Ask a physicist anything. (8)

[Chesterton]

Ah, I said velocity is relative, but acceleration is not. I can't say how fast you're moving, but I can certainly say how fast your movement is changing. So if you accelerate, it can be known. And since orbiting the Sun is a form of acceleration, it can be known.

Yes I understand that much. But still, you're taking a velocity which is relative and accelerating it...relative to what? Relative to its previous relative velocity. Do two relatives equal an absolute?

That's why geocentrism/heliocentrism isn't a matter of arbitrary choice - even if you define the Earth to be stationary, there's still the Sun's gravity pulling us towards it.

Ah, but our gravity is also pulling the Sun towards us, right?

Since we've defined the Earth to be stationary, we see the Sun move out from under us, hence we fall forever in a circle - we orbit.

But the universe is said to be expanding outwards in all directions. So if I here declare the tip of my nose to be the center of the universe, can you dispute that?

 

[Elendur]

Because c isn't a certain speed through distance, it's a speed through spacetime -
...

I'd never heard that in my physics classes, but things makes so much more sense now (possibly also due to me being better at norms within arbitrary spaces).

Thanks!

 

[Michael]

But the universe is said to be expanding outwards in all directions. So if I here declare the tip of my nose to be the center of the universe, can you dispute that?

Amusingly, the answer is actually "no".

Who's Afraid of the Dark? Alternatives to Dark Energy - The Nature of Reality

 

[Wiccan_Child]

Yes I understand that much. But still, you're taking a velocity which is relative and accelerating it...relative to what? Relative to its previous relative velocity. Do two relatives equal an absolute?

Pretty much: acceleration is a change in velocity relative to itself. For example, suppose I find myself in empty black space, and next to me is you. We are both floating there. Are we moving? Are we stationary? Suppose we had whatever equipment you desired floating next to us; how could we tell whether we're moving or not?

All we can say is that you and I are not moving relative to each other. It could be that you are moving relative to me - but how could we tell who is moving? Am I seeing you move away, or am I the one moving away?

But suppose, over time, you inch forwards. After 1 minute you're 1" away, after 2 minutes you're 3 inches away, after 3 minutes you're 6 inches away, etc (I have clocks and rulers, so I can measure these with accuracy). This means you are accelerating - the amount by which our position changes each second, is itself changing each second.

I may never know how fast you were originally moving (indeed, the Principle of Relativity implies that the concept is meaningless), but I can certainly measure your change in speed.

Ah, but our gravity is also pulling the Sun towards us, right?

Yes. Mechanically, we both orbit the centre of mass of the Earth-Sun system, and since the Sun is so very much heavier than Earth, it resides near the Sun's core. So the Earth orbits a point very near to the centre of the Sun - which is basically equivalent to heliocentrism.

But the universe is said to be expanding outwards in all directions. So if I here declare the tip of my nose to be the center of the universe, can you dispute that?

To clarify, every point in the universe is receding from every other point in the universe. To any one point (such as us on Earth), this manifests as 'everything is receding from us, and more distant things are receding faster'.

But can I dispute that the tip of your nose is the centre of the universe? Yes - I can claim that "the centre of the universe" is an incoherent concept, like 'before the start of time' or 'north of the north pole' - such things are logical paradoxes and cannot exist.

 

[Justatruthseeker]

Well, yes, but we build you GPS machines, so hush


Ah, I said velocity is relative, but acceleration is not. I can't say how fast you're moving, but I can certainly say how fast your movement is changing. So if you accelerate, it can be known. And since orbiting the Sun is a form of acceleration, it can be known. That's why geocentrism/heliocentrism isn't a matter of arbitrary choice - even if you define the Earth to be stationary, there's still the Sun's gravity pulling us towards it. Since we've defined the Earth to be stationary, we see the Sun move out from under us, hence we fall forever in a circle - we orbit.

