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someguy44
2006-Dec-06, 07:35 PM
I've been having this debate with someone at another forum and this is what he claims:

"But your statement that nothing can travel faster than the speed of light is definitely wrong. Einstein's laws only dicate what happens when you speed up from below the speed of light (because you need infinite amounts of energy to accelerate etc), it says nothing about starting above the speed of light. This is simple enough even for you to understand."

Is this true? I don't believe it, but can one of you scholars enlighten me?

Fazor
2006-Dec-06, 07:56 PM
What does he propose is already above the speed of light? and how did it get to be above the speed of light?

if you do a forum search on speed of light you'll find many posts on the topic of the speed of light (sorry i got to the point where I just wanna see how many times i can say "speed of light").

antoniseb
2006-Dec-06, 08:05 PM
Can there be a speed faster than the speed of light???
Sure 400,000 kilometers/second is a speed, and it is faster than 300,000 kilometers/second (roughly the speed of light). It may be that no object will be seen to have that speed as it goes passed another object, but it is (technically) a speed.

Also, if the current mainstream model of the universe is correct there are a lot of objects out there that are moving away from us at faster than the speed of light. We've even seen some of them, but they are very far away.

This doesn't answer your friend's issue, which is about tachyons, a hypothetical type of particle that is stuck moving faster than light because it can't slow down to 'c'.

grant hutchison
2006-Dec-06, 08:26 PM
Special relativity tells us that you can't accelerate a massive object (that is, one that has any mass at all) up to and through the speed of light: the energy requirement to reach the speed of light would be infinite.
But it allows, as far as I know, for objects moving faster than the speed of light, which would require infinite energy to slow them down to lightspeed. Such objects would have an imaginary rest mass (whatever that might be), and are the tachyons antoniseb mentions. So far, no-one has detected one, and people have looked.

If objects or information could be moved around faster than light, special relativity does predict a major problem for the Universe (or, at least, our view of the Universe): causality violation.
Relativity tells us that observers in motion relative to each other will disagree about which events are simultaneous, and about the length of time that elapses between events. But so long as all objects obey the lightspeed limit, no observer will ever see the order of two causally related events reversed: if one event causes another, there will be no observer, in any state of motion, who can observe the effect happen before the cause.
As soon as the lightspeed limit is broken, that goes out the window; the relationship between cause and effect can be reversed. The direction of time and the second law of thermodynamics become open to negotiation.
It's not a pleasant prospect for physics, and we don't seem to observer the Universe behaving in such an undisciplined way, so I think the majority view among physicists is that nature probably doesn't provide a means of achieving faster-than-light travel.

Grant Hutchison

Fazor
2006-Dec-06, 08:55 PM
Hmm...something triggered a question for me that I'm sure you all can be quick to answer. No object can accelerate to the speed of light. But that's the speed of light relative to what?

If something was moving at 3/4ths the speed of light one dirrection, and observer moving 3/4ths the speed of light the opposite direction, wouldn't the first object have a relative speed faster than the speed of light?

grant hutchison
2006-Dec-06, 09:33 PM
No object can accelerate to the speed of light. But that's the speed of light relative to what?The point, indeed the foundation, of special relativity is that every observer measures the speed of light to be the same, no matter what their state of motion. So someone aboard an accelerated spaceship would measure the same speed of light as someone on Earth would.


If something was moving at 3/4ths the speed of light one dirrection, and observer moving 3/4ths the speed of light the opposite direction, wouldn't the first object have a relative speed faster than the speed of light?No, relativistic velocities don't add like that. The "stationary" observer who measures the velocities you report would see only a couple of objects, both moving more slowly than lightspeed. Any observer travelling with one the objects would measure the relative speed of the other as being less than the speed of light, because of the distortions of time and space predicted by relativity. I think the closing velocity turns out to be 0.96 lightspeed, as measured by either moving observer, if I'm doing the sums correctly.

Grant Hutchison

Fazor
2006-Dec-06, 09:41 PM
So you're saying that even though relative to each other, they would be moving apart at greater the speed of light, you would still only observe a speed of approx. 96% the speed of light. Just making sure i'm reading that correctly.

