Can information travel faster than light?

  • Thread starter Mafarazzo
  • Start date
  • #1
2
0

Main Question or Discussion Point

Let's say theres a stick, and two persons are holding the extremities:

Person A ----------stick---------- Person B

Both are pulling the stick with equal forces. At a certain moment, Person B releases it and instantly Person A knows it becase he can now pull the stick to himself.

Why can't I say that information in this case traveled faster than light? It seems to me that there is no delay between Person B releasing and Person A being able to notice it.

I know this can't be true, but I fail to find the answer.
Thanks in advance.
 

Answers and Replies

  • #2
rcgldr
Homework Helper
8,674
509
The rate of information about the pull or lack of it usually propagtes at the speed of sound in the medium involved. Think of the stick as being a very dense slinky.

I seem to recall some sort of instant "communication" between pairs of electrons, such that some obverved effect on one of the electrons causes an immediate repsonse by the other (maybe it was the spin)? This couldn't be used to transfer information, but it was a "faster than light speed" response between two particles, although I don't remember the details. Perhaps someone could respond with a correct explanation?
 
Last edited:
  • #3
266
0
And you must remember that at small distances the speed of sound might as well be the speed of light when compared to the sensitivity of human perception. I.e. synaptic operations peak around 120 m/s.
 
  • #4
absolute rigidity

Let's say theres a stick, and two persons are holding the extremities:

Person A ----------stick---------- Person B

Both are pulling the stick with equal forces. At a certain moment, Person B releases it and instantly Person A knows it becase he can now pull the stick to himself.

Why can't I say that information in this case traveled faster than light? It seems to me that there is no delay between Person B releasing and Person A being able to notice it.

I know this can't be true, but I fail to find the answer.
Thanks in advance.
I have seen the same problem in the context of absolute rigidity of the stick a concept not in accordance with SR
 
  • #5
1,060
0
Hello Mafarazzo.

Quote:-

----Both are pulling the stick with equal forces. At a certain moment, Person B releases it and instantly Person A knows it becase he can now pull the stick to himself.----

I think this may not be true but my knowledge is insufficient to say for certain. It needs some consideration.

Matheinste
 
  • #6
paw
169
0
Let's say theres a stick, and two persons are holding the extremities:

Person A ----------stick---------- Person B

Both are pulling the stick with equal forces. At a certain moment, Person B releases it and instantly Person A knows it becase he can now pull the stick to himself.

Why can't I say that information in this case traveled faster than light? It seems to me that there is no delay between Person B releasing and Person A being able to notice it.

I know this can't be true, but I fail to find the answer.
Thanks in advance.
It only seems to be instant to human perception. If you think of a spring instead of a stick it's quite easy to see the information about the 'release' travels at the speed of a wave in the spring. With a stick the information travels at approximately the speed of sound (sound wave) in the stick. It's far, far slower than c.
 
  • #7
28
0
Let's say theres a stick, and two persons are holding the extremities:

Person A ----------stick---------- Person B

Both are pulling the stick with equal forces. At a certain moment, Person B releases it and instantly Person A knows it becase he can now pull the stick to himself.

Why can't I say that information in this case traveled faster than light? It seems to me that there is no delay between Person B releasing and Person A being able to notice it.

I know this can't be true, but I fail to find the answer.
Thanks in advance.
Consider what a stick is made of. Several atoms pushing and pulling against each other. When person B lets go, A does NOT observe it instantly. Rather, he observes it in the time it takes for the "information" to travel across all of the atoms. While this might seem instantaneous, it actually happens at the speed of light, via photons.

Of course, one could rewrite the problem to be with say, an electron, and to that I don't have an actual answer. It might be, simply, that motion travels through a completely rigid body at the speed of light.
 
  • #8
2,257
7
Last edited:
  • #9
russ_watters
Mentor
19,328
5,365
so if light moving through spacetime can be compared to a sound wave moving through matter then maybe c is not the speed at which information itself moves.
No (to both parts!). I have no idea what would lead you to think such a thing.
have you ever thought about why it is that microwaves are 100% blocked by the door of the microwave even though it is 90% open (you can see through it)?
I'm not sure why you would think that either. The wavelength of "light" used by a microwave oven is on the order of 12 cm. The effect of the screen across the door is quite similar to trying to kick a soccer ball through a chain link fence.
 
  • #10
107
0
Why can't I say that information in this case traveled faster than light? It seems to me that there is no delay between Person B releasing and Person A being able to notice it. I know this can't be true, but I fail to find the answer.
Quote from http://en.wikipedia.org/wiki/Speed_of_light#Faster-than-light_observations_and_experiments:
"It is generally considered that it is impossible for any information or matter to travel faster than c. The equations of relativity show that, for an object travelling faster than c, some physical quantities would be not represented by real numbers."

It might be, simply, that motion travels through a completely rigid body at the speed of light.
Correct me if I'm wrong, but I don't think there is such a thing as a completely rigid object. The wave would propagate through the stick at the speed of sound of that medium, not at c.

