I Topic about physics axioms, theory, laws etc..

[jbriggs444]

Did Dayton Miller conduct Michelson-Morley experiments in 1930's, and find some difference in light speeds, and prove speed of light depend of source speed?

No. He did experiments. They were flawed. The results could not be reproduced.

A quick trip to Google pulls up this article.

Observations involving binary star systems put the kibosh on theories where light speed depends on the speed of the source.

 

[Ibix]

Did Dayton Miller conduct Michelson-Morley experiments in 1930's, and find some difference in light speeds, and prove speed of light depend of source speed?

He did conduct experiments and he did think he had found something like you say, but he was mistaken. See https://www.arxiv.org/abs/physics/0608238, which is linked from the excellent "Experimental basis of SR FAQ" that's stickied in the Relativity forum. From the abstract: In short, this is every experimenter's nightmare: [Miller] was unknowingly looking at statistically insignificant patterns in his systematic drift that mimicked the appearance of a real signal.

 

[user079622]

No. He did experiments. They were flawed. The results could not be reproduced.

A quick trip to Google pulls up this article.

Observations involving binary star systems put the kibosh on theories where light speed depends on the speed of the source.

Today with modern devices is confirmed that light dont depend on source speed?

In Michelson-Morley experiment what speed they use for Earth? earth is not moving just around sun, galaxy is also moving, etc etc so if aether is absolute stationary we dont know what is earth absolute speed?

 

[Dale]

Yes, if you've never seen black swan, it doesn't mean it doesn't exist.

And fundamentally that is why the scientific method can never be said to “prove” a theory true. It is an inherent limitation of the scientific method.

So everything in Physics=maybe, I am disappointed by that fact

One thing that may help you to be less disappointed is to know is that there are different degrees of maybe.

Suppose now that we have two competing theories of flubnubitz. My theory that they always come up "tails" and Bob's theory that they come up "tails" randomly with a 50% probability. After gathering 100 experimental trials and finding that they all were "tails" I cannot say that either theory is "proved". Maybe my theory is correct, but maybe Bob's theory is correct and we were just unlucky the first 100 trials. But the maybe for my theory is a lot more believable than the maybe for Bob's theory.

So although we have not "proven" either theory, we have learned about both theories. We have not eliminated uncertainty entirely, but we have reduced it substantially.

 

[Ibix]

In Michelson-Morley experiment what speed they use for Earth?

They didn't use a speed. They aimed to measure the change in speed relative to the ether over a year, and found a null result.

 

[fresh_42]

From the German Wikipedia:

The first proof that the speed of light is finite was achieved by the Danish astronomer Ole Rømer in 1676. He found seasonally fluctuating travel times for clock signals from Jupiter (entry of Jupiter's moon Io into Jupiter's shadow), while on this side, the Earth's rotation served as a stable time reference. He determined a travel time of 22 minutes for the diameter of the Earth's orbit. The correct value is shorter (16 minutes 38 seconds). Since Rømer did not know the diameter of the Earth's orbit, he could not provide a value for the speed of light. Christiaan Huygens did this two years later. He related Rømer's travel time to the diameter of the Earth's orbit around the Sun, which Cassini had accidentally determined almost correctly in 1673 (see solar parallax for the gradual improvement of this value), and arrived at a speed of light of 213,000 km/s.

The first earthly determination of the speed of light was achieved by Armand Fizeau using the gear method. In 1849, he sent light through a rotating gear to a mirror several kilometers away, which reflected it back through the gear. Depending on the speed of the gear, the reflected light, which had passed through a gap in the gear on its way there, either falls on a tooth or passes through a gap again, and only then can it be seen. Fizeau's value was 5% too high.

 

[A.T.]

So everything in Physics=maybe, I am disappointed by that fact,

If you want certainty, try religion.

 

[Jaime Rudas]

I always thought that everything we learned in school of physics is 100% correct. Now I see that math and physics is very different in that aspect..
Can I say math is absolute truth, physics is just best current approximation for reality?

Yes, in the sense that we can differentiate between two types of truth: logical truth and factual truth. In a given logical system, statements are considered true if, using the rules of the system, they can be deduced from the axioms and postulates. Factual truth consists of the complete correspondence between reality and our idea of it. Thus, it is possible to establish the truth of a logical statement, but, in general, it isn't possible to establish the truth of a factual statement (although it is possible to establish its falsity).

 

[Jaime Rudas]

Is this axioms?

• The laws of physics are the same in all inertial frames.
• The speed of light in vacuum is constant in all inertial frames.

In one sense, they are postulates of physical theories, which are logical systems. In another sense, they are factual hypotheses that can be verified.

 

[Herman Trivilino]

Is this axioms?

• The laws of physics are the same in all inertial frames.
• The speed of light in vacuum is constant in all inertial frames.

