# How to determine speed with no frame of reference?

## Main Question or Discussion Point

You are standing on one. The two planets were closing in on a speed of 1x. You just accelerated one of the 2 planets towards the other with a super powerful rocket engine, and then sat on one of them. Now they are closing in on each other at speed 10x. The universe is otherwhise empty. The problem is, your memory got wiped after you did this. How would you determine which planet is moving faster? They are the same size and weight. No atmosphere. You are standing on one.

Since there is no star to light the thing up, let's say we installed lights on those planets.

It must appear if you are sitting on either one of them, that the other planet is coming towards you. Can we state here that neither entity has a absolue speed, and it is impossible to determine which one is moving?

Maybe something with einstein's equations? Since the one that is moving 10x the speed of the other has time slowing down relative to the other? (you can hop between the two while this is going on).

Any thoughts?

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phinds
Gold Member
2019 Award
"Motion" is meaningless without a frame of reference so, no, you cannot legitimately say that one is moving and one is not because it just isn't meaningful.

Now, if one is ACCELERATING, that's a different story since acceleration is not relative

Drakkith
Staff Emeritus
It must appear if you are sitting on either one of them, that the other planet is coming towards you. Can we state here that neither entity has a absolue speed, and it is impossible to determine which one is moving?
It is already impossible to state which one is 'absolutely' moving. It is equally valid to say that planet A is moving or that planet B is moving or that BOTH planets are moving.

phinds
Gold Member
2019 Award
It is already impossible to state which one is 'absolutely' moving. It is equally valid to say that planet A is moving or that planet B is moving or that BOTH planets are moving.
Ha! You're a day late and a dollar short! (well, OK, 30 seconds late and maybe only 50 cents short)

But according to einstein, one planet now has more energy right? So time would go at a different pace at both planets relative to eachother? So you could do something with that? Place a clock on one, and then go to the other, and also place a clock there (a very accurate one). And then after a while compare which clock has less time go by? That is the one that is going faster?

phinds
Gold Member
2019 Award
But according to einstein, one planet now has more energy right?
No, why do you think so?

So time would go at a different pace at both planets relative to eachother? So you could do something with that? Place a clock on one, and then go to the other, and also place a clock there (a very accurate one). And then after a while compare which clock has less time go by? That is the one that is going faster?
No they each see the other as having time moving more slowly, although it's impossible to say by how much since you have not specified the speed of either one. Your original statement of one moving 10 times faster than the other is meaningless since you did not reference that to anything.

hmmm Ok what if you measured how fast time went before you sped one of them up by 10x? (so distance closing 10x faster). And now you measure afterwards? Then you can tell right? (let's say you forgot everything else, but you wrote that one thing down).

Drakkith
Staff Emeritus
But according to einstein, one planet now has more energy right?
Nope. Kinetic energy is frame dependent. A proton moving at 99% the speed of light (relative to the Earth) will see the Earth as having an enormous amount of kinetic energy and itself as having none.

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phinds
Gold Member
2019 Award
hmmm Ok what if you measured how fast time went before you sped one of them up by 10x? (so distance closing 10x faster). And now you measure afterwards? Then you can tell right? (let's say you forgot everything else, but you wrote that one thing down).
Look, both Drakkith and I have told you that you are wrong in thinking that there is a lack of symmetry here (that is, that there is any difference between how A sees B vs how B sees A. I'll say it one more time. There isn't.

Clearly, our telling you that isn't sinking in so it would like be best for you to go back to your text(s) and study the basics some more.

DaveC426913
Gold Member
hmmm Ok what if you measured how fast time went before you sped one of them up by 10x? (so distance closing 10x faster). And now you measure afterwards? Then you can tell right? (let's say you forgot everything else, but you wrote that one thing down).
Nope. Still symmetrical.

Nugatory
Mentor
hmmm Ok what if you measured how fast time went before you sped one of them up by 10x? (so distance closing 10x faster). And now you measure afterwards? Then you can tell right? (let's say you forgot everything else, but you wrote that one thing down).
Be precise. Describe the exact measurements that we're making, and exactly when and how we make them. Remember that:
- there is no such thing as measuring "how fast time goes". All we can do is look at clocks and see what they read at any moment.
- if you're looking at a clock that isn't where you are, you have to allow for light travel time. If you see (light hits your eyes) a clock five light-minutes away read 11:00 AM at the same time that your clock reads 11:17 AM, you must conclude that the remote clock read 11:00 AM when your clock read 11:12 AM.

Remembering this, try describing the exact sequence of measurements you have in mind. What do you find?

Ok so the scenario now is:
2 planets slowly coming at each other. We both measure one year. When one clock is finished measuring a year, we find out the other one is not finished yet, and the difference is 1 hour. Or since there is no sun, we measure a fixed number of atomic resonations (saying it right?), and see which one is finished first. Again both have exact same size and mass.

Now we push one planet to speed up when the collision will happen, and it now appears they are moving 10x faster at each other. We forget which one we pushed, and now we measure one year again on both planets and see which one is finished first. We can find out which one was sped up by looking at the time difference before and after the great speeding up?

So if the planet where time was going slower by 1 hour (which means this one was moving faster, if they both have same size and mass), is now even further behind , it means this was the on that was sped up? If the opposite is now happening, the other one was sped up.

Im not very good at physics, but really you can state here that if you have a lot of objects in space, if the size and mass is the same, you could measure their speed relative to each other if you measure how long it would take for a year to pass. Or really measure a fix number of times similar atoms resonate, and which ones will be finished first?