I seem to recall some sort of instant "communication" between pairs of electrons, such that some obverved effect on one of the electrons causes an immediate repsonse by the other (maybe it was the spin)? This couldn't be used to transfer information, but it was a "faster than light speed" response between two particles, although I don't remember the details.
I think this is quantum entanglement. See this: http://en.wikipedia.org/wiki/Quantum_entanglement
 
Last edited:
  • #11
45
0
I seem to recall some sort of instant "communication" between pairs of electrons, such that some obverved effect on one of the electrons causes an immediate repsonse by the other (maybe it was the spin)? This couldn't be used to transfer information, but it was a "faster than light speed" response between two particles, although I don't remember the details. Perhaps someone could respond with a correct explanation?
If two entangled particles will change when the other does and we have the instruments to detect this change then it is information. Think of the particles two states as on and off 1 and 0. there is no reason that information could not be transfered like this.. Also if we detect the change then that is information in itself because we now know the state of its entangled partner has been changed
 
  • #12
28
0
If two entangled particles will change when the other does and we have the instruments to detect this change then it is information. Think of the particles two states as on and off 1 and 0. there is no reason that information could not be transfered like this.. Also if we detect the change then that is information in itself because we now know the state of its entangled partner has been changed
GRB, are fundamental particles not completely rigid? If they aren't, how so?

Neh, entanglement cannot be used to transfer information. Say Alice and Bob share a pair of entangled photons. If Alice makes a measurement on her photon, that means that Bob will always find a certain thing when he measures his. However, he cannot tell without communication with Alice whether or not she did anything.
 
  • #13
126
0
Is this not similar to:

- Telling Alice and Bob that they will each receive a sealed box, with one containing a blue ball and one containing a red ball. After walking 1000 paces away from each other, they are allowed to open their box and determine their ball's colour. Alice opens her box and sees that it's red, and so Bob's ball must be blue. That's not instantaneous information transfer over 2000 paces worth of space, but plain old mutual exclusivity. The owner's ball colour was determined at the event of entanglement, not at the event where the box was opened. My assumption here lies in the fact that quantum states cannot be shared due to the Pauli exclusion principle, hence mutual exclusivity by default.

My understanding of quantum mechanics is pretty primitive, so I must repeat that this is a question, not a statement.
 
Last edited:
  • #14
Fredrik
Staff Emeritus
Science Advisor
Gold Member
10,851
406
No, it's definitely not like that shalyaka. If we assume that it is, we can use that assumption to prove mathematical relationships called Bell inequalities. Experiments have been performed to test them, and the results are consistent with quantum mechanics and contradict the Bell inequalities.
 
  • #15
Fredrik
Staff Emeritus
Science Advisor
Gold Member
10,851
406
GRB, are fundamental particles not completely rigid? If they aren't, how so?
There are no "completely rigid" objects of non-zero size in SR. It's fairly easy to see this by trying to figure out how such an object would look in two different inertial frames before and after a boost.

A classical point particle would be rigid (because its size is exactly zero in every frame), but the interactions between them can still only propagate at maximum speed c.
 
  • #16
28
0
There are no "completely rigid" objects of non-zero size in SR. It's fairly easy to see this by trying to figure out how such an object would look in two different inertial frames before and after a boost.

A classical point particle would be rigid (because its size is exactly zero in every frame), but the interactions between them can still only propagate at maximum speed c.
So particles being rigid is only permissible in SR because they have zero size?
 
  • #17
Fredrik
Staff Emeritus
Science Advisor
Gold Member
10,851
406
I'd rather say that the concept of "rigid" doesn't make sense for objects of zero size.

Also, note that I was talking about classical point particles. The concept of "size" is tricky when we're talking about quantum particles. The closest thing to a "size" they can have is the width of a certain function, and the shape of that function can't be rigid. (The argument is the same as for any object).
 
  • #18
104
0
Nothing Moves Faster than "C"

I seem to recall some sort of instant "communication" between pairs of electrons, such that some observed effect on one of the electrons causes an immediate repsonse by the other (maybe it was the spin)? This couldn't be used to transfer information, but it was a "faster than light speed" response between two particles, although I don't remember the details. Perhaps someone could respond with a correct explanation?

Wrong this transfer of information was at the speed of light, not faster than it. It happens whens photons (and possibly electrons) are ejected from the same atom and are sent across (in this case it was done at 6.2 miles apart) and yet they feel each other's energy states.

However this wasn't done faster than "c". Believe, it would have made tremendous headlines had it did.
 
  • #19
147
0
Although information can not be trasmitted faster then the speed of light I believe it can be recived from distances further then 300,000,000 m in less then 1 second. (You can not break the laws of physics, but you can work around them.)
 
  • #20
2,946
0
Let's say theres a stick, and two persons are holding the extremities:

Person A ----------stick---------- Person B

Both are pulling the stick with equal forces. At a certain moment, Person B releases it and instantly Person A knows it becase he can now pull the stick to himself.

