Question about gravity and speed of light

[apurvmj]

Hi all,
After reading about Einstein's theory of relativity I have few questions as follows
1. Let's say I am in a space lab which is traveling at the speed of half of the speed of light. So when I try to measure the speed of light coming from space I record it as 'c'. Time is running slow (half with respect to stationary frame) in side of space lab, so if I conduct the experiment to measure the speed of light inside the space lab what result will I get, remember time has slowed to half.

2. Gravity is curvature in space caused by mass right? Then that curvature should be constant for all other small masses. That means photon follow the path along the curvature caused by huge mass. So if Earth is rotating in its orbit around the sun due to curvature in space caused by sun, then if I point a laser in space in the direction of Earth's orbit, shouldn't the photons of laser should also travel along with that curvature and start orbiting sun?

Forgive me if these questions are asked earlier and guide me to that thread.

Thanks in advance.

 

[phinds]

You misunderstand time dilation. It is never something YOU experience, it is something you observe in frames that are moving relative to you. The spaceship in that frame still measures time at one second per second just as you do and you both measure light as traveling at c.

Photons are very mildly perturbed by the sun because they have almost no momentum at all. The Earth is MUCH more strongly attracted to the sun. That is, the curvature is different for different things. For the Earth there is much larger curvature than there is for a photon.

 

[A.T.]

The Earth is MUCH more strongly attracted to the sun. That is, the curvature is different for different things.

The intrinsic space-time curvature caused by the Sun depends only on the Sun, and isn't different for different things. The extrinic curvature of the spatial trajectory of different things depends on their velocity.

 

[apurvmj]

Is It? but don't we apply time correction to GPS satellite to provide proper coordinates. And in 'Interstellar' movie time dilation is shown in the form of one of the actor getting 'physically' old. My understanding was time slows at great speed, so I too slow down at that speed so I don't feel the slowness but I am slow if person watches me from outside.

 

[A.T.]

1. Let's say I am in a space lab which is traveling at the speed of half of the speed of light. So when I try to measure the speed of light coming from space I record it as 'c'. Time is running slow (half with respect to stationary frame) in side of space lab, so if I conduct the experiment to measure the speed of light inside the space lab what result will I get, remember time has slowed to half.

Light speed measured with rulers and clocks at rest is always c. If you use moving locks and rulers, you get different values, but that isn't how speed is defined.

2. Gravity is curvature in space caused by mass right? Then that curvature should be constant for all other small masses. That means photon follow the path along the curvature caused by huge mass. So if Earth is rotating in its orbit around the sun due to curvature in space caused by sun, then if I point a laser in space in the direction of Earth's orbit, shouldn't the photons of laser should also travel along with that curvature and start orbiting sun?

You are confusing intrinsic space-time curvature and extrinsic curvature of spatial trajectories.

 

[A.T.]

Is It? but don't we apply time correction to GPS satellite to provide proper coordinates. And in 'Interstellar' movie time dilation is shown in the form of one of the actor getting 'physically' old. My understanding was time slows at great speed, so I too slow down at that speed so I don't feel the slowness but I am slow if person watches me from outside.

The things you mention involve also gravitational time dilation, which isn't symmetrical like kinetic time dilation.

 

[Nugatory]

Hi all,
After reading about Einstein's theory of relativity I have few questions as follows
1. Let's say I am in a space lab which is traveling at the speed of half of the speed of light

Travelling relative to what? It is meaningless to speak of any speed without saying what it is relative to - remember that as far as you in teh space lab are concerned, you are at not moving at all, it's just that if you look out the window you might see other things moving by relative to you.

Time is running slow (half with respect to stationary frame) in side of space lab, so if I conduct the experiment to measure the speed of light inside the space lab what result will I get, remember time has slowed to half.

Nope. is running at the exact same speed as always for you: Your clock ticks once a second. It's true that someone moving relative to you will find that your clock is running slow relative to his - but you'll say the exact same thing about his clock because as far as you're concerned, you're the one who is at rest and he is moving relative to you.

You will both measure the speed of light to be c, inside the ship, outside the ship, everywhere.

Gravity is curvature in space caused by mass right? Then that curvature should be constant for all other small masses. That means photon follow the path along the curvature caused by huge mass.

If you're trying to say that we can ignore the gravitational effects of the small masses because they're just a rounding error compared with the large mas... Then yes. Otherwise, I don't understand what you're trying to say here.

So if Earth is rotating in its orbit around the sun due to curvature in space caused by sun, then if I point a laser in space in the direction of Earth's orbit, shouldn't the photons of laser should also travel along with that curvature and start orbiting sun?

No. It's not space that is curved, it's spacetime. Although you've started your flash of light out in the same direction as the Earth's orbit in space, they're completely different directions in spacetime

 

[phinds]

The intrinsic space-time curvature caused by the Sun depends only on the Sun, and isn't different for different things. The extrinic curvature of the spatial trajectory of different things depends on their velocity.

