# Motion

#### Simpletruther

##### Well-known member
If motion is relative how can we know that we have accelerated rather than everything else slowing down?

#### The Pixie

##### Well-known member
If you are on a perfectly smooth and windowless train, you cannot it it is moving or stationary. It is all relative. This is (as I understand it) the basis of special relativity.

You can detect, however, acceleration. When you put your foot down in a car, you can feel that you are pushed back into the seat. When you brake abruptly, you are flung forward. You would feel these on that smooth and windowless train. However, you cannot tell the difference between constant acceleration and gravity, and that is the basis of general relativity.

#### Simpletruther

##### Well-known member
If you are on a perfectly smooth and windowless train, you cannot it it is moving or stationary. It is all relative. This is (as I understand it) the basis of special relativity.

You can detect, however, acceleration. When you put your foot down in a car, you can feel that you are pushed back into the seat. When you brake abruptly, you are flung forward. You would feel these on that smooth and windowless train. However, you cannot tell the difference between constant acceleration and gravity, and that is the basis of general relativity.
But how would you k ow f it’s acceleration versus deceleration?

#### The Pixie

##### Well-known member
But how would you k ow f it’s acceleration versus deceleration?
You would not.

#### Simpletruther

##### Well-known member
You would not.
Ok so when I apply that to relativity I don't get it.

It is said that a traveler moving near speed of light will age slower. But how can we know which one is moving?

#### Whateverman

##### Well-known member
But how would you k ow f it’s acceleration versus deceleration?
Acceleration and deceleration are the exact same thing: a change in velocity. And velocity is always defined is reference to something else (ie. the ground outside the train), without which there would literally be no way TO speed up or slow down.

#### Whateverman

##### Well-known member
It is said that a traveler moving near speed of light will age slower. But how can we know which one is moving?
It's relative. It's literally up to the points of reference.

Velocity = average displacement / average time.

To have displacement, you have to have a point of reference, and once you have a point of reference, you've got two things which can disagree about which one of them is moving.

If something is moving near the speed of light, it's moving towards or away from something. Those two somethings can legitimately argue about which one is in motion...

#### The Pixie

##### Well-known member
It is said that a traveler moving near speed of light will age slower. But how can we know which one is moving?
I am not an expert but...

Say Alfred is travelling at near the speed, while Brian is station. Time goes more slowly for Alfred, so he ages more slowly. Or so it seems to Brian.

To Alfred, he is the stationary one, and Brian is aging slowly.

#### LifeIn

##### Well-known member
It is said that a traveler moving near speed of light will age slower. But how can we know which one is moving?
The important consequence of relativity is philosophical as much as it is mathematical. That is that the question itself does not make sense, because just asking the question as it is phrased above assumes there is an absolute "one who is moving". When applied to the aging question, if two observers are passing each other at a substantial fraction of the speed of light, each of them will observe the othe to be aging more slowly. If this sounds like a contradiction, just ask yourself how each of them would go about measuring the aging rate of the other, given the limitations that communication cannot take place faster than the speed of light. Once you construct an actual operation definition of the measurment, you will see that there is no contradiction. (This is pretty much what The Pixie said just above.)

#### Simpletruther

##### Well-known member
I am not an expert but...

Say Alfred is travelling at near the speed, while Brian is station. Time goes more slowly for Alfred, so he ages more slowly. Or so it seems to Brian.

To Alfred, he is the stationary one, and Brian is aging slowly.

The important consequence of relativity is philosophical as much as it is mathematical. That is that the question itself does not make sense, because just asking the question as it is phrased above assumes there is an absolute "one who is moving". When applied to the aging question, if two observers are passing each other at a substantial fraction of the speed of light, each of them will observe the othe to be aging more slowly. If this sounds like a contradiction, just ask yourself how each of them would go about measuring the aging rate of the other, given the limitations that communication cannot take place faster than the speed of light. Once you construct an actual operation definition of the measurment, you will see that there is no contradiction. (This is pretty much what The Pixie said just above.)
According to explanations the space traveler would come back less aged than those he left.

So he is the one they approached light speed.

The question is how can we know it was him moving near light speed and not the others.

#### LifeIn

##### Well-known member
According to explanations the space traveler would come back less aged than those he left.

So he is the one they approached light speed.

The question is how can we know it was him moving near light speed and not the others.
Good question. We assume a round-trip then. Out and back again. But the only way to do that is to reverse velocity after going out. Once you do that, you are no longer dealing with uniform velocity. The change in direction from going out to coming back would result in a very large acceleration, which could be felt by the traveler. Meanwhile, the guy left on earth experienced no such acceleration, and he knows it too, because he did not feel it. So when the two are reunited, and the one that went out and back appears to have aged less, the two of them did not experience the same thing. The huge acceleration required to slow down and speed up in the opposite direction puts the scenario under General Relativity, which deals with the effects of acceleration/gravity. So the situation was not completely symmetrical.

#### Simpletruther

##### Well-known member
Good question. We assume a round-trip then. Out and back again. But the only way to do that is to reverse velocity after going out. Once you do that, you are no longer dealing with uniform velocity. The change in direction from going out to coming back would result in a very large acceleration, which could be felt by the traveler. Meanwhile, the guy left on earth experienced no such acceleration, and he knows it too, because he did not feel it. So when the two are reunited, and the one that went out and back appears to have aged less, the two of them did not experience the same thing. The huge acceleration required to slow down and speed up in the opposite direction puts the scenario under General Relativity, which deals with the effects of acceleration/gravity. So the situation was not completely symmetrical.
What about the idea that in the beginning he may have actually decelerated and had to accelerate to catch back up?

