# Noob question: If you travel as fast as light, time freezes so

1. Jun 2, 2010

### nap

Noob question: Physicists say if you travel as fast as light, time is frozen/stops, if so, how come we can see light coming towards us, when in reality that light is supposedly "frozen" in time?

2. Jun 2, 2010

### dulrich

The photon is frozen in its own time. If it were holding a clock, the clock would never move. In other words, the photon never changes.

3. Jun 2, 2010

### rcgldr

I recall reading that the explanation for this is the rules for objects that only exist at light speed, such as photons, are different than the (special relativity related) rules for sub-light speed objects, which can never acheive light speed. This is why photons can have a frequency and why that frequency is affected the same manner as an ordinary (sub light speed) clock, being slowed down in a strong gravitational field (call gravitational red shift in the case of photons).

4. Jun 8, 2010

### JDługosz

No, the same thing applies to both massive and massless forms. All objects have a wavelength. Gravity slows time for all processes.

5. Jun 9, 2010

### mikeph

The light is not frozen in time. Everything else is frozen in time according to the photon.

There is a difference between an observation of an object in our rest frame, and an observation of us from that object's rest frame. We see photons change all the time, because it is not us that are moving at the speed of light.

6. Jul 3, 2010

### nap

I asked my friend this and he said the only reason why time freezes if you go as fast as light is because we reference everything according to light, so if you were going at light speed on par with a light beam, you would be going "as fast" as the light beam, therefore time would appear to be "frozen" to you.

is this true, or really is it because protons are just frozen in time.

7. Jul 3, 2010

No, its not true. You cannot reference everything by the speed of light, because you always observer the speed of light to be the same - no matter how fast you are going.

There is no answer to the question "If you were going at the speed of light..." because you cannot go at the speed of light. In physics its an impossibility, and you cannot conclude any thing from an impossible premise. Its kind of like asking, "what if squares were circles, what then?". You cant conclude anything from that, it doesn't make sense.

Its a common question though, asking about what if at the speed of light. Thats because it takes a bit of study to see why it is nonsensical, it doesn't seem nonsensical on the surface of it. The better question to ask, one that you can conclude things from is "What happens when you get real close to the speed of light?". You can conclude things from that premise.

8. Jul 3, 2010

### Glen Bartusch

Obviously, if you consider velocities from the light's perspective, then every velocity you measure will be equal to 300km/sec, or the speed of light. this would be true even for light rays coming toward you as well as light rays moving away from you. This "everything is moving at light speed" would even include objects that aren't even in motion relative to you!
How is this even possible? how is it even possible that everything you see is going at the speed of light? It's possible because events that occur behind you and to your sides (including above and below you) are now taking place directly in front of you; in your direection of motion, all concentrated to a fine point.
As you move at the speed of light, things do not happen "behind" or "to the side" of you; all events happen directly in front of you!

9. Jul 3, 2010

### Glen Bartusch

suppose you're on the on-ramp of the Intergalactic Freeway, where all cars run at the speed of light. Sitting there you see all approaching cars in the eastbound lane moving at lightspeed. likewise, all cars in your westbound lane are also driving at light speed.
As you begin to accelerate, all stationary objects (trees, roads and the like) appear to move at your speed, either coming toward you or away from you after you pass it. According to relativity theory, however, all cars moving in your prevailing direction or away from you are ddeiving at light speed.
as you approach lightspeed yourself, what happens? First off, you develop a sort of tunnel vision. In other words, things behind you no longer exist: eastbound traffic now seems to be moving alongside westbound traffic, and the freaky thing is that the eastbound traffic is still moving at the speed of light! How is this possible? a quick glance at the dashboard clock on any of the eastbound cars will give you your answer: their clocks are moving like way too slow! to frustrate matters further, the guys driving the eastbound cars are likewise telling you that your dashboard clock is moving too slowly.
So there we have it. Photons don't "freeze in time;" rather, they move very fast and compensate by their clocks slowing down. If photons only moved at like 15 mph then they'd freeze in time. But they don't; they move at like 300km per second!