Why can't I say that information in this case traveled faster than light? It seems to me that there is no delay between Person B releasing and Person A being able to notice it.

I know this can't be true, but I fail to find the answer.
Thanks in advance.
I recall a paper by Einstein on this subject. The idea is that there are no rigid bodies in existance. In your description above it appears that you're thinking of the rod as a rigid body.

Regarding information traveling faster than light (ftl). There have been some articles in the physics literature about superluminal signals which is accomplished through the use of entagled states (a quantum phenomena). I can find some references to these articles if you'd like?

Pete
 
  • #21
104
0
Quantum Phenomena

Regarding Quantum Phenomena - I read that a physicist named John Bell came up with experimental ideas back in the 1960's trying to test a photon phenomena knows as the " Phenomena of Non-Locality". "Non-locality" basically says that some observed effect on one of the photons causes an immediate response by the other (no matter how far the photons are separated away from each other).

In the early 1980's, scientists were able to create experiments testing this phenomena. Scientists were able to induce an atom to emit two photons simultaneously in opposite directions. The common origin of these photons meant that, according to quantum physics equations, the photons were correlated with one another and remained in essence "entangled" - even when they were far apart.

The results of the experiments showed that measuring the properties of one of the photons affected the measuring of the other photon across the lab room instantaneously. This was also done with an experiment where two photons were separated 6.2 miles apart with the same result.

The two photons acted like one particle even though there were more than six miles apart.
 
  • #22
104
0
Not Faster Than Speed of Light

In the early 1980's, scientists were able to create experiments testing this phenomena. Scientists were able to induce an atom to emit two photons simultaneously in opposite directions. The common origin of these photons meant that, according to quantum physics equations, the photons were correlated with one another and remained in essence "entangled" - even when they were far apart.

The results of the experiments showed that measuring the properties of one of the photons affected the measuring of the other photon across the lab room instantaneously. This was also done with an experiment where two photons were separated 6.2 miles apart with the same result.

The two photons acted like one particle even though there were more than six miles apart.

However realize, even though some influences link the two photons together instantaneously, no useful information travels between them faster than the speed of light. A physicist by the name of Charles Bennett pointed out in a research paper of his in 1993, that what was happening was a secret message was being sent in principle, from photon A to photon B and vice-versa. This message was sent in two parts, none of which made sense on its own, but one which involves quantum entanglement. The whole message traveled no faster than the speed of light because both halves were needed, yet the quantum entanglement could not be separated without changing the message.
 
  • #23
2,946
0
  • #24
7
0
it does seem instentanious. but all of your brains sensors relay info to you by electic signals. electricity is slower then light. there for, you would not sense the stick breaking faster then it would take light to tell you this if your brain could directly prosses light rather then from light into your eyes into electrical signals or even slower sound into electric signals.
 
  • #25
Let's say theres a stick, and two persons are holding the extremities:

Person A ----------stick---------- Person B

Both are pulling the stick with equal forces. At a certain moment, Person B releases it and instantly Person A knows it becase he can now pull the stick to himself.

Why can't I say that information in this case traveled faster than light? It seems to me that there is no delay between Person B releasing and Person A being able to notice it.

I know this can't be true, but I fail to find the answer.
Thanks in advance.
Relativistically, no - information transmission is bound by the speed of light and meaningful superluminal signals and action-at-a-distance are prohibited by the postulates of the Albert Einstein. Quantum Mechanically, possibly - if the interaction on end B is entangled with a detector on end A, then probably, by the postulates of John S. Bell (note that this is just a slight statistical bias and not deliberate communication). However, because sticks interact and so are measured electromagnetically, whether by charge-force equalizations (vibration, rigidity, displacement) or directly by the transmission of light, they require light to communicate transitions. A signal (such as an electron) that interacts with and describes electromagnetic phenomena (the movement of the stick) needs light quanta to scatter and transfer momentum to it (or be absorbed by it) to accelerate, and this will not occur unless the light can catch up with and also push the signal. However there are still more loopholes...general relativity may permit wormholes which also allow instantaneous teleportation across vast distances (this would require a pretty rigid stick :tongue:)...

have you ever thought about why it is that microwaves are 100% blocked by the door of the microwave even though it is 90% open (you can see through it)?
It is a Faraday cage: conduction electrons absorb incident radiation like a polarizer grating.

Say Alice and Bob share a pair of entangled photons. If Alice makes a measurement on her photon, that means that Bob will always find a certain thing when he measures his. However, he cannot tell without communication with Alice whether or not she did anything.
:yuck:
 
Last edited:

Related Threads for: Can information travel faster than light?

Replies
5
Views
4K
Replies
5
Views
3K
Replies
6
Views
864
  • Last Post
Replies
8
Views
5K
  • Last Post
Replies
13
Views
3K
Replies
7
Views
2K
  • Last Post
2
Replies
42
Views
7K
Top