# Can you accelerate something to c using gravity?

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Is it possible to use gravity to accelerate something to the speed

of light?

past the swartzchild radius then could something accelerate to c?

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Is it possible to use gravity to accelerate something to the speed

of light?

past the swartzchild radius then could something accelerate to c?

No, nothing can be "accelerated to the speed of light".

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No, nothing can be "accelerated to the speed of light".

why not? With gravity, everything accelerates towards another mass at the same rate, regardless of its mass, so even if it gains more and more mass, then it still accelerates towards it at the same rate (actually it accelerates at a faster rate), and eventually it can get to the speed of light
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Wouldn't the two masses have to start accelerating towards one another from an infinite distance apart?

I'm imagining a universe consisting only of two equal masses set initially at some distance apart.

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Locally you cannot acclerate massive particles to the speed of light.

You can of course have apparant faster than the speed of light motion by warping space-time. However, such exotic things require lots of energy and negative masses.

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Is it possible to use gravity to accelerate something to the speed

of light?

why not? With gravity, everything accelerates towards another mass at the same rate, regardless of its mass, so even if it gains more and more mass, then it still accelerates towards it at the same rate (actually it accelerates at a faster rate), and eventually it can get to the speed of light

You are posting in the relativity forum but attempting to impose Newtonian mechanics.

Apply the correct formula and you will see for yourself why the question is meaningless as posed.

Edited by studiot
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You are posting in the relativity forum but attempting to impose Newtonian mechanics.

Apply the correct formula and you will see for yourself why the question is meaningless as posed.

Then what is the correct equation?
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You are posting in the relativity forum but attempting to impose Newtonian mechanics.

Apply the correct formula and you will see for yourself why the question is meaningless as posed.

Then what is the correct equation?

You are posting in the relativity forum but attempting to impose Newtonian mechanics.

Apply the correct formula and you will see for yourself why the question is meaningless as posed.

Then what is the correct equation?

And it is relativity because it uses special relativity

And it is relativity because it uses special relativity

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As said, it looks like you're applying Newtonian physics. Newtonian physics are good enough for most things, but things change somewhat when relative speeds are very large.

I say relative speed because I think that's all that matters; absolute speed being a meaningless concept - to the point that it doesn't exist. For example, ask yourself: what speed am I doing right now?

Then there's time dilation whereby the faster you go relative to something else the slower you will age relative to the something else (or to put it another way, you will see the something else aging much faster). I understand that according to Einstein you will also get more massive than the something else - something I further understand they have to account for in the LHC. Conclusion must therefore be that since absolute speed doesn't exist, then presumably absolute time and mass don't exist either! Leaving both mass and time to be nothing more than relative as well! Just an illusion mayhap?

Probably poses more questions than it answers.

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why not? With gravity, everything accelerates towards another mass at the same rate, regardless of its mass, so even if it gains more and more mass, then it still accelerates towards it at the same rate (actually it accelerates at a faster rate), and eventually it can get to the speed of light

There are many proofs, here is one of them:

In special relativity, the speed for accelerated motion is related to acceleration via the expression:

$v=\frac{at}{\sqrt{1+(at/c)^2}}$

Simple algebra, shows that for any acceleration $a$ and for any $t$, no matter how large, $v<c$.

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There are many proofs, here is one of them:

In special relativity, the speed for accelerated motion is related to acceleration via the expression:

$v=\frac{at}{\sqrt{1+(at/c)^2}}$

Simple algebra, shows that for any acceleration $a$ and for any $t$, no matter how large, $v<c$.

oh

There are many proofs, here is one of them:

In special relativity, the speed for accelerated motion is related to acceleration via the expression:

$v=\frac{at}{\sqrt{1+(at/c)^2}}$

Simple algebra, shows that for any acceleration $a$ and for any $t$, no matter how large, $v<c$.

oh
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It is my understanding that things accelerate towards centers of gravity, but once an object reaches the bottom of a 'gravity well' its speed will no longer grow (in fact the object will lose speed as it tries to leave the gravitational field). In order to accelerate something to the speed of light, you would probably need a field of infinite gravity, therefore you would need infinite mass, and infinite mass means that it would take infinite energy. All of this means that you cannot use gravity to accelerate an object to c for the same reason that you cannot accelerate a space craft to c using energy; you don't have access to an unlimited amount of energy.

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It is my understanding that things accelerate towards centers of gravity,...

Small point, but does it not depend what is meant by centre? If it's anything other than a infinitely small point it'll hit the surface. And if there was a hole such that it could continue through to the other side the force of gravity would reduce to the point of weightlessness at the centre.

Regarding that particular aspect: presumably there's no gravity at the centre of a black hole? But that's off subject so I shouldn't have posed the question.

Anyway, back to the subject. I agree that it's no different than acceleration by any other means; it'll need an unlimited amount of energy.

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All of this means that you cannot use gravity to accelerate an object to c for the same reason that you cannot accelerate a space craft to c using energy; you don't have access to an unlimited amount of energy.