I have another question but, but it starts to stray from this topic I don't want to hijack this post so i'll start a new one when I figure out how to word it :) thanks.

grant hutchison
2006-Dec-06, 09:58 PM
So you're saying that even though relative to each other, they would be moving apart at greater the speed of light, you would still only observe a speed of approx. 96% the speed of light.No, I'm saying that relative to each other they are not moving apart at greater than the speed of light: each sees the other moving at 0.96 lightspeed.
A third observer, who is moving relative to both of them, can clock each of them as travelling at 0.75 lightspeed relative to himself: but if he wants to work out their speed relative to each other, he isn't allowed, under special relativity, to just add V1+V2 in the familiar way. Instead, he has to calculate (V1+V2)/(1+V1.V2)/c2).

Grant Hutchison

Argos
2006-Dec-06, 10:01 PM
How Do You Add Velocities in Special Relativity? (http://math.ucr.edu/home/baez/physics/Relativity/SR/velocity.html)

Grey
2006-Dec-06, 10:55 PM
And here (http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/tachyons.html)'s a link from the Physics FAQ that talks about tachyons - hypothetical particles that always travel faster than light. Such particles have never been detected, but if they did exist, they'd have some weird properties, like an imaginary rest mass. And it turns out that even if they did exist, it still wouldn't be possible to send information with them superluminally.

Ken G
2006-Dec-08, 08:39 PM
So you're saying that even though relative to each other, they would be moving apart at greater the speed of light, you would still only observe a speed of approx. 96% the speed of light.

To clarify Grant's correction to this statement even further, the only meaning to 1.5c in this situation is that the stationary observer could calculate, in his/her own frame, the rate of increase of distance between the two others, and it would be 1.5c. But the stationary observer would know (if they know relativity) that each moving observer would only perceive the other as moving at 0.96c, trusting Grant's result. So if one interprets "speed" as meaning "the rate of increase of distance between two objects", then it can certainly be faster than c. However, that's not a measurement it's a calculation, and it doesn't violate special relativity, because SR is a theory about observation by an observer of things relative to themself. Thus "speed" in relativity means the speed of something measured relative to you, the observer. What calculations you may wish to do with those two 0.75c speeds is up to you. A classic example of this is the Big Bang interpreted in "comoving coordinates". In those coordinates, the cosmological speeds of everything that everyone measures are basically zero (as speed is only uniquely measurable for things at the same place as you when you choose global coordinates), yet they can calculate, using those global coordinates, the distance to other things as increasing faster than c with no violation of relativity. (Indeed, most of the universe that we've observed is indeed separating from us at faster than c right now, when comoving rulers are used to measure distances and comoving clocks are used to measure time. What it's not doing is whizzing by our location at a speed faster than c-- that's impossible, unless there are tachyons, and nobody is holding their breath there.)

Saluki
2006-Dec-08, 10:42 PM
And it turns out that even if they did exist, it still wouldn't be possible to send information with them superluminally.

What if we used a Tesla Purple Energy Shield (http://www.lifetechnology.org/teslashield.htm)?


With anodising, the field of the plating is changed and interacts with tachyons.

I bet you didn't think of that, did you?

Grey
2006-Dec-08, 11:37 PM
I bet you didn't think of that, did you?You are absolutely correct. I did not think of that. :eh:

Delvo
2006-Dec-09, 12:48 AM
A few years ago I read an article about the possibility that space, rather than being a smooth continuous background, is actually what you could "granular" or "pixellated", consisting of distinct points. There'd be no such thing as a fraction of a point or space that is outside of them because these would be the quanta of space, like the cells of a honeycomb. The counterpart idea for time is that time doesn't flow, but rather blips ahead one distinct unit ("instant"/"moment") at a time, like computer clock cycles or the "tick-tock" of a mechanical clock or the numbers on a football game clock that just blink from one to the next without any partial units or gradual sliding.

It's still not known whether this is really the way things are or not because these units are smaller than any other observed phenomenon and thus any other observed phenomenon is already several minimum units in size and/or duration. The units are the Planck length and the Planck time.

