# Will time stop if we were able to go as fast as the speed of light?

## Main Question or Discussion Point

Just saw a video about Einstein. He found out that if we go as fast as the speed of light, time will stop. Therfore, if we were able to build a divice that technically could go faster than the speed of light, it wouldn't be posible because time would stop the moment you reach the speed of light?
Cause when time stands still, you arent able to move, and everything stops.

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bcrowell
Staff Emeritus
Gold Member
FAQ: What does the world look like in a frame of reference moving at the speed of light?

This question has a long and honorable history. As a young student, Einstein tried to imagine what an electromagnetic wave would look like from the point of view of a motorcyclist riding alongside it. But we now know, thanks to Einstein himself, that it really doesn't make sense to talk about such observers.

The most straightforward argument is based on the positivist idea that concepts only mean something if you can define how to measure them operationally. If we accept this philosophical stance (which is by no means compatible with every concept we ever discuss in physics), then we need to be able to physically realize this frame in terms of an observer and measuring devices. But we can't. It would take an infinite amount of energy to accelerate Einstein and his motorcycle to the speed of light.

Since arguments from positivism can often kill off perfectly interesting and reasonable concepts, we might ask whether there are other reasons not to allow such frames. There are. One of the most basic geometrical ideas is intersection. In relativity, we expect that even if different observers disagree about many things, they agree about intersections of world-lines. Either the particles collided or they didn't. The arrow either hit the bull's-eye or it didn't. So although general relativity is far more permissive than Newtonian mechanics about changes of coordinates, there is a restriction that they should be smooth, one-to-one functions. If there was something like a Lorentz transformation for v=c, it wouldn't be one-to-one, so it wouldn't be mathematically compatible with the structure of relativity. (An easy way to see that it can't be one-to-one is that the length contraction would reduce a finite distance to a point.)

altairz said:
... moment you reach the speed of light?
As it stands that does not make sense. Another thing relativity says is that there is no absolute motion, so speeds can only be measured relative to something else. Consider two cars travelling in the same direction at 60Kph relative to the road. Their speeds relative to each other is very small. Velocity ( speed vector ) is something that only makes sense relative to another frame.

Fredrik
Staff Emeritus
Gold Member
Just saw a video about Einstein. He found out that if we go as fast as the speed of light, time will stop. Therfore, if we were able to build a divice that technically could go faster than the speed of light, it wouldn't be posible because time would stop the moment you reach the speed of light?
Cause when time stands still, you arent able to move, and everything stops.
Massive objects can't be accelerated to the speed of light. The energy required to accelerate any of them to speed v goes to infinity as v goes to c. This result is derived here. (But I should have added that the first few equalities aren't obvious truths, but rather definitions of what we mean by "work", "force" and "momentum" in SR).

Massless particles like photons can only move at the speed of light. So has time stopped for them? The problem with that question is the words "for them". There is no natural way to define a coordinate system that represents a massless particle's point of view. The method we use to define a massive particle's point of view doesn't work. More about that here. So that question doesn't make sense.

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FAQ: What does the world look like in a frame of reference moving at the speed of light?

This question has a long and honorable history. As a young student, Einstein tried to imagine what an electromagnetic wave would look like from the point of view of a motorcyclist riding alongside it. But we now know, thanks to Einstein himself, that it really doesn't make sense to talk about such observers.

The most straightforward argument is based on the positivist idea that concepts only mean something if you can define how to measure them operationally. If we accept this philosophical stance (which is by no means compatible with every concept we ever discuss in physics), then we need to be able to physically realize this frame in terms of an observer and measuring devices. But we can't. It would take an infinite amount of energy to accelerate Einstein and his motorcycle to the speed of light.

Since arguments from positivism can often kill off perfectly interesting and reasonable concepts, we might ask whether there are other reasons not to allow such frames. There are. One of the most basic geometrical ideas is intersection. In relativity, we expect that even if different observers disagree about many things, they agree about intersections of world-lines. Either the particles collided or they didn't. The arrow either hit the bull's-eye or it didn't. So although general relativity is far more permissive than Newtonian mechanics about changes of coordinates, there is a restriction that they should be smooth, one-to-one functions. If there was something like a Lorentz transformation for v=c, it wouldn't be one-to-one, so it wouldn't be mathematically compatible with the structure of relativity. (An easy way to see that it can't be one-to-one is that the length contraction would reduce a finite distance to a point.)
1. So we can't build a device that goes at the speed of light, because we would need an infinite amount of energy to be able to do it?