Sorry for longwinded explanation :) . Hope it is more clear now.

phinds
Gold Member
2019 Award
What is clear is that despite repeated attempts, we have not yet found a way to help you understand that you are simply wrong. You keep thinking that there is not symmetry between the two planets but there IS symmetry and there is no way you can tell from a point on either one of them, or from a 3rd observer not on either one of them, that there is any difference.

DURING the "pushing" of one of them you can tell them apart because one of them is accelerating and that breaks the symmetry (the other is not accelerating) but once you are not pushing any more the symmetry is back.

HallsofIvy
Homework Helper
You are missing the point. Speed is not absolute and neither is time. What times your clocks read depends on where they are. If you are standing on one of the planets, it is motionless relative to you and the other planet is moving. If you have one of your clocks with you and the other clock on the other planet, you will read its clock as being slower than the other. But if you were to move to the other planet, you would now read the clock on the original planet as slower. If you were at some third frame of reference, you would read the time on the planet that is moving faster relative to you as being slower.

Im not being a smart ass here, just trying to understand :) .

So what you are saying is, that after speeding up one planet, if you would arrange that after counting a certain amount of atomic resonations (let's say 10 million), you shoot a laser at the other planet. If what I am saying is right, the planet that did get the speed up, would receive the laser signal first. As they would take longer to finish counting those 10 million resonations.

But what you are saying, if they would both start counting 10 million resonations after the speed up, and then send a laser right away to the other planet, both would receive the laser signal at the exact same time?

Im asking because it was my impression that space time would be more distorted by faster speeds?

Also I made a mistake in my assumption, if one speeds up, it means mass is converted into energy, so I guess they will not be the same amount of mass after speeding up.

DaveC426913
Gold Member
jimmylegs: once the acceleration has stopped, the system is in perfect symmetry. Nothing you do (short of looking at some reference point external to the system) will distinguish which planet was the one accelerated. Whatever happened before, during the acceleration phase, does not break the symmetry.

Think through your scenarios, and any time you find you need to violate that, know that you are making an error in your logic.

The key to all this is understanding the basic tent of SR: that there is no such thing as absolute position or absolute motion. Every measurement of position and every measurement of motion must be qualified as to what point you are measuring it relative to.

People on Planet A consider themselves motionless, and, relative to them, Planet B is in motion.
People on Planet B consider themselves motionless, and, relative to them, Planet A is in motion.
They are both correct.

So what you are saying is, that after speeding up one planet, if you would arrange that after counting a certain amount of atomic resonations (let's say 10 million), you shoot a laser at the other planet. If what I am saying is right, the planet that did get the speed up,
No, they will both experience the same delay. It is symmetrical.

Im asking because it was my impression that space time would be more distorted by faster speeds?
Speed is relative. It is not fixed to some absolute reference point.
Each planet sees itself as stationary while the other is moving.

Also I made a mistake in my assumption, if one speeds up, it means mass is converted into energy, so I guess they will not be the same amount of mass after speeding up.
Yes they will. Each observes the other planet as the one in motion.

Moreso, you could get in a rocket and hover over planet A. You would observe planet B in motion, and time-modified and more massive.
You could then change your velocity (accelerate) so that you hover over planet B. You would observe planet A in motion, and time-modified and more massive.

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Thanks for the detailed response.

What it really boils down to is how much kinetic energy and mass is concentrated in a certain amount of spacetime? In my example mass would be converted into kinetic energy (assuming for a second a perfect conversion without heat), hence the flow of time would not be affected? Or really the speed of atomic resonations relative to eachother.

So you could have 2 objects of the same mass and size, except one of them has a extra piece of mass. If that piece of mass is converted into kinetic energy, that can be measured by counting the number of atomic resonations on both? Since they both look the same, and from both perspectives it looks as if only the other one is moving, you could measure the energy difference between both systems by using a atomic clock? And there should be a difference because of the conservation of energy?

So my mistake was that I assumed somehow one could magically speed up, while keeping it's mass steady, without help from the outside. You would need energy input from outside the system if that was the case?

phinds
Gold Member
2019 Award
You are still on the wrong track and still believing that there is an asymmetry. Since kinetic energy is a function of speed, I can only conclude that you still are not willing to see that there is no such thing as absolute speed. When you speed up an object in one frame of reference, you are slowing it down in another frame of reference. Thus the planet that you gave a push to has LOST kinetic energy in some frame of reference and gained it in others and none of that does you any good at all in differentiating between the two planets based only on them (and any clocks you put on them)

DaveC426913
Gold Member
jimmyleggs, you're digging a deeper hole for yourself as far as understanding relativity. Read more; theorize less.

Certainly you should go back through this thread and read what people have written.

Dale
Mentor
What it really boils down to is how much kinetic energy and mass is concentrated in a certain amount of spacetime?
No, it doesn't have anything to do with that. It has to do with symmetry. The laws of physics are the same in any inertial frame. That is one symmetry of the laws of nature.

There is no possible physical experiment using any possible mechanism which can distinguish between the scenario where A is at rest with B moving towards it or the scenario where B is at rest with A moving towards it. In the first scenario B has a lot of KE and in the second scenario A has a lot of KE, but due to the symmetry there is no possible experiment you can do which will distinguish the first from the second, even though it may seem like there should be.

EDIT: fixed my wording, see below

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phinds
Gold Member
2019 Award
There is no possible physical experiment using any possible mechanism which can distinguish between the scenario where A is at rest moving towards B or the scenario where B is at rest moving towards A. .
I think you mean A is at rest with B moving towards it / B is at rest with A moving towards it.

Dale
Mentor
I think you mean A is at rest with B moving towards it / B is at rest with A moving towards it.
Hehe, yes, that is what I meant :-)