One of the many things that article had me thinking about, as implications of the idea of the universe being structured that way, is that it gives an explanation for the speed of light. The ratio of these Planck units, the quantity you could just as well call the "Planck speed" if it hadn't already been named something else, is the speed of light, c. So, if the universe is really that way, then you can see how moving ahead one pixel of space per tick of the universe's clock would be the top speed, like a king on a chessboard being able to move only one square per "turn". That would mean that there's higher speed because you can't move to a non-adjacent point in one move and you can't get to the next point over in a fraction of a move, which would mean that tachyons can't exist and are nothing but a mathematical nonsense result from calculations with nonsense inputs.

Bearded One
2006-Dec-09, 12:50 AM
There are phenomena that move faster than light, shadows are a prime example. So yes, there can be speeds faster than light. Good luck transferring information with them though.

lti
2006-Dec-09, 02:57 AM
Bearded One, can you explain how a shadow moves faster than light?

Ken G
2006-Dec-09, 04:30 AM
That's true, a shadow can cross a distance d in a time much less than d/c, just as you can cause water from a hose to cross a wall faster than the speed of the water droplets themselves. It is debatable if this is a "speed", as a shadow is not an "object" (when I look at stars my "point of focus" can shift across many light years in a few seconds), so one must define the terms more carefully than is done in the OP.

Bearded One
2006-Dec-09, 10:04 AM
That's true, a shadow can cross a distance d in a time much less than d/c, just as you can cause water from a hose to cross a wall faster than the speed of the water droplets themselves. It is debatable if this is a "speed", as a shadow is not an "object" (when I look at stars my "point of focus" can shift across many light years in a few seconds), so one must define the terms more carefully than is done in the OP.

That's why I used the term "phenomena" instead of "things".

Grant Hutchison nailed the root of the problem well.

Personally I hate the phrase "faster than light". Light simply moves at the fastest speed observable in the Universe we occupy, so does gravity apparently. It's always moving that fast, to everybody, everywhere.

I've seen many Sci-fi/Space-Fantasy shows with gauges that read %c. Those are completely ridiculous as they only make sense to an outside observer. To those in the ship those gauges would always be stuck on zero.

The key is to get those gauges to even quiver to the people on the ship, even a minor quiver above zero would throw current physics to the dogs.

Frying Tiger
2006-Dec-09, 01:12 PM
I would assume the "%c" gauge is reading relative to an object, and thus it would register something.

Bearded One
2006-Dec-09, 01:14 PM
I would assume the "%c" gauge is reading relative to an object, and thus it would register something.

Such a reading would be useless as far as judging FTL speed though. We are all moving at near lightspeed WRT to distant Quasars.

Ken G
2006-Dec-09, 02:32 PM
Yes, sci fi definitely hates relativity. The %c is a joke, as is "warp speed", and the worst is, they always use an absolute concept of time on the various planets they visit ("stardate" would be a useful measure of time, but it wouldn't mimic time passage on the ship except while in orbit). I suppose we'll have to chalk the whole business up to "poetic license", but it removes the sci in favor of the fi.

buzgz
2006-Dec-09, 09:14 PM
I've been having this debate with someone at another forum and this is what he claims:

"But your statement that nothing can travel faster than the speed of light is definitely wrong. Einstein's laws only dicate what happens when you speed up from below the speed of light (because you need infinite amounts of energy to accelerate etc), it says nothing about starting above the speed of light. This is simple enough even for you to understand."

Is this true? I don't believe it, but can one of you scholars enlighten me?

Well, all objects outside the Hubble sphere recede faster than the speed of light. Does this count ?

Obviously, this must be true since the Hubble sphere is defined to be the distance beyond which the recession velocity exceeds the speed of light.

Interestingly, the Hubble sphere is not an horizon, and light that originated from objects receding faster than the speed of light is detected here.

foreignkid
2006-Dec-10, 10:24 PM
Also, if the current mainstream model of the universe is correct there are a lot of objects out there that are moving away from us at faster than the speed of light. We've even seen some of them, but they are very far away.

So, if , for example, an object was moving towards us at the speed of light, we could not see it it, since in the time that it takes the light to travel to us from the object, the object has already travelled that same distance and hit us.

