Why Doesn't Relativity Apply to Light?

[Line]

Why is it that relativity doesn't work with light? I'm trying to understand
if light is moving at you at the speed of light, and you are moving towards it at the speed of light why would your equipment measure it at the speed of light. Shouldn't it read twice the speed of light?

 

[jtbell]

(I'm going to use 0.9c for your velocity because massive objects can't travel at c.) In relativity, the "velocity addition" formula is different from what you're probably accustomed to. Instead of u' = u + v = c + 0.9c = 1.9c you have to use

\frac{u + v}{1 + uv/c^2} = \frac{0.9c + c}{1 + (0.9c)(c)/c^2} = c

 

[Line]

Ok but it doesn't explain it.

 

[MeJennifer]

Why is it that relativity doesn't work with light? I'm trying to understand
if light is moving at you at the speed of light, and you are moving towards it at the speed of light why would your equipment measure it at the speed of light. Shouldn't it read twice the speed of light?

Well think about it, how do you know you are moving towards it with a certain speed?
It is always relative to something else, and how do you know that it is you who is moving.
When someone states that something is going at x% of the speed of light it is always relative to something else. Everything with mass is always going at 0% of the speed of light in an absolute sense.

Only a change in movement, e.g. acceleration is absolute, but even there you can have the situation that some frames think you decelerate while others think you accelerate.

 

[pervect]

Why is it that relativity doesn't work with light? I'm trying to understand
if light is moving at you at the speed of light, and you are moving towards it at the speed of light why would your equipment measure it at the speed of light. Shouldn't it read twice the speed of light?

You can't move at the speed of light.

As other posters have mentioned, for any velocity lower than 'c', if you move towards the light source (or if the light source is moving towards you) you will still measure the speed of light as 'c'.

The later case has been measued in the laboratory - the speed of gamma rays emitted from VERY fast moving particles has been measured, and been found to be equal to 'c', regardless of the motion of the source.

 

[Emanresu]

The speed of light is constant no matter how you are moving or how the light source is moving. This is fact and has been well proven.

To answer your question then something has to give. It turns out that the things we expect to be always constant - time and distance - are not.

You have to get over this fact before you can start dealing with all the apparent paradoxes in relativity.

E.

 

[AnssiH]

I believe Line is trying to ask, what is it that justified the idea that the speed of light is isotropic in the first place.

After Einstein had considered simple emission theory (where speed of light is measured as C + V of the emitter), he couldn't really make it work and started to consider other options. Usually when we think about motion, we think about ordinary Newtonian velocity addition (C+V), but Einstein started to ponder on the possibility, that different "inertial frames" are symmetrical also by the speed of light they measure.

It is not immediately obvious how one could make that work. Like he put it himself, the "principle of relativity" (Newtonian) and the "principle of the constancy of the velocity of light" are two "apparently incompatible" principles. But there was a way out if that mess.

The key to relativity theory (and this is how relativity should be taught to mainstream), is to understand, that it is an arbitrary assumption about reality, that two events that happen in two different places "simultaneously", are really simultaneous in an absolute sense. When we receive information about event A and event B that happened in opposite directions, we cannot really say which one had happened first. So to replace Newtonian addition of velocities, Einstein could attach a different notion of "simultaneity" to each inertial frame, in such way that when a beam of light does hit an observer, we can figure out just when the light started its journey by claiming it moved at the speed C. And the way to make this work as a coherent whole, is to perform Lorentz-transformation on spacetime between inertial frames. (hence the idea that reality is a spacetime)

Note how the relativity is simultaneity is NOT about the order in which you observe events, but it is an assumption made about the real "now-moment" being relative, so to replace Newtonian velocity addition.

And when you consider motion this way, it just follows that by accelerating towards another object, your notion of simultaneity is also changing in such manner that you can never reach relative speeds more than C between each others. And what also follows is time dilation, and length contraction; The geometry of an object is defined by where each of its element is in space simultaneously, but as notion of simultaneity changes, the back and the front (to the direction of motion) of the object are found to exist closer to each others "simultaneously".

Well, by invoking the idea of length contraction may be saying too much. Because after all is said and done, you should be pretty careful with the ontological idea that there really is a spacetime instead of motion.

Here is a good longer read about how Einstein got to relativity:
http://www.aip.org/history/einstein/essay-einstein-relativity.htm

 

[AnssiH]

the closing speed of two anythings- like light and mirror for example-
can't be constant no matter if light is a wave traveling in a stationary
ether, a wave traveling in a moving aether, or a free particle.

To measure the speed of any thing, you must register its position at one moment, and then again at another moment.

If you allow for adjusting these "moments", you can always get to isotropic C for light (as long as its the absolutely fastest thing in the universe, for anything faster would be moving backwards in time in some inertial frames, and so it would break the whole logic of static spacetime).

You take a sound wave in a windless room. The sound wave travels at c in that
room always but if you move towards it, the closing speed is c+v. That +v is the difference between you standing still in the room and moving towards the speakers. Light speed is no different. If you standing still,
the light is c, you move towards it the light is c+v. Period.

If not, there's no difference between standing still and moving.

And there's no incompatability between the principle of relativity and
a constant speed of light. As long as this constancy is compared between
different REST frames. That is as long the source of the light and
the detector of the light are at rest with each other. No incompatability,
just confusion by the SR people.

Well, you are talking about a model that is completely different from SR. I personally think it is very much possible, if not even likely, that the speed of light is not constant, but the path it takes is a result of the complex "web of matter" that is present in the situation. But that is completely different matter, what is important here is that SR is logically valid model as far as current observations and useful predictions go. Emission theories are not completely unproblematic either, albeit somewhat possible.

Nevertheless, just saying "it is so" doesn't get us anywhere if we cannot show that it really is so.

 

[russ_watters]

Why is it that relativity doesn't work with light? I'm trying to understand
if light is moving at you at the speed of light, and you are moving towards it at the speed of light why would your equipment measure it at the speed of light. Shouldn't it read twice the speed of light?

Relativity does work for light - it is about light! That's the whole point (the reason it was developed) - to explain/deal with the observed behavior of light.

What you are really asking is why does light not work the way particles work and the answer is (redundantly): because it is light! It has no (rest) mass so it travels at C.

If, then, you are meaning to ask why light behaves like light -- well, why is anything the way it is? It just is. And that's not a question for science.

