# Is there such a thing as constant velocity ?

It seems to me that it is highly likely that everything in the universe is acted upon by gravity, unremittingly. Since gravity causes objects having mass to accelerate it would appear that a constant velocity for these type of objects is not possible. Can anyone explain my conundrum ?

ZapperZ
Staff Emeritus
It seems to me that it is highly likely that everything in the universe is acted upon by gravity, unremittingly. Since gravity causes objects having mass to accelerate it would appear that a constant velocity for these type of objects is not possible. Can anyone explain my conundrum ?

Gravity is such a minuscule force in particle physics. I would challenge you to show if it would even come up or detectable in many instances. For example, in particle accelerators, gravity NEVER, EVER, comes up in all of the modeling and design done. After passing through any accelerating structures, particle beams are always considered to be coasting at constant v. Only other factors such as space charge would come into play in affecting the trajectory, never gravity.

Zz.

Gravity is such a minuscule force in particle physics. I would challenge you to show if it would even come up or detectable in many instances. For example, in particle accelerators, gravity NEVER, EVER, comes up in all of the modeling and design done. After passing through any accelerating structures, particle beams are always considered to be coasting at constant v. Only other factors such as space charge would come into play in affecting the trajectory, never gravity.

Zz.

Then what I say is true for macroscopic objects ?

I guess then that the challenge for physicists is to explain why gravity has no effect at the level of particles, expecially the massive sort that would result when a particle is accelerated close to C. Does gravity perchance act in quanta that are too large for particles to detect ? Or maybe the models are inaccurate but the inaccuracy is not of consequence. If so does that imply that all equations using v may be slightly inaccurate ?

diazona
Homework Helper
Yeah, the inaccuracy introduced by gravity is inconsequential. Just as how when a cop measures a car's speed with a radar gun, if gravitational time dilation or something causes that reading to be off by one part in a billion, it doesn't matter. It's certainly not true that gravity has no effect at all on particles.

By the way, this doesn't mean that the equations themselves (for constant-velocity motion) are inaccurate; just that the assumptions underlying them don't quite hold true in real situations. But they're more than close enough.

ZapperZ
Staff Emeritus
Then what I say is true for macroscopic objects ?

I guess then that the challenge for physicists is to explain why gravity has no effect at the level of particles, expecially the massive sort that would result when a particle is accelerated close to C. Does gravity perchance act in quanta that are too large for particles to detect ? Or maybe the models are inaccurate but the inaccuracy is not of consequence. If so does that imply that all equations using v may be slightly inaccurate ?

Er... we HAVE detected gravitational potential effects via a http://physicsworld.com/cws/article/news/3525" [Broken]! Do a search! This is not unknown and we DO have the accuracy to detect such a thing.

But because of this also, we do know when such an effect is practically negligible! After all, can you tell me what effects that you can see here that are due to the gravity from Alpha Centauri?

Zz.

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Er... we HAVE detected gravitational potential effects via a http://physicsworld.com/cws/article/news/3525" [Broken]! But because of this also, we do know when such an effect is practically negligible! After all, can you tell me what effects that you can see here that are due to the gravity from Alpha Centauri?

Zz.

It seems obvious that Alpha Centauri would have little gravitional effect on the earth. What about the earth's gravity though ? I was taught that all objects on earth that have mass are accelerated by gravity at 32 ft per sec per sec. And it did not matter how massive the object was the acceleration was the same. You are telling me that particles with mass do not react in this manner which really surprises me.

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ZapperZ
Staff Emeritus
It seems obvious that Alpha Centauri would have little gravitional effect on the earth. What about the earth's gravity though ? I was taught that all objects on earth that have mass are accelerated by gravity at 32 ft per sec per sec. And it did not matter how massive the object was the acceleration was the same. You are telling me that particles with mass do not react in this manner which really surprises me.

1. Did you read my reference to the neutron drop experiment?

2. Can you tell me why all the codes for, say, electron accelerators has ZERO gravitational effects included in all the interactions and STILL, they get accurate description of the particle dynamics?

3. At what point is "too small" small enough for you to ignore? After all, you ignored effects from Alpha Centauri. How are you able to pick and choose what to ignore and what not to? It seems that you can, but for some odd reason, there are situations that you simply won't. Why is that?

Zz.,

It seems to me that it is highly likely that everything in the universe is acted upon by gravity, unremittingly. Since gravity causes objects having mass to accelerate it would appear that a constant velocity for these type of objects is not possible. Can anyone explain my conundrum ?

I remember learning the geometric definition of a line as a youngster. A line was drawn on the blackboard and the teacher said it is defined as infinite in both directions. Of course constant velocity over infinite time would require an infinite line and no acceleration due to gravity or any other force.

It seems to me Newton's theory of gravitation would curve all bodies based on the presence of another mass, even if the deflection is too small to measure, it should be there. I have often wondered about the implications of General Relativity, that is, how does a body know the shape of local space (acceleration rate) when moving without changing its mass? It is falling through space with knowledge of every other body's position in the Universe? I think so (although of course we suppose the effect of very remote bodies is neglibible compared to the effect of bodies nearby). Anyway I tend to agree with your comment.