Einstein constructed his special theory of relativity in 1905 using these as postulates. His goal was to show the consequences of accepting these two experimental behaviors.

So, they are postulates subject to experimental and observational verification. Einstein himself was interested in any experimental endeavors that might show that the speed of light from a distant star showed any dependence on the motion of the star. He knew that if the postulres were in any way invalid it would mean his entire theory would have to be modified.

 

[dextercioby]

Building a physical theory - the job of a theoretical physicist - requires two things to start: a set of definitions and a set of postulates, i.e. statements that cannot be directly logically proven, but can and must be used to derive results in the form of theorems/statements. We say that a theory of physics has two different but equivalent formulations, if the postulates of one formulation can be thorems in the other and viceversa.

Here's a question: if we superficially say that the theory of special relativity (as known today after about 120 years) is summarized by the two postulates:

• The laws of physics are the same in all inertial frames.
• The speed of light in vacuum is constant in all inertial frames.

what is the set of definitions necessary in a book/university course to be explicitely formulated before stating these postulates, for these two statements (together with the definitions I am asking about) to "begin producing results" such as the proposition: "the length of an object is not absolute, but inertial frame-dependent"?

 

[user079622]

He did conduct experiments and he did think he had found something like you say, but he was mistaken. See https://www.arxiv.org/abs/physics/0608238, which is linked from the excellent "Experimental basis of SR FAQ" that's stickied in the Relativity forum. From the abstract: In short, this is every experimenter's nightmare: [Miller] was unknowingly looking at statistically insignificant patterns in his systematic drift that mimicked the appearance of a real signal.

Sound:
1. If sound source move toward me, I measure higher frequency and same speed of sound.
2. If I move toward sound source, I measure higher frequency and higher speed of sound(relative to me, even sound speed relative to medium stay constant).

Light (vacuum):
3. If light source move toward me, I measure higher frequency and speed c .
4. If I move toward light source, I measure higher frequency and speed c

Why in 4. speed is not higher(like in wave 2.)? Any logical explanation why this happen?

"If light speed depended on observer velocity, causality would break down"
Why would causality brake down, if light wave from some event didn't come to your eye retina, that doesn't mean that this event didn't happen?What has EM wave with causality?

 

[Ibix]

Why in 4. speed is not higher(like in wave 2.)?

Depends what you mean. Why there is a speed that is the same in all frames is not known. It just appears to be how the universe works - Einstein postulated it, worked out the implications, and found that they matched existing experimental results. However, you can answer why light travels at the invariant speed, which is because it is massless.

"If light speed depended on observer velocity, causality would break down"

On it's own that makes no sense. Please link to the context of that claim.

 

[user079622]

On it's own that makes no sense. Please link to the context of that claim.

There is tone of content if speed of light is not constant it will brake causality.
EM wave don't have nothing to do with causality.
Why would speed of causality be speed of light?

 

[Ibix]

There is tone of content if speed of light is not constant it will brake causality.

Please link to some, then. The video you posted looks to be about the tachyonic anti-telephone, which shows how faster than light communication allows paradoxes assuming special relativity is true - that is, assuming the speed of light is invariant for all inertial observers.

Why would speed of causality be speed of light?

You have this the wrong way round. A direct consequence of assuming that there is a finite invariant speed is that nothing can travel faster than that without causal paradoxes (which the tachyonic anti-telephone thought experiment shows). Note that I did not mention light at all. Separately, you can ask why light travels at that invariant speed, which I answered in my last post: because it's massless.

It's unfortunate that the invariant speed is still called the speed of light, because that's really an artefact of the historical development of the theory. It's perfectly possible to write down the theory of a universe in which light does not travel at the invariant speed - the photon ends up having mass and can be stopped like any other particle, and causality is unaffected. In fact, that theory is used as a test theory to test our belief that the photon is massless (its mass is indistinguishable from 0kg to 50-something decimal places, IIRC).

 

[Herman Trivilino]

Why in 4. speed is not higher(like in wave 2.)? Any logical explanation why this happen?

Because in 4 the speed is less than c.

Here's a geometric analogy. If you stack wedges 1 and 2 (see figure) you can make the slope of the upper surface of the stack steeper. (And you can continue this practice if you add another wedge 3.) If you keep adding wedges the upper surface is vertical, but you can't increase the slope further by adding additional wedges.

Why does adding wedges not make the slope get steeper when the upper surface is vertical (like it does when the slope of the upper surface is not vertical)? Any logical explanation why this happens?

 

[Herman Trivilino]

Why would speed of causality be speed of light?

This question is better phrased as why is there a fastest speed? And why is that the speed of causality and also the speed of light?

 

[hutchphd]

e, I am disappointed by that fact, disappointed that everything what we learn is maybe wrong.