AnssiH - neophysique deleted the post you responded to and it would probably be best if you deleted your response to avoid confusion...

 

[Line]

And why is it that light can't rest?

 

[Line]

Well think about it, how do you know you are moving towards it with a certain speed?
It is always relative to something else, and how do you know that it is you who is moving.
When someone states that something is going at x% of the speed of light it is always relative to something else. Everything with mass is always going at 0% of the speed of light in an absolute sense.

Only a change in movement, e.g. acceleration is absolute, but even there you can have the situation that some frames think you decelerate while others think you accelerate.

No something goign X% of C is relative to the speed of light.And what's this that any mass is always movign at 0% of C. In that case they're not moving. WHat about cosmic rays, they move at near the speed of light.

 

[JesseM]

No something goign X% of C is relative to the speed of light.

No, MeJennifer is correct, you can only talk about about the speed of an object relative to another, or relative to a particular reference frame. When people use a phrase like "0.8c", what they mean is that in the reference frame they're using, the velocity of the object is 0.8 that of light in that particular reference frame, but in other reference frames it'd be a different fraction. Light travels at c in all reference frames, and relative to all observers--so if you are traveling at 0.8c in my reference frame, that doesn't mean you will only measure the light beam to be moving away from you at 0.2c, you will still measure the light beam to be moving away from you at exactly 1 c in your own reference frame.

 

[Line]

Can someone give me a down to Earth explanation why this works?

 

[Aether]

Can someone give me a down to Earth explanation why this works?

Newton's laws are valued for their simplicity, and they are reasonably accurate when relative speeds are low. An "inertial frame" is a coordinate system in which Newton's laws hold (e.g., we assume that Newton's laws are accurate, and we build a set of coordinate systems that make this so).

Newton's laws become less and less accurate as relative speeds increase, and they become completely wrong as speeds approach c. This is because we measure time with clocks and distance with "rulers" (theodolites), and these measuring devices are physically affected by speed changes.

Special relativity is a tool for describing mathematically how two or more inertial frames relate to one another. It is a mixture of mathematical and physical concepts.

 

[Sojourner01]

'Light' is a manifestation of disturbances in the electromagnetic field. Disturbances in the electromagnetic field propogate at a constantly-measured speed because they do. There isn't an explanation, that's just how fields behave. It's kind of like a first cause.

 

[Aether]

I personally think it is very much possible, if not even likely, that the speed of light is not constant...

Disturbances in the electromagnetic field propogate at a constantly-measured speed because they do. There isn't an explanation, that's just how fields behave. It's kind of like a first cause.

We can measure round-trip speeds in a coordinate-system independent way, and the round-trip speed of light is isotropic. We can not measure any one-way speed in a coordinate-system independent way (at least not so far).

 

[Sojourner01]

I'm not sure whether you're disagreeing with me or not, there. Am I being quoted because I said something inaccurate?

 

[Aether]

I'm not sure whether you're disagreeing with me or not, there. Am I being quoted because I said something inaccurate?

Your statement may be accurate if you are only talking about round-trip EM field propagation, but it is not accurate if you are also talking about one-way propagation. The difference is that we can actually "measure" round-trip speeds (isotropy) in a coordinate-system independent way, but not one-way speeds; at least not so far.

 

[Farsight]

Why is it that relativity doesn't work with light? I'm trying to understand if light is moving at you at the speed of light, and you are moving towards it at the speed of light why would your equipment measure it at the speed of light. Shouldn't it read twice the speed of light?

It's difficult to get a handle on all this if you're lumbered with light travels at the speed of light. It's rather like talking about bullets traveling at the speed of bullets. In a weird kind of way light doesn't move at any speed. What relativity is all about is how we perceive things like time and speed via comparison of electromagnetic propagation within our atoms, bodies, and clocks, against electromagnetic propagation over some perceived distance. All this can get very deep very quickly, so to keep it simple for now: instead of thinking light travels at the speed of light, think light travels at the speed of time.

 

[Doc Al]

All this can get very deep very quickly, so to keep it simple for now: instead of thinking light travels at the speed of light, think light travels at the speed of time.

Give us a break!

Much better would be to accept that fact that--per relativity--there is a "speed limit" built into the very structure of space-time itself. This speed limit, which happens to equal the speed of light in vacuum, affects the kinematic behavior of everything: light, bullets, you name it.

The implication for light itself is that, as has been mentioned several times in this thread, light must travel at speed c as measured by any inertial observer.

To really understand what this means you'll have to break down and learn some relativity. (I recommend N. David Mermin's latest pedagogical effort, "It's About Time".)

 

[rbj]

Can someone give me a down to Earth explanation why this works?

first, Line, do you understand that "light" is the propagation of electromagnetic (E&M) fields or "waves" and the physics that describes that propagation (called "Maxwell's Equations"). are you at that level? if not, i don't know quite where to begin with a down to Earth explanation.

i would not call the constancy of c (for all frames of reference) an axiom for which there is no idea why such principle exists (and we just notice it experimentally). it's because we can detect no intrinsic difference between different inertial frames of reference (two observers moving at constant velocities relative to each other both have equal claim to being "stationary", there is no good reason to say that one is absolutely stationary and the other is the one that is moving) and that the laws of physics, namely Maxwell's equations, apply to both frames of reference equally. if two different observers, neither accelerated and moving relatively to each other, are examing the very same beam of light (an electromagnetic wave), for both observers, when they apply and solve Maxwell's equations for the propagation of the EM wave, they both get the same speed of c out of solving Maxwell's eqs.

so we do have a good idea for why the speed of propagation of E&M is the same for all inertial observers that may or may not be moving relative to each other. it's because, we cannot tell the difference between a "moving" vacuum and "stationary" vacuum, that there is no difference between a moving and stationary vacuum and then there is not apparent reason for the observed speed of light to be different.

this is different than for sound. the physics of Maxwell's Equations make no reference to a medium that conducts the electromagnetic field (and, indeed, the Michaelson-Morley experiement failed to show that such a hypothetical medium, called "aether" exists - if it does exist, it seems to be moving around in the same frame of reference as the Earth going around the sun because no matter what time of day or season of the year, no one could detect with the M-M apparatus any motion through this aether). but for sound, the physics describe it as compressions and rarefractions of the air (or whatever other matter medium). there is no such thing as sound in a vacuum (but there is light). so if you feel the wind moving past your face from left to right (say at a speed of 20 m/s), you will also measure the speed of sound from a source on your left to be 20 m/s faster than sound from a source in front of you and 40 m/s faster than a sound that is at your right. now you can repeat that setup and get an identical result if there is no wind but you are moving (toward your left) through the air at a speed of 20 m/s. so the observer that is stationary (relative to the air) will look at a sound wave and measure it at something like 334 m/s, but you, moving through the air toward the source at 20 m/s will measure the speed of that very same sound wave to be 354 m/s.

