Can something move faster than light?

[HIENVN]

Okay, this question pops up now and then so let's have a new poll.

 

What do you think - can anything move faster than light?

 

Gravitational waves may move faster or slower than light; or these waves may move equal to light. Because nobody knows about gravitational waves, which are not discover yet, then I think your question is so far to have a right answer.

[TheFaithfulStone]

The best current estimate for the speed of gravity is .95c +- .25%

citation

 

I'll go out on a limb and say that's a pretty good indication that it's c exactly.

 

TFS

[i see your van Flandren, and raise you one John Baez!]

[CraigD]

What if … there was an extremely long something with very little mass… What might happen if it was given a gentle nudge. Would there instantaniously be a corresponding move at the other end, ie communication faster than light.

According to the current best accepted theory, no.

 

Unless the Very Long Thing was a single fundamental particle (eg: a gigantic electron), the nudge of the Near End would be due to an interaction of one or more bosons (in the case of a VLT made of atoms, or even of degenerate matter such as neutronium, magnetic force photons) with one or more fermions (electrons or quarks). These fermions in turn have the force of then nudge carried to the next fermions in the direction of the Far End, and so on, until the fermions of the FE are nudged. The bosons are limited to the speed of light ©, so the nudge can’t propagate from one end to the other any faster than c.

 

In most cases, it propagates much slower than c, as given by the speed of sound in a solid. For example, if the VLT was made of steel, the nudge would travel at about 5100 m/s, a modest (by astronomical standards) 0.000017 c. The less dense the material, the faster the speed of sound in it (by density^-2), but the lower bound for the density of a solid is on the order of 1/10 to 1/10,000 that of steel, giving an upper bound for the speed of sound in a solid of about 500000 m/s (0.002 c).

[TheFaithfulStone]

Okay, I'm going to lift up my skirt and show everyone my ignorance.

 

I know that the solid object thing wouldn't work, but if force through the VLT propagates at the speed of sound, how can anything accelerate faster than the speed of sound in itself?

 

If force through an object propagates at the speed of sound in a solid, does that mean that nothing can gravel faster than the speed of sound in itself?

 

That's obviously not true, so how does a rocket (or whatever) push a steel block faster than .002c?

 

At what point do sound wave force propagations give way to actual atomic level interactions?

 

:eek:

 

TFS

[CraigD]

I know that the solid object thing wouldn't work, but if force through the VLT propagates at the speed of sound, how can anything accelerate faster than the speed of sound in itself?

You appear to be confusing speed and acceleration. It’s not meaningful to speak of an acceleration (which is in units of (distance/time)/time) being greater of less than a speed (units of distance/time) (eg: "the acceleration of the VLT is less than the speed of sound in it"). Likewise, it doesn’t make sense to compare distances to speeds (eg: “the distance from Baltimore to Washington is 30 miles/hour”, or "The car's speed is 5 meters").

 

The speed of an object relative to another object is independent of the speed of sound in either object. The length of an object, divided by the speed of sound in it, gives the minimum difference in time between a force being applied to one end of the object, and being detected on the other.

If force through an object propagates at the speed of sound in a solid, does that mean that nothing can gravel faster than the speed of sound in itself?

No.

so how does a rocket (or whatever) push a steel block faster than .002c?

Looked at on a microscopic scale, the velocities of the individual atoms of are complicated. When it receives its initial push at the “tail end”, the component of the velocities in the direction of the push of the atoms in the tail end are briefly faster than in the “nose end”. Some time later, the nose atoms may have an average velocity in that direction slightly greater than the tail. The atoms in the middle may briefly have average velocities in a particular direction greater of less than their neighbors. The average speed of all of the atoms in the rocket, however, increases in a simple, easy-to-describe way, that doesn’t require taking the motion of individual atoms into account.

 

For typical forces and materials, the differences in velocity between atoms in different parts of the rocket are negligible, especially when one considers that, at a temperature well above absolute zero, all of the atoms are moving in effectively random directions at velocities on the order of several hundred m/s, changing direction with accelerations dwarfing that of the object as a whole (Working out the acceleration of gas molecules at room temperature show that they experience minimum accelerations of at minimum 10s of millions of m/s^2!)

At what point do sound wave force propagations give way to actual atomic level interactions?

Sound wave force propogations are actual atomic level interactions. It’s just convenient to refer to a large number of atomic interactions as a single sound wave, much as it’s convenient to refer to a large number of atoms as a single object.

[TheFaithfulStone]

:hihi:

 

Now that I think about it a little more, it's dead obvious isn't it?

 

The speed of sound in an object really has no bearing on it's overall speed.

