Debunking the Existence and Duration of Virtual Particles

[kith]

Thanks for your extensive comment, muppet.

So you seem to agree, that there is no clear distinction between real and virtual particles.

I don't really get the bottom line from the second part of your text. Maybe, I'll come back to it, if I have some more basic knowledge. I'll put Taylor on my list, but a 500-pages on nonrelativistic scattering theory alone, may exceed my reading capabilities. ;)

 

[muppet]

Thanks for your extensive comment, muppet.

So you seem to agree, that there is no clear distinction between real and virtual particles.

I don't really get the bottom line from the second part of your text. Maybe, I'll come back to it, if I have some more basic knowledge. I'll put Taylor on my list, but a 500-pages on nonrelativistic scattering theory alone, may exceed my reading capabilities. ;)

You mean the part about the leading order term in an expansion of the effective action in powers of hbar? You'll definitely come across that eventually. Read it as "quantum field theory reproduces classical field theory when Planck's constant goes to zero"

Happily, the discussion in Taylor I'm referring to is in chapter two; chapter one is mathematical preliminaries that I skipped and referred back to as and when needed. The full book is too much for my reading capabilities too- I picked it up as the treatment of scattering in the quantum field theory text by Peskin and Schroeder is based on 3 chapters of that book.

I suppose the best way to summarise my opinion on virtual particles is by analogy with ordinary NRQM.
If you have a simple harmonic oscillator potential, you expand your state in oscillators of different frequencies, and you'll be working with states that correspond exactly to the eigenstates of the system.
If you have a system "close to" a SHO, you can perturb around your idealised SHO; it's a good calculational scheme, and it gives you an intuition for the physics, but you're no longer talking about an exact correspondence.
If you have an arbitrary potential, you can still expand states in the SHO basis, as it's a perfectly good basis. But the calculations will be intractable and you'll have no understanding of the physical picture of the situation at all.

Free particles are the idealisation, analogous to the SHO. The virtual particles exchanged in scattering processes are exactly like perturbations around the SHO; they're an intutive way of understanding the result of poking a system you understand well, but you shouldn't do any rigorous philosophy treating them as entities in themselves. The confusion arises because what we think of as "states" and "potentials" in QFT are created using the same operators. But the "particle" states behave differently in the presence of interactions to our conception of what a particle should be, as they're no longer eigenstates of the Hamiltonian that actually describes the system. Any circumstance in which a state created by the action of a field behaves like we imagine a real particle should will be one in which we can neglect the effect of interactions upon the time evolution of that state. And situations exist where perturbation theory breaks down and the particle picture conveys almost no useful information, because we're no longer basing our approximation on a picture that describes the physics well.

As a totally unrelated aside, proofreading this post reminded me of an observation one of the permanent staff here in my university recently made to me:

I like the way you speak, in these closed sentences like something out of 'Yes Minister' that somehow eventually manage to end.

(For those unfamiliar with the reference: )
I hope it's not too verbose to be of some help.

 

[byron178]

The opening query of this post is the claim that virtual particles 'flat-out don't exist'. Hawking is a highly imaginative theorist, but his exploitation of flat-out non-existent particles is hardly comparable to that of exploiting the aether, or phlogiston.

The non-observation, incidentally, is not a falsification of Hawking evaporation proper. We need a positive observation of a radiation-signature of some process antithetical to Hawking evaporation: if x is happening, y cannot be happening.
_______

On, the other hand, I may have the cart before the horse: Hawking radiation would be as much evidence of virtual particles as Hawking BH-evaporation.

So its safe to say virtual particles don't exist in physical reality?also wouldn't they violate causality since they can travel faster than light?

 

[byron178]

I know virtual particles travel faster than light,and faster than light travel has to also deal with time travel backwards in time because from what i understand if a particle were to travel faster than light it will in one frame travel to the past,now do virtual particles travel backwards to the past from them traveling faster than light?

 

[Vanadium 50]

I know virtual particles travel faster than light

Well, they don't.

"It AIN'T so much the things we don't know that get us into trouble. It's the things we know that just ain't so."

 

[tiny-tim]

I know virtual particles travel faster than light …

no, https://www.physicsforums.com/library.php?do=view_item&itemid=287" in the coordinate-space representation (of a Feynman diagram) are on-mass-shell, so virtual photons travel at the speed of light, while virtual electrons travel at any and every speed slower than light

however, in the momentum-space representation, the virtual particles are both on- and off-mass-shell: both virtual photons and virtual electrons travel at all possible speeds

(of course, this is all just maths … virtual particles aren't real … the clue's in the name! )

and faster than light travel has to also deal with time travel backwards in time because from what i understand if a particle were to travel faster than light it will in one frame travel to the past

no, traveling is always forward in time

but a faster than light object traveling forward in time from A to B in one frame may be traveling forward in time from B to A in another frame

now do virtual particles travel backwards to the past from them traveling faster than light?

virtual particles don't actually exist, they don't have an A or B to travel between

i suspect you're thinking of the rule that an anti-particle (real or virtual) traveling forward in time can be thought of as a particle traveling backward in time

 

[ThomasT]

... a faster than light object traveling forward in time from A to B in one frame may be traveling forward in time from B to A in another frame

So whether an object is moving from A to B or from B to A depends solely on the observational frame of reference? I don't think so.

 

[yoron]

On the contrary one could argue that nonvirtual particles do not exist. Every particle is virtual since it is always en route from one interaction to the next.

Beautifully put. We put a lot of trust in 'propagation', don't we? And sometimes even seem to ignore indeterminacy (HUP), as being a principle of its own, instead referring to 'virtual particles' as if they were limited by 'time' to be unmeasurable. I used to like the idea of 'virtual particles' but?

I think I like indeterminacy more.

