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Gravity v Velocity v Time

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I'm a lay person with who reads science books by the likes of Brian Cox and Carlo Ravelli. I understand that time slows as gravity increases and time slows as velocity increases. Gravity and velocity can cancel each other out so time stays the same too. My question is that if gravity and velocity have the same effect on time, aren't gravity and velocity the same thing? Is this answered by Einstein's elevator analogy somehow?

Edited by TSRobot
Thinking

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No, they are not. They are totally different effects.

Regards Andrew

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Acceleration rather than velocity - Einstein did say that the effects of acceleration are indistinguishable from the effects of gravity in his General Theory of Relativity. Not really the same thing thought, i don't think.

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As Andrew says the distortion of time due to the influence of speed and gravity arises from two entirely separate mechanism.  The first, concerning speed, is described by Einstein's theory of Special Relativity while the second, concerning gravity, is described by his theory of General Relativity.  The special term in Special Relativity simply refers to a "special" or particular state of motion namely non accelerating (inertial). Here, the distortion or dilation of time is a consequence of two assumptions (Einstein's postulates) - 1  the speed of light in a vacuum is the same for all inertial observers (3 x10^8 m/s)  and 2  - the laws of motion (physics) are the same for all inertial observers (Galilean invariance) .  Those two postulates force the remarkable conclusion that time dilates for moving objects (inertial) when observed from a different inertial frame of reference. The actual derivation of the time dilation relationship is surprisingly straightforward (high school maths) and it's worth a quick google - search for time dilation and light clocks.

This link is not a bad starting point  -    SR Time Dilation

General relativity explores the relationship between gravity / the curvature of spacetime and time.  It is in the examination of that relationship that Einstein showed that time again is affected under the influence of gravity or acceleration (equivalence). The mathematics of general relativity/curvature of spacetime are somewhat more complicated that that for Special Relativity and not something that I could competently describe. But certainly the affects arise from two separate mechanism so again speed and gravity are not the same. Gravity and acceleration are however equivalent in this respect. Hope that helps or gives you something on which to base a further search.

Jim

Edited by saac
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So the effects CAN be the same, but the mechanisms are not? The book i read went a long way to explain these things and raises questions too, but that's science isn't it! Thanks for your explanation and time :-)

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Just to add to whatever confusion I would suggest you forget Gavity. Simple reason is What is Gravity ?

It is not the force between 2 objects, there is no force. That concept is a good approximation made by Newton and works fairly well but in term of GR there is no force.

I recall speaking to someone once and my question was: Do we use too many words and descriptions that are actually not applicable. After a few seconds thopught their reply was that very likely we do and we should not be doing so.

Question: Why does light not get out of a black hole?

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1 hour ago, ronin said:

I recall speaking to someone once and my question was: Do we use too many words and descriptions that are actually not applicable. After a few seconds thopught their reply was that very likely we do and we should not be doing so.

I think that largely depends on who the audience is and what models ideas/concepts they have already overtaken. But I appreciate what you are saying, as we have said before when concerning Physics, language can often be a barrier to understanding.  Then there is the tendency for us to force one view on something - a pertinent example being the nature of light. How often do we tackle a question concerning light by considering its classical nature ignoring it's other less intuitive character even though that may provide a more comfortable route to the solution. I know this is a common mistake that I make. Nature and physics won't bend to our will.

Jim

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To quote Dr Neil deGrass Tyson: "nature is under no obligation to make sense to you"

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On 10/24/2017 at 12:04, ronin said:

Question: Why does light not get out of a black hole?

In as few words I can muster.

Because all worldlines inside the black hole terminate on the "singularity".

Would more words help?

Regards Andrew

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On ‎23‎/‎10‎/‎2017 at 21:31, TSRobot said:

I understand that time slows as gravity increases and time slows as velocity increases. Gravity and velocity can cancel each other out so time stays the same too. My question is that if gravity and velocity have the same effect on time, aren't gravity and velocity the same thing? Is this answered by Einstein's elevator analogy somehow?

Strong gravity => slow clocks

High constant velocity => slow clocks

These two effects are unrelated.

If you consider how the Laws of Physics might appear in a freely falling elevator and are as smart as Einstein then you would be able to deduce the Principles Of General Relativity. One of these principles being the similarity between accelerated frames of reference and gravity. An accelerated frame of reference is of course one which has a non-constant velocity.

I hope this helps.

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So if it were possible to be completely stationary and under the influence of no gravity whatsoever then would that be the fastest that time can possibly pass? I understand that it would never be possible to test this because we don't have a stationary reference point outside our universe to tell us if we are stationary or not. I think read somewhere that the piece of space we are in is travelling quite fast anyhow. Still thinking ?

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Time always passes at one second per second for all ideal clocks. The differences arrise when you compare clocks that have travelled along different world lines.

Regards Andrew

PS consider two odometers each measures one mile per mile. If they go on different journeys and then get back together then they  will most likely record different distances. So it is with clocks.

Edited by andrew s
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22 hours ago, andrew s said:

Time always passes at one second per second for all ideal clocks. The differences arrise when you compare clocks that have travelled along different world lines.

Regards Andrew

PS consider two odometers each measures one mile per mile. If they go on different journeys and then get back together then they  will most likely record different distances. So it is with clocks.

I guess that means no simultaneously of anything really unless it's quantum and quantum is just fascinating. But bigger than quantum an inch equals and inch, a mile a mile and a beer a beer unless you move an inch then to know if your inch equals the last inch/mile/beer you have to check gravity and velocity and just drink the beer really.

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Check out the muon; their rate of decay says they should never reach the earth's surface. But we detect them all the time - this is because they travel at near the speed of light; from our perspective the muon's clock is running slow and from the muon's perspective the distance to the surface is shorter than we measure it.

It's all very bizarre, because we don't experience the universe at that speed, but fascinating all the same.

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As a non scientist i really love when folk put things down in an understandable way. Science shared is fab. Thanks

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A little chipping in.

The orig Q was also about "spin". this is a quantum property that is measurable but the word spin is just a handle..like colour is to quarks. The quantum spin behave a bit like classical spin which is why the term is used but as in all quantum realms is v different too. So try not to thing of it as something actually rotating like a top.... no point trying to picture anything in quantum tbh!!

Yes Physicists do use specific words and phases which are often not clear and too many maybe...our main language is mathematics so to explain outside that phrases are used.. eg entanglement, coherence, etc. Not often v useful to non-physicists. Its a challenge to get info across.....

Bck to orig. it was a bit of an apple and pear comparison (apple being v apt!) . gravity (by which I assume u mean the force that arises) (force will do for now,,  ) is a diff quantity to velocity..thts a distance/time.

why light doesnt escape a black hole (whic is neither a hole or black!) is simply the escape vel is greater than c.

good for reading but u cant beat doing a course and talking face to face with people in the field. OU is vv good......

P

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