# What happens if C is faster than the speed of light?

Hi all, I'm a basic novice at physics so please be gentle :)

As I understand it, C (from E = MC2) is the constant representing the maximum speed of information transmission in the theory of relativity. The Speed of Light in a vacume is atributed to C under the theory that, as light has no mass it has therefore the fastest possible attainable speed.
(Have I basically got that right?)

My query is this:-
Theoretically....
If there is an (undetected)energy that travels faster than the speed of light, therefore replacing C in E=MC2, would it effect the existing physics mathematics etc or would the existing math/theories be uneffected?

Related Special and General Relativity News on Phys.org
My understanding is we already know we can transmit data faster than the speed of light... But we just don't know where it is going. That stuff is tied to quantum physics of which I am but a baby. I believe those theories leave e=mc2 intact. Now could you transmit a photon using a "quantum" method? Light would travel faster than light. :)

It would certainly call for correction in SR and GR.

1.In SR&GR we calculate not distance, but spacetime time "distance", which is
ds^2=-c^2dt^2+dr^2, massive particles travel on time line intervals, what this means is that our speeds are always locally lower than c, hence
ds^2=-c^2dt^2+v^2dt^2
ds^2=(v^2-c^2)dt^2
So massive particles travel on spacetime intervals which are always complex numbers.

2.The photon travels on null lines
ds^2=(c^2-c^2)dt^2=0
Photons always travel on paths with 0 spacetime "length".
There really is no "reason" presented in SR&GR for photons to have this speed, it's more of an empirical thing

3.Any information, mass or wave that travels faster than c (locally), will violate casuality.
What is casuality? If I throw you a ball, and you catch it, everyone will agree on the order of the events. This is casuality, the cause is always observed to happen before the result.
However if I throw you a ball at faster than the speed of light, some observers will observe that you caught the ball before I threw it.

4.So far we have not yet observed any true faster than light travel, there are seemingly faster than light cases, but these can all be reconciled with the proper reasoning

@GarageDweller - now my curiosity is peaked. Many years ago when I read about the "successful" transmission of matter from one part of the room to another instantaneously, but with random destinations... I just did a search to find stuff on that. I noticed a term called, "entanglement". Does this mean the phenomenon I mention has been refined now to that term with the understanding that particles can "mirror" each other, but not exactly, and they aren't really travelling from one point to another without passing through the space between them?

This article seems to suggest mirroring photons could in fact allow the communication of information at many times the speed of light. I'm sure the devil is in the details, of which I do not understand. The answer must lie in your mention of the "observer".

http://blogs.discovermagazine.com/80beats/2008/08/13/entangled-particles-seem-to-communicate-instantly%E2%80%94and-befuddle-scientists/ [Broken]

The experiment suggests 10,000 times faster than light travel. This is not physically possible in our universe. It therefore supports the theory of our universe sitting atop a white hole singularity, by which everything is connected as part of our universal fabric.
Connection by a singularity is not faster than light time travel and no law has been broken,
entangled particles communicate via the singularity “short”.

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mfb
Mentor
Maybe things have changed in the last 15 years (lol) since I last read about moving a particle from one place to another faster than light. Sounds like it...
http://en.m.wikipedia.org/wiki/Quantum_teleportation
You cannot transfer any data (information you can work with) quicker than light with quantum teleportation.

the issue is that quantum entanglment does not deliver any information.
due to some physical reasons, if one photon takes state A, its engtangled photon will have to take B and vice versa. If the two photons are the same then it's a 50 50 game, hence you do not actually send any information this way

If the two entangled photons must take on a different state, couldn't you use that to transmit information? Or in your 50/50 comment are you saying that 50% of the time it will take on a different state and 50% of the time it will not?

Also, is a photon the only thing they can move/transmit? I thought 15 years ago they were able to do it with something other than a photon?

I found an article where it is claimed that a state has been transmitted in 2009. (maybe it's just a claim? or maybe it's just Fox news? lol)

They claim 90% accuracy.

