Why is the invariance of light a problem?

[bobie]

Surely I am missing something, can you explain what?

If we shoot a gun while traveling on a train, the speed of the bullet is its usual speed plus the speed of the train v_t, because the bullet inside is already traveling at v_t.

If we produce an EMR on the train (or on the Earth in the case of MMX) the "bullet" is created on the spot and has no previous speed.

So, why should the speed of light vary in any frame of reference? why look for the aether, why thw MMX? the speed of light depends only on properties of the medium and not on the speed of the emitter or the receiver.

 

[bp_psy]

In a medium the speed of mechanical waves is dependent on the speed of the observer.Imagine that you are moving at a constant velocity over a sea with waves .The speed of the waves that you measure will depend on your speed with respect to the bottom of the sea.The same is true for sound. The idea was that Galilean relativity should apply to light also. The fact that it doesn't was indeed very strange and the special theory of relativity required some very deep changes in our understanding of physics such as time dilation, length contraction and the idea of space time.

ps. What is EMR and MMX? Endoscopic mucosal resection and Matrix math extension doesn't make much sense to me.

 

[bobie]

ps. What is EMR and MMX? Endoscopic mucosal resection

https://www.physicsforums.com/showthread.php?t=683945&highlight=Michelson

We are talking about light, not sound or other

 

[bp_psy]

https://www.physicsforums.com/showthread.php?t=683945&highlight=Michelson

We are talking about light, not sound or other

Yes but at the time there was no reason to think that light should not obey Galilean relativity or that electromagnetic waves didn't really need a medium in which to propagate.They only knew that light could be described as a wave with a very high velocity. The difference in velocity between reference frames in which the speed of light could be measured at the time was also small so one could also not say that light didn't indeed obey Galilean relativity. The Michelson experiment showed that it didn't and special relativity showed how and why.The idea of an aether propagating wave was the best model at the time considering what was known.

 

[Saw]

If we produce an EMR on the train (or on the Earth in the case of MMX) the "bullet" is created on the spot and has no previous speed.

So, why should the speed of light vary in any frame of reference?

The fact that the speed of light is independent of the speed of the light source means that it is going to "behave in the same way" no matter whether it is flashed from the train or from the platform. Thus two beams projected from train and platform at the same time will always travel together.

But another thing is that "the same speed is measured" from the two reference frames. Speed is distance divided by time. Suppose that a measurement of the speed of any of those beams is carried out from the train and the platform. In the Galilean mindset, time is absolute, so the two frames will measure the same time. But one one is moving with regard to the other, so they *cannot" measure that light travels, in such time interval, the same distance. Hence it was expected that the measurements of light speed should be frame-dependent.

 

[bobie]

Thanks, you are all speaking in the past. I was not referring only to Galileo and MMX I am referring to relativity, too: why do we need it to justify the invariance of light. It is invariant because nothing can affect its speed.The way it appears to observers is a problem of the observer.
Where is the problem?

 

[Nugatory]

Thanks, you are all speaking in the past. I was not referring only to Galileo and MMX I am referring to relativity, too: why do we need it to justify the invariance of light. It is invariant because nothing can affect its speed.The way it appears to observers is a problem of the observer.
Where is the problem?

Yes, it really is that simple... in hindsight.
It took close to a half-century (roughly between 1860 and 1905) for physicists to figure this out, in part because the implications - relativity of simultaneity, differential aging, length contraction and time dilation - are so counterintuitive.

Even today, probably 90% of the traffic in the relativity forum is about helping people understand this stuff. So the problem isn't the invariance of the speed of light, it's understanding all the consequences of that invariance.

 

[bobie]

So the problem isn't the invariance of the speed of light, it's understanding all the consequences of that invariance.

Why are they a problem? why do we need relativity is my question, if problems arise when different observers get different result isn't just their problem?We suffer all kinds of illusions and distortions in this world.

 

[chingel]

Relativity is there to explain how the results will change for different observers, so that you can predict and understand what will happen in different scenarios for different people.

The constant speed of light can be a problem to believe because it means that time can change speed, e.g. one twin can age twice as fast as the other and so on. It was much easier to believe that if you move towards the light it would appear to be moving faster than believing that the space-time fabric distorts and time changes speed. On an everyday basis things don't seem to work that way, if you run after some moving object it appears to be moving slower compared to you.

 

[bobie]

The constant speed of light can be a problem to believe because it means that time can change speed,

Why must Time change speed? And if we speak of time changing anythind we are giving it an ontological status it has not. That is what I do not understand. Time is not an entity with powers or qualities, time is the sum of billions of physics principles not related to one another. Relativity treats time as an omnipotent entity.