now think of the same thing, but instead you two observers are out in some vacuum of space somewhere and are looking at the same beam of light. the other observer is holding the flashlight and measuring the speed of light to be 299792458 m/s. you are moving toward that observer at a speed of, say, 1000 m/s and looking at the very same beam of light that the other observer is. you are thinking that you would measure it at a speed of 299793458 m/s, right? but why should it be any different for you? you have equal claim to being stationary (and it's the guy with the flashlight is moving toward you at 1000 m/s). you cannot feel the vacuum moving past you at a speed of 1000 m/s, in fact there is no physical meaning to the vacuum moving past your face at 1000 m/s like it's a wind. you cannot tell the difference between you moving and the other guy as stationary or if the roles were reversed and there is no meaning to any notion of who is stationary absolutely and who is moving.

so then, if there is no meaningful difference, if both of you have equal claim to being stationary (and it's the other guy that is moving), then the laws of physics (particularly Maxwell's Equations) have to be exactly the same for both of you, both in a qualitative sense, and in a quantitative sense. both of you have the same permittivity of free space (\epsilon_0) and permealbility of free space (\mu_0). so when you apply Maxwell's equations to this E&M wave (of this flashlight beam), you will see that this changing E field is causing a changing B field which, in turn, is causing a changing E field which is causing a changing B field, etc. now for both of you, the laws (Maxwell's Eqs. and the parameters \epsilon_0 and \mu_0) are the same. now, it turns out, when we solved Maxwell's Equations for this case, we get a propagating wave and the wave speed is

c = \sqrt{\frac{1}{\epsilon_0 \mu_0}} [/itex]<br /> <br /> <b>but that&#039;s the same for both you and the other observer!</b> (even though you are both moving relative to the other.) there is no reason that the other guy should solve the Maxwell&#039;s equations and get a different c than you get (because you have the same \epsilon_0 and \mu_0)! even if you two are looking at the very same beam of light. this is as far down to Earth as i can put it. does it do it for you, Line?

 

[MeJennifer]

Wow, great explanation rbj!

 

[AnssiH]

Farsight is not far off the mark, it is much clearer to look at light as it exists in static sense in spacetime if you really want to understand relativity.

It is not like relativity is based on the idea that "nothing moves faster than light". This speed limit is just what naturally follows from the two postulates. It is rather topsy-turvy way to explain relativity to someone as if it is the assumption about the speed limit that causes relativistic effects.

Like I said, it is imperative to understand how relativity of simultaneity can replace Newtonian velocity addition. This is the key to grasping the idea, and everything else (like the speed limit) will follow. This is the unintuitive part of the theory, and this is exactly what makes it tick. It is rather abhorrent to reply to Line's question with "Because the formula to calculate motion is this and that". Having any arbitrary formula doesn't change reality. It is the formula that follows from the assumption about relativity of simultaneity!

So, Line, let's try a simple thought experiment for a fit.

I assume you are very much familiar with Newtonian relativity of motion.

Let's consider a lab frame where you are standing 10 light seconds away from a pole (You and the pole are at rest in lab frame).

Let's say the speed of light is mere 1 m/s. (the pole is 10 meters away from you)

Whenever you receive a light pulse from the pole, we assume the light started its journey 10 seconds ago. And indeed, we can verify this by placing a clock at the pole to register the moment the light departs. If the clock by the pole shows "0 seconds" when the light departs, an identical synchronized clock by your position shows "10 seconds" when the light is received.

So far all fine and well, but what if we suppose there is another observer (B), only he is moving towards the pole at 5 m/s. Let's consider a light pulse that you both receive at the very moment you pass each others. How could it be that the light was approaching him also at 10 m/s instead of 15 m/s? How to "lower" the speed of light from 15 m/s to 10 m/s?

Well, looking at the situation from B's inertial frame, the moment the light departed is not known. We get to the intuitive 15 m/s only by assuming the light departed at such and such moment. If we can assume the light departed much earlier, we can lower the "required" speed to 10 m/s. (Note how in B's frame the light moves much longer distance than 10 meters; after all, it is the pole that is moving towards B, and the light must have started its journey much earlier than when the distance between was reduced to 10 meters)

If there are clocks at rest in B's inertial frame that are registering where the light pulse was at different moments, the clocks would show - according to relativity - that the pulse was on its way much longer than 10 seconds.

This assumption about relativity of simultaneity (=notion of simultaneity is different in each inertial frame) also means that at the moment you two are passing each others, the clock at the pole has different reading in Bs inertial frame from that of yours. In your "now-moment" the clock reads 10 seconds (albeit you cannot see this yet), and in his "now-moment" it reads more than 10 seconds (obviously he cannot see it either).

It also immediately follows, that if you consider there to really exist any "now-moment", then you must also accept that if the observer now stops, the clock in his own "reality" snaps backwards in time to 10 seconds (beyond his observations of course). Hence the idea about static spacetime.

And finally, when you assume this sort of reality, from the point of view of that light pulse, time did not move at all during its "journey". When it departed, sure, the clock at the pole showed 0 seconds, but the clock in your wrist was already showing exactly 10 seconds and observer B was also already next to you. So in a very real sense, relativity says that the light did not move at all (hence what Farsight said). Its whole path merely "exists" in different inertial frame from yours, and in its own frame its whole motion exists "simultaneously". Only in our frame we find it in one place at one instant and in a new place at another instant.

Well, I hope this helps little bit. It may take a moment to really wrestle the idea in. If you have access to any 3D-modeling software, it may help to build spacetime diagrams which you can then simply scale to perform Lorentz-transformation (to get from one inertial frame to another properly). I'll explain you how if you want to.

-Anssi

 

[AnssiH]

so we do have a good idea for why the speed of propagation of E&M is the same for all inertial observers that may or may not be moving relative to each other. it's because, we cannot tell the difference between a "moving" vacuum and "stationary" vacuum, that there is no difference between a moving and stationary vacuum and then there is not apparent reason for the observed speed of light to be different.