It has previously been achieved between photons (a unit, or quantum, of electromagnetic radiation, such as light) over very large distances, between photons and ensembles of atoms, and between two nearby atoms through the intermediary action of a third.
None of those, however, provides a feasible means of holding and managing quantum information over long distances.
Now the JQI team, along with colleagues at the University of Michigan, has succeeded in teleporting a quantum state directly from one atom to another over a meter.

mfb
Mentor
If the two entangled photons must take on a different state, couldn't you use that to transmit information?
No, you cannot fix the state of a photon to represent your bit, otherwise you lose the entanglement.

Also, is a photon the only thing they can move/transmit?
Photons are nice for teleportation, they are quick, interact weakly with other stuff and can be transmitted via fibres. I would expect that it is possible to do similar things with electrons with shorter distances. Edit: Oh, you found an article.

Bill_K
Now the JQI team, along with colleagues at the University of Michigan, has succeeded in teleporting a quantum state directly from one atom to another over a meter.

atyy
Hi all, I'm a basic novice at physics so please be gentle :)

As I understand it, C (from E = MC2) is the constant representing the maximum speed of information transmission in the theory of relativity. The Speed of Light in a vacume is atributed to C under the theory that, as light has no mass it has therefore the fastest possible attainable speed.
(Have I basically got that right?)

My query is this:-
Theoretically....
If there is an (undetected)energy that travels faster than the speed of light, therefore replacing C in E=MC2, would it effect the existing physics mathematics etc or would the existing math/theories be uneffected?
If faster than light travel is detected, there are (at least) two different possibilities.

The first is that light is not massless, which would also mean that different frequencies of light travel at different speeds. In such a case, E=mc2 would still apply, and 'c' in the formula would remain a universal constant reflecting Lorentz symmetry, but 'c' in the formula would not also be the speed of light. The second possibility is that the Lorentz symmetry of our current laws of physics is not exact.

My follow on questions will further display my naivety on the subject. Dibs101, I hope the information generated from my questions isn't too far outside what you're looking for. At least we're still talking about the potential to transfer > C.

@mfb - Does this mean we cannot initiate the act of entanglement since we cannot set it? I mean if I can grab an arbitrary photon and cause entanglement and the 10km photon mirrors it.... with so many photons to choose from, why can't I just grab a series photons that represent the message I want to send. Not actually setting their state, just picking ones with the right state. (Although I don't know what a 'state' really means with respect to a photon) I would assume they have some complex state that allows you to differentiate a photon 10km away as being the mirror. Or is it a simple A/B state and when you cause entanglement on one, suddenly a photon 10km has a state change... and if you turn the switch on and off enough times you begin to realize you are the one turning the light on and off? Or do we really cause entanglement or just observe it happening at random?

?? Like I said, I'm a baby, I find it difficult to feed myself this type of information from reading without the ability to ask silly questions as it is so esoteric from my perspective.

OmCheeto
Gold Member
Hi all, I'm a basic novice at physics so please be gentle :)

As I understand it, C (from E = MC2) is the constant representing the maximum speed of information transmission in the theory of relativity. The Speed of Light in a vacume is atributed to C under the theory that, as light has no mass it has therefore the fastest possible attainable speed.
(Have I basically got that right?)

My query is this:-
Theoretically....
If there is an (undetected)energy that travels faster than the speed of light, therefore replacing C in E=MC2, would it effect the existing physics mathematics etc or would the existing math/theories be uneffected?
Good question. I asked some "goofy button" questions about 15 years ago, and got the following response:

munu said:
If photons have a small rest mass, they can no longer move at the speed we call "c". I know its confusing that in this situation "c" can no longer be described as the "velocity of light", but the situation is completely consistent and satisfactory, and is open to various experimental tests, which yield the limit of about 10-20 eV for the photon mass.
We bounced this idea around here in the "neutrinos faster than light" thread, but I think everything was deleted.

There is a link on John Baez's site that talks about required modifications to theories:

It is almost certainly impossible to do any experiment that would establish the photon rest mass to be exactly zero. The best we can hope to do is place limits on it. A non-zero rest mass would introduce a small damping factor in the inverse square Coulomb law of electrostatic forces. That means the electrostatic force would be weaker over very large distances.