So velocity (speed+direction) is relative, but acceleration (change in velocity) is not. So orbit (changing direction = changing velocity = acceleration) is not relative, it's absolute.

Orbits are an acceleration, and follow a very well known law.

Charged Particle in a Magnetic Field

The problem is nothing is stationary, so velocity can only be determined relative to something else.

So, using a thought experiment, let us assume we are perfectly stationary in space. We then accelerate to 1/2 of light speed, and stop all acceleration. How fast are we moving?

We then accelerate again to 1/2 of light speed, and stop all acceleration. How fast are we moving?

I'll let all of you quibble over this for a bit. Let the round of double-talk begin. Not aimed at you in particular, because like you I understand velocity is relative, while acceleration is absolute.

 

[Justatruthseeker]

Pretty much: acceleration is a change in velocity relative to itself. For example, suppose I find myself in empty black space, and next to me is you. We are both floating there. Are we moving? Are we stationary? Suppose we had whatever equipment you desired floating next to us; how could we tell whether we're moving or not?

All we can say is that you and I are not moving relative to each other. It could be that you are moving relative to me - but how could we tell who is moving? Am I seeing you move away, or am I the one moving away?

But suppose, over time, you inch forwards. After 1 minute you're 1" away, after 2 minutes you're 3 inches away, after 3 minutes you're 6 inches away, etc (I have clocks and rulers, so I can measure these with accuracy). This means you are accelerating - the amount by which our position changes each second, is itself changing each second.

I may never know how fast you were originally moving (indeed, the Principle of Relativity implies that the concept is meaningless), but I can certainly measure your change in speed.


Yes. Mechanically, we both orbit the centre of mass of the Earth-Sun system, and since the Sun is so very much heavier than Earth, it resides near the Sun's core. So the Earth orbits a point very near to the centre of the Sun - which is basically equivalent to heliocentrism.


To clarify, every point in the universe is receding from every other point in the universe. To any one point (such as us on Earth), this manifests as 'everything is receding from us, and more distant things are receding faster'.

But can I dispute that the tip of your nose is the centre of the universe? Yes - I can claim that "the centre of the universe" is an incoherent concept, like 'before the start of time' or 'north of the north pole' - such things are logical paradoxes and cannot exist.

And yet blueshift says not everything is receding. If 3 cars are traveling on the freeway in the same direction, and there are no stationary landmarks to judge position, and all you can observe is the cars, then if the first car is traveling faster than the second, it appears to be receding, from any frame of reference. likewise if the third car is traveling slower than the second car, it too appears to be receding, from any frame of reference. yet in reality all are traveling in the same direction.

So appearances of recession does not necessarily mean all are traveling in different directions, when one has no set stationary landmarks such as exists in space.

 

[Wiccan_Child]

And yet blueshift says not everything is receding.

Well obviously not everything is receding. The metric expansion of spacetime makes things recede on large scales, but on small scales local forces (gravity, electromagnetism, etc) keep things together.

If 3 cars are traveling on the freeway in the same direction, and there are no stationary landmarks to judge position, and all you can observe is the cars, then if the first car is traveling faster than the second, it appears to be receding, from any frame of reference. likewise if the third car is traveling slower than the second car, it too appears to be receding, from any frame of reference. yet in reality all are traveling in the same direction.

Don't forget there are four inertial reference frames here - you are forgetting that the road is also an inertial frame. So to say "in reality all are travelling in the same direction" is only true in the reference frame of the road.

So appearances of recession does not necessarily mean all are traveling in different directions, when one has no set stationary landmarks such as exists in space.

No. Recession means they are all moving away from a point relative to that point.

Remember that velocity is composed of speed (magnitude) and direction. So if velocity changes based on our choice of reference frame, then that must include direction - which reference frame we choose determines which direction things move.

And landmarks do exist in space - the stars. And galaxies. And the interstellar medium. And Birkland currents

 

[Wiccan_Child]

Orbits are an acceleration, and follow a very well known law.