Ah. Thanks for that correction.

 

[phinds]

By the way ampurvmj, you might find it helpful to keep in mind that you, right now as you read this, are traveling at .9999999c in the frame of reference of an "accelerated" particle at CERN. Do you feel time dilated? Has your watch slowed down?

 

[apurvmj]

ok... then what made lead actor in movie 'Interstellar' look younger than rest of the universe. Am I making sense?

 

[Nugatory]

And in 'Interstellar' movie time dilation is shown in the form of one of the actor getting 'physically' old. My understanding was time slows at great speed, so I too slow down at that speed so I don't feel the slowness but I am slow if person watches me from outside.

Interstellar is a movie not real science, and when you say "time slows down at great speeds" you are making a very common mistake, one that just about everyone makes when they first encounter relativity.

I said a few posts above that you must never speak of a speed without saying what it is relative to - that's the first step in making sense of relativity.

 

[Nugatory]

ok... then what made lead actor in movie 'Interstellar' look younger than rest of the universe. Am I making sense?

The screenwriter made it happen, the same way that in the Lord of the Rings movie Peter Jackson made Barad-Dur collapse when the ring fell into the lava.

To understand the real science, you'll have to back up and learn the real science behind time dilation and differential ageing. I very strongly recommend that you figure out the basic gravity-free relative-speed-only scenarios first - once you understand these you can build on it to take on the more complicated gravitational

 

[apurvmj]

My frame of reference is source of light towards which I am approaching.

 

[phinds]

My frame of reference is source of light towards which I am approaching.

You can't use light as a frame of reference because it doesn't have one.

Google the "twin paradox" for a discussion of relative aging. The AMOUNT of time passed differs for different paths through space-time even though it each of the paths time is passing at one second per second. This seems counterintuitive but it's all in the math and it does represent reality.

 

[apurvmj]

No friend I said source of light not light. Let say a star.

 

[phinds]

No friend I said source of light not light. Let say a star.

OK, my mistake. You have a valid frame of reference, but what's your point? Do you understand the answers you have been given?

 

[apurvmj]

 

[phinds]

You think the answers you have been given are hilarious?

 

[apurvmj]

Twin paradox says one of the twin will be younger if he is traveling at the speed high enough to notice time dilation.

 

[apurvmj]

about the "makeup' answer

 

[phinds]

Twin paradox says one of the twin will be younger if he is traveling at the speed high enough to notice time dilation.

NO. It says he will be younger. He does not "notice time dilation". Please re-read post #9 and think about it.

Also, read what I said in post #14 and think about that as well.

 

[apurvmj]

At phinds, I know he will not feel time dilation coz he is part of the system, he too will move like sloth for out side observer. So whoever is feeling time dilation its a observable and physical thing.
I found it difficult to understand that speed of light is constant with all frame of reference. Then what is the difference between photon and a particle of mass 1 gram traveling at speed of light in vacuum. That particle will still travel at speed of light unless acted by external force. So if I try to measure speed of that particle while moving toward it what should I observe?

 

[jtbell]

Then what is the difference between photon and a particle of mass 1 gram traveling at speed of light in vacuum.

The difference is that a particle of mass one gram cannot travel at the speed of light; whereas a photon cannot travel at any speed except the speed of light.

 

[apurvmj]

Ok let's say one gram particle travels at 0.9c. Now

 

[Janus]

Ok let's say one gram particle travels at 0.9c. Now

Okay, let's say there is a particle traveling at 0.9c relative to you. I am moving at 0.5c in the opposite direction.
You measure the particle as moving at 0.9c ( relative to you)
I will measure the particle as moving at 0.9655c ( relative to me).
This is because, under Relativity, velocities add by the rule of

$$w = \frac{u+v}{1+\frac{uv}{c^2}}$$

and not

w=u+v

If i was moving at 0.5c in the same direction as the particle (relative to you),
I would measure the particle as moving at 0.7273c (relative to me).

Note what would happen if I make either u or v equal to c. Then, no matter what the value of the other, the answer becomes c.

So the same rule for the addition for velocities applies to both light and sub-light particles.

 

[Shailesh Pincha]

Hi all,
After reading about Einstein's theory of relativity I have few questions as follows
1. Let's say I am in a space lab which is traveling at the speed of half of the speed of light. So when I try to measure the speed of light coming from space I record it as 'c'. Time is running slow (half with respect to stationary frame) in side of space lab, so if I conduct the experiment to measure the speed of light inside the space lab what result will I get, remember time has slowed to half.

