Light clock treating horizontal and vertical motion differently?

[foxeamonn1969]

In every example I have seen using the light clock, the moving observer is considered to be moving relative to the entire light clock, whereas the light beam is considered to be moving from one part of the apparatus to the other. But if the light clock was picked up and thrown, the entire clock would move, regardless of whether it was thrown horizontally or vertically. So why is the motion of the light beam being treated differently?

 

[Ibix]

I'm not sure what you mean. Surely the pulse is always moving from one part of the apparatus to another?

You certainly can consider a light clock oscillating parallel to its direction of motion, if that's what you mean. It's messier because you have to factor in length contraction.

 

[foxeamonn1969]

Pulse (and all other bodies) are moving relative to the centre of mass of the clock, which is a single point. Reverse the analysis and consider the light at rest and the clock moving. The top mirror will not move independently of the bottom mirror. So although the light will strike the top mirror, that is not what it is moving relative to.

 

[robphy]

consider the light at rest

In special relativity, there is no such reference frame (that is equivalent to that of a typical ["timelike"] observer)
where light is at rest [since a light-signal has a worldline whose tangent is "lightlike" or "null"].
The second postulate is that light travels with the same speed according to all inertial observers.

 

[Ibix]

Reverse the analysis and consider the light at rest

You can't - that would contradict the second postulate.

 

[foxeamonn1969]

A postulate isn't a fact. Consider there is no top mirror. The light would still be moving relative to the clock. What point on the clock would it be moving relative to?

 

[robphy]

The Michelson-Morley apparatus is essentially
a [typical] transverse light-clock (whose light-signal is perpendicular to the direction of relative motion with respect to the lab frame)
and
a longitudinal light-clock (whose light-signal is parallel...).

From a flash at the origin-event O, its light-signals travel in all spatial directions.
In analyzing "one particular light-clock",
one focuses on the light-signal that will reflect off that light-clock's mirror.

Although the light-cone of O is isotropic (independent of spatial direction),
the worldlines of the clock-mirrors are not isotropic since they depend on direction.

www.geogebra.org/m/XFXzXGTq


We know what the experiment revealed.

---

One can imagine not just a pair of perpendicular light-clocks, but a whole circle's [generally, sphere's] worth of light-clocks.

www.geogebra.org/m/pr63mk3j

---

For an animation, visit
www.youtube.com/watch?v=tIZeqRn7cmI

 

[Ibix]

A postulate isn't a fact.

This one appears to be a correct statement about our universe, however. And if you want to study relativity you need to accept its postulates, at least for that purpose.

What point on the clock would it be moving relative to?

Light is always moving relative to any material object. Given that you've removed everything except the bottom mirror, I'd say the light pulse is moving with respect to any point on the mirror. Or any other massive object you're thinking of but haven't mentioned.

 

[foxeamonn1969]

Michelson-Morley experiment is no different to trying to tell if you are moving by throwing a ball in different directions in an enclosed space. The original question I had was why is the motion of the observer being treated differently the motion of the light?

 

[Ibix]

The original question I had was why is the motion of the observer being treated differently the motion of the light?

It isn't, really. Light is postulated to be moving at the same speed in all frames, which makes the transformation of its velocity trivial. From that, via the light clock, you can derive a transformation law between the two frames that applies equally to all velocities, including that of light. It maps light speed to light speed (obviously, because that was a requirement we specified) but all other speeds change.

What observer are you thinking of, by the way? So far we just seem to have a (possibly broken because you removed one mirror) light clock and two frames of reference. We don't really need an observer here - unless you're using "observer" as a synonym for "frame", which I'd recommend against.

 

[foxeamonn1969]

A physically real observer who is moving relative to the light clock apparatus.

 

[Ibix]

Michelson-Morley experiment is no different to trying to tell if you are moving by throwing a ball in different directions in an enclosed space

Sort of. They were expecting to find movement relative to a hypothesised medium, but didn't find it, ruling out most simple models of such a medium (and pretty much all other models when combined with other results). As @robphy points out, it's two light clocks combined.