Charged Particle in a Magnetic Field

The problem is nothing is stationary, so velocity can only be determined relative to something else.

So, using a thought experiment, let us assume we are perfectly stationary in space. We then accelerate to 1/2 of light speed, and stop all acceleration. How fast are we moving?

We then accelerate again to 1/2 of light speed, and stop all acceleration. How fast are we moving?

I'm not sure I understand the problem.

We begin stationary.
Then we accelerate to 0.5c, so we're moving at 0.5c.
Then we accelerate to... 0.5c? So we don't accelerate at all? So we're still at 0.5c?

Presumably we start stationary by defining ourselves so, and our velocity after acceleration is measured relative to an object left at our starting position, or a convenient infinite ruler.

Am I missing something?

 

[Seipai]

Orbits are an acceleration, and follow a very well known law.

Charged Particle in a Magnetic Field

The problem is nothing is stationary, so velocity can only be determined relative to something else.

So, using a thought experiment, let us assume we are perfectly stationary in space. We then accelerate to 1/2 of light speed, and stop all acceleration. How fast are we moving?

We then accelerate again to 1/2 of light speed, and stop all acceleration. How fast are we moving?

I'll let all of you quibble over this for a bit. Let the round of double-talk begin. Not aimed at you in particular, because like you I understand velocity is relative, while acceleration is absolute.

Your problem in your two accelerations to the speed of light problem is that you are changing frames of reference.

You will have different answers dependent upon the frame of reference that the velocities are measured from.

 

[Chesterton]

Pretty much: acceleration is a change in velocity relative to itself. For example, suppose I find myself in empty black space, and next to me is you. We are both floating there. Are we moving? Are we stationary? Suppose we had whatever equipment you desired floating next to us; how could we tell whether we're moving or not?

All we can say is that you and I are not moving relative to each other. It could be that you are moving relative to me - but how could we tell who is moving? Am I seeing you move away, or am I the one moving away?

But suppose, over time, you inch forwards. After 1 minute you're 1" away, after 2 minutes you're 3 inches away, after 3 minutes you're 6 inches away, etc (I have clocks and rulers, so I can measure these with accuracy). This means you are accelerating - the amount by which our position changes each second, is itself changing each second.

I may never know how fast you were originally moving (indeed, the Principle of Relativity implies that the concept is meaningless), but I can certainly measure your change in speed.

Okay, I understand and agree, but...

Yes. Mechanically, we both orbit the centre of mass of the Earth-Sun system, and since the Sun is so very much heavier than Earth, it resides near the Sun's core. So the Earth orbits a point very near to the centre of the Sun - which is basically equivalent to heliocentrism.

In the first answer above, you used the idea of you and I floating in a black space, then in the immediately above answer, you talk of the sun and earth, which are also two bodies floating in black space. In the first case you said we can't tell who is moving relative to who, but in the second case you seem to say we can.

To clarify, every point in the universe is receding from every other point in the universe. To any one point (such as us on Earth), this manifests as 'everything is receding from us, and more distant things are receding faster'.

But can I dispute that the tip of your nose is the centre of the universe? Yes - I can claim that "the centre of the universe" is an incoherent concept, like 'before the start of time' or 'north of the north pole' - such things are logical paradoxes and cannot exist.

I disagree. If every point is receding from every other point, then my nose is as good a point as any to call center. But as far as being an incoherent concept, I'd say an additional reason you can't dispute that is that we only have a limited "observable universe". We really don't know what shape it is, or whether it's finite or infinite. I don't know how we can assert there is no center. (Are there math equations which address this?)

 

[technofox]

Why do people believe that we can't travel faster than the speed of light?

Personally I think it's just an energy issue, especially if the neutrino experiment ends up being reproduced that can prove they travel faster than the speed of light.

 

[florida2]

Why do people believe that we can't travel faster than the speed of light?