 

[tiny-tim]

So whether an object is moving from A to B or from B to A depends solely on the observational frame of reference? I don't think so.

think again! …

if we regard something as moving north at speed v faster than light, then an observer moving at speed slower than light but faster than c²/v will regard the same thing as moving south at a speed faster than light …

do the maths, and you'll agree ​

 

[byron178]

no, https://www.physicsforums.com/library.php?do=view_item&itemid=287" in the coordinate-space representation (of a Feynman diagram) are on-mass-shell, so virtual photons travel at the speed of light, while virtual electrons travel at any and every speed slower than light

however, in the momentum-space representation, the virtual particles are both on- and off-mass-shell: both virtual photons and virtual electrons travel at all possible speeds

(of course, this is all just maths … virtual particles aren't real … the clue's in the name! )no, traveling is always forward in time

but a faster than light object traveling forward in time from A to B in one frame may be traveling forward in time from B to A in another framevirtual particles don't actually exist, they don't have an A or B to travel between

i suspect you're thinking of the rule that an anti-particle (real or virtual) traveling forward in time can be thought of as a particle traveling backward in time

Doesn't relativity say that if matter or a particle were to travel faster than light it will in one frame of reference travel backwards in time? i also found this article maybe i misread it that says virtual particles travel faster than light. http://www.desy.de/user/projects/Physics/Quantum/virtual_particles.html

 

[tiny-tim]

Doesn't relativity say that if matter or a particle were to travel faster than light it will in one frame of reference travel backwards in time?

i don't really understand the concept of traveling backwards in time …

if an observer sees something moving, he sees it moving forward in time …

that's the way we see things …

why would an observer think that anything was traveling backward in time?

but yes, one observer may see an object traveling forward in time faster than light from A to B while another observer may see it traveling forward in time from B to A

(in other words: causality is not invariant )​

i also found this article maybe i misread it that says virtual particles travel faster than light. http://www.desy.de/user/projects/Physics/Quantum/virtual_particles.html

this usenet physics FAQ on https://www.physicsforums.com/library.php?do=view_item&itemid=287" is very misleading (it doesn't even consider whether they are simply a mathematical device) …

even the introductory section "What are virtual particles?" fails to answer its own question: after an initially promising overview, it (appropriately!) creates virtual particles out of nowhere without defining or describing them

as to faster-than-light, it says …

… the virtual photon's plane wave is seemingly created everywhere in space at once, and destroyed all at once. Therefore, the interaction can happen no matter how far the interacting particles are from each other.​

… from which it somehow gets to …

… the virtual photon can go from one interacting particle to the other faster than light!​

i'm not certain i understand what the first section means, and i am certain that the second section doesn't follow from it

as meoremuk says …

In my opinion, virtual particles are "made up" concepts.… Usually, the S-matrix is calculated from the Hamiltonian by using perturbation theory. These calculations involve a large number of rather complicated integrals.

In 1949 Feynman invented an ingenious technique of representing these integrals by diagrams. Each line and vertex in the diagram corresponded to a certain factor in the integrand. This technique enormously simplified manipulations with integrals of the perturbation theory.

The diagrams looked so nice that many people (including Feynman) started to use them to "explain" in layman terms what occurs in scattering events. Lines were interpreted as virtual particles that "move" between vertices, etc. etc. These explanations became so "sticky" that many people now believe that at large magnification a collision of electrons really looks like a web of virtual particles jumping back and forth. In my opinion, these beliefs have nothing to do with reality.

 

[sheaf]

In case it hasn't been posted before, http://profmattstrassler.com/articles-and-posts/particle-physics-basics/virtual-particles-what-are-they/" is quite informative on the subject of virtual particles.

 

[byron178]

i don't really understand the concept of traveling backwards in time …

if an observer sees something moving, he sees it moving forward in time …

that's the way we see things …

why would an observer think that anything was traveling backward in time?

but yes, one observer may see an object traveling forward in time faster than light from A to B while another observer may see it traveling forward in time from B to A

(in other words: causality is not invariant )​

this usenet physics FAQ on https://www.physicsforums.com/library.php?do=view_item&itemid=287" is very misleading (it doesn't even consider whether they are simply a mathematical device) …

even the introductory section "What are virtual particles?" fails to answer its own question: after an initially promising overview, it (appropriately!) creates virtual particles out of nowhere without defining or describing them

as to faster-than-light, it says …

… the virtual photon's plane wave is seemingly created everywhere in space at once, and destroyed all at once. Therefore, the interaction can happen no matter how far the interacting particles are from each other.​

… from which it somehow gets to …

… the virtual photon can go from one interacting particle to the other faster than light!​

i'm not certain i understand what the first section means, and i am certain that the second section doesn't follow from it

as meoremuk says …

So virtual particles travel faster than light but don't exist?

 

[Vanadium 50]

Byron, a number of people have noticed you keep posting the same question over and over and over and over and over...

Do you not understand the replies? If not, it would help us to know what you don't understand, or at least what your background in physics is.

 

[Constantin]

think again! …

if we regard something as moving north at speed v faster than light, then an observer moving at speed slower than light but faster than c²/v will regard the same thing as moving south at a speed faster than light …

do the maths, and you'll agree ​

I've been trying really hard to imagine this, but it seems I'm running out of brainpower.
I'm sure the maths are right, but can you help me imagine how this works ?

And if one observer sees the object moving north and another sees it moving south, then there should exist a third observer who sees the object as stationary, right ? But I still can't imagine it.

 

[byron178]

Byron, a number of people have noticed you keep posting the same question over and over and over and over and over...

Do you not understand the replies? If not, it would help us to know what you don't understand, or at least what your background in physics is.

I don't understand how virtual particles don't travel backwards in time when they travel faster than light.

 

[tiny-tim]

I'm sure the maths are right, but can you help me imagine how this works ?