2. Gravity is curvature in space caused by mass right? Then that curvature should be constant for all other small masses. That means photon follow the path along the curvature caused by huge mass. So if Earth is rotating in its orbit around the sun due to curvature in space caused by sun, then if I point a laser in space in the direction of Earth's orbit, shouldn't the photons of laser should also travel along with that curvature and start orbiting sun?

Forgive me if these questions are asked earlier and guide me to that thread.

Thanks in advance.

When you are in a space lab, it itself is a stationary frame and thus you can't perform an experiment which could prove that your time is running slow. It's always the observed quantities which change and not observer's.

For the second query may this help.

 

[apurvmj]

exactly, there is no way to know I am slowing unless I stop by my say imaginary twin and compare the watches. But when we meet after long time, my watch will be behind to that of my twin, that proves time was running slow for me, and so should the speed of light should be 0.5c inside the space lab.

 

[phinds]

exactly, there is no way to know I am slowing unless I stop by my say imaginary twin and compare the watches. But when we meet after long time, my watch will be behind to that of my twin, that proves time was running slow for me, and so should the speed of light should be 0.5c inside the space lab.

No, it most certainly does not "prove" that at all, it just proves that you still haven't figure out how special relativity works. Do you think all of us who keep telling you you are making a mistake are wrong? We've been at this for a while and you have not and yet you believe you are right and we are wrong. Does that really make sense to you?

 

[apurvmj]

phinds, by no means I mean you ppl are wrong. But found this theory is confusing.

 

[phinds]

phinds, by no means I mean you ppl are wrong. But found this theory is confusing.

Well, most of us DO find it confusing when we first encounter it. Several times when I was first learning the rudiments of cosmology and relativity and then again with quantum mechanics I would stomp around the room pulling my hair out and shouting THAT CAN"T BE RIGHT !

 

[HallsofIvy]

Twin paradox says one of the twin will be younger if he is traveling at the speed high enough to notice time dilation.

No, it doesn't. It says that each twin will see the other twin as younger. The only "paradox" occurs when you arrange for the two twins to be again together and motionless with respect to each other. And then which is younger will depend upon exactly how they returned to that situation.

 

[Jaami M.]

Travelling relative to what? It is meaningless to speak of any speed without saying what it is relative to - remember that as far as you in teh space lab are concerned, you are at not moving at all, it's just that if you look out the window you might see other things moving by relative to you.

Nugatory... No question is meaningless.
Technically he would be relative to "something" and that something is the light in his cosmological traveling lab. Light would be moving in all directions inside the lab. The reason he will be moving slower to relative light speed and space is because he is simply moving away from light at half the speed of light. Light will have to bounce off the object he sees then also have to catch up to his eyes at half its normal speed. (light speed travels 50% slower in the direction he moves away from it) remember that light travels at 186,000 mph, but from his new perspective it now tops at 93,000 mph (which is still astonishingly faster than sounds average speed of 760 mph) meaning that his interactions and perception wouldn't to much different from normal. MY QUESTION IS... IS SPEED AND TIME INTERTWINED?!.

 

[Drakkith]

Technically he would be relative to "something" and that something is the light in his cosmological traveling lab. Light would be moving in all directions inside the lab. The reason he will be moving slower to relative light speed and space is because he is simply moving away from light at half the speed of light. Light will have to bounce off the object he sees then also have to catch up to his eyes at half its normal speed. (light speed travels 50% slower in the direction he moves away from it) remember that light travels at 186,000 mph, but from his new perspective it now tops at 93,000 mph (which is still astonishingly faster than sounds average speed of 760 mph) meaning that his interactions and perception wouldn't to much different from normal.

This is entirely incorrect. As measured by the person aboard the station, the light moves at c, not at 1/2 c. You cannot define velocities relative to light and have any meaningful statement, as light moves at c for ALL inertial observers, no matter what their velocities relative to each other are.

 

[Jaami M.]

ok... then what made lead actor in movie 'Interstellar' look younger than rest of the universe. Am I making sense?

From the theories I've learned of and from my understanding it's because of the black hole he entered. Now whether it's possible to survive a black hole or not, when he left it he was in the future. This happened because of how time-space Bends and warps around the black hole, because of its Immense Gravitational Field, making "time" also warp. Ultimately making your perceptive relevance to other humans appear to move slower. From my understanding, if he ENTERED a BLACK HOLE, he should be teared limb from mole is to atom! Or at least his daughter should be long gone, and humans should be terraforming Mars and be fishing on Jupiters moon Europa by now! But remember that interstellar is just a movie. There are so many theories of black holes, from creation of another universe to space travel.

 

[Jaami M.]

This is entirely incorrect. As measured by the person aboard the station, the light moves at c, not at 1/2 c. You cannot define velocities relative to light and have any meaningful statement, as light moves at c for ALL inertial observers, no matter what their velocities relative to each other are.

Your completely right, sorry about that. But the argument is still valid via argument correct?