 

[Pythagorean]

Why are they a problem? why do we need relativity is my question, if problems arise when different observers get different result isn't just their problem?We suffer all kinds of illusions and distortions in this world.

The results aren't arbitrary. There's a systematic way in which observers get different results that can be predicted. So it's a meaningful difference, not a subjective one.

 

[bobie]

The results aren't arbitrary. There's a systematic way in which observers get different results that can be predicted. So it's a meaningful difference, not a subjective one.

Even the distortion of a stick in a liquid is systematic and predictable, but we do not conclude that space has changed, is warped. Where is really the problem? why can't it be the observer's problem?

 

[Bandersnatch]

Why are they a problem? why do we need relativity is my question, if problems arise when different observers get different result isn't just their problem?We suffer all kinds of illusions and distortions in this world.

Even the distortion of a stick in a liquid is systematic and predictable, but we do not conclude that space has changed, is warped. Where is really the problem? why can't it be the observer's problem?

Because these are not illusions but real physical effects.
The thickness of the atmosphere IS contracted for fast muons created in its upper parts, so that they reach Earth despite seemingly too short lifetime.
The time DOES pass at a different rate for communication and positioning satellites in Earth orbit, so it has to be corrected for.

 

[chingel]

Why must Time change speed? And if we speak of time changing anythind we are giving it an ontological status it has not. That is what I do not understand. Time is not an entity with powers or qualities, time is the sum of billions of physics principles not related to one another. Relativity treats time as an omnipotent entity.

Yes that is why people had a hard time getting a grasp of relativity, because among other things it says that the speed of time is something that can change. If one twin stays on Earth and the other goes on a round trip at a fast enough speed, it is possible that for one twin 10 years have passed but for the other 50 years have passed. And it is not that years just seem to have passed, they have really aged that much, one has more wrinkles. If they both had an animal with a lifespan of 5 years, one would be dead for a long time.

Such tests have been made with extremely accurate clocks on airplanes.

To get a sense that something must change, consider a spaceship moving past you at 0.5c. Once it is next to you, it emits a photon in the forward and another in the backward direction. You see these photons move at c, but the spaceship moves toward one photon at 0.5c. So it is a lot closer to that photon. However a guy on the spaceship sees both photons move away at the speed c, so that he is exactly in the middle of the two photons. So you think what is going on, the photons and the spaceship must be at a certain position and the spaceship is either in the middle of the photons or it is not. Therefore something must be changed. And it is not only that they somehow see it differently, they can measure the positions with all sorts of machines, for example the spaceship could have some detectors on really long sticks catching the photons and the results would be the same.

 

[Dale]

Even the distortion of a stick in a liquid is systematic and predictable, but we do not conclude that space has changed, is warped. Where is really the problem? why can't it be the observer's problem?

The problem is that the invariance of c is incompatible with the Galilean transform. So we had effectively been using the wrong transform for quite some time, and we needed to develop a new transform that was compatible with the invariance of c, the Lorentz transform.

 

[bobie]

If one twin stays on Earth and the other goes on a round trip at a fast enough speed, it is possible that for one twin 10 years have passed but for the other 50 years have passed. .

All effects can have many explanations. If I put a chiken in the fridge and another in the freezer, I might conclude that time has changed, as well, but that is not correct, of course.
I am not discussing what relativity says, I would like to understand what is the problem that cannot be solved without getting in the trouble of time changing.
Do you realize that if Time is considered as an absolute entity, all the phisical fenomena in a spaceship are altered? and that includes gravity and everything all the rules would change and not the age of the twins: the electrons in the atoms wouldn't get enough electrostatic energy and would collapse etc...?. Wouldn't that be catastrophic?

Thank you all for your patience

 

[bobie]

The problem is that the invariance of c is incompatible with the Galilean transform. .

Is that the main, real problem, Dalespam? You usually go to the core of the problem, could you expand on that? what does that mean?

 

[Dale]

You usually go to the core of the problem, could you expand on that? what does that mean?

Ever since Galileo it was understood that the same laws of physics work in multiple reference frames. These reference frames, called inertial frames, were assumed to be related by the Galilean transform. However, the Galilean transform says that there is no invariant speed, so the discovery that the speed of light was invariant was a direct contradiction of the Galilean transform. That meant that we needed to rethink our understanding of inertial frames and the laws of physics.

 

[bobie]

However, the Galilean transform says that there is no invariant speed, so the discovery that the speed of light was invariant was a direct contradiction of the Galilean transform.