See http://en.wikipedia.org/wiki/Velocity-addition_formula"

And if one observer sees the object moving north and another sees it moving south, then there should exist a third observer who sees the object as stationary, right ?

No, to see it as stationary, an observer would need to have the same velocity, v, ie also faster than light, wouldn't he?

But there would exist a third observer (with speed c²/v, of course, slower than light) who sees the object as moving infinitely fast!

So virtual particles travel faster than light …

byron, that's not true … read my https://www.physicsforums.com/showpost.php?p=3584038&postcount=256" again

 

[juanrga]

ive been reading on this forum that virtual particles flat out don't exist?then why is it said they exist for a certain amount of time?

Virtual particles are not detected as rest of particles. However, virtual particles appear in the equations of QFT and some people speak about their existence in this narrow sense.

In rigor, QFT does not say that «they exist for a certain amount of time», because «spacetime» in QFT has only a formal meaning.

 

[DevilsAvocado]

think again! …

if we regard something as moving north at speed v faster than light, then an observer moving at speed slower than light but faster than c²/v will regard the same thing as moving south at a speed faster than light …

This is probably one of the most astonishing claims I’ve seen in a while... are you sure this is not a 'mix-up' with RoS...??

do the maths, and you'll agree ​

Eh well, maybe, but let’s skip the advanced math for awhile, and use our common sense and pictures to begin with:

• We stand on the South Pole together with Alice and her brand new superluminal spaceship.
  
  
• Alice has decided to set a new speed record, traveling to the North Pole.
  
  
• Bob is flying his old "c²/v wreck" to inspect the event at the equator.
  
  
• Alice takes off at superluminal speed in direction towards the North Pole.
  
  
• Bob is dropping his jaw, because according to you; he will see Alice taking off at the North Pole, in direction towards the South Pole!

Now the question I have for you, my friend: – If Bob now travels with his old wreck to the South Pole to talk to Alice about what went wrong, where is Alice actually located, the North Pole or the South Pole?

...

 

[Fredrik]

This is probably one of the most astonishing claims I’ve seen in a while... are you sure this is not a 'mix-up' with RoS...??
...
Eh well, maybe, but let’s skip the advanced math for awhile, and use our common sense and pictures to begin with:

I'll use pictures, but I won't use common sense. Think about it in terms of spacetime diagrams. Let's make north the positive x direction. Imagine a tachyon gun that's fired at event A with x coordinate 0. The beam moving at speed v>1 hits the target at event B with x coordinate L>0. Now imagine the simultaneity lines of an observer moving at speed u in the positive x direction. (It has to be in the positive x direction). The larger the u, the more the simultaneity lines get "tilted" toward the line x=t representing light speed. The slope of the simultaneity lines can get arbitrarily close to the slope of the line x=t. So clearly, if u is large enough, there will be a simultaneity line that is "below" A and "above" B, meaning that in that guy's (comoving inertial) coordinate system, B is the earlier event.

Since the time axis is drawn in the "up" direction in a spacetime diagram, the world line of an object with speed u have slope dt/dx=1/u. If that object is the spaceship containing the observer, then its speed is <1 and its simultaneity lines make the same angle with the x-axis as its world line makes with the t axis. So the slope of the simultaneity lines is 1/u.

• Bob is dropping his jaw, because according to you; he will see Alice taking off at the North Pole, in direction towards the South Pole!

Now the question I have for you, my friend: – If Bob now travels with his old wreck to the South Pole to talk to Alice about what went wrong, where is Alice actually located, the North Pole or the South Pole?

That's an interesting question. The simplest possibility is that Alice doesn't exist anymore. In the south pole's comoving inertial frame, Alice (the tachyon) is created at A and destroyed at B. In Bob's (original) comoving inertial frame, Alice is created at B and destroyed at A. So A is an emission event in some coordinate systems and an absorption event in some coordinate systems. The same goes for B of course. When Bob has landed at the south pole, there is no Alice, at no point in "space" as defined by his new comoving inertial system, which is of course the one comoving with the south pole.

Suppose that the people at the south pole create Alice at an earlier time, and have "her" bounce back and forth between two tachyon mirrors until one of the mirrors is removed at event A. Suppose also that the people at the north pole are equipped to "catch" Alice at event B, and then have her bounce back and forth between two tachyon mirrors. In that case, the answer to your question must be that Alice is at the north pole.

On the other hand, if that scenario is possible, then it must also be possible to set things up so that from the point of view of Bob's original comoving frame, Alice is kept bouncing back and forth between tachyon mirrors at the north pole for some time before event B, where she is released, and later caught at event A, where she ends up bouncing back and forth between tachyon mirrors at the south pole. If this is how things were set up, then the answer to your question must be that Alice is at the south pole.

 

[DevilsAvocado]

I'll use pictures, but I won't use common sense.

Phew, that’s a big relief!

()

Think about it in terms of spacetime diagrams. Let's make north the positive x direction. Imagine a tachyon gun that's fired at event A with x coordinate 0. The beam moving at speed v>1 hits the target at event B with x coordinate L>0. Now imagine the simultaneity lines of an observer moving at speed u in the positive x direction. (It has to be in the positive x direction). The larger the u, the more the simultaneity lines get "tilted" toward the line x=t representing light speed. The slope of the simultaneity lines can get arbitrarily close to the slope of the line x=t. So clearly, if u is large enough, there will be a simultaneity line that is "below" A and "above" B, meaning that in that guy's (comoving inertial) coordinate system, B is the earlier event.

Since the time axis is drawn in the "up" direction in a spacetime diagram, the world line of an object with speed u have slope dt/dx=1/u. If that object is the spaceship containing the observer, then its speed is <1 and its simultaneity lines make the same angle with the x-axis as its world line makes with the t axis. So the slope of the simultaneity lines is 1/u.