I must add, that even if there was ordinary velocity addition to the speed of light, you couldn't tell which observer is moving. You could merely tell if you are moving relative to a given light source. (After all, Line's ordinary question was why there is no velocity addition)

Einstein's motivation, or at least part of it, to disgard velocity addition was that with relativity of simultaneity you can keep light from "mixing up" rather trivially (This all of course with rather naive idea about just what is "space" or "vacuum", but nevertheless). Of course with aether you could also keep light from mixing up, but that idea is even more naive.

And let it also be said that the constancy of speed of light is not a requirement for symmetry between different inertial frames. The constancy is a separate postulate on top of "ordinary" symmetry. Even if C was not isotropic, the laws of nature would be symmetrical (Maxwell's equations would need to be adjusted in some cases, but they would hold "within" any observer), or another way to put it, isotropic C does not make the measurements of light symmetrical between inertial frames (each measure different frequency in any case)

 

[Thrice]

Can someone give me a down to Earth explanation why this works?

Light is a strange thing. It has 0 mass and yet it carries momentum. In order to know where it is, you have to give up information about how fast it's travelling. And vice versa. It exhibits both wave-like and particle-like properties. How is that possible? And that's just the beginning

It has a well defined velocity, but no rest frame. Meaning you can't look at things from the photon's perspective. As you've probably heard, you hit a singularity when you try reach the photon's frame. This singularity suggests that thinking of light as an entity in itself moving through space might be a little misleading. Think of it as interaction between two massive bodies along a null spacetime interval. The two events - emitting light and absorbing it are simultaneous. That's why c is the maximum speed of interaction.

So think about it that way. You can move at any speed you want. Given enough fuel, you can get to anywhere in the universe in 5 min. The question of what time their clocks read when you get there is a little more complicated. Point here is however fast you're going, you cannot get past that singularity. You still take time to get where you're going.

 

[rbj]

I must add, that even if there was ordinary velocity addition to the speed of light, you couldn't tell which observer is moving. You could merely tell if you are moving relative to a given light source. (After all, Line's ordinary question was why there is no velocity addition)

well there is velocity "addition", it just that this:

\frac{v_1 + v_2}{1 + \frac{v_1 v_2}{c^2}}

is how you add it.

And let it also be said that the constancy of speed of light is not a requirement for symmetry between different inertial frames. The constancy is a separate postulate on top of "ordinary" symmetry. Even if C was not isotropic, the laws of nature would be symmetrical (Maxwell's equations would need to be adjusted in some cases, but they would hold "within" any observer), or another way to put it, isotropic C does not make the measurements of light symmetrical between inertial frames (each measure different frequency in any case)

i don't quite get that. if c is not isotropic, then at least one set of laws of nature (Maxwell's Equations) are not the same (and least not quantitatively) for every observer. in contrast, for sound, there is a quantitative difference in the physical equations describing sound propagation between one frame that is wind-free and another that moves through the air at a known velocity.

 

[Aether]

if c is not isotropic, then at least one set of laws of nature (Maxwell's Equations) are not the same (and least not quantitatively) for every observer.

If c is isotropic in at least one (preferred) inertial frame, then Maxwell's equations can be expressed in tensor notation, and they will agree with all experiments conducted in that frame. Then we may choose to do physics in any coordinate system as long as we do all the transformations correctly. For further details look http://bautforum.com/showpost.php?p=724212&postcount=124".

 

[Line]

OK so light doesn't move? SO in other words it's us who are moving at the speed of light?


ANd I though we didn't know if light was made of particles or waves.

 

[Thrice]

OK so light doesn't move?

Well ... You can think of it like it moves infinitely fast, roughly speaking. You can measure a finite speed of 300K km/s, but when you actually try reach that speed, you find you need an infinite amount of fuel or whatever so it's actually infinitely far away. That's roughly why saying "twice the speed of light" doesn't make sense.

Some of this is a lot easier to understand with math. I'm twisting it to make it easier & if I do it much more these fine people will run me out of the forum :-)

ANd I though we didn't know if light was made of particles or waves.

It has characteristics of both.

SO in other words it's us who are moving at the speed of light?

Not sure how you got that, but no.

 

[Epsilon Pi]

Light is a manifestation of energy, as someone wrote in this forum, light is an electromagnetic wave composed of massless photons.

EP

 

[AnssiH]

well there is velocity "addition", it just that this:

\frac{v_1 + v_2}{1 + \frac{v_1 v_2}{c^2}}

is how you add it.

Whoops, let me rephrase myself; There is no velocity addition to the motion of light.

That assertion was a response to Line's original question; "I'm trying to understand if light is moving at you at the speed of light, and you are moving towards it at the speed of light why would your equipment measure it at the speed of light. Shouldn't it read twice the speed of light?"

i don't quite get that. if c is not isotropic, then at least one set of laws of nature (Maxwell's Equations) are not the same (and least not quantitatively) for every observer. in contrast, for sound, there is a quantitative difference in the physical equations describing sound propagation between one frame that is wind-free and another that moves through the air at a known velocity.

That's valid if you consider the ether theory, but if you consider emitter theory (which is how I interpreted Line's velocity addition question), then there is no ether to reveal any universal motion against light. In velocity addition scheme Maxwell's laws would be found to be just what they are within any single system (like in experiments on earth). The refutations against emitter theory are different than those against ether theory.

Yet another twist to the tale comes from considerations about just what must be the relationship between matter and space in any scheme (I referred to "naive idea of space" earlier). There is no empirical data to give us any metaphysical "border" between an atom and space, and it seems quite useful to consider atoms in an extended sense, or that the "environment" between pieces of matter is a manifestation of the given pieces of matter themseves. I.e. that there is no such thing as empty space, but information simply moves in matter (in its extended sense).

This makes the borderline between different objects quite fuzzy, but so it appears to be in any case. This could provide some openings to consider how gravity exists and how QM phenomena exists (=light can know something about the state of the atom it is heading to). And yes, it does bring the tale very close to GR, only GR builds this picture in spacetime instead of just space (which may or may not be the correct way).

Well, the point I want to make is that relativity too, was laid onto quite simple assumptions about what is space, and AFAIK no one really investigated the possibility that matter is "extended". Hmmm, but this hasn't got that much to to do with Line's question anymore, so Line, don't confuse your head with such ideas before you get relativity. :)

 

[AnssiH]

OK so light doesn't move? SO in other words it's us who are moving at the speed of light?