10. Jul 9, 2010

### hope4all

The speed of light you cant reach as you try you yourself will slow down because you will gain mass. it is all apart of e=mc squared. (energy=mass times the square of the speed of light) this is a quote from einsteins big idea on nova:
"According to Einstein's special theory of relativity, objects gain mass as they accelerate to greater and greater speeds. Now, to get an object to move faster, you need to give it some sort of push. An object that has more mass needs a bigger push than an object with less mass. If an object reached the speed of light, it would have an infinite amount of mass and need an infinite amount of push, or acceleration, to keep it moving. No rocket engine, no matter how powerful, could do this. In fact, as far as we know, nothing can exceed the speed of light."

it also said later on in the documentary that no matter how fast you were going you would still see things going away from you at the speed of light. That was said by David Bodanis in it. really you should check out that pbs show it was awesome.

11. Jul 9, 2010

### my_wan

If you go so fast that a million years pass for your buddy for every second that passes for you, it'll look to you like it's your buddies clock that slowed down. Who is right? That depends on which one of you accelerates to the others location. If you go to your buddy, your clock will be the slow one. If he comes to you, his clock will be the slow one.

To the light still coming question: If your clock is really slow, then you could say light is moving slow also, it just doesn't look slow to you because time is also slow for you. It will not feel like slow motion, because you don't slow down, time does. So it will feel like a normal rate of time to you.

Light is never "frozen" in time. It is merely going the same speed relative to you no matter how fast or slow your clock gets. So the only way for light to be "frozen" in time, is for you to be "frozen" in time. But if you are "frozen" in time, how are you going to notice light is "frozen" in time?

12. Jul 9, 2010

### DaveC426913

There's a fair bit of misconception happening here.

1] No object with mass can ever reach the speed of light. No matter how long it accelerates, it will always be shy of c.
2] No matter how fast you travel, you will always measure light to move at c.
2] There is no such thing as the frame of reference of a photon. It is nonsensical to ask what the universe might look like to a photon.

13. Jul 10, 2010

### my_wan

Yes, I should have made that more clear in my last paragraph. I was trying too hard to make the point.

I just find it strange so many people think about time as if it's something external to them, and as if their perception of it has some absolute meaning.

14. Jul 10, 2010

### DaveC426913

That wasn't directed at you. Several earlier posters had some serious misconceptions about relativistic velocities.

15. Jul 10, 2010

### Ali Inam

Gravity was mentioned earlier, I dont think that gravity will attract that substance which has no mass. So as, when there will always be gravity, there will always be mass and there would not be possible for any thing to attain the speed of light

I think............

16. Jul 10, 2010

### bcrowell

Staff Emeritus
17. Jul 10, 2010

### DaveC426913

Gravity affects photons.

In Einsteinian general relativity, gravity is not a "force that pulls", gravity is the warpage of spacetime. Photons, like everything else, follow this warpage.

18. Jul 10, 2010

I often hear this restriction to massive objects attaining lightspeed. I don't believe massive objects can attain lightspeed and remain part of this universe but the rocket example seems flawed and I would appreciate being corrected if wrong.

Suppose a rocket is accelerating by streaming propellant out its nozzel. As it accelerates, the rocket gains mass by virtue of its kinetic energy increasing. (Call it "radiant mass" as opposed to "rest mass". Effective mass would be the sum of the two.) Well, it would seem that the fuel (and thus the expelled propellant) has also gained mass, making it all the more effective at accelerating the rocket. That is, the capacity of the fuel to accelerate seems relativistically invariant.

Rockets aside, consider a particle on a path straight toward the center of a black hole. As it accelerates, it gains mass. That would only seem to add to the "force" of attraction (or steepening of spatial slope) experienced by the particle. Rather than a barrier to attaining lightspeed, increasing mass would be an advantage. It appears that lightspeed is indeed attained by massive particles at the event horizon where they effectively leave our universe.

Last edited: Jul 10, 2010
19. Jul 10, 2010

### DaveC426913

There are far too many flaws and misunderstandings in this to address them all. I'll just touch on a few of the most egregious.

0] Please don't make up terms. There is no such thing as "radiant mass" or "effective mass".

1] Relativistic mass does not "count" in considering the effectiveness of a rocket exhaust. In the rocket's frame of reference, there is no increase in mass.

2] A heavy object and a light object fall in a gravitational field at the same speed, the heavier object does not fall faster. Galileo demonstrated this 400 years ago from the top of the Tower of Pisa.

20. Jul 11, 2010

:uhh:
I apologize for my stupidity. I forgot the apparent requirement at PF that the questioner must already know the answer prior to asking. Nevertheless, I thank you for condescending to reply to me, something others may have been too offended to do.
I’m not trying to add to the dictionary here, just establishing a term for use in the current conversation. I would have thought that was obvious from my use of quotes and the preceding notice, “Call it”. Please remember that we outsiders have to endure an ever increasing dictionary of rather whimsical sounding terms emerging from physicists (e.g. “top” & “bottom” quarks, or is it “truth” & “beauty”, I keep forgetting.) Regardless, we need words to communicate, especially about real entities.
Are you sure? I just searched the term “effective mass” on PF I got back 371 threads as a result!

