# Thought Experiment : Try and get Alice to "0%" the Speed of light

• B
• Justice Hunter
In summary, the conversation discusses the concept of being at rest in the universe and the implications of trying to reach a state of "0% the speed of light." It is argued that in order to truly be at rest, one would need to probe and know the state of every other object in the universe, which would require an infinite amount of information and energy. This raises questions about the nature of being at rest and the role of the principle of relativity in understanding motion and time.
Justice Hunter
TL;DR Summary
A thought experiment that challenges the user to think about slowing down Alice to "0%" the speed of light.
Someone asked a really interesting question on a comment thread somewhere's, and ever since, I could never really stop thinking about what the proper answer to it could be. It's a really basic question, but it unpacks a can of worms.

The exact question I read was the following
"if the motion of the planet going around the sun and the sun going around the galactic center create time like if we were static in space time would time actually pass as your not moving through space"

This question can be formalized into a rudimentary thought experiment.

We're all moving through the universe at some speed relative to the stuff around us in our causal light cones. Collectively the galaxy is probably hurtling through space at say, 10% the speed of light or whatever, the exact speed doesn't matter. One could ask the question of, instead of accelerating a mass faster towards 100% the speed of light...how does someone go slower...toward 0% the speed of light? The answer seems to be, that in order to go "0%" the speed of light, means that everything in the causal universe has to move in the same reference frame as you are. I thought about this and this essentially would make the entire universe become static, and this makes sense in answering the question.​

But the conjecture is that the above is impossible to do, much like how you can't go 100% the speed of light, you can't get an object to 0% the speed of light either in the same exact manner.

1) It requires you to decelerate the entire universe, (infinite amount of mass/energy.)
2) Uncertainty in information in altering the trajectory of everything to have the same reference frame. In order to change the reference frame of Alice to Bob, requires energy from Charlie, requires energy from David requires energy into infinity...
3) Then on top of that is just regular ol'quantum mechanical uncertainty in position and velocity...we simply CANT alter the trajectories of objects, even if we had all infinity of them available to us, because we can't resolve it to infinite accuracy.
4) All three reasons above, means one requires a machine, of infinite size, at the infinitely far away boundary of the universe in order to make Alice move at "0%" the speed of light. Sounds pretty familiar...

The above mimics the same restrictions one would have on accelerating an object with mass toward the speed of light...except, the infinity in this problem is a different kind of infinity...where it's not impossible to do these micro adjustments of relativistic frames to get things to stop in time...it just requires doing these micro-adjustments to the entire universe, which takes an infinite amount of energy. So what does that really say about the behavior of space-time?

The principle of relativity allows anybody to regard themself as "at rest", at least instantaneously, regardless of their state of motion. Attempting to reach "0% of the speed of light" is therefore either trivial (any state of motion can be considered "at rest"), or incoherent (you haven't specified speed relative to something).

The original quotation you cited appears to be nonsense. Attempting to explore the implications of nonsense is unlikely to be fruitful.

All observers experience time regardless of their state of motion.

Grasshopper, russ_watters, Vanadium 50 and 2 others
Alice is always going 0% of the speed of light in her own rest frame.

Edit: @Ibix with a slightly faster and much better reply!

Grasshopper, Ibix and vanhees71
Ibix said:
The principle of relativity allows anybody to regard themself as "at rest", at least instantaneously, regardless of their state of motion. Attempting to reach "0% of the speed of light" is therefore either trivial (any state of motion can be considered "at rest"), or incoherent (you haven't specified speed relative to something).

The original quotation you cited appears to be nonsense. Attempting to explore the implications of nonsense is unlikely to be fruitful.

All observers experience time regardless of their state of motion.

So let's say Alice is a particle "at rest" In order to be at rest requires probing it's state of rest, by probing another particle Bob, and knowing whether Bob's state is at rest requires probing Charlie etc...

You can't in fact "know" that Alice is at rest without requiring an infinite amount of probes to determine that information. This is not trivial by any means and it implies that the nature of being "at rest" is at the mercy of losing observables, yet again.

Justice Hunter said:
So let's say Alice is a particle "at rest" In order to be at rest requires probing it's state of rest, by probing another particle Bob, and knowing whether Bob's state is at rest requires probing Charlie etc...
No. In order to be at rest simply requires asserting that this is the case. What anything else is doing is irrelevant. This is the principle of relativity.

Grasshopper, russ_watters and Dale
etotheipi said:
How do you define "probing"? To be at rest is just to say the co-ordinates of the particle are not changing

Particle Alice, and Elevator Bob are in space. Alice and Bob appear "at rest." You probe observer Charlie determine that Alice and Bob are in fact, not at rest with respect to Charlie.

If you were to set Alice Bob and Charlie to be at rest with each other, then you probe David and realize that Alice Bob and Charlie aren't actually at rest again with respect to david...

In order to know if an object actually is at rest, requires knowing the state of everything in the system...an infinite amount of information, and in turn an infinite amount of energy to probe that information.

Implying that the coordinates of alice isn't changing, is assuming that those coordinates are actually static, which you can't know without probing everything in the system. Alice can in fact be moving through coordinate space, relative to something else in the system.

Last edited:
Justice, you REALLY should quit while you're behind. If you keep this up it's going to be clear that you are just trolling us (if it isn't all ready)

Grasshopper
Justice Hunter said:
an object actually is at rest
There is no such thing. This is what we are trying to tell you.

You can simply declare yourself to be at rest or not - it's up to you. There is no physical consequence to the decision. It only changes the maths you use to describe things.