Phew²... I think the 'magic' word here is tachyon, a hypothetical subatomic particle... I have absolutely no problem with that, and we can easily use 'common sense' to understand that there is nothing strange at all seeing a superluminal tachyon arriving at the end point, before we see it leaving the starting point. A no-brainer!

Also we can thank "the old one" for that macroscopic objects like Alice and her new spaceship is not allowed to travel faster than light, according to Einstein... Bob would have to spend all his free time at the dentist, repairing his jaw...

If I may become a little bit 'picky'; I would like to add that we would probably be able to distinguish the "real Alice" from her "mirage-redshifted-image", and by this we would be able to conclude that she is actually moving in the direction from the South Pole to the North Pole...

http://en.wikipedia.org/wiki/Tachyon"

Because a http://en.wikipedia.org/wiki/Tachyon" always moves faster than light, we cannot see it approaching. After a tachyon has passed nearby, we would be able to see two images of it, appearing and departing in opposite directions. The black line is the shock wave of Cherenkov radiation, shown only in one moment of time. This double image effect is most prominent for an observer located directly in the path of a superluminal object (in this example a sphere, shown in grey). The right hand bluish shape is the image formed by the blue-doppler shifted light arriving at the observer—who is located at the apex of the black Cherenkov lines—from the sphere as it approaches. The left-hand reddish image is formed from redshifted light that leaves the sphere after it passes the observer. Because the object arrives before the light, the observer sees nothing until the sphere starts to pass the observer, after which the image-as-seen-by-the-observer splits into two—one of the arriving sphere (to the right) and one of the departing sphere (to the left).

That's an interesting question. The simplest possibility is that Alice doesn't exist anymore.

I hear you; my brain has visited this "place" several times, reading thru this thread...

The serious (and perhaps simplest) answer is that Alice would cease to exist long before going > c. She have to accumulate infinite energy and would in this process acquire infinite mass = squashed in the heaviest 'black hole' of all times...

the answer to your question must be that Alice is at the north pole.
...
the answer to your question must be that Alice is at the south pole.

Now we’re talking! Thanks buddy!

()


P.S. If 'anyone' wishes to ask the question; "Does this mean that we can travel backwards in time?" the answer is as always NO.

 

[tiny-tim]

… let’s skip the advanced math for awhile…

it's not advanced, it's very elementary math … see the link

…, and use our common sense …

ok, and your common sense should tell you that Alice cannot go faster than light …

any argument based on the "common sense" concept of tachyonic Alice ageing is founded on nonsense, not common sense

(how would you apply the same "common sense" argument to a tachyonic particle without a concept of ageing? )

 

[DevilsAvocado]

it's not advanced, it's very elementary math … see the link


ok, and your common sense should tell you that Alice cannot go faster than light …

any argument based on the "common sense" concept of tachyonic Alice ageing is founded on nonsense, not common sense

(how would you apply the same "common sense" argument to a tachyonic particle without a concept of ageing? )

With all due respect, I think you missed the main point:

where is Alice actually located, the North Pole or the South Pole?

Please feel free to exchange "Alice" to anything that pleases you; "object", "something", tachyon, etc. The important question here is; how one "FTL object" can be detected/observed at two different end points, depending on the frame of reference?

I have never heard anything like it, and I would appreciate it very much if you could (at least in principle) describe how this works (i.e. more than "do the maths").

If you still refers to "do the maths" and this http://en.wikipedia.org/wiki/Velocity-addition_formula#Special_theory_of_relativity", it looks to me that you may not have the complete answer for this very simple question.

(Don’t worry about the conversation between me & Fredrik, there are some 'silly jokes' there, just for fun nothing more.)

 

[tiny-tim]

… where is Alice actually located, the North Pole or the South Pole?

Please feel free to exchange "Alice" to anything that pleases you; "object", "something", tachyon, etc. The important question here is; how one "FTL object" can be detected/observed at two different end points, depending on the frame of reference?

i'm sorry, i don't understand what the question is, nor what the difficulty is …

can you spell it out, please? ​

 

[tiny-tim]

hmm … how about i ask you a question instead …

i shine a laser beam at the the moon, so that the spot travels across the moon's surface faster than light (that's very easy! ) …

i say it travels from A to B faster than light, another observer says it travels from B to A faster than light …

where's the difficulty? ​

 

[DevilsAvocado]

i'm sorry, i don't understand what the question is, nor what the difficulty is …

can you spell it out, please? ​

Okay, your own words, my bolding:

if we regard something as moving north at speed v faster than light, then an observer moving at speed slower than light but faster than c²/v will regard the same thing as moving south at a speed faster than light …

Problem: I’m aware of Relativity of Simultaneity, but I have never heard of "Relativity of Direction".

Question: If "something" is moving north, let’s say from the South Pole to the North Pole, and another observer is experiencing this "something" as if moving south, i.e. from the North Pole to the South Pole. Where will this "something" finally end up, the South Pole or the North Pole?

Or to keep it simple: One object can’t be in two places at once.

 

[Constantin]

Problem: I’m aware of Relativity of Simultaneity, but I have never heard of "Relativity of Direction".

If you start moving faster than a certain object, it will appear reversing direction.
I'm not saying it's about moving faster than Alice in this case though, just that your problem is very simple.

One object can’t be in two places at once.

"Two places at once" is wrong. It will be in two different places for two different observers. Or for the same observer after he changes his motion. That's normal and very simple as well.

 

[tiny-tim]

Question: If "something" is moving north, let’s say from the South Pole to the North Pole, and another observer is experiencing this "something" as if moving south, i.e. from the North Pole to the South Pole. Where will this "something" finally end up, the South Pole or the North Pole?

yes!

one observer says it ends up at the South Pole, another observer says it ends up at the North Pole …

where's the difficulty? ​

Or to keep it simple: One object can’t be in two places at once.

nobody's saying it is

 

[DevilsAvocado]

one observer says it ends up at the South Pole, another observer says it ends up at the North Pole

Great, now we’re almost there. Instead of "saying" that it ends up at the South Pole and the North Pole, both observers travels to "their Pole" to check their measurement apparatus.