 

[Ibix]

A physically real observer who is moving relative to the light clock apparatus.

Ok. It doesn't seem to add anything to the analysis.

 

[FactChecker]

A postulate isn't a fact. Consider there is no top mirror. The light would still be moving relative to the clock. What point on the clock would it be moving relative to?

You're right, a postulate isn't necessarily a fact. This postulate was a way to explain experimental results. It is the simplest (naive?) explanation of the measured result that light speed is always measured to be c in an inertial reference frame, regardless of the velocity of that frame.

This postulate didn't just accept the experimental results at that time, it also led to predictions and explanations of many other results that have been verified. Among them are the atom bomb, accurate clocks for GPS devices, the orbit of Mercury, and the bending of light passing by the Sun.

IMO, it can be regarded as fact until someone comes up with a better theory.

 

[foxeamonn1969]

re : michelson-morley experiment. As long as the experimental apparatus is in contact with the ground, it becomes an attempt to detect inertial motion, which will not work regardless of whether the experimenter is using light-clocks or tennis balls. The question I had is based on whether artificial distinctions are being made between different types of motion because people are looking for a certain answer.

You're right, a postulate isn't necessarily a fact. This postulate was a way to explain experimental results. It is the simplest (naive?) explanation of the measured result that light speed is always measured to be c in an inertial reference frame, regardless of the velocity of that frame.

This postulate didn't just accept the experimental results at that time, it also led to predictions and explanations of many other results that have been verified. Among them are the atom bomb, accurate clocks for GPS devices, the orbit of Mercury, and the bending of light passing by the Sun.

IMO, it can be regarded as fact until someone comes up with a better theory.

A frame of reference is an invented concept, so any postulate can be ascribed to it. Predictions are then verified using these invented concepts. I don't advocate total scepticism, but the chances of imaginary concepts conforming to objective reality are slim.

 

[robphy]

A frame of reference is an invented concept, so any postulate can be ascribed to it. Predictions are then verified using these invented concepts. I don't advocate total scepticism, but the chances of imaginary concepts conforming to objective reality are slim.

And so, a model (motivated by physically observed phenomena)
and a set of tools to analyze the model
lead to testable predictions,
which (within its range of application) agrees with many experiments.
Many other candidate models (possibly promising when formulated),
however, do not agree with these many experiments.

It's not that "reality has to be as model says,,,"
but "reality appears to be as-if-it-is what the model says".
Until we find a better model, we stick with it
(continually subjecting it to new tests, as well as making predicts which we can check).

 

[Ibix]

The question I had is based on whether artificial distinctions are being made between different types of motion because people are looking for a certain answer.

And the answer is no.

Inertial motion is easiest to analyse, and the form of relativity Einstein initially derived (and that is usually taught in introductory relativity courses still) does work with inertial frames only. Objects can accelerate, but the frames must be inertial. Minkowski's view of Einstein's work led Einstein to realise that he needed to reframe his equations in terms of tensors. That's a significant step up in mathematical complexity, but handles arbitrary coordinate systems just fine, and takes you most of the way to general relativity.

I don't advocate total scepticism, but the chances of imaginary concepts conforming to objective reality are slim.

A frame is no more or less an invented concept than anything else in physics. And it's easily physically realisable as a network of rods and clocks.

 

[Dale]

The original question I had was why is the motion of the observer being treated differently the motion of the light?

Because the light moves at $c$ with respect to any inertial observer.

 

[Dale]

I don't advocate total scepticism, but the chances of imaginary concepts conforming to objective reality are slim.

Indeed. So when we find that our concepts do conform to objective reality we can reject the assumption that they are imaginary.

In science we use many conventions, which are human choices. We choose the sign of a charge, the zero of a potential, the origin and axes of an inertial frame. Those conventions could reasonably be described as imaginary.

But that does not imply that every quantity calculated after choosing a convention is also imaginary. After choosing the sign of a charge, the direction of the calculated force is physically meaningful. After choosing the zero for PE, changes in PE are physically meaningful. After choosing an inertial frame, the velocities relative to that frame are physically meaningful.

