# Two questions

#### Saiarii

I have been wondering two things. 1. Why is it that the speed of light, is regarded as some sort of holy grail that will open up everything for mankind (this is the general feeling I get when I read/watch various things on the subject). I mean from my general understanding "the speed of light" is just that. Much like the "speed of sound". 2. Is it (and if it is, how is) it possible to generate gravity without centrifugal force? I ask the second question because I've always been interested since watching many programs as a child where they would have gravity on star ships and space ships, then rooms without gravity on planets (you walk in and can drift about and then go to the door and walk away)

Sorry if these are in the wrong section.

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#### Kazza_765

I don't know about the speed of light being regarded as a holy grail, but it is quite different to the speed of sound. If I throw a ball to you at 20km/h, you catch it at 20km/h. If you were running towards me at 10km/h when you caught it, it would be like catching a ball at 30km/h, and if you were running away from me at 20km/h it would never reach you.

Light is different. No matter what speed two observers are moving at they will always measure light to be travelling at the same speed. This is not the result of some theory but an experimental result confirmed many many times. The theory of relativity looks at the consequences of this and we find that Space and Time bend, people can't agree on the order of events, clocks run slow and distances get smaller. The speed of light also acts as a universal speed limit, no object with mass can ever travel faster than it.

#### Dr.Brain

1) Everything we see is basically the reflected light , the light travels to us and this is what we call vision. Everything from 'time-travel' to 'fastest way of communication ' is bounded by the limit 'c' . This is the limit around which if an object can travel can increase its lifetime biologically . Because of this limit it will take us another thousands of years to communicate with our nearest of intelligent civilisations as the radiowaves we use 'retire before they spend'.

2) The only possible way 'mg' force could act was that is it was a pseudo force.

Refer to :

http://www.doxlab.co.nr

for more on (2)

#### pervect

Staff Emeritus
Saiarii said:
I have been wondering two things. 1. Why is it that the speed of light, is regarded as some sort of holy grail that will open up everything for mankind (this is the general feeling I get when I read/watch various things on the subject). I mean from my general understanding "the speed of light" is just that. Much like the "speed of sound".
The speed of light is much more fundamental than the speed of sound according to our best current theories. That said, I think people who are hung up about 'c' as a limit have their heads waaaay up in the clouds. I think part of this misguided concern is due to the current length of the human lifespan. If technology increases this significantly (and if we can also refrain from blowing ourselves up after reaching the required technology, not necessarily an easy task with the resulting population increase that would be expected as a result), we will see more realistic patience with the amount of time interstellar travel is likely going to take by avalable means.

2. Is it (and if it is, how is) it possible to generate gravity without centrifugal force? I ask the second question because I've always been interested since watching many programs as a child where they would have gravity on star ships and space ships, then rooms without gravity on planets (you walk in and can drift about and then go to the door and walk away)

Sorry if these are in the wrong section.
There are a few theoretical ideas, but nothing remoetly practical. All of the ideas will involve manipulating ultra-dense materials (on the order of electron degenerate matter in density, white dwarf star class densities).

Simple approaches will involve just forming the hyper-dense material in sheets. More exotic approaches might involve circulating this white-dwarf star material in a torroiid at appreciable fractions of the speed of light. This later approach is especially unlikely to be practical. The theory behind this later idea is to attempt to take advantage of gravitomagnetism. Robert Forward has written a lot about the general topic of gravity control at a popular level, he is a good reference if you want popular level wrting from a serious physicist about this topic.

Both of the approaches above will require extremely large masses to produce any sort of artificial gravity. Rotation remains the best bet for artifical gravity, hands down because it will not require extremely large masses to implement, just strong wires (carbon nanotubes come to mind).

#### Saiarii

Thanks for the replies. @Kazza, I'm not sure how that would work....I mean if it takes light approximately 8 minutes to reach us from the sun, then you had a craft that can reach half the speed of light instantly, would it not take the same light 12 minutes to reach the craft, that would have only taken 8 to reach earth? Effectively making the light seem slower?

Thanks for the link Dr.Brain and the information about Robert Forward pervect.

#### pervect

Staff Emeritus
For a specific reference from Forward, check out "Indistinguishable from magic", a collection of serious science essays mixed together with fiction.

#### EnumaElish

Homework Helper
What about non-centric (i.e. "straight-line") acceleration for generating artificial gravity? Author Poul Anderson appears to use this idea in his 1970 book tau zero. Isn't this the point of the theory of General Relativity?