The same would be valid in the opposite. If the same object travelled away from us, we still would not be able to see it... or would we...

I AM CONFUSED!!:cry:

**sarcastically**
Thanks, A. Einstein!

Ken G
2006-Dec-10, 11:02 PM
So, if , for example, an object was moving towards us at the speed of light, we could not see it it, since in the time that it takes the light to travel to us from the object, the object has already travelled that same distance and hit us.That's true-- the object and its light get to us at the same time.


The same would be valid in the opposite. If the same object travelled away from us, we still would not be able to see it... or would we...The reason we couldn't see this object is different-- the light from it would be so highly redshifted and so heavily diluted in intensity that it would be undetectable, but that's different from the speed issue. If we could detect that light, it would get to us in the normal time it takes light to travel that distance, it wouldn't matter that the emitter was moving away from us.

buzgz
2006-Dec-11, 08:50 PM
..... but that's different from the speed issue. If we could detect that light, it would get to us in the normal time it takes light to travel that distance, it wouldn't matter that the emitter was moving away from us.

Maybe. I suppose we could imagine an object close by receding at c, and I would agree with your interpretation.

But, if the object receding at c was doing so because it was being carried with the Hubble flow (practically, much more easily imagined), it would be located on the Hubble sphere. Then I would not be so sure about the length of time for the light to travel here.

Light emitted by superluminally receding objects have a local velocity of c, but since the velocity of recession at that distance is greater than c, the light has a net velocity away from us. In other words, the speed of photons from this region is not constant, but is v=v(rec)-c.

Using this reasoning, I would suppose that for an object receding at exactly the speed of light, the photons would (initially) have a net velocity of zero toward us. Notice I have specified the initial velocity is net zero. I think the light will reach us eventually since the Hubble sphere is still expanding, but it will take much longer than if the light was emitted from a "stationary" object.

Note that a "stationary" object located on the Hubble sphere would have to have an initial velocity of c toward us to appear "stationary" (no red shift).

Does any of this make sense ?

Ken G
2006-Dec-11, 10:23 PM
But, if the object receding at c was doing so because it was being carried with the Hubble flow (practically, much more easily imagined), it would be located on the Hubble sphere. Then I would not be so sure about the length of time for the light to travel here. The "Hubble flow", the way you are using the term, is not a real thing, it is merely a coordinate system that expands with time. There's no "reason" why the object is receding at c, there is only what you mean by the term "receding at c", which is itself coordinate dependent. So what you are really saying is, depending on what we mean by "receding at c", we can get different answers for the time it takes to get here, and that's true. What I meant was, receding from us at near c in an inertial reference frame of the type used in special relativity. On cosmological scales, there is no such global reference frame, and a more sophisticated meaning of "receding at c" must be explicitly specified.



Light emitted by superluminally receding objects have a local velocity of c, but since the velocity of recession at that distance is greater than c, the light has a net velocity away from us. You are using, as your "velocity of recession", the rate of change of proper distance with local proper time. That is indeed one conventionally chosen meaning.

I think the light will reach us eventually since the Hubble sphere is still expanding, but it will take much longer than if the light was emitted from a "stationary" object.I'm not sure what you mean by "stationary". The time it takes light to reach us will never depend on the speed of the object that emits it, when speed is measured relative to other objects at that same location. Light from all objects at any location emitted at the same time will always get to us at the same time, no matter how the objects are moving.

Delvo
2006-Dec-11, 10:53 PM
Does any of this make sense ?It made sense from a Newtonian perspective (because Newton, like us in our everyday lives, could only observe objects that were moving at practically zero compared to the speed of light), but it didn't count for the way space and time stretch and squeeze at higher speeds. Light emitted from any object toward us still always approaches us at the speed of light, no matter what speed the object was moving at or in what direction. It's not like an object you throw out the back of a moving vehicle and subtract the vehicle's speed from the object's speed. It's always the same no matter where you take your observations from because the speed of the "vehicle" it was thrown out of has nothing to do with it at all.