I understand that, DaleSpam, but why can't we find other explanations not touchin time, which is a very tricky issue? could you respond to my doubts in post #17?

 

[PAllen]

All effects can have many explanations. If I put a chiken in the fridge and another in the freezer, I might conclude that time has changed, as well, but that is not correct, of course.
I am not discussing what relativity says, I would like to understand what is the problem that cannot be solved without getting in the trouble of time changing.
Do you realize that if Time is considered as an absolute entity, all the phisical fenomena in a spaceship are altered? and that includes gravity and everything all the rules would change and not the age of the twins: the electrons in the atoms wouldn't get enough electrostatic energy and would collapse etc...?. Wouldn't that be catastrophic?

Thank you all for your patience

There are a set of phenomena predicted by SR that have been observed. There do exist alternate explanatory model, but they are considered by most highly contrived compared to SR (e.g. LET). SR is accepted because it accounts for all observations in the simplest way known.

A few of many phenomena, that all fall out with a couple of assumptions and trivial math from SR:


1) If two labs separate and meet after following different patterns of motion, all physical processes will have evolved differently between them, but remain consistent with within each lab. That is, the relative motion has affected mechanical clocks, radioactive decay, biological processes, etc. all identically. This universality of rate of change is simply given the name time.

2) Muons created in the upper atmosphere will reach the ground even thought their decay rate at rest suggests half should be decayed every 600 meters of travel.

So, the question is, do you look for separate explanations for a bunch of observed phenomena, or accept SR which derives all of these from a couple of trivial assumptions?

 

[Dale]

why can't we find other explanations not touchin time, which is a very tricky issue?

Nobody has found such an alternative explanation which is experimentally distinct from relativity. Obviously, not finding something doesn't prove it's non existence, however the existing experimental evidence puts severe constraints on any alternative explanations. Any alternative explanations must reduce to the Lorentz transform in the relativistic limit, just as the Lorentz transform must reduce to the Galilean transform in the Newtonian limit.

could you respond to my doubts in post #17?

A "chicken decay" clock is sensitive to temperature, but there are other clocks (e.g. Radioactive decay) which are not. In contrast, all clocks, regardless of their mechanism, are sensitive to time dilation. Experimentally, what is the difference between all clocks of every mechanism slowing down equally and time slowing down?

 

[bobie]

1) If two labs separate and meet after following different patterns of motion, all physical processes

So, the question is, do you look for separate explanations for a bunch of observed phenomena, or accept SR which derives all of these from a couple of trivial assumptions?

Of course yes, if it were so simple. But if you accept the assumption that all processes in the world can be altered in a tiny fraction of the world you are opening up a n endless Pandora-box of trouble.

What happens to processes (nearly all) that are time-dependent? forces like gravity and electric?
A ship orbiting the Earth at 9.9999.. c vould have no gravity, no atomic bonds, what would happen to radio communications? etc...

 

[bobie]

A "chicken decay" clock is sensitive to temperature, but there are other clocks (e.g. Radioactive decay) which are not.

I didn't mean that to be a positive example but just a fallacy, my doubts regard the fact that all processes slow down , that supposes omnipotence, see post #23

 

[Dale]

But if you accept the assumption that all processes in the world can be altered in a tiny fraction of the world

That is not an assumption of relativity. There are only two assumptions: the principle of relativity and the invariance of c. Everything else is derived, not assumed.

But also I don't recognize any derived result corresponding to what you describe here. Perhaps it is just a matter of wording, but I don't know what you are referring to.

 

[Dale]

my doubts regard the fact that all processes slow down

The experimental evidence is clear. EM processes, strong and weak nuclear processes, and gravitational processes all display time dilation. There is no room for doubt concerning the observed experimental facts. All processes slow down as predicted by SR.

that supposes omnipotence, see post #23

I don't know where you get this strange comment. No omnipotence is assumed, only the two postulates.

Please keep your comments reasonable or I will have to close the thread. There is no suggestion in the scientific literature which supports this very strange comment about omnipotence.

 

[bobie]

. Everything else is derived, not assumed.
..., but I don't know what you are referring to.

what happens when energy from outside reaches inside? is Earth gravitational pull changed? if it is, the ship could not orbit I suppose. And the very speed of the ship, shall it be meaured at the same time with two standards, one inside and one outside?

 

[PAllen]

what happens when energy from outside reaches inside? is Earth gravitational pull changed? if it is, the ship could not orbit I suppose. And the very speed of the ship, shall it be meaured at the same time with two standards, one inside and one outside?

