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Mass and angular momentum, left ambiguous by Einstein, get defined

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Re: Mass and angular momentum, left ambiguous by Einstein, get defined

#11

Post by CamperBob2 »

tasty_freeze wrote: post #10
codethief wrote: post #8

Earlier quoted context omitted.

> But it's an axiom. I don't agree. If light traveled instantaneously in one direction, then if we looked in the opposite direction (where such light originates from) we would be seeing stars and galaxies at much more recent time (now). Also, their light would have traveled a much longer distance (due to the ongoing expansion of space) and so would be redshifted much more. All in all what we see on the sky would look…

I get what you are saying, it suggests there is no preferred direction, but it is just an inference. The point is one cannot create an experiment to measure the speed of light other than a round trip average speed.

Wasn't that the whole idea behind Michelson-Morley ( https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_exper...)?

Re: Mass and angular momentum, left ambiguous by Einstein, get defined

#12

Post by db48x »

CamperBob2 wrote: post #11
tasty_freeze wrote: post #10

Earlier quoted context omitted.

I get what you are saying, it suggests there is no preferred direction, but it is just an inference. The point is one cannot create an experiment to measure the speed of light other than a round trip average speed.

Wasn't that the whole idea behind Michelson-Morley ( https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_exper... )?

Yes, that is an example of a measurement of the two-way speed of light. What is impossible, however, is a measurement of the one-way speed of light.

Re: Mass and angular momentum, left ambiguous by Einstein, get defined

#13

Post by kolinko »

tasty_freeze wrote: post #10
codethief wrote: post #8

Earlier quoted context omitted.

> But it's an axiom. I don't agree. If light traveled instantaneously in one direction, then if we looked in the opposite direction (where such light originates from) we would be seeing stars and galaxies at much more recent time (now). Also, their light would have traveled a much longer distance (due to the ongoing expansion of space) and so would be redshifted much more. All in all what we see on the sky would look…

I get what you are saying, it suggests there is no preferred direction, but it is just an inference. The point is one cannot create an experiment to measure the speed of light other than a round trip average speed.

Red shift would be different in various directions then. We would also see fewer young galaxies / stars in the direction in which light flies faster.

Re: Mass and angular momentum, left ambiguous by Einstein, get defined

#14

Post by gnramires »

qsdf38100 wrote: post #4
shadowgovt wrote: post #2

The most eye-opening thing I've learned about relativity in the past few years is that the notion that space has no preferential direction is an axiom in the theory. There's nothing about the way we measure the speed of light that would disambiguate if light traveled instantaneously in one direction and at half the measured speed of light in the other. We just don't have a way to know, because time measurements requi…

Veritasium has a nice video about it on YouTube. We can only measure the average back and forth speed of light.

I'm not sure that claim is true. Take this experiment, where A sends B a message following two paths:

  /-->--B (/ and \ are mirrors)
  |     |
  |     |
  \--
with speeds: (assume lengths are all 1m)

  /--cr--B
  |      |
  cu    cu
  |      |
  \--cl--A
The time it takes for path 1 (left,up,right) is cl+cu+cr. The time it takes for path 2 is cu. B can measure the difference cl+cu+cr-cu = cl+cr. A can compare cl+cr to cu: if cl+cr != 2cu the velocity is not isotropic. To see that's always possible, A and B can simply bounce back path 2, so B receives pulses every 2cu (and hence can measure and compare to other time intervals).

Directional speed of light would be very weird, I think it'd show up everywhere in experiments if it weren't true.

---

Also, I think there's a notable distinction in physics: they are usually called postulates to distinguish from axioms. A postulate is an assumption about a physical theory (usually something simple and "beautiful" -- mathematically neat and satisfying Occam's razor); if a theory doesn't match reality, one of its postulates is incorrect. An axiom in mathematics of course can't be proven wrong. Because axioms are the basis for your mathematical theory describing reality, they can't be incorrect (as long as they form a mathematically consistent theory); the most could happen is they're insufficient to describe reality (you need other axioms and another mathematical theory), but they're not (somewhat) falsifiable in the sense of physical postulates.

Re: Mass and angular momentum, left ambiguous by Einstein, get defined

#15

Post by CamperBob2 »

db48x wrote: post #12
CamperBob2 wrote: post #11

Earlier quoted context omitted.

