Can something move faster than light?

[mocnarf]

Question:

 

If an observer, could see three fixed points "A" "B" and "C" and these three points are aligned on a straight line with a fixed distance between "A" and "C", with point "B" in the middle between point "A" and point "C". Now, suppose a photon is shot at point "B" from both point "A" and "C" at the same time. The observer would see both photons travelling towards point "B" at the speed of light.

Question: At what speed would the observer see the distance between the photons shrink?

 

Does this not set up a conundrum? If the observer see's the distance shrinking between the two photons occurring at the speed of light, then the speed of the photon's travelling towards point "B" must be 1/2 the speed of light. Or, the distance between point "A" and point "C" must be increasing at 1/2 the speed of light, which would mean that there can be no fixed distance between two points.

 

Your thoughts.....?

[Zythryn]

As I recall, information is already able to break the speed of light. If someone has more information on the following please feel free to chime in.

 

I recall reading tests done with the quantum state of two particles. The two particles could be 'bound' at a quantum level. Then when the two particles were at a greater distance from each other, altering the quantum state of one would simultaneously alter the state of the other.

 

While this is not moving matter, it is moving information which I suggest qualifies as 'something' :hihi:

 

Mark

[CraigD]

If an observer, could see three fixed points "A" "B" and "C" and these three points are aligned on a straight line with a fixed distance between "A" and "C", with point "B" in the middle between point "A" and point "C". Now, suppose a photon is shot at point "B" from both point "A" and "C" at the same time. The observer would see both photons travelling towards point "B" at the speed of light.

Question: At what speed would the observer see the distance between the photons shrink?

The observer would see the distance between the 2 photons shrinking at 2 times the speed of light ©.

Does this not set up a conundrum?

No. There are some technical details necessary for an observer to “see a photon” – there must be at least a short pulse of many photons, and some sort of mechanism to send some photons toward the observer rather than toward C (such as a diffuse reflective cloud between A, B, and C), and the observer must account for the time it takes light to travel between the moving pulse and him – but these issues can be resolved with simple, non-relativitistic mechanics.

If the observer see's the distance shrinking between the two photons occurring at the speed of light, then the speed of the photon's travelling towards point "B" must be 1/2 the speed of light.

Since the observer sees the distance shrinking by 2 c, the speed of the photons remains the expected 1 c, and the conundrum is resolved.

 

Good questions, mocnarf!

[TheFaithfulStone]

As I recall, information is already able to break the speed of light. If someone has more information on the following please feel free to chime in.

 

Nope - there is now way to send information in quantum entanglement. For one, as Will pointed out to me a while back in this thread it isn't nessecarily true that the wavefunction collapse is a causal effect. It's impossible at relativistic speeds to talk about who measured "first." so the casuality of the relationship is in question, anyway.

 

Let's say that we have two entangled particles, and I measure mine. The result is random. Now I know what the measurement on yours will be - before you measure it (in my frame) but you have no way of knowing what this is. When you measure yours you have no way of knowing whether you've collapsed the wavefunction, or whether I already have. Even if you could solve the program (say with an equidistant pulsar...) you'd not be able to communicate with this method.

 

In any case, there isn't any way I can control what answer I get - so If I measure the spin and get 2-1-2-0-1 - you'll know what the answer is, but it won't be meaningful to you, because it's impossible for me to encode any information in a bunch of random q-bits.

 

Not that that was particularly lucid, but there you are. People smarter than I can explain this better.

 

TFS

[jpittelo]

Consider a particle moving at speed v=a*c , 0<a<1, along a distance of 1 light year for an observer at rest respect. to this length.

 

Then, as seen from the moving observer, the length is contracted : D=Sqrt(1-a^2)*1ly...which means that the observer at his clock travels the distance in the time :

 

T=Sqrt(1-a^2)*1year...

 

hence foir every a>0, the observer is "as if it/she/he was moving faster than c ?...Does this make any sense ?

[TheFaithfulStone]

Are you saying that for anybody travelling at relativistic velocities it appears to take them less time than it would take light to travel an equivalent distance? I think we might have a language barrier...

 

That is if I'm going .99c for 4 light years, it only seems to take me a year and a half. (I didn't do the math I have no Idea if that's accurate.)

 

Time dilation is a pretty well established consequence of special relativity.

 

But for outside observers the speed is still less than c and it still takes more than the time it takes light.

 

TFS

[Qfwfq]

Not that that was particularly lucid, but there you are.