Can you please describe some precise scenario that troubles you. The above is completely incomprehensible. I have a feeling that the "Pandora's box of trouble" you refer to earlier is nothing but a collection of misunderstandings on your part of what SR predicts.

 

[bobie]

Can you please describe some precise scenario that troubles you

Sure that is just what I am asking:
a ship is orbiting the Earth near the moon at 0.9 c or so and completes an orbit in 2π seconds, let's say that time goes about 100 times slower. what is the speed of the ship, inside it is 100 greater then c?
The pull of gravity from the Earth reaches the nearer side of the ship at regular time, then gets inside the ship to the farther side at 1/100 of time what is the gravity pull on the ship?

Thenks a lot PAllen

 

[PAllen]

Sure that is just what I am asking:
a ship is orbiting the Earth near the moon at 0.9 c or so and completes an orbit in 2π seconds, let's say that time goes about 100 times slower. what is the speed of the ship, inside it is 100 greater then c?
The pull of gravity from the Earth reaches the nearer side of the ship at regular time, then gets inside the ship to the farther side at 1/100 of time what is the gravity pull on the ship?

Thenks a lot PAllen

First point is that there is no orbit around the Earth with speed .9c. You would need to compress the Earth to near its Schwarzschild radius to produce such an orbit, and such an orbit would have a radius of roughly a centimeter. I hope it is ok to posit a rocket with superthrusters firing so as to force a circular path at .9c.

The speed of the ship measured by the ship is zero (by definition, you are not moving relative to yourself). The acceleration of the ship (measured gee forces inside it) would be enormous, impossible to withstand (over 100 million gee), even at the moon's orbital distance. This is just the thrust required to keep the rocket moving in a circle of the specified radius. The presence of the Earth is essentially irrelevant because its gravity is so weak compared to this.

Do you possibly mean what speed does the rocket measure for the earth? In a sense you can say the Earth would be considered to be moving at > c by the rocket: it would reach the same angular position periodically, and its distance would be considered such that this implies a speed much greater than c for the earth. There is no problem with this. All sorts of 'strange' things are possible in non-inertial frames (the rocket is an extremely non-inertial frame). For example, in a merry go round frame, distant mountains appear to move at >>c. So what? There is not 'relativity' between non-inertial and inertial frames.

Let's try to leave gravity out of this because that requires general relativity, while you seem to have very little understanding of special relativity. Also, what I glean from your comments about gravity implies you understand little about that as well.

 

[bobie]

First point is that there is no orbit around the Earth with speed .9c.

What if the ship is orbiting a BH with a mass that theoretically would provite the necessary pull?

 

[PAllen]

What if the ship is orbiting a BH with a mass that theoretically would provite the necessary pull?

Sure, but then you have a problem in strong field general relativity, not special relativity (and it would have to be a supermassive BH to have an moon sized orbit of .9c). Further, concepts of special relativity (specifically, global inertial frames) do not apply. Even though the rocket is in free fall, there is no corresponding inertial frame (except at an instant of time; not for an orbit) for it because these don't exist globally in general relativity.

If we assume the mass of the rocket is very small compared to the BH and is small enough to treat the rocket as a test body, then spacetime around the BH can be considered static (if it is a non-rotating BH). Then there is no concept of gravity traveling. The orbit is just a geodesic of the spacetime. It is true that by some definitions, the rocket could consider the BH as 'moving' at speed > c. However, so what? In general relativity, the speed of light being c and all bodies moving less than c is strictly a local property not a global one. Thus, the rocket would find that all physics in and right near the rocket over short periods of time would match those as if the rocket were in 'empty space'. Globally, GR predicts all sorts of effects different from SR.

 

[Dale]

what happens when energy from outside reaches inside?

It is still conserved. It may be transformed to another form, depending on the details, but the conservation of energy applies in all frames.

is Earth gravitational pull changed? if it is, the ship could not orbit I suppose.

An orbit in one frame is an orbit in all frames. They are called geodesics, and the math ensures that all frames agree on whether or not a given path is a geodesic.

And the very speed of the ship, shall it be meaured at the same time with two standards, one inside and one outside?

Yes. There is certainly nothing new in that. That has been the case since Galileo.

 

[Dale]

Moderator's note: I have deleted several off topic posts about $\epsilon_0$ and $\mu_0$, including my own. Let's not detract from the OP's discussion while he or she is actively involved.

 

[bobie]

An orbit in one frame is an orbit in all frames. They are called geodesics, and the math ensures that all frames agree on whether or not a given path is a geodesic.