2. But if we break all laws of physics, and say there was a divice that could ride the beam light, would then time stand still for that divice? (just wonder what would happen IF it was possible :P)

3. As the speed of light itself, lenght shrinks to zero, and time stands still. Why?

1. Yes

2. You are asking what the laws of physics would predect if the laws of physics weren't present. That is a paradox.

turin
Homework Helper
... say there was a divice that could ride the beam light, would then time stand still for that divice?
Actually, that is basically how some particle accelerators work. The "divice" would be a particle, or bunch of them. The "beam light" would be an RF wave. The particles sort of surf along the wave. The particles are not quite traveling at the speed of light, but very VERY close to it. (I'm not sure about the details, though.)

So far, I am only aware of using relatively stable particles in colliders, so the time that they experience isn't a very important issue. However, I have heard talk of a muon collider. Muons "die" very quickly, just microseconds after they are "born". Light cannot even travel 1 km in that small amount of time, not much room for acceleration up to interesting energy levels. However, if the muons are given enough energy (large enough gamma factor), then their "heart rate" can be slowed down enough that they can make a trip of several km (possibly 100's of km) before "dying". So, while time does not stop, it can be greatly slowed down in a practical relevant way (from the point of view of the experimenters).

The known evidence points to a limiting case, that as an object approaches the speed of light, time experienced by that object approaches a standstill in a very physical real way. I think that's as good as we can know it.

Just saw a video about Einstein. He found out that if we go as fast as the speed of light, time will stop. Therfore, if we were able to build a divice that technically could go faster than the speed of light, it wouldn't be posible because time would stop the moment you reach the speed of light?
Cause when time stands still, you arent able to move, and everything stops.
Simply put time would stop only to those people not moving with you. You would be able to breath and move and to you time would be happening normally I believe. But when you stopped it would be near the point in time when you started. I'm not certain if time absolutely stops at lightspeed or is going really really slow. I'm new to physics too, but I do know its impossible to go that fast, we can go as close as possible though. .999999999999999% etc etc

But physics is an ever-changing set of laws. Its entirely possible that in 100 years we will view the light speed limit as we viewed the sound barrier 100 years ago. There are theories of variable speed of light in the beginning of the universe. And there are alternative theories coming out now to relativity. Stay tuned in, keep reading the material at this site lol. You never know what we'll discover tomorrow let alone years from now.

turin
Homework Helper
... when you stopped it would be near the point in time when you started.
To the contrary, you would be very far into the future of the universe when you stop. See the imfamous twin paradox.

To the contrary, you would be very far into the future of the universe when you stop. See the imfamous twin paradox.
yeah my bad I knew that, exactly the opposite. From say earths vantage it would seem that your time stopped or u just froze if they could see you somehow.

from the way I see it, it isn't an issue of if amounts of energy. there is no possibility for mass to be = to c. the confusion arises from the fact that we can calculate velocities relativistically below c. and make the incorrect assumption of attaining c.

I'll admit to being a complete neophyte to the advanced math and esp. the correct terms for physics. So forgive me if I'm stating the obvious to others.

also I wouldn't say the time stops even at c, it's the fact that limit of c is space time. time is not universal but velocity in reference to space time is and at the limit velocity is no longer a value.

The photon then is everywhere and nowhere like a fuzzy foam covering the space time. which is why light is not free of the force if gravity. to have c's vector altered by gravity means that gravity is playing a part of space time 'topology' and the path light takes must abide by the locations of the photon. this also can be stated that distance, time and velocity and possibly gravity are a zero point. and that leads me to that light from what we observe is not = c. light carries information which is energy and mass, otherwise we could not distinguish distance between objects in the vacuum of space.

Would the we be moving at the speed of light in reference to a planet/observer that is beyond the visible limit of the universe? Granted they could never observe us due to expansion of the universe.

turin
Homework Helper
Granted they could never observe us ...
This renders the question beyond the scope of physics.