Here's a simpler problem to get you started thinking about light and speed in the right context, since the one you're beginning with isn't exactly beginner-level. Two space ships fly away from each other, each one going at some high fraction (more than half) of the speed of light. Don't overcomplicate by worrying about having them try to shine lights at each other or anything like that for the moment. Just try to figure out what their combined "separation speed" is. It's not greater than the speed of light. It's just a higher fraction of it.

Speeds are not truly distance divided by time; they're just fractions of c...

publius
2006-Dec-11, 11:04 PM
You are using, as your "velocity of recession", the rate of change of proper distance with local proper time. That is indeed one conventionally chosen meaning.
I'm not sure what you mean by "stationary". The time it takes light to reach us will never depend on the speed of the object that emits it, when speed is measured relative to other objects at that same location. Light from all objects at any location emitted at the same time will always get to us at the same time, no matter how the objects are moving.

Ken,

This reminded me of a curve ball the Born metric threw me (I don't know if you saw the "metric fun fact" thread, but "Born" is the proper name for what I previously called the Coriolis metric. Max Born was apparently the first to explicitly derive it or at least do serious work with it).

Anyway, outside the stationary limit, r > c/w, light, <ahem>, ...moves backwards.......<cough>. :lol: Sitting in our swivel chair rotating around, if we watch a light beam fired in the direction of rotation past the limit, we'd actually see it move backwards in our coordinates. If that beam was fired at another observer (and say for simplicity both were actually inertial), we'd both moving at a very high speed in the opposite direction of rotation. If the lead observers fired a beam of light toward the trailing observer, we'd see the light go backwards, and the trailing observer actually catch up to it.

The trailing part of the wave train would be "ahead" of the foward part, and our trailing obsever would catch up to the fore end first. In our coordinates, the trailing observer catches up and passes the light wave train.

If those two characters were exactly at the stationary limit, light wouldn't "move" at all in our coordinates. The lead observer would stretch out a wavetrain as he moved, and that wavetrain would remain completely stationary in our coordinates. The trailing would catch up with it, and "read it" as he went passed it! :lol:

Most everyone reading the above is probably screaming and throwing things at the screen now, "NO!!! That can't be". Well, that is frame dragging. :lol: We all know one of Einstein's thought experiments was imagining some observer moving at light speed watching a beam of light, and realizing that Maxwell would not allow one to see such a stationary wave train of E and B like that.

Well, Maxwell is modified when frame dragging is afoot in the metric. You can have both E and B due to a stationary charge or current distribution.....

-Richard

buzgz
2006-Dec-12, 12:50 AM
Light from all objects at any location emitted at the same time will always get to us at the same time, no matter how the objects are moving.

You're correct, of course. I can only claim good whiskey mixed with bad physics yields ???

I will wait awhile before I address your other comments.

Ken G
2006-Dec-12, 03:38 AM
Anyway, outside the stationary limit, r > c/w, light, <ahem>, ...moves backwards.......<cough>.

That's quite true, as long as we recognize that this statement requires a specification of what is meant by "moves". That's becoming kind of the theme of this thread! As you clearly laid out, this is what is known as a coordinate speed-- motion relative to a coordinate system. It is not motion relative to the observer themself-- if you swivel in your chair, nothing is actually set into motion relative to you, only to your perception of your own coordinate system. Kind of the opposite of the standard Big Bang coordinate system-- that is chosen to perceive as stationary things that are quite naturally thought of as moving!

Ken G
2006-Dec-12, 03:39 AM
You're correct, of course. I can only claim good whiskey mixed with bad physics yields ???


That's still better than good physics with bad whiskey...

Grey
2006-Dec-12, 03:20 PM
That's still better than good physics with bad whiskey...Hmm, but what would we get if we had good physics and good whiskey? Perhaps an experiment is in order... :)

Ken G
2006-Dec-12, 05:45 PM
Single malt? Talisker might be the next best thing to the Nobel prize...

buzgz
2006-Dec-12, 09:12 PM
Speeds are not truly distance divided by time; they're just fractions of c...

This seems to indicate that you think velocities of separation cannot exceed the speed of light. If that is what you intended to imply, then we are in disagreement.

Hubble's law predicts superluminal recession at large distances, specifically at distances D>c/H. Of course, all galaxies that are receding from us are at rest locally, and it is the expansion of space that produces superluminal recession velocities at large distances.