Of course we know this flight is technically impossible, but just to explore what would happen:

If time is, say, 1/10 of the time outside the ships should get 1/10 of the regular acceleration
When the radio waves are picked up inside the ship they they get in at a shorter wave but the words jam ten times quicker, but they cannot understand English any more because their brain is 10 times slower, right?
They cross the moon 10 times in a second ( their second) so, as the know the the orbit is 2π c, they are moving at 10 c in their frame?

Thanks for your kind patience, Dalespam

 

[PAllen]

Of course we know this flight is technically impossible, but just to explore what would happen:

If time is, say, 1/10 of the time outside the ships should get 1/10 of the regular acceleration

I cannot attach any meaning to this statement. Please try to be specific, using accepted definitions.

When the radio waves are picked up inside the ship they they get in at a shorter wave but the words jam ten times quicker, but they cannot understand English any more because their brain is 10 times slower, right?

If a ship is orbiting (a supermassive BH; even for a neutron star, there is no orbit that is .9c) at .9c, radio waves sent from a far away ship would be received blueshifted by the orbiting ship. There is one factor affecting wavelength (shorter than at source), frequency (greater than at source), interval between signals (smaller than at source). It is not clear if you are suggesting these affects add up - they do not, they all correlate with the same factor. However, for people inside the orbiting ship (talking, turning on lights, etc.) everything is indistinguishable from if they were floating far away from any massive body. It is only signals received from far away that are received differently (as described above) from the way they emitted. Note, that if such a signal were slowed down by the given factor, then the English would be perfectly comprehensible.

Note, also, that we cannot talk about signals received from the BH, because that is impossible.

They cross the moon 10 times in a second ( their second) so, as the know the the orbit is 2π c, they are moving at 10 c in their frame?

Near a BH, geometry is non-euclidean, the ratio of circumference to radius is no longer 2 pi, and distance to a BH is challenging to define in a way that can be measured.

I suggest you accept my accelerating scenario and stick to SR rather than GR. There I already explained that the ship would consider the Earth (not the other way around as you keep saying) to be moving faster than c and this is no more surprising for such a non-inertial coordinates than a merry go round observer seeing distant mountains moving faster than c.

[edit: The question of how the ship would measure the speed of the moon, assuming the ship accelerating in a circle near the moon's orbit, passing it periodically, is actually quite complicated. If you measure the moon's passing locally, as it passes you, it will be .9c. This is accounted for by distance contraction. However, to model the moon's average speed over an orbit from the rocket point of view, you need a complete coordinate system. This is actually quite complicated for such a circular accelerating rocket, but it doesn't really matter what the result might be - there is no expectation that motion of bodies must be less than c in highly non-inertial coordinates. What is true, is that the moon would never overtake any light signal in any direction, in any coordinates. This is because in non-inertial coordinates, light may move many times faster than c]

Thanks for your kind patience, Dalespam

 

[russ_watters]

Of course yes, if it were so simple. But if you accept the assumption that all processes in the world can be altered in a tiny fraction of the world you are opening up a n endless Pandora-box of trouble.

Why do you believe that when others are saying exactly the opposite is true? How is needing a lot of different explanations simpler than only needing one?

What happens to processes (nearly all) that are time-dependent? forces like gravity and electric?
A ship orbiting the Earth at 9.9999.. c vould have no gravity, no atomic bonds, what would happen to radio communications? etc...

Nothing. That's the point: relativity is what enables them to always work the same way. That is in fact one of its postulates.

 

[Dale]

If time is, say, 1/10 of the time outside the ships should get 1/10 of the regular acceleration

I would have to work it out, it may be a different factor. However, the short answer is that coordinate acceleration can indeed differ from proper acceleration. All frames will agree on proper acceleration, but, obviously, different frames will disagree on the coordinate acceleration.

When the radio waves are picked up inside the ship they they get in at a shorter wave but the words jam ten times quicker, but they cannot understand English any more because their brain is 10 times slower, right?

Sure. I don't see why you think that a persons inability to understand a blue shifted transmission represents a problem for a physical theory.

They cross the moon 10 times in a second ( their second) so, as the know the the orbit is 2π c, they are moving at 10 c in their frame?

By definition they are at rest in their frame.

The math guarantees that an object which has a timelike worldline (slower than c in a coordinate independent sense) will have a timelike worldline in all frames. Similarly, an object with a lightlike worldline (traveling at c in a coordinate independent sense) will have a lightlike worldline in all frames.

Furthermore, if two timelike objects pass right next to each other then they will both agree on their relative velocity at the meeting.