What will they find?

 

[tiny-tim]

Great, now we’re almost there. Instead of "saying" that it ends up at the South Pole and the North Pole, both observers travels to "their Pole" to check their measurement apparatus.

What will they find?

you mean one observer (with the given velocity) times his start so as to reach A when the spot of light does, and the other observer (with the other given velocity) times his start so as to reach B when the spot of light does?

then the first one will find he's just in time to catch the spotlight before it turns off, and so will the second one!

 

[DevilsAvocado]

you mean one observer (with the given velocity) times his start so as to reach A when the spot of light does, and the other observer (with the other given velocity) times his start so as to reach B when the spot of light does?

then the first one will find he's just in time to catch the spotlight before it turns off, and so will the second one!

ehhhhh "catch the spotlight"...

Look, I don’t know what’s going here... but it’s clearly a waste of time...

Take care and I wish you luck as Science Advisor.

 

[Constantin]

I agree such thought experiments are not easy to understand. I didn't understand it either at the beginning. But the explanations were quite good and helpful.

 

[Fredrik]

It's interesting to examine some of the details of a version of the laser pointer thought experiment that tiny-tim suggested. Suppose that a laser pointer shines at a fixed point on the moon for a while, and is then rotated so that the red dot on the moon moves faster than light to a new location. Suppose that the laser pointer is then constrained to keep shining at that point for some time. Let's call the event where the red dot begins to move A, and the event where it stops moving B. How would these things be described in an inertial system in which B occurs before A?

At first there's one dot, at the location on the moon where event A will occur at a later time. After a while, a new dot appears at event B, and immediately splits in two. So now there are three dots. One of the two new ones stays at the location on the moon where event B occured. The other one moves at a superluminal speed toward the other location. When it merges with the red dot that's already there, both of them disappear. So now there's only one red dot again.

This is all easy to see in a spacetime diagram.

 

[tiny-tim]

… So now there are three dots. One of the two new ones stays at the location on the moon where event B occured. The other one moves at a superluminal speed toward the other location. When it merges with the red dot that's already there, both of them disappear. So now there's only one red dot again.

oooh, nice, Fredrik!

 

[byron178]

hmm … how about i ask you a question instead …

i shine a laser beam at the the moon, so that the spot travels across the moon's surface faster than light (that's very easy! ) …

i say it travels from A to B faster than light, another observer says it travels from B to A faster than light …

where's the difficulty? ​

so the laser "time traveled backwards in time".and is it really time travel?doesn't this also violate causality? i also got this off of wiki.If a laser beam is swept quickly across a distant object, the spot of light can move faster than c, although the initial movement of the spot is delayed because of the time it takes light to get to the distant object at the speed c. However, the only physical entities that are moving are the laser and its emitted light, which travels at the speed c from the laser to the various positions of the spot. Similarly, a shadow projected onto a distant object can be made to move faster than c, after a delay in time.[38] In neither case does any matter, energy, or information travel faster than light.[39]

 

[DevilsAvocado]

It's interesting to examine some of the details of a version of the laser pointer thought experiment that tiny-tim suggested. Suppose that a laser pointer shines at a fixed point on the moon for a while, and is then rotated so that the red dot on the moon moves faster than light to a new location. Suppose that the laser pointer is then constrained to keep shining at that point for some time. Let's call the event where the red dot begins to move A, and the event where it stops moving B. How would these things be described in an inertial system in which B occurs before A?

At first there's one dot, at the location on the moon where event A will occur at a later time. After a while, a new dot appears at event B, and immediately splits in two. So now there are three dots. One of the two new ones stays at the location on the moon where event B occured. The other one moves at a superluminal speed toward the other location. When it merges with the red dot that's already there, both of them disappear. So now there's only one red dot again.

This is all easy to see in a spacetime diagram.

But Fredrik, moving laser dots and shadows don’t qualify for FTL, they are not physical objects and you cannot use it to send information FTL. If it was, any freak with a laser pointer would qualify for an instant Nobel Prize in Physics (probably why tiny-tim is so fond of it ).

My 'reaction' is due to the fact that the 'casual reader' could easily get the impression this "superluminal-dual-reversed-direction" is some form of empirical fact, at the same level as the very real consequences of Relativity of Simultaneity and other effects in SR & GR. All you have to do is "think again" and "do the maths".

This is wrong.

And I think anyone following these latest posts understands that the one calling out for others to "think again", hasn’t done so himself.

The problem, as I see it, is that we are talking about purely hypothetical features as some form of "no-brainer-every-day-experience". And as we have seen from latest posts – you run into hilarious paradoxes when trying to 'implement' this in a real world with real physical objects, in exactly the same way as you run into the unsolvable http://en.wikipedia.org/wiki/Grandfather_paradox" when claiming that Special and General Relativity might allow time travel.

I have 'reconstructed' my Gedankenexperiment to close all "spotlight-loopholes" and any other "time-consuming-uncertainties" into a bulletproof and somewhat 'brutal' version...

Superluminal Gedankenexperiment II

Adolf is very a cruel dictator that hates science, and most of all he hates the famous physicists Albert & Niels.

Adolf has stolen a brand new weapon from the defeated enemy; the so-called Titanium Tachyon Bullet (TTB) which is superluminal and very lethal, and always hits its target. Adolf has planned the execution to be as degrading as possible for Albert & Niels, and for the other captured physicists who are forced to inspect the event in different frame of reference.