Well sort of; some people like to think of it as if we are moving through spacetime at the speed of light. At rest we move through time dimension and objects that are moving in our frame use some of that speed to move in space dimensions also.

But while it may seem like a fun idea, I would not encourage that sort of interpretation. It doesn't really give you the whole picture (you have to understand what it means to change direction in space), and any idea of "motion" inside static spacetime is rather unelegant, to the point of being invalid as an ontological interpretation (while as a tool to understand real phenomena it may be just fine).

So, if you kind of get how relativity of simultaneity could offer a way to constant C, I think you are ready to look at what it means to change your direction (velocity) in spacetime. You can undestand this too without any math (contrary to the popular belief :)

http://en.wikipedia.org/wiki/Lorentz_transforms

Look at the animated spacetime diagram. This does not express space, but one dimension of space (horizontal) and one dimension of "time" (vertical). The dots are not objects but events that happen in one particular location in space at one particlar moment of time.

The lower diagonal lines represent the motion of information from spacetime reaching you (bottom lines). I.e. the events on that line are what you "see" at one instant.

If you imagine a straight horizontal line in the center, that would represent the "now-moment", or at least what you would assume to exist at one particular moment.

Notice how, when you change direction (accelerate to left or right in space), the whole spacetime diagram is scaled (in 45 degree angle) instead of just tilted. In velocity addition you would simply tilt the diagram, but in Lorentz-transformation you scale it.

Notice also how in some cases some events move from below that horizontal line (past) back to above the line (future), and notice how even then, you can never see this (at the diagonal lines)

And that's what it means to change direction in the case that simultaneity is relative to different directions of motion.

 

[Line]

SO not only do we not know if light is particle or wave but photons don't have mass? Is this like electrons hwhich have mass but it's too small to matter or it has no mass whatsoever?

And light has no frame for itself? DO we have the right frame? Cause from what I understand we are spinning around the Milkyway Galaxy at the speed of light. And who knows how fast the universe is moving. So just the galaxy moving at the speed of light means that light really moves at twice C.

 

[Line]

Farsight is not far off the mark, it is much clearer to look at light as it exists in static sense in spacetime if you really want to understand relativity.

It is not like relativity is based on the idea that "nothing moves faster than light". This speed limit is just what naturally follows from the two postulates. It is rather topsy-turvy way to explain relativity to someone as if it is the assumption about the speed limit that causes relativistic effects.

Like I said, it is imperative to understand how relativity of simultaneity can replace Newtonian velocity addition. This is the key to grasping the idea, and everything else (like the speed limit) will follow. This is the unintuitive part of the theory, and this is exactly what makes it tick. It is rather abhorrent to reply to Line's question with "Because the formula to calculate motion is this and that". Having any arbitrary formula doesn't change reality. It is the formula that follows from the assumption about relativity of simultaneity!

So, Line, let's try a simple thought experiment for a fit.

I assume you are very much familiar with Newtonian relativity of motion.

Let's consider a lab frame where you are standing 10 light seconds away from a pole (You and the pole are at rest in lab frame).

Let's say the speed of light is mere 1 m/s. (the pole is 10 meters away from you)

Whenever you receive a light pulse from the pole, we assume the light started its journey 10 seconds ago. And indeed, we can verify this by placing a clock at the pole to register the moment the light departs. If the clock by the pole shows "0 seconds" when the light departs, an identical synchronized clock by your position shows "10 seconds" when the light is received.

So far all fine and well, but what if we suppose there is another observer (B), only he is moving towards the pole at 5 m/s. Let's consider a light pulse that you both receive at the very moment you pass each others. How could it be that the light was approaching him also at 10 m/s instead of 15 m/s? How to "lower" the speed of light from 15 m/s to 10 m/s?

Well, looking at the situation from B's inertial frame, the moment the light departed is not known. We get to the intuitive 15 m/s only by assuming the light departed at such and such moment. If we can assume the light departed much earlier, we can lower the "required" speed to 10 m/s. (Note how in B's frame the light moves much longer distance than 10 meters; after all, it is the pole that is moving towards B, and the light must have started its journey much earlier than when the distance between was reduced to 10 meters)

If there are clocks at rest in B's inertial frame that are registering where the light pulse was at different moments, the clocks would show - according to relativity - that the pulse was on its way much longer than 10 seconds.

This assumption about relativity of simultaneity (=notion of simultaneity is different in each inertial frame) also means that at the moment you two are passing each others, the clock at the pole has different reading in Bs inertial frame from that of yours. In your "now-moment" the clock reads 10 seconds (albeit you cannot see this yet), and in his "now-moment" it reads more than 10 seconds (obviously he cannot see it either).

It also immediately follows, that if you consider there to really exist any "now-moment", then you must also accept that if the observer now stops, the clock in his own "reality" snaps backwards in time to 10 seconds (beyond his observations of course). Hence the idea about static spacetime.

And finally, when you assume this sort of reality, from the point of view of that light pulse, time did not move at all during its "journey". When it departed, sure, the clock at the pole showed 0 seconds, but the clock in your wrist was already showing exactly 10 seconds and observer B was also already next to you. So in a very real sense, relativity says that the light did not move at all (hence what Farsight said). Its whole path merely "exists" in different inertial frame from yours, and in its own frame its whole motion exists "simultaneously". Only in our frame we find it in one place at one instant and in a new place at another instant.

Well, I hope this helps little bit. It may take a moment to really wrestle the idea in. If you have access to any 3D-modeling software, it may help to build spacetime diagrams which you can then simply scale to perform Lorentz-transformation (to get from one inertial frame to another properly). I'll explain you how if you want to.

-Anssi

I would just sya B's equipment is broken. Is this real and proven? Is there any use for this?

 

[Aether]

Is this real and proven? Is there any use for this?

This is a mixture of mathematical and physical concepts. The two "postulates" are non-physical assumptions (e.g., they are not real, and can not be proven by experiment) that make all the rest which is real and proven by experiment easier to handle mathematically...sort of like arbitrarily placing a set of x,y,z axes anywhere you please in that they are non-physical assumptions that help you to put numbers to things that are physical.

 

[AnssiH]

SO not only do we not know if light is particle or wave but photons don't have mass? Is this like electrons hwhich have mass but it's too small to matter or it has no mass whatsoever?