Edit: of course, when we're falling through space alonside a body in a certain way, we observe the velocity of the body as constant.

1. Did you read my reference to the neutron drop experiment?

Yes.

2. Can you tell me why all the codes for, say, electron accelerators has ZERO gravitational effects included in all the interactions and STILL, they get accurate description of the particle dynamics?

No.

3. At what point is "too small"
small enough for you to ignore? After all, you ignored effects from Alpha Centauri. How are you able to pick and choose what to ignore and what not to? It seems that you can, but for some odd reason, there are situations that you simply won't. Why is that?
If I drop a cannonball and an apple simultaneously from the same height both will reach the ground at the same time. That is, the amount of mass has no bearing on the effect that gravity has on an object. I wonder why particles are different.

diazona
Homework Helper
If I drop a cannonball and an apple simultaneously from the same height both will reach the ground at the same time. That is, the amount of mass has no bearing on the effect that gravity has on an object. I wonder why particles are different.
They're not different. Why would you think they are?

They're not different. Why would you think they are?

According to Zapper in the first reply to the thread; "After passing through any accelerating structures, particle beams are always considered to be coasting at constant v. Only other factors such as space charge would come into play in affecting the trajectory, never gravity.".

cepheid
Staff Emeritus
Gold Member
According to Zapper in the first reply to the thread; "After passing through any accelerating structures, particle beams are always considered to be coasting at constant v. Only other factors such as space charge would come into play in affecting the trajectory, never gravity.".

I know that particles in accelerators have highly relativistic (very close to c) speeds. Even if moving at only 0.5c, and starting from a height at which it would normally fall to the ground in 1 second, a particle would move 25 times the radius of the Earth (in its direction of motion) in that time interval. In an accelerator, something would happen to it long before then. Is that the answer?

I know that particles in accelerators have highly relativistic (very close to c) speeds. Even if moving at only 0.5c, and starting from a height at which it would normally fall to the ground in 1 second, a particle would move 25 times the radius of the Earth (in its direction of motion) in that time interval. In an accelerator, something would happen to it long before then. Is that the answer?

Okayyy ! So it is true to say that there is no such thing as constant velocity.

diazona
Homework Helper
Okayyy ! So it is true to say that there is no such thing as constant velocity.
There most certainly is such a thing as constant velocity. What you mean to say, I'm pretty sure, is that no real object travels at constant velocity. That I can see being true (except at particular points in space where gravity "cancels out").

cepheid
Staff Emeritus
Gold Member
Yes, that is exactly what I was going to reply with. If you're going insist on talking about the way things are *in principle*, then a constant velocity certainly exists in that sense, because it is mathematically well defined. What you are asking is whether this type of motion occurs in nature. I suspect that there are many many instances in nature in which the answer is, "yes, any deviations from a constant velocity are negligible." It seems like all of the measurements experimental physicists are trying to conduct these days are tricky things requiring high precision (all the easy measurements have been done!). Yet, somebody (who I believe is a professional particle physicist?) tells you that people in his profession can ignore gravity and still be assured that they are getting the right answer, that there is no measurement that one could possibly perform to detect any deviations between reality and their models based upon the effects of gravity (i.e. such deviations are so small as to be always unmeasurable and irrelevant). What does that tell you?

So what I am being told is that because of gravity a constant velocity is not possible in nature. It is helpful though as a mathematical concept. Also at very high velocities the deviation caused by gravity is tiny and can be/is ignored in practice.

I guess then the deviation it is not involved in the difficulty of getting two particles to collide at near light speeds.

diazona
Homework Helper
That sounds like a reasonable way of putting it.

Also at very high velocities the deviation caused by gravity is tiny and can be/is ignored in practice

True if the particles don't travel very far or exist for very long. But measurements show that the Sun's gravity bends the light from distant stars during an eclipse. I think the concept of constant velocity is treated in terms of Newtonian versus Non-Newtonian reference frames. My understanding is space is curved in Einstein's universe so even light cannot follow a straight path ...

True if the particles don't travel very far or exist for very long. But measurements show that the Sun's gravity bends the light from distant stars during an eclipse. I think the concept of constant velocity is treated in terms of Newtonian versus Non-Newtonian reference frames. My understanding is space is curved in Einstein's universe so even light cannot follow a straight path ...

The bending of light by gravity is really weird. Bending implies acceleration but, of course, seemingly well-established theory has it that light has a constant velocity.

The bending of light by gravity is really weird. Bending implies acceleration but, of course, seemingly well-established theory has it that light has a constant velocity.

Yeah, but SystemTheory ended by saying that Einstein postulated that the universe itself is curved....So maybe it's straigt line light that is accelerating.

Yeah, but SystemTheory ended by saying that Einstein postulated that the universe itself is curved....So maybe it's straigt line light that is accelerating.

It seems to me you cannot have straight line on a curve ?