While I agree there is no recognized term, “radiant mass” I believe I am referring to a real quantity. Perhaps you could tell me what to call it? Typically, relativistic mass (m’) is given as a product of a velocity dependant term ($$\gamma$$) and the rest mass (m). m’ increases with velocity of m. Another way to think of this is the difference m’-m = $$\Delta$$m. I find that isolating the rest mass in moving reference frames (i.e. m’=m+$$\Delta$$m) to be helpful in considering the absolute nature of lightspeed. That is rest mass, also referred to as “invariant mass”, is itself constant in all reference frames just like the speed of light! So when I ask myself, “Speed c is constant relative to what?” I have the answer, “Relative to rest mass.” in every frame.

That still leaves me with what to call $$\Delta$$m. If you can’t give me a pre-existing term, I’m sticking with “radiant mass” which I happen to like. Radiant mass would also be a better term for the “relativistic mass” of a photon since photons have no rest mass. You see the problem there. m’ for a photon just can’t be a product of $$\gamma$$ times m. But m’ for a photon could easily survive as a sum m’= 0+$$\Delta$$m. That is, the relativistic mass of a photon is its radiant mass.
Then you see the source of my confusion. If we claim the rocket’s mass is invariant in its own frame, why are we then using infinite mass as a barrier to lightspeed? I could understand this concern if power were being applied from an external (at rest) source. In the rocket’s frame F=ma should give continuous acceleration as long as the fuel holds out, regardless of proximity to light speed. (Of course I could be wrong, even expectantly so, but that’s why I ask.)
The Pisa myth is so good, it ought to be true, so I concede it. But to your point, increasing relativistic mass as a particle descends into a black hole is no advantage. But neither is it a barrier to lightspeed. Acceleration should continue regardless of proximity to lightspeed. For neutrinos, initial velocity is already pretty close. No?

Last edited: Jul 11, 2010
21. Jul 11, 2010

### Mentz114

(my bold).

The people on the rocket will think they are accelerating continuously, but measured from any frame, they will not achieve light speed. It has nothing to do with mass increase - it's the Lorentz transformation.

22. Jul 11, 2010

### DaveC426913

That statement makes no sense. You're not the asker, you're the answerer. Wouldn't it behoove you to have the answer? All I expect is that you stick to known physics, and if you don't know it, at least put a proviso that you don't.

It's probably not, but the lesson is still valid.

Velocity does continue to increase; it just never reaches c.

Last edited: Jul 12, 2010
23. Jul 11, 2010

### Oldfart

I keep hearing about objects following paths in spacetime, rather than being pulled by gravity. Yet the moon and photons, for instance, will obviously not follow the same path no matter how we arrange the experiment. So it seems one cannot "map" spacetime in any universal way. Are we simply dealing with a problem of semantics, is there a spacetime that is unique to every object that moves through it, or what? It outwardly appears that spacetime itself "pulls" if gravity doesn't. I am thoroughly confused, but please be gentle as this is my first post here. Thanks...

24. Jul 12, 2010

### Phrak

In rickety were concerned with reaction mass. This is the relativistic mass, responsible for impulse. Even in ion engine's the difference between intrinsic mass and relativistic mass is negligible, but it is the correct quantity to use.

And by the way, what happened to the cranial flip top?

25. Jul 12, 2010

### DaveC426913

Why do you think this is counter-intuitive? Why would you expect the Moon, which is moving at ~1km/s to move on the same path as light, which is moving at 300,000km/s?

Think about putting on a golf green. Let's say the hole is in a slight depression in the green.

If you putt slow enough, then even a near-miss of several inches will cause the ball to curve inward, deflected by the downward the slope of the depression, and fall into the hole. Right?

But if you putt real hard, the ball is barely deflected at all. You could have the ball pass within a fraction of an inch of hole but, due to its large forward velocity, it will hardly deflect at all (it's not even in the the depression for long enough). So the ball will continue past the hole, its course barely deflected at all.

This is a pretty good approximation of how the Moon and light rays follow distinct paths in the same gravity well (except that the Moon is in a stable orbit, and does not fall in to the Earth.).