Grey
2006-Dec-12, 10:56 PM
Single malt? Talisker might be the next best thing to the Nobel prize...Talisker is a fine, fine beverage.* A Nobel prize would probably be better (if nothing else, think of just how many bottles you could purchase with the prize money!), but since I'm unlikely to ever receive the latter, at least I can console myself with the former. :)

* Though there are certainly other excellent single malts, and for myself at least, I'd be very hard pressed to choose a favorite.

Ken G
2006-Dec-12, 11:13 PM
I wonder what is the record number of single malt Scotch's (not to choose one in particular) that has ever been bought with Nobel funds? Is that prize therefore an elaborate way of making TNT to buy scotch?

publius
2006-Dec-12, 11:28 PM
Scotch is too hifalutin (that's the correct spelling, I assure you ) for my simple blood. Jim Beam is my speed (the green label, "Jim Beam's Choice", is my favorite, but it's hard to find and they don't advertise it much), and then ol' Jack Daniel's product, although they dropped that to 80 proof so they could see it all over the country and not have to maintain different proof mixtures.

But, I assure you all, there's no liquor like the "tax free" kind (that's made correctly -- tax free corn squeezin's is a dying art) that comes in mason jars. Something about being illegal makes it so much better.

This is family forum with young un's readin' and therefore I can't tell this story in any detail, but back in college, I introduced some city boy buddies of mine to that tax-free liqour. I let 'em drink it like "store bought" stuff without saying a word..........boy, that was a night they'll remember (or remember not remembering) for the rest of their lives.

But it's a dying art, and I can't find any locally nowadays (well, stuff that wouldn't make you go blind). I've actually been thinking about trying it myself --- well, learning about it of course. As it is illegal, I would never, ever, never, ever even think of actually making it, of course.


-Richard

buzgz
2006-Dec-12, 11:34 PM
Is it illegal to make the "tax free" kind, or is it just illegal to sell it ?

From the little I've sampled, it ought to be illegal to drink it.

publius
2006-Dec-12, 11:44 PM
Is it illegal to make the "tax free" kind, or is it just illegal to sell it ?


That depends on your jurisdiction, but as I understand it, Uncle Sam (that's the alcohol part of the vaunted Bureau of Alcohol, Tobacco, and Firearms) wants his cut if it's for drinking. In many areas, fermenting your own beer or wine is legal for your own use, but distilling it is another matter.


From the little I've sampled, it ought to be illegal to drink it.

And that's because got a batch made by someone who didn't know what he was doing. It's a dying art, but the old timers who knew how could make some good stuff.

-Richard

punkrockbong151
2006-Dec-13, 05:11 PM
well, i beleive easily that there is a speed faster than the speed of light, even if it isn't so easy to accomplish, i'm not really sure if there is infinite speed, because at a certain speed everything around u slows, so i geuss nothing can stop 100% percent, and wether we have the resourses, the time, the science, or even the comprehension to, i beleive there is a speed faster then light

publius
2006-Dec-14, 12:43 AM
Is it illegal to make the "tax free" kind, or is it just illegal to sell it ?

From the little I've sampled, it ought to be illegal to drink it.


I just checked this out to make sure. It is indeed flat illegal in the US to distill alcohol (at least for drinking purposes -- there was talk about changing things to allow people to distill for ethanol fuel -- if something is added to make it undrinkable, it might be different ) without a license, and there are federal volume taxes on it. IOW, you must have a federal license and pay so much per volume of distillate, so running a still is still a federal crime. The revenuers (BATF, or whatever they call it now after all the federali alphabet soup reshuffling) can still come after you. :lol:

And then there are state laws and taxes on top of that that vary from state to state.

But, it is completely legal to ferment your own, you just can't sell it without licenses and taxes and all that.

That is sort of funny. You can produce a compound using nature's own little buggers, but you can't legally concentrate that compound. The idea is the stronger stuff is more dangerous, more likely to cause unruly and bad behavior. But, from what I gather, at least in the US, most of that unruly behavior, drunken driving and all that is due to beer, which has the least alcohol content of all.

-Richard