Personally I think it's just an energy issue, especially if the neutrino experiment ends up being reproduced that can prove they travel faster than the speed of light.

Firstly, I believe the neutrino experiment was shown to be down to measurement error.

Nothing accelerate to a speed greater than that of light. You can have all the energy in the universe and you still won't. That is because mass increses as you get closer to light speed. Therefore, for anything with mass it would require an infinite amount of energy to accelerate to light speed.

Faster than light travel by simply accelerating more and more is impossible. However warp drives and wormholes could theoretically get around this!

 

[essentialsaltes]

Why do people believe that we can't travel faster than the speed of light?

Personally I think it's just an energy issue, especially if the neutrino experiment ends up being reproduced that can prove they travel faster than the speed of light.

Things that are massless (like photons) travel at the speed of light.

Things that have mass can never reach the speed of light, because it would take an infinite amount of energy.

It is hypothetically possible that there are particles that always travel faster than light. So-called tachyons. Tachyons cannot slow down to reach the speed of light (it would take an infinite amount of energy to slow them down to c).

 

[Wiccan_Child]

Okay, I understand and agree, but...



In the first answer above, you used the idea of you and I floating in a black space, then in the immediately above answer, you talk of the sun and earth, which are also two bodies floating in black space. In the first case you said we can't tell who is moving relative to who, but in the second case you seem to say we can.

Ah, not quite. In the first case, we can tell who's moving relative to who, but we can't tell how fast we're absolutely moving. I can say you're moving 10 mph faster than me, but not whether I'm at 100 and you're at 110, or I'm at 150 and you're at 160, or whatever. Only our relative velocity is known.

In the second case, we have the added complication of gravity. Gravity accelerates, which is objective (whereas velocity is only relative). In the second case, even if we arbitrarily define the Earth to be stationary when we start our stopwatch, it is inescapable that it accelerates towards the Sun thereafter.

So heliocentrism is about acceleration, which is objective and therefore not arbitrary.

I disagree. If every point is receding from every other point, then my nose is as good a point as any to call center.

But it isn't. It's not a special point, it's an arbitrary one. It is no more the 'centre' of the universe than an arbitrary dot on the surface of an expanding balloon is the 'centre' of the balloon. The expansion is away from every point simultaneously.

But as far as being an incoherent concept, I'd say an additional reason you can't dispute that is that we only have a limited "observable universe". We really don't know what shape it is, or whether it's finite or infinite. I don't know how we can assert there is no center. (Are there math equations which address this?)

The observable universe does have a centre - the Earth. More accurately, your eyes. My eyes delineate a slightly different observable universe.

Minutephysics did an excellent video on this very topic:

http://www.youtube.com/watch?v=5NU2t5zlxQQ

 

[Tinker Grey]

From here.

Many of the equations on this page contain that radical in the denominator. If your velocity reaches c, the equations are undefined because you are dividing by zero.

 

[Wiccan_Child]

Why do people believe that we can't travel faster than the speed of light?

According to general relativity, an object of mass m and velocity v has an energy E equal to γmc[sup]2[/sup], where 'γ' (gamma) is the Lorentz factor:

(image courtesy of Wikipedia)

When v is zero, γ is 1, so our energy is the famous E = mc[sup]2[/sup].

The problem is that as v approaches c, γ approaches infinity. To get v all the way to c requires making γ equal to infinity, which means you need infinite energy - which is impossible. Moreover, to make v go above c means that when you calculate γ you end up taking the square root of a negative number... which is meaningless.

Physically, then, it's an energy problem. The energy required to accelerate to the speed of light is infinitely large, so can never be done. This also precludes travelling faster than light.

Personally I think it's just an energy issue,

It is... but not in the way you think . We say it's 'impossible', not 'difficult'.

especially if the neutrino experiment ends up being reproduced that can prove they travel faster than the speed of light.

