeed why they MUST communicate faster than the speed of light. To understand this it is helpful to imagine each box has two compartments. On compartment always contain an animal (dog or cat) and the other always contains a sock (red or blue) and the following rules apply:

1) Opening one compartment of a box to see what is inside locks the other compartment for good. You can never find out was inside the other compartment of the box.
2) If Anne sees a red sock then Bob will ALWAYS see a blue sock if he opens the sock compartment of his box.
3) If Anne sees a blue sock then Bob will ALWAYS see a red sock if he opens the sock compartment of his box.
4) If Anne sees a cat then Bob will ALWAYS see a dog if he opens the animal compartment of his box.
5) If Anne sees a dog then Bob will ALWAYS see a red sock if he opens the sock compartment of his box.
6) If Anne sees a dog then Bob will SOMETIMES also see a dog if he opens the animal compartment of his box.

The rules are exactly the same if the names of the observers are switched and it does not matter who opens their box first.

Now imagine a third person (Harry) who has the job of loading the boxes for Anne and Bob so that no matter which compartments they choose to open, the above rules are obeyed. Harry's job is easy if only the first 5 rules are considered but rule 6 is the killer. In order to obey rule 6 Harry must occasionally load a dog into both Anne's and Bob's boxes. This is fine if both Anne and Bob choose to open their respective animal compartments. But what if Anne finds a dog and Bob opens the sock compartment? Rule 5 says the Bob should find a red sock so Harry loads a red sock into Bob's sock compartment to cover that eventuality. If both Anne and Bob open the sock compartments they should find opposite coloured socks to obey rules 1 and 2 so harry loads Anne's sock compartment with a blue sock. But what if Bob opens the animal compartment and finds a dog and Anne decides to open her sock compartment? Rule 5 (with the names reversed) says Anne should find a red sock in her box, but Harry has put a blue sock in there already to comply with rules 1,2 and 6. Clearly Harry can not pre-load the boxes to cover all the eventualities without violating one of the rules. This is where the quantum wierdness comes in. Bell's inequalities mathematically prove there is no realistic deterministic method to pre load the boxes to satisfy quantum rules. Harry's pre-loading the boxes is what is sometimes referred to as "hidden variables". Experimental evidence rules out hidden predetermined variables and this is often referred to as violating Bell's inequalities in the literature. The only logical explanation of quantum entanglement that agrees with actual experiments is that the particles (the boxes) communicate faster than the speed of light with each other and opening one box and seeing what is inside instantaneously changes the contents of the other box.

In fact quantum entanglement is a little wierder than I have already described. I described each box as having two compartments of which you are only allowed to open one compartment to make it easier to visualise. A better description is a box with only one compartment and you have to ask a question before you open the box. So if Anne asks "What kind of animal is in my box?" she will find either a dog or a cat when she opens the box and no sign of socks anywhere in the box. If Anne asks "What colour socks are in my box?" she will find either a red sock or a blue sock and no hint of a dog or a cat or traces of fur in her box. What she sees when she opens the box depends on what question she asks of the box before she opens it. So if Anne asks what type of animal is her box and opens it to discover a dog, a signal is sent to Bob's box saying the contents are allowed to be cat, dog or red sock but blue sock is not allowed. If Bob asks what kind of sock is in his box then there is 100% probability of there being a red sock inside and nothing else. If he asked what kind of animal is in his box then the it may be a cat or sometimes a dog. So this is wierder than Schrodinger's half alive, half dead cat because Bob's box contains an entity that is part dog, part cat and part sock and whether it materialises as a living animal or a red sock when Bob opens the box depends on what question Bob asks before he opens the box. I hope this analogy gives you an idea of just how weird or magic quantum effects are ;)

 

[Sam G]

I don't really understand your apparatus. I don't understand what a 'long end' is, or how many lengths the light or the rod is moving.

But the upshot is that the light will take one minute to travel one length of your device. The switch will actually take much longer than one minute - it will take as long as the speed of sound travels through that material, which might be only a few thousand miles per hour.

When I say 'long end' I mean the opposite end from the laser.

I'm imagining the rod is 1 light minute in length and would only move about a quarter of an inch.

Still, your answer is much removed from what I was thinking. I imagined the rod would move all at once but, you say it would move even slower than what I'd thought you'd say.

How do both ends of the rod not move at the same time?

 

[MeJennifer]

This is despite the fact the socks CAN communicate causally with each other faster than the speed of light. (I justify this below)

I would not go as far as calling that a fact.

At any rate this discussion seems to be appropriate in the Quantum Mechanics section of this forum.

 

[yuiop]

When I say 'long end' I mean the opposite end from the laser.

I'm imagining the rod is 1 light minute in length and would only move about a quarter of an inch.

Still, your answer is much removed from what I was thinking. I imagined the rod would move all at once but, you say it would move even slower than what I'd thought you'd say.

How do both ends of the rod not move at the same time?

Imagine a very rubbery or springy rod. When you hit one end it compresses so that for a short time one end is moving while the other is not. A steel rod seems very rigid but it can still be compressed. Relativity states there is no such thing as an infinitely rigid or incompressible rod. Instaneous movement of both ends of a rod requires a rod to be infinitely rigid.

 

[Sam G]

Imagine a very rubbery or springy rod. When you hit one end it compresses so that for a short time one end is moving while the other is not. A steel rod seems very rigid but it can still be compressed. Relativity states there is no such thing as an infinitely rigid or incompressible rod. Instaneous movement of both ends of a rod requires a rod to be infinitely rigid.

Ok. I think I understand that. But it sure is weird. What would cause steel, or say I make it from diamonds, to loose it's rigidness?

