# How does the LIGO experiment affect SpaceTime?

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• physicsnoobie79
In summary, the LIGO experiment measures the warping of space caused by gravitational waves. However, it is also affected by the warping of time in the LIGO area, which can complicate the measurements taken.
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.

physicsnoobie79 said:
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.

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.

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.

berkeman
@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?

physicsnoobie79 said:
@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.

jim mcnamara
PeroK said:
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.

berkeman
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physicsnoobie79 said:
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)

## 1. How does the LIGO experiment detect gravitational waves?

The LIGO experiment uses a laser interferometer to detect tiny changes in the length of its arms caused by passing gravitational waves. When a gravitational wave passes through the detector, it causes the arms to stretch and compress, which can be measured by the interference pattern of the laser beams.

## 2. How does the detection of gravitational waves impact our understanding of SpaceTime?

The detection of gravitational waves confirms the existence of ripples in the fabric of SpaceTime, as predicted by Einstein's theory of general relativity. This provides evidence for the idea that the fabric of SpaceTime is not static, but can be distorted by massive objects moving through it.

## 3. How does the LIGO experiment affect our understanding of black holes?

The LIGO experiment has allowed us to directly observe the collision of two black holes, providing evidence for their existence and the merging process. It has also allowed us to study the properties of black holes, such as their mass and spin, and has furthered our understanding of how they interact with their surroundings.

## 4. How does the LIGO experiment impact future space exploration?

The detection of gravitational waves by the LIGO experiment has opened up a new field of astronomy, allowing us to observe the universe in a completely different way. This technology could potentially be used in future space missions to study distant objects and phenomena, such as merging black holes or neutron stars.

## 5. How does the LIGO experiment contribute to the advancement of science and technology?

The LIGO experiment has pushed the boundaries of science and technology, requiring incredibly precise measurements and advanced engineering. It has also led to advancements in fields such as laser technology and data analysis, and has inspired further research and development in the study of gravitational waves and their effects on SpaceTime.

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