B How does the LIGO experiment affect SpaceTime?

[physicsnoobie79]

TL;DR Summary

If LIGO measures the warping of space, doesn't the warping of time compensate this so it cannot be measured?

I'm just a layperson with a keen interest a couple of notches above popular science.

As far as I understand SpaceTime is an attribute where if you change one attribute (space or time) then the other attribute is affected. E.g. as you approach the speed of light, the time passing of other things will be being to slow down. To the point where once you're at the speed of light, the time of other things will stop.

So, with the LIGO experiment, it uses lasers to measure the ripples from gravitational waves. These ripples stretch/contract space which can be measured by the lasers.

So my question is this, if space is being warped by gravitational waves and that is being measured by lasers, doesn't the related time warping it causes also affect the measurements taken such that it cancels out anyway we can measure this?

For example, if the distance between the two LIGO sensors is increased then this distance can be measured but if the time in that area is slowed, then the time taken for the measure will also increase and thus 100% cancel out the increased distance?

Maybe I've got things completely wrong or I've got my wires crossed. And maybe this might be beyond my comprehension. However, I'm still keen on trying to understand this.

Thanks in advance.

 

[PeroK]

Summary: If LIGO measures the warping of space, doesn't the warping of time compensate this so it cannot be measured?

I'm just a layperson with a keen interest a couple of notches above popular science.

As far as I understand SpaceTime is an attribute where if you change one attribute (space or time) then the other attribute is affected. E.g. as you approach the speed of light, the time passing of other things will be being to slow down. To the point where once you're at the speed of light, the time of other things will stop.

Maybe I've got things completely wrong or I've got my wires crossed. And maybe this might be beyond my comprehension. However, I'm still keen on trying to understand this.

Thanks in advance.

To be honest your understanding of relativity is very much at the popular science level.

And, in fact, like most people who learn from pop science sources, you have misunderstood the key concepts.

Just to take two points. There is no such thing as absolute motion. All motion is relative. When you talk about "time slowing down" for an object in motion, that effect is entirely symmetrical and you could equally well say your time is slowing down relative to that object's measurements.

Moreover, no material object can atain the speed of light relative to another. The concept of time stopping at the speed of light is, I'm afraid, a pop science myth that is not supported by the mathematics.

These basic confusions undermine your analysis of the LIGO experiment.

 

[PeroK]

PS your analysis is based on Special Relativity concepts, whereas gravitational waves are very much a General Relativity phenomenon, related to the changing metric of local spacetime.

 

[physicsnoobie79]

@PeroK thank you for your reply. I understand the first part of what you said but my choice of words and explanation in my opening post may not convey that. Hopefully these two points indicate this:

1) I understand that anything with mass cannot reach speed of light due to the need for infinite energy. However, photons (which are massless) travel at the speed of light and thus from their perspective, the universe is frozen in time, no?

2) Also, I also understand if Object A is traveling at 50% the speed of light relative to Object B. From, A's perspective, B's clock will appear to be slower than their own. But from B's perspective, A's clock will appear to be slower than their own. Hopefully this is accurate too?

So if the above two are accurate, then onto my confusion with the LIGO experiment. If either of the above two are incorrect, then time to give up and get back to my day job (lol).

The LIGO experiment measures the distance between two objects using a very accurate laser. It detects gravitational waves by the change in the distance between those two objects (i.e. the space is affected). However, gravitational waves (and gravity generally) affect SpaceTime. So if this is the case, what effect does the gravitational waves have on time in the LIGO area for an observer that is at the experimental sensors? Surely it can't just affect distance/space without having some affect on time?

 

[PeroK]

@PeroK thank you for your reply. I understand the first part of what you said but my choice of words and explanation in my opening post may not convey that. Hopefully these two points indicate this:

1) I understand that anything with mass cannot reach speed of light due to the need for infinite energy. However, photons (which are massless) travel at the speed of light and thus from their perspective, the universe is frozen in time, no?

2) Also, I also understand if Object A is traveling at 50% the speed of light relative to Object B. From, A's perspective, B's clock will appear to be slower than their own. But from B's perspective, A's clock will appear to be slower than their own. Hopefully this is accurate too?

So if the above two are accurate, then onto my confusion with the LIGO experiment. If either of the above two are incorrect, then time to give up and get back to my day job (lol).

The LIGO experiment measures the distance between two objects using a very accurate laser. It detects gravitational waves by the change in the distance between those two objects (i.e. the space is affected). However, gravitational waves (and gravity generally) affect SpaceTime. So if this is the case, what effect does the gravitational waves have on time in the LIGO area for an observer that is at the experimental sensors? Surely it can't just affect distance/space without having some effect on time?

To get to the main point: a gravitational wave changes the local spacetime metric. With the right experiment this can be detected. To see how, you really need to look at the specifics of the wave and the metric.

But, it's not just time dilation and an equal and opposite length contraction. These are really only concepts from the flat spacetime of SR.

 

[physicsnoobie79]

To get to the main point: a gravitational wave changes the local spacetime metric. With the right experiment this can be detected. To see how, you really need to look at the specifics of the wave and the metric.

But, it's not just time dilation and an equal and opposite length contraction. These are really only concepts from the flat spacetime of SR.

Gotcha. Thank you for your reply :-D

 

[Torbert]

https://www.ligo.caltech.edu/page/research-developmentA light duty overview with enough information to formulate questions from.

https://www.kavlifoundation.org/how-ligo-works
A lighter duty that hints at the sequential action of the two arms in getting a signal.

 

[phinds]

However, photons (which are massless) travel at the speed of light and thus from their perspective, the universe is frozen in time, no?

No. This is another pop-sci falsehood. The problem with is that "from the perspective of" in correct technical terms means "in the inertial frame of reference of", so for light to HAVE an inertial frame of reference, then you have posited a frame of reference in which light has zero velocity, BUT light travels at c in all inertial frames of reference, SO ... now you have posited a frame of reference in which light simultaneously is at rest and is traveling at c. Not possible.

The short version of all that is to just say that light doesn't HAVE a frame of reference (point of view)