# Does Force Travel Faster Than Light?

• B
• MrSimp
In summary: But does anyone think they have an answer, as I'm genuinely quite curious to learn what people who know more than me have to say about it, without it being written off as impossible to answer, simply because the premise would be difficult to replicate. Say this was done with an atom, instead of a long bar.Welcome to PF.
MrSimp
TL;DR Summary
Came up with a thought experiment, and since I'm only 18 I assumed there likely would be a simple answer that someone older than me would know. I went to my high school physics teacher and his response made me more interested.
Imagine a plane 1 light minute across. Now imagine 1 person on either and of that plane. Between them, is a thin indestructible bar that is 1 light minute in length. In the center of this plane, a simple device has a wire that leads to a motion sensor on the left side of the bar. If the left side of the bar moves, 30 seconds later, the device in the center will make it known. Perhaps a green light on the left side will activate. Vice versa for the right side, another sensor is present on the other end and another light is present on the right side of this same device.

My question:
To my understanding, if the person on the left side of the bar pushed the bar forward by 1 meter, the device 30 light seconds away would light up on its left side after a delay of 30 seconds. But since the bar is being moved forward, towards the opposing end, the right side would have to move too. My question is, would both lights on this device go off at the same time (meaning by pushing one end of this bar, the other end moved in unison), or would there be a delay of 1 minute 30 second for the right-sided light, since the force would travel along at the speed of light.

I asked this to my physics teacher, and he compared the question to asking, "If I were to turn off the sun's gravity, would all the planets start drifting away at the same time, or would Mercury start to drift several minutes before Jupiter." He wondered if this question was about general or special relativity. We both understood the speed of light to be a hard limit, but if this hypothetical, indestructible, 1-light-minute-long bar existed, what would happen in this experiment? Would both lights go off at the same time, meaning both ends of the bar moved, or not?

My teacher made another good point though, he said that asking this question is like asking "If dragons were real, could they breath fire?" The question itself, like with turning off the sun's gravity, already breaks the laws of physics with its premise. Meaning, its answer lay outside the laws of physics too, the entire question is pure fantasy, only possible in an imagination, improvable.

But does anyone think they have an answer, as I'm genuinely quite curious to learn what people who know more than me have to say about it, without it being written off as impossible to answer, simply because the premise would be difficult to replicate. Say this was done with an atom, instead of a long bar.

MrSimp said:
since the bar is being moved forward, towards the opposing end, the right side would have to move too.
But the bar does not move all at once. The internal forces in the bar that cause the rest of the bar to move when you push on one end take time to travel--and they cannot travel any faster than light. So the far end of the bar will not move until at least one light travel time after the end that is pushed. (In any real bar, it will take longer for the far end of the bar to move, because the speed of propagation of internal forces inside real materials, which is basically the speed of sound in the material, is much slower than the speed of light.)

In short, the concept of an "infinitely rigid object", where pushing on any part of it causes the whole object to move all at once, is impossible in relativity.

Edit: The above is saying the same thing as the Insights article that @berkeman linked to.

vanhees71 and PeroK
MrSimp said:
Thank you so much! This is the perfect answer.
Keep asking good questions. PF is a great place to learn.

MrSimp said:
My teacher made another good point though, he said that asking this question is like asking "If dragons were real, could they breath fire?" The question itself, like with turning off the sun's gravity, already breaks the laws of physics with its premise. Meaning, its answer lay outside the laws of physics too, the entire question is pure fantasy, only possible in an imagination, improvable.
It is even a bit further than that. There is nothing per se prohibiting the existence of dragons. SR predicts that there are no perfectly rigid rods (which is the appropriate term, "indestructible" just means it cannot be destroyed, not that it can't be bent or compressed) so asking what SR would say about them is quite literally asking what SR predicts if it doesn't apply.

MrSimp said:
he compared the question to asking, "If I were to turn off the sun's gravity, would all the planets start drifting away at the same time, or would Mercury start to drift several minutes before Jupiter."
Like the other question, this one contains an impossible hidden assumption. In this case, it is not possible to “turn off” gravity or to make mass disappear.

This was a fun video, showing experimentally the speed of "force" through a "solid" rod

PAllen, PeroK and MrSimp

## 1. Does force travel faster than light?

According to Einstein's theory of relativity, nothing can travel faster than the speed of light. This includes force, as it is a form of energy. Therefore, force cannot travel faster than light.

## 2. What is the speed of light?

The speed of light is approximately 299,792,458 meters per second in a vacuum. This is considered to be the fastest possible speed in the universe.

## 3. Can anything travel faster than the speed of light?

Based on our current understanding of physics, it is not possible for anything to travel faster than the speed of light. The laws of relativity apply to all objects and forces in the universe.

## 4. Why is the speed of light considered to be a universal speed limit?

The speed of light is considered to be a universal speed limit because it is a fundamental constant of the universe. It is the maximum speed at which energy, information, and matter can travel.

## 5. How do scientists study the speed of light and its limitations?

Scientists use a variety of methods to study the speed of light and its limitations. These include experiments with particles and light, observations of astronomical events, and mathematical models based on the laws of relativity.

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