This is what Adolf has set up:

• Albert is placed to the north, and Niels is placed to the south.
• The inspecting physicists are moving between Albert & Niels, in different frame of reference.
• Adolf is now firing the Titanium Tachyon Bullet in secret, from either north or south.
• When the TTB has traveled halfway, it’s spotted by the physicists.
• Some of them will see the TTB going north to kill Albert.
• Some of them will see the TTB going south to kill Niels.

One Titanium Tachyon Bullet going in two directions north/south

At this point we have 4 possible future options, which could happen in this kind of hypothetical superluminal reality:

• Albert is hit and killed, Niels survives.
• Niels is hit and killed, Albert survives.
• Both Albert and Niels is hit and killed.
• Both Albert and Niels survives.

​

And as we all can see, none of these options are logical satisfying – it’s a superluminal paradox!

(Personally I would love if both Albert & Niels were still alive, of course!)

 

[tiny-tim]

But Fredrik, moving laser dots and shadows , they are not physical objects and you cannot use it to send information FTL.

correct

… don’t qualify for FTL …

of course they qualify for FTL!

any tachyon (your bullet, for example) follows a curve in space-time, and a moving laser dot can be made to follow the same curve …

so we can unambiguously (and beyond criticism) study the laser dot, and be sure that any description of its course in any frame applies also to the tachyon!

you agree? ​

My 'reaction' is due to the fact that the 'casual reader' could easily get the impression this "superluminal-dual-reversed-direction" is some form of empirical fact, at the same level as the very real consequences of Relativity of Simultaneity and other effects in SR & GR.

dunno what you mean by "empirical"

but Frederik and I are simply pointing out the easy-to-calculate description of particular superluminal paths in the frames of different observers

have you checked the maths??​

This is what Adolf has set up:

• Albert is placed to the north, and Niels is placed to the south.
• The inspecting physicists are moving between Albert & Niels, in different frame of reference.
• Adolf is now firing the Titanium Tachyon Bullet in secret, from either north or south.
• When the TTB has traveled halfway, it’s spotted by the physicists.
• Some of them will see the TTB going north to kill Albert.
• Some of them will see the TTB going south to kill Niels.

is Adolf between them?

if so, you're wrong …

• Some of them will see the TTB going north to kill Albert.
• Some of them will see the TTB going south to kill Adolf.

and if the physicists had spotted it earlier, they would have seen it kill all three of them!

where's the difficulty? ​

 

[Fredrik]

But Fredrik, moving laser dots and shadows don’t qualify for FTL, they are not physical objects and you cannot use it to send information FTL.

Of course you can't use them to send information, but a) they're still moving faster than light, so it's certainly FTL, and b) we haven't been talking about sending information; we have only been talking about "objects" moving FTL.

My 'reaction' is due to the fact that the 'casual reader' could easily get the impression this "superluminal-dual-reversed-direction" is some form of empirical fact, at the same level as the very real consequences of Relativity of Simultaneity and other effects in SR & GR. All you have to do is "think again" and "do the maths".

This is wrong.

What do you mean it's wrong? It's predicted by the same part of the same theory that predicts time dilation, and it is relativity of simultaneity.

The problem, as I see it, is that we are talking about purely hypothetical features as some form of "no-brainer-every-day-experience".

We're just talking about what the theory predicts, so there's no problem. From a theorist's point of view, we're just talking about straight lines in \mathbb R^4.

And as we have seen from latest posts – you run into hilarious paradoxes when trying to 'implement' this in a real world with real physical objects, in exactly the same way as you run into the unsolvable http://en.wikipedia.org/wiki/Grandfather_paradox" when claiming that Special and General Relativity might allow time travel.

Real physical objects moving FTL? What does that mean? SR associates positive mass only with timelike curves. Particles moving as described by spacelike curves would by definition have m²<0. So if your definition of "real physical object" includes a mass that isn't an imaginary number, then your real physical FTL objects immediately contradict SR.

the so-called Titanium Tachyon Bullet (TTB) which is superluminal and very lethal,

Titanium consists of atoms with mass m>0, so if you want to discuss titanium moving FTL, you will have to invent a new theory in which m>0 particles can move FTL. But for the sake of the discussion, I'll pretend that you said "unobtainium" rather than titanium, so that we can at least try to figure out what SR says about your scenario.

• Adolf is now firing the Titanium Tachyon Bullet in secret, from either north or south.
• When the TTB has traveled halfway, it’s spotted by the physicists.
• Some of them will see the TTB going north to kill Albert.
• Some of them will see the TTB going south to kill Niels.

First of all, you can't "fire" this bullet. It's either moving >c at all times or <c at all times. But we can imagine a gun that produces a TTB, so I'll do that. If some of the observers see this TTB going north to kill Albert, the others will see it going south to enter the barrel of the gun and be destroyed there, right after Adolf pulls the trigger.

And as we all can see, none of these options are logical satisfying – it’s a superluminal paradox!

Actually, it's just a poorly specified scenario (sorry). But the picture of the bullets was nice. I suggest that you draw a spacetime diagram for the next scenario you want to think about. If the diagram can't be drawn, the scenario doesn't make sense (in SR).

 

[byron178]

Of course you can't use them to send information, but a) they're still moving faster than light, so it's certainly FTL, and b) we haven't been talking about sending information; we have only been talking about "objects" moving FTL.


What do you mean it's wrong? It's predicted by the same part of the same theory that predicts time dilation, and it is relativity of simultaneity.


We're just talking about what the theory predicts, so there's no problem. From a theorist's point of view, we're just talking about straight lines in \mathbb R^4.


Real physical objects moving FTL? What does that mean? SR associates positive mass only with timelike curves. Particles moving as described by spacelike curves would by definition have m²<0. So if your definition of "real physical object" includes a mass that isn't an imaginary number, then your real physical FTL objects moving FTL immediately contradict SR.