Let's say I change from a rod to a long string on a pulley at both ends. Would the same rules apply?

 

[MeJennifer]

What would cause steel, or say I make it from diamonds, to loose it's rigidness?

Acceleration!

 

[yuiop]

Ok. I think I understand that. But it sure is weird. What would cause steel, or say I make it from diamonds, to loose it's rigidness?

Let's say I change from a rod to a long string on a pulley at both ends. Would the same rules apply?

Yes.

Imagine a rubber band with a weight on the end. You can pull one end while the other remains stationary. A cable made of the toughest material such as carbon nontubes or kevlar that is one light minute long is pretty much like an elastic band. Relativity effectily puts a limit on how tough a material can be.

Look at a slow motion film of a car in a crash test and you will see steel is not at all rigid.

 

[yuiop]

I would not go as far as calling that a fact.

At any rate this discussion seems to be appropriate in the Quantum Mechanics section of this forum.

I think discussing the faster than light communications of entangled particles is very relevant to a thread titled "Can information travel faster than light?".


Entangled particles communicating faster than light is the simplest explanation. Bell said and experiment prove that there is no explanation of quantum entanglement experimental observations that is both local and realist. Local means that entangled particles are limited to communicating at the speed of light. If you insist on an explanation that does not require entangled particles to communicate faster than the speed of light then you have to accept a non-realist explanation.

Non realist explanations include:

a) Determinism: Anne and Bob (or any of us) have no free will and what measurements they carry out is predetermined even before the entangled photons are emitted or that the entangled photons rather than influencing each other influence the minds and decisions of Anne and Bob.

b) Many Worlds: Every time a measurement is made the universe splits of into parallel realities covering all possible outcomes and then somehow discards the parallel worlds where the outcomes do not agree with quantum rules.

c) Time manipulation: If the outcome does not agree with quantum rules then the universe rewinds erasing all memory of the quantum violation and has another go until it gets it right. Alternatively, when Anne or Bob make a measurement a signal goes back into the past causeing a photon (of the correct tiype) to be emitted. In this way the future causes the past rather than the other way round that most of us fondly imagine.

Bell's theorum basically means you have a stark choice. Either you have to believe that quantum mechanics allows faster than light communication at the quantum level or you have to believe one of the even wierder and more complex non-realist explanations.

The non-local pill is easier to swallow when you realize that just because entangled particles can communicate or influence each other faster than the speed of light it, the effect does not allow sentient observers to communicate information about events faster than the speed of light.

 

[DaveC426913]

What would cause steel, or say I make it from diamonds, to loose it's rigidness?

You're misunderstanding. Steel and diamond only seem rigid from the scale of humans. Moving one end only seems to move the other end instantly because we can't detect such a short delay.

If you have a one foot length of steel and you moved one end of it, the other end would not move until 0.000067 seconds later. If the bar were diamond, the delay would only be 0.000025s.

This is the way of all solid matter.


http://hyperphysics.phy-astr.gsu.edu/Hbase/Tables/soundv.html

 

[Sam G]

If you have a one foot length of steel and you moved one end of it, the other end would not move until 0.000067 seconds later. If the bar were diamond, the delay would only be 0.000025s.

Dang, I didn't know that. Thanks for the link ...this is heavy stuff!

 

[yuiop]

In an earlier post I suggested that entangled photons can effectively communicate with each other at faster than light speeds in a manner that described as "causal". One objection to anything communicating instantaneously is that if one observer sees event A causing effect B then another observer in a different reference frame will see effect event B before cause event A happens. This is obviously ridiculous and in many caes it violate the second law of thermodymics. It turns out that as far as communicating entagled photons are concerned there is a symmetry involved and the reversal of cause and effect for entangled photons is always plausible. If Anne measuring Up spin in her entangled particle causes Bob's particle to have Down spin, then there is no difference if Bob makes the first measurement and his observation of Down spin in his entangled particle causes Anne's entangled particle to have Up spin. It is only when larger systems of interacting particles are considered that the second law of thermodynamics comes into play. If Anne fires a gun at Bob causing Bob to die then it would not be reasonable for any observer to see the death of Bob causing Anne to pull the trigger of her gun at a later time. This would not be a good defence for Anne if she is charged with Bob's murder

Just to recap, in case there is any confusion. The faster than light communication between entangled particles does not enable sentient obervers such as Anne and Bob (when he was alive) to communicate information about events at faster than light speeds. The universe seems to differentiate between the communication channels open to simple quantum particles and the channels available to sentient observers. The dividing line seems to be the complexity of the systems. If we assume that sentient beings that can analyse, compute and predict are effectively computing systems then there must be a minimum complexity to a computing system that takes the system out of the quantum regime due to decoherance of the interacting particles of the complex system. From this, it could possibly be concluded that the observers of special relativity have to be sentient observers and a single entangled photon does not qualify as a sentient observer and the rules of special relativity do not apply to entangled photons.

 

[russ_watters]

Dang, I didn't know that. Thanks for the link ...this is heavy stuff!

I bet you did know it, you just didn't realize it: a spring has a certain elasticity and you clearly understand that - use a stiffer material or make the coils thicker and the whole spring gets stiffer. I'm sure you understand this. What holds you up is extending that concept. If the spring bends (not enough to cause permanent deformation), what happens inside it? The part on the outside of the bend must expand and the part inside must contract, right? So you know that the molecules can get pulled apart and pushed together as you bend a piece of metal. Why not also when you just push on a straight rod of the same material?

A lot of what understanding science is is understanding the domain of applicability of a concept and being able to expand the idea to cover it's whole domain. And also to not overexpand it.