In the mathematical descriptions light has got no mass, but it causes a momentum on any material it hits. Ontological descriptions can vary. For example, you can consider inertia to be an emergent phenomenon of some sort, and electromagnetism to be just one component of that.

Also it is not necessarily correct to say "we don't know if light is a particle or a wave". Rather we should say it is neither, but it does exhibit some properties of both. Namely, it appears to move as a particle but all its possible trajectories seem to interfere with each others. I think any reasonable ontological interpretation of the math of QED should probably let go of any ideas about photons with identity. (Although alternatively, a spacetime interpretation of QM effects could say something about why the wave/particle duality might exist; https://www.physicsforums.com/showthread.php?t=130623 )

In any case, it is not necessary to understand these properties of light to get the idea behind relativity.

And light has no frame for itself? DO we have the right frame? Cause from what I understand we are spinning around the Milkyway Galaxy at the speed of light. And who knows how fast the universe is moving. So just the galaxy moving at the speed of light means that light really moves at twice C.

Well, the thing that Newtonian relativity shows already is that it is nonsensical to invoke the idea of absolute motion. Motion, as a semantical concept, can only make sense if it is expressed as relative between two objects.

Physicists got, again, hung up on the idea of absolute motion when they imagined ether. It would have been pretty odd if space was like a giant backdrop with identity to its locations. And it would have been pretty odd if space was behaving like matter does. There's no reason to assume that. "Empty space" is just one big unnecessary metaphysical entity if you think of reality that way. Better let it go and look at objects as stable constructions that can have motion in relation to each others but not in relation to any backdrop. This assumption does not yet lead to relativity of time, mind you.

(It is not given that the backdrop doesn't exist, but there is no indication of such a thing, and consequently, if you assume it does exist, then your descriptions of the laws of nature and explanations of various phenomena become very complex)

I would just sya B's equipment is broken. Is this real and proven? Is there any use for this?

Well, it is not directly proven. In fact, the logic doesn't allow for direct observation of relativity of simultaneity. It just allows for observation of time dilation. There are time dilation observations that agree with the predictions of relativity, but it is possible to construct models that make the same predictions with absolute simultaneity (they may not be mathematically as elegant, but one can argue that they are ontologically more elegant).

So, we have observed, for example, that objects in inertial acceleration suffer time dilation, and we have some indications that objects in motion relative to us, will suffer time dilation in our frame (cosmic particles with very short life times seem to live longer than expected, which can be explained by time dilation).

Anyway, I would say it is required to learn the ins and outs of the logic behind relativity, before you can hope to see what elements of it are not necessarily set in stone, and how various experiments have been interpreted according to relativity, and what needs to be interpreted differently in other schemes. Any "other scheme" probably must also say something about inertia and gravity (and their equivalence), and it certainly helps if it can also say something about QM phenomena.

 

[Aether]

Thank-you, AnssiH. That's a fair summary.

Any "other scheme" probably must also say something about inertia and gravity (and their equivalence), and it certainly helps if it can also say something about QM phenomena.

http://www.arxiv.org/hep-th/0501149" a nice paper that was published in a special edition of Annalen der Physik on the 100th anniversary of Einstein's paper on special relativity: C.D. Froggatt et al., Derivation of Poincare Invariance from general quantum field theory, Annalen der Physik, Volume 14, Issue 1-3 , Pages 115 - 147 (2005); hep-th/0501149.

In the present work we shall present a model which has an absolute time without either Lorentz invariance or rotational invariance at the outset. Nevertheless, under very general assumptions, it essentially leads to the familiar Poincare invariant quantum electrodynamics.

 

[gonegahgah]

If, then, you are meaning to ask why light behaves like light -- well, why is anything the way it is? It just is. And that's not a question for science.

What a great answer! Please beam me back to the dark ages.

Can someone give me a down to Earth explanation why this works?

Hi Line, I think I may be able to help.

Although my answer doesn't use terms like 'frame' I will still explain in normal language what a 'frame' is.

A frame is like a picture that you are in. Unlike a normal picture the frame can have things moving in it around you. The frame itself does not move. For the frame the second thing that does not move is the observer (you or the detector). This is because the observer defines the frame. If the observer changes speed or direction relative to the other things in the picture then the frame remains with them. In essense, for a frame, the observer never changes speeds, only the other things in the frame.

So the key thing to remember is that the observer defines the frame. If another observer is moving around in your frame then for them they have their own stationary frame around themselves and it is in fact you moving in their frame; not them.

Does that explain frames for you? Not that it matters but it is nice for you to understand what it means and not have it adding confusion.

Understanding that each observer observes things relative to themselves is one of the founding principles of SR.

Other people here have been kind enough to explain what this means but I will explain it differently for them and you.

Things are all relative to themselves (which establishes the fixed frame around themself of course, but...) so that when anything accelerates it is accelerating relative to you - even if you think you are doing the accelerating.

According to SR only light can move at the speed of light relative to you; nothing else can be moving at the speed of light relative to you (or in your frame). And also nothing can be moving at the speed of light - except light - relative to all other observer's around you (relative to their fixed unmoving frame around themself).

What does this mean? Well easy example. Let's say observer B is moving away from you at half the speed of light. Let's jump to observer B. For observer B it is actually you moving away from them at half the speed of light (in their frame).

Now to help explain things better, let's say that on the opposite side of observer B from you is another observer C. Observer C is moving away from observer B at half the speed of light in the opposite direction (in observer B's frame).

So on one side of observer B is you moving away at half the speed of light and on their opposite side is observer C moving away at half the speed of light. According to SR this is perfectly allowable as neither you nor observer C are moving at the speed of light relative to observer B in their frame. Cool.

Does this mean that observer C is moving away from you at the speed of light? No according to SR because nothing except light can move away or towards you at the speed of light except light. Instead for you (in your frame) observer C will only be moving away at a calculatable greater amount than half the speed of light (obviously it is greater than half because observer B is moving away from you at half light speed and C is further on than B and moving away).

So for you observer C will achieve less distance for the same amount of time than observer C will achieve for observer B.
Let's explain. Observer B will always find itself midway between you and observer C; at the same time observer C will never be twice as far away from you as observer B.

This paradox is supposed to be fixed by aspects of time dilation. So although for observer B he will get the same time report from you as he gets from observer C; for you you will not get the same time report from observer C that you get from observer B if they were both able to report to you at the same time.