 

TFS

[Boerseun]

I think that any given object can't accellerate faster than a soundwave would propagate in the same material, although it can easily travel faster than the wave relative to some other object. For instance, if we say that the speed of sound in steel is 5,000 km/h, then how can Voyagers I & II be travelling at more than 40,000 km/h? Simple - according to their own frames of reference, they are standing perfectly still and they can still accellerate.

 

I suppose you can see it as a pressure wave in front of the accelerating atoms. If the wave propagates at 5,000km/h but the force at the back of the mass is applied at 5,001km/h, sideways motion will be less of an effort than trying to accellerate the wave forward, so energy will be released as the material will shatter sideways at the point where the force is trying to overtake the pressure wave.

[Bagpi]

Questions:

1) What speed do two light (beams) approach each other from opposite directions.

2) If light has no mass what makes the black and white thingy in the light bulb rotate and why bother with a proposal to power space ships with a big sail.

3) If light has no mass how is it bent (slowed down) by muckle heavy things.

4) If light does indeed have mass...how can it travel at the speed of light without violating lots of laws.

5) If light does have mass and can be slowed down does this mean some stuff out there might be closer than we think.

6) Somebody stoped light recently in a near absolute zero degrees clowd of sodium..then started it up again.............what now???

Very puzzled I am

Keith (Bagpi)

[TheFaithfulStone]

Questions:

1) What speed do two light (beams) approach each other from opposite directions.

2) If light has no mass what makes the black and white thingy in the light bulb rotate and why bother with a proposal to power space ships with a big sail.

3) If light has no mass how is it bent (slowed down) by muckle heavy things.

4) If light does indeed have mass...how can it travel at the speed of light without violating lots of laws.

5) If light does have mass and can be slowed down does this mean some stuff out there might be closer than we think.

6) Somebody stoped light recently in a near absolute zero degrees clowd of sodium..then started it up again.............what now???

Very puzzled I am

Keith (Bagpi)

 

1. c According to either observer. Actually, that's not strictly accurate. If you had two things approaching each other at the 99.9% the speed of light, they would appear to a third observer to be traveling towards each other at 2c. If two things were approaching each other at exactly the speed of light, they would already have hit each other before a measurement could be made, so it's not really meaningful to talk about how fast it seems to them, since it seems infinitely fast.

 

2. Differential air pressure actually, not light pressure - but I believe the whole "solar sail" thing works because when an atom "reflects" a photon, it actually gives one off in the opposite direction and "keeps" some of the energy as momentum.

3. Light isn't bent, space itself is bent. Light always travels in a straight line, it's just that sometime the definition of "straight" changes.

4. Light doesn't have mass.

 

I'm not familiar enough with how the "light stopping" experiments work, so I'll leave those to somebody else - but to "stop" light doesn't require that it has mass.

 

TFS

[Bagpi]

I have not got my head around the quote thingy yet so have pasted...........

QUOTE:

According to either observer. Actually, that's not strictly accurate. If you had two things approaching each other at the 99.9% the speed of light, they would appear to a third observer to be traveling towards each other at 2c. If two things were approaching each other at exactly the speed of light, they would already have hit each other before a measurement could be made, so it's not really meaningful to talk about how fast it seems to them, since it seems infinitely fast.

 

So if they were a light year apart the distance between them would be reducing at what rate at 100% c, if you were a Deity looking on. Surely, said Deity would not see an instantaneous coming together as light itself would then appear to have travelled faster than c.

Aaaaaand...why does something with no mass give a monkeys about a blach hole and all the gravity inherent in such a thing.

Still puzzled.

Keith

[TheFaithfulStone]

If you were a deity looking on, you would each of them moving at c relative to you. If you were to measure the perceived distance between them, you would find that it was decreasing at 2c yes.

 

If you were "onboard" one of the photons, traveling at c, then time dilation would be at it's max, and you would effectively arrive as soon as you left.

 

If you were traveling at say .8c on a space ship, approaching another spaceship that was also traveling at .8c - your rate of closing with the other vessel would be .9756c

 

Which is given by velocity addition formula which is

 

V = (U + V')/(1+UV'/c^2)

 

Where U & V' are the two velocities.

 

That's special relativity.

 

This is general relativity:

 

Remember the gravity deforms space time itself. Light isn't be "pulled" toward the gravity well, it's still traveling in a straight line. Although this is an inexact analogy, imagine a grid drawn on a rubber sheet. Put a heavy object on the rubber sheet and it pulls the sheet down and deforms the grid. Now run your finger along the grid - notice that even when it moves perfectly along a "straight line" as defined by the grid, to you it looks like it's not going "straight" anymore.