Likewise, the behavior of static magnetic fields would be modified. An upper limit to the photon mass can be inferred through satellite measurements of planetary magnetic fields. The Charge Composition Explorer spacecraft was used to derive an upper limit of 6 × 10−16 eV with high certainty. This was slightly improved in 1998 by Roderic Lakes in a laboratory experiment that looked for anomalous forces on a Cavendish balance. The new limit is 7 × 10−17 eV. Studies of galactic magnetic fields suggest a much better limit of less than 3 × 10−27 eV, but there is some doubt about the validity of this method.
The way I interpret this, is that the smaller the rest mass a particle/wave has, the closer it can come to traveling at the "invariant" speed "c".

Caveat: I am a complete and utter layman in this topic. My opinion(s) should therefore be looked at with a great deal of suspicion.

bcrowell
Staff Emeritus
Gold Member
If you start from the assumption that spacetime is symmetric in certain sensible ways (in particular, symmetry between different frames of reference), then you get either special relativity or Galilean relativity. In SR, there is some invariant speed, which we call c. Galilean relativity corresponds to the case where c is infinite. None of this has anything to do with light. We have a FAQ about this:

Having established this, there is no reasonable way to have the speed of light be unequal to c, without being inconsistent with experiment.

You could have the speed of light not be a fixed number. This doesn't work, because of the null results of the Michelson-Morley experiment and searches for anomalous optical effects from double-star systems.

If you want the speed of light to be a fixed number, but not equal to the universal speed c, then it's going to depend on what frame you're in. But this violates symmetry between different frames of reference, which was one of our starting assumptions (and can be motivated, e.g., by the null result of Michelson-Morley).

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mfb
Mentor
@sday: To get entangled photons 10km away, you first have to locally produce entangled photons and send one of them to the other lab.
- If you already know their states at this point, there is nothing mysterious at all - it is like a regular data transmission.
- If you do not know their states, you can measure them in lab A, and be sure that lab B will measure the opposite result. This does not require any delay between the measurements. But you cannot transmit information in this way.
There is a good classical analogy in terms of information transfer: Write "1" and "0" on different sheets of paper, put them in different envelopes, mix them, send one to A and one to B. If A now opens one of the envelopes, it knows what B will measure. But A cannot transmit any data in this way. The quantum system has more features than the envelopes, so the analogy is not perfect, but I still like it.

Quantum teleportation now allows to transfer the state of a particle from A to B - including information which might be known to A. However, it requires a classical way to send data from A to B, so you do not get any FTL-communication here.

@mfb - So what makes the entangled photons.... entangled. I mean once you bring the entangled photon to the lab miles away, what can you do experimentally to prove they are entangled? What happens if you disturb one of them... or destroy one (convert it to something else)? Can you see that disturbance in any way on the other photon?

mfb
Mentor
what can you do experimentally to prove they are entangled?
With a single photon pair, you cannot be sure that entanglement worked. Without entanglement, you get the same measurement in 50% of the cases (just randomly one of two results), with perfect entanglement, you always get the opposite measurement. Every realistic entanglement experiment will get some number in between and can calculate the efficiency based on that.

What happens if you disturb one of them... or destroy one (convert it to something else)? Can you see that disturbance in any way on the other photon?
Depends on your interaction with the photon. A measurement is similar to a destruction. The measurement at B does not depend on your treatment of the photon at A in a unique way. It depends on the treatment and the measurement result obtained at A.

@mfb - I'm still confused. If measuring Photon A is like destruction, how do you know it was like B? Do you mean that once you measure A, you have a value now, say 123. Now the two are no longer entangled, but if you go back and measure B it is likely to also have a value of 123? And when you do that to enough pairs, more than 50% of them end up matching?

mfb
Mentor
Well, the usual measurement is polarisation. Let's say you measure "vertical" at A. In this case, B will always be horizontal. In a real setup, for vertical A you might measure something like 95% horizontal, which corresponds to 10% noise (=> 5% horizontal, 5% vertical).
The difference between the classical envelopes and entanglement is: You can measure in different directions, too. If you measure "\" versus "/", B is always the opposite of A, too. This shows that there is no (unknown) fixed polarisation of the photons, you influence them (both!) via the way you measure them.