Titanium consists of atoms with mass m>0, so if you want to discuss titanium moving FTL, you will have to invent a new theory in which m>0 particles can move FTL. But for the sake of the discussion, I'll pretend that you said "unobtainium" rather than titanium, so that we can at least try to figure out what SR says about your scenario.


First of all, you can't "fire" this bullet. It's either moving >c at all times or <c at all times. But we can imagine a gun that produces a TTB, so I'll do that. If some of the observers see this TTB going north to kill Albert, the others will see it going south to enter the barrel of the gun and be destroyed there, right after Adolf pulls the trigger.


Actually, it's just a poorly specified scenario (sorry). But the picture of the bullets was nice. I suggest that you draw a spacetime diagram for the next scenario you want to think about. If the diagram can't be drawn, the scenario doesn't make sense (in SR).

can someone help me with why a group or phase velocity can exceed the speed of light?

 

[DevilsAvocado]

But we can imagine a gun that produces a TTB, so I'll do that.

Thankee, I was about to give up...

If some of the observers see this TTB going north to kill Albert, the others will see it going south to enter the barrel of the gun and be destroyed there, right after Adolf pulls the trigger.

Aha! Gotcha!

Serious Fredrik, I’m not running for someone’s "scalp" here, I’m just here to learn and hopefully others will do the same, in the 'process'.

Now to the point, as we can see from your 'acknowledgment' above; there’s one TTB performing two tasks at two separate locations in space... it doesn’t matter that the experiment 'goofed'. This was the main point I wanted to draw attention to.

And I hope you admit there’s something 'strange' going on here... to say at least...

AFAICT, this doesn’t work according to current knowledge.

So, AFAICT, the only way to have an "object" moving at FTL in two different directions, at once, is to treat "it" as 'unreal' or virtual, and then we’re back to the OP question, thank god!

Thanks for taking the time; (I think) I learned something new today!

 

[DevilsAvocado]

where's the difficulty? ​

No worries mate, everything is cleared up.

 

[Fredrik]

Now to the point, as we can see from your 'acknowledgment' above; there’s one TTB performing two tasks at two separate locations in space... it doesn’t matter that the experiment 'goofed'. This was the main point I wanted to draw attention to.

If the TTB is created when the gun fires and destroyed when it interacts with Albert's body, its world line is a single spacelike straight line segment. In no coordinate system is it in two places at once. On the other hand, if we use the kind of setup I described earlier, where the TTB is kept bouncing between two tachyon mirrors and then released to travel to a different location where it's trapped bouncing between two tachyon mirrors again, then in some coordinate systems, the number of TTBs changes from 1 to 3 and then back to 1 again. (Very easy to see in a spacetime diagram). And...hehe...I just realized something cool when I visualized that spacetime diagram in my head. We can change that to "the number of TTBs changes from 1 to N and then back to 1 again", for any odd integer N≥3 by making the TTB and the observer fast enough. (Speed →∞ for the TTB and speed →c for the observer).

And I hope you admit there’s something 'strange' going on here... to say at least...

Yes, but these are still just easily derived consequences on SR. If you want to see some really strange (self-contradictory) consequences of FTL messages in SR, check out this post. (It contains a typo that's corrected in #138).

This thought experiment looks like proof by contradiction that FTL messages can't exist in a SR universe, but the argument actually has loopholes. For example, if the time it takes to emit and/or detect a tachyon grows at least linearly with the distance it travels, there's nothing self-contradictory about this setup. OK, but it still rules out the possibility of tachyons that can be emitted and detected quickly, right? Wrong. The correct conclusion (a long discussion with JesseM in another thread helped me see this) is that there's no theory of matter in Minkowski spacetime such that its equations of motion have solutions that describe experiments similar to this one. There can still be a theory of tachyonic matter interacting with normal matter in Minkowski spacetime, but if it has a solution where someone begins to build this setup, something will happen that prevents the experiment from being carried out. The possibilities include such things as equipment malfunction, the Earth getting destroyed by a comet that's been on a collision course with us since it was formed billions of years ago, or the experimenter simply choosing not to go through with it.

I'm not sure what to make of this counterargument. I have a feeling that it's possible to come up with some good counter-counterarguments, but I haven't found one that really works.

 

[byron178]

If the TTB is created when the gun fires and destroyed when it interacts with Albert's body, its world line is a single spacelike straight line segment. In no coordinate system is it in two places at once. On the other hand, if we use the kind of setup I described earlier, where the TTB is kept bouncing between two tachyon mirrors and then released to travel to a different location where it's trapped bouncing between two tachyon mirrors again, then in some coordinate systems, the number of TTBs changes from 1 to 3 and then back to 1 again. (Very easy to see in a spacetime diagram). And...hehe...I just realized something cool when I visualized that spacetime diagram in my head. We can change that to "the number of TTBs changes from 1 to N and then back to 1 again", for any odd integer N≥3 by making the TTB and the observer fast enough. (Speed →∞ for the TTB and speed →c for the observer).


Yes, but these are still just easily derived consequences on SR. If you want to see some really strange (self-contradictory) consequences of FTL messages in SR, check out this post. (It contains a typo that's corrected in #138).

This thought experiment looks like proof by contradiction that FTL messages can't exist in a SR universe, but the argument actually has loopholes. For example, if the time it takes to emit and/or detect a tachyon grows at least linearly with the distance it travels, there's nothing self-contradictory about this setup. OK, but it still rules out the possibility of tachyons that can be emitted and detected quickly, right? Wrong. The correct conclusion (a long discussion with JesseM in another thread helped me see this) is that there's no theory of matter in Minkowski spacetime such that its equations of motion have solutions that describe experiments similar to this one. There can still be a theory of tachyonic matter interacting with normal matter in Minkowski spacetime, but if it has a solution where someone begins to build this setup, something will happen that prevents the experiment from being carried out. The possibilities include such things as equipment malfunction, the Earth getting destroyed by a comet that's been on a collision course with us since it was formed billions of years ago, or the experimenter simply choosing not to go through with it.