#### εllipse

I'm not sure what you mean by the "point" of general relativity, but GR is a theory of gravity; the best theory of gravity we have so far, so if there is a "point" of GR, I'd say it is to define a theory of gravity which is consistent with special relativity. Another nice thing GR does is allow us to formulate the laws of physics to work for any Gaussian coordinate system, not just inertial reference frames in Minkowskian geometry. Einstein used both linear and angular accelerations in his thought experiments, so the equivalence principle applies equally to angular accelerations as it does to linear ones.

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#### EnumaElish

Homework Helper
Thank you;

What I meant was, "why should the search for artificial gravity be confined to 'angular acceleration' given that the 'equivalance principle' of GR is not specific to angular movement and applies to linear movement as well?" Badly expressed.

#### εllipse

It shouldn't be confined to angular acceleration, but the problem with linear acceleration is you actually have to go somewhere. With angular acceleration you can just set up in the solar system somewhere. Linear acceleration as a source of artificial gravity may be more suited to trips to other stars, but once you get there, you'll probably want the space craft to have the ability to have gravity without linear acceleration, so you can stick around near the star for a while and learn about its planets.

Another problem with linear acceleration is time goes by a lot faster for those you left behind, over a long period of time. If you sat out in the solar system using angular acceleration as a source of gravity, then you'd age just as fast as people on Earth. However, if you went zooming off somewhere, you'd age a lot slower.

And also, linear acceleration would waste a lot more fuel.

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#### EnumaElish

Homework Helper
εllipse said:
It shouldn't be confined to angular acceleration, but the problem with linear acceleration is you actually have to go somewhere. With angular acceleration you can just set up in the solar system somewhere. Linear acceleration as a source of artificial gravity may be more suited to trips to other stars, but once you get there, you'll probably want the space craft to have the ability to have gravity without linear acceleration, so you can stick around near the star for a while and learn about its planets.
Agree.

Another problem with linear acceleration is time goes by a lot faster for those you left behind, over a long period of time. If you sat out in the solar system using angular acceleration as a source of gravity, then you'd age just as fast as people on Earth. However, if you went zooming off somewhere, you'd age a lot slower.
Really??? Does SR predict (or is it consistent with) "no time dilation on a non-linear trajectory"? I thought time dilation was non-directional and depended only on the speed (with which two frames are moving relative to one another).

#### JesseM

EnumaElish said:
Really??? Does SR predict (or is it consistent with) "no time dilation on a non-linear trajectory"? I thought time dilation was non-directional and depended only on the speed (with which two frames are moving relative to one another).
Time dilation is indeed dependent only on speed, but if you accelerate in a circle your speed will be constant in the rest frame of the center of the circle (and unless the circle is really huge your speed will be such a small fraction of light speed that you won't age noticeably slower than people on earth), while if you accelerate in a linear way you will get arbitrarily close to light speed in the frame you began to accelerate, and if you try to return to your point of origin you'll end up very far in the future. See http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html [Broken] for some quantitative examples.

Another issue is that accelerating a large mass at 1 G for significant amounts of time requires a huge amount of energy, whereas once something is accelerating in a circle, it will keep doing so indefinitely thanks to angular momentum.

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#### EnumaElish

Homework Helper
JesseM said:
if you accelerate in a circle your speed will be constant in the rest frame of the center of the circle ...
Uh, trigonometry has always presented a special challenge to my problem-solving abilities, and this may be one such instance. This quote implies that you can accelerate yet have constant speed; contrary to my understanding that acceleration is defined as $$a \equiv {\partial v} / {\partial t}$$, so $$a > 0 \text{ implies } v \uparrow \text{, that is, non-constant } v$$.

#### jtbell

Mentor
EnumaElish said:
This quote implies that you can accelerate yet have constant speed; contrary to my understanding that acceleration is defined as $$a \equiv {\partial v} / {\partial t}$$, so $$a > 0 \text{ implies } v \uparrow \text{, that is, non-constant } v$$.
Velocity is a vector. It has magnitude (which physicists call "speed") and direction. You can change velocity by changing either its magnitude or its direction, or both, and thereby have acceleration:

$$\vec a = \frac {d \vec v} {dt}$$

Uniform circular motion is the classic example of acceleration which arises from change in direction only, with constant speed.

#### russ_watters

Mentor
Saiarii said:
Thanks for the replies. @Kazza, I'm not sure how that would work....I mean if it takes light approximately 8 minutes to reach us from the sun, then you had a craft that can reach half the speed of light instantly, would it not take the same light 12 minutes to reach the craft, that would have only taken 8 to reach earth? Effectively making the light seem slower?
No, because both your perception of the passage of time and your perception of the distance the light traveled will change. But at that speed anyway, those effects are fairly small, so all you'd measure is that the light traveled 1.5 AU in 12 minutes instead of 1 AU in 8 - same speed.

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