If faster than light travel is detected, there are (at least) two different possibilities.

The first is that light is not massless, which would also mean that different frequencies of light travel at different speeds. In such a case, E=mc2 would still apply, and 'c' in the formula would remain a universal constant reflecting Lorentz symmetry, but 'c' in the formula would not also be the speed of light. The second possibility is that the Lorentz symmetry of our current laws of physics is not exact.
Thanks atyy(& OmCheeto),

I see now that my question would have been better formed by asking what would happen to the physics if light is not massless.

My question is:-
Suppose for example that light is not massless & has a velocity of 0.5C....would the universe as we understand it through our math, theories, laws & observations still remain generally true?

It would certainly call for correction in SR and GR.

3.Any information, mass or wave that travels faster than c (locally), will violate casuality.
What is casuality? If I throw you a ball, and you catch it, everyone will agree on the order of the events. This is casuality, the cause is always observed to happen before the result.
However if I throw you a ball at faster than the speed of light, some observers will observe that you caught the ball before I threw it.
I am supposing that c is faster than the speed of light. In this supposition, causality would not be violated.

Thank mfb for trying to explain it to me. I think it's a topic that would need a more real-time dialog for me to grasp what I'm missing. I don't have enough base knowledge to connect the dots.

mfb
Mentor
My question is:-
Suppose for example that light is not massless & has a velocity of 0.5C....would the universe as we understand it through our math, theories, laws & observations still remain generally true?
It would probably ruin quantum electrodynamics - this predicts a massless photon, and it might need major changes to get massive photons.
0.5 c relative to what? Once you want to slow light in some way, it cannot have the same speed everywhere.

There are experimental tests of the photon mass, based on http://pdglive.lbl.gov/popupblockdata.brl?nodein=S000M&inscript=Y&fsizein=1 [Broken], with 10-18 eV as best upper limit (which does not use any external assumptions about the galaxy). For all photons with an energy significantly larger than this value, the speed would be nearly the speed of light (in frames with this photon energy). Therefore, most photon mass measurements use low-frequency waves, as the mass is more important there.

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I think it's a topic that would need a more real-time dialog for me to grasp what I'm missing. I don't have enough base knowledge to connect the dots.
(my apology to the original poster since this reply is somewhat off-topic, I just want to reply to sday about entanglement)

Hi sday! In addition to what mfb said, here's an analogy to describe entanglement;

Imagine two coins, each rotating fast (the coins represent photons, and the rotation represents their indefinite state, in superposition). Further imagine that you just know that they are rotating, not exactly how they are rotating.

Now you (Alice) measure one of the coins, that is by stopping its rotation, forcing it into a definite state, which can either be heads or tails. You can not decide/influence the outcome of the measurement, all you can do is measure. Let's say you measure heads.

Later someone else (Bob) measure the other coin (and the same rules apply). This coin will be measured as tails. On the other hand, if the result of the first coin would be tails, the second coin would be heads. This is entanglement.

From this, the following applies:

1. Since you can't influence any outcome (you can only measure it), you can not use this to send any classical information between Alice and Bob.

2. Imagine you do the same procedure with many pairs of coins. You can not look at the measurements of only one party (Alice or Bob) to determine if the coins were entangled. Alice will always see her measurements as completely random. Bob will always see his measurements as completely random. But if all measurements are brought together and compared pair by pair, an entanglement can be discovered.

Note 1: The description above is only an analogy, the issues are of course more detailed than this (e.g. using photons and different polarizer settings). If you are interested, I suggest you read about Bell's theorem and Bell inequalities here and/or DrChinese's page here (he's a member on PF).

Note 2: You can also search for "entanglement" in the PF Quantum Physics subforum. There are MANY threads on this topic . You can also start your own thread about it there, if you like.

Note 3: Quantum teleportation is about teleportation of states of objects, not of objects themselves. Anyway, a classical information channel is needed to exploit it.

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