Grasshopper, russ_watters and Dale
Ibix said:
There is no such thing. This is what we are trying to tell you.

You can simply declare yourself to be at rest or not - it's up to you. There is no physical consequence to the decision. It only changes the maths you use to describe things.

I mean, you can't actually declare that you are at rest, because you need to know an infinite amount of information to do so...and physically you can't have a system where you are moving at 0% the speed of light, because it requires an infinite amount of energy to move all components in a system into your reference frame.

So there is a physical consequence to declaring something is "at rest," as well as having to lose observables.

Justice Hunter said:
I mean, you can't actually declare that you are at rest, because you need to know an infinite amount of information to do so...
This is infinitely overcomplicating matters.

Grasshopper, vanhees71 and phinds
Justice Hunter said:
because you need to know an infinite amount of information to do so
Simply repeating something that is wrong does not make it right.

I can always declare myself to be at rest because there is no experiment that I can do whose outcome is affected by that decision. I can detect that something else is moving relative to me (or some other reference), but I am always free to declare that I am moving and it is stationary, I am stationary and it is moving, or a bit of both.

If you think otherwise, you need to come up with a measurement that you can make that will be different if you assume you are moving. Note that things like "I'd measure the kinetic energy of an object" don't work - you can only measure its kinetic energy by measuring the energy needed to stop it, which means you need an assumption of the state of rest.

Dale and russ_watters
A.T. said:
This is infinitely overcomplicating matters.
Ibix said:
Simply repeating something that is wrong does not make it right.

I can always declare myself to be at rest because there is no experiment that I can do whose outcome is affected by that decision. I can detect that something else is moving relative to me (or some other reference), but I am always free to declare that I am moving and it is stationary, I am stationary and it is moving, or a bit of both.

If you think otherwise, you need to come up with a measurement that you can make that will be different if you assume you are moving. Note that things like "I'd measure the kinetic energy of an object" don't work - you can only measure its kinetic energy by measuring the energy needed to stop it, which means you need an assumption of the state of rest.
Think of it this way then.

Prove that right now, you aren't moving at 99% the speed of light.

The answer is, that you can't. You have no observables...you need infinite information and infinite energy to probe that question. The speed of light is always at 300km/s in all reference frames.

Justice Hunter said:
Prove that right now, you aren't moving at 99% the speed of light.
I AM moving at 99% of the speed of light right now and so are you. In fact, more like 99.9999% (relative to a particle in the CERN accelerator). We are both also moving at 0% of the speed of light relative to the chairs that we are sitting in.

You really need to stop your ridiculous argument and study some actual physics.

Grasshopper, Dale, Ibix and 1 other person
Ibix said:
Simply repeating something that is wrong does not make it right.
To be fair, he didn't simply repeat it. He bolded it as well.

Grasshopper, Ibix, vanhees71 and 2 others
phinds said:
I AM moving at 99% of the speed of light right now and so are you. In fact, more like 99.9999% (relative to a particle in the CERN accelerator). We are both also moving at 0% of the speed of light relative to the chairs that we are sitting in.
Yes exactly. In order to know the answer, requires probing Geneva and probing your chair, and probing everything else in the universe.

So back to the original thought experiment, is that if you wanted to "stop time", by placing everything in your reference frame, then you need to orchestrate the displacement of all components of everything in the universe, which is impossible to do, just like how you can't reach the speed of light, you can't go 0% the speed of light either because it requires infinite knowledge of the system, and energy comparable to the size of the system.

weirdoguy
Justice Hunter said:
Yes exactly. In order to know the answer, requires probing Geneva and probing your chair, and probing everything else in the universe.

So back to the original thought experiment, is that if you wanted to "stop time", by placing everything in your reference frame, then you need to orchestrate the displacement of all components of everything in the universe, which is impossible to do, just like how you can't reach the speed of light, you can't go 0% the speed of light either because it requires infinite knowledge of the system, and energy comparable to the size of the system.
The original quote you gave is nonsense. The arguments you are making in this thread are nonsense. The responses to you in this thread have explained why. Enough is enough.

Thread closed.

Grasshopper, jbriggs444, Klystron and 5 others

## 1. What is a thought experiment?

A thought experiment is a mental exercise or hypothetical scenario used to explore and understand a scientific concept or theory. It involves using logic and reasoning to imagine a situation and predict the outcome without actually performing the experiment in reality.

## 2. How is the speed of light related to thought experiments?

The speed of light, denoted as "c", is a fundamental constant in physics that represents the maximum speed at which energy, matter, and information can travel in the universe. Thought experiments often use the speed of light as a reference point to explore the consequences of hypothetical scenarios and test the limits of our understanding of the laws of physics.

## 3. What does it mean to "0%" the speed of light?

To "0%" the speed of light means to reach a velocity of 0 meters per second relative to the speed of light. This is impossible according to Einstein's theory of relativity, which states that nothing can travel faster than the speed of light. It is a hypothetical scenario used in thought experiments to explore the consequences of breaking this fundamental law.

## 4. How can Alice reach "0%" the speed of light in a thought experiment?

In a thought experiment, Alice can reach "0%" the speed of light by imagining a scenario where she is traveling at a constant velocity and then suddenly stops. This would result in her relative velocity to the speed of light being 0 meters per second. However, in reality, this is impossible to achieve as it would require infinite energy.

## 5. What can we learn from thought experiments about reaching "0%" the speed of light?

Thought experiments allow us to explore the consequences of breaking the laws of physics and help us understand the limitations of our current scientific understanding. By imagining scenarios where we can reach "0%" the speed of light, we can gain insights into the nature of space, time, and the universe itself.

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