Yes, I mentioned earlier that there are multiple valid explanations. I gave a kinematic one earlier in the thread. Here is your explanation, in mathematical terms:

$E^2+(pc)^2=(mc^2)^2$

$p=\frac{mv}{\sqrt{1-(v/c)^2}}$

From the above two, one gets:

$E^2+(\frac{mvc}{\sqrt{1-(v/c)^2}})^2=(mc^2)^2$

Explicitate $v$:

$v=c \sqrt{1-\frac{m^2c^4}{E^2}}<c$

Edited by xyzt
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I think Endercreeper01 is viewing it as a classical physics problem. But as we know classical physics is only good enough at everyday speeds and masses.

Anyway. my maths is rubbish so I would appreciate to what p and E refers. I presume v, c and m refers to velocity, speed of light and mass.

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Small point, but does it not depend what is meant by centre? If it's anything other than a infinitely small point it'll hit the surface.

There may be nothing at the center of gravity. For example, the center of gravity of a sitting person is in front of that person.

And if there was a hole such that it could continue through to the other side the force of gravity would reduce to the point of weightlessness at the centre.

Weightless relative to that mass, yes.

Regarding that particular aspect: presumably there's no gravity at the centre of a black hole?

No, there is plenty of gravity, but at that point gravity pulling from all directions is equal.
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I think Endercreeper01 is viewing it as a classical physics problem. But as we know classical physics is only good enough at everyday speeds and masses.

Anyway. my maths is rubbish so I would appreciate to what p and E refers. I presume v, c and m refers to velocity, speed of light and mass.

E=total energy, p=momentum, you got the other 3 right.

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Logical answer: yes, passing the event horizon of a black hole. This matches newton and general relativity.

However special relativity takes priority in this community and any example of c being broken is Chalked up to the lorentz transform.... An item can pass the EH of a black hole from one side at NEAR c... But God stretches time just enough to make sure Einstein stays correct. Two objects can approach from opposite ends, and although each will have a velocity near c relative to the black hole.... Their velocity relative to each other will be approximately equal to the velocity relative to the black hole.... Even though they're approaching eachother.

Cuz Lorentz transform>rational thought.

Edited by Didymus
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Cuz Lorentz transform>rational thought.

We can see that locally we require the constant speed of light as general relativity reduces to special relativity in "small enough" regions.

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Delbert, on 12 Aug 2013 - 7:31 PM, said:

Small point, but does it not depend what is meant by centre?

It should not be forgotton that the centre refers to the cente of the system, not any particular paticipating particle.

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We can see that locally we require the constant speed of light as general relativity reduces to special relativity in "small enough" regions.

Can you define that? What situation makes more rational sense to explain the change we observe as an alteration of time and space as opposed to an alteration of a process?

I.e. muons... we define their halflife as unchangeable... yet observe them last longer than expected. Therefore, we conclude that they couldn't have changed, but that time around them must have changed by the degree anticipated by Einstein. What if his numbers were fine, but his explanation was backwards, and instead of them decaying at the same rate, but the rate expanding.... the function was extended by that same ratio... but it was the function itself, rather than time?

Using a different explanation of the same math, it's quantitatively identical... but logically sound. Whereas the assertion that muons tell time more accurately than time itself is fundamentally ridiculous.

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What is time if not a measure of the rate at which things happen?

Saying that time has remained the same but the rate at which all processes take place has altered by the same amount may give equivalent answers, but then how are you defining time?

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What is time if not a measure of the rate at which things happen?

I think that's almost right. The rate at which things happen determines time, I think would be more profound.

Take the GPS satellites for example. I understand because they are farther away from Earth in a lower gravity field their clocks run faster (things happen faster). But if you or me were sitting on said satellite we wouldn't feel different - we would feel time the same as we were back on Earth. But what we would notice upon looking down on Earth would be earthlings moving slow. In other words whatever time frame we're in, we would feel normal, because our 'normal' is the rate of our processes.

Obviously the difference on a GPS satellite is miniscule, but the principle is whatever time frame we are in in the universe, we will experience the flow of time the same as we are now here on this good Earth - it's everywhere else in a different time frame that we would see to be different.

I think if this apparent illusionary state of affairs weren't the case, then we would be in a special place in the universe, which I understand has been shown to be incorrect.

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I think if this apparent illusionary state of affairs weren't the case, then we would be in a special place in the universe, which I understand has been shown to be incorrect.

That hasn't been shown to be incorrect. It is just a reasonable assumption: the cosmological principle.

Well, reasonable if you are not a geocentrist, I suupose.

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Can you define that?

You need some differential geometry to do that. But basically we have some special coordinates that we can use called normal coordinates. In a sence, normal coordinates are the right generalisation of inertial frames in special relativity. In such coordinates in the neghbourhood of a point p, the metric at p reduces to the Minkowski metric and the Christoffel symbols vanish. This gives us a mathematical notion of the equivalence princical and how non-gravitational physics reduces to special relativity locally.

What situation makes more rational sense to explain the change we observe as an alteration of time and space as opposed to an alteration of a process?

Basically all obserbvers will agree on certian things, but not everything, like simultaneous events or the actual values of measureables. Because of this, any change of the events/processes themselves must reproduce the fact that not all observers agree on everything. And what about an observer comoving with the event? What change will he see and why?

Edited by ajb

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