 

[Dale]

This thread is reopened. Participants are reminded that all posts must be consistent with the professional scientific literature.

 

[FactChecker]

re : michelson-morley experiment. As long as the experimental apparatus is in contact with the ground, it becomes an attempt to detect inertial motion, which will not work regardless of whether the experimenter is using light-clocks or tennis balls. The question I had is based on whether artificial distinctions are being made between different types of motion because people are looking for a certain answer.

A frame of reference is an invented concept, so any postulate can be ascribed to it.

An inertial frame of reference is a mathematical construct to describe an undeniable physical fact. Anyone can feel acceleration. Acceleration exists and affects physical objects, whether there is a human to witness it or not. Inventing a mathematical reference frame to describe the existence or absence of acceleration is not an arbitrary definition.

Predictions are then verified using these invented concepts. I don't advocate total scepticism, but the chances of imaginary concepts conforming to objective reality are slim.

An inertial reference frame is designed to match objective reality. Probabilities do not enter in.

 

[PeroK]

A frame of reference is an invented concept, so any postulate can be ascribed to it. Predictions are then verified using these invented concepts. I don't advocate total scepticism, but the chances of imaginary concepts conforming to objective reality are slim.

Let's take that as your argument that physics generally is no good. So, what do we do? Get out the new-age crystals? Rely on ancient wisdom (air, earth, fire, water) and put everything down to the capriciousness of the gods?

What is the point of this thread? You don't believe in theoretical physics. So what? Why not go and and study something you get on with?

 

[Janus]

A postulate isn't a fact. Consider there is no top mirror. The light would still be moving relative to the clock. What point on the clock would it be moving relative to?

While a postulate is not a fact, it we can make predictions as what would happen if it were true. This, in turn allows us to devise real life experiment to test whether these predictions hold, and whether or not the postulate is valid. To date, all real life observations align with this postulate being valid.

 

[Ibix]

Because the light moves at $c$ with respect to any inertial observer.

Not sure I agree here, although this is definitely a matter of preference for description.

I would say that we simply assert that there is a system of physics where the velocity transformation rule maps speed $c$ to itself but maps 0 to $v$. So I think we are treating the velocities the same (although we haven't worked out the details yet). If no such system existed there'd be contradictions in the maths, but in fact out drop the Lorentz transforms which we can then test against experiment (as we have done).

 

[A.T.]

the chances of imaginary concepts conforming to objective reality are slim

What is "conforming to objective reality" supposed to mean here exactly? Physics makes quantitative predictions, which are compared to objective measurements. This is how we test if it conforms to objective reality.

 

[Herman Trivilino]

A postulate isn't a fact.

That's not relevant! What we're doing here is using the two postulates to draw conclusions.

This entire thread is a discussion of the consequences of the postulates. If we reject the postulates then we reject the conclusions, and thus none of what you're discussing is meaningful.

 

[Dale]

It isn’t relevant and it isn’t correct either. Some postulates are facts. Being a postulate doesn’t make something a fact, but it doesn’t prevent it from being a fact either.

 

[FactChecker]

Everything we believe boils down to acceptance of a set of postulates. In reality this may all be a dream of the Flying Spaghetti Monster. (Sorry. I am a Pastafarian and could not resist.)

 

[Herman Trivilino]

Some postulates are facts.

Einstein was very careful about the way he worded the 2nd Postulate. He stated the speed of light (in a vacuum) is indepedendent of the speed of the source.

Not the speed of the observer, although it seemed to follow immediately that it is. Even more so today as we have well over a century of experimental and observational support. So much so that it does indeed seem to be a fact.

Throughout his life he was always interested in whether or not astronomers could detect the slightest dependence in the speed of star light on the speed of the star.

 

[FactChecker]

IMO, it is false that the Michelson-Morley experiments were artificially designed to give the result supporting the postulate of a constant speed of light, Neither Michelson nor Morley expected the results that they got.