I'm not sure what to make of this counterargument. I have a feeling that it's possible to come up with some good counter-counterarguments, but I haven't found one that really works.

Fredrik,What moves backwards in time with the group velocity?

 

[DevilsAvocado]

... I'm not sure what to make of this counterargument. I have a feeling that it's possible to come up with some good counter-counterarguments, but I haven't found one that really works.

Thanks a lot Fredrik for your ideas and comments, very interesting. I must read every post in the thread you provided.

*So little time so much to do*

My first thought regarding "counterarguments" is the http://en.wikipedia.org/wiki/Novikov_self-consistency_principle" to solve the problem of paradoxes in time travel. Maybe it’s possible to impose this on the TTB... (my feeling is that you had a track along these lines).

My natural feeling (for what it’s worth) is that it just feels 'unnatural'... Ockham's razor'ish...

[And now I take one step back]

I woke up with an 'epiphany' this morning! Now I know what went 'wrong' when we discussed moving laser dots, shadows and FTL "objects". I hope I can explain it easily:

The main 'epiphany point' is that moving laser dots and shadows are events happening in spacetime, not "objects" moving in spacetime. And suddenly everything makes perfect sense, and we can even use RoS (at ≤ c) to explain it all. Heck! We can even use Einstein's old train thought experiment from 1917 to visualize it, and it works like a dream!

"[URL for large 1024x1577 picture

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In this experiment, lightning simultaneous strikes both train ends a "time 0", i.e. if we were to implement this with a laser pointer, it would mean 'infinite' speed... But we could 'easily' () move this experiment to the surface of the moon, and at first put the laser spot at one end of the train, and then move it to the other end. This will cause just a slight difference to Einstein's 1917 experiment (time of events), but otherwise – it’s exactly the same thing!

... I hope you agree ... :shy:

 

[byron178]

I need some clairafacation,do virtual particles time travel backwards in time when they travel faster than light,because relativity says that if a particle were to travel faster than light then in one frame it will travel backwards in time,but I've been some research and have not heard of virtual particles going backwards in time or coming from future to present.

 

[tiny-tim]

I need some clairafacation,do virtual particles time travel backwards in time when they travel faster than light,because relativity says that if a particle were to travel faster than light then in one frame it will travel backwards in time,but I've been some research and have not heard of virtual particles going backwards in time or coming from future to present.

hi byron178!

(i'm sorry no-one's answered you for some time )

only some virtual particles (in the momentum representation only) travel faster than light (virtual particles have all possible speeds, both slower and faster than light)

a virtual particle has no "time of its own" (unlike us, say), so we prefer to say that a virtual particle is exchanged between A and B (rather than going from A to B or vice versa)

anything traveling faster than light breaks causality, in that some observers say it moves from A to B, while others say it moves from B to A

if it had a well-defined "direction of ageing" (a human for example starts looking like a baby, and finishes looking like an old person), then some observers would say it was ageing backwards, ie it was going backwards in its own time while of course going forward in the observer's time

but no material can travel faster than light, so there isn't a well-defined "direction of ageing", and every observer simply says "it's going forward in my time, and it doesn't appear to have any time of its own"

(of course, virtual particles are just mathematical artefacts that help in the calculations, so this is a bit like discussing how many angels can dance on the head of a pin )

 

[byron178]

hi byron178!

(i'm sorry no-one's answered you for some time )

only some virtual particles (in the momentum representation only) travel faster than light (virtual particles have all possible speeds, both slower and faster than light)

a virtual particle has no "time of its own" (unlike us, say), so we prefer to say that a virtual particle is exchanged between A and B (rather than going from A to B or vice versa)

anything traveling faster than light breaks causality, in that some observers say it moves from A to B, while others say it moves from B to A

if it had a well-defined "direction of ageing" (a human for example starts looking like a baby, and finishes looking like an old person), then some observers would say it was ageing backwards, ie it was going backwards in its own time while of course going forward in the observer's time

but no material can travel faster than light, so there isn't a well-defined "direction of ageing", and every observer simply says "it's going forward in my time, and it doesn't appear to have any time of its own"

(of course, virtual particles are just mathematical artefacts that help in the calculations, so this is a bit like discussing how many angels can dance on the head of a pin )

Tiny-Tim,So what your saying is that virtual particles travel faster than light but don't travel backwards in time to the past,is this what your saying?

 

[Drakkith]

Tiny-Tim,So what your saying is that virtual particles travel faster than light but don't travel backwards in time to the past,is this what your saying?

I'm not sure, but it sounds to me like he's saying they don't actually travel through space.

 

[tiny-tim]

Tiny-Tim,So what your saying is that virtual particles travel faster than light but don't travel backwards in time to the past,is this what your saying?

in each observer's time, everything travels toward the future

if it had its "own time" (like a human), it could travel toward the past in its own time while traveling toward the future in the observer's time, in other words the observer would say it was getting younger

but it doesn't have its "own time"

I'm not sure, but it sounds to me like he's saying they don't actually travel through space.

hi Drakkith!

it depends what you mean by "travel" …

(eg in general relativity, do we say that an object travels along its world-line, or merely that it has a world-line? )

each observer certainly regards it as traveling (from A to B or from B to A), but since different observers can't agree (for faster-than-light travel) on the direction, is it really traveling (or is it really only "being exchanged")?

(of course, i repeat: virtual particles aren't real, they're just mathematical artefacts that help in the calculations )

 

[Drakkith]

As your tiny sized text at the bottom says, they aren't real(which I agree with), so why is this even an issue? More variations in interpretations?