After OPERA fixed their mistakes, it turned out the speed of light was not violated. It was an experimental error, that's all.

 

[Chesterton]

Ah, not quite. In the first case, we can tell who's moving relative to who, but we can't tell how fast we're absolutely moving. I can say you're moving 10 mph faster than me, but not whether I'm at 100 and you're at 110, or I'm at 150 and you're at 160, or whatever. Only our relative velocity is known.

In the second case, we have the added complication of gravity. Gravity accelerates, which is objective (whereas velocity is only relative). In the second case, even if we arbitrarily define the Earth to be stationary when we start our stopwatch, it is inescapable that it accelerates towards the Sun thereafter.

So heliocentrism is about acceleration, which is objective and therefore not arbitrary.


But it isn't. It's not a special point, it's an arbitrary one. It is no more the 'centre' of the universe than an arbitrary dot on the surface of an expanding balloon is the 'centre' of the balloon. The expansion is away from every point simultaneously.


The observable universe does have a centre - the Earth. More accurately, your eyes. My eyes delineate a slightly different observable universe.

Minutephysics did an excellent video on this very topic:

How Big is the Universe? - YouTube

I give up for now. Seems like we're agreeing and disagreeing at the same time. Seems like we're playing word games, but I can't tell if I'm playing games with you, or you with me, or we're both playing.

Thank you, but my head hurts. I'm going to watch some kitten videos on YouTube.

 

[technofox]

According to general relativity, an object of mass m and velocity v has an energy E equal to ?mc[sup]2[/sup], where '?' (gamma) is the Lorentz factor: (image courtesy of Wikipedia) When v is zero, ? is 1, so our energy is the famous E = mc[sup]2[/sup]. The problem is that as v approaches c, ? approaches infinity. To get v all the way to c requires making ? equal to infinity, which means you need infinite energy - which is impossible. Moreover, to make v go above c means that when you calculate ? you end up taking the square root of a negative number... which is meaningless. Physically, then, it's an energy problem. The energy required to accelerate to the speed of light is infinitely large, so can never be done. This also precludes travelling faster than light. It is... but not in the way you think . We say it's 'impossible', not 'difficult'. After OPERA fixed their mistakes, it turned out the speed of light was not violated. It was an experimental error, that's all.

Thank you and Chesterton for those answers.

Now my next question. If a quantum singularity to contain all of the mass and energy in the universe and some of the mass is annihilated during the Big Bang, obviously it becomes energy, what would happen if the universe was able to collapse in on itself to form the next Big Bang? Would it lead to a smaller universe or one of equal size?

 

[essentialsaltes]

Thank you and Chesterton for those answers.

Now my next question. If a quantum singularity to contain all of the mass and energy in the universe and some of the mass is annihilated during the Big Bang, obviously it becomes energy, what would happen if the universe was able to collapse in on itself to form the next Big Bang? Would it lead to a smaller universe or one of equal size?

Short answer: No one knows.

Slightly longer answer: By size do you mean spatial dimensions or mass content?

A) Spatial dimension: the universe is infinite. If there were some sort of Big Crunch/New Big Bang, and the universe is more or less like ours, then it would also be infinite in spatial extent. There is no widely accepted theory (that's putting it mildly) that explains what would happen in this situation. Perhaps a finite universe could come out. No one knows.

B) Mass content. Although you speak of mass being 'annihilated', the total mass/energy of the universe remains constant. That's the point of Einstein's famous equation that E=mc^2. Mass and energy are equivalent. So no mass or energy is lost as the universe 'ages'. If there were some sort of Big Crunch/New Big Bang, and the conservation of energy applies to that process, then the new universe would have the same mass/energy content as ours. There is no widely accepted theory (that's putting it mildly) that explains what would happen in this situation. Perhaps the new universe would have more or less mass/energy. No one knows.

 

[technofox]

What would happen if an anti-proton came in contact with a neutron?