# I Question about gravity and speed of light

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1. Jul 18, 2015

### apurvmj

Hi all,
After reading about Einstein's theory of relativity I have few questions as follows
1. Lets 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.

2. Jul 18, 2015

### 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 space ship 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.

3. Jul 18, 2015

### A.T.

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.

4. Jul 18, 2015

### 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.

5. Jul 18, 2015

### A.T.

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.

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

6. Jul 18, 2015

### A.T.

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

7. Jul 18, 2015

### Staff: Mentor

Travelling relative to what? It is meaningless to speak of any speed with out 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.

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.

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.

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

8. Jul 18, 2015

### phinds

Ah. Thanks for that correction.

9. Jul 18, 2015

### 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?

10. Jul 18, 2015

### apurvmj

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

11. Jul 18, 2015

### Staff: Mentor

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.

12. Jul 18, 2015

### Staff: Mentor

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

13. Jul 18, 2015

### apurvmj

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

14. Jul 18, 2015

### phinds

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.

15. Jul 18, 2015

### apurvmj

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

16. Jul 18, 2015

### phinds

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

17. Jul 18, 2015

### apurvmj

18. Jul 18, 2015

### phinds

You think the answers you have been given are hilarious?

19. Jul 18, 2015

### apurvmj

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

20. Jul 18, 2015

### apurvmj

21. Jul 18, 2015

### phinds

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.

22. Jul 18, 2015

### 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?

23. Jul 18, 2015

### Staff: Mentor

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.

24. Jul 18, 2015

### apurvmj

Ok lets say one gram particle travels at 0.9c. Now

25. Jul 18, 2015

### Janus

Staff Emeritus
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.