Wasn't that the whole idea behind Michelson-Morley ( https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_exper... )?

Yes, that is an example of a measurement of the two-way speed of light. What is impossible, however, is a measurement of the one-way speed of light.

M-M compared the speed in one direction to the speed at right angles. That's not quite the same thing as a one-way measurement, but it's also not the same thing as a two-way measurement along the same path.

Re: Mass and angular momentum, left ambiguous by Einstein, get defined

#16

Post by db48x »

CamperBob2 wrote: post #15
db48x wrote: post #12

Earlier quoted context omitted.

Yes, that is an example of a measurement of the two-way speed of light. What is impossible, however, is a measurement of the one-way speed of light.

M-M compared the speed in one direction to the speed at right angles. That's not quite the same thing as a one-way measurement, but it's also not the same thing as a two-way measurement along the same path.

Right, but the way that they measured the speed in any direction was by making a two-way measurement. They sent a pulse of light out to be reflected and timed how long it took for it to return.

Re: Mass and angular momentum, left ambiguous by Einstein, get defined

#17

Post by naasking »

qubex wrote: post #9
shadowgovt wrote: post #2

The most eye-opening thing I've learned about relativity in the past few years is that the notion that space has no preferential direction is an axiom in the theory. There's nothing about the way we measure the speed of light that would disambiguate if light traveled instantaneously in one direction and at half the measured speed of light in the other. We just don't have a way to know, because time measurements requi…

That’s not an axiom, it’s an assumption. Anyway, it’s the assumption of isotropy.

An axiom is an assumption: https://en.wikipedia.org/wiki/Axiom

Re: Mass and angular momentum, left ambiguous by Einstein, get defined

#18

Post by Beldin »

codethief wrote: post #8
shadowgovt wrote: post #2

The most eye-opening thing I've learned about relativity in the past few years is that the notion that space has no preferential direction is an axiom in the theory. There's nothing about the way we measure the speed of light that would disambiguate if light traveled instantaneously in one direction and at half the measured speed of light in the other. We just don't have a way to know, because time measurements requi…

> But it's an axiom. I don't agree. If light traveled instantaneously in one direction, then if we looked in the opposite direction (where such light originates from) we would be seeing stars and galaxies at much more recent time (now). Also, their light would have traveled a much longer distance (due to the ongoing expansion of space) and so would be redshifted much more. All in all what we see on the sky would look…

Good point! Simplified (trying to explain to myself), if light travelled slower in one direction than others, some objects would be less far away from us and each other and thus their gravitational effects would be different.

Basically, our cosmological horizon would be significantly closer in one direction than others - and all objects (= mass) in that direction would be as well.

There might be a very specific way in which matter could be distributed so that this is actually true but indistinguishable from uniform light speed in all directions, from our specific point of view. But I'd expect the effects to be noticeably different even within a fairly limited time range (decades, perhaps centuries).

Re: Mass and angular momentum, left ambiguous by Einstein, get defined

#19

Post by alex_smart »

shadowgovt wrote: post #2

The most eye-opening thing I've learned about relativity in the past few years is that the notion that space has no preferential direction is an axiom in the theory. There's nothing about the way we measure the speed of light that would disambiguate if light traveled instantaneously in one direction and at half the measured speed of light in the other. We just don't have a way to know, because time measurements requi…

Could you not, at least theoretically, create two synchronized clocks and physically transport one of them to another location and then throw a beam of light from the location of the first clock and measure the time of arrival at the second clock? Am I missing something?

Edit: What I was missing was time dilation. Physically transporting the clocks would mean that they are no longer synchronized.

Re: Mass and angular momentum, left ambiguous by Einstein, get defined

#20

Post by [deleted] »

gnramires wrote: post #14
qsdf38100 wrote: post #4

Earlier quoted context omitted.

Veritasium has a nice video about it on YouTube. We can only measure the average back and forth speed of light.

I'm not sure that claim is true. Take this experiment, where A sends B a message following two paths: /-->--B (/ and \ are mirrors) | | | | \-- with speeds: (assume lengths are all 1m) /--cr--B | | cu cu | | \--cl--A The time it takes for path 1 (left,up,right) is cl+cu+cr. The time it takes for path 2 is cu. B can measure the difference cl+cu+cr-cu = cl+cr. A can compare cl+cr to cu: if cl+cr != 2cu the velocity is…

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