So you can see that in B's frame he has you and observer C moving away from each other at the speed of light but not from him. But in your frame you do not see the same thing. In the same amount of time that B sees you and C moving apart at light speed, neither you nor observer C see the same thing in your frames. Instead you will measure C moving slower than twice the speed of B and observer C will measure you moving at less than twice the speed of B; and as such less than the speed of light in your and C's frames.

Now I'm going to spoil things after what has hopefully been the most helpful explanation here. This is because personally I believe this whole theory of SR is the Theory of BS. However, I hope that I have been able to make this more understandable for you.

 

[Doc Al]

In Newtonian/Galilian Relativity, the speed of light c measured in a moving
frame is viewed by a rest frame as c+v. This is simple arithmetic
logic.

In Special Relativity, the speed of light c measured in a moving frame
is viewed by a rest frame as c. What happened to the V then between
the moving frame and the stationary frame? Good question. What happened is that the SR rest observer rescaled the c+ v to c.
Example: 3+1 = 4 is now 3.25/1.25 = 3. Problem is the original
velocity between the frames used the same set of rulers and clocks
for viewing each other. For no logical reason, SR proposed a
new set of rulers and clocks to re-view the same events in the moving frame so that the speed of light would become c. What it ends up
saying is that 3= 3 and 4 = 3.

Not quite. Everyone uses their own, identically constructed, rulers and clocks. True, under SR, rulers and clocks in one moving frame are observed to "shrink" and operate slowly according to measurements made using "stationary" rulers and clocks. But no one is saying that 4 = 3; arithmetic still works just fine.

 

[AnssiH]

Things are all relative to themselves (which establishes the fixed frame around themself of course, but...) so that when anything accelerates it is accelerating relative to you - even if you think you are doing the accelerating.

I want to clarify, to avoid confusion, that acceleration is not relative but absolute. The rate of acceleration is not absolute, but when something accelerates, it really does accelerate (obviously; he also feels the acceleration). Other objects in his frame are still in rest.

I also want to stress that "nothing moves faster than the speed of light" is not a fundamental postulate from which all the other effects spring. Nothing moves faster than light is an effect which springs from the relativity of simultaneity, which springs from the assumption that C is isotropic.

Now I'm going to spoil things after what has hopefully been the most helpful explanation here. This is because personally I believe this whole theory of SR is the Theory of BS.

Yeah well, my hopes are not particularly high either.

 

[russ_watters]

What a great answer! Please beam me back to the dark ages.

Learn to understand the difference between science and philosophy before throwing around one-liners like that. All "why" lines of questioning eventually end up in the realm of philosophy and it is quite common for people just learning about Relativity to understand the concept but not accept it because they don't like the "why". That's what it appeared to me that the OP was after.

 

[gonegahgah]

Russ,

I don't understand your reply. From your post "It just is" you seem to be the one pushing philosophy and not science. You seem to be adherring to the idea of science as a religion of faith rather than as a tool towards ultimately explaining the mechanism.

In this respect I think you have attempted to be obstructive and the least helpful to the original poster in this thread.

 

[gonegahgah]

Just to help clarify things further...

Let's say that you and another spaceship leave the Earth side by side. You are both traveling at one tenth the speed of light away from Earth.
You are designated spaceship A and the other spaceship is shaceship B.

You decide to accelerate away from spaceship B until they are falling behind you at one tenth the speed of light. Soon spaceship B notices and accelerates until he catches up to you.

You repeat this again and again spaceship B is falling behind you at one tenth the speed of light. Again he notices and catches up to you.

You do this again and again. In total you do this eleven times. Each time spaceship B finds himself falling behind you at one tenth the speed of light. This works under SR because spaceship B is moving relative to your frame of reference.

So you've done this eleven times and you think to yourself "well I've accelerated away to be moving away from spaceship B at one tenth the speed of light eleven times so I should be now moving away from Earth at over the speed of light". However you turn around and to your surprise the Earth is still only falling behind you and spaceship B at less than the speed of light.

I hope this clarifies the Theory of (B)SR for you further Line.

 

[JesseM]

Just to help clarify things further...

Let's say that you and another spaceship leave the Earth side by side. You are both traveling at one tenth the speed of light away from Earth.
You are designated spaceship A and the other spaceship is shaceship B.

You decide to accelerate away from spaceship B until they are falling behind you at one tenth the speed of light. Soon spaceship B notices and accelerates until he catches up to you.

You repeat this again and again spaceship B is falling behind you at one tenth the speed of light. Again he notices and catches up to you.

You do this again and again. In total you do this eleven times. Each time spaceship B finds himself falling behind you at one tenth the speed of light. This works under SR because spaceship B is moving relative to your frame of reference.

So you've done this eleven times and you think to yourself "well I've accelerated away to be moving away from spaceship B at one tenth the speed of light eleven times so I should be now moving away from Earth at over the speed of light". However you turn around and to your surprise the Earth is still only falling behind you and spaceship B at less than the speed of light.

I hope this clarifies the Theory of (B)SR for you further Line.

You forgot to include the fact that each time you change speeds, you make use of a different set of clocks and rulers to measure your speed relative to the other ship, and to the Earth--a set which is at rest relative to you at your current velocity, and where all the clocks in the set have been synchronized using the assumption that light moves at the same speed in all directions in the set's own rest frame. Time dilation and lorentz contraction will mean that each set measures every other set to have its rulers shrunk and its clocks slowed down, and the synchronization rule means that each set will see the clocks of every other set to be out-of-sync. Given this, it should not be so surprising that the velocities won't add the same way they do in Newtonian physics, where rulers and clocks don't change depending on their speed, and you can synchronize clocks in a way that won't cause disagreements between reference frames (just set bring them to a common location and synchronize them there, then since there's no time dilation you can be sure they'll stay synchronized when you move them apart).

There is nothing magical about this. In fact, even if we lived in a purely Newtonian world with no time dilation or length contraction, and there was only one frame where light traveled at the same speed in all directions (the rest frame of the ether, say), if we had an observer who set up rulers and clocks in this frame, and for any other observer moving at velocity v relative to him we gave the moving observer phony gag rulers shrunk by \sqrt{1 - v^2/c^2} and phony gag clocks whose ticks were extended by 1/\sqrt{1 - v^2/c^2}, and instructed them to synchronize their clocks by making the (false, in this universe) assumption that light traveled at the same speed in all directions in their own rest frame, then the measurements made on the distorted rulers and clocks would mirror those made on normal rulers and clocks moving at different velocities in relativity. For example, all observers would measure light to have the same coordinate velocity, and if the observer with the normal rulers and clocks measured a missile to be moving at 0.8c, then an observer moving at 0.6c in the opposite direction in the normal observer's frame would measure the missile to be moving at (0.8c + 0.6c)/(1 + 0.8*0.6) = about 0.946c on his system of artificially-shrunk rulers and artificially-slowed-down (and desynchronized) clocks.