You do not understand the unique genius of the scientific method, and therefore you do not understand anything much about physics. The fact that every fact we learn may be wrong is the entire point and power of hard science. It redounds to social as well as technological realms. Your disappointment illustrates this insufficient understanding which, alas, is widespread even among the "educated". Physics has little to do with a certain pile of facts.

 

[Herman Trivilino]

The fact that every fact we learn may be wrong is the entire point and power of hard science. It redounds to social as well as technological realms.

Failure to understand this is perhaps the greatest shortcoming of our society.

 

[user079622]

However, you can answer why light travels at the invariant speed, which is because it is massless.

Yes that make sense.
But why this is speed of causality?
Event happen before observer see this, event happen and doesn't care when you observed it.
For event, observer is irrelevant factor.

 

[Ibix]

But why this is speed of causality?

Nothing can travel faster so nothing can be affected earlier.

If there is a car accident, it's common for people to have other crashes because they're looking at the accident. The later crashes are caused by the first one. But if there is a car crash one light second away and you crash half a second later it cannot have been caused by the other accident because you can't have seen it at the time the accident happened. The invariance of the speed of light enforces that all frames will agree you couldn't have seen the crash yet at the time you crashed (or vice versa).

 

[Dale]

What has EM wave with causality?

EM waves can cause things.

If there is a finite invariant speed then anything massless travels at that invariant speed. If there is an invariant speed and causality then the invariant speed is the fastest speed at which any causal influence can travel. Light is the first massless thing that we understood, so it was the first thing we dealt with whose speed is the invariant speed. So we call the invariant speed the speed of light.

EM wave don't have nothing to do with causality.
Why would speed of causality be speed of light?

EM waves can cause things so of course they have something to do with causality.

The speed of causality is yet another term for the invariant speed. I don’t like that term because causality can also travel slower. But despite my opinion, it is a recognized term for the invariant speed

 

[user079622]

EM waves can cause things.

If there is a finite invariant speed then anything massless travels at that invariant speed. If there is an invariant speed and causality then the invariant speed is the fastest speed at which any causal influence can travel. Light is the first massless thing that we understood, so it was the first thing we dealt with whose speed is the invariant speed. So we call the invariant speed the speed of light.

EM waves can cause things so of course they have something to do with causality.

The speed of causality is yet another term for the invariant speed. I don’t like that term because causality can also travel slower. But despite my opinion, it is a recognized term for the invariant speed

Queen of UK become instantaneous queen of Australia.
The other thing is that people in Australia will have to wait a few milliseconds for information to reach them via TV.

 

[A.T.]

Queen of UK become instantaneous queen of Australia.

Instantaneous in which frame of reference?

The other thing is that people in Australia will have to wait a few milliseconds for information to reach them via TV.

Signal delay is removed in the usual simultaneity conventions.

 

[Ibix]

Queen of UK become instantaneous queen of Australia.

The late, great Sir Terry Pratchett hypothesised the existence of queons and kingons, particles that carry monarchy, and their antiparticle, the republicon. He notes that plans for an instantaneous communication device based on torturing a small king were interrupted by the bar closing.

Royalty is not a physical property that can be measured nor transmitted. It's a bunch of human made rules.

 

[WWGD]

I always thought that everything we learned in school of physics is 100% correct. Now I see that math and physics is very different in that aspect..
Can I say math is absolute truth, physics is just best current approximation for reality?

Depends on which Math you refer to, as it splits on every undecidable statement. Choice or no Choice, Archimedean axiom holds in Standard Model for Reals but not in other models such as the Hyperreals, etc.

 

[Dale]

Queen of UK become instantaneous queen of Australia.
The other thing is that people in Australia will have to wait a few milliseconds for information to reach them via TV.

And what physical measurement can her being queen of Australia cause in those milliseconds? Think about that carefully. What physical measurements can someone perform in Australia whose outcome depends on who is queen? What will be the outcome of those measurements be in those milliseconds?

She may instantaneously be queen of Australia, but that cannot cause any effect in Australia faster than light.

 

[fresh_42]

Queen of UK become instantaneous queen of Australia.
The other thing is that people in Australia will have to wait a few milliseconds for information to reach them via TV.

Reading a line in a book changes the information instantaneously, but the physical information that it changed is not instantaneous. We can only measure physical quantities. We do not have any information about what has or has not happened in that line before reading it. And this bit of information transfer takes time.

People in Australia actually never know whether they have a queen or a king in a given moment in time.

 

[Jaime Rudas]

Instantaneous in which frame of reference?

Westminster Abbey

 

[jbriggs444]

Westminster Abbey

Since the rest frame of Westminster Abbey is rotating, that leaves some remaining troubles with simultaneity.