I'll try to put it more simply. I'll more or less outline the point of view of J. S. Bell:

 

Whoever measures the state of one of the particles, cannot decide the outcome of this measurement but only see what it was. The impossibility of planning future outcomes makes it totally useless as a way of sending information of your choice to someone measuring the states at the other particle.

 

Now, that said, I'll add that one could consider the correlation as necessarily implying comunication of information between the two particles (mutually, as the spacelike distance implies the before-after ambiguity) but that is a subtley different thing from saying that we can send the info. The violation of Bell's inequalities is, in fact, a violation of the requisite of local realism (rather than of local causality). That is what is disturbing.

[Qfwfq]

But for outside observers the speed is still less than c

and for all observers. Just do the math properly. ;)

[TheFaithfulStone]

:doh:

 

Yes. It just take less time to get there than you'd think.

 

Relativity is wierd.

 

The violation of Bell's inequalities is, in fact, a violation of the requisite of local realism (rather than of local causality). That is what is disturbing.

 

What's that mean? That the particles are not nessecarily confined to a single "location?"

 

TFS

[CraigD]

Consider a particle moving at speed v=a*c , 0<a<1 …

hence foir every a>0, the observer is "as if it/she/he was moving faster than c ?...Does this make any sense ?

Change it to “for every a > 2^.5/2 (half the square root of 2, or a little more than .7), it appears to the accelerated observer that he has traveled faster than c”, and the statement agrees with the predictions of Special Relativity.

 

In short, SR tells us that you can move any distance in as little time (as measured by you, the accelerated observer) as you would expect to according to ordinary, classical Newtonian physics. Relativity only deviates from classical mechanics in what it predicts an un-accelerated (or differently accelerated) observer measure your travel time to be, and the speed that you measure objects not traveling with you to be moving. If you don’t care about these things, you don’t need to use SR.

 

For example, to travel 1 light years (about 10^16 meters) at a constant acceleration of 10 m/s/s (a comfortable 1 g), just solve

Distance = (Acceleration/2) * Time^2

For Time:

10^16 = (10/2) * Time^2

Time = (2*10^15)^.5 = ~ 44721359 seconds = ~ 2 years

For 10 light years, Time = ~ 4.5 years.

 

I think common misunderstandings about the implications of SR – in particular, that it demands that travel to very distant points take a very long time, as perceived by the traveler – is the cause of much of the distaste for the theory by many Science and SF enthusiasts. I think this distaste is much of the reason for the invention of such improbable fictional technologies as “hyperdrive,” and why a surprising 55% of this science site voted “Can something move faster than light? Yes” in this thread’s poll.

[Qfwfq]

In short, SR tells us that you can move any distance in as little time (as measured by you, the accelerated observer) as you would expect to according to ordinary, classical Newtonian physics.

But that distance, as measured by the same observer, will not be greter than ct.

 

Relativity only deviates from classical mechanics in what it predicts an un-accelerated (or differently accelerated) observer measure your travel time to be, and the speed that you measure objects not traveling with you to be moving. If you don’t care about these things, you don’t need to use SR.

:hihi:

 

Question: If you're a train passenger, how do you reckon the speed you're moving at?

 

Nastier question: What is your speed, "as measured by you"?

[ldsoftwaresteve]

Thank goodness I'm not the only one confused by SR.

Much of the dialogue concerns the perception (measurement) of velocity and if I get the implication, doesn't limit velocity to the speed of light at all.

I'm assuming that we can still infer relative speeds in excess of c, is that correct?

Also, now this is going to sound really strange but SR doesn't imply a causal connection to perception, does it?

Some stuff I can visualize, like color shifts due to the increased 'energy' of photons striking my retina. Perhaps even energy shifts into a spectrum I'm blind to.

One more scenario and then I'll shut up: What if the Milky Way Galaxie is moving at the speed of light 'towards' another galaxy. Would we be aware of it? Since we assume it can't happen, we wouldn't look for it right?

[TheFaithfulStone]

What if the Milky Way Galaxie is moving at the speed of light 'towards' another galaxy. Would we be aware of it? Since we assume it can't happen, we wouldn't look for it right?

 

Oo! oo! I know that one! TRUE.

 

LOOK OUT!

 

A galaxy moving toward us will have a blue shift - and in fact, there is a "speeding train" in the form of Andromeda.

 

Thankfully, the tunnel is about 3 billion year long - so you know... I wouldn't worry too much about that.

