Could we detect an intrinsic change in the flow of time?

[koti]

 

 

The clock drift in this scenario (as I am understanding it) is affected by distance, and therefore has a time component, not by time in the same location. Interfering two lasers with a several km path delay is trivial to attempt in fiber (and increases the time delay by ~50%, owing to the index of refraction slowing the light), though you run into problems with the coherence of the beams at some point*. LIGO was comparing beams that had traversed nominally the same distance, so AFAIK they didn't have that problem. They also went to great lengths to stabilize the system to reduce noise. Typical table-top systems allow you to measure a small fraction of a fringe. So if you're doing this with a 1 micron laser, measuring a fringe shift of a couple parts in 10^8 to get you to your 5.95E-15 meters sounds pretty hard.

 

*~10 km requires a laser linewidth of ~10 kHz. So 1000 km will require 100 Hz.

 

Fiber optic does sound more rational to use in this scenario.

By "laser linewidth" do you mean beam focus (divergence) ?

[substitutematerials]

 

 

The clock drift in this scenario (as I am understanding it) is affected by distance, and therefore has a time component, not by time in the same location. Interfering two lasers with a several km path delay is trivial to attempt in fiber (and increases the time delay by ~50%, owing to the index of refraction slowing the light), though you run into problems with the coherence of the beams at some point*. LIGO was comparing beams that had traversed nominally the same distance, so AFAIK they didn't have that problem. They also went to great lengths to stabilize the system to reduce noise. Typical table-top systems allow you to measure a small fraction of a fringe. So if you're doing this with a 1 micron laser, measuring a fringe shift of a couple parts in 10^8 to get you to your 5.95E-15 meters sounds pretty hard.

 

*~10 km requires a laser linewidth of ~10 kHz. So 1000 km will require 100 Hz.

 

 

Wow this is great Swanson, thanks. I had to do some reading on lasers. If the effect manifested as a discrepancy in length as I was thinking, it looks like it would be hard to set up an experiment without LIGO level precision. However if Strange is right:

 

 

If there were a difference in time, then it would appear as a slow drifting in and out of phase.

then maybe a several km fiber delay inserted into one side of an interferometer would work, because we could observe the interference pattern change over time. If we use 10 km of fiber, and the 50% delay due to the index of the glass you mentioned, this calculation (possibly butchered) suggests that a 500nm laser will go a half-wave out of phase in 4.3 million seconds, or ~50 days. Would this be immeasurable due to frequency instability of the laser? It seems to me that general relativistic dilation could be ignored because the single laser source is necessarily at a single gravitational potential.

But if it does phase in and out, how the laser "know" where it should be in that process? What if you interrupt and restart the beam? I'm not positive that this is a logically consistent idea.

Edited October 20, 2016 by substitutematerials

[swansont]

Fiber optic does sound more rational to use in this scenario.

By "laser linewidth" do you mean beam focus (divergence) ?

Not rational if one is insisting that this affect the speed of light in a vacuum.

 

Linewidth is the frequency width. How monochromatic the light is. a 100 kHz laser is one whose photons are generally within 100 kHz of each other. Meaning that two photons, if they've traveled far enough, will be 180º out of phase with each other, and at that point the light has completely lost its coherence.

[koti]

Linewidth is the frequency width. How monochromatic the light is. a 100 kHz laser is one whose photons are generally within 100 kHz of each other. Meaning that two photons, if they've traveled far enough, will be 180º out of phase with each other, and at that point the light has completely lost its coherence.

Thanks Swansont, I wasn't aware of this. I will need to read up more on this.

Not rational if one is insisting that this affect the speed of light in a vacuum.

I am only partially grasping the concept of this exercise. Do you think it comes down to that? If yes theres something crucial Im missing.

[swansont]

I am only partially grasping the concept of this exercise. Do you think it comes down to that? If yes theres something crucial Im missing.

Well, part of it is the notion that they are measuring the limit of c in a vacuum, but not doing the experiment in a vacuum.

 

The other part is that they give an answer that's inconsistent with "light sometimes travels slower than c" because they point out they are not using the length that the light travels. They are using the starting point and ending point, and not accounting for the fact that the light is not traveling the shortest path. So their conclusion is consistent with photons traveling at c, but changing direction along the path when beams are not comprised of plane waves.

[koti]

I havent realised that the two notions which you point out are an integral part of this excersise. These two notions do render the experiment useless :/ Thanks for the post.

[substitutematerials]

Well, part of it is the notion that they are measuring the limit of c in a vacuum, but not doing the experiment in a vacuum.

 

The other part is that they give an answer that's inconsistent with "light sometimes travels slower than c" because they point out they are not using the length that the light travels. They are using the starting point and ending point, and not accounting for the fact that the light is not traveling the shortest path. So their conclusion is consistent with photons traveling at c, but changing direction along the path when beams are not comprised of plane waves.

 

 

I don't think the hypothetical effect necessitates a change in c. In fact the constancy of c is necessary for the doppler measurements in the Pioneers' telemetry. In the clock acceleration interpretation, the spacecraft are not actually slowing down, the deceleration is then an artifact of the clock error.

 

I think you could use refractive index to add light delay in an experiment, as long as you can precisely account for it. The only criteria is a delay between the 2 clock signals or laser wavelengths; the vacuum is not inherently necessary I don't think.

[swansont]

 

 

I don't think the hypothetical effect necessitates a change in c. In fact the constancy of c is necessary for the doppler measurements in the Pioneers' telemetry. In the clock acceleration interpretation, the spacecraft are not actually slowing down, the deceleration is then an artifact of the clock error.

 

I think you could use refractive index to add light delay in an experiment, as long as you can precisely account for it. The only criteria is a delay between the 2 clock signals or laser wavelengths; the vacuum is not inherently necessary I don't think.

 

 

If you are proposing a change in time but not a corresponding change in length, how does c remain constant?

[substitutematerials]

I think you are right, clocks ticking at a slower rate necessitate a change in length to preserve c. This is the case in the transformation from expanding coordinates to time-dilated coordinates for cosmological expansion, correct? So if we add length to the fiberoptic cable, or more bounces in the beam path at LIGO, a clock acceleration would cause us to observe the light traveling just slightly further or less than the actual rest length that we added. Totally nonsensical?

[michel123456]

 

 

If you are proposing a change in time but not a corresponding change in length, how does c remain constant?

But even in the accepted model, space is expanding alone. How is c conserved in this case?

[swansont]

But even in the accepted model, space is expanding alone. How is c conserved in this case?

 

 

Space is expanding. That's not the same as some length metric changing. A remote galaxy is e.g. 10 BLY away when it was 5 at some previous time. If lengths stretched instead we'd say it was still 5 BLY away.

[michel123456]

 

 

Space is expanding. That's not the same as some length metric changing. A remote galaxy is e.g. 10 BLY away when it was 5 at some previous time. If lengths stretched instead we'd say it was still 5 BLY away.

Too bad for me I will never understand that.

To me the concept of expanding space IS a change in the metric. It is a scale factor.

[koti]

Too bad for me I will never understand that.

To me the concept of expanding space IS a change in the metric. It is a scale factor.

For lack of a better word, the lenght metric is "contained" within space-time so it's not the same. I guess.

I too have difficulties in grasping some of Swansont's posts. Sometimes I read them 5 times and still don't get it. Thats what you get for not being a physicist.