 

[Doc Al]

Just to help clarify things further...

Let's say that you and another spaceship leave the Earth side by side. You are both traveling at one tenth the speed of light away from Earth.
You are designated spaceship A and the other spaceship is shaceship B.

You decide to accelerate away from spaceship B until they are falling behind you at one tenth the speed of light. Soon spaceship B notices and accelerates until he catches up to you.

At which time both spaceships watch the Earth recede at a speed of less than two tenths the speed of light. Being savvy space travelers, this hardly surprises them since they understand relativity.

You repeat this again and again spaceship B is falling behind you at one tenth the speed of light. Again he notices and catches up to you.

You do this again and again. In total you do this eleven times. Each time spaceship B finds himself falling behind you at one tenth the speed of light. This works under SR because spaceship B is moving relative to your frame of reference.

So you've done this eleven times and you think to yourself "well I've accelerated away to be moving away from spaceship B at one tenth the speed of light eleven times so I should be now moving away from Earth at over the speed of light". However you turn around and to your surprise the Earth is still only falling behind you and spaceship B at less than the speed of light.

Perhaps "you", having been smuggled aboard from your outpost on some planet still struggling in its pre-relativistic dark age, are surprised at how velocities add. Not so the others.

I hope this clarifies the Theory of (B)SR for you further Line.

The only thing this clarifies is your complete lack of understanding of SR and how velocities really add up.

 

[russ_watters]

In this respect I think you have attempted to be obstructive and the least helpful to the original poster in this thread.

This thread has done just fine without me and will again - I'm out. If you want to discuss this further, we can do so over PM.

 

[gonegahgah]

Hi Doc

Nothing I have said contradicts SR in my explanations of it so please don't be embarrassing to yourself in providing non-rebuttals.

Hi Jesse

I haven't forgotten. However for each person on their spaceship their clock seems to be running normally to themselves and their ruler seems to still be the same length. No matter how fast we travel relative to anything else, we will still measure our elbow to hand to be about 30cm as ours is the preferred frame.

So with the same clock and ruler we will be able to increase our speed relative to spaceship B to being one tenth the speed of light faster no matter how many times spaceship B then catches up again. We can keep doing this again and again. But according to SR this will not result in an equal or greater than speed of light relative to the Earth for our ships. This is as you say for our clocks and rulers relative to the Earth nor for the Earth's clocks or rulers relative to us.

This is what SR says.

 

[JesseM]

Hi Jesse

I haven't forgotten. However for each person on their spaceship their clock seems to be running normally to themselves and their ruler seems to still be the same length. No matter how fast we travel relative to anything else, we will still measure our elbow to hand to be about 30cm as ours is the preferred frame.

Sure, but it's really better to think in terms of using a series of different grids of rulers and clocks, since "inertial frames" are supposed to be coordinate systems that move inertially throughout space and time. Each time you change velocities, it's true that once you return to moving inertially, your ruler will measure the same length as a ruler which has been moving at that velocity for all time, and your clock will tick at the same speed as a clock which has been moving at that speed for all time, so perhaps this distinction isn't so important. Then again, you do have to worry about simulataneity--if you synchronize your clocks using Einstein's light-signal method while moving at one velocity, then accelerate to a new velocity, your clocks will probably no longer be in sync as measured by a set of clocks which have always been moving at your current velocity and which were also synchronized using Einstein's light-signal method (I say 'probably' because it would depend on the details of how different parts of your ship were moving relative to each other during the acceleration itself). So at the very least, you do need to resynchronize your clocks each time you change velocities.

So with the same clock and ruler we will be able to increase our speed relative to spaceship B to being one tenth the speed of light faster no matter how many times spaceship B then catches up again. We can keep doing this again and again. But according to SR this will not result in an equal or greater than speed of light relative to the Earth for our ships.

But in the Earth's frame, your rulers keep shrinking and the clocks keep slowing down each time you accelerate to a new velocity, and each time you resynchronize your clocks after attaining a new velocity they get progressively more out-of-sync...so why should an Earth-observer be surprised to find that your increase in speed relative to the Earth grows smaller and smaller with each jump, even though by your own measurements each jump increases your speed by 0.1c relative to your previous speed?

To bring things back to the Newtonian example, suppose the phony gag rulers were built to actually shrink or expand depending on their speed relative to the observer with the normal rulers and clocks, and suppose the phony gag clocks were built to slow down or speed up depending on their speed relative to him. Also suppose that each time the observer with the gag rulers and clocks changes velocities, he resynchronizes his clocks using the assumption that light travels at the same speed in all directions in his current rest frame (which, again, would actually be a false assumption in this Newtonian world--light would actually only travel at the same speed in all directions in the frame of the observer with the normal rulers and clocks, who we can assume is in the rest frame of the ether). Now, do you agree that in this purely Newtonian situation, we'd get exactly the same results as in the relativistic situation with unmodified rulers and clocks? In other words, do you agree that if the second observer always increments his speed by 0.1c as measured on his own (distorted) rulers and clocks, then the Earth-observer with the normal rulers and clocks will measure him to increase his speed by smaller and smaller increments each time, never reaching the speed of light?

 

[Doc Al]

Nothing I have said contradicts SR in my explanations of it so please don't be embarrassing to yourself in providing non-rebuttals.

If that were true, what is the point of your posts?

 

[pess5]

OK,

So I'm new to your forum here, and trying to figure out how to use this interface. I see gonegahgah seems unconvinced that Einstein got it right. I have found this to generally arise from lack of understanding of some of the finer points of the theory, shortsightness, or the adherence to absolute simultaneity. A see at least a few folks here are up on Relativity, so that's very good.

How does one add a quote into the respone? I see the checkbox here in the QUICK REPLY window, but it does not allow me to check it. When I go GO ADVANCED, I see nothing regarding unfortunately QUOTE.

thanx