So he may have appeared to go on a space trip but actually just slowed down while the people staying home kept moving

How can we know the difference?

#### LifeIn

##### Well-known member
What about the idea that in the beginning he may have actually decelerated and had to accelerate to catch back up?

So he may have appeared to go on a space trip but actually just slowed down while the people staying home kept moving

How can we know the difference?
The difference is that one of them will have felt the acceleration/deceleration and the other one - the one that did not go on a space trip, will have felt nothing.

#### Simpletruther

##### Well-known member
The difference is that one of them will have felt the acceleration/deceleration and the other one - the one that did not go on a space trip, will have felt nothing.
Yes but the one that felt nothing may have already been moving near light speed. And simply kept going. While the "traveler" simply slowed down and moved away.

#### Temujin

##### Well-known member
Yes but the one that felt nothing may have already been moving near light speed. And simply kept going. While the "traveler" simply slowed down and moved away.
Doesn't matter. Speed is relative. Acceleration and deceleration are particular.

For a fictional take on this, try "Tau Zero" by Poul Anderson.

#### LifeIn

##### Well-known member
Yes but the one that felt nothing may have already been moving near light speed.
The statement makes no sense, given the understanding of relativity. One can only speak about quantities that can be measured. So every time you mention a speed, make sure to also mention exactly how that speed is to be measured. There is no such thing as absolute speed independent of the observer.

And simply kept going. While the "traveler" simply slowed down and moved away.
To compare ages at the end requires an eventual reunion. Whatever scenario you describe, make sure to include how the reunion takes place. When it does take place, whichever of them has undergone the most overall acceleration/gravity will be the one who has appeared to have aged less.

#### Cisco Qid

##### Active member
If motion is relative how can we know that we have accelerated rather than everything else slowing down?
Relative motion does not deal with acceleration but rather with bodies at constant velocities. For example, if two bodies are each moving at 80 mph toward each other relative to an observer. An observer moving with one of the bodies might think his body was stationary in his frame of reference while the other was moving at 160 mph in his direction. This is the same in both Galilean Relativity and Special Relativity. But Special Relativity adds the premise that the speed of light is the same in all reference frames. This brings in such results as, time dilation and space contraction at speeds near the speed of light. Acceleration is a change in the motion and is dealt with separately and is not relative. Acceleration is converted into deceleration by simple changing the plus or minus. For example, and acceleration of 15 feet/sec/sec would be -15 feet/sec/sec in the opposite direction and either one could be deceleration depending on how you define your coordinate system.

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#### Cisco Qid

##### Active member
Relative motion does not deal with acceleration but rather with bodies at constant velocities. For example, if two bodies are each moving at 80 mph toward each other relative to an observer. An observer moving with one of the bodies might think his body was stationary in his frame of reference while the other was moving at 160 mph in his direction. This is the same in both Galilean Relativity and Special Relativity. But Special Relativity adds the premise that the speed of light is the same in all reference frames. This brings in such results as, time dilation and space contraction at speeds near the speed of light. Acceleration is a change in the motion and is dealt with separately and is not relative. Acceleration is converted into deceleration by simple changing the plus or minus. For example, and acceleration of 15 feet/sec/sec would be -15 feet/sec/sec in the opposite direction and either one could be deceleration depending on how you define your coordinate system.
Einstein described that the happiest moment in his life was when he realized that a body in free fall felt no force or as he put it, some one jumping off a roof top would feel no force while dropping. It was at that moment that he knew everything about General Relativity. He realized that gravity was not a force and it took him years to learn the math to express his idea in mathematical form which we refer to as "Einstein field equations" which used Tensors in their expression. These equations could then be used to make predictions that could not be made with simply the idea alone such as explaining Mercury's orbital perihelion progression.

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Redundant

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#### Electric Skeptic

##### Well-known member
Relative motion does not deal with acceleration but rather with bodies at constant velocities. For example, if two bodies are each moving at 80 mph toward each other relative to an observer. An observer moving with one of the bodies might think his body was stationary in his frame of reference while the other was moving at 160 mph in his direction. This is the same in both Galilean Relativity and Special Relativity. But Special Relativity adds the premise that the speed of light is the same in all reference frames. This brings in such results as, time dilation and space contraction at speeds near the speed of light. Acceleration is a change in the motion and is dealt with separately and is not relative. Acceleration is converted into deceleration by simple changing the plus or minus. For example, and acceleration of 15 feet/sec/sec would be -15 feet/sec/sec in the opposite direction and either one could be deceleration depending on how you define your coordinate system.
Can the above explain an anomaly I've never understood?

A car drives at 100 mph into an immovable object. You can imagine the mess the car is in.

Another car drives at 50 mph into an immovable object. Obviously, the car is a mess - but nowhere near as bad a mess as the 100 mph car.

Two cars drive straight at each other at 50 mph. Relative to each other, they are travelling at 100 mph. When they collide, will the damage to the cards be approximately equivalent to the 100 mph car above, or the 50 mph car above?