 

TFS

[CraigD]

1) What speed do two light (beams) approach each other from opposite directions.

I mostly agree with The Faithful Stone’s explanation, except that an observer distant from to approaching light beams can in principle observe them, so long as some of the beams photons are scattered/reflected in their direction, such as by interstellar dust/gas

2) If light has no mass what makes the black and white thingy in the light bulb rotate and why bother with a proposal to power space ships with a big sail.

I agree with TFS’s explanation.

 

Bagpi appears to be referring to a Crooks radiometer. The accepted explanation of why these work is somewhat complicated (see linked), but involves the flow of air around the vanes due to different heating of their black and white sides.

 

Crooks (1832-1919) believed his device worked due to the pressure of photons impacting the vanes. It’s odd that he would have made this mistake, since, if this were the case, the vanes would spin in the opposite direction, since the more reflective white side would receive more momentum than the black side. If you make a Crooks radiometer with a more perfect vacuum than the usual kind, they stop working – while light pressure could, in principle, spin the vanes, the simple pin-on-glass bearing has more friction than the force of light on the small vanes can overcome.

3) If light has no mass how is it bent (slowed down) by muckle heavy things. [“why does something with no mass give a monkeys about a blach hole and all the gravity inherent in such a thing.”]

This is what the theory of General Relativity explains.

 

Though a detailed explanation is best read from sources such as the linked wikipedia article, a very brief one is that GR asserts that force due to acceleration and force due to gravity are, from the point-of-view of light, indistinguishable. So, since you can make a beam of light appear to curve by moving a box that it passes through, the same curving is seen when it passes through a gravitational field.

4) If light does indeed have mass...how can it travel at the speed of light without violating lots of laws.

5) If light does have mass and can be slowed down does this mean some stuff out there might be closer than we think.

As TFS states, light (the photon) has zero mass. If it had non-zero mass, it would violate lots of laws by traveling at lightspeed.

6) Somebody stoped light recently in a near absolute zero degrees clowd of sodium..then started it up again.............what now???

Light always travels slower than c through a non-vacuum. What researchers such as those Bagpi describes have done recently is produce substances where light is slowed to nearly or exactly zero – that is, produced a material with an infinite refractive index.

 

The reason for both the ordinary and the extraordinary slowing of light in a non-vacuum is that photons interact with matter. This interaction takes time, so slow their passage throught the matter. I give a lengthy explanation of this in post #8 in the 4741 thread – a good thread to read with questions like Bagpi’s.

[Guest jamongo]

I don't kow.

 

How do we know there are not "some things" that already move faster than the speed of light?

We wouldn't be able to detect them, would we?

Humans have a limited range of detecting sound waves, while other animals, dogs for example, have a more extended range. A poor example, perhaps, but it makes the point.

I just hesitate to state that the end of something has been obtained. History has proven us wrong so many times.

[CraigD]

How do we know there are not "some things" that already move faster than the speed of light?

We wouldn't be able to detect them, would we?

Why not?

 

It something that moved in excess of the speed of light interacted with something that we can detect, we’d be able determine the faster-than-light thing by its effect on the not-faster-than-light one. Many well-understood particles are known only by their interaction with large ensembles of measurable ones, such as photons and electrons.

 

If a faster-than-light (or a not-faster-than-light) something has absolute no interaction with anything that can in any way interact with us – if it’s not measurable – it can in a useful and reasonable sense be said not to exist.

[learnin to learn]

but if one somehow moves faster than light their mass alters right?:Glasses:

[CraigD]

but if one somehow moves faster than light their mass alters right?:confused:

If one moves (changes velocity) at all, their mass, as measured by someone who does not (the observer), alters, according the formula:

[math]M = \frac{M_0}{\sqr{1-\frac{V^2}{c^2}}}[/math]

Where [math]M[/math] is the measured mass, [math]M_0[/math] is one mass as measured when stationary ([math]V[/math]=0) relative to the observer, [math]V[/math] is the velocity relative to the observer, as measured by them, and [math]c[/math] is the speed of light in vacuum.

 

Notice that if [math]V[/math] exceeds [math]c[/math], [math]M[/math] gains a factor of [math]\sqr{-1}[/math] – that is, it becomes an imaginary, or complex, number. Nobody I’ve ever heard has a convincing interpretation of what “imaginary mass” means – though a lot of people have convincing interpretations of what it doesn’t mean, including “an ordinary mass moving backward in time”, “existing in an alternate universe”, or “infinitely larger than infinity”. For this and other reasons, most people conclude that it doesn’t make sense to assume one can move faster than light (though not according to this thread’s poll!)

[learnin to learn]

I see. imaginary mass does seem unlikely.