 

TFS

[CraigD]

Re: Travel time as perceived by an accelerated traveler - Today, 03:35 AM

In short, SR tells us that you can move any distance in as little time (as measured by you, the accelerated observer) as you would expect to according to ordinary, classical Newtonian physics.[/Quote]But that distance, as measured by the same observer, will not be greater than ct.[/Quote]For the accelerated observer, that distance depends on when she measures it.

 

Let's define Ttrip to be the time, as measured by the traveler, that the trip takes; Dbefore to be the distance of the trip as measured by the traveler before beginning it. In these terms, Dbefore > Ttrip*c

 

Now, Let's define Dduring to be to be the distance of the trip as measured by the traveler when she has accelerated to her maximum speed, as measured by an unaccelerated observer, Vmax. Dduring = Dbefore*(1-(Vmax/c)^2)^.5, the famous relativistic length contraction.

Working out the not-as-simple-as-before math reveals that Qfwfq is correct (as usual):

Dduring < Ttrip*c

 

Note that in all of my post # 155 examples imply a simplified “trip” where my speed upon arrival is Vmax. This is a poor match for the intuitive meaning of a “trip”, which involves stopping when one reaches ones destination. Using this definition and the previous examples, a 1 g, 2 lightyear trip would take about 4 years, a 20 LY trip about 9.

Relativity only deviates from classical mechanics in what it predicts an un-accelerated (or differently accelerated) observer measure your travel time to be, and the speed that you measure objects not traveling with you to be moving. If you don’t care about these things, you don’t need to use SR.

Question: If you're a train passenger, how do you reckon the speed you're moving at?

 

Nastier question: What is your speed, "as measured by you"?

These are indeed nasty questions, serving, I think, to illustrate how vague and inappropriate ordinary language is for discussing mechanics. Until the day arrives that we’re all speaking Espiranto-based General Semantics ( :lightbulb ), however, we’ll just have to muddle on as best we can.

 

My point is that one of the delightful consequences of Special Relativity is that, for some very practical questions such as “If I accelerate in a specific way for a specific time, how long will it take me to reach a specific point in space” the answer can be found using simple mechanics. Questions such as “what will things look like to me or an observer not traveling with me” involve less simple, non-relativistic calculations, but similarly avoid paradoxes and contradictions of observed reality.

[CraigD]

What if the Milky Way Galaxie is moving at the speed of light 'towards' another galaxy. Would we be aware of it? Since we assume it can't happen, we wouldn't look for it right?

If another galaxy – or anything, for that matter – is moving towards us at the speed of light, there is no point in trying to look for it – by the time we saw it, it would be here! (and something awful and exotic would happen) This example is analogous to attempting to detect the approach of a supersonic aircraft by listening for its sound (but more awful and exotic).

 

If something – a galaxy, or even a single glowing ember - is moving toward us at a tiny fraction less than the speed of light, we’d see it coming, but its light would be so blueshifted that it would be devastatingly high-energy, ultra-ultra-violet, reducing us and everything around us to a plasma of our component atoms. Fortunately, the universe doesn’t appear to have any light-emitting matter moving this fast, and the laws of mechanics make it very, very hard for such a thing to be done.

[Qfwfq]

These are indeed nasty questions, serving, I think, to illustrate how vague and inappropriate ordinary language is for discussing mechanics.

Uhm, they were meant to be answerable with quite simple English, and by applying the principle of relativity. The brunt of SR can be discussed without speaking "Esperanto-based General Semantics" but with the right mathematical formalism.

 

The answer to the "even nastier" question is none other than: exactly zero! A train passenger looking out the window will see trees, houses and many other things whizzing along at a smart pace and may be able to make some estimate of their velocity by judging distances and marking times. For some very odd reason however, most train passengers firmly believe that these things are not moving at all.

 

Questions such as “what will things look like to me or an observer not traveling with me” involve less simple, non-relativistic calculations, but similarly avoid paradoxes and contradictions of observed reality.

I'm not too sure what you mean there. :lightbulb

[chamilton333]

past the speed of light cannot be reached. light itself can only go its speed because of the law that the faster you go, the slower time goes. if you go the speed of light, time stops, and so you go the speed of light. i think the speed of light is acually infinity if you are the thing going the speed of light, but to the observer, or even precise instruments, you are going slower. the only time this has probobly been reached is during the big bang, when the whole universe blew apart.